Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 01, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	Bellicum Pharmaceuticals, Inc.
Entity Central Index Key	0001358403
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	true
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Ex Transition	true
Entity Small Business	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		44,320,559
Entity Well-Known Seasonal Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 217,815,748

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 43,695	$ 38,839
Investment securities, available for sale	49,304	60,057
Accounts receivable, interest and other receivables	909	320
Prepaid expenses and other current assets	1,387	2,434
Total current assets	95,295	101,650
Investment securities, available for sale - long-term	0	1,368
Property and equipment, net	20,878	25,942
Restricted cash	4,973	6,190
Other assets	355	378
TOTAL ASSETS	121,501	135,528
Current liabilities:
Accounts payable	3,774	3,287
Accrued expenses and other current liabilities	8,589	6,392
Current portion of capital lease obligations	40	31
Current portion of deferred revenue	2,983	2,049
Current portion of deferred rent	418	397
Total current liabilities	15,804	12,156
Long-term liabilities:
Long-term debt, net of deferred financing costs	35,832	34,946
Capital lease obligations	91	131
Deferred revenue	0	2,054
Deferred rent	1,296	1,593
TOTAL LIABILITIES	53,023	50,880
Commitments and Contingencies
Stockholders’ Equity:
Preferred stock: $0.01 par value; 10,000,000 shares authorized: no shares issued and outstanding	0	0
Common stock: $0.01 par value; 200,000,000 shares authorized at December 31, 2018 and 2017; 44,242,059 shares issued and 43,564,596 shares outstanding at December 31, 2018; 33,962,640 shares issued and 33,285,177 shares outstanding at December 31, 2017	442	340
Treasury stock: 677,463 shares held at December 31, 2018 and 2017	(5,056)	(5,056)
Additional paid-in capital	493,784	411,922
Accumulated other comprehensive loss	(144)	(46)
Accumulated deficit	(420,548)	(322,512)
Total stockholders’ equity	68,478	84,648
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 121,501	$ 135,528

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value (in usd per share)	$ 0.01	$ 0.01
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in usd per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	200,000,000	200,000,000
Common stock, issued (in shares)	44,242,059	33,962,640
Common stock, outstanding (in shares)	43,564,596	33,285,177
Treasury stock (in shares)	677,463	677,463

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
REVENUES
Total revenues	$ 1,120	$ 185
OPERATING EXPENSES
Research and development	71,152	65,663
License fees	436	864
General and administrative	24,998	21,045
Total operating expenses	96,586	87,572
LOSS FROM OPERATIONS	(95,466)	(87,387)
OTHER INCOME (EXPENSE)
Interest income	1,639	1,055
Interest expense	(4,199)	(3,672)
Gain/(Loss) on disposal of assets	(10)	11
Loss on extinguishment of debt	0	(1,786)
Total other expense	(2,570)	(4,392)
NET LOSS	$ (98,036)	$ (91,779)
Net loss per share attributable to common shareholders - basic and diluted (in usd per share)	$ (2.44)	$ (2.89)
Weighted-average shares outstanding-basic and diluted (in shares)	40,230,580	31,714,164
Net Loss	$ (98,036)	$ (91,779)
Other comprehensive loss:
Unrealized gain (loss) on securities, net	1	(63)
Foreign currency translation adjustment	(99)	0
Comprehensive loss	$ (98,134)	(91,842)
Grants
REVENUES
Total revenues		$ 185

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance, beginning of period (in shares) at Dec. 31, 2016		27,833,028	677,463
Balance, beginning of period at Dec. 31, 2016	$ 96,574	$ 278	$ (5,056)	$ 332,068	$ (230,733)	$ 17
Increase (Decrease) in Stockholders' Equity
Share-based compensation	$ 13,569			13,569
Exercise of stock options (in shares)	344,958	344,958
Exercise of stock options	$ 1,486	$ 4		1,482
Issuance of common stock - Employee Stock Purchase Plan (in shares)		34,654
Issuance of common stock - Employee Stock Purchase Plan	293			293
Issuance of common stock in a public offering, net (in shares)		5,750,000
Issuance of common stock in a public offering, net	64,568	$ 58		64,510
Comprehensive loss	(91,842)				(91,779)	(63)
Balance, end of period (in shares) at Dec. 31, 2017		33,962,640	677,463
Balance, end of period at Dec. 31, 2017	84,648	$ 340	$ (5,056)	411,922	(322,512)	(46)
Increase (Decrease) in Stockholders' Equity
Share-based compensation	$ 13,824			13,824
Exercise of stock options (in shares)	1,044,450	1,016,803
Exercise of stock options	$ 3,270	$ 10		3,260
Issuance of common stock - Employee Stock Purchase Plan (in shares)		45,390
Issuance of common stock - Employee Stock Purchase Plan	205			205
Issuance of common stock in a public offering, net (in shares)		9,200,000
Issuance of common stock in a public offering, net	64,665	$ 92		64,573
Issuance of common stock upon vesting of restricted stock units (in shares)		17,226
Comprehensive loss	(98,134)				(98,036)	(98)
Balance, end of period (in shares) at Dec. 31, 2018		44,242,059	677,463
Balance, end of period at Dec. 31, 2018	$ 68,478	$ 442	$ (5,056)	$ 493,784	$ (420,548)	$ (144)

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (98,036)	$ (91,779)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation	13,824	13,569
Depreciation expense	6,698	3,564
Amortization of premium on investment securities, net	94	302
Amortization of lease liability	(276)	(102)
Amortization of deferred financing costs	886	774
Loss on extinguishment of debt	0	1,786
(Gain)/Loss on disposal of property and equipment	10	(11)
Changes in operating assets and liabilities:
Accounts receivable	(589)	14
Prepaid expenses and other current assets	1,047	(930)
Other assets	23	(95)
Accounts payable	460	(512)
Accrued liabilities and other	2,197	(3,667)
Deferred revenue – grants	(1,120)	4,103
NET CASH USED IN OPERATING ACTIVITIES	(74,782)	(72,984)
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sale of investment securities	71,362	63,737
Purchases of investment securities	(59,335)	(54,895)
Proceeds on disposition of equipment	0	39
Purchases of property and equipment	(1,617)	(12,134)
CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	10,410	(3,253)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from debt	0	45,000
Payment on debt	0	(32,688)
Payment of debt issuance costs	0	(150)
Payment on capital lease obligations	(31)	(23)
Proceeds from issuance of common stock, net	64,860	64,860
Proceeds from exercise of stock options	3,270	1,486
Proceeds from issuance of common stock - ESPP	205	293
Payment of issuance costs on common stock	(195)	(292)
NET CASH PROVIDED BY FINANCING ACTIVITIES	68,109	78,486
EFFECT OF EXCHANGE RATE CHANGES ON CASH	(98)	0
NET CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	3,639	2,249
CASH, CASH EQUIVALENTS AND RESTRICTED CASH AT BEGINNING OF YEAR	45,029	42,780
CASH, CASH EQUIVALENTS AND RESTRICTED CASH AT END OF YEAR	48,668	45,029
SUPPLEMENTAL CASH FLOW INFORMATION:
Cash paid during the period for interest	3,025	2,951
NON-CASH INVESTING AND FINANCING ACTIVITIES
Purchases of property and equipment in accounts payables and accrued liabilities	27	872
Accrued debt issuance costs	0	3,045
Capital lease obligations incurred for equipment	$ 0	$ 23

ORGANIZATION AND BUSINESS DESCRIPTION	12 Months Ended
ORGANIZATION AND BUSINESS DESCRIPTION	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION AND BUSINESS DESCRIPTION	ORGANIZATION AND BUSINESS DESCRIPTION Bellicum Pharmaceuticals, Inc., (“Bellicum”), was incorporated in Delaware in July 2004 and is based in Houston, Texas. Bellicum is a clinical stage biopharmaceutical company focused on discovering and developing novel cellular immunotherapies for various forms of cancer, including both hematological cancers and solid tumors, as well as orphan inherited blood disorders. Bellicum is devoting substantially all of its present efforts to developing next-generation product candidates in some of the most important areas of cellular immunotherapy, including CAR T and hematopoietic stem cell transplantation. In 2017, Bellicum formed two wholly-owned subsidiaries, Bellicum Pharma Limited, a private limited company organized under the laws of the United Kingdom, and Bellicum Europe GmbH, a private limited liability company organized under Swiss law. In 2018, Bellicum formed Bellicum Pharma GmbH, a private limited liability company organized under German law. All were formed for the purpose of developing product candidates in Europe. Bellicum, Bellicum Pharma Limited, Bellicum Europe GmbH and Bellicum Pharma GmbH are collectively referred to herein as the “Company”.

BASIS OF PRESENTATION AND MANAGEMENT PLANS	12 Months Ended
BASIS OF PRESENTATION AND MANAGEMENT PLANS	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
BASIS OF PRESENTATION AND MANAGEMENT PLANS	BASIS OF PRESENTATION AND MANAGEMENT PLANS Going Concern The consolidated financial statements have been prepared on a going concern basis, which assumes the Company will continue to realize its assets and discharge its liabilities in the normal course of business. The continuation of the Company as a going concern is dependent upon the ability of the Company to obtain necessary funding to continue operations. As of December 31, 2018, the Company has incurred an accumulated deficit of $420.5 million since inception and had not yet generated any revenue from operations. Additionally, , the Company continues to expend cash to continue its R&D efforts. Management anticipates that its cash on hand as of December 31, 2018, grants and other cash inflows will be insufficient to fund its operations within one year from the financial statement issuance date and therefore, substantial doubt about the entity’s ability to continue as a going concern exists. These consolidated financial statements do not include any adjustments to the recoverability and classification of recorded asset amounts and classification of liabilities that might be necessary should the Company be unable to continue as a going concern. The Company may seek additional funding through a combination of equity offerings, debt financings, government or other third-party funding, commercialization, marketing and distribution arrangements, other collaborations, strategic alliances and licensing arrangements and delay planned cash outlays or a combination thereof. Management cannot be certain that such events or a combination thereof can be achieved. The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America. Any reference in these footnotes to applicable guidance is meant to refer to the authoritative U.S. generally accepted accounting principles (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The Company is subject to risks common to companies in the biotechnology industry and the future success of the Company is dependent on its ability to successfully complete the development of, and obtain regulatory approval for, its product candidates, manage the growth of the organization, obtain additional financing necessary in order to develop, launch and commercialize its product candidates, and compete successfully with other companies in its industry.

SIGNIFICANT ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

SIGNIFICANT ACCOUNTING POLICIES

Use of Estimates

The preparation of the financial statements in accordance with GAAP requires management to make certain estimates and judgments that affect the reported amounts of assets, liabilities, and expenses. Actual results could differ from those estimates.

Consolidation

All financial information presented includes the accounts of the Company and its wholly-owned subsidiaries, neither of which have had any material activity to date. All significant intercompany balances and transactions have been eliminated in consolidation.

Revenue Recognition

The Company has not yet generated any revenue from product sales. The Company’s source of revenue in 2018 and 2017 has been from grants. When grant funds are received after costs have been incurred, the Company accrues revenue and records a grant receivable. Cash received from grants in advance of incurring qualifying costs is recorded as deferred revenue and recognized as revenue when qualifying costs are incurred.

Cash and Cash Equivalents

The Company considers all short-term, highly liquid investments with maturity of three months or less from the date of purchase to be cash equivalents.

Investment Securities

Consistent with its investment policy, the Company invests its cash allocated to fund its short-term liquidity requirements with prominent financial institutions in bank depository accounts and institutional money market funds. The Company invests the remainder of its cash in corporate debt securities and municipal bonds rated at least A quality or equivalent, U.S. Treasury notes and bonds and U.S. and state government agency-backed securities.

The Company determines the appropriate classification of investment securities based on whether they represent the investment of funds available for current operations, as defined in ASC 210-10-45-1 and ASC 210-10-45-2. The Company reevaluates its classification as of each balance sheet date. All investment securities owned are classified as available-for-sale. The cost of securities sold is based on the specific identification method. Investment securities are recorded as of each balance sheet date at fair value, with unrealized gains and, to the extent deemed temporary, unrealized losses reported as accumulated other comprehensive gain (loss), a separate component of stockholders' equity. Interest and dividend income on investment securities, accretion of discounts and amortization of premiums and realized gains and losses are included in interest income in the statements of operations and comprehensive income (loss).

An investment security is considered to be impaired when a decline in fair value below its cost basis is determined to be other than temporary. The Company evaluates whether a decline in fair value of an investment security is below its cost basis is other than temporary using available evidence. In the event that the cost basis of the investment security exceeds its fair value, the Company evaluates, among other factors, the amount and duration of the period that the fair value is less than the cost basis, the financial health of and business outlook for the issuer, including industry and sector performance, and operational and financing cash flow factors, overall market conditions and trends, the Company’s intent to sell the investment security and whether it is more likely than not the Company would be required to sell the investment security before its anticipated recovery. If a decline in fair value is determined to be other than temporary, the Company records an impairment charge in the statement of operations and comprehensive loss and establishes a new cost basis in the investment.

Property and Equipment

Leasehold improvements, furniture, equipment and software are recorded at cost and are depreciated using the straight-line method over the estimated useful lives of the related assets, which range from three to five years. Leasehold improvements are amortized over the shorter of the estimated useful life or the remaining lease term.

Impairment of Long-Lived Assets

The Company reviews long-lived assets for impairment when events or changes in business conditions indicate that their carrying value may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to the estimated undiscounted future cash flows expected to be generated by the asset. If the carrying amount of an asset exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. There were no impairment charges related to long-lived assets for the years ended December 31, 2018 and 2017.

Debt Issuance Costs

Costs related to debt issuance are presented in the balance sheet as a direct deduction from the carrying amount of the debt liability, consistent with debt discounts and are amortized using the effective interest method. Amortization of debt issuance costs are included in interest expense.

Rent and Deferred Rent

The Company recognizes rent expense for leases with increasing annual rents on a straight-line basis over the term of the lease. The amount of rent expense in excess of cash payments is classified as deferred rent. Any lease incentives received are deferred and amortized over the term of the lease.

Fair Value of Financial Instruments

Accounting standards include disclosure requirements around fair values used for certain financial instruments and establish a fair value hierarchy. The three-tier hierarchy prioritizes valuation inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market, as described further in Note 5.

The Company believes the recorded values of its financial instruments, including cash and cash equivalents, accounts payable and accrued liabilities approximate their fair values due to the short-term nature of these instruments.

Financial Instruments and Credit Risks

Financial instruments that potentially subject the Company to credit risk include cash and cash equivalents, investment securities, and accounts receivable. Cash is deposited in demand accounts in federally insured domestic institutions to minimize risk. Insurance is provided through the Federal Deposit Insurance Corporation (“FDIC”) and Security Investor Protection Corporation (“SIPC”). Although the balances in these accounts exceed the federally insured limit from time to time, the Company has not incurred losses related to these deposits.

Equity Issuance Costs

Equity issuance costs represent costs paid to third parties in order to obtain equity financing. These costs have been netted against the proceeds of the equity issuances.

Licenses and Patents

Licenses and patent costs for technologies that are utilized in research and development and have no alternative future use are expensed as incurred. Costs related to the license of patents from third parties and internally developed patents are classified as research and development expenses. Legal costs related to patent applications and maintenance are classified as general and administrative expenses.

Clinical Trials

The Company estimates its clinical trial expense accrual for a given period based on the number of patients enrolled at each site, estimated cost per patient, and the length of time each patient has been in the trial, less amounts previously billed. These accruals are recorded in accrued expenses and other current liabilities, and the related expense is recorded in research and development expense.

Research and Development

Research and development expenses consist of expenses incurred in performing research and development activities, including compensation and benefits for research and development employees and consultants, facilities expenses, overhead expenses, cost of laboratory supplies, manufacturing expenses, fees paid to third parties and other outside expenses.

Research and development costs are expensed as incurred. Clinical trial and other development costs incurred by third parties are expensed as the contracted work is performed. The Company accrues for costs incurred as the services are being provided by monitoring the status of the clinical trial or project and the invoices received from its external service providers. The Company estimates depend on the timeliness and accuracy of the data provided by the vendors regarding the status of each project and total project spending. The Company adjusts its accrual as actual costs become known. Where contingent milestone payments are due to third parties under research and development arrangements, the milestone payment obligations are expensed when the milestone events are achieved.

Collaboration Agreements

The Company enters into collaboration agreements that include varying arrangements regarding which parties perform and bear the costs of research and development activities. The Company may share the costs of research and development activities with a collaborator, or the Company may be reimbursed for all or a significant portion of the costs of the Company's research and development activities. The Company records its internal and third-party development costs associated with these collaborations as research and development expenses. When the Company is entitled to reimbursement of all or a portion of the research and development expenses that it incurs under a collaboration, the Company records those reimbursable amounts as a deduction to the research and development expenses. If the collaboration is a cost-sharing arrangement in which both the Company and its collaborator perform development work and share costs, the Company also recognizes, as research and development expenses in the period when its collaborator incurs development expenses, the portion of the collaborator's development expenses that the Company is obligated to reimburse.

Contract Manufacturing Services

Contract manufacturing services are expensed as incurred. Prepaid expenses are capitalized and amortized as services are performed.

Share-Based Compensation

The Company accounts for its share-based compensation in accordance with ASC 718, Compensation — Stock Compensation, which requires the measurement and recognition of compensation expense for all share-based payment awards made to employees and directors to be recognized in the financial statements, based on their fair value. The Company measures share-based compensation to consultants in accordance with ASC 505-50, Equity-Based Payments to Non-Employees, and recognizes the fair value of the award over the period the services are rendered.

The Company uses the Black-Scholes option-pricing model to estimate the fair value of stock option awards. The fair value is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award on a straight-line basis.

Income Taxes

Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. This method also requires the recognition of future tax benefits such as net operating loss and tax credit carry forwards, to the extent that realization of such benefits is more likely than not. A valuation allowance is recorded when the realization of future tax benefits is uncertain. The Company records a valuation allowance for the full amount of deferred tax assets, which would otherwise be recorded for tax benefits relating to the operating loss and tax credit carryforwards, as realization of such deferred tax assets cannot be determined to be more likely than not.

Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the statement of operations in the period that includes the enactment date.

On December 22, 2017, the U.S. government enacted the Tax Cuts and Jobs Act. The Tax Cuts and Jobs Act significantly revised how U.S. taxes corporations. The provisions of the Tax Cuts and Jobs Act that impacted the Company include, but are not limited to, (1) lowered the U. S. corporate income tax rate from 35% to 21%, (2) limitations on the maximum deduction for net operating loss (NOL) carryforwards generated in tax years beginning after December 31, 2017, to 80 percent of a taxpayer’s taxable income and (3) elimination of certain business expenditures. In conjunction with the Tax Cuts and Jobs Act, the SEC staff issued SAB 118, which allowed companies to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company previously reported provisional amounts of the income tax effects of the Tax Cuts and Jobs Act for which the accounting was incomplete, but a reasonable estimate could be determined. During the year ended December 31, 2018, our accounting for the income tax effects of the Tax Cuts and Jobs Act was completed without material changes to the previously provided estimates.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740, Income Taxes. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2018 and 2017, the Company had no uncertain tax positions and no interest or penalties have been charged to the Company for the years ended December 31, 2018 and 2017. If incurred, the Company will classify any interest and penalties as a component of interest expense and operating expense, respectively. The Company is subject to routine audits by taxing jurisdictions; however, there are currently no audits for any tax periods in progress. The tax years 2005 through 2018 remain open to examination by the Internal Revenue Service.

Comprehensive Loss

Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period, from transactions, and other events and circumstances from non-owner sources. Components of comprehensive income (loss) includes, among other items, unrealized gains and losses on the changes in fair value of investments and unrealized gains and losses on the change in foreign currency exchange rates. These components are added, net of their related tax effect, to the reported net income (loss) to arrive at comprehensive income (loss). The components of accumulated other comprehensive loss at December 31, 2018 and 2017, on the Company’s balance sheet was comprised of the net unrealized holding losses on the Company’s investment securities, and the effect of changes in foreign currency exchange rates. See Note 5 for further detail of the unrealized holding gains and losses on the Company’s investment securities.

Net Loss and Net Loss per Share of Common Stock Attributable to Common Stockholders

Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period without consideration for common stock equivalents.

The following outstanding shares of common stock equivalents were excluded from the computations of diluted net loss per shares of common stock attributable to common stockholders for the periods presented as the effect of including such securities would be anti-dilutive.



	Number of shares
	December 31, 2018	December 31, 2017
Options to purchase common stock	5,759,246	5,286,472
Unvested shares of restricted stock units	246,155	111,250
Unvested shares of restricted stock	—	29,413
Total common stock equivalents	6,005,401	5,427,135

Application of New Accounting Standards

During 2017, the Company adopted ASU No. 2016-09, “Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting”, which is intended to simplify the financial reporting of the income tax impacts of share-based compensation arrangements. The classification guidance under ASU No. 2016-09 requires the recognition of excess tax benefits from share-based compensation arrangements as a discrete item within income tax benefit rather than additional paid-in capital and the classification guidance requiring presentation of excess tax benefits from share-based compensation arrangements as an operating activity on the statement of cash flows, rather than as a financing activity.

The adoption of ASU No 2016-09 had no net tax impact on the Company's financial statements because the Company has established a valuation allowance against the entire net deferred tax asset as of December 31, 2017. The Company has adjusted the net operating loss carryforward to include the excess tax benefits that were not previously recognized offset by a full valuation allowance. Further, the Company has elected to continue to estimate the number of stock-based awards expected to vest, as permitted by ASU 2016-09, rather than electing to account for forfeitures as they occur.

In August 2016, the FASB issued ASU 2016-15, “Classification of Certain Cash Receipts and Cash Payments,” which provides guidance on the classification of certain cash receipts and payments in the statement of cash flows. The pronouncement is effective for annual periods beginning after December 15, 2017, and interim periods within those annual periods. Earlier application is permitted in any interim or annual period. The Company adopted this standard in 2017, with no material effect upon its financial statements.

New Accounting Requirements and Disclosures

In February 2016, the FASB issued ASU No. 2016-02, “Leases,” which requires companies that lease assets to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, in its balance sheet. The pronouncement will also require additional disclosures about the amount, timing and uncertainty of cash flows arising from leases. This pronouncement is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018, and early adoption is permitted.

ASC 842 was previously required to be adopted using the modified retrospective approach. However, in July 2018, the FASB issued ASU 2018-11, which allows for retrospective application with the recognition of a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Under this option, entities would not need to apply ASC 842 (along with its disclosure requirements) to the comparative prior periods presented.

ASC 842 is effective for the Company in the first quarter of 2019 and management expects that most of its operating leases (primarily office space) will be recognized as operating lease liabilities and right of use assets on its balance sheet. The Company has elected to adopt certain of the optional practical expedients, including the package of practical expedients, which, among other things, gives the option to not reassess: 1) whether expired or existing contracts are or contain leases; 2) the lease classification for expired or existing leases; and 3) initial direct costs for existing leases. Management has evaluated the impact of the adoption of this standard and expects to record right of use assets of $4.9 million and lease obligations of $6.6 million.

In 2018, the FASB issued ASU No. 2018-07, “Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting”, which changes the measurement date for share-based awards to the grant date, instead of the previous requirement to remeasure the awards through the performance completion date. ASU No. 2018-07 is effective for the Company for fiscal years beginning after December 31, 2018, including interim periods within that fiscal year. Early adoption is permitted. The Company does not believe adopting ASU No. 2018-07 will have a material impact on its consolidated financial statements.

In 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement”, which modifies fair value disclosures and removes some disclosure requirements for both public and private companies. In addition, public companies are subject to some new disclosure requirements which requires to disclose the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. ASU No. 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not believe adopting ASU No. 2018-13 will have a material impact on its consolidated financial statements.

CASH AND CASH EQUIVALENTS AND RESTRICTED CASH

12 Months Ended

Dec. 31, 2018

Cash and Cash Equivalents [Abstract]

CASH, CASH EQUIVALENTS AND RESTRICTED CASH

At December 31, 2018 and 2017, the Company maintained $5.0 million and $6.2 million as restricted cash.

During 2017, $4.2 million was received from the Cancer Prevention and Research Institute of Texas, or “CPRIT”, and is being held in a separate account to be used for costs solely related to the CPRIT grant. Release of the CPRIT funds are subject to the terms of the grant agreement and requirements therein and require the authorization of CPRIT. During 2018, CPRIT authorized the release of $0.8 million of restricted funds from the CPRIT account, leaving a balance of $3.4 million at December 31, 2018. For more information about the CPRIT grant, see Note 9.

The remaining $1.6 million of restricted cash as of December 31, 2018 and the $2.0 million in 2017 is held in escrow to cover specific construction of manufacturing improvement costs related to the facility lease. The release of the escrowed funds is subject to the terms of the escrow agreement and requirements therein including approval by both the Company and the landlord based on authorized completion of certain aspects of the manufacturing improvements.

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same such amounts shown in the statements of cash flows.



	December 31, 2018		December 31, 2017
	(in thousands)
Cash and cash equivalents ⁽¹⁾	$	43,695	$	38,839
Restricted cash, noncurrent	4,973		6,190
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$	48,668	$	45,029

(1) As of December 31, 2018 and 2017, the Company invested approximately $25.0 million and $25.6 million, respectively, in cash equivalent instruments.

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES

The Company follows ASC, Topic 820, Fair Value Measurements and Disclosures, or ASC 820, for application to financial assets. ASC 820 defines fair value, provides a consistent framework for measuring fair value under GAAP and requires fair value financial statement disclosures. ASC 820 applies only to the measurement and disclosure of financial assets that are required or permitted to be measured and reported at fair value under other ASC topics (except for standards that relate to share-based payments such as ASC Topic 718, Compensation – Stock Compensation).

The valuation techniques required by ASC 820 may be based on either observable or unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, and unobservable inputs reflect the Company’s market assumptions.

These inputs are classified into the following hierarchy:

Level 1 Inputs – quoted prices (unadjusted) in active markets for identical assets that the reporting entity has the ability to access at the measurement date;

Level 2 Inputs – inputs other than quoted prices included within Level 1 that are observable for the asset, either directly or indirectly; and

Level 3 Inputs – unobservable inputs for the assets.

The following tables present the Company’s investment securities (including, if applicable, those classified on the Company’s balance sheet as cash equivalents) that are measured at fair value on a recurring basis as of December 31, 2018 and 2017:



			Fair Value Measurements at Reporting Date Using
	Balance at December 31, 2018		Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
	(in thousands)
Cash Equivalents:
Money market funds	$	24,953	$	24,953	$	—	$	—
Total Cash Equivalents	$	24,953	$	24,953	$	—	$	—

Investment Securities:
U.S. government agency-backed securities	$	7,383	$	—	$	7,383	$	—
Corporate debt securities	41,921		—		41,921		—
Total Investment Securities	$	49,304	$	—	$	49,304	$	—

			Fair Value Measurements at Reporting Date Using
	Balance at December 31, 2017		Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
	(in thousands)
Cash Equivalents:
Money market funds	$	25,550	$	25,550	$	—	$	—
Total Cash Equivalents	$	25,550	$	25,550	$	—	$	—

Investment Securities:
U.S. government agency-backed securities	$	22,604	$	—	$	22,604	$	—
Corporate debt securities	38,821		—		38,821		—
Total Investment Securities	$	61,425	$	—	$	61,425	$	—

U.S. Treasury, U.S. government agency-backed securities, corporate debt securities and municipal bonds are valued based on various observable inputs such as benchmark yields, reported trades, broker/dealer quotes, benchmark securities and bids.



Investment securities, all classified as available-for-sale, consisted of the following as of December 31, 2018 and 2017:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Aggregate Estimated Fair Value
December 31, 2018	(in thousands)
U.S. government agency-backed securities	$	7,382	$	2	$	(1	)	$	7,383
Corporate debt securities	41,968				(47		)	41,921
Total	$	49,350	$	2	$	(48	)	$	49,304

December 31, 2017
U.S. government agency-backed securities	$	22,632	$	—	$	(28	)	$	22,604
Corporate debt securities	38,839		13		(31		)	38,821
Total	$	61,471	$	13	$	(59	)	$	61,425

The Company's investment securities as of December 31, 2018, will reach maturity between January 2019 and November 2019, with a weighted-average maturity date in April 2019.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

DEBT

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

DEBT

Hercules Loan

On March 10, 2016 (the “Hercules Closing Date”), the Company entered into a Loan and Security Agreement (the “Hercules Loan Agreement”) with Hercules Capital, Inc., Hercules Technology II, L.P., and Hercules Technology III, L.P., or collectively, “Hercules”, as a lender, under which the Company borrowed $15.0 million. The Company borrowed an additional $5.0 million and $10.0 million on September 15, 2016 and March 8, 2017, respectively. The total debt was secured by a lien covering substantially all of the assets of the Company, excluding intellectual property, but including proceeds from the sale, license, or disposition of intellectual property. The Company paid expenses related to the loan of $0.3 million which, along with a final facility charge of $2.1 million was recorded as deferred financing costs. Interest expense for the year ended December 31, 2017 included $0.8 million of amortized deferred financing costs.

On December 21, 2017, the Company repaid the outstanding balance, accrued interest and Final Hercules Facility Charge totaling $32.9 million, which included a prepayment charge of $1.8 million with proceeds from a new loan from Oxford Finance, LLC, discussed below.

Oxford Loan

On December 21, 2017 (the “Oxford Closing Date”), the Company entered into a loan and security agreement (the “Oxford Loan Agreement”) with Oxford Finance LLC, as the collateral agent and a lender, pursuant to which the Company borrowed $35.0 million in a single term loan (the “Oxford Loan”) on the Oxford Closing Date. As discussed above, on the Oxford Closing Date, the Company used approximately $32.9 million of the proceeds from the Oxford Loan to repay its indebtedness to Hercules.

The Company’s obligations under the Oxford Loan Agreement are secured by a first priority security interest in substantially all of the Company’s current and future assets, other than its intellectual property. The Company has also agreed not to encumber its intellectual property assets, except as permitted by the Oxford Loan Agreement. The Oxford Loan matures on December 1, 2022 (the “Oxford Maturity Date”) and will be interest-only through January 31, 2020, followed by 35 equal monthly payments of principal and unpaid accrued interest. The Oxford Loan bears interest at a floating per annum rate equal to (i) 7.25% plus (ii) the greater of (a) the 30-day U.S. Dollar LIBOR rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue and (b) 1.25%. The interest rate on amounts borrowed under the Oxford Loan Agreement was 12.09% at December 31, 2018.

The Company will be required to make a final payment of 8.70% of the principal amount of the Oxford Loan borrowed (the “Oxford Final Payment Fee”), payable on the earlier of (i) the Oxford Maturity Date, (ii) the acceleration of the Oxford Loan, or (iii) the prepayment of the Oxford Loan. The Company may prepay all, but not less than all, of the borrowed amounts, provided that the Company will be obligated to pay a prepayment fee equal to (i) 3.00% of the outstanding principal balance if prepaid on or before the first anniversary of the Closing Date, (ii) 2.00% of the outstanding principal balance, if prepaid after the first anniversary and before the second anniversary of the Closing Date, and (iii) 1.00% of the outstanding principal balance prepaid thereafter and prior to the Maturity Date (each, a “Prepayment Fee”). While any amounts are outstanding under the Oxford Loan Agreement, the Company is subject to a number of affirmative and restrictive covenants, including covenants regarding delivery of financial statements, payment of taxes, maintenance of insurance, dispositions of property, business combinations or acquisitions, incurrence of additional indebtedness and transactions with affiliates, among other customary covenants. The Company is also restricted from paying dividends or making other distributions or payments of its capital stock, subject to limited exceptions. Upon the occurrence of certain events, including but not limited to the Company’s failure to satisfy its payment obligations under the Oxford Loan Agreement, the breach of certain of its other covenants under the Oxford Loan Agreement, or the occurrence of a material adverse change, the collateral agent will have the right, among other remedies, to declare all principal and interest immediately due and payable, and the lender will have the right to receive the Oxford Final Payment Fee and, if the payment of principal and interest is due prior to the Oxford Maturity Date, a Prepayment Fee.

The Company paid expenses related to the Oxford Loan Agreement of $0.1 million, which, along with the final facility charge of $3.0 million, have been recorded as deferred financing costs, which offset long-term debt on the Company's balance sheet. The deferred financing costs are being amortized over the term of the loan as interest expense. During the year ended December 31, 2018, interest expense included $0.9 million of amortized deferred financing costs compared to less than $22,000 for the year ended December 31, 2017.



The total gross payments due under the Company's debt arrangements are as follows:
	As of December 31, 2018
Year	(in thousands)
2019	$	—
2020	11,000
2021	12,000
2022	12,000
Thereafter	3,045
Total debt	$	38,045
Less deferred financing costs	(2,213		)
Less current portion	—
Total long-term debt	$	35,832

Management believes that the carrying value of the debt facility approximates its fair value, as the Company's debt facility bears interest at a rate that approximates prevailing market rates for instruments with similar characteristics. The fair value of the Company's debt facility is determined under Level 2 in the fair value hierarchy.

GRANT REVENUE	12 Months Ended
GRANT REVENUE	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
GRANT REVENUE	GRANT REVENUE Cancer Research Grant Contract On August 9, 2017, the Company entered into a Cancer Research Grant Contract (the “CPRIT Agreement”) with CPRIT, pursuant to which CPRIT awarded a grant of approximately $16.9 million to the Company to fund development of rivo-cel for hematologic cancer (the “CPRIT Award”). The CPRIT Award is contingent upon funds being available during the term of the Agreement and subject to CPRIT’s ability to perform its obligations under the Agreement. The Company and CPRIT will retain joint ownership over any intellectual property developed under the Agreement. With respect to non-commercial use of any intellectual property developed under the CPRIT Agreement (the “CPRIT Project Results”), the Company agreed to grant to CPRIT a sublicensable, nonexclusive, irrevocable, royalty-free, perpetual worldwide license to any intellectual property of the Company that is necessary to exploit the CPRIT Project Results. The CPRIT Agreement permits the Company to license any CPRIT Project Results, subject to the Company retaining an exclusive sublicensable license to exploit the CPRIT Project Results for non-commercial purposes. The Company is obligated to make revenue-sharing payments to CPRIT with respect to net sales of any product covered by the Agreement, up to a maximum repayment of 400% of the aggregate amount paid to the Company by CPRIT under the CPRIT Agreement. The payments are determined as a percentage of net sales ranging from the low to mid-single digits, which may be reduced if the Company is required to obtain a license from a third party to sell any such product. In addition, upon meeting the foregoing limitation on revenue-sharing payments, the Company agreed to make continued revenue-sharing payments to CPRIT of less than 1% of net sales. The CPRIT Agreement will expire on February 29, 2020 unless terminated earlier by: mutual consent of the parties; CPRIT upon an event of default by the Company as specified in the CPRIT Agreement; CPRIT if allocated funds become unavailable or CPRIT is unable to obtain additional funds; or the Company in its sole discretion. During 2017, the Company received $4.2 million in advance funding from CPRIT, which was recorded as deferred revenue. As of December 31, 2018 and 2017, the Company incurred expenses and accrued revenue of $1.1 million and $0.1 million, respectively, for work performed under the CPRIT grant. NIH Grant During 2013, the Company entered into a grant agreement with the NIH. The grant was a modular multi-year grant with funds being awarded each year based on the progress of the program being funded. Grant money is not received until expenses for the program are incurred. We have been awarded approximately $1.4 million to date. The grant expired March 31, 2017.

STOCKHOLDERS' EQUITY	12 Months Ended
STOCKHOLDERS' EQUITY	Dec. 31, 2018
Equity [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS' EQUITY On March 29, 2017, the Company completed an underwritten public offering of 5,750,000 shares of its common stock at a price of $12.00 per share, for an aggregate offering size of $69.0 million, pursuant to a registration statement on Form S-3. The net proceeds to the Company, after deducting underwriting discounts, and commissions and offering expenses were approximately $64.6 million. These costs have been recorded as a reduction of the proceeds received from the offering. On April 20, 2018, the Company completed an underwritten public offering of 9,200,000 shares of its common stock at a price of $7.50 per share, for an aggregate offering size of $69.0 million, pursuant to a registration statement on Form S-3. The net proceeds to the Company, after deducting underwriting discounts, and commissions and offering expenses was approximately $64.7 million. These costs have been recorded as a reduction of the proceeds received from the offering. On October 5, 2018, the Company entered into an Open Market Sale Agreement^SM with Jefferies LLC, as sales agent, pursuant to which the Company may offer and sell, from time to time, through Jefferies, shares of the Company’s common stock having an aggregate offering price of up to $60.0 million. The shares will be offered and sold pursuant to the Company’s shelf registration statement on Form S-3.

SHARE-BASED COMPENSATION PLANS

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

SHARE-BASED COMPENSATION PLANS

The Company has four share-based compensation plans, which authorize the granting of shares of common stock and options to purchase common stock to employees and directors of the Company, as well as non-employee consultants, and allows the holder of the option to purchase common stock at a stated exercise price. Options vest according to the terms of the grant, which may be immediately or based on the passage of time, generally over four years, and have a term of up to 10 years. Unexercised stock options terminate on the expiration date of the grant. The Company recognizes the share-based compensation expense over the requisite service period of the individual grantees, which generally equals the vesting period.

At December 31, 2018, the Company had share-based awards outstanding under four share-based compensation plans, as follows:

2006 Stock Option Plan

The 2006 Stock Option Plan (the “2006 Plan”) provided for the issuance of non-qualified stock options to employees, including officers, non-employee directors and consultants to the Company. As of December 31, 2018, there were 6,882 shares of common stock reserved for issuance pursuant to outstanding options granted under the 2006 Plan. The 2006 Plan was terminated by the Board in October 2014.

2011 Stock Option Plan

The 2011 Stock Option Plan (the “2011 Plan”) provided for the issuance of incentive and non-qualified stock options to employees, including officers, non-employee directors and consultants to the Company. As of December 31, 2018, there were 512,015 shares of common stock reserved for issuance pursuant to outstanding options granted under the 2011 Plan. The 2011 Plan replaced the 2006 Plan. The 2011 Plan terminated upon the effectiveness of the 2014 Plan described below.

2014 Equity Incentive Plan

The 2014 Equity Incentive Plan (the “2014 Plan”) became effective in December 2014, upon the closing of the IPO. The 2014 Plan provides for the issuance of equity awards, including incentive and non-qualified stock options and restricted stock awards to employees, including officers, non-employee directors and consultants to the Company or its affiliates. The 2014 Plan also provides for the grant of performance cash awards and performance-based stock awards.

On June 14, 2017, the stockholders approved an amendment to the 2014 Plan to, among other things, increase the number of shares of common stock authorized for issuance under the 2014 Plan by 3,100,000 shares and eliminate the prior provision in the 2014 Plan that allowed the Company’s Board of Directors to reprice stock options without stockholder approval.

The aggregate number of shares of common stock that are authorized for issuance under the 2014 Plan is 6,090,354 shares, plus any shares subject to outstanding options that were granted under the 2011 Plan or 2006 Plan that are forfeited, terminated, expired or are otherwise not issued. As of December 31, 2018, there were 5,486,504 outstanding awards, comprised of 4,015,349 options, 1,225,000 inducement option awards, 194,905 restricted stock units and 51,250 inducement restricted stock units. At December 31, 2017, there were 3,646,113 outstanding awards, comprised 2,875,450 options, 630,000 inducement option awards, 96,250 restricted stock units, 29,413 shares of restricted stock and 15,000 inducement restricted stock units outstanding. As of December 31, 2018, there were 2,059,399 shares remaining to be issued under 2014 Plan.

2014 Employee Stock Purchase Plan

The 2014 Employee Stock Purchase Plan (the “ESPP”) provides for eligible Company employees, as defined by the ESPP, to be given an opportunity to purchase the Company's common stock at a discount, through payroll deductions, with stock purchases being made upon defined purchase dates. The ESPP authorizes the issuance of up to 550,000 shares of the Company's common stock to participating employees, and allows eligible employees to purchase shares of common stock at a 15% discount from the grant date fair market value. During the year ended December 31, 2018 and 2017, 45,390 and 34,654 shares were purchased under the ESPP.

As of December 31, 2018, there were 414,637 shares remaining to be issued.



A summary of activity within the ESPP follows:	Year Ended December 31,
	2018		2017
	(amounts in thousands, except per share)
Deductions from employees	$	221	$	276
Share-based compensation expense recognized	$	138	$	225
Remaining share-based compensation expense	$	464	$	264
Proceeds received by the Company for ESPP	$	205	$	293
Weighted-average purchase price per common share	$	4.53	$	8.43
Number of shares purchased by employees under ESPP	45,390		34,654

Share-Based Compensation Expense

The valuation of the share-based compensation awards is a significant accounting estimate that requires the use of judgment and assumptions that are likely to have a material impact on the financial statements. The fair value of option grants is determined using the Black-Scholes option-pricing model. Expected volatilities utilized in the model are based on the Company’s historical trading volatility. Similarly, the dividend yield is based on historical experience and the estimate of future dividend yields. The risk-free interest rate is derived from the U.S. Treasury yield curve in effect at the time of grant. The expected term of the options is based on the average period the stock options are expected to remain outstanding. As the Company does not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior, the expected term is calculated as the midpoint between the weighted-average vesting term and the contractual expiration period also known as the simplified method.

The fair values of the option grants have been estimated, with the following weighted-average assumptions:



	Year Ended December 31,
	2018		2017
Risk-free interest rate	2.67	%	2.06	%
Volatility	72	%	72	%
Expected life (years)	6.08		6.08
Expected dividend yield	0	%	0	%

Share-based compensation for the years ended December 31, 2018 and 2017, are as follows:



	Year Ended December 31,
	2018		2017
	(in thousands)
General and administrative	$	7,479	$	7,128
Research and development	6,345		6,441
Total	$	13,824	$	13,569

Stock option activity for the years ended December 31, 2018 and 2017 is as follows:



Options	Outstanding Stock Options	Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value⁽¹⁾(in thousands)
Balance at December 31, 2016	4,532,120	$	12.37	7.58	$	20,453
Granted	1,504,625	$	11.67
Exercised	(344,958)	$	4.31		$	—
Forfeited	(405,315)	$	16.97
Balance at December 31, 2017	5,286,472	$	12.35	7.35	$	7,223
Granted⁽²⁾	2,623,191	$	6.88
Exercised	(1,044,450)	$	3.20
Forfeited	(1,105,967)	$	15.56
Balance at December 31, 2018	5,759,246	$	10.90	8.09	$	87
Exercisable as of December 31, 2018	2,447,034	$	14.28	6.73	$	87

⁽¹⁾The aggregate intrinsic value as of December 31, 2018, is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at December 31, 2018.

⁽²⁾Includes 595,000 of inducement option awards granted in 2018.

The following table summarizes the stock award activity for all stock plans during the year ended December 31, 2018 and 2017:



Restricted Stock Shares	Outstanding Restricted Stock Awards and Units		Outstanding Aggregate Intrinsic Value (1) (in thousands)
Balance at December 31, 2016	58,825		$	801
Granted	117,500
Vested	(29,412	)	330
Forfeited	(6,250	)
Balance at December 31, 2017⁽²⁾	140,663		$	1,183
Granted⁽³⁾	211,250
Vested	(57,227	)	420
Forfeited	(48,531	)
Balance at December 31, 2018⁽²⁾	246,155		$	719

⁽¹⁾The aggregate intrinsic value as of December 31, 2018, is calculated as the fair value of restricted stock and restricted stock units at December 31, 2018.

⁽²⁾At December 31, 2018, there were no shares of unvested restricted common stock outstanding, compared with 29,413 shares outstanding at December 31, 2017.

⁽³⁾Includes 40,000 of inducement restricted stock units granted during 2018.

The following table includes share-based payment activity for the years ended December 31, 2018 and 2017:



	Year Ended December 31,
	2018		2017
	(in thousands, except per share)
Weighted-average grant date fair value of options granted	$	4.51	$	7.55
Weighted-average grant date fair value of restricted stock units granted	$	7.14	$	12.59
Total fair value of restricted shares vested	$	907	$	330
Cash received by Company upon option exercises	$	3,270	$	1,486

The following table summarizes the options outstanding and exercisable at December 31, 2018:



Options Outstanding					Options Exercisable
Exercise Price	Total Shares	Weighted- Average Remaining Contractual Term (in years)	Weighted-Average Exercise Price		Total Shares	Weighted- Average Remaining Contractual Term (in years)	Weighted-Average Exercise Price
$0.51 to $6.17	1,154,806	8.59	$	4.21	197,806	3.14	$	2.48
$6.18 to $7.84	1,267,975	8.38	$	7.53	373,146	6.25	$	7.48
$7.85 to $11.76	1,206,985	8.66	$	9.87	326,720	8.03	$	10.85
$11.77 to $18.24	1,010,001	7.80	$	12.88	571,090	7.66	$	13.05
$18.25 to $24.48	1,119,479	5.49	$	20.95	978,272	5.32	$	21.12
Total	5,759,246	8.09	$	10.90	2,447,034	6.73	$	14.28

At December 31, 2018, total compensation cost not yet recognized was $16.3 million and the weighted average period over which this amount is expected to be recognized is 2.51 years. The aggregate fair value of options and restricted shares vesting in the years ended December 31, 2018 and 2017 was $13.9 million and $12.2 million, respectively.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

Operating Lease Agreements

The Company leases its office and manufacturing facilities under non-cancelable operating leases that expire January 31, 2020 and August 31, 2026, respectively. Rent expense for non-cancelable operating leases with scheduled rent increases is recognized on a straight-line basis over the terms of the leases. Improvement reimbursements from the landlord of $2.5 million are being amortized on a straight-line basis into rent expense over the terms of the leases. The difference between required lease payments and rent expense has been recorded as deferred rent. Rent expense was $2.1 million in 2018 and $2.1 million in 2017. Deferred rent was $1.7 million as of December 31, 2018 and $2.0 million as of December 31, 2017.

Escrow agreement related to the construction of the Company's manufacturing facility

Pursuant to the escrow agreements related to the Company's lease dated May 2015, the Company agreed to deposit into escrow a total of approximately $11.0 million, which represents 110% of the Company’s then-remaining share of the estimated build-out costs. The funds were deposited into an escrow account in December 2016 and reported as restricted cash. The escrow balance in restricted cash as of December 31, 2018 and 2017 was $1.6 million and 2.0 million, respectively.

Capital Lease Agreements - Equipment

The Company entered into multiple office equipment leases during both 2016 and 2015, which expire in 2021. The office equipment leases are being accounted for as capital leases under FASB Topic ASC 840 - Leases. The present value of the minimum lease payments is greater than 90% or more than the fair market value of the leased equipment and the lease terms are 6 years or the remaining term of the lease.

Aggregate future minimum annual payments under operating and capital leases at December 31, 2018, are as follows:



Year	Operating Leases		Capital Leases
	(in thousands)
2019	$	2,087	$	68
2020	1,112		68
2021	1,055		43
2022	1,094		—
2023	1,133		—
Thereafter	3,222		—
Total minimum rentals	$	9,703	$	179

Co-Development and Co-Commercialization Agreement - Adaptimmune Therapeutics plc

On December 16, 2016, the Company entered into a Co-Development and Co-Commercialization Agreement with and Adaptimmune Therapeutics plc (Adaptimmune) in order to facilitate a staged collaboration to evaluate, develop and commercialize next generation T cell therapies. Under the Agreement, the parties agreed to evaluate the Company’s GoTCR technology (inducible MyD88/CD40 co-stimulation, or iMC) with Adaptimmune’s affinity-optimized SPEAR® T cells for the potential to create enhanced TCR product candidates. Depending on results of the preclinical proof-of-concept phase, the parties expect to progress to a two-target co-development and co-commercialization phase. To the extent necessary, and in furtherance of the parties’ proof-of-concept and co-development efforts, the parties granted each other a royalty-free, non-transferable, non-exclusive license covering their respective technologies for purposes of facilitating such proof-of-concept and co-development efforts. In addition, as to covered therapies developed under the agreement, the parties granted each other a reciprocal exclusive license for the commercialization of such therapies. With respect to any joint commercialization of a covered therapy, the parties agreed to negotiate in good faith the commercially reasonable terms of a co-commercialization agreement. The parties also agreed that any such agreement shall provide for, among other things, equal sharing of the costs of any such joint commercialization and the calculation of profit shares as set forth in the Agreement. The Agreement will expire on a country-by-country basis once the parties cease commercialization of the T cell therapies covered by the Agreement, unless earlier terminated by either party for material breach, non-performance or cessation of development, bankruptcy/insolvency, or failure to progress to co-development phase. There were no expenses recognized under the Adaptimmune agreement for the years ended December 31, 2018 and 2017.

Collaboration Agreement - OPBG

In October 2016, the Company entered into a collaboration agreement with Ospedale Pediatrico Bambino Gesú (“OPBG”), pursuant to which the Company and OPBG agreed to collaborate on research projects and early stage clinical trials for the design and development of various T cell immunotherapies. As consideration for OPBG’s performance of the research under the agreement and grant of certain licenses to the Company, the Company agreed to fund an aggregate of up to $4.7 million in project costs payable to OPBG or certain third-party service providers, as applicable, over the term of the research, estimated to be 4 years. With respect to any inventions arising from the research, OPBG agreed to grant the Company an exclusive license to any such inventions, the terms of which will be set forth in a separate agreement. In addition, OPBG granted the Company paid-up, worldwide co-exclusive licenses for non-commercial development of OPBG’s CD19 and GD2 CAR T technologies, as well as paid-up, worldwide exclusive licenses to commercialize its CD19 and GD2 CAR T technologies, each to be governed by a separate agreement. The expenses recognized under the OPBG Collaboration Agreement were $0.6 million and $0.7 million for the years ended December 31, 2018 and 2017, respectively. The collaboration agreement with OPBG expired on December 31, 2018.

Collaboration Agreement - Leiden

In May 2016, the Company and Academisch Ziekenhuis Leiden (“Leiden”) entered into a research collaboration agreement pursuant to which the Company will provide Leiden with financial support for research to discover and validate high-affinity TCR product candidates targeting several cancer-associated antigens. The Company agreed to pay Leiden an aggregate of EURO 2.5 million in quarterly installments during the three-year term of the research, which will be recognized as services are incurred. With respect to any inventions arising from the research that are relevant to or useful for any high affinity TCR that is studied in the research, Leiden granted the Company an exclusive option to obtain an exclusive, worldwide license to practice and exploit such inventions. The parties agreed to negotiate in good faith the commercially reasonable terms of each such license agreement entered into between the parties, based on terms similar to those set forth in the previously executed license agreement between the parties and those specified in the agreement. The expenses recognized under the Leiden license agreement were $1.1 million for each of the years ended December 31, 2018 and 2017, respectively.

License Agreement - Baylor

In March 2016, the Company and Baylor College of Medicine (“BCM”) entered into two additional license agreements pursuant to which the Company obtained exclusive rights to technologies and patent rights owned by BCM. The Company paid BCM a nonrefundable license fee of $0.1 million and could incur additional payments upon the achievement of certain milestone events as set forth in the agreement. If the Company is successful in developing any of the licensed technologies, resulting sales would be subject to a royalty payment in the low single digits. During each of the years ended December 31, 2018 and 2017, $22,500 was recognized under Baylor License Agreement.

License Agreement - Agensys, Inc.

On December 10, 2015, the Company and Agensys, Inc. (“Agensys”), entered into a license agreement (the “Agensys Agreement”), pursuant to which (i) Agensys granted the Company, within the field of cell and gene therapy of diseases in humans, an exclusive, worldwide license and sublicense to its patent rights directed to prostate stem cell antigen 1 (“PSCA”) and related antibodies, and (ii) the Company granted Agensys a non-exclusive, fully paid license to the Company’s patents directed to inventions that were made by the Company in the course of developing the Company’s licensed products, solely for use with Agensys therapeutic products containing a soluble antibody that binds to PSCA or, to the extent not based upon the Company’s other proprietary technology, to non-therapeutic applications of antibodies not used within the field. As consideration for the rights granted to the Company under the Agreement, the Company agreed to pay to Agensys a non-refundable upfront fee of $3,000,000, which was included in license fee expense. The Company is also required to make aggregate milestone payments to Agensys of up to (i) $5,000,000 upon the first achievement of certain specified clinical milestones for its licensed products, (ii) $50,000,000 upon the achievement of certain specified clinical milestones for each licensed product, and (iii) $75,000,000 upon the achievement of certain sales milestones for each licensed product. The Agreement additionally provides that the Company will pay to Agensys a royalty that ranges from the mid to high single digits based on the level of annual net sales of licensed products by the Company, its affiliates or permitted sublicensees. The royalty payments are subject to reduction under specified circumstances. These milestone and royalty payments will be expensed as incurred. Under the Agreement, Agensys also was granted the option to obtain an exclusive license, on a product-by-product basis, from the Company to commercialize in Japan each licensed product developed under the Agensys Agreement that has completed a phase 2 clinical trial. As to each such licensed product, if Agensys or its affiliate, Astellas Pharma, Inc., exercises the option, the Agensys Agreement provides that the Company will be paid an option exercise fee of $5,000,000. In addition, the Agensys Agreement provides that the Company will be paid a royalty that ranges from the mid to high single digits based on the level of annual net sales in Japan of each such licensed product. If the option is exercised, the aggregate milestone payments payable by the Company to Agensys, described above with respect to each licensed product, would be reduced by up to an aggregate of $65,000,000 upon the achievement of certain specified clinical and sales milestones. The Agensys Agreement will terminate upon the expiration of the last royalty term for the products covered by the Agensys Agreement, which is the earlier of (i) the date of expiration or abandonment of the last valid claim within the licensed patent rights covering any licensed products under the Agreement, (ii) the expiration of regulatory exclusivity as to a licensed product, and (iii) 10 years after the first commercial sale of a licensed product. Either party may terminate the Agensys Agreement upon a material breach by the other party that remains uncured following 60 days after the date of written notice of such breach (or 30 days if such material breach is related to failure to make payment of amounts due under the Agensys Agreement) or upon certain insolvency events. In addition, Agensys may terminate the Agensys Agreement immediately upon written notice to the Company if the Company or any of its affiliates or permitted sublicensees commences an interference proceeding or challenges the validity or enforceability of any of Agensys’ patent rights. There were no expenses recognized under the Agensys Agreement for the years ended December 31, 2018 and 2017.

License Agreement - BioVec

On June 10, 2015, the Company and BioVec Pharma, Inc. (“BioVec”) entered into a license agreement (the “BioVec Agreement”) pursuant to which BioVec agreed to supply the Company with certain proprietary cell lines and granted to the Company a non-exclusive, worldwide license to certain of its patent rights and related know-how related to such proprietary cell lines. As consideration for the products supplied and rights granted to the Company under the BioVec Agreement, the Company agreed to pay to BioVec an upfront fee of $100,000 within ten business days of the effective date of the BioVec Agreement and a fee of $300,000 within ten business days of its receipt of the first release of GMP lot of the products licensed under the BioVec Agreement. In addition, the Company agreed to pay to BioVec an annual fee of $150,000, commencing 30 days following the first filing of an Investigational New Drug Application (an IND filing), or its foreign equivalent, for a product covered by the license; with such annual fees being creditable against any royalties payable by the Company to BioVec under the BioVec Agreement. The Company also is required to make a $250,000 milestone payment to BioVec for each of the first three licensed products to enter into a clinical phase trial and one-time milestone payments of $2,000,000 upon receipt of a registration granted by the Federal Drug Administration or European Medicines Agency on each of the Company’s first three licensed products. The BioVec Agreement additionally provides that the Company will pay to BioVec a royalty in the low single digits on net sales of products covered by the BioVec Agreement. The Company may also grant sublicenses under the licensed patent rights and know-how to third parties for limited purposes related to the use, sale and other exploitation of the products licensed under the BioVec Agreement. The BioVec Agreement will continue until terminated. The BioVec Agreement may be terminated by the Company, in its sole discretion, at any time upon 90 days written notice to BioVec. Either party may terminate the BioVec Agreement in the event of a breach by the other party of any material provision of the BioVec Agreement that remains uncured on the date that is 60 days after written notice of such failure or upon certain insolvency events that remain uncured following the date that is 30 days after the date of written notice to a party regarding such insolvency event. The Company recognized expenses of $0.2 million and $0.7 million, in connection with the BioVec License Agreement for the years ended December 31, 2018 and 2017, respectively.

License Agreement - Leiden

On April 23, 2015, the Company and Academisch Ziekenhuis Leiden, also acting under the name Leiden University Medical Centre (Leiden), entered into a license agreement (the Leiden Agreement), pursuant to which Leiden granted to the Company an exclusive, worldwide license to its patent rights covering high affinity T-cell receptors targeting preferentially-expressed antigen in melanoma, (PRAME) and POU2AF1 epitopes. The license granted under the Leiden Agreement is subject to certain restrictions and to Leiden’s retained right to use the licensed patents solely for academic research and teaching purposes, including research collaborations by Leiden with academic, non-profit research third parties; provided that Leiden provides 30 days advance written notice to the Company of such academic research collaborations. As consideration for the rights granted to the Company under the Leiden Agreement, the Company agreed to pay to Leiden an aggregate of EUR 0.1 million in upfront fees within 30 days of the effective date of the Leiden Agreement. In addition, the Company agreed to pay to Leiden, beginning on the eighth anniversary of the effective date of the Leiden Agreement, annual minimum royalty payments of EUR 30.0 thousand. The Company also is required to make milestone payments to Leiden of up to an aggregate of EUR 1.0 million for each of the first licensed product that is specific to PRAME and to POU2AF1. The Leiden Agreement additionally provides that the Company will pay to Leiden a royalty in the low single digits on net sales of products covered by the Leiden Agreement. If the Company enters into a sublicensing agreement with a third party related to a product covered by the Leiden Agreement, the Company agreed to pay Leiden a percentage ranging in the low double digits on all non-royalty income received from sublicensing revenue directly attributable to the sublicense, dependent on whether the Company is in phase 1/2, phase 2 or phase 3 at the time that the Company enters into any such sublicensing agreement. Under the Leiden Agreement, the Company and Leiden entered into a sponsored research agreement, pursuant to which the Company is required to pay Leiden up to EUR 0.3 million over a three year period during the term of the sponsored research agreement. The Leiden Agreement will expire upon the expiration of the last patent included in the licensed patent rights. The Leiden Agreement may be terminated earlier upon mutual written agreement between the Company and Leiden, and at any time by the Company upon six months written notice to Leiden. Leiden may terminate the Leiden Agreement in the event of a failure by the Company to pay any amounts due under the Leiden Agreement that remains uncured on the date that is 30 days after written notice of such failure. Either party may terminate the Leiden Agreement upon a material breach by the other party that remains uncured following 30 days after the date of written notice of such breach or upon certain insolvency events that remain uncured following the date that is 45 days after the date of written notice to a party of such insolvency event. The Company paid milestone payments under the Leiden Agreement of $0.1 million in each of the years ended December 31, 2018 and 2017, respectively.

Employment agreements

The Company has signed agreements with 20 of its officers and key employees to provide certain benefits in the event of a “change of control” as defined in these agreements and the occurrence of certain other events. The agreements provide for continued base salary payments for 3 to 18 months and a lump-sum annual cash bonus. The continued base salary and annual cash bonus portion of the agreements would aggregate approximately $6.7 million and $4.8 million at the rate of compensation in effect at December 31, 2018 and 2017, respectively. In addition, the agreements provide for continuation of certain insurance and other benefits for periods of 3 to 18 months.

Litigation

On February 6, 2018, a purported securities class action complaint captioned Nipun Kakkar v. Bellicum Pharmaceuticals, Inc., Rick Fair and Alan Musso was filed against the Company, and certain of its officers in the U.S. District Court for the Southern District of Texas, Houston Division. A second substantially similar class action was filed on March 14, 2018 by plaintiff Frances Rudy against the same defendants in the same court. The lawsuits purport to assert class action claims on behalf of purchasers of the Company's securities during the period from May 8, 2017 through January 30, 2018. The complaints allege that the defendants violated the Securities Exchange Act of 1934, as amended (the "Exchange Act"), by making materially false and misleading statements concerning the Company's clinical trials being conducted in the U.S. to assess rivo-cel (rivogenlecleucel, formerly known as BPX-501) as an adjunct T-cell therapy administered after allogeneic hematopoietic stem cell transplantation. The complaints purport to assert claims for violations of Sections 10(b) and 20(a) of the Exchange Act and Rule 10b-5 promulgated thereunder. The complaints seek, on behalf of the purported class, an unspecified amount of monetary damages, interest, fees and expenses of attorneys and experts, and other relief. On April 9, 2018, the District Court consolidated the two lawsuits under the Kakkar action and motions were filed by putative class members for appointment as lead plaintiff and approval of lead counsel.

On July 19, 2018, a purported shareholder derivative complaint captioned Seung Paik v. Richard A. Fair, et al. was filed against the Company’s directors and certain of the Company’s officers in the U.S. District Court for the Southern District of Texas, Houston Division. The lawsuit purports to seek damages on behalf of the Company against the individual defendants for breach of fiduciary duty, waste, unjust enrichment and violations of Section 14(a) of the Exchange Act. The complaint alleges that the defendants caused or allowed the Company to disseminate misstatements regarding the clinical trials for rivo-cel and to make false or misleading statements in the proxy materials for the Company’s 2017 annual meeting of stockholders. On October 3, 2018, the District Court granted the Company’s motion to stay the derivative cause of action until reinstated on motion of the parties.

The District Court conducted a status hearing on February 21, 2019 to hear arguments on competing motions from putative class members for appointment as lead plaintiff. The Court has yet to rule on the motions.

INCOME TAXES

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

INCOME TAXES

On December 22, 2017, the U.S. government enacted the Tax Cuts and Jobs Act. The Tax Cuts and Jobs Act significantly revised how U.S. taxes corporations.

The provisions of the Tax Cuts and Jobs Act that impacted the Company include, but are not limited to, (1) lowered the U. S. corporate income tax rate from 35% to 21%, (2) limitations on the maximum deduction for net operating loss (NOL) carryforwards generated in tax years beginning after December 31, 2017, to 80 percent of a taxpayer’s taxable income and (3) elimination of certain business expenditures. In conjunction with the Tax Cuts and Jobs Act, the SEC staff issued SAB 118, which allowed companies to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company previously reported provisional amounts of the income tax effects of the Tax Cuts and Jobs Act t for which the accounting was incomplete, but a reasonable estimate could be determined. During the year ended December 31, 2018, our accounting for the income tax effects of the Tax Cuts and Jobs Act was completed without material changes to the previously provided estimates.

The reconciliation between federal income taxes at the statutory rate and the Company’s income tax expense for the year is as follows:



	December 31,
	2018			2017
	(in thousands)
Tax benefit at statutory rate	$	(20,608	)	$	(31,204	)
Stock options	730			860
Other	128			(36		)
U.S. tax reform rate change	—			37,848
Stock based compensation	1,483			333
Deferred tax valuation allowances	21,606			(1,824		)
Research and development credit	(3,339		)	(5,977		)
Income tax expense	$	—		$	—

Deferred income taxes reflect the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes, and the amounts used for income tax purposes. Significant components of the Company’s deferred taxes as of December 31, 2018 and 2017 are as follows:



	December 31,
	2018			2017
	(in thousands)
Deferred tax assets:
Federal net operating loss carryforward	$	63,624		$	46,772
Stock compensation	4,533			4,900
Intangible assets	8,392			8,820
Research and development credit	13,612			10,273
Other	2,858			648
Total deferred tax assets	93,019			71,413
Valuation allowance	(93,019		)	(71,413		)
Net deferred tax assets	$	—		$	—

As of December 31, 2018 and 2017, the Company had gross U.S. federal income tax net operating loss (NOL) carryforwards of $303.0 million and $221.2 million, respectively and as of December 31, 2018, the Company had $2.4 million of U.K. net operating loss carryforwards. As of December 31, 2018 and 2017, the Company had U.S. federal research tax credits of $8.9 million and $6.7 million, respectively and Texas research tax credits of $4.7 million and $3.6 million, respectively. $81.8 million of the U.S. federal net operating loss carryovers have no expiration date and the remaining begin to expire in 2025. The foreign net operating loss carryovers have no expiration date. The U.S. Federal and state research credits will begin to expire in 2028 and 2034 respectively.

The Internal Revenue Code Section 382 limits NOL and tax credit carry forwards when an ownership change of more than 50% of the value of the stock in a loss corporation occurs. Accordingly, the ability to utilize remaining NOL and tax credit carryforwards may be significantly restricted.

In assessing the realization of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax asset will be realized. The ultimate realization of deferred tax assets is dependent upon the Company attaining future taxable income during periods in which those temporary differences become deductible.

Due to the uncertainty surrounding the realization of the benefits of its deferred assets, including NOL carryforwards, the Company has provided a 100% valuation allowance on its deferred tax assets at December 31, 2018 and 2017. The changes in the valuation allowance were an increase of $21.6 million and a decrease of $1.8 million for the years ended December 31, 2018 and 2017, respectively.

SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
SIGNIFICANT ACCOUNTING POLICIES (Policies)	Dec. 31, 2018
Accounting Policies [Abstract]
Going Concern	Going Concern The consolidated financial statements have been prepared on a going concern basis, which assumes the Company will continue to realize its assets and discharge its liabilities in the normal course of business. The continuation of the Company as a going concern is dependent upon the ability of the Company to obtain necessary funding to continue operations. As of December 31, 2018, the Company has incurred an accumulated deficit of $420.5 million since inception and had not yet generated any revenue from operations. Additionally, , the Company continues to expend cash to continue its R&D efforts. Management anticipates that its cash on hand as of December 31, 2018, grants and other cash inflows will be insufficient to fund its operations within one year from the financial statement issuance date and therefore, substantial doubt about the entity’s ability to continue as a going concern exists. These consolidated financial statements do not include any adjustments to the recoverability and classification of recorded asset amounts and classification of liabilities that might be necessary should the Company be unable to continue as a going concern. The Company may seek additional funding through a combination of equity offerings, debt financings, government or other third-party funding, commercialization, marketing and distribution arrangements, other collaborations, strategic alliances and licensing arrangements and delay planned cash outlays or a combination thereof. Management cannot be certain that such events or a combination thereof can be achieved.
Basis of Presentation	The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America. Any reference in these footnotes to applicable guidance is meant to refer to the authoritative U.S. generally accepted accounting principles (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).
Use of Estimates	Use of Estimates The preparation of the financial statements in accordance with GAAP requires management to make certain estimates and judgments that affect the reported amounts of assets, liabilities, and expenses. Actual results could differ from those estimates.
Consolidation	Consolidation All financial information presented includes the accounts of the Company and its wholly-owned subsidiaries, neither of which have had any material activity to date. All significant intercompany balances and transactions have been eliminated in consolidation.
Revenue Recognition, Licenses and Patents	Revenue Recognition The Company has not yet generated any revenue from product sales. The Company’s source of revenue in 2018 and 2017 has been from grants. When grant funds are received after costs have been incurred, the Company accrues revenue and records a grant receivable. Cash received from grants in advance of incurring qualifying costs is recorded as deferred revenue and recognized as revenue when qualifying costs are incurred. Licenses and Patents Licenses and patent costs for technologies that are utilized in research and development and have no alternative future use are expensed as incurred. Costs related to the license of patents from third parties and internally developed patents are classified as research and development expenses. Legal costs related to patent applications and maintenance are classified as general and administrative expenses.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all short-term, highly liquid investments with maturity of three months or less from the date of purchase to be cash equivalents.
Investment Securities	Investment Securities Consistent with its investment policy, the Company invests its cash allocated to fund its short-term liquidity requirements with prominent financial institutions in bank depository accounts and institutional money market funds. The Company invests the remainder of its cash in corporate debt securities and municipal bonds rated at least A quality or equivalent, U.S. Treasury notes and bonds and U.S. and state government agency-backed securities. The Company determines the appropriate classification of investment securities based on whether they represent the investment of funds available for current operations, as defined in ASC 210-10-45-1 and ASC 210-10-45-2. The Company reevaluates its classification as of each balance sheet date. All investment securities owned are classified as available-for-sale. The cost of securities sold is based on the specific identification method. Investment securities are recorded as of each balance sheet date at fair value, with unrealized gains and, to the extent deemed temporary, unrealized losses reported as accumulated other comprehensive gain (loss), a separate component of stockholders' equity. Interest and dividend income on investment securities, accretion of discounts and amortization of premiums and realized gains and losses are included in interest income in the statements of operations and comprehensive income (loss). An investment security is considered to be impaired when a decline in fair value below its cost basis is determined to be other than temporary. The Company evaluates whether a decline in fair value of an investment security is below its cost basis is other than temporary using available evidence. In the event that the cost basis of the investment security exceeds its fair value, the Company evaluates, among other factors, the amount and duration of the period that the fair value is less than the cost basis, the financial health of and business outlook for the issuer, including industry and sector performance, and operational and financing cash flow factors, overall market conditions and trends, the Company’s intent to sell the investment security and whether it is more likely than not the Company would be required to sell the investment security before its anticipated recovery. If a decline in fair value is determined to be other than temporary, the Company records an impairment charge in the statement of operations and comprehensive loss and establishes a new cost basis in the investment.
Property and Equipment	Property and Equipment Leasehold improvements, furniture, equipment and software are recorded at cost and are depreciated using the straight-line method over the estimated useful lives of the related assets, which range from three to five years. Leasehold improvements are amortized over the shorter of the estimated useful life or the remaining lease term.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment when events or changes in business conditions indicate that their carrying value may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to the estimated undiscounted future cash flows expected to be generated by the asset. If the carrying amount of an asset exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. There were no impairment charges related to long-lived assets for the years ended December 31, 2018 and 2017.
Debt Issuance Costs	Debt Issuance Costs Costs related to debt issuance are presented in the balance sheet as a direct deduction from the carrying amount of the debt liability, consistent with debt discounts and are amortized using the effective interest method. Amortization of debt issuance costs are included in interest expense.
Rent and Deferred Rent	Rent and Deferred Rent The Company recognizes rent expense for leases with increasing annual rents on a straight-line basis over the term of the lease. The amount of rent expense in excess of cash payments is classified as deferred rent. Any lease incentives received are deferred and amortized over the term of the lease.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Accounting standards include disclosure requirements around fair values used for certain financial instruments and establish a fair value hierarchy. The three-tier hierarchy prioritizes valuation inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market, as described further in Note 5. The Company believes the recorded values of its financial instruments, including cash and cash equivalents, accounts payable and accrued liabilities approximate their fair values due to the short-term nature of these instruments.
Financial Instruments and Credit Risks	Financial Instruments and Credit Risks Financial instruments that potentially subject the Company to credit risk include cash and cash equivalents, investment securities, and accounts receivable. Cash is deposited in demand accounts in federally insured domestic institutions to minimize risk. Insurance is provided through the Federal Deposit Insurance Corporation (“FDIC”) and Security Investor Protection Corporation (“SIPC”). Although the balances in these accounts exceed the federally insured limit from time to time, the Company has not incurred losses related to these deposits.
Equity Issuance Costs	Equity Issuance Costs Equity issuance costs represent costs paid to third parties in order to obtain equity financing. These costs have been netted against the proceeds of the equity issuances.
Clinical Trials	Clinical Trials The Company estimates its clinical trial expense accrual for a given period based on the number of patients enrolled at each site, estimated cost per patient, and the length of time each patient has been in the trial, less amounts previously billed. These accruals are recorded in accrued expenses and other current liabilities, and the related expense is recorded in research and development expense.
Research and Development	Research and Development Research and development expenses consist of expenses incurred in performing research and development activities, including compensation and benefits for research and development employees and consultants, facilities expenses, overhead expenses, cost of laboratory supplies, manufacturing expenses, fees paid to third parties and other outside expenses. Research and development costs are expensed as incurred. Clinical trial and other development costs incurred by third parties are expensed as the contracted work is performed. The Company accrues for costs incurred as the services are being provided by monitoring the status of the clinical trial or project and the invoices received from its external service providers. The Company estimates depend on the timeliness and accuracy of the data provided by the vendors regarding the status of each project and total project spending. The Company adjusts its accrual as actual costs become known. Where contingent milestone payments are due to third parties under research and development arrangements, the milestone payment obligations are expensed when the milestone events are achieved.
Collaboration Agreements	Collaboration Agreements The Company enters into collaboration agreements that include varying arrangements regarding which parties perform and bear the costs of research and development activities. The Company may share the costs of research and development activities with a collaborator, or the Company may be reimbursed for all or a significant portion of the costs of the Company's research and development activities. The Company records its internal and third-party development costs associated with these collaborations as research and development expenses. When the Company is entitled to reimbursement of all or a portion of the research and development expenses that it incurs under a collaboration, the Company records those reimbursable amounts as a deduction to the research and development expenses. If the collaboration is a cost-sharing arrangement in which both the Company and its collaborator perform development work and share costs, the Company also recognizes, as research and development expenses in the period when its collaborator incurs development expenses, the portion of the collaborator's development expenses that the Company is obligated to reimburse.
Contract Manufacturing Services	Contract Manufacturing Services Contract manufacturing services are expensed as incurred. Prepaid expenses are capitalized and amortized as services are performed.
Share-Based Compensation	Share-Based Compensation The Company accounts for its share-based compensation in accordance with ASC 718, Compensation — Stock Compensation, which requires the measurement and recognition of compensation expense for all share-based payment awards made to employees and directors to be recognized in the financial statements, based on their fair value. The Company measures share-based compensation to consultants in accordance with ASC 505-50, Equity-Based Payments to Non-Employees, and recognizes the fair value of the award over the period the services are rendered. The Company uses the Black-Scholes option-pricing model to estimate the fair value of stock option awards. The fair value is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award on a straight-line basis.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. This method also requires the recognition of future tax benefits such as net operating loss and tax credit carry forwards, to the extent that realization of such benefits is more likely than not. A valuation allowance is recorded when the realization of future tax benefits is uncertain. The Company records a valuation allowance for the full amount of deferred tax assets, which would otherwise be recorded for tax benefits relating to the operating loss and tax credit carryforwards, as realization of such deferred tax assets cannot be determined to be more likely than not. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the statement of operations in the period that includes the enactment date. On December 22, 2017, the U.S. government enacted the Tax Cuts and Jobs Act. The Tax Cuts and Jobs Act significantly revised how U.S. taxes corporations. The provisions of the Tax Cuts and Jobs Act that impacted the Company include, but are not limited to, (1) lowered the U. S. corporate income tax rate from 35% to 21%, (2) limitations on the maximum deduction for net operating loss (NOL) carryforwards generated in tax years beginning after December 31, 2017, to 80 percent of a taxpayer’s taxable income and (3) elimination of certain business expenditures. In conjunction with the Tax Cuts and Jobs Act, the SEC staff issued SAB 118, which allowed companies to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company previously reported provisional amounts of the income tax effects of the Tax Cuts and Jobs Act for which the accounting was incomplete, but a reasonable estimate could be determined. During the year ended December 31, 2018, our accounting for the income tax effects of the Tax Cuts and Jobs Act was completed without material changes to the previously provided estimates. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740, Income Taxes. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2018 and 2017, the Company had no uncertain tax positions and no interest or penalties have been charged to the Company for the years ended December 31, 2018 and 2017. If incurred, the Company will classify any interest and penalties as a component of interest expense and operating expense, respectively. The Company is subject to routine audits by taxing jurisdictions; however, there are currently no audits for any tax periods in progress. The tax years 2005 through 2018 remain open to examination by the Internal Revenue Service.
Comprehensive Loss	Comprehensive Loss Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period, from transactions, and other events and circumstances from non-owner sources. Components of comprehensive income (loss) includes, among other items, unrealized gains and losses on the changes in fair value of investments and unrealized gains and losses on the change in foreign currency exchange rates. These components are added, net of their related tax effect, to the reported net income (loss) to arrive at comprehensive income (loss). The components of accumulated other comprehensive loss at December 31, 2018 and 2017, on the Company’s balance sheet was comprised of the net unrealized holding losses on the Company’s investment securities, and the effect of changes in foreign currency exchange rates. See Note 5 for further detail of the unrealized holding gains and losses on the Company’s investment securities.
Net Loss and Net Loss per Share of Common Stock Attributable to Common Stockholders	Net Loss and Net Loss per Share of Common Stock Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period without consideration for common stock equivalents.
New Accounting Standards, Requirements and Disclosures	Application of New Accounting Standards During 2017, the Company adopted ASU No. 2016-09, “Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting”, which is intended to simplify the financial reporting of the income tax impacts of share-based compensation arrangements. The classification guidance under ASU No. 2016-09 requires the recognition of excess tax benefits from share-based compensation arrangements as a discrete item within income tax benefit rather than additional paid-in capital and the classification guidance requiring presentation of excess tax benefits from share-based compensation arrangements as an operating activity on the statement of cash flows, rather than as a financing activity. The adoption of ASU No 2016-09 had no net tax impact on the Company's financial statements because the Company has established a valuation allowance against the entire net deferred tax asset as of December 31, 2017. The Company has adjusted the net operating loss carryforward to include the excess tax benefits that were not previously recognized offset by a full valuation allowance. Further, the Company has elected to continue to estimate the number of stock-based awards expected to vest, as permitted by ASU 2016-09, rather than electing to account for forfeitures as they occur. In August 2016, the FASB issued ASU 2016-15, “Classification of Certain Cash Receipts and Cash Payments,” which provides guidance on the classification of certain cash receipts and payments in the statement of cash flows. The pronouncement is effective for annual periods beginning after December 15, 2017, and interim periods within those annual periods. Earlier application is permitted in any interim or annual period. The Company adopted this standard in 2017, with no material effect upon its financial statements. New Accounting Requirements and Disclosures In February 2016, the FASB issued ASU No. 2016-02, “Leases,” which requires companies that lease assets to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, in its balance sheet. The pronouncement will also require additional disclosures about the amount, timing and uncertainty of cash flows arising from leases. This pronouncement is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018, and early adoption is permitted. ASC 842 was previously required to be adopted using the modified retrospective approach. However, in July 2018, the FASB issued ASU 2018-11, which allows for retrospective application with the recognition of a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Under this option, entities would not need to apply ASC 842 (along with its disclosure requirements) to the comparative prior periods presented. ASC 842 is effective for the Company in the first quarter of 2019 and management expects that most of its operating leases (primarily office space) will be recognized as operating lease liabilities and right of use assets on its balance sheet. The Company has elected to adopt certain of the optional practical expedients, including the package of practical expedients, which, among other things, gives the option to not reassess: 1) whether expired or existing contracts are or contain leases; 2) the lease classification for expired or existing leases; and 3) initial direct costs for existing leases. Management has evaluated the impact of the adoption of this standard and expects to record right of use assets of $4.9 million and lease obligations of $6.6 million. In 2018, the FASB issued ASU No. 2018-07, “Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting”, which changes the measurement date for share-based awards to the grant date, instead of the previous requirement to remeasure the awards through the performance completion date. ASU No. 2018-07 is effective for the Company for fiscal years beginning after December 31, 2018, including interim periods within that fiscal year. Early adoption is permitted. The Company does not believe adopting ASU No. 2018-07 will have a material impact on its consolidated financial statements. In 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement”, which modifies fair value disclosures and removes some disclosure requirements for both public and private companies. In addition, public companies are subject to some new disclosure requirements which requires to disclose the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. ASU No. 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not believe adopting ASU No. 2018-13 will have a material impact on its consolidated financial statements.

CASH AND CASH EQUIVALENTS AND RESTRICTED CASH (Tables)

12 Months Ended

Dec. 31, 2018

Cash and Cash Equivalents [Abstract]

Schedule of Cash, Cash Equivalents and Restricted Cash



	December 31, 2018		December 31, 2017
	(in thousands)
Cash and cash equivalents ⁽¹⁾	$	43,695	$	38,839
Restricted cash, noncurrent	4,973		6,190
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$	48,668	$	45,029

(1) As of December 31, 2018 and 2017, the Company invested approximately $25.0 million and $25.6 million, respectively, in cash equivalent instruments.

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of assets measured at fair value on a recurring basis



			Fair Value Measurements at Reporting Date Using
	Balance at December 31, 2018		Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
	(in thousands)
Cash Equivalents:
Money market funds	$	24,953	$	24,953	$	—	$	—
Total Cash Equivalents	$	24,953	$	24,953	$	—	$	—

Investment Securities:
U.S. government agency-backed securities	$	7,383	$	—	$	7,383	$	—
Corporate debt securities	41,921		—		41,921		—
Total Investment Securities	$	49,304	$	—	$	49,304	$	—

			Fair Value Measurements at Reporting Date Using
	Balance at December 31, 2017		Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
	(in thousands)
Cash Equivalents:
Money market funds	$	25,550	$	25,550	$	—	$	—
Total Cash Equivalents	$	25,550	$	25,550	$	—	$	—

Investment Securities:
U.S. government agency-backed securities	$	22,604	$	—	$	22,604	$	—
Corporate debt securities	38,821		—		38,821		—
Total Investment Securities	$	61,425	$	—	$	61,425	$	—

Summary of available-for-sale securities



Investment securities, all classified as available-for-sale, consisted of the following as of December 31, 2018 and 2017:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Aggregate Estimated Fair Value
December 31, 2018	(in thousands)
U.S. government agency-backed securities	$	7,382	$	2	$	(1	)	$	7,383
Corporate debt securities	41,968				(47		)	41,921
Total	$	49,350	$	2	$	(48	)	$	49,304

December 31, 2017
U.S. government agency-backed securities	$	22,632	$	—	$	(28	)	$	22,604
Corporate debt securities	38,839		13		(31		)	38,821
Total	$	61,471	$	13	$	(59	)	$	61,425

SHARE-BASED COMPENSATION PLANS (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of ESPP Activity



A summary of activity within the ESPP follows:	Year Ended December 31,
	2018		2017
	(amounts in thousands, except per share)
Deductions from employees	$	221	$	276
Share-based compensation expense recognized	$	138	$	225
Remaining share-based compensation expense	$	464	$	264
Proceeds received by the Company for ESPP	$	205	$	293
Weighted-average purchase price per common share	$	4.53	$	8.43
Number of shares purchased by employees under ESPP	45,390		34,654

Weighted-Average Valuation Assumptions

The fair values of the option grants have been estimated, with the following weighted-average assumptions:



	Year Ended December 31,
	2018		2017
Risk-free interest rate	2.67	%	2.06	%
Volatility	72	%	72	%
Expected life (years)	6.08		6.08
Expected dividend yield	0	%	0	%

Schedule Share-based Compensation

Share-based compensation for the years ended December 31, 2018 and 2017, are as follows:



	Year Ended December 31,
	2018		2017
	(in thousands)
General and administrative	$	7,479	$	7,128
Research and development	6,345		6,441
Total	$	13,824	$	13,569

Schedule of Stock Option Activity

Stock option activity for the years ended December 31, 2018 and 2017 is as follows:



Options	Outstanding Stock Options	Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value⁽¹⁾(in thousands)
Balance at December 31, 2016	4,532,120	$	12.37	7.58	$	20,453
Granted	1,504,625	$	11.67
Exercised	(344,958)	$	4.31		$	—
Forfeited	(405,315)	$	16.97
Balance at December 31, 2017	5,286,472	$	12.35	7.35	$	7,223
Granted⁽²⁾	2,623,191	$	6.88
Exercised	(1,044,450)	$	3.20
Forfeited	(1,105,967)	$	15.56
Balance at December 31, 2018	5,759,246	$	10.90	8.09	$	87
Exercisable as of December 31, 2018	2,447,034	$	14.28	6.73	$	87

⁽²⁾Includes 595,000 of inducement option awards granted in 2018.

Schedule of Restricted Stock Award Activity

The following table summarizes the stock award activity for all stock plans during the year ended December 31, 2018 and 2017:



Restricted Stock Shares	Outstanding Restricted Stock Awards and Units		Outstanding Aggregate Intrinsic Value (1) (in thousands)
Balance at December 31, 2016	58,825		$	801
Granted	117,500
Vested	(29,412	)	330
Forfeited	(6,250	)
Balance at December 31, 2017⁽²⁾	140,663		$	1,183
Granted⁽³⁾	211,250
Vested	(57,227	)	420
Forfeited	(48,531	)
Balance at December 31, 2018⁽²⁾	246,155		$	719

⁽¹⁾The aggregate intrinsic value as of December 31, 2018, is calculated as the fair value of restricted stock and restricted stock units at December 31, 2018.

⁽²⁾At December 31, 2018, there were no shares of unvested restricted common stock outstanding, compared with 29,413 shares outstanding at December 31, 2017.

⁽³⁾Includes 40,000 of inducement restricted stock units granted during 2018.

Schedule of Share-based Payment Activity

The following table includes share-based payment activity for the years ended December 31, 2018 and 2017:



	Year Ended December 31,
	2018		2017
	(in thousands, except per share)
Weighted-average grant date fair value of options granted	$	4.51	$	7.55
Weighted-average grant date fair value of restricted stock units granted	$	7.14	$	12.59
Total fair value of restricted shares vested	$	907	$	330
Cash received by Company upon option exercises	$	3,270	$	1,486

Summary of Options Outstanding and Exercisable

The following table summarizes the options outstanding and exercisable at December 31, 2018:



Options Outstanding					Options Exercisable
Exercise Price	Total Shares	Weighted- Average Remaining Contractual Term (in years)	Weighted-Average Exercise Price		Total Shares	Weighted- Average Remaining Contractual Term (in years)	Weighted-Average Exercise Price
$0.51 to $6.17	1,154,806	8.59	$	4.21	197,806	3.14	$	2.48
$6.18 to $7.84	1,267,975	8.38	$	7.53	373,146	6.25	$	7.48
$7.85 to $11.76	1,206,985	8.66	$	9.87	326,720	8.03	$	10.85
$11.77 to $18.24	1,010,001	7.80	$	12.88	571,090	7.66	$	13.05
$18.25 to $24.48	1,119,479	5.49	$	20.95	978,272	5.32	$	21.12
Total	5,759,246	8.09	$	10.90	2,447,034	6.73	$	14.28

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments for Capital Leases

Aggregate future minimum annual payments under operating and capital leases at December 31, 2018, are as follows:



Year	Operating Leases		Capital Leases
	(in thousands)
2019	$	2,087	$	68
2020	1,112		68
2021	1,055		43
2022	1,094		—
2023	1,133		—
Thereafter	3,222		—
Total minimum rentals	$	9,703	$	179

Schedule of Future Minimum Rental Payments for Operating Leases

Aggregate future minimum annual payments under operating and capital leases at December 31, 2018, are as follows:



Year	Operating Leases		Capital Leases
	(in thousands)
2019	$	2,087	$	68
2020	1,112		68
2021	1,055		43
2022	1,094		—
2023	1,133		—
Thereafter	3,222		—
Total minimum rentals	$	9,703	$	179

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Reconciliation of income tax expense

The reconciliation between federal income taxes at the statutory rate and the Company’s income tax expense for the year is as follows:



	December 31,
	2018			2017
	(in thousands)
Tax benefit at statutory rate	$	(20,608	)	$	(31,204	)
Stock options	730			860
Other	128			(36		)
U.S. tax reform rate change	—			37,848
Stock based compensation	1,483			333
Deferred tax valuation allowances	21,606			(1,824		)
Research and development credit	(3,339		)	(5,977		)
Income tax expense	$	—		$	—

Significant components of deferred taxes

Significant components of the Company’s deferred taxes as of December 31, 2018 and 2017 are as follows:



	December 31,
	2018			2017
	(in thousands)
Deferred tax assets:
Federal net operating loss carryforward	$	63,624		$	46,772
Stock compensation	4,533			4,900
Intangible assets	8,392			8,820
Research and development credit	13,612			10,273
Other	2,858			648
Total deferred tax assets	93,019			71,413
Valuation allowance	(93,019		)	(71,413		)
Net deferred tax assets	$	—		$	—

SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Property, Plant and Equipment [Line Items]
Asset impairment charges	$ 0	$ 0
Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Accounting Standards Update 2016-02 \| Scenario, Forecast
Property, Plant and Equipment [Line Items]
Right of use assets			$ 4,900,000
Lease obligations			$ 6,600,000

SIGNIFICANT ACCOUNTING POLICIES - Antidilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents (in shares)	6,005,401	5,427,135
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents (in shares)	5,759,246	5,286,472
Unvested shares of restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents (in shares)	246,155	111,250
Unvested shares of restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents (in shares)	0	29,413

CASH AND CASH EQUIVALENTS AND RESTRICTED CASH Cash and Cash Equivalents - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, noncurrent	$ 4,973	$ 6,190
Restricted Cash and Cash Equivalents	4,973	6,190
Cash equivalent instruments	25,000	25,600
Grant costs
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, noncurrent	3,400	4,200
Release of restricted cash	800
Leasehold improvement costs
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, noncurrent	$ 1,600	$ 2,000

CASH AND CASH EQUIVALENTS AND RESTRICTED CASH Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 43,695	$ 38,839
Restricted cash, noncurrent	4,973	6,190
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$ 48,668	$ 45,029	$ 42,780

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	$ 24,953	$ 25,550
Investment Securities	49,304	61,425
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	24,953	25,550
Investment Securities	0	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	0
Investment Securities	49,304	61,425
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	0
Investment Securities	0	0
U.S. government agency-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	7,383	22,604
U.S. government agency-backed securities \| Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	0	0
U.S. government agency-backed securities \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	7,383	22,604
U.S. government agency-backed securities \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	0	0
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	41,921	38,821
Corporate debt securities \| Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	0	0
Corporate debt securities \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	41,921	38,821
Corporate debt securities \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment Securities	0	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	24,953	25,550
Money market funds \| Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	24,953	25,550
Money market funds \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	0
Money market funds \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	$ 0	$ 0

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 49,350	$ 61,471
Gross Unrealized Gains	2	13
Gross Unrealized Losses	(48)	(59)
Aggregate Estimated Fair Value	49,304	61,425
U.S. government agency-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,382	22,632
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(1)	(28)
Aggregate Estimated Fair Value	7,383	22,604
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	41,968	38,839
Gross Unrealized Gains		13
Gross Unrealized Losses	(47)	(31)
Aggregate Estimated Fair Value	$ 41,921	$ 38,821

PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total	$ 35,869	$ 34,238
Less: accumulated depreciation	(14,991)	(8,296)
Property and equipment, net	20,878	25,942
Depreciation expense	6,698	3,564
Leasehold improvements	$ 2,500	2,500
Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	3 years
Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Total	$ 21,633	21,462
Lab equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Total	$ 8,471	7,766
Office furniture
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Total	$ 1,704	1,701
Manufacturing equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Total	$ 1,890	1,815
Computer and office equipment
Property, Plant and Equipment [Line Items]
Total	$ 1,606	1,074
Computer and office equipment \| Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	3 years
Computer and office equipment \| Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Equipment held under capital leases
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Total	$ 204	204
Software
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	3 years
Total	$ 361	$ 216

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued payroll	$ 3,430	$ 2,682
Accrued patient treatment costs	2,053	1,392
Accrued manufacturing costs	546	370
Accrued construction costs	457	565
Accrued other	2,103	1,383
Total accrued expenses and other current liabilities	$ 8,589	$ 6,392

DEBT - Narrative (Details)		12 Months Ended
DEBT - Narrative (Details)	Dec. 21, 2017 USD ($) installment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 08, 2017 USD ($)	Sep. 15, 2016 USD ($)	Mar. 10, 2016 USD ($)
Debt Instrument [Line Items]
Payment of debt issuance costs		$ 0	$ 150,000
Amortization of deferred financing costs		886,000	774,000
Payment on debt		0	32,688,000
Hercules Loan
Debt Instrument [Line Items]
Face amount borrowed						$ 15,000,000.0
Debt instrument, additional borrowing, second draw					$ 5,000,000.0
Debt instrument, additional borrowing, third draw				$ 10,000,000.0
Payment of debt issuance costs				300,000
Final facility charge payment				$ 2,100,000
Amortization of deferred financing costs			800,000
Payment on debt	$ 32,900,000
Debt prepayment cost	1,800,000
Oxford Loan
Debt Instrument [Line Items]
Face amount borrowed	35,000,000.0
Payment of debt issuance costs	100,000
Final facility charge payment	$ 3,000,000
Amortization of deferred financing costs		$ 900,000	$ 22,000
Number of periodic payments \| installment	35
Interest rate at end of period		12.09%
Final payment, percent of principal	8.70%
Early repayment fee, percentage of prepaid amount, period one	3.00%
Early repayment fee, percentage of prepaid amount, period two	2.00%
Early repayment fee, percentage of prepaid amount, period three	1.00%
Oxford Loan \| Floating per Annum Rate
Debt Instrument [Line Items]
Basis spread on variable rate - plus (minus)	7.25%
Oxford Loan \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate - plus (minus)	1.25%

DEBT - Schedule of Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
2019	$ 0
2020	11,000
2021	12,000
2022	12,000
Thereafter	3,045
Total debt	38,045
Less deferred financing costs	(2,213)
Less current portion	0
Total long-term debt	$ 35,832	$ 34,946

GRANT REVENUE (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended		72 Months Ended
GRANT REVENUE (Details) - USD ($) $ in Thousands	Aug. 09, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Accrued revenue		$ 312	$ 292	$ 362	$ 154	$ (69)	$ 126	$ 0	$ 128	$ 1,120	$ 185
Cancer Prevention and Research Institute of Texas
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Grants awarded	$ 16,900
Revenue-sharing payments, maximum percent	1.00%
Deferred grant revenue						$ 4,200					4,200
Maximum \| Cancer Prevention and Research Institute of Texas
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue-sharing payments, maximum percent	400.00%
Grants
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Accrued revenue											185
Grants \| National Institutes of Health Grant
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Accrued revenue										$ 1,120	$ 100	$ 1,400

STOCKHOLDERS' EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Oct. 05, 2018	Apr. 20, 2018	Mar. 29, 2017	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Proceeds from issuance of common stock				$ 205	$ 293
Jefferies LLC
Class of Stock [Line Items]
Proceeds from issuance of common stock	$ 60,000
Follow-Up Offering
Class of Stock [Line Items]
Issuance of common stock (in shares)		9,200,000	5,750,000
Issuance of common stock, price (in usd per share)		$ 7.50	$ 12.00
Proceeds from issuance of common stock		$ 69,000	$ 69,000
Proceeds from issuance of common stock, net		$ 64,700	$ 64,600

SHARE-BASED COMPENSATION PLANS - Narrative (Details) $ in Millions		12 Months Ended
	Jun. 14, 2017 shares	Dec. 31, 2018 USD ($) plan shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 shares
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of share-based compensation plans \| plan		4
Options outstanding (in shares)		5,759,246	5,286,472	4,532,120
ESPP common stock discount		15.00%
Compensation cost not yet recognized \| $		$ 16.3
Compensation cost not yet recognized, period for recognition		2 years 6 months 4 days
Aggregate fair value of fair value of options vesting \| $		$ 13.9	$ 12.2
Stock options
Share-based Goods and Nonemployee Services Transaction [Line Items]
Vesting period		4 years
Award expiration period (up to)		10 years
2006 Stock Option Plan \| Stock options
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of shares remaining to be issued		6,882
2011 Stock Option Plan \| Stock options
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of shares remaining to be issued		512,015
2014 Equity Incentive Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of shares remaining to be issued		2,059,399
Additional shares authorized (in shares)	3,100,000
Number of shares authorized for issuance		6,090,354
Awards outstanding (in shares)		5,486,504	3,646,113
2014 Equity Incentive Plan \| Stock options
Share-based Goods and Nonemployee Services Transaction [Line Items]
Options outstanding (in shares)		4,015,349	2,875,450
2014 Equity Incentive Plan \| Inducement stock options
Share-based Goods and Nonemployee Services Transaction [Line Items]
Options outstanding (in shares)		1,225,000	630,000
2014 Equity Incentive Plan \| Restricted stock units
Share-based Goods and Nonemployee Services Transaction [Line Items]
Awards other than options outstanding (in shares)		194,905	96,250
2014 Equity Incentive Plan \| Restricted stock
Share-based Goods and Nonemployee Services Transaction [Line Items]
Awards other than options outstanding (in shares)			29,413
2014 Equity Incentive Plan \| Inducement restricted stock units
Share-based Goods and Nonemployee Services Transaction [Line Items]
Awards other than options outstanding (in shares)		51,250	15,000
2014 Employee Stock Purchase Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of shares remaining to be issued		414,637
Number of shares authorized for issuance		550,000
Issuance of common stock - ESPP (in shares)		45,390	34,654
Restricted stock
Share-based Goods and Nonemployee Services Transaction [Line Items]
Awards other than options outstanding (in shares)		0	29,413

SHARE-BASED COMPENSATION PLANS - ESPP Activity (Details) - 2014 Employee Stock Purchase Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deductions from employees	$ 221	$ 276
Share-based compensation expense recognized	138	225
Remaining share-based compensation expense	464	264
Proceeds received by the Company for ESPP	$ 205	$ 293
Weighted-average purchase price per common share (in dollars per share)	$ 4.53	$ 8.43
Number of shares purchased by employees under ESPP (in shares)	45,390	34,654

SHARE-BASED COMPENSATION PLANS - Weighted-Average Valuation Assumptions (Details) - Stock options	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.67%	2.06%
Volatility	72.00%	72.00%
Expected life (years)	6 years 29 days	6 years 29 days
Expected dividend yield	0.00%	0.00%

SHARE-BASED COMPENSATION PLANS - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Outstanding Stock Options
Balance (in shares)	5,286,472	4,532,120
Granted (in shares)	2,623,191	1,504,625
Exercised (in shares)	(1,044,450)	(344,958)
Forfeited (in shares)	(1,105,967)	(405,315)
Balance (in shares)	5,759,246	5,286,472	4,532,120
Exercisable at end of period (in shares)	2,447,034
Weighted-Average Exercise Price
Balance (usd per share)	$ 12.35	$ 12.37
Granted (usd per share)	6.88	11.67
Exercised (usd per share)	3.20	4.31
Forfeited (usd per share)	15.56	16.97
Balance (usd per share)	10.90	$ 12.35	$ 12.37
Exercisable at end of period (usd per share)	$ 14.28
Outstanding Stock Options, Other Disclosures
Weighted-average remaining contractual term, outstanding	8 years 1 month 2 days	7 years 4 months 6 days	7 years 6 months 29 days
Weighted-average remaining contractual term, exercisable	6 years 8 months 23 days
Aggregate intrinsic value, outstanding	$ 87	$ 7,223	$ 20,453
Aggregate intrinsic value, exercised		$ 0
Aggregate intrinsic value, exercisable	$ 87
Inducement stock options
Outstanding Stock Options
Granted (in shares)	595,000

SHARE-BASED COMPENSATION PLANS - Restricted Stock Award Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted stock and restricted stock units
Outstanding Restricted Stock Awards and Units
Beginning Balance (in shares)	140,663	58,825
Granted (in shares)	211,250	117,500
Vested (in shares)	(57,227)	(29,412)
Forfeited (in shares)	(48,531)	(6,250)
Ending Balance (in shares)	246,155	140,663
Outstanding Aggregate Intrinsic Value
Balance (in usd per share)	$ 719	$ 1,183	$ 801
Vested (in usd per share)	420	330
Balance (in usd per share)	$ 719	$ 1,183
Inducement restricted stock units
Outstanding Restricted Stock Awards and Units
Granted (in shares)	40,000

SHARE-BASED COMPENSATION PLANS - Share-Based Payment Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash received by Company upon option exercises	$ 3,270	$ 1,486
Unvested shares of restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value of options granted (in usd per share)	$ 4.51	$ 7.55
Weighted-average grant date fair value of restricted stock units granted (in usd per share)	$ 7.14	$ 12.59
Total fair value of restricted shares vested	$ 907	$ 330
Cash received by Company upon option exercises	$ 3,270	$ 1,486

SHARE-BASED COMPENSATION PLANS - Stock Options Outstanding and Exercisable (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding (in shares)	5,759,246
Weighted-average remaining contractual term, outstanding	8 years 1 month 2 days	7 years 4 months 6 days	7 years 6 months 29 days
Weighted-average exercise price, outstanding (in usd per share)	$ 10.90
Exercisable (in shares)	2,447,034
Weighted-average remaining contractual term, exercisable	6 years 8 months 23 days
Weighted-Average Exercise Price (in USD per share)	$ 14.28
$0.51 to $6.17
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in USD per share)	0.51
Exercise Price - Maximum (in USD per share)	$ 6.17
Outstanding (in shares)	1,154,806
Weighted-average remaining contractual term, outstanding	8 years 7 months 2 days
Weighted-average exercise price, outstanding (in usd per share)	$ 4.21
Exercisable (in shares)	197,806
Weighted-average remaining contractual term, exercisable	3 years 1 month 20 days
Weighted-Average Exercise Price (in USD per share)	$ 2.48
$6.18 to $7.84
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in USD per share)	6.18
Exercise Price - Maximum (in USD per share)	$ 7.84
Outstanding (in shares)	1,267,975
Weighted-average remaining contractual term, outstanding	8 years 4 months 17 days
Weighted-average exercise price, outstanding (in usd per share)	$ 7.53
Exercisable (in shares)	373,146
Weighted-average remaining contractual term, exercisable	6 years 3 months
Weighted-Average Exercise Price (in USD per share)	$ 7.48
$7.85 to $11.76
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in USD per share)	7.85
Exercise Price - Maximum (in USD per share)	$ 11.76
Outstanding (in shares)	1,206,985
Weighted-average remaining contractual term, outstanding	8 years 7 months 28 days
Weighted-average exercise price, outstanding (in usd per share)	$ 9.87
Exercisable (in shares)	326,720
Weighted-average remaining contractual term, exercisable	8 years 11 days
Weighted-Average Exercise Price (in USD per share)	$ 10.85
$11.77 to $18.24
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in USD per share)	11.77
Exercise Price - Maximum (in USD per share)	$ 18.24
Outstanding (in shares)	1,010,001
Weighted-average remaining contractual term, outstanding	7 years 9 months 18 days
Weighted-average exercise price, outstanding (in usd per share)	$ 12.88
Exercisable (in shares)	571,090
Weighted-average remaining contractual term, exercisable	7 years 7 months 28 days
Weighted-Average Exercise Price (in USD per share)	$ 13.05
$18.25 to $24.48
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in USD per share)	18.25
Exercise Price - Maximum (in USD per share)	$ 24.48
Outstanding (in shares)	1,119,479
Weighted-average remaining contractual term, outstanding	5 years 5 months 27 days
Weighted-average exercise price, outstanding (in usd per share)	$ 20.95
Exercisable (in shares)	978,272
Weighted-average remaining contractual term, exercisable	5 years 3 months 26 days
Weighted-Average Exercise Price (in USD per share)	$ 21.12

COMMITMENTS AND CONTINGENCIES - Lease Agreements, Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2015	Dec. 31, 2018	Dec. 31, 2017
Property Subject to or Available for Operating Lease [Line Items]
Landlord reimbursements		$ 2,500
Rent expense		2,100	$ 2,100
Deferred rent		1,700	2,000
Restricted cash		$ 4,973	6,190
Present value of minimum lease payments, percentage of fair market value of leased equipment (greater than)		90.00%
Capital lease, term		6 years
Operating Lease Dated May 2015
Property Subject to or Available for Operating Lease [Line Items]
Estimated build-out costs (as a percent)	110.00%
Restricted cash	$ 11,000
Leasehold improvement costs
Property Subject to or Available for Operating Lease [Line Items]
Restricted cash		$ 1,600	$ 2,000

COMMITMENTS AND CONTINGENCIES - Aggregate Future Minimum Annual Payments Under Operating and Capital Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leases
2019	$ 2,087
2020	1,112
2021	1,055
2022	1,094
2023	1,133
Thereafter	3,222
Total minimum rentals	9,703
Capital Leases
2019	68
2020	68
2021	43
2022	0
2023	0
Thereafter	0
Total minimum rentals	$ 179

COMMITMENTS AND CONTINGENCIES - Collaboration Agreement - Narrative (Details) $ in Thousands, € in Millions	1 Months Ended		12 Months Ended
	Oct. 31, 2016 USD ($)	May 31, 2016 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Research and development			$ 71,152	$ 65,663
Adaptimmune Therapeutics plc \| Collaborative Arrangement
Long-term Purchase Commitment [Line Items]
License agreement expense			0	0
OPBG \| Collaborative Arrangement
Long-term Purchase Commitment [Line Items]
License agreement expense			$ 600	700
OPBG \| Research and Development Arrangement
Long-term Purchase Commitment [Line Items]
Amount agreed to fund (up to)	$ 4,700
Research term	4 years
Leiden \| Collaborative Arrangement
Long-term Purchase Commitment [Line Items]
Amount agreed to fund (up to) \| €		€ 2.5
Research term		3 years
License agreement expense				$ 1,100

COMMITMENTS AND CONTINGENCIES - License Agreements - Narrative (Details) - Licensing Agreements				1 Months Ended	12 Months Ended
	Dec. 10, 2015 USD ($)	Jun. 10, 2015 USD ($) product	Apr. 23, 2015 EUR (€)	Mar. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Baylor
Long-term Purchase Commitment [Line Items]
Nonrefundable license fee				$ 100,000
License agreement expense					$ 15,000	$ 22,500
Agensys
Long-term Purchase Commitment [Line Items]
License agreement expense					0	0
Nonrefundable upfront fee	$ 3,000,000
Milestone payments upon first achievement of specified clinical milestones	5,000,000
Milestone payments upon achievement of specified clinical milestones for each licensed product	50,000,000
Milestone payments upon achievement of sales milestones	75,000,000
Option exercise fee	5,000,000
Milestone payments reduced upon exercise of option	$ 65,000,000
Termination period, number of years after first commercial sale of licensed product	10 years
Termination period, notice of failure on uncured items	60 days
Period of notice of failure on uncured items, if material breach is related to failure to make payments	30 days
BioVec
Long-term Purchase Commitment [Line Items]
License agreement expense					200,000	700,000
Nonrefundable upfront fee		$ 100,000
Milestone payments upon first achievement of specified clinical milestones		$ 250,000
Termination period, notice of failure on uncured items		60 days
Upfront fee payment period, number of days from effective date		10 days
License costs due upon first release of product		$ 300,000
License costs due upon first release of product, period of payment		10 days
License agreement, annual fee		$ 150,000
License agreement, annual fee period, from first IND filing		30 days
Milestone payments, number of initial products \| product		3
License agreement, milestone payments upon receipt of FDA or EMA registration		$ 2,000,000
Termination notice period for any other breach		90 days
Termination period, after insolvency event		30 days
Leiden
Long-term Purchase Commitment [Line Items]
License agreement expense					$ 100,000	$ 100,000
Nonrefundable upfront fee \| €			€ 100,000
Milestone payments upon first achievement of specified clinical milestones \| €			€ 1,000,000
Termination period, notice of failure on uncured items			30 days
Upfront fee payment period, number of days from effective date			30 days
Termination notice period for any other breach			6 months
Termination period, after insolvency event			45 days
License agreement, advance notice required for research collaborations			30 days
License agreement, minimum annual royalty payments beginning on eighth anniversary \| €			€ 30,000
License agreement, payments required under sponsored research agreement \| €			€ 300,000
License agreement, termination period after written notice of failure			30 days

COMMITMENTS AND CONTINGENCIES - Employment Agreement - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) employe	Dec. 31, 2017 USD ($)
Other Commitments [Line Items]
Postemployment Agreement, Number Of Officers and Key Employees \| employe	20
Estimated annual base salary and cash bonus compensation in the event of a change in control \| $	$ 6.7	$ 4.8
Minimum
Other Commitments [Line Items]
Postemployment benefits, estimated liability, period	3 months
Continuation of certain insurance and other benefits, period	3 months
Maximum
Other Commitments [Line Items]
Postemployment benefits, estimated liability, period	18 months
Continuation of certain insurance and other benefits, period	18 months

INCOME TAXES - Reconciliation of income tax expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax benefit at statutory rate	$ (20,608)	$ (31,204)
Stock options	730	860
Other	128	(36)
U.S. tax reform rate change	0	37,848
Stock based compensation	1,483	333
Deferred tax valuation allowances	21,606	(1,824)
Research and development credit	(3,339)	(5,977)
Income tax expense	$ 0	$ 0

INCOME TAXES - Significant components of deferred taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Federal net operating loss carryforward	$ 63,624	$ 46,772
Stock compensation	4,533	4,900
Intangible assets	8,392	8,820
Research and development credit	13,612	10,273
Other	2,858	648
Total deferred tax assets	93,019	71,413
Valuation allowance	(93,019)	(71,413)
Net deferred tax assets	$ 0	$ 0

INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Research tax credits	$ 13,612	$ 10,273
Net operating loss carryforwards, no expiration date	81,800
Increase (decrease) in valuation allowance	21,600	(1,800)
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards	303,000	221,200
Research tax credits	8,900	6,700
Foreign
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards	2,400	2,300
Texas
Operating Loss Carryforwards [Line Items]
Research tax credits	$ 4,700	$ 3,600

SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 312	$ 292	$ 362	$ 154	$ (69)	$ 126	$ 0	$ 128	$ 1,120	$ 185
Loss from operations	(26,567)	(23,228)	(23,567)	(22,104)	(19,445)	(22,705)	(23,788)	(21,449)	(95,466)	(87,387)
Net loss	$ (27,220)	$ (23,801)	$ (24,175)	$ (22,840)	$ (21,918)	$ (23,431)	$ (24,457)	$ (21,973)	$ (98,036)	$ (91,779)
Net loss per share attributable to common shareholders - basic and diluted (in usd per share)	$ (0.63)	$ (0.55)	$ (0.60)	$ (0.68)	$ (0.66)	$ (0.71)	$ (0.74)	$ (0.80)	$ (2.44)	$ (2.89)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

SUBSEQUENT EVENTS (Details)		12 Months Ended
SUBSEQUENT EVENTS (Details)	Feb. 28, 2019 USD ($)	Dec. 31, 2018	Mar. 01, 2019 ft²
Subsequent Event [Line Items]
Term of lease		6 years
CBR Systems \| Subsequent Event
Subsequent Event [Line Items]
Sublease, area of land covered (in sqft) \| ft²			13,943
Term of lease	41 months
Monthly base rent \| $	$ 43,223
Annual percentage increase to monthly base rent	3.00%

SUBSEQUENT EVENTS	12 Months Ended
SUBSEQUENT EVENTS	Dec. 31, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS On February 28, 2019 the Company entered into a sublease agreement with CBR Systems, Inc. to lease an aggregate of 13,943 square feet commencing on March 1, 2019 for 41 months. The Company is required to remit monthly base rent of approximately $43,223 commencing on May 1, 2019 which will increase by an average approximate rate of 3% each year. The Company evaluated this lease for accounting treatment under ASC 842 - leases and determined that the lease is an operating lease.