Bellicum Pharmaceuticals (BLCM) 10-K 31 Mar 21 2020 FY Annual report Financial data

Cover

Cover - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 22, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-36783
Entity Registrant Name	Bellicum Pharmaceuticals, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-1450200
Entity Address, Address Line One	3730 Kirby Drive, Suite 1200
Entity Address, City or Town	Houston
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77098
City Area Code	832
Local Phone Number	384-1100
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	BLCM
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	false
Entity Public Float			$ 31,731,375
Entity Common Stock, Shares Outstanding (in shares)		8,318,273
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to its 2021 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K. Such Proxy Statement will be filed with the U.S. Securities and Exchange Commission within 120 days following the Registrant’s fiscal year ended December 31, 2020.
Entity Central Index Key	0001358403
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 35,495	$ 91,028
Restricted cash	1,501	2,788
Accounts receivable, interest and other receivables	2	303
Prepaid expenses and other current assets	802	884
Assets held for sale	1,643	16,851
Total current assets	39,443	111,854
Operating lease right-of-use assets	645	1,042
Property and equipment, net	189	2,529
Other assets	307	825
Total assets	40,584	116,250
Current liabilities:
Accounts payable	891	2,643
Accrued expenses and other current liabilities	4,165	9,770
Warrant derivative liability	10,345	52,184
Private placement option liability	7,803	12,094
Current portion of long-term debt	0	11,000
Current portion of lease liabilities	825	454
Liabilities held for sale	672	6,273
Total current liabilities	24,701	94,418
Long-term liabilities:
Long-term debt, net of deferred issuance costs	0	25,717
Long-term lease liabilities	344	864
Total liabilities	25,045	120,999
Commitments and contingencies
Preferred stock: $0.01 par value; 10,000,000 shares authorized	18,036	21,468
Stockholders’ deficit:
Common stock, $0.01 par value; 80,000,000 shares authorized at December 31, 2020 and December 31, 2019, respectively; 8,385,650 shares issued and 8,317,904 shares outstanding at December 31, 2020; 5,076,593 shares issued and 5,008,846 shares outstanding at December 31, 2019	84	507
Treasury stock: 67,746 shares held at December 31, 2020 and December 31, 2019	(5,056)	(5,056)
Additional paid-in capital	543,561	511,684
Accumulated other comprehensive loss	(339)	(327)
Accumulated deficit	(540,747)	(533,025)
Total stockholders’ deficit	(2,497)	(26,217)
Total liabilities, preferred stock and stockholders' deficit	$ 40,584	$ 116,250

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Preferred stock, par value (in usd per share)	$ 0.01	$ 0.01
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Common stock, par value (in USD per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	80,000,000	80,000,000
Common stock, issued (in shares)	8,385,650	5,076,593
Common stock, outstanding (in shares)	8,317,904	5,008,846
Treasury stock (in shares)	67,746	67,746
Convertible Preferred Stock
Preferred stock, par value (in usd per share)	$ 0.01	$ 0.01
Preferred stock, authorized (in shares)	1,517,500	1,517,500
Preferred stock, issued (in shares)	452,000	538,000
Preferred stock, outstanding (in shares)	452,000	538,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenues
Revenues	$ 500	$ 7,143
Operating expenses
Research and development	39,052	64,535
General and administrative	15,531	29,972
Total operating expenses	54,583	94,507
Impairment of property and equipment	(1,265)	0
Gain on dispositions, net	3,656	0
Total other operating income	2,391	0
Loss from operations	(51,692)	(87,364)
Other income (expense):
Interest income	387	1,351
Interest expense	(2,659)	(4,280)
Change in fair value of warrant and private placement option liabilities	46,130	(19,192)
Gain on extinguishment of debt	112	0
Other expense	0	(2,992)
Total other income (expense)	43,970	(25,113)
Net loss	$ (7,722)	$ (112,477)
Net loss per common share attributable to common shareholders, basic and diluted (in usd per share)	$ (1.34)	$ (24.01)
Weighted-average shares outstanding-basic and diluted (in shares)	5,760,159	4,684,711
Net loss	$ (7,722)	$ (112,477)
Other comprehensive loss:
Unrealized gain on available-for-sale securities, net of tax	0	45
Foreign currency translation adjustment	(12)	(228)
Comprehensive loss	(7,734)	(112,660)
Grants
Revenues
Revenues	0	2,143
License fee revenue
Revenues
Revenues	$ 500	$ 5,000

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders’ (Deficit) Equity $ in Thousands	USD ($)shares	Common StockUSD ($)shares	Treasury StockUSD ($)shares	Additional Paid-In CapitalUSD ($)	Accumulated DeficitUSD ($)	Accumulated Other Comprehensive LossUSD ($)	Series 1 PreferredUSD ($)shares
Preferred stock, beginning balance (in shares) at Dec. 31, 2018 | shares							0
Preferred stock, beginning balance at Dec. 31, 2018							$ 0
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock in public offering, net (in shares) | shares							575,000
Issuance of redeemable convertible preferred stock in public offering, net							$ 22,944
Conversion of redeemable convertible preferred stock into common stock (in shares) | shares		370,000					(37,000)
Conversion of redeemable convertible preferred stock into common stock	$ 1,476	$ 37		$ 1,439			$ (1,476)
Preferred stock, ending balance (in shares) at Dec. 31, 2019 | shares							538,000
Preferred stock, ending balance at Dec. 31, 2019	21,468						$ 21,468
Balance, beginning of period (in shares) at Dec. 31, 2018 | shares		4,424,205	67,746
Balance, beginning of period at Dec. 31, 2018	68,478	$ 442	$ (5,056)	493,784	$ (420,548)	$ (144)
Increase (Decrease) in Stockholders' Equity
Share-based compensation	$ 7,338			7,338
Exercise of stock options (in shares) | shares	220	2,985
Exercise of stock options	$ 76	$ 0		76
Issuance of common stock - Employee Stock Purchase Plan (in shares) | shares		8,000
Issuance of common stock - Employee Stock Purchase Plan	98	$ 1		97
Issuance of common stock upon vesting of restricted stock units (in shares) | shares		12,287
Issuance of common stock upon vesting of restricted stock units	0	$ 1		(1)
Issuance of common stock in open market transactions, net of issuance costs (in shares) | shares		259,116
Issuance of common stock in open market transactions, net of issuance costs	8,977	$ 26		8,951
Conversion of redeemable convertible preferred stock into common stock (in shares) | shares		370,000					(37,000)
Conversion of redeemable convertible preferred stock into common stock	1,476	$ 37		1,439			$ (1,476)
Comprehensive loss	(112,660)				(112,477)	(183)
Balance, end of period (in shares) at Dec. 31, 2019 | shares		5,076,593	67,746
Balance, end of period at Dec. 31, 2019	(26,217)	$ 507	$ (5,056)	511,684	(533,025)	(327)
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of redeemable convertible preferred stock into common stock (in shares) | shares		860,000					(86,000)
Conversion of redeemable convertible preferred stock into common stock	3,432	$ 9		3,423			$ (3,432)
Preferred stock, ending balance (in shares) at Dec. 31, 2020 | shares							452,000
Preferred stock, ending balance at Dec. 31, 2020	18,036						$ 18,036
Increase (Decrease) in Stockholders' Equity
Share-based compensation	$ 5,031			5,031
1-for-10 reverse stock split		$ (457)		457
Reverse stock split ratio		0.01
Exercise of stock options (in shares) | shares	0
Issuance of common stock - Employee Stock Purchase Plan (in shares) | shares		14,975
Issuance of common stock - Employee Stock Purchase Plan	$ 78	$ 0		78
Issuance of common stock upon vesting of restricted stock units (in shares) | shares		1,406
Issuance of common stock upon vesting of restricted stock units	0	$ 0		0
Issuance of common stock and pre-funded warrants exercise, net of issuance costs (in shares) | shares		2,432,676
Issuance of common stock and pre-funded warrants exercise, net of issuance costs	22,913	$ 25		22,888
Conversion of redeemable convertible preferred stock into common stock (in shares) | shares		860,000					(86,000)
Conversion of redeemable convertible preferred stock into common stock	3,432	$ 9		3,423			$ (3,432)
Comprehensive loss	(7,734)				(7,722)	(12)
Balance, end of period (in shares) at Dec. 31, 2020 | shares		8,385,650	67,746
Balance, end of period at Dec. 31, 2020	$ (2,497)	$ 84	$ (5,056)	$ 543,561	$ (540,747)	$ (339)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (7,722)	$ (112,477)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation	5,031	7,338
Depreciation and amortization expense	1,472	7,175
Change in fair value of warrant and private placement option liabilities	(46,130)	19,192
Impairment of intangible assets	0	2,064
Amortization of discount on investment securities, net	0	(30)
Amortization of right-of-use assets	402	1,331
Accretion of lease liability	414	804
Impairment of property and equipment	1,265	0
Amortization of deferred issuance costs	550	885
Gain on extinguishment of debt	(112)	0
(Gain) loss on dispositions, net	(3,656)	6
Warrant and private placement option issuance costs	0	3,047
Changes in operating assets and liabilities:
Accounts receivable, interest and other receivables	301	606
Prepaid expenses and other assets	563	(1,096)
Accounts payable	(1,917)	(1,131)
Accrued liabilities and other	(7,111)	(2,300)
Deferred revenue	0	(2,983)
Net cash used in operating activities	(56,650)	(77,569)
Cash flows from investing activities:
Proceeds from sale of investment securities	0	49,379
Proceeds from sale of property and equipment	14,705	0
Purchases of property and equipment	(625)	(522)
Cash provided by investing activities	14,080	48,857
Cash flows from financing activities:
Payment on debt	(37,155)	0
Proceeds from issuance of common stock in a public offering, net	0	8,977
Proceeds from issuance of redeemable convertible preferred stock in a public offering, net	0	22,944
Proceeds from issuance of warrants in a public offering, net	0	30,888
Proceeds received from private placement option, net	0	11,152
Proceeds from exercise of stock options	12	76
Proceeds from issuance of stock from employee stock purchase plan	78	98
Proceeds from issuance of common stock and pre-funded warrants exercise, net	22,901	0
Payment on financing lease obligations	(74)	(47)
Net cash (used in) provided by financing activities	(14,238)	74,088
Effect of exchange rate changes on cash	(12)	(228)
Net change in cash, cash equivalents and restricted cash	(56,820)	45,148
Cash, cash equivalents and restricted cash at beginning of period	93,816	48,668
Cash, cash equivalents and restricted cash at end of period	36,996	93,816
Supplemental cash flow information:
Cash paid during the period for interest	2,340	3,201
Non-cash investing and financing activities:
Financing leases incurred for equipment	0	167
Conversion of redeemable preferred stock into common stock	3,432	1,476
Reclassification of property and equipment, net to assets held for sale	2,300	12,039
Leasehold improvements paid by landlord	$ 113	$ 0

ORGANIZATION, BASIS OF PRESENTA

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Organization Bellicum Pharmaceuticals, Inc. (“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. Bellicum is devoting substantially all of its present efforts to developing next-generation product candidates in areas of cellular immunotherapy, including CAR-T therapy. Bellicum has two wholly-owned subsidiaries, Bellicum Pharma Limited, a private limited company organized under the laws of the United Kingdom, and Bellicum Pharma GmbH, a private limited liability company organized under German law. Both were formed for the purpose of developing product candidates in Europe. Bellicum, Bellicum Pharma Limited and Bellicum Pharma GmbH are collectively referred to herein as the “Company”. All intercompany balances and transactions among the consolidated entities have been eliminated in consolidation. Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker. The Company has determined that it has one operating and reporting segment as it allocates resources and assesses financial performance on a consolidated basis. The Company’s chief operating decision maker is its Chief Executive Officer who manages operations and reviews the financial information as a single operating segment for purposes of allocating resources and evaluating its financial performance. Reverse Stock Split On February 5, 2020, the Company filed a Certificate of Amendment of the Amended and Restated Certificate of Incorporation with the Secretary of State of the State of Delaware to (i) effect a reverse stock split of all issued and outstanding shares of the Company’s common stock at a ratio of 1-for-10 and (ii) reduce the number of authorized shares of the Company’s common stock from 200,000,000 to 40,000,000. The accompanying consolidated financial statements and notes to the consolidated financial statements gives retroactive effect to the reverse stock split for all periods presented. On June 15, 2020, the Company filed with the Secretary of State of the State of Delaware a Second Certificate of Amendment to the Company’s Amended and Restated Certificate of Incorporation to increase the authorized number of shares of the Company’s common stock from 40,000,000 shares to 80,000,000 shares. Basis of Presentation The accompanying financial statements have been prepared in conformity with the authoritative U.S. generally accepted accounting principles (“GAAP”). The accompanying financial statements have been prepared on a basis that assumes that the Company will continue as a going concern, and do not include any adjustments that may result from the outcome of this uncertainty. This basis of accounting contemplates the recovery of the Company’s assets and the satisfaction of the Company’s liabilities and commitments in the normal course of business and does not include any adjustments to reflect the possible future effects of the recoverability and classification of recorded asset amounts or amounts and classification of liabilities that might be necessary should the Company be unable to continue as a going concern. The Company has experienced net losses since its inception and if the Company does not successfully obtain regulatory approval and commercialize any of its product candidates, the Company will not be able to achieve profitability. As of December 31, 2020, the Company has an accumulated deficit of $540.7 million. 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. The Company believes that its current capital resources, which consist of cash and cash equivalents, are sufficient to fund operations through at least the next twelve months from the date the accompanying financial statements are issued based on the expected cash burn rate. The Company may be required to raise additional capital to fund future operations through the sale of additional equity, incurrence of additional debt allowed under existing debt arrangements, the entry into licensing or collaboration agreements with partners, grants or other sources of financing. Sufficient funds may not be available to the Company at all or on attractive terms when needed from equity or debt financings. If the Company is unable to obtain additional funding from these or other sources when needed, or to the extent needed, it may be necessary to significantly reduce its controllable and variable expenditures and current rate of spending through reductions in staff and delaying, scaling back, or suspending certain research and development, sales and marketing programs and other operational goals. 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, revenue recognition, and expenses. Actual results could differ materially from those estimates. Revenue Recognition The Company’s only source of revenue in 2020 was from its licensing agreement with The University of Texas MD Anderson Cancer Center, (“MD Anderson”). In 2019, the Company earned revenue from its licensing agreement with The University of Texas MD Anderson Cancer Center, (“MD Anderson”) and from grants. Prior to 2019, the Company's only source of revenue was from grants. Grant Revenue When grant funds are received after costs have been incurred, the Company records revenue and a corresponding 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. License Revenue The promised services in license agreements consist of license rights to the Company’s intellectual property. When management believes the license to its intellectual property and products has stand-alone value, the Company recognizes revenue attributed to the license upon delivery. The Company recognizes revenue when control of the promised goods or services is transferred to its customers, in an amount that reflects the consideration to which the Company expects to be entitled in exchange for the goods or services. In order to achieve that core principle, a five-step approach is applied: (1) identify the contract with a customer, (2) identify the performance obligations in the contract, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations in the contract, and (5) recognize revenue allocated to each performance obligation when the Company satisfies the performance obligation. A performance obligation is a promise in a contract to transfer a distinct good or service to the customer, and is the unit of account for revenue recognition. The Company may provide options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. License agreements generally include certain milestone payments. The Company utilizes the “most likely amount” method to estimate the amount of variable consideration, to predict the amount of consideration to which it will be entitled for its license agreement. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. Milestone, annual maintenance, and royalty payments that are not within the control of the Company or the licensee, such as those dependent upon receipt of regulatory approval, are not considered to be probable of achievement until the triggering event occurs. At the end of each reporting period, the Company reevaluates the probability of achievement of each and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue and net loss in the period of adjustment. To date, the Company has not recognized any development, regulatory or commercial milestones or royalty revenue resulting from its license agreement. Consideration that would be received for optional goods and/or services is excluded from the transaction price at contract inception. License Agreement On January 22, 2019, the Company entered into a licensing and commercialization agreement with MD Anderson (the "MD Anderson License Agreement"). Under the MD Anderson License Agreement, the Company granted MD Anderson non-exclusive rights in certain Caspase-9 and related technologies and use of a small molecule known as rimiducid. During the fourth quarter of 2019, and under the terms of the MD Anderson License Agreement, MD Anderson exercised an option to grant a non-exclusive sublicense of the rights licensed by the Company to MD Anderson under the MD Anderson License Agreement. MD Anderson, as a result of this exercise, granted a sublicense that entitled the Company to receive as consideration an upfront payment of $5.0 million in license fees as well as additional future annual maintenance fees, milestone payments related to the achievement of pre-specified development, regulatory, and commercialization events, and royalties of two percent on net sales of licensed products. During the fourth quarter of 2019, the Company recognized $5.0 million of license fee revenue as delivery of the license occurred and the license to its Caspase-9 intellectual property has stand-alone value. During 2020, the Company has received $0.5 million of maintenance fees under the MD Anderson License Agreement. Cancer Research Grant Contract On August 9, 2017, the Company entered into a Cancer Research Grant Contract (the “CPRIT Agreement”) with the Cancer Prevention Research Institute of Texas (“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 CPRIT Agreement and subject to CPRIT’s ability to perform its obligations under the CPRIT Agreement. During 2017, the Company received $4.2 million in advance funding from CPRIT, which was recorded as deferred revenue. During the years ended December 31, 2020 and 2019, the Company incurred expenses and recognized revenue of $0.0 million and $2.1 million, respectively, for work performed under the CPRIT grant. The CPRIT Agreement was due to expire on February 29, 2020, but was terminated early by the Company on January 31, 2020. Upon termination of the CPRIT Agreement, the Company reclassified the remaining unexpended award proceeds of $0.8 million from deferred revenue to accrued liabilities. During the year ended December 31, 2020, the Company returned the remaining unexpended award proceeds to CPRIT and released the accrued liability. Cash, Cash Equivalents, and Restricted Cash The Company considers all short-term, highly liquid investments with maturity of three months or less from the date of purchase and that can be liquidated without prior notice or penalty, to be cash equivalents. 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. (in thousands) December 31, 2020 December 31, 2019 Cash and cash equivalents $ 35,495 $ 91,028 Restricted cash 1,501 2,788 Total cash, cash equivalents and restricted cash shown in the statements of cash flows $ 36,996 $ 93,816 In addition to the restricted cash held and released by CPRIT, there was $1.1 million of restricted cash as of December 31, 2019 in escrow to cover specific construction of manufacturing improvement costs related to the facility lease. The release of the funds was subject to the authorized completion of certain aspects of the manufacturing improvements. The funds were released during year ended December 31, 2020. In connection with the closing of the Asset Purchase Agreement with M.D. Anderson on April 14, 2020, $1.5 million of the cash proceeds received are subject to certain escrow provisions and recorded as restricted cash. The funds are required to be held for a period of up to 18 months subsequent to the April 14, 2020 closing date. Dispositions and Derecognition of Liabilities Disposition of Assets and Liabilities Held for Sale In 2019, the Company completed the buildout of manufacturing space at its leased headquarters in Houston, Texas and began in-house clinical supply manufacturing. The facility included capacity in excess of the Company's anticipated current and near-term manufacturing needs and management decided to seek a partner for the facility with the goal of reducing the Company's costs while maintaining dedicated cell therapy manufacturing capacity to support the Company's product candidates. The disposal of the assets and liabilities of such facility was completed on April 14, 2020, at a purchase price of $15.0 million. The disposal group consisted of property and equipment, net of $12.0 million, right-of-use assets of $4.8 million, current portion of lease liabilities of $1.4 million and long-term lease liabilities of $4.6 million. During the year ended December 31, 2020, the Company recognized a net gain of $3.8 million in connection with the disposal, which is presented within Gain on dispositions, net within the accompanying consolidated statements of operations and comprehensive loss. 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. 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 based on quoted prices in active markets, with unrealized gains and, to the extent deemed temporary, unrealized losses reported as accumulated other comprehensive gain (loss), a separate component of stockholders' (deficit) 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 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. To date, the Company has not identified any other than temporary declines in the fair value of its investment securities. Assets Held for Sale In the fourth quarter of 2020, in connection with the Company's restructuring plan, Management elected to seek an exit to its leased R&D facility in Houston, Texas. As a result of this decision, the Company reclassified the assets and liabilities associated with the leased facility as held for sale. The reclassified assets and liabilities included a right-of-use asset of $0.5 million, property and equipment of $2.3 million and the related lease liability of $0.7 million. Based on the cost to exit the lease and the net realizable value of the related assets the Company recognized an impairment charge of $1.3 million in 2020. Property and Equipment 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 3 to 5 years. Leasehold improvements are amortized over the shorter of the estimated useful life or the remaining lease term. Property and equipment consisted of the following: (in thousands, except useful lives) Estimated Useful Lives December 31, 2020 December 31, 2019 Leasehold improvements 5 Years $ 167 $ 3,944 Lab equipment 5 Years 530 5,459 Office furniture 5 Years — 392 Manufacturing equipment 5 Years 138 395 Computer and office equipment 3 to 5 Years 834 1,595 Equipment held under capital leases 5 Years — 270 Software 3 Years 94 385 Total 1,763 12,440 Less: accumulated depreciation (1,574) (9,911) Property and equipment, net $ 189 $ 2,529 During the years ended December 31, 2020 and 2019, the Company recorded $1.5 million and $7.0 million of depreciation expense, respectively. Intangible Assets Non-refundable upfront payments related to a supply agreement with future benefits have been capitalized as an intangible asset, presented in other assets on the accompanying consolidated balance sheets and amortized over the term of the agreement. The amortization of the intangible asset is included in operating expenses in the accompanying consolidated statements of operations and comprehensive loss. During the fourth quarter of 2019, the Company recorded $2.1 million of impairment charges related to the non-refundable upfront payments for the Miltenyi supply agreement that had been capitalized as an intangible asset. The Company recorded the impairment charge as a “Research and development” operating expense within the accompanying consolidated statements of operations and comprehensive loss. There were no other impairment charges related to long-lived assets for the years ended December 31, 2020 and 2019. 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. Accrued Expenses and Other Current Liabilities Accrued expenses and other liabilities consist of the following: December 31, 2020 December 31, 2019 Accrued payroll $ 1,029 $ 2,032 Accrued patient treatment costs 899 1,162 Accrued manufacturing costs 24 2,230 Accrued professional services 294 654 Accrued obligations under material supply agreements — 1,121 Accrued other 1,919 2,571 Total accrued expenses and other current liabilities $ 4,165 $ 9,770 Debt Issuance Costs Costs related to debt issuance are presented in the accompanying consolidated balance sheets 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 in the accompanying statements of operations and comprehensive loss. Warrant Derivatives Freestanding warrants exercisable for multiple underlying instruments are classified as liabilities in the accompanying consolidated balance sheets. The Company accounts for these warrants at fair value on the date of issuance and are subject to re-measurement to fair value at each balance sheet date. Any change in fair value is recognized as a component of other income (expense) on the accompanying consolidated statements of operations and comprehensive loss. The Company estimates the fair value of these liabilities using the binomial option model, adjusted for the effect of dilution, because it embodies all of the requisite assumptions (including trading volatility, estimated terms, dilution and risk-free rates) necessary to determine the fair value of this instrument. The Company will continue to adjust the liability for changes in fair value until the earlier of the exercise or expiration of the warrants or a change in control, as defined. The warrants are freely exercisable at any time from the issuance date until the expiration date, provided exercise does not cause a warrant holder to exceed a pre-determined beneficial ownership limit. The Company estimates the fair value of these liabilities using the Black-Scholes valuation technique, which utilizes assumptions including (i) the fair value of the underlying stock at the valuation measurement date, (ii) volatility of the price of the underlying stock, (iii) the expected term, and (iv) risk-free interest rates. Private Placement Option The Company has entered into a security purchase agreement that contains a call option on preferred shares that are puttable outside the control of the Company. The Company recorded the option as a liability and measured the fair value of the option at the time of issuance. The Company will re-measure the option to fair value at each balance sheet date and record changes in fair value in other income (expense) in the accompanying consolidated statement of operations and comprehensive loss at each reporting period. Offering expenses arising from the issuance of the private placement option were expensed as incurred. The Company estimates the fair value of these liabilities using a binomial lattice model, which utilizes assumptions including (i) the fair value of the underlying stock at the valuation measurement date, (ii) volatility of the price of the underlying stock, (iii) the expected term, and (iv) risk-free interest rates. Preferred Stock Preferred shares issued by the Company that are subject to mandatory redemption are classified as liability instruments in the accompanying consolidated balance sheets and are measured at fair value at the date of issuance. Conditionally redeemable preferred shares (including preferred shares that feature redemption rights that are either within the control of the holder or subject to redemption upon the occurrence of uncertain events not solely within the Company’s control) are classified within mezzanine equity in the accompanying consolidated balance sheets. At all other times, preferred shares are classified within stockholders’ (deficit) equity. Operating Leases At the inception of a contractual arrangement, the Company determines whether the contract contains a lease by assessing whether there is an identified asset and whether the contract conveys the right to control the use of the identified asset in exchange for consideration over a period of time. If both criteria are met, upon lease commencement, the Company records a lease liability which represents the Company’s obligation to make lease payments arising from the lease, and a corresponding right-of-use (“ROU”) asset which represents the Company’s right to use an underlying asset during the lease term. Operating leases are recognized as right-of-use, or ROU, assets and operating lease liabilities on the balance sheet based on the present value of the future minimum lease payments over the lease term at commencement date calculated using the Company's incremental borrowing rate applicable to the underlying asset unless the implicit rate is readily determinable. Any lease incentives received are deferred and recorded as a reduction of the ROU asset and amortized over the term of the lease. Rent expense, comprised of amortization of the ROU asset and the implicit interest accreted on the operating lease liability, is recognized on a straight-line basis over the lease term. The Company determines the lease term as the noncancellable period of the lease and may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise such options. Leases with a term of 12 months or less are not recognized on the balance sheets. Fair Value of Financial Instruments Fair value is defined as the exchange price that would be received for an asset or an exit price paid to transfer a liability in an orderly transaction between market participants in a principal market on the measurement date. Accounting standards include disclosure requirements around fair values used for certain financial instruments and establish a fair value hierarchy. The three-tier hierarchy defines a three-tiered valuation hierarchy for disclosures that 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 2. 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 categorization of a financial instrument within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The Company believes the recorded values of its financial instruments, including cash and cash equivalents, accounts payable, accrued liabilities, and debt 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 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 and Security Investor Protection Corporation. 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 in the accompanying consolidated statements of operations and comprehensive loss. 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 share-based compensation based on the measurement and recognition of compensation expense for all share-based payment awards made to employees, directors and consultants to be recognized in the financial statements, based on their fair value. The Company calculates the fair value of stock options on the date of grant using the Black-Scholes pricing model, which requires a number of estimates, including the expected life of awards, interest rates, stock volatility and other assumptions.

FAIR VALUE OF MEASUREMENTS AND

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES	FAI R VALU E O F MEASUREMENTS AND INVESTMENT SECURITIES Investment Securities The following tables present the Company’s investment securities (including 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, 2020 and 2019: Fair Value at December 31, 2020 Fair Value at December 31, 2019 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Cash Equivalents: Money market funds and treasury bills $ 27,463 $ — $ — $ 77,170 $ — $ — Total Cash Equivalents $ 27,463 $ — $ — $ 77,170 $ — $ — As of December 31, 2020, the $1.5 million of restricted cash, on the Company's balance sheet is held in a money market fund. Money market funds, 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. Warrant Derivative Liability and Private Placement Option Liability The Company's financial liabilities recorded at fair value on a recurring basis include the fair values of the warrant derivative liability and the private placement option liability. As of December 31, 2020, the fair values of the warrant derivative liability and the private placement option liability are classified as current liabilities in the accompanying consolidated balance sheets. These liabilities will be shown as current liabilities on the balance sheet when it is deemed more probable than not by management to be exercised within one year. Inputs used to determine estimated fair value (Level 3) of the warrants include the fair value of the underlying stock relative to the warrant exercise price at the valuation measurement date, volatility of the price of the underlying stock, the expected term of the warrants, and risk-free interest rates. The fair value of the warrants has been estimated with the following weighted-average assumptions, including the most sensitive input, volatility: December 31, 2020 December 31, 2019 Risk-free interest rate 0.46 % 1.83 % Volatility 90 % 78.67 % Expected life (years) 5.64 6.64 The fair value of the private placement option has been estimated with the following weighted-average assumptions, including the most sensitive input, volatility: December 31, 2020 Risk-free interest rate 0.77% Volatility of underlying stock price 90.00% Expected term (years) 7.00 The following table provides the warrant derivative and private placement option liabilities reported at fair value and measured on a recurring basis: Fair Value at December 31, 2020 Fair Value at December 31, 2019 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Warrant derivative liability $ — $ — $ 10,345 $ — $ — $ 52,184 Private placement option liability — — 7,803 — — 12,094 Total fair value $ — $ — $ 18,148 $ — $ — $ 64,278

LEASES

LEASES	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
LEASES	LEASES The Company determines whether an arrangement is a lease at its inception. Operating leases relate primarily to office space and manufacturing facilities with remaining lease terms of two The Company entered into a lease agreement for office and laboratory space in Houston, Texas commencing in July 2020 and expiring in 2023. The Company recorded ROU assets of $0.5 million and a corresponding lease liability of $0.6 million upon lease commencement. In October 2020, in connection with the Company's restructuring plan, Management elected to seek an exit to its leased office and laboratory space in Houston, Texas. As a result of this decision, the Company reclassified the assets and liabilities associated with the leased facility as held for sale. The reclassified assets included a right-of-use asset of $0.5 million, property and equipment of $2.3 million, and related lease liability of $0.7 million. Based on the cost to exit the lease and the net realizable value of the related assets the Company recognized an impairment charge of $1.3 million in 2020. As most of the Company's leases do not provide an implicit rate, the Company's incremental borrowing rate is based on the information available at lease commencement date and was used to determine the present value of lease payments. Components of lease cost are as follows: (in thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Finance lease cost: Amortization of leased assets $ 74 $ 61 Interest on lease liabilities 21 25 Operating lease cost 814 2,135 Short-term lease cost 582 296 Total lease cost $ 1,491 $ 2,517 Weighted-average remaining lease term: Operating leases 1.7 years 5.2 years Finance leases 1.4 years 2.4 years Weighted-average discount rate: Operating leases 11.47 % 12.10 % Finance leases 13.05 % 13.40 % Supplemental cash flow information and non-cash activity related to the Company's operating and finance leases are as follows: (in thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 950 $ 2,378 Operating cash flows from finance leases 21 25 Financing cash flows from finance leases 74 47 Non-cash activity: Right-of-use assets obtained in exchange for lease obligations $ — $ 2,263 Maturities of lease liabilities by year for leases are as follows: (in thousands) Operating Leases Financing Leases 2021 $ 499 $ 90 2022 306 39 2023 14 — Total lease payments 819 129 Less: Imputed interest (79) (12) Present value of lease liabilities $ 740 $ 117
LEASES	LEASES The Company determines whether an arrangement is a lease at its inception. Operating leases relate primarily to office space and manufacturing facilities with remaining lease terms of two The Company entered into a lease agreement for office and laboratory space in Houston, Texas commencing in July 2020 and expiring in 2023. The Company recorded ROU assets of $0.5 million and a corresponding lease liability of $0.6 million upon lease commencement. In October 2020, in connection with the Company's restructuring plan, Management elected to seek an exit to its leased office and laboratory space in Houston, Texas. As a result of this decision, the Company reclassified the assets and liabilities associated with the leased facility as held for sale. The reclassified assets included a right-of-use asset of $0.5 million, property and equipment of $2.3 million, and related lease liability of $0.7 million. Based on the cost to exit the lease and the net realizable value of the related assets the Company recognized an impairment charge of $1.3 million in 2020. As most of the Company's leases do not provide an implicit rate, the Company's incremental borrowing rate is based on the information available at lease commencement date and was used to determine the present value of lease payments. Components of lease cost are as follows: (in thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Finance lease cost: Amortization of leased assets $ 74 $ 61 Interest on lease liabilities 21 25 Operating lease cost 814 2,135 Short-term lease cost 582 296 Total lease cost $ 1,491 $ 2,517 Weighted-average remaining lease term: Operating leases 1.7 years 5.2 years Finance leases 1.4 years 2.4 years Weighted-average discount rate: Operating leases 11.47 % 12.10 % Finance leases 13.05 % 13.40 % Supplemental cash flow information and non-cash activity related to the Company's operating and finance leases are as follows: (in thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 950 $ 2,378 Operating cash flows from finance leases 21 25 Financing cash flows from finance leases 74 47 Non-cash activity: Right-of-use assets obtained in exchange for lease obligations $ — $ 2,263 Maturities of lease liabilities by year for leases are as follows: (in thousands) Operating Leases Financing Leases 2021 $ 499 $ 90 2022 306 39 2023 14 — Total lease payments 819 129 Less: Imputed interest (79) (12) Present value of lease liabilities $ 740 $ 117

DEBT

DEBT	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
DEBT	DEBT 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 the lenders listed on Schedule 1.1 thereto or otherwise party thereto from time to time (the "Lenders"), pursuant to which the Company borrowed $35.0 million in a single term loan (the “Oxford Loan”) on the Oxford Closing Date. On the Oxford Closing Date, the Company used approximately $32.9 million of the proceeds from the Oxford Loan to repay its indebtedness to a previous lender. On December 24, 2019, the Company entered into a First Amendment to Loan and Security Agreement (the “First Amendment”) with Oxford, in connection with the Asset Sale with M.D. Anderson. On March 31, 2020, the Company entered into a Second Amendment to Loan and Security Agreement (the "Second Amendment") with Oxford Finance LLC, in connection with the Asset Sale. The loan and security agreement with Oxford, as amended by the First and Second Amendment, is referred to as the "Oxford Loan Agreement." The Company’s obligations under the Oxford Loan Agreement were secured by a first priority security interest in substantially all of the Company’s current and future assets, including its intellectual property. The Company also agreed not to encumber its intellectual property assets, except as permitted by the Oxford Loan Agreement. The Oxford Loan originally matured on December 1, 2022 (the “Oxford Maturity Date”) and was originally interest-only through January 31, 2020, followed by 35 equal monthly payments of principal and unpaid accrued interest. The Oxford Loan bore 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%. Pursuant to the Second Amendment, the Loan Agreement was amended to, among other things: (i) provide for Oxford's and the Lenders’ consent to the Company’s consummation of the Asset Sale with M.D. Anderson, provided such sale occurs on or prior to June 30, 2020; (ii) if such Asset Sale occurs on or prior to June 30, 2020, extend the interest-only period through as late as July 31, 2021; (iii) if Asset Sale closes on or prior to June 30, 2020, provide for a partial repayment to the Lenders of a significant percentage of the proceeds of the asset sale that varies in accordance with the timing of closing and the associated amortization schedule, a portion of which will be applied as partial payment of the Final Payment Percentage (as defined in the Loan Agreement); and (iv) grant the Lenders and Oxford a security interest in the Company’s intellectual property as of the date of the Second Amendment, in each case as set forth in the Second Amendment. The sale of certain assets subject to the Second Amendment closed on April 14, 2020. Pursuant to the Second Amendment, the closing of the Asset Sale to M.D. Anderson triggered the Company’s obligation to provide partial repayment to the Lenders of $7.0 million, of which $0.6 million was applied as partial payment of the Oxford Final Payment Fee. The interest-only period was extended through December 31, 2020. 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, were recorded as deferred issuance costs, which offset long-term debt on the Company's consolidated balance sheet. The deferred issuance costs were being amortized over the term of the loan as interest expense using the effective interest method. During the years ended December 31, 2020 and 2019, interest expense of amortized deferred issuance costs included $0.6 million and $0.9 million, respectively. During November 2020, the Company entered into an agreement for the early settlement of all debt obligations under the Oxford Loan Agreement. During the same month, the Company remitted payment of $27.4 million, which included full repayment of the outstanding principal balance, the Oxford Final Payment Fee, and accrued interest. The Company recorded a gain on extinguishment of $0.1 million in the accompanying statements of operations and comprehensive loss.

PUBLIC OFFERING AND PRIVATE PLA

PUBLIC OFFERING AND PRIVATE PLACEMENT	12 Months Ended
	Dec. 31, 2020
Statement of Stockholders' Equity [Abstract]
PUBLIC OFFERING AND PRIVATE PLACEMENT	PUBLIC OFFERING AND PRIVATE PLACEMENT November 2020 Underwritten Offering In November 2020, the Company closed an underwritten offering of 1,040,000 shares of its common stock, pre-funded warrants to purchase 3,109,378 shares of its common stock, and accompanying common warrants to purchase up to an aggregate of 4,149,378 shares of its common stock. Each share of common stock and pre-funded warrant to purchase one share of common stock was sold together with a common warrant to purchase one share of common stock. The public offering price of each share of common stock and accompanying common warrant was $6.025 and $6.024 for each pre-funded warrant. The pre-funded warrants were immediately exercisable at a price of $0.001 per share of common stock. The common warrants were immediately exercisable at an exercise price of $6.50 per share of common stock and will expire five years from the date of issuance. The shares of common stock and pre-funded warrants, and the accompanying common warrants, were issued separately and were immediately separable upon issuance. The gross proceeds to the Company were approximately $25.0 million before deducting underwriting discounts and commissions and other offering expenses. August 2019 Public Offering On August 16, 2019, the Company entered into an underwriting agreement (the “Underwriting Agreement”) with Jefferies LLC and Wells Fargo Securities, LLC, as representatives of the several underwriters named therein (the “Underwriters”), relating to an underwritten public offering (the “Offering”) of 575,000 shares of the Series 1 Redeemable Convertible Non-Voting Preferred Stock of the Company (the “Series 1 Preferred Stock”) and warrants (the “Public Warrants”) to purchase up to 5,750,000 shares of its common stock. Each share of Series 1 Preferred Stock was being sold together with a warrant to purchase 10 shares of common stock at a combined price to the public of $100.00. Under certain circumstances, each warrant to purchase 10 shares of common stock will be exercisable, at the irrevocable election of the holder, for one share of Series 1 Preferred Stock. The offering closed on August 21, 2019, and the net proceeds to the Company from the Offering was approximately $53.8 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company, and excluding any proceeds that the Company may receive upon exercise of the Public Warrants. All of the Public Warrants sold in the Offering have an exercise price of $13.00 per share of common stock or, in certain circumstances, for $130.00 per share of Series 1 Preferred Stock, subject to proportional adjustments in the event of stock splits or combinations or similar events. The Public Warrants were immediately exercisable upon issuance, provided that the holder is prohibited, subject to certain exceptions, from exercising a warrant for shares of common stock to the extent that immediately prior to or after giving effect to such exercise, the holder, together with its affiliates and other attribution parties, would own more than 9.99% of the total number of shares of common stock then issued and outstanding, which percentage may be changed at the holder’s election to a lower percentage at any time or to a higher percentage not to exceed 19.99% upon 61 days’ notice to the Company. The Public Warrants will expire on August 21, 2026, unless exercised prior to that date. Private Placement On August 16, 2019, the Company entered into a securities purchase agreement (the “Securities Purchase Agreement”) with certain institutional investors named therein (the “Purchasers”), pursuant to which the Company agreed to issue in a private placement (i) 350,000 shares of its Series 2 Redeemable Convertible Non-Voting Preferred Stock (the “Series 2 Preferred Stock”), at a purchase price of $100.00 per share, and related warrants (the “Private Warrants”) to purchase up to 2,800,000 shares of common stock at an exercise price of $10.00 per share, and (ii) 250,000 shares of its Series 3 Redeemable Convertible Non-Voting Preferred Stock (the “Series 3 Preferred Stock” and, together with the Series 1 Preferred Stock and Series 2 Preferred Stock, the “Preferred Stock”), at a purchase price of $140.00 per share, and related warrants (also, “Private Warrants”) to purchase up to 875,000 shares of common stock at an exercise price of $14.00 per share. The purchase and sale of the securities issuable under the private placement agreement may occur in two or more separate closings, each to be conducted at the Purchasers’ discretion within five days’ notice to the Company. The purchase and sale was subject to the Company’s obtaining stockholder approval for additional authorized shares of Common Stock or a reverse stock split (the “Required Stockholder Approval”), which occurred in the first quarter of 2020. The right of the Purchasers to purchase such securities will expire two and a half years after the Required Stockholder Approval, on June 15, 2022, with respect to the Series 2 Preferred Stock, and three years after such stockholder approval, on January 15, 2023, with respect to the Series 3 Preferred Stock, if not exercised prior to that date. The Company received $11.2 million in net option fee proceeds, net of offering costs, upon the execution of the Securities Purchase Agreement. Total offering costs incurred by the Company related to the Public Warrants, Private Warrants and options amounted to $3.0 million. The following table reflects the fair value roll forward reconciliation of the warrant derivative and private placement option liabilities for the period ended December 31, 2020: (in thousands) Warrant Derivative Liability Private Placement Option Liability Total Balance, December 31, 2019 $ 52,184 $ 12,094 $ 64,278 Change in fair value (41,839) (4,291) (46,130) Balance, December 31, 2020 $ 10,345 $ 7,803 $ 18,148

REDEEMABLE CONVERTIBLE PREFERRE

REDEEMABLE CONVERTIBLE PREFERRED STOCK	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
REDEEMABLE CONVERTIBLE PREFERRED STOCK	REDEEMABLE CONVERTIBLE PREFERRED STOCK In August 2019, the Company sold Series 1 Preferred Stock pursuant to the Offering. The Company has 10,000,000 authorized shares of preferred stock with a par value of $0.01, of which the Company has designated 1,517,500 shares as Series 1 redeemable convertible non-voting preferred stock, 350,000 shares as Series 2 redeemable convertible non-voting preferred stock and 250,000 shares as Series 3 redeemable convertible non-voting preferred stock. There were 452,000 shares and 538,000 shares of Series 1 Preferred Stock issued and outstanding as of December 31, 2020 and 2019, respectively. There were no shares of Series 2 or 3 Preferred Stock issued and outstanding as of December 31, 2020 and 2019. The Series 1 Preferred Stock was issued together with warrants for a combined purchase price of $100.00 per share of Series 1 Preferred Stock and one warrant to purchase 10 shares of common stock. During the year ended December 31, 2020 and 2019, 86,000 shares and 37,000 shares, respectively of Series 1 Preferred Stock were converted to common stock. As of December 31, 2020 and 2019, the Company classified the Series 1 preferred stock within mezzanine equity, as the Series 1 preferred stock is redeemable at the option of the holders upon passage of time, which is outside of the Company’s control to prevent. The Series 1 Preferred Stock is not currently redeemable and is only redeemable upon a fundamental change at a redemption price. The Company does not believe a fundamental change is considered probable until it occurs. Subsequent adjustment of the amount presented within mezzanine equity to its redemption amount is unnecessary if it is not probable that the instrument will become redeemable. As (i) the Series 1 Preferred Stock is only redeemable upon a fundamental change, the occurrence of which is not probable, and (ii) the occurrence of Transition Date (defined below) is probable, the Company did not accrete the Series 1 Preferred Stock to its redemption amount. Optional Conversion Each share of Preferred Stock is initially convertible into 10 shares of Common Stock. The conversion price at which Preferred Stock may be converted into shares of common stock, is subject to adjustment in connection with certain specified events. Redemption Until the applicable Transition Date (defined below), at any time on or after the date that is the fifth (5th) anniversary of the initial issue date of the applicable series of preferred stock, all or any portion of the preferred stock is redeemable at the option of the holder at a redemption price of $100.00 per share (for Series 1 and Series 2 Preferred Stock) and $140.00 per share (for Series 3 Preferred Stock). The “Transition Date” means: • With respect to the Series 1 Preferred Stock, the first date following August 21, 2021, on which each of the Conditions (as defined below) is met (the “Series 1 Transition Date”); • With respect to the Series 2 Preferred Stock, the first date following the six-month anniversary of the Series 1 Transition Date on which each of the Conditions is met (the “Series 2 Transition Date”); and • With respect to the Series 3 Preferred Stock, the first date following the six-month anniversary of the Series 2 Transition Date on which each of the Conditions is met. The “Conditions” mean: (1) the closing price of the Company’s common stock has been equal to or exceeded $25.00 per share for 180 calendar days (for determining if the Conditions are met for the Series 1 Preferred Stock and Series 2 Preferred Stock) and $35.00 per share (for the Series 3 Preferred Stock) for 180 calendar days; (2) the 50-day average trading volume of the Company’s common stock on the Nasdaq stock market is greater than 50,000 shares; and (3) a Phase 3 or Phase 2 pivotal clinical trial for one of the Company’s CAR-T product candidates has been initiated, meaning that at least one clinical trial site has been activated. Dividends Shares of Preferred Stock will be entitled to receive dividends equal to (on an as-if-converted-to-common stock basis), and in the same form and manner as, dividends actually paid on shares of common stock. Liquidation Until the applicable Transition Date, in the event of a liquidation, dissolution, winding up or deemed liquidation, holders of the Preferred Stock will receive a payment equal to the applicable per share purchase price of their Preferred Stock before any proceeds are distributed to the holders of Common Stock. The liquidation preferences, protective voting provisions and redemption rights of the Preferred Stock will terminate upon the occurrence of certain events. Voting Shares of Preferred Stock will generally have no voting rights, except to the extent expressly provided in the Company’s certificate of incorporation or as otherwise required by law.

STOCKHOLDERS' EQUITY AND SHARE-

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS	STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS Stockholder's Equity On October 5, 2018, the Company entered into an Open Market Sale Agreement (the “Sale Agreement”) with Jefferies LLC ("Jefferies"), 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 prospective supplement to its shelf registration statement on Form S-3 (the “Prospective Supplement”). During the year ended December 31, 2019, the Company received $9.0 million in net proceeds from the sale of 259,115 shares of its common stock in the open market. On August 16, 2019, in connection with the Public Offering, the Company delivered written notice to Jefferies that the Company was suspending and terminating the Prospectus Supplement related to the shares of its common stock issuable pursuant to the Sale Agreement. The Company will not make any sales of its securities pursuant to the Sales Agreement, unless and until a new prospectus supplement is filed. Other than the termination of the Prospectus Supplement, the Sale Agreement remains in full force and effect. Share-Based Compensation Plans The Company has five share-based compensation plans, including the 2019 Equity Incentive Plan the ("2019 Plan") which was adopted in June 2019. Each plan authorizes 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. The only plan under which the Company may currently grant equity awards is the 2019 Equity Incentive Plan although there remain outstanding awards under the other four plans. 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. 2019 Equity Incentive Plan The 2019 Plan, is designed to secure and retain the services of the Company’s employees and directors. The 2019 Plan is successor to and continuation of the 2014 Equity Incentive Plan, as amended, the ("2014 Plan"), and no additional awards may be issued from the 2014 Plan. Subject to adjustment for certain changes in the Company’s capitalization, the aggregate number of shares of common stock that may be issued under the 2019 Plan (the "Share Reserve") will not exceed the sum of (i) 250,000 new shares, plus (ii) an additional 600,000 shares that were approved at the Company’s Special Meeting of Stockholders in January 2020, and plus (iii) an additional 500,000 shares that were approved at the Company's Annual meeting of Stockholders in June 2020, and plus (iv) the Prior Plans’ Returning Shares, as defined in the 2019 Plan documents, in an amount not to exceed 600,540 shares, including any stock award granted under the 2014 Plan, 2011 Stock Option Plan, as amended, or 2006 Stock Option Plan, as amended, that were outstanding as of the date the 2019 Plan was approved by the Company's stockholders, as such shares become available from time to time. The following shares of common stock, or the 2019 Plan Returning Shares, will also become available again for issuance under the 2019 Plan: (i) any shares subject to a stock award granted under the 2019 Plan that are not issued because such stock award expires or otherwise terminates without all of the shares covered by such stock award having been issued; (ii) any shares subject to a stock award granted under the 2019 Plan that are not issued because such stock award is settled in cash; and (iii) any shares issued pursuant to a stock award granted under the 2019 Plan that are forfeited back to or repurchased by the Company because of a failure to vest. The 2019 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock awards, and other stock awards. At December 31, 2020 and 2019, outstanding awards were comprised of the following: December 31, 2020 December 31, 2019 Options 1,425,207 471,282 Inducement option awards 85,761 96,560 Restricted stock units 129,361 5,609 Inducement restricted stock units outstanding 500 750 Total outstanding awards 1,640,829 574,201 Grant Date Fair Value The valuation of the share-based compensation awards is a significant accounting estimate that requires the use of judgments 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 historical volatility of the Company’s common stock. 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 value of the option grants has been estimated, with the following weighted-average assumptions: Year Ended December 31, 2020 December 31, 2019 Options granted 1,352,595 276,830 Weighted-average exercise price 7.59 26.12 Weighted-average grant date fair value 5.29 16.99 Assumptions: Risk-free interest rate 0.82 % 2.23 % Volatility 85 % 72 % Expected life (years) 5.83 6.04 Expected dividend yield — % — % Share-Based Compensation Activity The following table summarizes the stock option activity for all stock plans during the year ended December 31, 2020 and 2019 as follows: Options Outstanding Stock Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2018 575,924 $ 109.01 8.09 $ 87 Granted 276,830 $ 26.12 Exercised (220) $ 25.50 Forfeited (284,692) $ 93.81 Balance at December 31, 2019 567,842 $ 76.25 7.82 $ 12 Granted 1,352,595 $ 7.59 Exercised — Forfeited (409,469) $ 29.86 Balance at December 31, 2020 1,510,968 $ 27.36 8.19 $ 379 Exercisable at December 31, 2020 339,972 $ 91.97 5.61 $ — For the years ended December 31, 2020 and 2019, the Company received cash of less than $0.1 million and $0.1 million, respectively, upon option exercises. The following table summarizes the options outstanding and exercisable at December 31, 2020: 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 $2.97 to $5.05 676,000 9.95 $ 2.97 — 0.00 $ — $5.06 to $12.90 125,301 9.40 $ 8.41 1,549 8.83 $ 9.62 $12.91 to $15.85 307,904 6.75 $ 13.60 — 0.00 $ — $15.86 to $81.25 217,682 6.61 $ 42.24 163,911 6.12 $ 44.40 $81.26 to $234.70 184,081 5.19 $ 135.21 174,512 5.01 $ 137.38 Total 1,510,968 8.19 $ 27.36 339,972 5.61 $ 91.97 The following table summarizes the stock award activity for all restricted stock units during the year ended December 31, 2020: Awards Outstanding Restricted Stock Awards and Units Weighted-Average Grant Date Fair Value Per Share Outstanding Aggregate Intrinsic Value (in thousands) Total Fair Value of Restricted Awards Vested (in thousands) Balance at December 31, 2018 24,615 $ 80.23 $ 719 Granted 3,000 $ 33.30 Vested (14,478) $ 64.16 $ 240 $ 929 Forfeited (6,778) $ 82.44 Balance at December 31, 2019 6,359 $ 92.29 $ 82 Granted 239,023 $ 4.70 Vested (2,309) $ 96.14 $ 30 $ 222 Forfeited (113,212) $ 6.73 Balance at December 31, 2020 129,861 $ 5.59 $ 458 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 55,000 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 lesser of the grant date or purchase date fair market value. A summary of activity within the ESPP follows: Year Ended (in thousands except share data) December 31, 2020 December 31, 2019 Deductions from employees $ 56 $ 70 Share-based compensation expense recognized $ 96 $ 95 Remaining share-based compensation expense $ — $ 206 Proceeds received by the Company for ESPP $ 78 $ 98 Weighted-average purchase price per common share $ 5.21 $ 12.25 Number of shares purchased by employees under ESPP 14,975 8,000 As of December 31, 2020, there were 18,488 shares available for issuance under the ESPP. Share-Based Compensation Expense Share-based compensation expense by classification for December 31, 2020 and 2019 are as follows: Year Ended (in thousands) December 31, 2020 December 31, 2019 General and administrative $ 3,170 $ 4,017 Research and development 1,861 3,321 Total $ 5,031 $ 7,338 At December 31, 2020, total compensation cost not yet recognized was $5.3 million and the weighted-average period over which this amount is expected to be recognized is 2.03 years. The aggregate fair value of options and restricted shares vesting in the years ended December 31, 2020 and 2019 was $4.8 million and $8.9 million, respectively.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES 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. 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 $100,000 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. 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 $3.0 million, which was included in license fee expense. The Company is also required to make aggregate milestone payments to Agensys of up to (i) $5 million upon the first achievement of certain specified clinical milestones for its licensed products, (ii) $50 million upon the achievement of certain specified clinical milestones for each licensed product, and (iii) $75 million 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 million. 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 million 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. 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 ten Litigation Securities Litigation On August 11, 2020, the following derivative complaints were voluntarily dismissed by the plaintiffs following the May 2020 dismissal of the securities class action complaint captioned Nipun Kakkar v. Bellicum Pharmaceuticals, Inc., Rick Fair and Alan Musso, and are no longer pending: (i) a purported shareholder derivative complaint captioned Seung Paik v. Richard A. Fair, et al. against the Company’s directors and certain of the Company’s officers filed on July 19, 2018 in the U.S. District Court for the Southern District of Texas, Houston Division; and (ii) another purported shareholder derivative complaint captioned Scott Ludovissy and Ann Gordon Trammell v. Richard A. Fair, et al. filed July 8, 2019 against the same defendants in the same court. On September 18, 2020, an additional purported derivative complaint captioned Mildred Taylor and Jessica Armor v. Richard A. Fair, et al. filed November 1, 2019 in the District of Delaware was voluntarily dismissed by the plaintiffs.

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The reconciliation between federal income taxes at the statutory U.S. federal income tax rate and the Company’s income tax expense for the year is as follows: (in thousands) December 31, 2020 December 31, 2019 Tax benefit at statutory rate $ (1,658) $ (23,591) Other 200 (294) Stock based compensation 642 2,674 Offering issuance costs and changes in fair value of warrants and private placement option (9,687) 4,657 Deferred tax valuation allowances 11,690 19,542 Research and development credit (1,187) (2,988) 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, 2020 and 2019 are as follows: (in thousands) December 31, 2020 December 31, 2019 Deferred tax assets (liabilities): Net operating loss carryforward $ 94,132 $ 81,960 Stock compensation 3,539 3,270 Intangible assets 7,682 8,077 Research and development credit 17,787 16,601 Operating lease assets (213) (1,229) Operating lease liabilities 354 1,538 Other 976 2,336 Total deferred tax assets, net of deferred tax liabilities 124,257 112,553 Valuation allowance (124,257) (112,553) Net deferred tax $ — $ — Net operating loss carryforwards and research tax credits as of December 31, 2020 and 2019 are as follows: (in thousands) December 31, 2020 December 31, 2019 U.S. federal income tax net operating loss carryforwards $ 448,250 $ 390,286 U.K. net operating loss carryforwards $ 133 $ — U.S. federal research tax credits $ 12,535 $ 11,348 Texas research tax credits $ 5,252 $ 5,252 The Company has $227.0 million of U.S. federal net operating loss carryovers that have no expiration date and the remaining begin to expire in 2025. The U.S. Federal and state research credits will begin to expire in 2028 and 2034 respectively. No study has been performed on the research and development (R&D) credits and gross R&D credits in the amount of $17.8 million could be limited based on review by the Internal Revenue Service. 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 net deferred tax assets at December 31, 2020 and 2019. The changes in the valuation allowance was an increase of $11.7 million and an increase of $19.5 million for the years ended December 31, 2020 and 2019, respectively.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS Houston R&D Facility On February 2, 2021 the Company executed a term sheet to exit the lease of the R&D facility on Reed Road in Houston, Texas. Under the terms of the agreement a third party will assume the lease for the facility. In addition, the third party will provide $1.1 million of consideration to the Company for substantially all of the property, plant and equipment associated with the location. The consideration will include $0.9 million of cash and an unsecured promissory note for $0.2 million. On February 26, 2021 the Company completed the transaction as contemplated in the aforementioned term sheet. The Company fully reserved for the transaction through the impairment recognized in 2020. South San Francisco Facility On March 15, 2021 the Company entered an agreement to terminate its sub-lease of the South San Francisco office space contingent upon consent of the prime lessor. The decision to exit this lease reflects the ability of the Company to carry on administrative function of the Company remotely as proven through necessity during the pandemic. Under the terms of the agreement, the company will pay a lease termination fee of $0.9 million. As of December 31, 2020, the lease termination fee has been fully accrued as lease expense. The Company does not anticipate additional material expenses associated with exiting the lease. Compliance with NASDAQ Listing Rules On November 23, 2020, the Company received notice (the "Notice") from The Nasdaq Stock Market LLC ("Nasdaq) advising the Company that for the last 30 consecutive business days preceding the date of the Notice, the Company's Market Value of Listed Securities ("MVLS") had been below the minimum of $35,000,000 required for continued listing on The Nasdaq Capital Market pursuant to Nasdaq Listing Rule 5550(b)(2) (the "MVLS Requirement"). On February 16, 2021, the Company received notification from Nasdaq that the Company had regained compliance with the MVLS Requirement.

ORGANIZATION, BASIS OF PRESEN_2

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	Basis of PresentationThe accompanying financial statements have been prepared in conformity with the authoritative U.S. generally accepted accounting principles (“GAAP”).
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, revenue recognition, and expenses. Actual results could differ materially from those estimates.
Revenue Recognition	Revenue Recognition The Company’s only source of revenue in 2020 was from its licensing agreement with The University of Texas MD Anderson Cancer Center, (“MD Anderson”). In 2019, the Company earned revenue from its licensing agreement with The University of Texas MD Anderson Cancer Center, (“MD Anderson”) and from grants. Prior to 2019, the Company's only source of revenue was from grants. Grant Revenue When grant funds are received after costs have been incurred, the Company records revenue and a corresponding 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. License Revenue The promised services in license agreements consist of license rights to the Company’s intellectual property. When management believes the license to its intellectual property and products has stand-alone value, the Company recognizes revenue attributed to the license upon delivery. The Company recognizes revenue when control of the promised goods or services is transferred to its customers, in an amount that reflects the consideration to which the Company expects to be entitled in exchange for the goods or services. In order to achieve that core principle, a five-step approach is applied: (1) identify the contract with a customer, (2) identify the performance obligations in the contract, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations in the contract, and (5) recognize revenue allocated to each performance obligation when the Company satisfies the performance obligation. A performance obligation is a promise in a contract to transfer a distinct good or service to the customer, and is the unit of account for revenue recognition. The Company may provide options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. License agreements generally include certain milestone payments. The Company utilizes the “most likely amount” method to estimate the amount of variable consideration, to predict the amount of consideration to which it will be entitled for its license agreement. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. Milestone, annual maintenance, and royalty payments that are not within the control of the Company or the licensee, such as those dependent upon receipt of regulatory approval, are not considered to be probable of achievement until the triggering event occurs. At the end of each reporting period, the Company reevaluates the probability of achievement of each and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue and net loss in the period of adjustment. To date, the Company has not recognized any development, regulatory or commercial milestones or royalty revenue resulting from its license agreement. Consideration that would be received for optional goods and/or services is excluded from the transaction price at contract inception. License Agreement On January 22, 2019, the Company entered into a licensing and commercialization agreement with MD Anderson (the "MD Anderson License Agreement"). Under the MD Anderson License Agreement, the Company granted MD Anderson non-exclusive rights in certain Caspase-9 and related technologies and use of a small molecule known as rimiducid. During the fourth quarter of 2019, and under the terms of the MD Anderson License Agreement, MD Anderson exercised an option to grant a non-exclusive sublicense of the rights licensed by the Company to MD Anderson under the MD Anderson License Agreement. MD Anderson, as a result of this exercise, granted a sublicense that entitled the Company to receive as consideration an upfront payment of $5.0 million in license fees as well as additional future annual maintenance fees, milestone payments related to the achievement of pre-specified development, regulatory, and commercialization events, and royalties of two percent on net sales of licensed products. During the fourth quarter of 2019, the Company recognized $5.0 million of license fee revenue as delivery of the license occurred and the license to its Caspase-9 intellectual property has stand-alone value. During 2020, the Company has received $0.5 million of maintenance fees under the MD Anderson License Agreement. Cancer Research Grant Contract On August 9, 2017, the Company entered into a Cancer Research Grant Contract (the “CPRIT Agreement”) with the Cancer Prevention Research Institute of Texas (“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 CPRIT Agreement and subject to CPRIT’s ability to perform its obligations under the CPRIT Agreement. During 2017, the Company received $4.2 million in advance funding from CPRIT, which was recorded as deferred revenue. During the years ended December 31, 2020 and 2019, the Company incurred expenses and recognized revenue of $0.0 million and $2.1 million, respectively, for work performed under the CPRIT grant. The CPRIT Agreement was due to expire on February 29, 2020, but was terminated early by the Company on January 31, 2020. Upon termination of the CPRIT Agreement, the Company reclassified the remaining unexpended award proceeds of $0.8 million from deferred revenue to accrued liabilities. During the year ended December 31, 2020, the Company returned the remaining unexpended award proceeds to CPRIT and released the accrued liability.
Cash, Cash Equivalents, and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all short-term, highly liquid investments with maturity of three months or less from the date of purchase and that can be liquidated without prior notice or penalty, 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. 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 based on quoted prices in active markets, with unrealized gains and, to the extent deemed temporary, unrealized losses reported as accumulated other comprehensive gain (loss), a separate component of stockholders' (deficit) 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 loss.
Property and Equipment	Property and Equipment 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 3 to 5 years. Leasehold improvements are amortized over the shorter of the estimated useful life or the remaining lease term.
Intangible Assets	Intangible Assets Non-refundable upfront payments related to a supply agreement with future benefits have been capitalized as an intangible asset, presented in other assets on the accompanying consolidated balance sheets and amortized over the term of the agreement. The amortization of the intangible asset is included in operating expenses in the accompanying consolidated statements of operations and comprehensive loss.
Impairment of Long-Lived Assets	Impairment of Long-Lived AssetsThe 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.
Debt Issuance Costs	Debt Issuance Costs Costs related to debt issuance are presented in the accompanying consolidated balance sheets 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 in the accompanying statements of operations and comprehensive loss.
Warrant Derivatives	Warrant Derivatives Freestanding warrants exercisable for multiple underlying instruments are classified as liabilities in the accompanying consolidated balance sheets. The Company accounts for these warrants at fair value on the date of issuance and are subject to re-measurement to fair value at each balance sheet date. Any change in fair value is recognized as a component of other income (expense) on the accompanying consolidated statements of operations and comprehensive loss. The Company estimates the fair value of these liabilities using the binomial option model, adjusted for the effect of dilution, because it embodies all of the requisite assumptions (including trading volatility, estimated terms, dilution and risk-free rates) necessary to determine the fair value of this instrument. The Company will continue to adjust the liability for changes in fair value until the earlier of the exercise or expiration of the warrants or a change in control, as defined. The warrants are freely exercisable at any time from the issuance date until the expiration date, provided exercise does not cause a warrant holder to exceed a pre-determined beneficial ownership limit. The Company estimates the fair value of these liabilities using the Black-Scholes valuation technique, which utilizes assumptions including (i) the fair value of the underlying stock at the valuation measurement date, (ii) volatility of the price of the underlying stock, (iii) the expected term, and (iv) risk-free interest rates.
Private Placement Option	Private Placement Option The Company has entered into a security purchase agreement that contains a call option on preferred shares that are puttable outside the control of the Company. The Company recorded the option as a liability and measured the fair value of the option at the time of issuance. The Company will re-measure the option to fair value at each balance sheet date and record changes in fair value in other income (expense) in the accompanying consolidated statement of operations and comprehensive loss at each reporting period. Offering expenses arising from the issuance of the private placement option were expensed as incurred. The Company estimates the fair value of these liabilities using a binomial lattice model, which utilizes assumptions including (i) the fair value of the underlying stock at the valuation measurement date, (ii) volatility of the price of the underlying stock, (iii) the expected term, and (iv) risk-free interest rates.
Preferred stock	Preferred Stock Preferred shares issued by the Company that are subject to mandatory redemption are classified as liability instruments in the accompanying consolidated balance sheets and are measured at fair value at the date of issuance. Conditionally redeemable preferred shares (including preferred shares that feature redemption rights that are either within the control of the holder or subject to redemption upon the occurrence of uncertain events not solely within the Company’s control) are classified within mezzanine equity in the accompanying consolidated balance sheets. At all other times, preferred shares are classified within stockholders’ (deficit) equity.
Operating Leases	Operating Leases At the inception of a contractual arrangement, the Company determines whether the contract contains a lease by assessing whether there is an identified asset and whether the contract conveys the right to control the use of the identified asset in exchange for consideration over a period of time. If both criteria are met, upon lease commencement, the Company records a lease liability which represents the Company’s obligation to make lease payments arising from the lease, and a corresponding right-of-use (“ROU”) asset which represents the Company’s right to use an underlying asset during the lease term. Operating leases are recognized as right-of-use, or ROU, assets and operating lease liabilities on the balance sheet based on the present value of the future minimum lease payments over the lease term at commencement date calculated using the Company's incremental borrowing rate applicable to the underlying asset unless the implicit rate is readily determinable. Any lease incentives received are deferred and recorded as a reduction of the ROU asset and amortized over the term of the lease. Rent expense, comprised of amortization of the ROU asset and the implicit interest accreted on the operating lease liability, is recognized on a straight-line basis over the lease term. The Company determines the lease term as the noncancellable period of the lease and may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise such options. Leases with a term of 12 months or less are not recognized on the balance sheets.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined as the exchange price that would be received for an asset or an exit price paid to transfer a liability in an orderly transaction between market participants in a principal market on the measurement date. Accounting standards include disclosure requirements around fair values used for certain financial instruments and establish a fair value hierarchy. The three-tier hierarchy defines a three-tiered valuation hierarchy for disclosures that 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 2. 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 categorization of a financial instrument within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The Company believes the recorded values of its financial instruments, including cash and cash equivalents, accounts payable, accrued liabilities, and debt 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 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 and Security Investor Protection Corporation. 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.
Licenses and Patents, Research and Development	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. 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.
General and administrative expenses	Legal costs related to patent applications and maintenance are classified as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.
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 share-based compensation based on the measurement and recognition of compensation expense for all share-based payment awards made to employees, directors and consultants to be recognized in the financial statements, based on their fair value.
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. As of December 31, 2020, the Company had recorded a full valuation allowance on its net U.S. and foreign deferred tax assets because the Company expects that it is more likely than not that its deferred tax assets will not be realized in the foreseeable future. Should the actual amounts differ from our estimates, the amount of the valuation allowance could be materially impacted. The Company accounts for uncertain tax positions in accordance with the provisions of the Accounting Standards Codification (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, 2020 and 2019, the Company had no uncertain tax positions and no interest or penalties have been charged for the years ended December 31, 2020 and 2019. 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 2020 remain open to examination by the U.S. Internal Revenue Service.
Comprehensive Loss	Comprehensive LossComprehensive 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 other comprehensive 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 loss to arrive at comprehensive loss.
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. Diluted earnings per share is based on the treasury stock method and includes the effect from potential issuance of ordinary shares, such as shares issuable pursuant to the conversion of preferred stock to common stock, exercise of warrants to purchase common stock, exercise of stock options, and vesting of restricted stock units.
New Accounting Requirements and Disclosures	New Accounting Requirements and Disclosures Fair Value Measurement In August 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 expect the adoption of this standard to have a material effect on its financial statements. Financial Instruments – Credit Losses In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments , which requires the measurement of all expected credit losses for financial assets including trade receivables held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. ASU No. 2016-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company adopted the standard effective January 1, 2020 with no material effect on its financial statements. Subsequent to the issuance of ASU 2016-13, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments - Credit Losses. This ASU does not change the core principle of the guidance in ASU 2016-13, instead these amendments are intended to clarify and improve operability of certain topics included within the credit losses guidance. The FASB also subsequently issued ASU No. 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Derivatives and Hedging (Topic 815), and Financial Instruments (Topic 842), which did not change the core principle of the guidance in ASU 2016-13 but clarified that expected recoveries of amounts previously written off and expected to be written off should be included in the valuation account and should not exceed amounts previously written off and expected to be written off. The guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019 for public business entities, excluding smaller reporting companies. Early adoption is permitted. As a smaller reporting company, the guidance will be effective for the Company during the first quarter of 2023. The Company is in the process of assessing the impact adoption will have on its consolidated financial statements. Income Taxes In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740). The guidance eliminates certain exceptions for recognizing deferred taxes for investments, performing intraperiod allocation and calculating income taxes in interim periods. This guidance also includes guidance to reduce complexity in certain areas, including recognizing deferred taxes for tax goodwill and allocating taxes to members of a consolidated group. ASU 2019-12 is effective for annual and interim periods in fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company is currently evaluating the impact this change will have on its consolidated financial statements. Investments

ORGANIZATION, BASIS OF PRESEN_3

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash, Cash Equivalents and Restricted Cash	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. (in thousands) December 31, 2020 December 31, 2019 Cash and cash equivalents $ 35,495 $ 91,028 Restricted cash 1,501 2,788 Total cash, cash equivalents and restricted cash shown in the statements of cash flows $ 36,996 $ 93,816
Cash, Cash Equivalents and Restricted Cash	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. (in thousands) December 31, 2020 December 31, 2019 Cash and cash equivalents $ 35,495 $ 91,028 Restricted cash 1,501 2,788 Total cash, cash equivalents and restricted cash shown in the statements of cash flows $ 36,996 $ 93,816
Property and Equipment	Property and equipment consisted of the following: (in thousands, except useful lives) Estimated Useful Lives December 31, 2020 December 31, 2019 Leasehold improvements 5 Years $ 167 $ 3,944 Lab equipment 5 Years 530 5,459 Office furniture 5 Years — 392 Manufacturing equipment 5 Years 138 395 Computer and office equipment 3 to 5 Years 834 1,595 Equipment held under capital leases 5 Years — 270 Software 3 Years 94 385 Total 1,763 12,440 Less: accumulated depreciation (1,574) (9,911) Property and equipment, net $ 189 $ 2,529
Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consist of the following: December 31, 2020 December 31, 2019 Accrued payroll $ 1,029 $ 2,032 Accrued patient treatment costs 899 1,162 Accrued manufacturing costs 24 2,230 Accrued professional services 294 654 Accrued obligations under material supply agreements — 1,121 Accrued other 1,919 2,571 Total accrued expenses and other current liabilities $ 4,165 $ 9,770
Earnings Per Share, Potentially Dilutive Securities	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. December 31, 2020 December 31, 2019 Common stock equivalents: Number of Shares Redeemable convertible series 1 preferred stock 4,520,000 5,380,000 Warrants to purchase common stock 11,616,080 5,750,000 Private placement option 9,675,000 — Options to purchase common stock 1,510,968 567,842 Unvested shares of restricted stock units 129,861 6,359 Total common stock equivalents 27,451,909 11,704,201

FAIR VALUE OF MEASUREMENTS AN_2

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Assets Measured at Fair Value on a Recurring Basis	The following tables present the Company’s investment securities (including 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, 2020 and 2019: Fair Value at December 31, 2020 Fair Value at December 31, 2019 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Cash Equivalents: Money market funds and treasury bills $ 27,463 $ — $ — $ 77,170 $ — $ — Total Cash Equivalents $ 27,463 $ — $ — $ 77,170 $ — $ — The following table provides the warrant derivative and private placement option liabilities reported at fair value and measured on a recurring basis: Fair Value at December 31, 2020 Fair Value at December 31, 2019 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Warrant derivative liability $ — $ — $ 10,345 $ — $ — $ 52,184 Private placement option liability — — 7,803 — — 12,094 Total fair value $ — $ — $ 18,148 $ — $ — $ 64,278
Schedule of Derivative Liabilities at Fair Value	The fair value of the warrants has been estimated with the following weighted-average assumptions, including the most sensitive input, volatility: December 31, 2020 December 31, 2019 Risk-free interest rate 0.46 % 1.83 % Volatility 90 % 78.67 % Expected life (years) 5.64 6.64 The fair value of the private placement option has been estimated with the following weighted-average assumptions, including the most sensitive input, volatility: December 31, 2020 Risk-free interest rate 0.77% Volatility of underlying stock price 90.00% Expected term (years) 7.00 (in thousands) Warrant Derivative Liability Private Placement Option Liability Total Balance, December 31, 2019 $ 52,184 $ 12,094 $ 64,278 Change in fair value (41,839) (4,291) (46,130) Balance, December 31, 2020 $ 10,345 $ 7,803 $ 18,148

LEASES (Tables)

LEASES (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Components of Lease Cost	Components of lease cost are as follows: (in thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Finance lease cost: Amortization of leased assets $ 74 $ 61 Interest on lease liabilities 21 25 Operating lease cost 814 2,135 Short-term lease cost 582 296 Total lease cost $ 1,491 $ 2,517 Weighted-average remaining lease term: Operating leases 1.7 years 5.2 years Finance leases 1.4 years 2.4 years Weighted-average discount rate: Operating leases 11.47 % 12.10 % Finance leases 13.05 % 13.40 % Supplemental cash flow information and non-cash activity related to the Company's operating and finance leases are as follows: (in thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 950 $ 2,378 Operating cash flows from finance leases 21 25 Financing cash flows from finance leases 74 47 Non-cash activity: Right-of-use assets obtained in exchange for lease obligations $ — $ 2,263
Maturity of Finance Lease Liabilities	Maturities of lease liabilities by year for leases are as follows: (in thousands) Operating Leases Financing Leases 2021 $ 499 $ 90 2022 306 39 2023 14 — Total lease payments 819 129 Less: Imputed interest (79) (12) Present value of lease liabilities $ 740 $ 117
Maturity of Operating Lease Liabilities	Maturities of lease liabilities by year for leases are as follows: (in thousands) Operating Leases Financing Leases 2021 $ 499 $ 90 2022 306 39 2023 14 — Total lease payments 819 129 Less: Imputed interest (79) (12) Present value of lease liabilities $ 740 $ 117

PUBLIC OFFERING AND PRIVATE P_2

PUBLIC OFFERING AND PRIVATE PLACEMENT (Tables)	12 Months Ended
	Dec. 31, 2020
Statement of Stockholders' Equity [Abstract]
Schedule of Derivative Liabilities at Fair Value	The fair value of the warrants has been estimated with the following weighted-average assumptions, including the most sensitive input, volatility: December 31, 2020 December 31, 2019 Risk-free interest rate 0.46 % 1.83 % Volatility 90 % 78.67 % Expected life (years) 5.64 6.64 The fair value of the private placement option has been estimated with the following weighted-average assumptions, including the most sensitive input, volatility: December 31, 2020 Risk-free interest rate 0.77% Volatility of underlying stock price 90.00% Expected term (years) 7.00 (in thousands) Warrant Derivative Liability Private Placement Option Liability Total Balance, December 31, 2019 $ 52,184 $ 12,094 $ 64,278 Change in fair value (41,839) (4,291) (46,130) Balance, December 31, 2020 $ 10,345 $ 7,803 $ 18,148

STOCKHOLDERS' EQUITY AND SHAR_2

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Outstanding Awards	At December 31, 2020 and 2019, outstanding awards were comprised of the following: December 31, 2020 December 31, 2019 Options 1,425,207 471,282 Inducement option awards 85,761 96,560 Restricted stock units 129,361 5,609 Inducement restricted stock units outstanding 500 750 Total outstanding awards 1,640,829 574,201
Weighted-Average Valuation Assumptions	The fair value of the option grants has been estimated, with the following weighted-average assumptions: Year Ended December 31, 2020 December 31, 2019 Options granted 1,352,595 276,830 Weighted-average exercise price 7.59 26.12 Weighted-average grant date fair value 5.29 16.99 Assumptions: Risk-free interest rate 0.82 % 2.23 % Volatility 85 % 72 % Expected life (years) 5.83 6.04 Expected dividend yield — % — %
Schedule of Stock Option Activity	The following table summarizes the stock option activity for all stock plans during the year ended December 31, 2020 and 2019 as follows: Options Outstanding Stock Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2018 575,924 $ 109.01 8.09 $ 87 Granted 276,830 $ 26.12 Exercised (220) $ 25.50 Forfeited (284,692) $ 93.81 Balance at December 31, 2019 567,842 $ 76.25 7.82 $ 12 Granted 1,352,595 $ 7.59 Exercised — Forfeited (409,469) $ 29.86 Balance at December 31, 2020 1,510,968 $ 27.36 8.19 $ 379 Exercisable at December 31, 2020 339,972 $ 91.97 5.61 $ —
Summary of Options Outstanding and Exercisable	The following table summarizes the options outstanding and exercisable at December 31, 2020: 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 $2.97 to $5.05 676,000 9.95 $ 2.97 — 0.00 $ — $5.06 to $12.90 125,301 9.40 $ 8.41 1,549 8.83 $ 9.62 $12.91 to $15.85 307,904 6.75 $ 13.60 — 0.00 $ — $15.86 to $81.25 217,682 6.61 $ 42.24 163,911 6.12 $ 44.40 $81.26 to $234.70 184,081 5.19 $ 135.21 174,512 5.01 $ 137.38 Total 1,510,968 8.19 $ 27.36 339,972 5.61 $ 91.97
Schedule of Restricted Stock Unit Activity	The following table summarizes the stock award activity for all restricted stock units during the year ended December 31, 2020: Awards Outstanding Restricted Stock Awards and Units Weighted-Average Grant Date Fair Value Per Share Outstanding Aggregate Intrinsic Value (in thousands) Total Fair Value of Restricted Awards Vested (in thousands) Balance at December 31, 2018 24,615 $ 80.23 $ 719 Granted 3,000 $ 33.30 Vested (14,478) $ 64.16 $ 240 $ 929 Forfeited (6,778) $ 82.44 Balance at December 31, 2019 6,359 $ 92.29 $ 82 Granted 239,023 $ 4.70 Vested (2,309) $ 96.14 $ 30 $ 222 Forfeited (113,212) $ 6.73 Balance at December 31, 2020 129,861 $ 5.59 $ 458
Summary of ESPP Activity	A summary of activity within the ESPP follows: Year Ended (in thousands except share data) December 31, 2020 December 31, 2019 Deductions from employees $ 56 $ 70 Share-based compensation expense recognized $ 96 $ 95 Remaining share-based compensation expense $ — $ 206 Proceeds received by the Company for ESPP $ 78 $ 98 Weighted-average purchase price per common share $ 5.21 $ 12.25 Number of shares purchased by employees under ESPP 14,975 8,000
Schedule Share-based Compensation	Share-based compensation expense by classification for December 31, 2020 and 2019 are as follows: Year Ended (in thousands) December 31, 2020 December 31, 2019 General and administrative $ 3,170 $ 4,017 Research and development 1,861 3,321 Total $ 5,031 $ 7,338

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Reconciliation of Income Tax Expense	The reconciliation between federal income taxes at the statutory U.S. federal income tax rate and the Company’s income tax expense for the year is as follows: (in thousands) December 31, 2020 December 31, 2019 Tax benefit at statutory rate $ (1,658) $ (23,591) Other 200 (294) Stock based compensation 642 2,674 Offering issuance costs and changes in fair value of warrants and private placement option (9,687) 4,657 Deferred tax valuation allowances 11,690 19,542 Research and development credit (1,187) (2,988) Income tax expense $ — $ —
Significant Components of Deferred Taxes	Significant components of the Company’s deferred taxes as of December 31, 2020 and 2019 are as follows: (in thousands) December 31, 2020 December 31, 2019 Deferred tax assets (liabilities): Net operating loss carryforward $ 94,132 $ 81,960 Stock compensation 3,539 3,270 Intangible assets 7,682 8,077 Research and development credit 17,787 16,601 Operating lease assets (213) (1,229) Operating lease liabilities 354 1,538 Other 976 2,336 Total deferred tax assets, net of deferred tax liabilities 124,257 112,553 Valuation allowance (124,257) (112,553) Net deferred tax $ — $ —
Summary of Net Operating Loss Carryforwards	Net operating loss carryforwards and research tax credits as of December 31, 2020 and 2019 are as follows: (in thousands) December 31, 2020 December 31, 2019 U.S. federal income tax net operating loss carryforwards $ 448,250 $ 390,286 U.K. net operating loss carryforwards $ 133 $ — U.S. federal research tax credits $ 12,535 $ 11,348 Texas research tax credits $ 5,252 $ 5,252

ORGANIZATION, BASIS OF PRESEN_4

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)	Apr. 14, 2020USD ($)	Feb. 05, 2020shares	Dec. 31, 2019USD ($)shares	Dec. 31, 2020USD ($)segmentshares	Dec. 31, 2019USD ($)shares	Oct. 31, 2020USD ($)	Jun. 15, 2020shares	Jun. 14, 2020shares	Feb. 04, 2020shares	Jan. 31, 2020USD ($)	Dec. 31, 2017USD ($)	Aug. 09, 2017USD ($)
Class of Stock [Line Items]
Number of operating segments | segment				1
Number of reportable segments | segment				1
Reverse stock split ratio		0.1
Common stock, authorized (in shares) | shares		40,000,000	80,000,000	80,000,000	80,000,000		80,000,000	40,000,000	200,000,000
Accumulated deficit			$ (533,025,000)	$ (540,747,000)	$ (533,025,000)
License fee revenue				500,000	7,143,000
Accrued expenses and other current liabilities			9,770,000	4,165,000	9,770,000
Gain on dispositions, net				3,656,000	0
Operating lease right-of-use assets			1,042,000	645,000	1,042,000
Property and equipment, net			2,529,000	189,000	2,529,000
Lease liability				740,000
Depreciation expense				1,500,000	7,000,000
Impairment of intangible assets			2,100,000	0	2,064,000
Disposal Group, Held-for-sale, Not Discontinued Operations
Class of Stock [Line Items]
Assets held for sale, property and equipment	$ 12,000,000
Assets held for sale, right-of-use assets	4,800,000
Assets held for sale, current lease liabilities	1,400,000
Assets held for sale, long-term lease liabilities	4,600,000
Gain on dispositions, net				3,800,000
2130 W. Holcombe Blvd. Assets
Class of Stock [Line Items]
Cash proceeds subject to escrow provisions	1,500,000
Amount paid upon closing of asset sale	$ 15,000,000
R&D Facility | Disposal Group, Held-for-sale, Not Discontinued Operations
Class of Stock [Line Items]
Operating lease right-of-use assets				500,000		$ 500,000
Property and equipment, net				2,300,000		2,300,000
Lease liability				700,000		$ 700,000
Impairment of property and equipment				1,300,000
Held in escrow
Class of Stock [Line Items]
Restricted cash, non-current			1,100,000		1,100,000
CPRIT Agreement
Class of Stock [Line Items]
Grants awarded												$ 16,900,000
Deferred grant revenue											$ 4,200,000
License fee revenue
Class of Stock [Line Items]
License fee revenue				500,000	5,000,000
License fee revenue | MD Anderson License Agreement
Class of Stock [Line Items]
License fee revenue			$ 5,000,000
Maintenance fee received				500,000
Grants
Class of Stock [Line Items]
License fee revenue				0	2,143,000
Grants | CPRIT Agreement
Class of Stock [Line Items]
License fee revenue				$ 0	$ 2,100,000
Accrued expenses and other current liabilities										$ 800,000

ORGANIZATION, BASIS OF PRESEN_5

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents	$ 35,495	$ 91,028
Restricted cash	1,501	2,788
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$ 36,996	$ 93,816	$ 48,668

ORGANIZATION, BASIS OF PRESEN_6

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment	$ 1,763	$ 12,440
Less: accumulated depreciation	(1,574)	(9,911)
Property and equipment, net	$ 189	2,529
Leasehold improvements
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Property and equipment	$ 167	3,944
Lab equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Property and equipment	$ 530	5,459
Office furniture
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Property and equipment	$ 0	392
Manufacturing equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Property and equipment	$ 138	395
Computer and office equipment
Property, Plant and Equipment [Line Items]
Property and equipment	$ 834	1,595
Equipment held under capital leases
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Property and equipment	$ 0	270
Software
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	3 years
Property and equipment	$ 94	$ 385
Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		3 years
Minimum | Computer and office equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		3 years
Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Maximum | Computer and office equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years

ORGANIZATION, BASIS OF PRESEN_7

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll	$ 1,029	$ 2,032
Accrued patient treatment costs	899	1,162
Accrued manufacturing costs	24	2,230
Accrued professional services	294	654
Accrued obligations under material supply agreements	0	1,121
Accrued other	1,919	2,571
Total accrued expenses and other current liabilities	$ 4,165	$ 9,770

ORGANIZATION, BASIS OF PRESEN_8

ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share, Potentially Dilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Total common stock equivalents (in shares)	27,451,909	11,704,201
Redeemable convertible series 1 preferred stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Total common stock equivalents (in shares)	4,520,000	5,380,000
Warrants to purchase common stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Total common stock equivalents (in shares)	11,616,080	5,750,000
Private placement option
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Total common stock equivalents (in shares)	9,675,000	0
Options to purchase common stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Total common stock equivalents (in shares)	1,510,968	567,842
Unvested shares of restricted stock units
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Total common stock equivalents (in shares)	129,861	6,359

FAIR VALUE OF MEASUREMENTS AN_3

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted cash, noncurrent	$ 1,500
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	27,463	$ 77,170
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	0
Money market funds and treasury bills | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	27,463	77,170
Money market funds and treasury bills | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	0
Money market funds and treasury bills | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	$ 0	$ 0

FAIR VALUE OF MEASUREMENTS AN_4

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Fair Value of Warrants (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Warrant Derivative Liability
Derivative [Line Items]
Risk-free interest rate	0.46%	1.83%
Volatility	90.00%	78.67%
Expected life (years)	5 years 7 months 20 days	6 years 7 months 20 days
Private Placement Option Liability
Derivative [Line Items]
Risk-free interest rate	0.77%
Volatility	90.00%
Expected life (years)	7 years

FAIR VALUE OF MEASUREMENTS AN_5

FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Derivative Liabilities Reported at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	$ 10,345	$ 52,184
Private placement option liability	7,803	12,094
Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	0	0
Private placement option liability	0	0
Total fair value	0	0
Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	0	0
Private placement option liability	0	0
Total fair value	0	0
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	10,345	52,184
Private placement option liability	7,803	12,094
Total fair value	$ 18,148	$ 64,278

LEASES - Narrative (Details)

LEASES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Oct. 31, 2020	Jul. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	$ 645			$ 1,042
Lease liability	740
Property and equipment, net	189			$ 2,529
Disposal Group, Held-for-sale, Not Discontinued Operations | R&D Facility
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	500	$ 500
Lease liability	700	700
Property and equipment, net	2,300	$ 2,300
Impairment of property and equipment	$ 1,300
Houston, Texas | Office space
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets			$ 500
Lease liability			$ 600
Minimum
Lessee, Lease, Description [Line Items]
Remaining lease term	2 years
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease term	3 years

LEASES - Components of Lease Co

LEASES - Components of Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finance lease cost:
Amortization of leased assets	$ 74	$ 61
Interest on lease liabilities	21	25
Operating lease cost	814	2,135
Short-term lease cost	582	296
Total lease cost	$ 1,491	$ 2,517
Weighted-average remaining lease term, operating leases (in years)	1 year 8 months 12 days	5 years 2 months 12 days
Weighted-average remaining lease term, finance leases (in years)	1 year 4 months 24 days	2 years 4 months 24 days
Weighted-average discount rate, operating leases	11.47%	12.10%
Weighted-average discount rate, finance leases	13.05%	13.40%

LEASES - Supplemental Cash Flow

LEASES - Supplemental Cash Flow Information Related to Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 950	$ 2,378
Operating cash flows from finance leases	21	25
Financing cash flows from finance leases	74	47
Non-cash activity:
Right-of-use assets obtained in exchange for lease obligations	$ 0	$ 2,263

LEASES - Maturity of Lease Liab

LEASES - Maturity of Lease Liabilities (Details) $ in Thousands	Dec. 31, 2020USD ($)
Operating Leases
2021	$ 499
2022	306
2023	14
Total lease payments	819
Less: Imputed interest	(79)
Present value of lease liabilities	$ 740
Operating Lease, Liability, Statement of Financial Position [Extensible List]	Long-term lease liabilities
Financing Leases
2021	$ 90
2022	39
2023	0
Total lease payments	129
Less: Imputed interest	(12)
Present value of lease liabilities	$ 117

DEBT - Narrative (Details)

DEBT - Narrative (Details)	Apr. 14, 2020USD ($)	Dec. 21, 2017USD ($)installment	Nov. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Debt Instrument [Line Items]
Payment on debt				$ 37,155,000	$ 0
Amortization of deferred issuance costs				550,000	885,000
Gain on extinguishment of debt				112,000	0
Oxford Loan
Debt Instrument [Line Items]
Face amount borrowed		$ 35,000,000
Payment on debt	$ 7,000,000		$ 27,400,000
Number of periodic payments | installment		35
Partial payment of final payment percentage	$ 600,000
Payment of debt issuance costs		$ 100,000
Final facility charge payment		$ 3,000,000
Amortization of deferred issuance costs				$ 600,000	$ 900,000
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%
Hercules Loan
Debt Instrument [Line Items]
Payment on debt		$ 32,900,000

PUBLIC OFFERING AND PRIVATE P_3

PUBLIC OFFERING AND PRIVATE PLACEMENT - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Aug. 16, 2019	Nov. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Mar. 31, 2020
Class of Stock [Line Items]
Common stock, issued (in shares)		1,040,000	8,385,650	5,076,593
Shares issuable per warrants (in shares)		3,109,378
Warrants to purchase aggregate shares of common stock (in shares)		4,149,378	239,023	3,000
Common stock, par value (in USD per share)		$ 0.001	$ 0.01	$ 0.01
Warrant exercise price (in USD per share)		$ 6.50	$ 91.97
Expiration period (in years)		5 years
Proceeds from issuance of common stock		$ 25,000
Proceeds from issuance of redeemable convertible preferred stock in a public offering, net			$ 53,800
Proceeds upon entering into private placement agreement			0	$ 11,152
Total offering costs related to warrants and options			$ 0	$ 2,992
Minimum
Class of Stock [Line Items]
Price of each share of common stock and accompanying common warrant (in USD per share)		$ 6.024
Maximum
Class of Stock [Line Items]
Price of each share of common stock and accompanying common warrant (in USD per share)		$ 6.025
Series 1 Preferred
Class of Stock [Line Items]
Shares issuable per warrants (in shares)			10
Public Offering
Class of Stock [Line Items]
Shares issuable under warrants (in shares)	5,750,000
Price per share (in dollars per share)	$ 100
Public Offering | Series 1 Preferred
Class of Stock [Line Items]
Number of shares of common stock sold in offering (in shares)	575,000
Warrants exercise price (in dollars per share)	$ 130
Public Offering | Common Stock
Class of Stock [Line Items]
Shares issuable per warrants (in shares)	10
Warrants exercise price (in dollars per share)	$ 13
Public Offering | Common Stock | Minimum
Class of Stock [Line Items]
Warrants, limitations on ownership after exercise			9.99%
Public Offering | Common Stock | Maximum
Class of Stock [Line Items]
Warrants, limitations on ownership after exercise			19.99%
Private placement option | Series 2 redeemable convertible non-voting preferred stock
Class of Stock [Line Items]
Number of shares of common stock sold in offering (in shares)	350,000
Shares issuable under warrants (in shares)	2,800,000
Price per share (in dollars per share)	$ 100
Warrants exercise price (in dollars per share)	$ 10
Warrants outstanding, term (in years)					2 years 6 months
Private placement option | Series 3 redeemable convertible non-voting preferred stock
Class of Stock [Line Items]
Number of shares of common stock sold in offering (in shares)	250,000
Warrants outstanding, term (in years)					3 years
Private placement option | Preferred Stock
Class of Stock [Line Items]
Shares issuable under warrants (in shares)	875,000
Price per share (in dollars per share)	$ 140		$ 140
Warrants exercise price (in dollars per share)	$ 14

PUBLIC OFFERING AND PRIVATE P_4

PUBLIC OFFERING AND PRIVATE PLACEMENT - Fair Value of Derivative Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Warrant Derivative Liabilities, Unobservable Input Reconciliation [Roll Forward]
Change in fair value of warrant and private placement option liabilities	$ (46,130)	$ 19,192
Level 3
Fair Value, Warrant Derivative Liabilities, Unobservable Input Reconciliation [Roll Forward]
Fair value of warrants	64,278
Change in fair value of warrant and private placement option liabilities	(46,130)
Fair value of warrants	18,148	64,278
Warrant Derivative Liability | Level 3
Fair Value, Warrant Derivative Liabilities, Unobservable Input Reconciliation [Roll Forward]
Fair value of warrants	52,184
Change in fair value of warrant and private placement option liabilities	(41,839)
Fair value of warrants	10,345	52,184
Private Placement Option Liability | Level 3
Fair Value, Warrant Derivative Liabilities, Unobservable Input Reconciliation [Roll Forward]
Fair value of warrants	12,094
Change in fair value of warrant and private placement option liabilities	(4,291)
Fair value of warrants	$ 7,803	$ 12,094

REDEEMABLE CONVERTIBLE PREFER_2

REDEEMABLE CONVERTIBLE PREFERRED STOCK - Narrative (Details) - $ / shares	Aug. 16, 2019	Dec. 31, 2020	Dec. 31, 2019	Nov. 30, 2020	Aug. 31, 2019
Class of Stock [Line Items]
Preferred stock, authorized (in shares)					10,000,000
Preferred stock, par value (in usd per share)					$ 0.01
Shares issuable per warrants (in shares)				3,109,378
50-day trading volume on Nasdaq stock market (in shares)		50,000
Series 1 Preferred
Class of Stock [Line Items]
Preferred stock, authorized (in shares)					1,517,500
Preferred stock, outstanding (in shares)		452,000	538,000
Preferred stock, issued (in shares)		452,000	538,000
Shares issuable per warrants (in shares)		10
Conversion of redeemable convertible preferred stock into common stock (in shares)		(86,000)	(37,000)
Series 2 redeemable convertible non-voting preferred stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		0	0
Preferred stock, issued (in shares)		0	0
Series 2 redeemable convertible non-voting preferred stock | Private placement option
Class of Stock [Line Items]
Number of shares of common stock sold in offering (in shares)	350,000
Price per share (in dollars per share)	$ 100
Series 3 redeemable convertible non-voting preferred stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		0	0
Preferred stock, issued (in shares)		0	0
Closing price of common stock (in usd per share)		$ 35
Series 3 redeemable convertible non-voting preferred stock | Private placement option
Class of Stock [Line Items]
Number of shares of common stock sold in offering (in shares)	250,000
Series 1 and 2 Preferred Stock
Class of Stock [Line Items]
Price per share (in dollars per share)		100
Closing price of common stock (in usd per share)		25
Preferred Stock | Private placement option
Class of Stock [Line Items]
Price per share (in dollars per share)	$ 140	$ 140

STOCKHOLDERS' EQUITY AND SHAR_3

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Narrative (Details) $ in Thousands	Oct. 05, 2018USD ($)	Nov. 30, 2020USD ($)shares	Jun. 30, 2020shares	Jan. 31, 2020shares	Dec. 31, 2020USD ($)planshares	Dec. 31, 2019USD ($)shares
Share-based Goods and Nonemployee Services Transaction [Line Items]
Proceeds from issuance of common stock | $		$ 25,000
Common stock, issued (in shares)		1,040,000			8,385,650	5,076,593
Number of share-based compensation plans | plan					5
Expiration period (in years)		5 years
Proceeds from exercise of stock options | $					$ 12	$ 76
Compensation cost not yet recognized | $					$ 5,300
Period of recognition (in years)					2 years 10 days
Aggregate fair value of options and restricted shares vesting | $					$ 4,800	8,900
2019 Equity Incentive Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of shares authorized (in shares)					250,000
Additional authorized shares approved (in shares)			500,000	600,000
2019 Equity Incentive Plan | Maximum
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of shares authorized (in shares)				600,540
Options
Share-based Goods and Nonemployee Services Transaction [Line Items]
Vesting period (in years)					4 years
Expiration period (in years)					10 years
Employee Stock | 2014 Employee Stock Purchase Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Number of shares authorized (in shares)					55,000
ESPP common stock discount					15.00%
Shares available for issuance under ESPP (in shares)					18,488
Jefferies LLC
Share-based Goods and Nonemployee Services Transaction [Line Items]
Proceeds from issuance of common stock | $	$ 60,000					$ 9,000
Common stock, issued (in shares)						259,115

STOCKHOLDERS' EQUITY AND SHAR_4

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Outstanding Awards (Details) - shares	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Goods and Nonemployee Services Transaction [Line Items]
Options outstanding (in shares)	1,510,968	567,842	575,924
2019 Equity Incentive Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Total awards outstanding (in shares)	1,640,829	574,201
Options | 2019 Equity Incentive Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Options outstanding (in shares)	1,425,207	471,282
Inducement option awards | 2019 Equity Incentive Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Options outstanding (in shares)	85,761	96,560
Restricted stock units | 2019 Equity Incentive Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Awards other than options outstanding (in shares)	129,361	5,609
Inducement restricted stock units outstanding | 2019 Equity Incentive Plan
Share-based Goods and Nonemployee Services Transaction [Line Items]
Awards other than options outstanding (in shares)	500	750

STOCKHOLDERS' EQUITY AND SHAR_5

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Weighted-Average Valuation Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	1,352,595	276,830
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	1,352,595	276,830
Weighted-average exercise price (in usd per share)	$ 7.59	$ 26.12
Weighted-average grant date fair value of options granted (in usd per share)	$ 5.29	$ 16.99
Risk-free interest rate	0.82%	2.23%
Volatility	85.00%	72.00%
Expected life (years)	5 years 9 months 29 days	6 years 14 days
Expected dividend yield	0.00%	0.00%

STOCKHOLDERS' EQUITY AND SHAR_6

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 30, 2020
Outstanding Stock Options
Balance (in shares)	567,842	575,924
Granted (in shares)	1,352,595	276,830
Exercised (in shares)	0	(220)
Forfeited (in shares)	(409,469)	(284,692)
Balance (in shares)	1,510,968	567,842	575,924
Exercisable at end of period (in shares)	339,972
Weighted Average Exercise Price
Balance (usd per share)	$ 76.25	$ 109.01
Granted (usd per share)	7.59	26.12
Exercised (usd per share)		25.50
Forfeited (usd per share)	29.86	93.81
Balance (usd per share)	27.36	$ 76.25	$ 109.01
Exercisable at end of period (in USD per share)	$ 91.97			$ 6.50
Outstanding Stock Options, Other Disclosures
Weighted-average remaining contractual term, outstanding (in years)	8 years 2 months 8 days	7 years 9 months 25 days	8 years 1 month 2 days
Weighted-average remaining contractual term, exercisable (in years)	5 years 7 months 9 days
Aggregate intrinsic value, outstanding	$ 379	$ 12	$ 87
Aggregate intrinsic value, exercisable	$ 0

STOCKHOLDERS' EQUITY AND SHAR_7

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Stock Options Outstanding and Exercisable (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding (in shares)	1,510,968
Weighted-average remaining contractual term, outstanding (in years)	8 years 2 months 8 days	7 years 9 months 25 days	8 years 1 month 2 days
Weighted-average exercise price, outstanding (in usd per share)	$ 27.36
Exercisable (in shares)	339,972
Weighted-average remaining contractual term, exercisable	5 years 7 months 9 days
Weighted-average exercise price (in usd per share)	$ 91.97
$2.97 to $5.05
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in usd per share)	2.97
Exercise price - Maximum (in usd per share)	$ 5.05
Outstanding (in shares)	676,000
Weighted-average remaining contractual term, outstanding (in years)	9 years 11 months 12 days
Weighted-average exercise price, outstanding (in usd per share)	$ 2.97
Exercisable (in shares)	0
Weighted-average remaining contractual term, exercisable	0 years
Weighted-average exercise price (in usd per share)	$ 0
$5.06 to $12.90
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in usd per share)	5.06
Exercise price - Maximum (in usd per share)	$ 12.90
Outstanding (in shares)	125,301
Weighted-average remaining contractual term, outstanding (in years)	9 years 4 months 24 days
Weighted-average exercise price, outstanding (in usd per share)	$ 8.41
Exercisable (in shares)	1,549
Weighted-average remaining contractual term, exercisable	8 years 9 months 29 days
Weighted-average exercise price (in usd per share)	$ 9.62
$12.91 to $15.85
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in usd per share)	12.91
Exercise price - Maximum (in usd per share)	$ 15.85
Outstanding (in shares)	307,904
Weighted-average remaining contractual term, outstanding (in years)	6 years 9 months
Weighted-average exercise price, outstanding (in usd per share)	$ 13.60
Exercisable (in shares)	0
Weighted-average remaining contractual term, exercisable	0 years
Weighted-average exercise price (in usd per share)	$ 0
$15.86 to $81.25
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in usd per share)	15.86
Exercise price - Maximum (in usd per share)	$ 81.25
Outstanding (in shares)	217,682
Weighted-average remaining contractual term, outstanding (in years)	6 years 7 months 9 days
Weighted-average exercise price, outstanding (in usd per share)	$ 42.24
Exercisable (in shares)	163,911
Weighted-average remaining contractual term, exercisable	6 years 1 month 13 days
Weighted-average exercise price (in usd per share)	$ 44.40
$81.26 to $234.70
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise Price - Minimum (in usd per share)	81.26
Exercise price - Maximum (in usd per share)	$ 234.70
Outstanding (in shares)	184,081
Weighted-average remaining contractual term, outstanding (in years)	5 years 2 months 8 days
Weighted-average exercise price, outstanding (in usd per share)	$ 135.21
Exercisable (in shares)	174,512
Weighted-average remaining contractual term, exercisable	5 years 3 days
Weighted-average exercise price (in usd per share)	$ 137.38

STOCKHOLDERS' EQUITY AND SHAR_8

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Restricted Stock Award Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Outstanding Restricted Stock Awards and Units
Beginning balance (in shares)		6,359	24,615
Granted (in shares)	4,149,378	239,023	3,000
Vested (in shares)		(2,309)	(14,478)
Forfeited (in shares)		(113,212)	(6,778)
Ending balance (in shares)		129,861	6,359
Weighted-Average Grant Date Fair Value Per Share
Beginning balance (in usd per share)		$ 92.29	$ 80.23
Granted (in usd per share)		4.70	33.30
Vested (in usd per share)		96.14	64.16
Forfeited (in usd per share)		6.73	82.44
Ending balance (in usd per share)		$ 5.59	$ 92.29
Outstanding Aggregate Intrinsic Value
Beginning balance (in usd per share)		$ 458	$ 82	$ 719
Vested (in usd per share)		30	240
Ending balance (in usd per share)		458	82
Total fair value of restricted shares vested		$ 222	$ 929

STOCKHOLDERS' EQUITY AND SHAR_9

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - ESPP Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense recognized	$ 5,031	$ 7,338
Remaining share-based compensation expense	5,031	7,338
Proceeds received by the Company for ESPP	78	98
Employee Stock | 2014 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deductions from employees	56	70
Share-based compensation expense recognized	96	95
Remaining share-based compensation expense	0	206
Proceeds received by the Company for ESPP	$ 78	$ 98
Weighted-average purchase price per common share (in dollars per share)	$ 5.21	$ 12.25
Number of shares purchased by employees under ESPP (in shares)	14,975	8,000

STOCKHOLDERS' EQUITY AND SHA_10

STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Expense by Classification (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense recognized	$ 5,031	$ 7,338
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense recognized	3,170	4,017
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense recognized	$ 1,861	$ 3,321

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - Licensing Agreements	Dec. 10, 2015USD ($)	Jun. 10, 2015USD ($)product	Mar. 31, 2016USD ($)
Baylor
Long-term Purchase Commitment [Line Items]
Nonrefundable license fee			$ 100,000
Agensys
Long-term Purchase Commitment [Line Items]
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]
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

INCOME TAXES - Reconciliation o

INCOME TAXES - Reconciliation of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Tax benefit at statutory rate	$ (1,658)	$ (23,591)
Other	200	(294)
Stock based compensation	642	2,674
Offering issuance costs and changes in fair value of warrants and private placement option	(9,687)	4,657
Deferred tax valuation allowances	11,690	19,542
Research and development credit	(1,187)	(2,988)
Income tax expense	$ 0	$ 0

INCOME TAXES - Significant Comp

INCOME TAXES - Significant Components of Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets (liabilities):
Net operating loss carryforward	$ 94,132	$ 81,960
Stock compensation	3,539	3,270
Intangible assets	7,682	8,077
Research and development credit	17,787	16,601
Operating lease assets	(213)	(1,229)
Operating lease liabilities	354	1,538
Other	976	2,336
Total deferred tax assets, net of deferred tax liabilities	124,257	112,553
Valuation allowance	(124,257)	(112,553)
Net deferred tax	$ 0	$ 0

INCOME TAXES - Operating Loss C

INCOME TAXES - Operating Loss Carryforwards and Research Tax Credits (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Research tax credits	$ 17,787	$ 16,601
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards	448,250	390,286
Research tax credits	12,535	11,348
Foreign
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards	133	0
Texas
Operating Loss Carryforwards [Line Items]
Research tax credits	$ 5,252	$ 5,252

INCOME TAXES - Narrative (Detai

INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Net operating loss carryforwards, no expiration date	$ 227,000
Research tax credits	17,787	$ 16,601
Increase (decrease) in valuation allowance	$ 11,700	$ 19,500

SUBSEQUENT EVENTS (Details)

SUBSEQUENT EVENTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 02, 2021
San Francisco Office Space
Subsequent Event [Line Items]
Lease termination fee	$ 0.9
Subsequent Event | Reed Road, Houston, TX
Subsequent Event [Line Items]
Lease exit agreement, consideration receivable		$ 1.1
Lease exit agreement, cash receivable		0.9
Lease exit agreement, note receivable		$ 0.2