ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands				1 Months Ended	3 Months Ended				12 Months Ended
	Mar. 15, 2021 USD ($)	Apr. 14, 2020 USD ($)	Feb. 05, 2020 shares	Jun. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2021 USD ($) segment subsidiary shares	Dec. 31, 2020 USD ($) shares	Mar. 26, 2021 USD ($)	Feb. 26, 2021 USD ($)	Oct. 31, 2020 USD ($)	Jun. 15, 2020 shares	Jun. 14, 2020 shares	Feb. 04, 2020 shares
Class of Stock [Line Items]
Number of wholly-owned subsidiaries formed | subsidiary									2
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								$ (540,747)	$ (550,452)	$ (540,747)
Operating lease right-of-use assets								645	0	645
Property and equipment, net								189	12	189
Depreciation expense									100	1,500
Accrued clinical research costs								1,562	$ 479	1,562
Scenario, Adjustment
Class of Stock [Line Items]
Accrued clinical research costs								1,500		1,500
Accrued manufacturing costs and accrued professional fees, current								100		100
Minimum
Class of Stock [Line Items]
Estimated useful lives									3 years
Maximum
Class of Stock [Line Items]
Estimated useful lives									5 years
Lab equipment
Class of Stock [Line Items]
Estimated useful lives									5 years
2130 W. Holcombe Blvd. Assets
Class of Stock [Line Items]
Cash proceeds subject to escrow provisions		$ 1,500
R&D Facility | Disposal Group, Held-for-sale, Not Discontinued Operations
Class of Stock [Line Items]
Operating lease right-of-use assets								500		500			$ 500
Property and equipment, net								2,300		2,300			2,300
Lease liability								700		700			$ 700
Accretion of lease liability								$ 1,300		$ 1,300
Reed Road, Houston, TX
Class of Stock [Line Items]
Amount paid upon closing of asset sale												$ 1,100
Lease exit agreement, cash receivable												900
Lease exit agreement, note receivable												$ 200
San Francisco Office Space
Class of Stock [Line Items]
Operating lease right-of-use assets											$ 600
Lease liability											1,000
Lease termination fee	$ 900
Security deposits refund	$ 200			$ 200
Loss on termination of lease							$ 500		$ 500
San Francisco Office Space | Lab equipment
Class of Stock [Line Items]
Operating lease right-of-use assets											$ 100
MD Anderson
Class of Stock [Line Items]
Upfront payment received					$ 5,000
Takeda
Class of Stock [Line Items]
Additional revenue from sales						$ 700