Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (69,192)	$ (62,879)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	9,439	6,609
Depreciation and amortization	213	191
Changes in fair value of warrant liabilities	0	16,240
Securities issuance obligation		1,485
Asset impairment		106
Other	78	108
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(3,263)	(586)
Accounts payable	(1,358)	531
Accrued, other and operating lease liabilities	369	(1,624)
Deferred revenue		200
Net cash used in operating activities	(63,714)	(39,619)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(40)	(12)
Net cash used in investing activities	(40)	(12)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock from At-The-Market equity offering, net of offering costs	55,364	1,140
Proceeds from exercise of common stock warrants	810
Proceeds from exercise of common stock options	677
Net cash provided by financing activities	56,851	1,140
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	(84)	(23)
NET DECREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(6,987)	(38,514)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — Beginning of period	104,355	147,828
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — End of period	97,368	109,314
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes, net	51	17
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Right-of-use asset obtained in exchange for operating lease obligation	730
Unpaid deferred financing costs in accrued and other liabilities	$ 7	94
Issuance of common stock and common stock warrant in connection with an amendment to the asset purchase agreement		11,970
Reclassification of warrant liabilities to equity		$ 62,175