Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (94,659)	$ (80,902)	$ (88,275)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	12,890	9,470	5,695
Depreciation and amortization	305	238	83
Changes in fair value of warrant liabilities	0	16,240	20,926
Securities issuance obligation		1,485	10,485
Asset impairment		106
Warrant issuance costs			1,279
Term loan repayment fee			438
Other	(39)	201	161
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(622)	(33)	336
Accrued, other and operating lease liabilities	3,270	(366)	(2,034)
Accounts payable	(109)	1,194	(277)
Net cash used in operating activities	(78,964)	(52,367)	(51,183)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(72)	(12)	(39)
Net cash used in investing activities	(72)	(12)	(39)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from exercise of common stock options	847		445
Proceeds from exercise of common stock warrants	810
Proceeds from public offering, net of offering costs			97,731
Payment of term loan and repayment fee			(5,438)
Net cash provided by financing activities	79,808	8,901	92,738
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	(78)	5	(34)
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	694	(43,473)	41,482
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — Beginning of period	104,355	147,828	106,346
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — End of period	105,049	104,355	147,828
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for (refund of) income taxes, net	66	28	(69)
Cash paid for interest			336
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Right-of-use asset obtained in exchange for operating lease obligation	730		771
Property and equipment purchases included in accounts payable	62
Offering costs not yet paid	54	10
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
Issuance costs of convertible voting preferred stock and warrants included in accrued and other liabilities			$ 268
At-The-Market Offering [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of offering costs	$ 78,151	$ 8,901