Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (40,545)	$ (48,374)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	6,159	4,535
Depreciation and amortization	123	113
Changes in fair value of warrant liabilities		16,240
Securities issuance obligation		1,485
Other	21	74
Changes in operating assets and liabilities:
Prepaid expenses and other assets	310	972
Accrued, other and operating lease liabilities	623	(546)
Accounts payable	(657)	976
Deferred revenue		300
Net cash used in operating activities	(33,966)	(24,225)
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	19,632
Proceeds from exercise of common stock warrants	660
Proceeds from exercise of common stock options	401
Net cash provided by financing activities	20,693
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	(52)	(58)
NET DECREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(13,365)	(24,295)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — Beginning of period	104,355	147,828
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — End of period	90,990	123,533
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes, net	33	14
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	$ 100	15
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