Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (52,807)	$ (33,285)	$ (21,994)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,050	759	492
Depreciation and amortization expense	103	103	71
Loss on transfer of nonfinancial assets	3,900	0	0
Non-cash lease expense	569	0	0
Accretion of debt discount	695	152	157
Loss on extinguishment of debt	566	229	0
Loss on preferred stock repurchase liability	0	0	587
Change in fair value of preferred stock warrant and derivative liability	105	3,487	(1,266)
Changes in operating assets and liabilities:
Prepaid expenses, other current assets and research & development incentive receivable	109	279	402
Accounts payable	(2,752)	1,307	(1,169)
Accrued expenses	160	1,549	1,409
Lease liabilities	(753)	0	0
Net cash used in operating activities	(48,055)	(25,420)	(21,311)
Cash flows from investing activities:
Cash, cash equivalents and restricted cash acquired in connection with the Merger	26,406	0	0
Proceeds from transfer of non-financial assets	1,000	0	0
Purchase of property, equipment and intangible assets	(174)	0	(378)
Net cash provided by (used in) investing activities	27,232	0	(378)
Cash flows from financing activities:
Proceeds from exercise of stock options and warrants	792	7	9
Proceeds from borrowings under loan and security agreements, net of issuance costs	9,849	9,908	6,000
Proceeds from issuance of Series B convertible preferred stock, net of issuance costs	0	4,461	27,413
Repurchase of Series Seed convertible preferred stock	0	(1,126)	0
Repayments of borrowings under loan and security agreement	(9,368)	(6,380)	0
Proceeds from sale of common stock and warrants, net of issuance costs	139,388	0	0
Net cash provided by financing activities	140,661	6,870	33,422
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(250)	0	0
Net increase (decrease) in cash, cash equivalents and restricted cash	119,588	(18,550)	11,733
Cash, cash equivalents and restricted cash at beginning of period	8,498	27,048	15,315
Cash, cash equivalents and restricted cash at end of period	128,086	8,498	27,048
Supplemental disclosure of cash flow information
Cash paid for interest	1,284	540	282
Supplemental disclosure of non-cash investing and financing activities:
Purchase of property, equipment and intangible assets included in accounts payable	40	0	0
Issuance costs related to sale of common stock and warrants, not yet paid	775
Conversion of convertible preferred stock into common stock	64,675	0	0
Conversion of redeemable common stock into common stock	734	0	0
Conversion of convertible preferred stock warrants into common stock warrants	5,235	0	0
Fair value of net assets acquired in the Merger in exchange for common shares, excluding cash acquired	19,952	0	0
Initial fair value of derivative liability in connection with loan and security agreement	0	18	0
Issuance of warrants in connection with Series B convertible preferred stock financing	0	172	817
Issuance of warrants in connection with loan and security agreement	$ 0	$ 132	$ 0