Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (51,181,000)	$ (44,731,000)	$ (44,675,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	856,000	1,062,000	1,149,000
Stock-based compensation	7,397,000	6,111,000	4,890,000
Amortization of premiums (accretion of discounts) on short-term investments	85,000	(29,000)	(787,000)
Amortization of debt discount	172,000	191,000	189,000
Impairment of property and equipment	727,000	0	0
Changes in operating assets and liabilities:
Prepaid and other assets	(1,564,000)	108,000	536,000
Accounts payable	276,000	(374,000)	127,000
Accrued expenses	1,459,000	(2,419,000)	1,656,000
Accrued compensation	(477,000)	1,334,000	(42,000)
Right-of-use assets and lease liabilities, net	(168,000)	(127,000)	32,000
Net cash used in operating activities	(42,418,000)	(38,874,000)	(36,925,000)
Cash flows from investing activities:
Purchases of short-term investments	0	(68,581,000)	(85,004,000)
Maturities of short-term investments	55,490,000	48,500,000	114,000,000
Purchases of property and equipment	(295,000)	(134,000)	(700,000)
Proceeds from disposition	768,000	0
Net cash provided by (used in) investing activities	55,963,000	(20,215,000)	28,296,000
Cash flows from financing activities:
Proceeds from issuance of common stock and pre-funded warrants, net of issuance costs	32,207,000	64,187,000
Proceeds from issuance of long-term debt, net of cash issuance costs	674,000
Proceeds from short-term debt		1,126,000
Principal payments on short-term debt		(1,126,000)
Proceeds from issuance of common stock under employee stock purchase plan	213,000	270,000	242,000
Proceeds from exercise of stock options	6,000	206,000	5,000
Payments of debt issuance costs			(52,000)
Net cash provided by financing activities	33,100,000	64,663,000	195,000
Net change in cash, cash equivalents and restricted cash	46,645,000	5,574,000	(8,434,000)
Cash, cash equivalents and restricted cash at beginning of period	31,469,000	25,895,000	34,329,000
Cash, cash equivalents and restricted cash at end of period	78,114,000	31,469,000	25,895,000
Cash and cash equivalents at end of period	77,412,000	30,767,000	25,194,000
Restricted cash at end of period	702,000	702,000	701,000
Supplemental cash flow information:
Cash paid for interest	1,422,000	1,373,000	1,394,000
Supplemental disclosure of non-cash investing and financing activities:
Purchase of property and equipment in accounts payable and accrued expenses	$ 85,000	$ 67,000	$ 76,000