Condensed Balance Sheets (Current Period Unaudited) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Assets
Cash and cash equivalents	$ 100,242	$ 112,346
Marketable securities	1,338	0
Accounts and other receivables	3,320	1,528
Prepaid expenses and other current assets	880	1,182
Total current assets	105,780	115,056
Property and equipment, net	835	884
Operating lease right-of-use assets	1,090	1,141
Long-term investments	17,564	15,422
Other long-term assets	437	438
Total assets	125,706	132,941
Liabilities and stockholders' equity
Accounts payable and accrued expenses	2,582	3,038
Current portion of operating lease liabilities	356	356
Other current liabilities	7	836
Total current liabilities	2,945	4,230
Derivative and warrant liability	89	330
Operating lease liabilities, less current portion	1,144	1,232
Other long-term liabilities	29	31
Total liabilities	4,207	5,823
Commitments and contingencies (Note D)
Stockholders’ equity:
Common stock, $0.0001 par value, 250,000,000 shares authorized, 35,333,450 shares issued and 34,423,497 shares outstanding as of March 31, 2022 (unaudited); 35,325,801 shares issued and 35,005,640 shares outstanding as of December 31, 2021	3	4
Additional paid-in capital	397,925	396,957
Treasury stock, at cost	(7,536)	(2,814)
Accumulated deficit	(268,893)	(267,029)
Total stockholders' equity	121,499	127,118
Total liabilities and stockholders' equity	125,706	132,941
Undesignated Preferred Stock [Member]
Stockholders’ equity:
Undesignated preferred stock, $0.0001 par value, 10,000,000 shares authorized, no shares issued or outstanding as of March 31, 2022 (unaudited) or December 31, 2021	$ 0	$ 0