BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 76,054	$ 115,069
Receivables	1,111	4,665
Prepaid expenses and other current assets	1,666	10,855
Total current assets	78,831	130,589
Property and equipment, net	10,941	10,231
Right-of-use asset	4,420	4,650
Deposits	42	130
Other non-current assets	631	745
Total assets	94,865	146,345
Current liabilities:
Accounts payable	1,368	960
Current portion of long-term debt	7,868	0
Accrued expenses	6,076	16,589
Lease liability - current portion	729	819
Total current liabilities	16,041	18,368
Long-term debt	16,250	0
Lease liability - noncurrent portion	4,518	3,995
Total liabilities	36,809	22,363
Commitments and contingencies (Note 9)
Stockholders' equity:
Common stock, $0.001 par value; 350,000,000 shares authorized; 216,127,443 shares issued and outstanding at December 31, 2021 and 250,000,000 shares authorized, 214,591,906 shares issued and outstanding at December 31, 2020	216	215
Additional paid-in capital	900,693	887,868
Accumulated deficit	(842,852)	(764,101)
Total stockholders' equity	58,057	123,982
Total liabilities and stockholders' equity	$ 94,865	$ 146,345