CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash Flows from Operating Activities:
Net loss	$ (33,283)	$ (47,731)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	2,868	4,454
Issuance of common stock for services rendered	92	77
Accretion of discounts on short-term investments	(377)
Depreciation and amortization expense	94	395
Loss on disposal of property and equipment	(10)	497
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,298)	1,881
Accounts payable, accrued expenses, and other liabilities	(1,257)	455
Deferred revenue		(472)
Net cash used in operating activities	(34,171)	(40,444)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(44,447)
Proceeds from maturity of available-for-sale securities	35,850
Proceeds from the sale of property and equipment	11	193
Purchases of property and equipment	(11)	(71)
Net cash (used in) provided by investing activities	(8,597)	122
Cash Flows from Financing Activities:
Proceeds from equity financings, net of issuance costs	3,857
Proceeds from employee stock purchases	97	205
Proceeds from exercise of common stock options and warrants		10,166
Payments on note payable		(209)
Other	(6)	(7)
Net cash provided by financing activities	3,948	10,155
Net decrease in cash and cash equivalents	(38,820)	(30,167)
Cash, cash equivalents and restricted cash, beginning of period	71,431	112,940
Cash, cash equivalents and restricted cash, end of period	$ 32,611	$ 82,773