CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash Flows from Operating Activities:
Net loss	$ (22,150)	$ (36,126)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,905	3,127
Issuance of common stock for services rendered	59	45
Accretion of discounts on short-term investments	(353)
Unrealized gain on available-for-sale securities	2
Depreciation and amortization expense	67	321
Gain on disposal of property and equipment	(11)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(423)	69
Accounts payable, accrued expenses, and other liabilities	(2,426)	4,243
Deferred revenue		(331)
Net cash used in operating activities	(23,330)	(28,652)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(35,486)
Proceeds from maturity of available-for-sale securities	23,350
Proceeds from the sale of property and equipment	11
Purchases of property and equipment	(11)	(42)
Net cash used in investing activities	(12,136)	(42)
Cash Flows from Financing Activities:
Proceeds from equity financings, net of issuance costs	3,857
Proceeds from employee stock purchases	68	159
Proceeds from exercise of common stock options and warrants		10,166
Payments on note payable		(209)
Other	(6)	(5)
Net cash provided by financing activities	3,919	10,111
Net decrease in cash and cash equivalents	(31,547)	(18,583)
Cash, cash equivalents and restricted cash, beginning of period	71,431	112,940
Cash, cash equivalents and restricted cash, end of period	$ 39,884	$ 94,357