Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (6,137)	$ (10,667)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	167	184
Stock-based compensation	571	928
Debt discount accretion and non-cash interest expense	201	81
Accretion of discount of available-for-sale investment securities	(106)	(64)
Amortization of right-of-use asset	169
Changes in operating assets and liabilities
Collaboration receivable	(630)
Prepaid expenses and other assets	97	(40)
Accounts payable and accrued expenses	(985)	(999)
Contract liability	630
Net cash used in operating activities	(6,023)	(10,577)
Cash flows from investing activities:
Purchases of property and equipment	(10)	(498)
Purchases of available-for-sale investment securities	(13,995)	(7,988)
Maturities of available-for-sale investment securities	10,650	19,900
Net cash (used in) provided by investing activities	(3,355)	11,414
Cash flows from financing activities:
Proceeds from issuance of common stock through option exercises		8
Proceeds from issuance of common stock through at the market offerings, net of offering costs	1,381
Repayments on borrowing	(2,000)
Net cash (used in) provided by financing activities	(619)	8
Net change in cash and cash equivalents	(9,997)	845
Cash and cash equivalents at beginning of period	22,962	21,091
Cash and cash equivalents at the end of period	$ 12,965	$ 21,936