Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (21,833)	$ (26,366)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	5,971	5,674
Depreciation	36	50
Amortization of operating lease right-of-use asset	246	231
Amortization of debt discount	301	234
Amortization, net, of premiums and discounts on investments	9	(109)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(449)	339
Accounts payable	(1,101)	1,726
Accrued expenses	(1,170)	(281)
Operating lease liabilities	(245)	(231)
Taxes payable	(5)	(79)
Net cash used in operating activities	(18,240)	(18,812)
Cash flows from investing activities:
Purchases of marketable securities		(33,590)
Proceeds from maturities of marketable securities	27,000	6,750
Purchase of property and equipment		(16)
Cash received (paid) for long-term deposits	42	(9)
Net cash provided by (used in) investing activities	27,042	(26,865)
Cash flows from financing activities:
Proceeds from underwritten public offering, net of issuance costs		455
Proceeds from debt financing obligation	797	15,000
Payment of debt issuance costs		(276)
Repayment of term loan principal	(2,083)
Proceeds from exercises of stock options	64	116
Payment for settlement of taxes upon vesting of restricted stock units	(119)	(1,120)
Proceeds from advances from collaboration partners	402
Net cash (used in) provided by financing activities	(939)	46,397
Increase in cash, cash equivalents and restricted cash	7,863	720
Cash, cash equivalents and restricted cash at the beginning of the period	22,536	48,651
Cash, cash equivalents and restricted cash at the end of the period	30,399	49,371
Reconciliation of cash, cash equivalents and restricted cash to condensed consolidated balance sheets:
Cash and cash equivalents	30,347	49,326
Restricted cash	52	45
Cash, cash equivalents and restricted cash at the end of the period	30,399	49,371
Supplemental disclosure of cash flow activities:
Cash paid for interest	481	400
Cash paid for income taxes	$ 5	79
Supplemental disclosure of non-cash financing activities:
Non-cash acquisition of treasury stock		95
Non-cash issuance of common stock upon exercise of warrants		178
Fair value of warrants issued in connection with long-term debt		421
Deferred public offering costs in accounts payable and accrued expenses		172
Underwritten Public Offering
Cash flows from financing activities:
Proceeds from underwritten public offering, net of issuance costs		$ 32,222