Unaudited Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (9,070)	$ (15,312)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	39	7
Stock-based compensation expense	833	616
Write off of deferred offering costs	0	111
Amortization of investment premium	165	0
Amortization of debt discount	200	0
Change in fair value of warrant liability	(4,447)	(101)
Changes in deferred rent	(7)	0
Changes in operating assets and liabilities:
Prepaid expenses, other assets, and deferred costs	(1,326)	(1,003)
Accounts payable and accrued expenses	(411)	278
Accrued severance and retention cost obligations	0	(2,419)
Deferred revenue	(129)	(128)
Net cash used in operating activities	(14,153)	(17,951)
Cash flows from investing activities:
Maturities of investments	25,497	0
Purchases of property and equipment	(2)	(14)
Purchase of investments	(44,622)	(12,472)
Net cash used in investing activities	(19,127)	(12,486)
Cash flows from financing activities:
Proceeds from common stock issued	5,224	23,077
Payments of deferred offering costs and underwriting discounts and commissions	(168)	(1,765)
Proceeds from employee stock purchase plan issuance	18	5
Net cash provided by financing activities	5,074	21,317
Net decrease in cash and cash equivalents	(28,206)	(9,120)
Cash and cash equivalents, beginning of period	35,656	46,985
Cash and cash equivalents, end of period	7,450	37,865
Supplemental cash flow information:
Cash paid for interest	709	0
Cash received for interest	307	67
Noncash financing and investing activities:
Deferred offering costs included in accounts payable and accrued expenses	0	205
Deferred offering costs reclassified to additional-paid-in capital	$ 15	$ 65