Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (28,741)	$ (30,335)	$ (16,077)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	19	18	12
Noncash stock bonus			80
Noncash stock based compensation	3,425	2,965	1,381
Loss on extinguishment of debt		1,036
Noncash interest	946	590	185
Change in fair value of warrants	(234)	110	(348)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	922	(1,209)	(2,335)
Accounts payable and accrued expenses	362	1,347	2,599
Net cash used in operating activities	(23,301)	(25,478)	(14,503)
Cash flows from investing activities
Acquisition of property and equipment	(31)	(290)	(96)
Net cash used in investing activities	(31)	(290)	(96)
Cash flows from financing activities
Proceeds from convertible bridge notes			3,000
Proceeds from issuance of term loan		16,265
Repayment of term loan		(15,784)
Principal payments of long-term debt	(985)
Return of principal payments of long-term debt	985
Proceeds from issuance of common stock, in public offering, net of offering costs	37,528		49,744
Proceeds from issuance of common stock in private placement, net of offering costs		19,330
Cash paid for financing costs	(175)	(423)	(150)
Proceeds from exercise of stock options	334	593	67
Net cash provided by financing activities	37,687	19,981	52,661
Net increase (decrease) in cash and cash equivalents	14,355	(5,787)	38,062
Cash and cash equivalents, beginning of year	34,395	40,182	2,120
Cash and cash equivalents, end of year	48,750	34,395	40,182
Supplemental cash flow information
Interest paid	$ 1,500	1,474	1,380
Supplemental disclosure of noncash financing activities
Fair value of common stock warrants issued		$ 1,184
Conversion of preferred stock into common stock			69,233
Conversion of notes payable and interest into common stock			$ 3,021