Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities
Net loss	$ (30,335)	$ (16,077)	$ (14,321)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	18	12	12
Noncash stock bonus		80	80
Noncash stock based compensation	2,965	1,381	1,338
Loss on extinguishment of debt	1,036
Noncash interest	590	185	133
Change in fair value of warrants	110	(348)	81
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,209)	(2,335)	107
Accounts payable and accrued expenses	1,347	2,599	(449)
Net cash used in operating activities	(25,478)	(14,503)	(13,019)
Cash flows from investing activities
Acquisition of property and equipment	(290)	(96)	(4,945)
Net cash used in investing activities	(290)	(96)	(4,945)
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)
Proceeds from issuance of common stock, in initial public offering, net of offering costs		49,744
Proceeds from issuance of common stock in private placement, net of offering costs	19,330
Cash paid for financing costs	(423)	(150)
Proceeds from exercise of stock options	593	67	70
Net cash provided by financing activities	19,981	52,661	70
Net increase (decrease) in cash and cash equivalents	(5,787)	38,062	(17,894)
Cash and cash equivalents, beginning of year	40,182	2,120	20,014
Cash and cash equivalents, end of year	34,395	40,182	2,120
Supplemental cash flow information
Interest paid	1,474	1,380	$ 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