Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities
Net loss	$ (14,540)	$ (9,833)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	93	130
Stock-based compensation expense	655	373
Noncash interest expense	123
Changes in assets and liabilities:
Prepaid expenses and other current assets	(174)	(454)
Accounts payable	649	522
Accrued expenses	(11)	(491)
Deferred rent	(18)	(10)
Net cash used in operating activities	(13,223)	(9,763)
Investing activities
Purchases of property and equipment	(35)	(157)
Net cash used in investing activities	(35)	(157)
Financing activities
Proceeds from initial public offering, net of issuance costs	62,763
Proceeds from issuance of preferred stock, net of issuance costs	12,331
Proceeds from exercise of common stock options	51	22
Proceeds from borrowings	5,000
Debt issuance costs	(7)
Net cash provided by financing activities	80,138	22
Net increase (decrease) in cash and cash equivalents	66,880	(9,898)
Cash and cash equivalents, beginning of period	14,668	30,747
Cash and cash equivalents, end of period	81,548	$ 20,849
Supplemental disclosures of cash flow information
Cash paid for interest	254
Noncash financing activities
Warrants for the purchase of series B preferred stock issued in conjunction with credit facility	110
Initial public offering costs in accounts payable and accrued liabilities	970
Reclassification of deferred IPO costs from non-current assets to additional paid-in capital	1,787
Reclassification of warrant liability to additional paid-in capital	$ 206