Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net loss	$ (23,024)	$ (15,858)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	130	190
Stock-based compensation expense	1,143	605
Noncash interest expense	209	21
Changes in assets and liabilities:
Prepaid expenses and other current assets	(237)	(126)
Other assets	2
Accounts payable	619	544
Accrued expenses	112	151
Deferred rent	(4)	(19)
Net cash used in operating activities	(21,050)	(14,492)
Investing activities
Purchases of available-for-sale securities		(4,976)
Purchases of property and equipment	(60)	(159)
Net cash used in investing activities	(60)	(5,135)
Financing activities
Proceeds from initial public offering, net of issuance costs	61,776
Proceeds from issuance of preferred stock, net of issuance costs	12,331
Proceeds from exercise of common stock options	51	110
Proceeds from borrowings	5,000	5,000
Debt issuance costs	(7)	(282)
Net cash provided by financing activities	79,151	4,828
Net increase (decrease) in cash and cash equivalents	58,041	(14,799)
Cash and cash equivalents, beginning of period	14,668	30,474
Cash and cash equivalents, end of period	72,709	15,675
Supplemental disclosure of cash flow information
Cash paid for interest	500	1
Non-cash financing activities
Warrants for the purchase of series B preferred stock issued in conjunction with credit facility	107	$ 110
Initial public offering costs in accounts payable and accrued liabilities	32
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