STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (40,636)	$ (41,050)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	412	259
Stock-based compensation	6,850	6,165
Change in fair value of derivative liabilities	(46)	1,292
(Increase) decrease in:
Receivables	(1,002)	8
Prepaid expenses and other current assets	5,406	(6,660)
Other noncurrent assets	(4,684)	(3)
Increase (decrease) in:
Accounts payable	(3,148)	1,368
Accrued expenses	(532)	1,002
Contract liabilities	(146)	(4,792)
Deferred rent	(112)	(102)
Net cash used in operating activities	(37,638)	(42,513)
Cash flows from investing activities:
Purchases of property and equipment	(545)	(461)
Net cash used in investing activities	(545)	(461)
Cash flows from financing activities:
Proceeds from exercise of stock options	240	99
Repurchase of restricted common stock	(1,275)	(1,042)
Proceeds from issuance of common stock, net		47,270
Net cash provided by (used in) financing activities	(1,035)	46,327
Net increase (decrease) in cash, cash equivalents, and restricted cash	(39,218)	3,353
Cash, cash equivalents, and restricted cash, beginning of period	71,335	81,441
Cash, cash equivalents, and restricted cash, end of period	32,117	84,794
Supplementary disclosure of cash flow information:
Cash paid for interest	0	0
Cash paid for income taxes	0	0
Series 1 Preferred Stock
Supplementary disclosure of noncash investing and financing activities:
Payment of dividends in preferred stock	$ 16,656	$ 13,939