CONSOLIDATED 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	$ (38,023)	$ (36,676)	$ (43,002)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,791	6,345	6,219
Depreciation and amortization	11	27	29
Amortization of debt discount and debt issuance costs	359	135	367
Write-off debt discount upon note repayment	27
Decrease in fair value of warrant liability		(3,543)	(3,892)
Changes in operating assets and liabilities:
Prepaids and other assets	(85)	660	(43)
Accounts payable	(582)	(724)	2,224
Accrued clinical expense	(520)	(1,158)	(1,638)
Accrued compensation	394	(681)	568
Other liabilities	169
Other accrued liabilities	(1,062)	(186)	1,674
Deferred revenue	(2,441)	(2,930)	(5,687)
Net cash used in operating activities	(36,962)	(38,731)	(43,181)
Cash flows from investing activities
Purchases of property and equipment			(48)
Purchases of marketable securities	(35,530)	(35,683)	(42,463)
Sale and maturities of marketable securities	20,531	36,930	45,836
Net cash (used in) provided by investing activities	(14,999)	1,247	3,325
Cash flows from financing activities
Proceeds from notes payable	15,000
Principal payments on notes payable and final payment	(7,983)	(1,642)	(9,356)
Payment of financing fees and debt issuance costs	(266)
Proceeds from issuance of convertible preferred stock offering, net	5,592	16,459
Proceeds from issuance of common stock and warrants	20,367	8,709	40,024
Proceeds from issuance of common stock through controlled equity offering facilities, net	269	18,125	14,292
Proceeds from exercise of warrants, stock options and stock purchase rights	152	533	1,961
Net cash provided by financing activities	33,131	42,184	46,921
Net (decrease) increase in cash and cash equivalents	(18,830)	4,700	7,065
Cash and cash equivalents at beginning of period	26,886	22,186	15,121
Cash and cash equivalents at end of period	8,056	26,886	22,186
Supplemental disclosure of cash flow information
Interest paid	1,239	631	1,221
Supplemental disclosure of non-cash activities
Transfer of fair value of exercised warrants to additional paid-in capital		100	496
Conversion of preferred stock to common stock	(3,243)
Fair value of warrants issued in connection with notes payable	$ 536
Cashless exercise of warrants		$ 4,486	$ 9,337