Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (6,391)	$ (6,623)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	135	146
Derivatives (gain) loss	(241)	1,047
Common stock issued to 401(k) plan	51	51
Share-based compensation expense	1,315	1,153
Non-cash investment (income) expense, net	12	(70)
Changes in operating assets and liabilities:
Prepaid expenses	(489)	(433)
Other assets	1	3
Accounts payable	27	(83)
Accrued expenses and other liabilities	(536)	512
Net cash used in operating activities	(6,140)	(4,157)
Cash flows from investing activities:
Purchases of marketable securities	(6,761)	(2,452)
Sales of marketable securities	5,950	2,760
Purchases of property and equipment	(16)	(50)
Net cash (used in) provided by investing activities	(827)	258
Cash flows from financing activities:
Proceeds from exercise of stock options	17	36
Proceeds from issuance of stock under ESPP	29
Repayment of loan payable	(103)	(96)
Proceeds from issuance of common stock and warrants		29,910
Net cash (used in) provided by financing activities	(57)	29,850
Increase (decrease) in cash and cash equivalents	(7,024)	25,951
Cash and cash equivalents at beginning of period	21,464	14,920
Cash and cash equivalents at end of period	14,440	40,871
Supplemental disclosure of cash flow information and non-cash transactions:
Cash paid for interest	$ 20	$ 28