Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities:
Net loss	$ (11,378)	$ (10,201)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	7	16
Stock-based compensation	642	627
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	993	(829)
Accounts payable and accrued expenses	(403)	(618)
Net cash used in operating activities	(10,139)	(11,005)
Investing activities:
Purchase of short-term investments		(18,915)
Sale of short-term investments	8,512	7,802
Net cash provided by (used in) investing activities	8,512	(11,113)
Decrease in cash and cash equivalents	(1,627)	(22,118)
Cash and cash equivalents at beginning of period	3,535	27,285
Cash and cash equivalents at end of period	$ 1,908	$ 5,167