Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities:
Net loss	$ (3,958)	$ (3,325)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	3	5
Stock-based compensation	240	192
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	233	(1,192)
Accounts payable and accrued expenses	(221)	(79)
Net cash used in operating activities	(3,703)	(4,399)
Investing activities:
Purchase of short-term investments		(12,610)
Sale of short-term investments	6,209
Net cash provided by (used in) investing activities	6,209	(12,610)
Increase (decrease) in cash and cash equivalents	2,506	(17,009)
Cash and cash equivalents at beginning of period	3,535	30,031
Cash and cash equivalents at end of period	$ 6,041	$ 13,022