Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accrued Expenses
Research and development expenses	$ 1,250	$ 1,166
Employee compensation expenses	515	1,732
Vixen contract payable.	100	100
Professional fees	109	77
Other	59	303
Total accrued expenses	$ 2,033	$ 3,378