Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accrued Expenses
Research and development expenses	$ 1,187	$ 1,166
Employee compensation expenses	1,074	1,732
Vixen contract payable.	100	100
Professional fees	100	77
Other	60	303
Total accrued expenses	$ 2,521	$ 3,378