Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 1,606	$ 304
Accrued external research and development expenses	6,361	430
Accrued legal and professional fees	370	106
Other	85	129
Total accrued expenses and other current liabilities	$ 8,422	$ 969