Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 2,559	$ 2,514
Accrued external research and development expenses	2,742	2,055
Accrued professional fees	1,723	867
Current portion of operating lease liabilities	684	640
Other	90	68
Accrued expenses and other current liabilities	$ 7,798	$ 6,144