Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 966	$ 304
Accrued external research and development expenses	5,182	430
Accrued legal and professional fees	199	106
Liability for early exercise of stock options and restricted stock	225
Other	60	129
Total accrued expenses and other current liabilities	$ 6,632	$ 969