Income taxes - Components of net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets
Net operating loss	$ 41,727	$ 27,749
Stock‑based compensation	2,603
Accrued clinical trials	176	322
Accrued compensation	509	582
Accrued expenses and other, net	383	374
Total deferred tax assets	45,398	29,027
Less valuation allowance	(45,398)	(28,963)
Net deferred tax assets		64
Deferred tax liabilities
Stock-based compensation and other, net		(64)
Net deferred taxes	$ 0
Net operating loss carryforwards with no taxable income limitation		$ 7,100