Income taxes - Components of net deferred tax assets (Details) - USD ($) $ in Thousands |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Deferred tax assets | ||
| Net operating loss | $ 27,749 | $ 13,920 |
| Accrued clinical trials | 322 | 82 |
| Accrued compensation | 582 | 409 |
| Accrued expenses and other, net | 374 | 456 |
| Total deferred tax assets | 29,027 | 14,867 |
| Less valuation allowance | (28,963) | (14,688) |
| Net deferred tax assets | 64 | 179 |
| Deferred tax liabilities | ||
| Stock-based compensation and other, net | (64) | $ (179) |
| Net deferred taxes | 0 | |
| Net operating loss carryforwards with no taxable income limitation | $ 7,100 |