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Accrued expenses (Details) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Accrued expenses    
Professional fees $ 207,917 $ 80,771
Compensation and benefits 961,790 624,298
Research and development 4,992,311 2,178,086
Other 480,535 245,617
Total accrued expenses $ 6,642,553 $ 3,128,772