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Accrued expenses (Details) - USD ($)
Sep. 30, 2019
Dec. 31, 2018
Accrued expenses    
Professional fees $ 2,982,289 $ 80,771
Compensation and benefits 575,262 624,298
Research and development 2,153,881 2,178,086
Other 135,932 245,617
Total accrued expenses $ 5,847,364 $ 3,128,772