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Accrued expenses (Details) - USD ($)
Jun. 30, 2020
Dec. 31, 2019
Accrued expenses    
Professional fees $ 143,611 $ 207,917
Compensation and benefits 809,203 961,790
Research and development 6,528,641 4,992,311
Other 1,346,405 480,535
Total accrued expenses $ 8,827,860 $ 6,642,553