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Statements of Cash Flows - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating activities      
Net loss $ (48,273,633) $ (33,281,068) $ (31,809,838)
Adjustments to reconcile net loss to net cash used in operating activities:      
Depreciation 275,123 263,541 190,540
Loss on disposal of property and equipment 168    
Stock-based compensation expense 4,587,098 3,760,802 2,964,767
Changes in assets and liabilities      
Prepaid expenses and other current assets 943,360 (2,816,381) (36,693)
Prepaid research and development expenses (1,356,243) 555,167 (63,000)
Deposits     (52,320)
Accounts payable 16,488 1,061,880 1,000,975
Accrued expenses and bonuses 551,146 724,797 (2,504,179)
Deferred rent (74,637) (37,099) 578,483
Net cash used in operating activities (43,331,130) (29,768,361) (29,731,265)
Investing activities      
Purchases of property and equipment (125,618) (294,107) (704,202)
Net cash used in investing activities (125,618) (294,107) (704,202)
Financing activities      
Proceeds from issuance of common stock, net of issuance costs 128,425,080 113,572,189 23,601,955
Proceeds from exercise of stock options and warrants 1,024,525 373,376 72,593
Net cash provided by financing activities 129,449,605 113,945,565 23,674,548
Net change in cash and cash equivalents 85,992,857 83,883,097 (6,760,919)
Cash and cash equivalents, beginning of period 123,924,738 40,041,641 46,802,560
Cash and cash equivalents, end of period $ 209,917,595 123,924,738 40,041,641
Non-cash investing activities and financing activities      
Property acquisition costs included in accounts payable and accrued expenses   20,000 $ 21,292
Issuance costs associated with financing included in accounts payable and accrued expenses   $ 25,005