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Unaudited Statements of Cash Flows - USD ($)
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Operating activities    
Net loss $ (14,083,897) $ (11,512,844)
Adjustments to reconcile net loss to net cash used in operating activities:    
Depreciation 70,441 68,757
Amortization of operating lease right-of-use asset 149,686  
Stock-based compensation expense 1,382,443 1,115,830
Changes in assets and liabilities    
Prepaid expenses and other current assets (561,897) (182,401)
Accounts payable (832,560) 1,206,896
Accrued expenses and bonuses (300,817) (887,737)
Operating lease liabilities (171,084) (17,325)
Net cash used in operating activities (14,347,685) (10,208,824)
Investing activities    
Purchases of property and equipment (39,538) (16,618)
Net cash used in investing activities (39,538) (16,618)
Financing activities    
Proceeds from issuance of common stock, net of issuance costs   128,634,000
Proceeds from exercise of stock options and warrants 30,554 298,789
Net cash provided by financing activities 30,554 128,932,789
Net change in cash and cash equivalents (14,356,669) 118,707,347
Cash and cash equivalents, beginning of period 209,917,595 123,924,738
Cash and cash equivalents, end of period 195,560,926 242,632,085
Non-cash investing activities and financing activities    
Property acquisition costs included in accrued expenses $ 35,918  
Issuance costs associated with financing included in accrued expenses   $ 198,000