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Unaudited Statements of Cash Flows - USD ($)
 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Operating activities    
Net loss $ (35,464,168) $ (43,165,594)
Adjustments to reconcile net loss to net cash used in operating activities:    
Depreciation 202,399 211,391
Non-cash lease expense 504,504 459,599
Stock-based compensation expense 5,274,173 4,488,985
Changes in assets and liabilities:    
Prepaid expenses and other current assets 1,553,529 (1,642,917)
Accounts payable (362,050) (1,256,031)
Accrued expenses (850,891) 2,244,062
Noncurrent accrued expenses 180,117  
Operating lease liabilities (593,511) (530,818)
Net cash used in operating activities (29,555,898) (39,191,323)
Investing activities    
Purchases of property and equipment (30,332) (132,571)
Net cash used in investing activities (30,332) (132,571)
Financing activities    
Proceeds from issuance of common stock, net of issuance costs 14,119,553  
Proceeds from exercise of stock options 135,583 293,091
Net cash provided by financing activities 14,255,136 293,091
Net change in cash and cash equivalents (15,331,094) (39,030,803)
Cash and cash equivalents, beginning of period 158,201,441 209,917,595
Cash and cash equivalents, end of period 142,870,347 $ 170,886,792
Non-cash investing and financing activities    
Property acquisition costs included in accounts payable $ 10,688