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Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Payables And Accruals [Abstract]    
Accrued external research and development expenses $ 3,209 $ 4,541
Accrued payroll and related expenses 885 2,379
Accrued professional fees 470 917
Accrued other 635 426
Accrued expenses and other current liabilities $ 5,199 $ 8,263