Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
R&D tax credit receivable	$ 891	$ 834
Prepaid clinical trial costs	1,067	2,129
VAT receivable	292	196
Prepaid insurance	289	158
Forward currency exchange derivative	4	40
Deposits and other	154	194
Total prepaid expenses and other current assets	$ 2,697	$ 3,551