Income Taxes - Schedule of Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 18,819,077	$ 23,146,178
Accrued expenses	204,599	18,400
Stock-based compensation	96,578	96,570
Research and development credits	3,038,863	2,670,688
Other	13,953	20,784
Total deferred tax assets	22,173,070	25,952,620
Deferred tax liabilities:
Fixed assets	3,606	8,434
Total deferred tax liabilities	3,606	8,434
Net deferred tax assets before valuation allowance	22,169,464	25,944,186
Less valuation allowance	$ (22,169,464)	$ (25,944,186)