Subsequent Events - Summary of Actual Balance Sheet Data and Pro Forma Balance Sheet Data to Reflect Activities Related to Offering (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Deferred public offering and other financing costs	$ 269,613	$ 521,538
Accrued interest	21,960	34,894
Convertible notes payable, current portion (net of discount of $404,437 and $675,589 at December 31, 2017 and 2016, respectively)	3,450,540	3,269,582
Debt conversion feature liability	1,397,597	731,048
Stockholders’ equity:
Common stock, $0.00001 par value: 300,000,000 shares authorized at December 31, 2017 and 2016; 35,817,104 shares issued and outstanding at December 31, 2017 and 20,823,873 shares issued and outstanding at December 31, 2016	358	208
Additional paid-in capital	94,339,179	68,326,818
Accumulated deficit	(80,854,932)	(60,277,267)
Accumulated other comprehensive loss	(19,769)	(6,892)
Total stockholders’ equity	13,464,836	$ 8,042,867
February 2018 Offering [Member]
Subsequent Event [Line Items]
Cash, cash equivalents and investments	20,575,956
Deferred public offering and other financing costs	269,613
Debt	4,870,097
Accrued interest	21,960
Convertible notes payable, current portion (net of discount of $404,437 and $675,589 at December 31, 2017 and 2016, respectively)	3,450,540
Debt conversion feature liability	1,397,597
Stockholders’ equity:
Common stock, $0.00001 par value: 300,000,000 shares authorized at December 31, 2017 and 2016; 35,817,104 shares issued and outstanding at December 31, 2017 and 20,823,873 shares issued and outstanding at December 31, 2016	358
Additional paid-in capital	94,339,179
Accumulated deficit	(80,854,932)
Accumulated other comprehensive loss	(19,769)
Total stockholders’ equity	13,464,836
February 2018 Offering [Member] | Pro Forma [Member]
Subsequent Event [Line Items]
Cash, cash equivalents and investments	79,198,456
Deferred public offering and other financing costs	269,613
Debt	4,870,097
Accrued interest	21,960
Convertible notes payable, current portion (net of discount of $404,437 and $675,589 at December 31, 2017 and 2016, respectively)	3,450,540
Debt conversion feature liability	1,397,597
Stockholders’ equity:
Common stock, $0.00001 par value: 300,000,000 shares authorized at December 31, 2017 and 2016; 35,817,104 shares issued and outstanding at December 31, 2017 and 20,823,873 shares issued and outstanding at December 31, 2016	485
Additional paid-in capital	152,961,552
Accumulated deficit	(80,854,932)
Accumulated other comprehensive loss	(19,769)
Total stockholders’ equity	$ 72,087,336