Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity - Additional Information (Detail)

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Stockholders' Equity - Additional Information (Detail) - USD ($)
 1 Months Ended 9 Months Ended
Sep. 25, 2018
Feb. 27, 2018
Aug. 01, 2017
Jun. 23, 2017
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock         3,331,735 3,521,735  
Proceeds from public offering           $ 48,061,000 $ 57,716,000
Term Loan Facility | Perceptive Credit Facility              
Class Of Warrant Or Right [Line Items]              
Warrant exercisable period   7 years          
Common stock exercise price   $ 9.5792          
Number of shares to be issued upon exercise of warrant   190,000          
2017 Warrants              
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock         377,273 377,273  
Common stock exercise price           $ 39.82  
Common Stock              
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock           3,521,735  
Common Stock | Term Loan Facility | Perceptive Credit Facility              
Class Of Warrant Or Right [Line Items]              
Warrant exercisable period   7 years          
Common stock exercise price   $ 9.58          
Valuation of equity classified warrants recorded in additional paid-n capital   $ 1,500,000          
Expected dividend rate   0.00%          
Expected volatility   105.00%          
Risk free interest rate   2.83%          
Expected term   7 years          
Number of shares to be issued upon exercise of warrant   190,000          
Common Stock | 2017 Warrants              
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock     377,273        
Common stock exercise price     $ 39.82        
Change in fair value of common stock warrants           $ 916,000  
Takeda Pharmaceuticals Inc              
Class Of Warrant Or Right [Line Items]              
Aggregate Purchase Price       $ 20,000,000      
Each unit of shares transaction of common stock       2,922,993      
Purchase price per share       $ 6.84      
Wedbush Warrant Valuation              
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock         57,930    
Common stock exercise price         $ 6.84    
Valuation of equity classified warrants recorded in additional paid-n capital         $ 400,000    
Expected dividend rate         0.00%    
Expected volatility         108.00%    
Risk free interest rate         2.33%    
Expected term         7 years    
Private Placement              
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock         2,954,462 2,954,462  
Common stock exercise price           $ 6.84  
Private Placement | Common Stock              
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock     2,896,532        
Common stock exercise price     $ 6.84        
Valuation of equity classified warrants recorded in additional paid-n capital     $ 16,300,000        
Expected dividend rate     0.00%        
Expected volatility     147.00%        
Risk free interest rate     2.07%        
Expected term     7 years        
Private Placement | Longitude Venture Partners III, L.P.              
Class Of Warrant Or Right [Line Items]              
Number of aggregate units sold     5,793,063        
Aggregate Purchase Price     $ 40,000,000        
Each unit of shares transaction of common stock     1        
Warrants to purchase shares of common stock     0.5     2,896,532  
Sale of units, description and its composition           the Company sold an aggregate of 5,793,063 units (the “Units”) accredited investors having an aggregate purchase price of $40.0 million, each such Unit consisting of (i) one (1) share (the “Shares”) of the Company’s common stock and (ii) a warrant (the “Private Placement Warrants”) to purchase 0.5 shares of the Company’s common stock (the “Private Placement”).  
Purchase price per unit     $ 6.9048        
Warrant exercisable period     7 years        
Common stock exercise price     $ 6.8423        
Private Placement | Wedbush Agreement | Common Stock              
Class Of Warrant Or Right [Line Items]              
Warrants to purchase shares of common stock         57,930 57,930  
Warrant exercisable period         7 years    
Common stock exercise price         $ 6.8423    
Underwritten Public Offering              
Class Of Warrant Or Right [Line Items]              
Shares issued under public offering 9,430,000            
Shares issued, price per share $ 5.50            
Proceeds from public offering $ 48,100,000            
Over-Allotment Option              
Class Of Warrant Or Right [Line Items]              
Shares issued under public offering 1,230,000