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Equity - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Jan. 29, 2019
Sep. 20, 2018
Aug. 01, 2018
May 09, 2018
Apr. 03, 2018
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Class Of Stock [Line Items]                  
Warrant to purchase shares of common stock         38,400        
Issuance of stock for services, shares           121,617   200,009  
Issuance of stock for services           $ 192,274   $ 553,303  
Preferred stock shares authorized           10,000,000      
Preferred stock, par value           $ 0.001      
Preferred stock, share outstanding           0      
Non-voting common stock conversion ratio         100.00%        
Common stock, shares authorized         200,000,000 200,000,000   200,000,000  
Common stock, par value         $ 0.001 $ 0.001   $ 0.001  
Common stock, shares outstanding           15,560,765   15,239,148  
Shares outstanding           5,905,583   4,535,681 2,628,749
Share-based compensation arrangement by share-based payment award, options, grants           1,461,715   2,034,525  
Options expired or cancelled           91,813   127,593  
Share-based compensation arrangement by share-based payment award, expiration period           10 years      
Share-based compensation arrangement by share-based payment award, options, outstanding, weighted average exercise price           $ 2.94   $ 3.31 $ 1.31
Share-based compensation arrangement by share-based payment award, options, outstanding, weighted average remaining contractual term           8 years 1 month 28 days   7 years 9 months 29 days  
Employees                  
Class Of Stock [Line Items]                  
Share-based compensation arrangement by share-based payment award, options, grants           1,461,715      
2018 Equity Incentive Plan                  
Class Of Stock [Line Items]                  
Common stock reserved for issuance           4,160,000      
Common stock incentive plan description           The number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each year, beginning on January 1, 2019 by 5% of the total number of shares of the Company's Common Stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the administrator      
Annual percentage increase in number of shares of common stock reserved for issuance         5.00%        
2009 Plan                  
Class Of Stock [Line Items]                  
Common stock reserved for issuance           554,963      
Shares available for future grants           0      
2009 Plan | Stock Options                  
Class Of Stock [Line Items]                  
Shares outstanding           0      
2018 Employee Stock Purchase Plan                  
Class Of Stock [Line Items]                  
Annual percentage increase in number of shares of common stock reserved for issuance           2.00%      
Shares authorized for the issuance of common stock to purchase rights granted to employees           208,500      
Employee stock purchase plan description           The number of shares of common stock reserved for issuance will automatically increase on January 1 of each calendar year, from January 1, 2019 by the lesser of 2% of the total number of shares of our common stock outstanding on December 31 of the preceding calendar year or a number determined by the administrator of the ESPP.      
World Wide Holdings L L C                  
Class Of Stock [Line Items]                  
Exercised of warrant to purchase shares   50,000              
Common stock issued upon warrant exercise   34,615              
Cancelled of remaining comon stock,shares   15,385              
Warrant                  
Class Of Stock [Line Items]                  
Granted warrants to purchase common stock, shares           0   3,166,491  
Granted warrants to purchase common stock, price               $ 4.47  
Common stock issued upon warrant exercise               34,615  
Cancelled of remaining comon stock,shares               15,385  
Maximum                  
Class Of Stock [Line Items]                  
Share-based compensation arrangement by share-based payment award, award vesting period           48 months      
Share-based compensation arrangement by share-based payment award, options, outstanding, weighted average exercise price           $ 9.80      
Share-based compensation arrangement by share-based payment award, options, vested, weighted average grant date fair value           7.93      
Maximum | Stock Options                  
Class Of Stock [Line Items]                  
Common stock share price           $ 1.62      
Minimum                  
Class Of Stock [Line Items]                  
Share-based compensation arrangement by share-based payment award, award vesting period           12 months      
Share-based compensation arrangement by share-based payment award, options, outstanding, weighted average exercise price           $ 0.001      
Share-based compensation arrangement by share-based payment award, options, vested, weighted average grant date fair value           0.001      
Minimum | Stock Options                  
Class Of Stock [Line Items]                  
Common stock share price           $ 1.45      
IPO                  
Class Of Stock [Line Items]                  
Stock issued during period, shares, new issues         1,280,000        
Sale of stock, price per share         $ 5.00        
Estimated net proceeds of common stock         $ 5,025,000        
Underwriting discounts, commissions and offering expenses         $ 895,000        
Warrant issued to purchase common stock, percentage         3.00%        
All preferred stock shares issued             1,394,953    
Preferred stock, shares includes all series of shares were converted to common stock             9,324,177    
Preferred stock shares authorized           10,000,000      
Preferred stock, par value           $ 0.001      
Preferred stock, share outstanding           0      
Granted warrants to purchase common stock, shares               38,400  
Granted warrants to purchase common stock, price               $ 6.25  
Private Placement                  
Class Of Stock [Line Items]                  
Stock issued during period, shares, new issues       828,500          
Sale of stock, price per share       $ 12.07          
Warrants exercise price per share       $ 15.62          
Net proceeds from commissions and expenses       $ 9,250,000          
Granted warrants to purchase common stock, shares               2,283,740  
Granted warrants to purchase common stock, price               $ 4.25  
Private Placement | Securities Purchase Agreement                  
Class Of Stock [Line Items]                  
Stock issued during period, shares, new issues     1,174,440            
Warrants exercise price per share     $ 4.25            
Warrants exercisable for common stock     2,283,740            
Common stock reserved for issuance     150,000            
Private Placement | Maximum                  
Class Of Stock [Line Items]                  
Warrant to purchase shares of common stock       621,376          
Private Placement | Minimum                  
Class Of Stock [Line Items]                  
Warrants exercise price per share       $ 4.25          
IPO and Private Placement                  
Class Of Stock [Line Items]                  
Stock issued during period, shares, new issues           200,000   3,282,940  
Issuance of stock for cash               $ 16,400,000  
Consideration of Service                  
Class Of Stock [Line Items]                  
Granted warrants to purchase common stock, shares               844,352  
Granted warrants to purchase common stock, price               $ 5.00  
Granted warrants to purchase common stock, value               $ 2,203,506  
Consideration of Service | Cancer Revolution L L C                  
Class Of Stock [Line Items]                  
Granted warrants to purchase common stock, shares               425,000  
Granted warrants to purchase common stock, price               $ 5.00  
Consideration of Service | Cancer Biotech L L C                  
Class Of Stock [Line Items]                  
Granted warrants to purchase common stock, shares               225,000  
Granted warrants to purchase common stock, price               $ 5.00  
Consideration of Service | Inception Capital Management L L C                  
Class Of Stock [Line Items]                  
Granted warrants to purchase common stock, shares               144,352  
Granted warrants to purchase common stock, price               $ 5.00  
Consideration of Service | World Wide Holdings L L C                  
Class Of Stock [Line Items]                  
Granted warrants to purchase common stock, shares               50,000  
Granted warrants to purchase common stock, price               $ 5.00  
Consideration of Service | FundAthena                  
Class Of Stock [Line Items]                  
Granted warrants to purchase common stock, shares 6,000                
Granted warrants to purchase common stock, price $ 5.00                
Granted warrants to purchase common stock, value $ 30,000                
Warrant shares issued           0