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Significant Inputs and Assumptions Used in Binomial Lattice Model for Derivative Liability (Detail)

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Significant Inputs and Assumptions Used in Binomial Lattice Model for Derivative Liability (Detail) - $ / shares
 1 Months Ended 12 Months Ended
Apr. 30, 2015
Feb. 06, 2015
Sep. 26, 2014
May. 31, 2015
May. 31, 2015
May. 31, 2014
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]            
Contractual conversion rate       $ 0.15 $ 0.15  
Adjusted conversion price $ 0.675          
Contractual term to maturity (years)         6 months  
Expected volatility         80.68%  
Risk-free rate         0.12%  
Minimum            
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]            
Contractual conversion rate           $ 0.66
Contractual term to maturity (years)           3 years
Expected volatility           78.00%
Risk-free rate           0.64%
Maximum            
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]            
Contractual conversion rate           $ 0.72
Contractual term to maturity (years)           5 years
Expected volatility           93.00%
Risk-free rate           1.42%
Derivative Liability            
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]            
Quoted market price on valuation date   $ 0.96 $ 0.79 0.99    
Contractual conversion rate   1.00 1.00 1.00 $ 1.00  
Adjusted conversion price [1]   $ 1.0000 $ 0.9759 $ 0.6750    
Contractual term to maturity (years)   5 months 27 days 2 years      
Expected volatility   124.00% 123.00%      
Contractual interest rate   2.00% 5.00%      
Risk-free rate   0.045% 0.59%      
Risk-adjusted rate   2.78% 2.69% 2.80%    
Probability of event of default   5.00% 5.00% 5.00%    
Derivative Liability | Minimum            
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]            
Contractual term to maturity (years)       2 months 5 days    
Expected volatility       73.00%    
Contractual interest rate       1.20%    
Risk-free rate       0.01%    
Derivative Liability | Maximum            
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]            
Contractual term to maturity (years)       1 year 3 months 29 days    
Expected volatility       105.00%    
Contractual interest rate       5.00%    
Risk-free rate       0.35%    
[1] The adjusted conversion price input used in the Binomial Lattice Model considers both i) the reduction of the conversion price to $0.675 on April 30, 2015, as result of the short-term convertible notes offering in which Common Stock was sold for a weighted average price of $0.75 and ii) potential adjustment to the stated conversion price due to a future dilutive issuance. This input was calculated using a probability-weighted approach which considered the likelihood of various scenarios occurring including (i) potential success or failure of various phases for PRO 140, (ii) the probability the Company will enter into a future financing and (iii) and the potential price of a future financing.