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Note 6 - Stockholders' Equity
 6 Months Ended
Jun. 30, 2016
Notes to Financial Statements  
Stockholders' Equity Note Disclosure [Text Block]
6. Stockholders’ Equity
 
2015 Common Stock Offering
 
In June 2015, we sold approximately 5,520,000 shares of our common stock at a public offering price of approximately $24.75 per share. We received total net proceeds of approximately $128,199,000 (net of approximately $8,421,000 in issuance costs) from the sale of the common stock.
 
Private Placement
Warrants
 
In June 2011, we sold shares of common stock and warrants to purchase common stock in a private placement. A total of 4,000,000 warrants to purchase common stock at an exercise price of $3.60 per share were issued as part of this private placement.
 
During the six months ended June 30, 2016, warrant holders exercised 2,965,477 warrants under the cashless exercise provision in each such holder’s warrant, which resulted in the net issuance of 2,395,700 shares of common stock and no net cash proceeds to us. During the six months ended June 30, 2015, warrant holders exercised 260,480 warrants under the cashless exercise provision in each such holder’s warrant, which resulted in the net issuance of 209,607 shares of common stock and no net cash proceeds to us. As of June 30, 2016, all warrants from the June 2011 Private Placement have been exercised.
 
Stock Option
Activity
 
The following table summarizes the stock option activity for the six months ended June 30, 2016:
 
 
 
 
 
 
 
 
 
 
 
Weighted-
 
 
 
 
 
 
 
 
 
 
 
Average
 
 
 
 
 
 
 
Weighted-
 
 
Remaining
 
 
 
 
 
 
 
Average
 
 
Contractual
 
 
 
Shares
 
 
Exercise
 
 
Term
 
 
 
(in thousands)
 
 
Price
 
 
(Years)
 
Balance at January 1, 2016
     8,435     $ 13.64       7.94  
Granted
     1,080     $ 20.76          
Exercised
     (531
)
   $ 8.96          
Expired and forfeited
     (237
)
   $ 24.81          
Balance at June 30, 2016
     8,747     $ 14.50       7.91  
 
For the six months ended June 30, 2016, 530,840 shares of common stock were issued pursuant to the exercise of stock options, resulting in proceeds to us of approximately $4,755,000. For the six months ended June 30, 2015, 713,381 shares of common stock were issued pursuant to the exercise of stock options, resulting in proceeds to us of approximately $6,306,000.
 
Stock-Based Compensation
 
The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements for the three and six months ended June 30, 2016 and 2015 (in thousands):
 
 
 
Three Months Ended
June 30
,
 
 
Six Months Ended
June 30,
 
 
 
2016
 
 
2015
 
 
2016
 
 
2015
 
                                 
Research and development
   $ 2,329     $ 919     $ 4,402     $ 1,725  
General and administrative
     1,798       1,470       3,465       2,472  
Sales and marketing
     1,704       752       3,319       1,494  
Stock-based compensation expense included in operating expenses
   $ 5,831     $ 3,141     $ 11,186     $ 5,691  
                                 
Impact on basic and diluted net loss per share
   $ 0.16     $ 0.10     $ 0.31     $ 0.19  
 
As of June 30, 2016, there was approximately $65,205,000 of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.2 years.
 
We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:
 
 
 
June 30,
 
 
 
20
16
 
 
20
15
 
Risk-free interest rate
     1.5 %     1.7 %
Dividend yield
     0.0 %     0.0 %
Volatility
     90.6 %     91.0 %
Expected life (years)
     6       6  
 
We estimate the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan at the beginning of each new offering period using the Black-Scholes option pricing model with the following assumptions:
 
 
 
June 30,
 
 
 
20
16
 
 
20
15
 
Risk-free interest rate
     0.4 %     0.1 %
Dividend yield
     0.0 %     0.0 %
Volatility
     85.1 %     59.1 %
Expected life (months)
     6       6