|6 Months Ended
Jun. 30, 2015
Total stock-based compensation expense related to all of our share-based payments that we recognized for the three and six months ended June 30, 2015 and 2014 were as follows (in thousands):
The fair value of each option award is estimated on the date of grant using the Black-Scholes option pricing model. We have segregated option awards into the following three homogenous groups for the purposes of determining fair values of options: officers and directors, all other employees, and consultants.
We determined weighted-average valuation assumptions separately for each of these groups as follows:
Pursuant to FASB ASC 718, we are required to estimate the amount of expected forfeitures when calculating compensation costs. We estimated the forfeiture rate using our historical experience with non-vested options. We adjust our stock-based compensation expense as actual forfeitures occur, review our estimated forfeiture rates each quarter and make changes to our estimate as appropriate.
The following table summarizes the weighted-average assumptions relating to options granted pursuant to our equity incentive plans for the three and six months ended June 30, 2015 and 2014:
The exercise price of stock options is at the market price of our common stock on the date immediately preceding the date of grant. Options become exercisable at varying dates and generally expire 10 years from the date of grant. We granted options to purchase 3,793,420 shares of common stock during the six months ended June 30, 2015, with a grant-date weighted-average fair value of $1.40 per share. Of the 3,793,420 common stock options granted, 1,175,000 shares were related to performance-based stock option awards which will vest upon the achievement of a corporate performance-based milestone. We granted options to purchase 3,397,275 shares of common stock during the six months ended June 30, 2014, with a grant-date weighted-average fair value of $2.35 per share. Of the 3,397,275 common stock options granted, 950,000 shares were related to performance-based stock option awards, of which only 700,000 shares remain outstanding, which will vest upon the achievement of certain corporate performance-based milestones. As of June 30, 2015, there was approximately $7.3 million of total unrecognized stock-based compensation cost, net of estimated forfeitures, related to unvested options granted under our equity incentive plans. At June 30, 2015, there were 4,272,986 shares of common stock available for future grant under our equity incentive plans and 155,299 options to purchase shares were exercised during the six months ended June 30, 2015.
Employee Stock Purchase Plan
The fair value of awards granted under our Purchase Plan is estimated on the date of grant using the Black-Scholes option pricing model, which uses weighted- average assumptions. Our Purchase Plan provides for a twenty-four month offering period comprised of four six-month purchase periods with a look-back option. A look-back option is a provision in our Purchase Plan under which eligible employees can purchase shares of our common stock at a price per share equal to the lesser of 85% of the fair market value on the first day of the offering period or 85% of the fair market value on the purchase date. Our Purchase Plan also includes a feature that provides for a new offering period to begin when the fair market value of our common stock on any purchase date during an offering period falls below the fair market value of our common stock on the first day of such offering period. This feature is called a “reset.” Participants are automatically enrolled in the new offering period. We had a “reset” on January 2, 2015 because the fair market value of our stock on December 31, 2014 was lower than the fair market value of our stock on July 1, 2014, the first day of the offering period. We applied modification accounting in accordance with ASC Topic No. 718, Stock Compensation, to determine the incremental fair value associated with this Purchase Plan “reset” and will recognize the related stock-based compensation expense according to FASB ASC Subtopic No. 718-50, Employee Share Purchase Plan. The total incremental fair value for this Purchase Plan “reset” was approximately $792,000, and is being recognized from January 2, 2015 to December 31, 2016.
As of June 30, 2015, there were approximately 3,290,397 shares reserved for future issuance under the Purchase Plan. The following table summarizes the weighted-average assumptions related to our Purchase Plan for the six months ended June 30, 2015 and 2014. Expected volatilities for our Purchase Plan are based on the historical volatility of our stock. Expected term represents the weighted-average of the purchase periods within the offering period. The risk-free interest rate for periods within the expected term is based on U.S. Treasury constant maturity rates.