5.              Stock-based Compensation

Total stock-based compensation expense related to all of our share-based payments that we recognized in the quarters ended March 31, 2014 and 2013 was 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:

·                  Volatility—We estimated volatility using our historical share price performance over the expected life of the option. We also considered other factors, such as implied volatility, our current clinical trials and other company activities that may affect the volatility of our stock in the future. We determined that at this time historical volatility is more indicative of our expected future stock performance than implied volatility.

·                  Expected term—For options granted to consultants, we use the contractual term of the option, which is generally ten years, for the initial valuation of the option and the remaining contractual term of the option for the succeeding periods. We analyzed various historical data to determine the applicable expected term for each of the other option groups. This data included: (1) for exercised options, the term of the options from option grant date to exercise date; (2) for cancelled options, the term of the options from option grant date to cancellation date, excluding non-vested option forfeitures; and (3) for options that remained outstanding at the balance sheet date, the term of the options from option grant date to the end of the reporting period and the estimated remaining term of the options. The consideration and calculation of the above data gave us reasonable estimates of the expected term for each employee group. We also considered the vesting schedules of the options granted and factors surrounding exercise behavior of the option groups, our current market price and company activity that may affect our market price. In addition, we considered the optionee type (i.e., officers and directors or all other employees) and other factors that may affect the expected term of the option.

·                  Risk-free interest rate—The risk-free interest rate is based on U.S. Treasury constant maturity rates with similar terms to the expected term of the options for each option group.

·                  Dividend yield—The expected dividend yield is 0% as we have not paid and do not expect to pay dividends in the future.

Pursuant to Financial Accounting Standards Board (FASB) Accounting Standards Codification (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 months ended March 31, 2014 and 2013:

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 3,188,150 shares of common stock during the three months ended March 31, 2014, with a grant-date weighted-average fair value of $2.36 per share. Of the 3,188,150 common stock options granted, 950,000 shares were related to performance-based stock option awards which will vest upon the achievement of certain corporate performance-based milestones related to the progress and success of the Phase 3 clinical program of fostamatinib in ITP. We granted options to purchase 2,018,691 shares of common stock during the three months ended March 31, 2013, with a grant-date weighted-average fair value of $4.02 per share.  As of March 31, 2014, there was approximately $12.0 million of total unrecognized stock-based compensation cost, net of estimated forfeitures, related to unvested options granted under our equity incentive plans. At March 31, 2014, there were 6,290,247 shares of common stock available for future grant under our equity incentive plans and 6,968 options to purchase shares were exercised during the three months ended March 31, 2014.

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, 2014 because the fair market value of our stock on December 31, 2013 was lower than the fair market value of our stock on July 1, 2013, 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 $577,000, and is being recognized from January 2, 2014 to December 31, 2015.

As of March 31, 2014, there were approximately 84,030 shares reserved for future issuance under the Purchase Plan. The following table summarizes the weighted-average assumptions related to our Purchase Plan for the three months ended March 31, 2014 and 2013. 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.