6.Stock-based Compensation

 

Total stock-based compensation expense related to all of our share-based payments that we recognized for the three months ended March 31, 2018 and 2017 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 account for forfeitures as they occur.

 

We determined weighted-average valuation assumptions separately for each of these groups as follows:

 

 

 

 

 

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, 2018 and 2017:

 

 

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 3,396,975 shares of common stock during the three months ended March 31, 2018 with a grant-date weighted-average fair value of $2.79 per share. As of March 31, 2018, we have 2,672,500  shares related to outstanding performance-based stock option awards with a grant date fair value of $5.7 million which will vest upon achievement of certain corporate performance-based milestones. Of this amount, 1,160,000 shares related to performance-based stock option awards wherein the achievement of the corresponding corporate-based milestones was probable.  Accordingly, we recognized $1.6 million in stock-based compensation expense, of which $402,000 was recorded in the first quarter of 2018.  As of March 31, 2018, there was approximately $4.2 million of unrecognized compensation cost related to these outstanding performance stock options.

 

As of March  31, 2018, there was approximately $14.2 million of unrecognized stock-based compensation cost related to all unvested time-based and performance-based stock options granted under our equity incentive plans.

 

At March  31, 2018, there were 10,544,883 shares of common stock available for future grant under our equity incentive plans and 652,891 options to purchase shares were exercised during the three months ended March 31, 2018.

 

Employee Stock Purchase Plan

 

Our Purchase Plan permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering periods. The price at which the stock is purchased is equal to the lesser of 85% of the fair market value of the common stock on the first day of the offering or 85% of the fair market value of our common stock on the purchase date. The initial offering period commenced on the effective date of our initial public offering.

 

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 July 1, 2016 because the fair market value of our stock on June 30, 2016 was lower than the fair market value of our stock on January 5, 2015, 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 Plans. The total incremental fair value for this Purchase Plan “reset” was approximately $1.0 million and is being recognized as expense from July 1, 2016 to June 30, 2018.

 

As of March  31, 2018, there were approximately 2,115,568 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, 2018 and 2017. 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.