|6 Months Ended
Jun. 30, 2018
Total stock-based compensation related to all of our share-based payments that we recognized for the three and six months ended June 30, 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 and six months ended June 30, 2018 and 2017:
Starting in May 2018, the exercise price of stock options granted under our 2018 Plan is at the market price of our common stock on the date of grant. The exercise price of stock options under our prior equity incentive plans 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 4,486,225 shares of common stock during the six months ended June 30, 2018 with a grant-date weighted-average fair value of $2.68 per share. As of June 30, 2018, we have 2,790,000 shares related to outstanding performance-based stock option awards with a grant date fair value of $5.9 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 that have vested or wherein the achievement of the corresponding corporate-based milestones was probable. Accordingly, we recognized $1.6 million in stock-based compensation expense, of which $488,000 was recorded during the six months ended June 30, 2018.
As of June 30, 2018, there was approximately $15.5 million of unrecognized stock-based compensation cost related to all unvested time-based and performance-based stock options granted under our equity incentive plans, of which approximately $4.3 million related to the unvested performance stock options.
At June 30, 2018, there were 14,597,716 shares of common stock available for future grant under our equity incentive plan and 823,130 options to purchase shares were exercised during the six months ended June 30, 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.
As of June 30, 2018, there were 1,790,451 shares reserved for future issuance under the Purchase Plan and there were $2.2 million of unrecognized stock-based compensation cost related to our employee stock purchase plan. The following table summarizes the weighted-average assumptions related to our Purchase Plan for the three and six months ended June 30, 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.