8. Stock-based compensation

2014 Stock Plan

Pieris granted 88,853 and 1,157,734 options to employees, consultants, and directors under its 2014 employee, director, and consultant equity incentive plan, (the “2014 Plan”) during the three months ended six months ended June 30, 2016, respectively. The 2014 Plan was terminated on June 28, 2016 when the Company adopted its 2016 employee, director and consultant equity incentive plan, (the “2016 Plan”). Therefore, no options were granted for the three and six months ended June 30, 2017 under the 2014 Plan.

2016 Stock Plan

In June 2016, the Company adopted the 2016 Plan which provides for the grant of stock options, restricted and unrestricted stock awards, and other stock-based awards to employees of the Company, non-employee directors of the Company, and certain other consultants performing services for the Company as designated by the Compensation Committee of the Board of Directors or the Board of Directors. The vesting periods of equity incentives issued under the 2016 Plan are determined by the Compensation Committee of the Company’s Board of Directors, with stock options generally vesting over a four-year period.

The Company granted 118,367 and 1,258,755 options to employees and directors under the 2016 Plan during the three months ended June 30, 2017 and the six months ended June 30, 2017, respectively. No options were granted under the 2016 Plan during the same periods or 2016. As of June 30, 2017, there were 2,094,670 shares available for future grant under the 2016 Plan. The shares available for future grant under the 2016 Plan include 217,530 shares which were forfeited under the 2016 Plan and 11,208 shares which were forfeited under the 2014 Plan. These forfeited shares are available for future issuance under the 2016 Plan.

Stock-based compensation expense was $0.6 million and $1.3 million for the three months ended June 30, 2017 and the six months ended June 30, 2017, respectively. For the three months ended June 30, 2016 and the six months ended June 30, 2016, stock based compensation expense was $0.6 million and $1.0 million, respectively.

Total stock-based compensation expense was recorded to operating expenses based upon the functional responsibilities of the individuals holding the respective options as follows:

There were an aggregate of 15,000 and 15,000 options exercised under the 2014 Plan during the three months ended June 30, 2017 and the six months ended June 30, 2017, respectively, for which the Company received $30,000 in cash. There were 74,462 and 74,462 options exercised under the 2016 Plan during the three months ended June 30, 2017 and the six months ended June 30, 2017, respectively, for which the Company received $147,950 in cash. No options were exercised during the 2016 periods.

The Company uses the Black-Scholes option pricing model to determine the estimated fair value for stock-based awards. Option-pricing models require the input of various subjective assumptions, including the option’s expected life, expected dividend yield, price volatility, risk free interest rate, and forfeitures of the underlying stock. Accordingly, the weighted-average fair value of the options granted was $2.08 and $1.39 for the three months ended June 30, 2017 and the six months ended June 30, 2017. The weighted-average fair value of the options granted was $1.09 and $1.01 for the three months ended June 30, 2016 and the six months ended June 30, 2016.

The calculation was based on the following assumptions:

Option-pricing models require the input of various subjective assumptions, including the option’s expected life and the price volatility of the underlying stock. Pieris’ estimated expected stock price volatility is based on the average volatilities of other guideline companies in the same industry. Pieris’ expected term of options granted during the three and six months ended June 30, 2017 and 2016, respectively was derived using the SEC’s simplified method. The risk-free rate for the expected term of the option is based on the U.S. Treasury yield curve in effect at the time of grant.

The Company’s stock options have a maximum term of ten years from the date of grant. Stock options granted under the 2016 Plan may be either incentive stock options, or nonqualified stock options. The exercise price of stock options granted under the 2016 Plan must be at least equal to the fair market value of the common stock on the date of grant.