9. Stock-Based Payments

2016 Stock Incentive Plan

The 2016 Stock Incentive Plan (the “2016 Plan”) was adopted by the board of directors on December 15, 2015 and approved by the stockholders on June 17, 2016, and became effective on July 6, 2016 upon the closing of the Company’s initial public offering (“IPO”). The 2016 Plan replaced the 2012 Equity Incentive Plan (the “2012 Plan”). Any options or awards outstanding under the 2012 Plan remained outstanding and effective. Under the 2016 Plan, the Company may grant incentive stock options, non-statutory stock options, stock appreciation rights, restricted stock, restricted stock units and other stock-based awards. The number of shares of the Company’s common stock reserved for issuance under the 2016 Plan automatically increases on the first day of each calendar year, through the 2025 calendar year, in an amount equal to the least of (i) 1,600,000 shares of common stock, (ii) 4.0% of the outstanding shares of common stock as of such date, or (iii) such lesser amount as specified by the board of directors. This number is subject to adjustment in the event of a stock split, stock dividend or other change in the Company’s capitalization. For the calendar year beginning January 1, 2019, the number of shares reserved for issuance under the 2016 Plan was increased by 1,350,634 shares. At March 31, 2019, 2,223,902 shares remained available for future issuance under the 2016 Plan. Under the 2016 Plan, stock options may not be granted at less than fair value on the date of grant.

2016 Employee Stock Purchase Plan

The 2016 Employee Stock Purchase Plan (the “2016 ESPP”) was adopted by the board of directors on December 15, 2015 and approved by the stockholders on June 17, 2016, and became effective on July 6, 2016 upon the closing of the IPO. The number of shares of the Company’s common stock reserved for issuance under the 2016 ESPP automatically increases on the first day of each calendar year through the 2025 calendar year, in an amount equal to the least of (i) 1,173,333 shares of the Company’s common stock, (ii) 1.0% of the total number of shares of the Company’s common stock outstanding on the first day of the applicable year, and (iii) an amount determined by the Company’s board of directors. For the calendar year beginning January 1, 2019, the number of shares reserved for issuance under the 2016 ESPP was increased by 337,658 shares. At March 31, 2019, 1,422,414 shares remained available for future issuance under the 2016 ESPP.

Stock Options

Terms of stock option agreements, including vesting requirements, are determined by the board of directors, subject to the provisions of the 2016 Plan. Stock option awards granted by the Company generally vest over four years, with 25% vesting on the first anniversary of the vesting commencement date and 75% vesting ratably, on a monthly basis, over the remaining three years. Such awards are exercisable from the date of grant for a period of ten years.

The Company has granted stock options to management for which vesting accelerates upon the achievement of performance-based criteria. Milestone events are specific to the Company’s corporate goals, including but not limited to certain clinical development milestones and the Company’s ability to execute on its corporate development and financing strategies. Stock-based compensation expense associated with these performance-based stock options is recognized based on the accelerated attribution model. Management evaluates when the achievement of a performance-based milestone is probable based on the expected satisfaction of the performance conditions as of the reporting date. Notwithstanding any vesting in accordance with the achievement of performance-based milestones, such awards vest in full on the sixth anniversary of the vesting commencement date. The Company did not record any additional stock-based compensation expense related to the achievement of performance-based milestones during the three months ended March 31, 2019. As of March 31, 2019, there was $1.0 million of unrecognized stock-based compensation expense related to the performance-based stock options granted to management, with an expected recognition period of 3.5 years.

The Company has granted options to purchase 75,000 shares of common stock to an advisor that vest upon the achievement of performance-based criteria. As of March 31, 2019, no such performance-based criteria had been achieved. As of March 31, 2019, there was $0.3 million of unrecognized compensation cost related to this option, with a remaining contractual period of 7.5 years.

A summary of the status of stock options as of December 31, 2018 and March 31, 2019 and changes during the three months ended March 31, 2019 is presented below:

The intrinsic value of stock options exercised during the three months ended March 31, 2019 and 2018 was $4,800 and $0.7 million, respectively.

As of March 31, 2019, there was $18.6 million of total unrecognized compensation cost related to non-vested stock options granted to employees, excluding those stock option grants subject to the achievement of performance milestones, which is expected to be recognized over a weighted-average period of 3.1 years. 

Restricted Stock Units

From time to time, upon approval by the Company’s board of directors, certain employees have been granted restricted stock units with time-based vesting criteria. The majority of these restricted stock units vest annually over a four-year term with 25% vesting on each anniversary of the grant date. Those restricted stock units granted to the Company’s executive officers during the quarter ended March 31, 2019, vest in full on March 31, 2022. The fair value of restricted stock units is calculated based on the closing sale price of the Company’s common stock on the date of grant.

A summary of the status of restricted stock units as of December 31, 2018 and March 31, 2019 and changes during the three months ended March 31, 2019 is presented below:

As of March 31, 2019, there was $6.9 million of unrecognized stock-based compensation expense related to outstanding restricted stock units, with an expected recognition period of 3.4 years.

Stock-based Compensation Expense

The fair value of each stock option granted was estimated on the date of grant using the Black-Scholes option-pricing model based on the following weighted-average assumptions:

The weighted-average grant date fair value per share of options granted in the three months ended March 31, 2019 and 2018 was $5.16 and $7.93, respectively.

The following table summarizes the stock-based compensation expense for stock options and restricted stock units granted to employees and non-employees recorded in the Company’s condensed consolidated statements of operations:

Due to an operating loss, the Company does not record tax benefits associated with stock‑based compensation or option exercises. Tax benefits will be recorded when realized.