APLS 2017 Annual Report

102 The following table summarizes the Company’s stock option activity: Weighted - Weighted - Weighted - Average Average Average Exercise Grant Date Contractual Aggregate Price Fair Value Life Intrinsic Shares Per Share Per Share (in years) Value Outstanding, January 1, 2015 2,343,090 $ 2.50 $ 1.56 7.52 $ 578,128 Granted 210,970 4.82 3.26 9.50 65,000 Exercised (732) 2.35 1.43 — 1,031 Forfeited (79,113) 2.13 1.54 — 128,250 Outstanding, December 31, 2015 2,474,215 2.71 1.71 7.01 2,870,775 Granted 1,380,463 3.26 1.98 9.37 — Forfeited (23,441) 2.43 1.13 9.83 — Outstanding, December 31, 2016 3,831,237 2.84 1.75 7.20 243,000 Granted 2,940,444 6.94 3.61 9.67 41,878,351 Exercised (46,881) 3.51 2.25 — 852,818 Forfeited (369,219) 2.56 1.22 — 7,066,915 Outstanding, December 31, 2017 6,355,581 4.78 2.66 7.64 107,534,863 Options exercisable, December 31, 2015 1,519,866 2.43 1.58 6.10 2,058,419 Expected to vest, December 31, 2015 954,349 3.16 1.90 8.46 812,365 Options exercisable, December 31, 2016 2,048,318 2.56 1.64 5.76 243,000 Expected to vest, December 31, 2016 1,782,919 3.16 1.90 8.85 — Options exercisable, December 31, 2017 3,156,433 3.67 2.12 5.90 56,918,296 Expected to vest, December 31, 2017 3,199,149 5.88 3.19 9.35 50,616,567 The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the common stock as of December 31, 2017. Estimated fair values of the common stock at the time of the grants between May 12, 2010 and December 31, 2017 were between $1.71 and $14.00. Total share-based compensation expense recognized was as follows: Year Ended December 31, 2015 2016 2017 Research and development $ 171,388 $ 377,776 $ 2,678,956 General and administrative 374,313 701,212 2,739,754 Total share-based compensation expense $ 545,701 $ 1,078,988 $ 5,418,710 At December 31, 2015, 2016 and 2017, unrecognized compensation expense related to unvested options, net of estimated forfeitures, was $1,673,110, $2,910,870 and $10,415,787, respectively, which the Company expects to recognize over an estimated weighted-average period of 2.36, 2.85, and 3.14 years, respectively. As of December 31, 2017, the future amortization of unearned share-based compensation costs will be $4,693,212 in 2018, $2,548,514 in 2019, $1,949,248 in 2020 and $1,224,813 in 2021. The assumptions used in the Monte Carlo and Black-Scholes models to estimate the grant date fair value are as follows: Year Ended December 31, 2015 2016 2017 Risk-free interest rate 1.61 - 1.87% 1.21 - 1.60% 1.21 - 2.30% Dividend yield 0% 0% 0% Volatility 78.1 - 93.5% 52.0 - 78.1% 51.6 - 55.8% Expected terms (years) 5.4 - 6.2 5.2 - 5.7 5.3 - 7.0 2017 Employee Stock Purchase Plan In October 2017, the Company’s board of directors adopted and the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (“ESPP”), which became effective upon the IPO and provides participating employees with the opportunity to purchase up to an aggregate of 468,823 shares of our common stock. The number of shares of our common stock reserved for issuance