Kodiak Sciences Inc. Notes to Consolidated Financial Statements (in thousands, except share and per share data) 105 The Company estimated the fair value of the 2021 Feb PSO using the Black-Scholes valuation model. Significant assumptions utilized in estimating the fair value of 2021 Feb PSO include an expected volatility of 66%, a risk-free rate of 0.66%, expected dividend yield of 0%, and expected term of 5.94 years. In August 2021, the Company granted 478,750 performance-based stock options ("2021 Aug PSO"). These stock options will vest upon the achievement of specific f clinical development milestones with the percentage of shares earned being dependent on the relative total stockholder return over the performance period. As of December 31, 2023, the requisite performance criteria for the 2021 Aug PSO was not achieved and the shares were cancelled and returned to the plan. No stock-based compensation expense was recognized related to these stock options. Performance-based awards are recorded as expense beginning when vesting events are determined to be probabl a e. Stock-based compensation expense recognized during the years ended December 31, 2023, 2022 and 2021 for the performance-based equity awards was $2.2 million, $6.3 million and $11.4 million, respectively. 2018 Empl m oy l ee Share Purchase Plan l In August 2018, the Company adopted the 2018 Employee Share Purchase Plan (“ESPP”), which became effe f ctive on the business day prior to the effe f ctiveness of the registration statement relating to the IPO. A total of 460,000 shares of common stock were initially reserved for issuance under the ESPP. The initial offe f ring period of the ESPP was authorized by the Company’s board of directors and commenced on January 4, 2021. Each offe f ring period is twelve months long, with two purchase periods. ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the enrollment date or (2) the fair market value of the common stock on the exercise date. The Company issued 50,821 shares under the ESPP during the year ended December 31, 2023. Stock-based compensation expense recognized during the year ended December 31, 2023, 2022 and 2021 for the ESPP was $0.2 million, $0.2 million and $0.3 million, respectively. Stoc t k-Ba - sed Compensation Expe x nse Stock-based compensation is classified in the consolidated statements of operations and comprehensive loss as follows (in thousands): Year Ended December 31, 2023 2022 2021 Research and development $ 44,014 $ 59,288 $ 33,237 General and administrative 44,542 46,745 28,148 Total stock-based compensation $ 88,556 $ 106,033 $ 61,385 As of December 31, 2023, the Company had $200.9 million of unrecognized compensation expense related to unvested share-based awards including options granted under the LTPIP Program and ESPP, which is expected to be recognized over a weighted-average period of 2.87 years. Shares Subje b ct to Repu e rchase The Company has a right of repurchase with respect to unvested shares issued upon early exercise of options at an amount equal to the lower of (1) the exercise price of each restricted share being repurchased and (2) the fair market value of such restricted share at the time the Company’s right of repurchase is exercised. The Company’s right to repurchase these shares laps a es as those shares vest over the requisite service period. Shares purchased by employees pursuant to the early exercise of stock options are not deemed, for accounting purpos r es, to be issued until those shares vest according to their respective vesting schedules. Cash received for early exercised stock options is recorded as accrue r d liabi a lities and other current liabilities on the consolidated balance sheet and is reclassified to common stock and additional paid-in capi a tal as such shares vest. At December 31, 2023, there were no early exercised stock options subj u ect to the Company’s right of repurchase.