Employee Stock Purchase Plan

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

On August 17, 2021, a special meeting of the Company’s stockholders was held to approve the Merger and related matters, at which the Company's stockholders considered and approved the Company’s 2021 ESPP which permits participants to contribute up to 15% of their eligible compensation during defined rolling six-month offering periods to purchase the Company’s common stock. The purchase price of the shares will be 85% of the lower of the fair market value of the Company’s common stock on the first day of trading of the offering period or on the applicable purchase date. Upon approval of the 2021 ESPP by the stockholders, Aerpio’s Amended and Restated 2017 Employee Stock Purchase Plan terminated. An aggregate of 519,563 shares of common stock was initially reserved for issuance under the 2021 ESPP. The number of shares of common stock available for issuance under the 2021 ESPP is increased on the first day of each fiscal year beginning with the 2022 fiscal year in an amount equal to the least of (i) 310,617 shares of common stock, (ii) one percent (1%) of the outstanding shares of all classes of common stock on the last day of the immediately preceding fiscal year, or (iii) an amount to be determined by the board of directors or its designated committee no later than the last day of the immediately preceding fiscal year. On January 1, 2024, 245,542 shares of common stock were added to the 2021 ESPP. Shares of common stock issuable under the 2021 ESPP will be authorized, but unissued, or reacquired shares of common stock. If the Company’s capital structure changes because of a stock dividend, stock split or similar event, the number of shares that can be issued under the 2021 ESPP will be appropriately adjusted. The Company opened enrollment into the ESPP in May 2022.

The Company uses the Black-Scholes model to determine the estimated fair value for purchases under the 2021 ESPP. Black-Scholes models require the input of various assumptions, including the expected life, expected dividend yield, price volatility and risk-free interest rate of the underlying stock. The expected volatility used in calculating the estimated fair value for purchases under the 2021 ESPP is based on the historical volatility of the Company's common stock.

The calculation was based on the following assumptions:


	Year Ended December 31,
	2023	2022
Strike price (per share)	$4.00 - $5.98	$11.40 - $11.66
Risk-free interest rate	5.24% - 5.41%	1.54% - 4.40%
Expected volatility	60.99% - 176.11%	92.59% - 105.68%
Expected term (in years)	0.5	0.5
Expected dividend yield	—	—

As of December 31, 2023 and 2022, 659,146 and 492,361 shares of common stock were available for issuance under the 2021 ESPP, respectively. The Company had an outstanding liability of $0.1 million and $76,000 at December 31, 2023 and 2022, respectively, which will be recognized over six months. During the years ended December 31, 2023 and 2022, 77,565 and 27,202 shares were issued under the 2021 ESPP, respectively.