X0206 3 2017-11-01 0 0001701108 Spero Therapeutics, Inc. SPRO 0001720875 Sendek Joel C/O SPERO THERAPEUTICS, INC. 675 MASSACHUSETTS AVENUE, 14TH FLOOR CAMBRIDGE MA 02139 0 1 0 0 Chief Financial Officer Series C Preferred Stock Common Stock 10181 I Joel D. Sendek Retained Annuity Trust No. 1 Stock Option (Right to Buy) 5.90 2027-07-05 Common Stock 67852 D Stock Option (Right to Buy) 5.90 2027-07-05 Common Stock 99981 D The shares of Series C preferred stock are convertible into the number of shares of common stock shown in Column 3, calculated using the conversion ratio set forth in the Issuer's Amended and Restated Certificate of Incorporation, as amended, at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The shares have no expiration date. The share number gives effect to the 1-for-6.0774 reverse stock split of the Issuer's common stock effected on October 20, 2017. The shares underlying this option vest as to 25% on May 1, 2018, with the remainder vesting in 36 equal monthly installments thereafter, subject to the Reporting Person's continued service through the applicable vesting date. The number of underlying shares of common stock and the exercise price reported reflect a 1-for-6.0774 reverse stock split of the Issuer's common stock effected on October 20, 2017. The shares underlying this option vest as to 25% on May 1, 2018, with the remainder vesting in 36 equal monthly installments thereafter, subject to the Reporting Person's continued service through the applicable vesting date. Exhibit 24.1 - Power of Attorney /s/ Matthew Tikonoff, Attorney-in-Fact for Joel Sendek 2017-11-01