Sec Form 4 Filing - Awad Antoine @ SYNLOGIC, INC. - 2023-11-10

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Awad Antoine 2. Issuer Name and Ticker or Trading Symbol SYNLOGIC, INC. [ SYBX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chief Operating Officer (Last) (First) (Middle) C/O SYNLOGIC, INC., 301 BINNEY STREET, SUITE 402 3. Date of Earliest Transaction (MM/DD/YY) 11/10/2023 (Street) CAMBRIDGE, MA02142 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (right to buy) $ 130.35 ( 1 ) 11/10/2023 D 3,666 ( 2 ) 02/28/2029 Common Stock 3,666 ( 1 ) 0 D Stock Option (right to buy) $ 128.85 ( 1 ) 11/10/2023 D 666 ( 2 ) 03/05/2029 Common Stock 666 ( 1 ) 0 D Stock Option (right to buy) $ 9.3 ( 1 ) 11/10/2023 D 6,666 ( 3 ) 03/16/2033 Common Stock 6,666 ( 1 ) 0 D Stock Option (right to buy) $ 25.5 ( 1 ) 11/10/2023 D 4,666 ( 4 ) 03/10/2030 Common Stock 4,666 ( 1 ) 0 D Stock Option (right to buy) $ 27.75 ( 1 ) 11/10/2023 D 4,666 ( 5 ) 07/14/2030 Common Stock 4,666 ( 1 ) 0 D Stock Option (right to buy) $ 52.5 ( 1 ) 11/10/2023 D 4,666 ( 6 ) 03/10/2031 Common Stock 4,666 ( 1 ) 0 D Stock Option (right to buy) $ 29.25 ( 1 ) 11/10/2023 D 11,031 ( 7 ) 02/04/2032 Common Stock 11,031 ( 1 ) 0 D Stock Option (right to buy) $ 9.3 ( 1 ) 11/10/2023 D 16,666 ( 8 ) 03/16/2033 Common Stock 16,666 ( 1 ) 0 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 3,666 ( 2 ) 02/28/2029 Common Stock 3,666 ( 1 ) 3,666 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 666 ( 2 ) 03/05/2029 Common Stock 666 ( 1 ) 666 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 6,666 ( 3 ) 03/16/2033 Common Stock 6,666 ( 1 ) 6,666 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 4,666 ( 4 ) 03/10/2030 Common Stock 4,666 ( 1 ) 4,666 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 4,666 ( 5 ) 07/14/2030 Common Stock 4,666 ( 1 ) 4,666 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 4,666 ( 6 ) 03/10/2031 Common Stock 4,666 ( 1 ) 4,666 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 11,031 ( 7 ) 02/04/2032 Common Stock 11,031 ( 1 ) 11,031 D Stock Option (right to buy) $ 1.85 ( 1 ) 11/10/2023 A 16,666 ( 8 ) 03/16/2033 Common Stock 16,666 ( 1 ) 16,666 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Awad Antoine C/O SYNLOGIC, INC. 301 BINNEY STREET, SUITE 402 CAMBRIDGE, MA02142 Chief Operating Officer Signatures /s/ Antoine Awad 11/14/2023 Signature of Reporting Person Date

Explanation of Responses:

( 1 )In accordance with the terms of the Synlogic, Inc. 2015 Equity Incentive Award Plan and the Company's 2017 Stock Incentive Plan (together the "Plans"), the Board of Directors of Synlogic, Inc. approved a stock option repricing pursuant to which option awards granted under the Plans were amended to reduce the exercise price to $1.85 per share, the closing stock price of the Company's Common Stock on November 10, 2023. Except for the modified exercise price, all other terms and conditions of each of the option awards will continue as set forth in the Plan and the applicable award agreements. ( 2 )The shares subject to this option are fully vested as of the date hereof. ( 3 )These performance-based stock options, granted on March 16, 2023, vest when the achievement of certain clinical and regulatory development milestones are met. ( 4 )The shares subject to this option vest in approximately equal monthly installments beginning on April 1, 2020 through April 1, 2024. ( 5 )The shares subject to this option vest in approximately equal monthly installments beginning on July 1, 2020 through July 1, 2024. ( 6 )The shares subject to this option vest in approximately equal monthly installments beginning on April 1, 2021 through April 1, 2025. ( 7 )The shares subject to this option vest in approximately equal monthly installments beginning on February 4, 2022 through February 4, 2026. ( 8 )The shares subject to this option vest in approximately equal monthly installments beginning on April 1, 2023 through April 1, 2027.

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