UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934 (Amendment No.  )

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

☐	Preliminary Proxy Statement
☐	Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))
☐	Definitive Proxy Statement
☒	Definitive Additional Materials
☐	Soliciting Material under §240.14a-12

THESEUS PHARMACEUTICALS, INC.
(Name of Registrant as Specified In Its Charter)
(Name of Person(s) Filing Proxy Statement, if other than the Registrant)
Payment of Filing Fee (Check the appropriate box):
☒	No fee required.
☐	Fee paid previously with preliminary materials.
☐	Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11.

Your Vote Counts! THESEUS PHARMACEUTICALS, INC. 2022 Annual Meeting Vote by June 9, 2022 11:59 PM EDT THESEUS PHARMACEUTICALS, INC. 314 MAIN ST CAMBRIDGE, MASSACHUSETTS 02142 D80459-P74003 You invested in THESEUS PHARMACEUTICALS, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the stockholder meeting to be held on June 10, 2022. Get informed before you vote View the Notice and Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 27, 2022. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. Smartphone users Point your camera here and vote without entering a control number Vote Virtually at the Meeting* June 10, 2022 10:00 a.m., Eastern Daylight Time Virtually at: www.virtualshareholdermeeting.com/THRX2022 *Please check the meeting materials for any special requirements for meeting attendance. V1.1 For complete information and to vote, visit www.ProxyVote.com Control #

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Board Recommends Voting Items D80460-P74003 Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Sign up for E-delivery”. 1. To elect two Class I directors to serve until the 2025 Annual Meeting of stockholders or until their respective successors have been elected or appointed. Nominees: 01) Iain D. Dukes 02) Kathy Yi For 2. To ratify the appointment of Ernst & Young LLP as the independent registered public accounting firm for the fiscal year ending December 31, 2022. For NOTE: Such other business as may properly come before the meeting or any adjournment thereof.