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Theseus Pharmaceuticals Initial Dose Escalation Results from Ongoing Phase 1/2 Trial of THE-630 Accepted for Online Publication at 2023 ASCO Annual Meeting

-- Company to host virtual investor event to review updated data on May 25, 2023 at 5:30pm ET --

CAMBRIDGE, Mass., April 26, 2023 /PRNewswire/ -- Theseus Pharmaceuticals, Inc. (NASDAQ: THRX) (Theseus or the Company), a clinical-stage biopharmaceutical company focused on improving the lives of cancer patients through the discovery, development, and commercialization of transformative targeted therapies, today announced that initial dose escalation data from the ongoing phase 1/2 study of THE-630 in advanced gastrointestinal stromal tumors (GIST) were accepted for online publication at the 2023 American Society of Clinical Oncology (ASCO) Annual Meeting, taking place June 2-6, in Chicago, Illinois.

The abstract selected for online publication was submitted on February 14, 2023. On May 25, 2023, Theseus plans to host a virtual investor event to present data with an updated cutoff date. The data to be presented from the ongoing dose escalation study are expected to include preliminary safety, pharmacokinetic (PK), and clinical activity data through cohort 6, as well as an analysis of circulating tumor DNA (ctDNA) data through cohort 5.

THE-630 is a pan-variant tyrosine kinase inhibitor (TKI) of the receptor tyrosine kinase KIT, designed for patients with GIST that have developed resistance to earlier lines of therapy. The primary objectives of the phase 1 dose escalation portion of the study are to evaluate the safety profile of THE-630, including the determination of a recommended phase 2 dose (RP2D) in GIST patients who have received imatinib and at least one other TKI. Secondary objectives include determining the PK profile of THE-630, and to characterize preliminary evidence of antitumor activity.

Virtual Investor Event

Theseus will host a virtual investor event to review these new data on Thursday, May 25, 2023, beginning at 5:30pm ET. The event will be webcast live and can be accessed in the Events section of the Company's investor relations website at ir.theseusrx.com. A replay of the webcast will be archived and available for 30 days following the event.

About Theseus Pharmaceuticals, Inc.

Theseus is a clinical-stage biopharmaceutical company focused on improving the lives of cancer patients through the discovery, development, and commercialization of transformative targeted therapies. Theseus is working to outsmart cancer resistance by developing pan-variant tyrosine kinase inhibitors (TKIs) to target all classes of cancer-causing and resistance mutations that lead to clinically relevant variants in a particular protein in a given type of cancer. Theseus' lead product candidate, THE-630, is a pan-variant KIT inhibitor for the treatment of patients with advanced gastrointestinal stromal tumors (GIST), whose cancer has developed resistance to earlier lines of kinase inhibitor therapy. Theseus is also developing THE-349, a fourth-generation, selective epidermal growth factor receptor (EGFR) inhibitor for C797X-mediated resistance to first- or later-line osimertinib treatment in patients with non-small cell lung cancer (NSCLC), and a pan-variant BCR-ABL inhibitor for the treatment of relapsed/refractory chronic myeloid leukemia (CML) and newly diagnosed Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL). For more information, visit www.theseusrx.com.

Cautionary Statement Regarding Forward Looking Statements

Certain statements included in this press release are not historical facts but are forward-looking statements for purposes of the safe harbor provisions under the United States Private Securities Litigation Reform Act of 1995. Forward-looking statements generally are accompanied by words such as "believe," "may," "will," "estimate," "anticipate," "expect," "plan," "predict," "potential," "seem," "outlook," "continue," "intend," and similar expressions that predict or indicate future events or trends or that are not statements of historical matters, but the absence of these words does not mean that a statement is not forward-looking. These forward-looking statements include, but are not limited to, statements regarding: Theseus' strategy, future operations, prospects and plans; the structure and timing of its preclinical studies and clinical trials, the timing for the presentation of preliminary data for THE-630, and Theseus' plans regarding the phase 1/2 dose escalation and expansion clinical trial for THE-630.

Actual results may differ materially from those indicated by such forward-looking statements as a result of various important risks, uncertainties and other factors, including, but not limited to: uncertainties inherent in preclinical studies and clinical trials; risks and uncertainties regarding whether results from preclinical studies and clinical trials will be predictive of the results of future trials; risks related to the expected timing of submissions to regulatory authorities; and other risks, uncertainties and other factors such as those described from time to time in the reports Theseus files with the Securities and Exchange Commission (SEC), including Theseus' Form 10-K for the year ended December 31, 2022 and subsequent Quarterly Reports on Form 10-Q which are on file with the SEC and available on the SEC's website at https://www.sec.gov/. However, new risk factors and uncertainties may emerge from time to time which may cause actual results to differ materially from those anticipated or implied by the forward-looking statements in this press release, and it is not possible to predict all risk factors and uncertainties. Accordingly, readers are cautioned not to place undue reliance on these forward-looking statements. Any forward-looking statements contained in this press release are based on the current expectations of Theseus' management team and speak only as of the date hereof, and Theseus specifically disclaims any obligation to update any forward-looking statement, whether as a result of new information, future events or otherwise, except as required by law.

Media Contact
Christen Baglaneas
Director, Corporate Communications & Investor Relations
Theseus Pharmaceuticals
857-706-4993
christen.baglaneas@theseusrx.com

Investor Contact
Josh Rappaport
Stern Investor Relations
212-362-1200
josh.rappaport@sternir.com

SOURCE Theseus Pharmaceuticals