Symbiotic-GYN-18: A Study to Learn About the Study Medicine Called PF-08634404 in Combination With Chemotherapy in Adult Participants With Advanced or Recurrent MMR-proficient Endometrial Cancer

AN INTERVENTIONAL PHASE 3, OPEN LABEL, RANDOMIZED STUDY TO EVALUATE EFFICACY AND SAFETY OF PF-08634404 IN COMBINATION WITH CHEMOTHERAPY VERSUS PEMBROLIZUMAB IN COMBINATION WITH CHEMOTHERAPY IN ADULT PARTICIPANTS WITH ADVANCED OR RECURRENT MISMATCH REPAIR PROFICIENT ENDOMETRIAL CANCER

This study is being conducted to assess whether the study medicine PF 08634404, given in combination with chemotherapy, improves outcomes compared with another medicine called pembrolizumab plus chemotherapy. Chemotherapy is a type of cancer treatment that uses medicines to destroy cancer cells or stop them from growing.

Our bodies have a built-in DNA "spell-checker," called the mismatch repair (MMR) system, that fixes genetic mistakes. In most endometrial cancers, this system works normally, and these cancers are called MMR-proficient (pMMR). However, pMMR tumors are harder for the immune system to recognize and attack. When endometrial cancer has spread beyond the uterus or comes back after previous treatment, it is called advanced or recurrent endometrial cancer. This study is for adults with mismatch repair-pMMR advanced or recurrent endometrial cancer.

Participants must meet key criteria, including:

• Women who are 18 years or older, and not pregnant at the time of joining the study
• pMMR endometrial cancer only
• Measurable Stage III disease, Stage IV disease (with or without measurable disease) per FIGO staging, a system doctors use to describe how far cancer has spread in the body, or recurrent endometrial cancer
• Has not received chemotherapy except for chemotherapy given after the main surgery and more than 6 months before relapse
• Be in good enough health to receive study treatment. Approximately 600 adult women will be enrolled. Each participant will be randomly assigned (like a flip of the coin) to one of two treatment groups, with about half in each group. The study is open, meaning both the doctors and participants know what treatment is being given. Participants will receive their assigned treatment through intravenous infusions (medicine is given directly into a vein). The treatment will be given in cycles.

Experimental Group will receive new study medicine called PF-08634404 plus chemotherapy. It will be followed by PF 08634404 alone for up to 2 years (35 cycles).

Control Group will receive an approved medicine called pembrolizumab plus chemotherapy. It will be followed by pembrolizumab alone for up to 2 years (20 cycles).

The study will include regular visits for:

• Participants will have regular visits to the study site for treatment, health checks, and tests.
• After stopping treatment, participants will come for a final visit within a month to check their health and review any reactions.
• Follow-up will continue every 12 weeks by phone or in person or by reviewing health records. This helps check health and any new treatments.
• Tests will be done every 9 weeks during the first 104 weeks to see how the cancer is responding. After that, tests will be done every 12 weeks.

Study Overview

• Status: Not yet recruiting
• Conditions: Endometrial Neoplasms; Endometrial Cancer
• Intervention / Treatment: Biological: PF-08634404; Drug: Standard of Care Chemotherapy; Biological: Pembrolizumab

• Study Type: Interventional
• Enrollment (Estimated): 600
• Phase: Phase 3

Contacts and Locations

• Study Contact: Name: Pfizer CT.gov Call Center; Phone Number: 1-800-718-1021; Email: ClinicalTrials.gov_Inquiries@pfizer.com

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

• Women ≥18 years of age who are confirmed not pregnant at screening.
• Histologically or cytologically confirmed endometrial cancer that is recurrent or advanced; carcinosarcomas are eligible but pure sarcomas are excluded.
• Newly diagnosed FIGO Stage III disease with measurable disease per RECIST v1.1, newly diagnosed FIGO Stage IV disease with or without measurable disease, or recurrent disease with or without measurable disease per RECIST v1.1 for which curative treatment with surgery and/or radiotherapy is unlikely.
• For participants with recurrent disease, prior adjuvant systemic anti-cancer therapy is allowed if relapse occurred more than 6 months after the last dose.
• Must provide tumor tissue for prospective central assessment of MMR and p53 status.
• Proficient mismatch repair (pMMR) endometrial cancer as determined by central testing.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.
• Life expectancy of at least 12 weeks.
• Adequate organ function as defined in the protocol.

Exclusion Criteria:

• Prior systemic therapy for first-line advanced or recurrent endometrial cancer.
• Prior immunotherapy or anti-angiogenic therapy.
• Deficient mismatch repair (dMMR) endometrial cancer by central testing.
• Active or untreated central nervous system (CNS) metastases; participants with previously treated and clinically stable brain metastases may be eligible if not requiring steroids and without evidence of progression.
• Clinically significant risk of bleeding or fistula, including a history of severe bleeding disorders.
• History of another malignancy within 3 years, except for cancers with negligible risk of recurrence or death.
• History of allogeneic organ or hematopoietic stem cell transplantation; active autoimmune disease requiring systemic treatment within the past 2 years; or history of immunodeficiency
• Interstitial lung disease, pneumonitis, or clinically significant pulmonary disease
• Uncontrolled or significant cardiovascular, metabolic, renal, hepatic, or vascular disease
• Active or uncontrolled infection.
• Recent major surgery or severe trauma within protocol-defined washout periods.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: PF-08634404 plus Chemotherapy; PF-08634404 plus Chemotherapy, followed by PF-08634404 maintenance therapy	Biological: PF-08634404; Solution for IV infusion; Drug: Standard of Care Chemotherapy; Solution for IV infusion
Active Comparator: Pembrolizumab plus Chemotherapy; Pembrolizumab plus Chemotherapy, followed by Pembrolizumab maintenance therapy	Drug: Standard of Care Chemotherapy; Solution for IV infusion; Biological: Pembrolizumab; Solution for IV infusion; Other Names: keytruda

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Progression-Free Survival (PFS) using RECIST v1.1 as assessed by Blinded Independent Central Review (BICR)	PFS by BICR is defined as the time from the date of randomization to the date of first documented disease progression per RECIST v1.1 as assessed by BICR, or death due to any cause, whichever occurs first.	Approximately 36 months

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Overall Survival (OS)	OS is defined as the time from the date of randomization to the date of death due to any cause.	Approximately 56 months
PFS using RECIST v1.1 as assessed by investigator	PFS by investigator is defined as the time from the date of randomization to the date of first documented disease progression per RECIST v1.1 as assessed by investigator, or death due to any cause, whichever occurs first. PFS by investigator will be analyzed with the same methodology as for PFS by BICR.	Approximately 36 months
Objective Response Rate (ORR) as assessed by BICR and investigator	Proportion of participants with confirmed Complete Response (CR) or Partial Response (PR) per RECIST v1.1, assessed by BICR and by investigator	Approximately 56 months
Duration of Response (DoR) as assessed by BICR and investigator	Time from first documented CR or PR to disease progression or death, per RECIST v1.1, assessed by BICR and by investigator	Approximately 56 months
Number of participants with treatment-emergent adverse events	AEs as characterized by type, frequency, intensity as graded by NCI CTCAE version 5.0, timing, seriousness, and relationship to study intervention(s)	Through end of the study and up to approximately 56 months
Number of participants with laboratory abnormalities	Laboratory abnormalities as characterized by type, frequency, severity (as graded by NCI CTCAE version 5.0), and timing. For laboratory tests without CTCAE grade definitions, results will be categorized as normal, high, low, or not done and be listed.	Through end of the study and up to approximately 56 months
Serum concentrations of PF-08634404	Pre-dose and post-dose serum concentrations of PF-08634404	Through end of the study and up to approximately 56 months
Incidence of ADA against PF-08634404	Incidence of anti-drug antibodies (ADA) to PF-08634404	Through end of the study and up to approximately 56 months
Change from baseline in the global health status/quality of life (QoL) and physical functioning scale of the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire-Core 30 (EORTC QLQ-C30)	The EORTC QLQ-C30 is a questionnaire for quantitative measure of health-related quality of life pertinent to participants with a broad range of cancers who are participating in international clinical trials. The core instrument is a 30-item questionnaire consisting of the following: 5 Functional scales (physical, role, cognitive, emotional, social); 3 Symptom scales (fatigue, pain, nausea, and vomiting, shortness of breath, loss of appetite, sleep disturbance, constipation, diarrhea) and financial impact of the disease; 2 Global items (global health status, overall HRQoL) All of the scales and single-item measures range in score from 0 to 100. High scale score represents a higher response level.	Baseline, up to approximately 56 months
Change from baseline in the back pain/pelvis pain, GI symptoms, and urological symptoms scales of the EORTC QLQ EN24	The Endometrial Cancer Module (EORTC QLQ-EN24) is a disease-specific measure that is supplemental to the EORTC QLQ-C30 to capture patient's experiences with endometrial cancer-specific symptoms and impacts. The recall period is the past week. The 24-item questionnaire consists of the following: Symptom scales/items (Lymphoedema, urological symptoms, gastrointestinal symptoms, poor body image, and sexual/vaginal problems, pain in back and pelvis, tingling/numbness, muscular pain, hair loss, and taste change); Functional scales (sexual interest, sexual activity, and sexual enjoyment) Higher scores for functional scales represent high level of functioning, while higher scores for symptoms scales represent higher levels of symptoms/problems.	Baseline, up to approximately 56 months
Time to definitive deterioration in the global health status/QoL and physical functioning scale of the EORTC QLQ-C30.	The European Organisation for Research and Treatment of Cancer Quality of Life-Core 30 Questionnaire (EORTC QLQ-C30), is a validated 30-item self-administered core questionnaire designed to assess HRQoL, functioning, and symptoms in all cancer patients or survivors. Time to deterioration is defined as time from baseline (defined as day 1 of cycle 1 before first application of maintenance therapy) to the date of the first clinically meaningful deterioration i.e. a decrease by ≥10 points for the functional scales and an increase by ≥ 10 points for the symptom scales or death in: Physical functioning score; Role functioning score	Approximately 56 months

Collaborators and Investigators

• Sponsor: Pfizer
• Collaborators: GOG Foundation
• Investigators: Study Director: Pfizer CT.gov Call Center, Pfizer

Publications and helpful links

Helpful Links

• To obtain contact information for a study center near you, click here.

Study record dates

Study Major Dates

• Study Start (Estimated): October 9, 2026
• Primary Completion (Estimated): June 1, 2029
• Study Completion (Estimated): January 30, 2031

Study Registration Dates

• First Submitted: May 5, 2026
• First Submitted That Met QC Criteria: May 5, 2026
• First Posted (Actual): May 11, 2026

Study Record Updates

• Last Update Posted (Actual): May 19, 2026
• Last Update Submitted That Met QC Criteria: May 18, 2026
• Last Verified: May 1, 2026

More Information

Terms related to this study

• Keywords: pMMR; Endometrial Neoplasms; Recurrent Endometrial Carcinoma; Advanced Endometrial Carcinoma; Advanced or Recurrent Endometrial Cancer
• Additional Relevant MeSH Terms: Urogenital Diseases; Genital Diseases; Urogenital Neoplasms; Neoplasms by Site; Neoplasms; Female Urogenital Diseases; Female Urogenital Diseases and Pregnancy Complications; Uterine Diseases; Genital Diseases, Female; Genital Neoplasms, Female; Uterine Neoplasms; Endometrial Neoplasms; pembrolizumab
• Other Study ID Numbers: C6461018; 2026-525584-40-00 (Registry Identifier: CTIS (EU)); GOG-3145 (Other Identifier: APGOT); ENGOT-en37 (Other Identifier: APGOT); APGOT-EN6 (Other Identifier: APGOT)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical_trials/trial_data_and_results/data_requests.

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: Yes
• Studies a U.S. FDA-regulated device product: No
• product manufactured in and exported from the U.S.: No