This is a Phase III, 2-arm, randomized, open label, multicenter, global study assessing the efficacy and safety of puxitatug samrotecan compared to physician's choice of chemotherapy (doxorubicin or paclitaxel) in participants with B7-H4 selected advanced/metastatic EC that progressed following platinum based chemotherapy and anti-PD-1/anti-PD-L1 therapy.

The target population of interest in this study is participants with B7-H4-selected advanced/metastatic EC who have progressed on or after platinum-based chemotherapy and anti-PD-1/anti-PD-L1 therapy, either separately or in combination and should have received no more than 2 prior lines of therapy in advanced/metastatic setting. Participants will be randomized in a 1:1 ratio to Puxi-Sam (arm A) or physician's choice of chemotherapy (arm B; doxorubicin or paclitaxel). The total study size will be approximately 700 eligible participants.

During the treatment period, participants will receive Puxi-Sam IV Day 1 Q3W (Arm A) or either doxorubicin treatment IV Day 1 Q3W or paclitaxel treatment IV on Days 1, 8, and 15 in 28-day cycle (Arm B).

This study aims to see if Puxi-Sam allows participants to live longer without their endometrial cancer getting worse, or simply to live longer, compared to participants receiving standard of care chemotherapy. This study is also looking to see how the treatment and the endometrial cancer affects participants' quality of life.

The main inclusion criteria include but are not limited to the following:

• Histologically confirmed diagnosis of endometrial carcinoma or carcinosarcoma.
• Recurrent/metastatic EC ie, with radiological or objective evidence of recurrence or progression.
• Has received prior platinum-based chemotherapy and anti-programmed cell death 1 protein (PD-1)/anti- programmed cell death ligand 1 (PD-L1) therapy, either separately or in combination.
• A WHO/ECOG performance status of 0 or 1 at Screening.
• Has radiographically measurable disease by RECIST 1.1

The main exclusion criteria include but are not limited to the following:

• Had uterine sarcomas or uterine neuroendocrine carcinoma.
• Has had a recurrence of endometrial carcinoma or carcinosarcoma more than > 12 months after completing platinum-based therapy administered in the curative-intent setting without any additional platinum-based therapy received in the recurrent setting.
• Had previously received treatment with any therapy (approved or investigational) that contained a TOP1i including ADCs .
• Had previously received treatment with Puxi-Sam or another B7-H4 targeting agent.
• History of (non-infectious) ILD/pneumonitis that required steroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at screening.
• Has clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses.
• Active or previously documented autoimmune or inflammatory disorders

Female

Qualified researchers can request access to anonymized individual patient-level data from AstraZeneca group of companies sponsored clinical trials via the request portal Vivli.org. All requests will be evaluated as per the AZ disclosure commitment: https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

Yes, indicates that AZ are accepting requests for IPD, but this does not mean all requests will be shared.

AstraZeneca will meet or exceed data availability as per the commitments made to the EFPIA PhRMA Data Sharing Principles. For details of our timelines, please rerefer to our disclosure commitment at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure.

When a request has been approved AstraZeneca will provide access to the anonymized individual patient-level data via secure research environment Vivli.org. Signed Data Usage Agreement (non-negotiable contract for data accessors) must be in place before accessing requested information.

Open label