This study is designed to assess efficacy and safety of T-DXd adjuvant therapy, with or without radiotherapy, post-surgery in anticancer treatment naïve (including neoadjuvant therapy) endometrial cancer with various HER2 expression levels.

This is a global, multicenter, open-label, phase 3 study to evaluate the efficacy and safety of T-DXd versus SoC chemotherapy with or without radiotherapy as adjuvant treatment in participants with HER2-expressing (IHC 3+/2+) endometrial cancer. Participants will be randomized 1:1 to either T-DXd or SoC chemotherapy. The primary objective will assess disease-free survival as assessed radiographically by BICR or by histopathologic confirmation of disease recurrence per local assessment.

T-DXd (Enhertu®) is a HER2 directed ADC composed of a humanized anti-HER2 IgG1 mAb with the same amino acid sequence as trastuzumab, which includes a plasma-stable, selectively cleavable linker and potent topoisomerase I inhibitor payload (a derivative of exatecan) that leverages the clinically validated DXd ADC technology.

Key Inclusion Criteria

• Adults ≥18 years at the time the ICF is signed (Please follow local regulatory requirements if the legal age of consent for trial participation is >18 years old)
• Has histologically confirmed diagnosis of epithelial endometrial carcinoma. All histology's are allowed except for sarcomas (carcinosarcomas are allowed).
• Is newly diagnosed FIGO 2023 Stage IIC (including Stage IICmp53abn) or Stage III Note: FIGO 2023 Stage IIC includes disease with aggressive histological types (aggressive histological types are composed of high-grade EECs (grade 3), serous, clear cell, undifferentiated, mixed, mesonephric-like, gastrointestinal mucinous type carcinomas, and carcinosarcomas) with any myometrial involvement. FIGO 2023 Stage III includes disease with local and/or regional spread of the tumor of any histological subtype.
• Has HER2-expression (IHC 3+/2+) per 2016 ASCO-CAP gastric cancer IHC scoring guidelines as confirmed by central laboratory testing.
• Has adequate archived tumor tissue sample (sample from surgery is strongly recommended) available for assessment of HER2 status by central laboratory.

Key Exclusion Criteria

• Has uterine mesenchymal tumor such as an endometrial stromal sarcoma, leiomyosarcoma, or other types of pure sarcomas. Adenosarcomas are also not allowed.
• Has recurrent or FIGO 2023 Stage IV
• Has measurable residual tumor after surgery as determined by BICR assessment.
• Is known to have a POLE mutation from an approved and/or validated local test, according to local regulations, if available
• Has a medical history of MI within 6 months before randomization/enrollment, symptomatic CHF (NYHA Class II to IV). Participants with troponin levels above ULN at SCR (as defined by the manufacturer), and without any MI related symptoms should have a cardiologic consultation during SCR Period to rule out MI.
• Has a QTcF prolongation to > 480 msec based on average of the SCR triplicate12-lead ECG. Clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses including, but not limited to, any underlying pulmonary disorder (ie, pulmonary emboli within 3 months of the trial enrollment, severe asthma, severe COPD, restrictive lung disease, pleural effusion, etc.) and any autoimmune, connective tissue, or inflammatory disorder with potential pulmonary involvement (eg, rheumatoid arthritis, Sjogren's syndrome, sarcoidosis, etc.), or prior pneumonectomy.
• Has a history of (noninfectious) ILD/pneumonitis that required corticosteroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at SCR.

De-identified individual participant data (IPD) on completed studies and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Completed studies that has reached a global end or completion with all data set collected and analyzed, and for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Access Criteria: Formal request from qualified scientific and medical researchers on IPD and clinical study documents on completed clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.

This is an open-label study.