A Study of Dato-DXd in Inoperable or Metastatic Hormone Receptor-positive, HER2 IHC 0 Breast Cancer

This is a Phase IIIb, single-arm, open-label, multicentre, multinational study assessing the efficacy and safety of Dato-DXd in participants with HR-positive, HER2 IHC 0, locally advanced inoperable or metastatic breast cancer refractory to endocrine therapy. Refer to the study schema (Figure 1) and SoA (Section 1.3) for overall design and required assessments. Day 1 is defined as the first day of study treatment. Approximately 100 participants will be treated in this study with Dato-DXd 6 mg/kg as an intravenous infusion every 3 weeks (21-day cycle), up to a maximum of 540 mg for patients ≥ 90kg. Participants will receive study intervention until RECIST 1.1 defined radiological progression by investigator unless there is evidence of unacceptable toxicity or if the participant requests to stop the study treatment. Written consent is required from all participants for newly acquired tumour biopsy collection at baseline or for providing a tumour biopsy sample collected within 6 weeks prior to initiating Dato-DXd and, if feasible, at progression. This sample will be used to assess for HER2 and TROP2 expression and exploratory analysis of biomarkers of response. Participants are also expected to consent for repeated liquid biopsy at baseline, during the treatment period, and at progression to evaluate biomarkers of response and resistance. (Sample collection in China will comply with local regulatory requirements.) Tumour imaging will occur every 8 weeks (± 1 week) after the first dose of study drug for 48 weeks and then every 12 weeks (± 1 week) thereafter until RECIST 1.1 disease progression as assessed by the investigator or the start of subsequent anticancer therapy.