A Phase 1b Study of T-DXd Combinations in HER2-low Advanced or Metastatic Breast Cancer

DESTINY-Breast 08 will investigate the safety, tolerability, PK and preliminary anti-tumour activity of T-DXd in combination with other therapies in patients with Metastatic HER2-low Advanced or Metastatic Breast Cancer

This study is modular in design allowing assessment of the safety, tolerability, PK and preliminary anti-tumour activity of T-DXd in combination with other therapies. Combination-treatment modules will have 2 parts: a dose-finding phase (Part 1), and a dose expansion phase (Part 2); the Part 2 dose-expansion phase will use the RP2D determined in Part 1. The target population of interest in this study is patients with HER2-low (IHC 1+ or IHC 2+/ISH -) (as per ASCO/CAP 2018 guidelines) advanced/MBC. Part 1 of each module will enroll patients with locally confirmed HER2-low advanced/MBC in second-line or later (≥ 2L) settings Part 2 of each module will enroll patients with HER2-low MBC who have either not received prior treatment, or received only 1 prior treatment (depending on the module-specific exclusion criteria) for advanced/metastatic disease

Key Inclusion Criteria: * Patients must be at least 18 years of age * Male or female patients who have pathologically documented breast cancer that: 1. Has a history of HER2-low expression, defined as IHC 2+/ISH- or IHC 1+ (ISH- or untested) with a validated assay 2. Is documented as HR+ (either ER and/or PgR positive \[ER or PgR ≥1%\]) or ER and PgR negative (ER and PgR \<1%) per ASCO/CAP guidelines in the metastatic setting * Patient must have adequate tumor sample for biomarker assessment * ECOG Performance Status of 0 or 1 For patients with HR+ disease: Part 1: At least 1 prior treatment line of ET with or without a targeted therapy (such as CDK4/6, mTOR or PI3-K inhibitors), and at least 1 prior line of chemotherapy for MBC are required. Part 2: Only 1 prior treatment line of ET with or without a targeted therapy (such as CDK4/6, mTOR or PI3-K inhibitors) for MBC is allowed. No prior chemotherapy in the metastatic setting is allowed. Note there are no patients with HR+ disease in Part 2 of Modules 2 and 3. For patients with HR- disease: Part 1: At least 1 prior line of chemotherapy for MBC is required. Note there are no patients with HR- disease in Part 1 of Modules 4 and 5. Part 2: For Module 2, no prior lines of therapy for MBC are allowed, and for Modules 1 and 3, only 1 prior line of chemotherapy for MBC is allowed. Note there are no patients with HR- disease in Part 2 of Modules 4 and 5. Key Exclusion Criteria: * Uncontrolled intercurrent illness * Uncontrolled or siginificant cardiovascular disease * 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. * Lung-specific intercurrent clinically significant illnesses * Has spinal cord compression or clinically active central nervous system metastases * Active primary immunodeficiency * Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals * Prior treatment with ADC that comprises of an exatecan derivative that is a topoisomerase I inhibitor.