A Study Evaluating the Efficacy and Safety of Multiple Treatment Combinations in Participants With Breast Cancer

This is a Phase Ib/II, open-label, multicenter, randomized umbrella study in participants with breast cancer. The study is designed with the flexibility to open new treatment arms as new treatments become available, close existing treatment arms that demonstrate minimal clinical activity or unacceptable toxicity, or modify the patient population. Cohort 1 will focus on participants with inoperable, locally advanced or metastatic, estrogen receptor-positive (ER+), HER2-negative breast cancer who had disease progression during or following treatment with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i; e.g., palbociclib, ribociclib, abemaciclib) in the first- or second-line setting. Cohort 2 will focus on inoperable, locally advanced or metastatic, ER+, HER2-positive breast cancer with previous progression to standard-of-care anti-HER2 therapies, of which one was a trastuzumab-and-taxane-based systemic therapy (including in the early setting if recurrence occurred within 6 months of finishing adjuvant therapy) and one was a HER2-targeting antibody-drug conjugate (ADC; e.g., ado-trastuzumab emtansine or trastuzumab-deruxtecan) or a HER2-targeting tyrosine kinase inhibitor (TKI; e.g., tucatinib, lapatinib, pyrotinib, or neratinib). Cohort 3 will focus on inoperable, locally advanced or metastatic, ER+, HER2-negative, PIK3CA-mutated breast cancer with resistance to adjuvant endocrine therapy.