BGB-21447 (Bcl-2 Inhibitor) Combinations for Adults With Hormone-Receptor Positive (HR+)/Human Epidermal Growth Factor Receptor 2 Negative (HER2-) Metastatic Breast Cancer

This new study will check how safe and helpful a potential anticancer drug called BGB-21447 (Bcl-2i) is. This drug will be tested in combination with fulvestrant, with or without BGB-43395 (CDK4i), in adults with metastatic breast cancer. HR+/HER2- tumors account for approximately 70% of all breast cancers and are responsible for most breast cancer-related deaths. While CDK4/6 inhibitors combined with endocrine therapy have improved outcomes for patients with HR+/HER2- metastatic breast cancer, patients eventually develop progressive disease on these therapies and require new treatments. BGB-21447 is an oral drug that is highly potent and selectively stops a protein called B-cell lymphoma-2 (Bcl-2). Bcl-2 proteins are often overexpressed in some cancers (like HR+ breast cancer) by keeping the cancer cells from dying. Disrupting this pathway is believed to lead to cell death. BGB-43395 is an oral drug that selectively stops a protein called cyclin-dependent kinase 4 (CDK4). CDK4 is a type of protein that regulates cell growth and division in your body. Fulvestrant is a treatment that blocks estrogen receptors and reduces estrogen production. Fulvestrant has been approved to treat hormone receptor positive metastatic breast cancer to help stop the cancer cells from growing. This combination might be a good way to fight cancer, aiming to give patients the best possible treatment. The study is designed to see if this combination is safe and works well.