This is a dose escalation and dose expansion study to assess the safety and tolerability of BGB-21447 (a B-cell leukemia/lymphoma 2 inhibitor, Bcl-2i) in combination with fulvestrant, with or without BGB-43395 (cyclin-dependent kinase 4 inhibitor, CDK4i), in adults with HR+/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.

Inclusion Criteria:

• Histologically or cytologically confirmed HR+/HER2- metastatic breast cancer. Part 1A and 1B: Participants must have received ≥ 1 prior line(s) of treatment for advanced/metastatic disease, including prior endocrine therapy and CDK4/6 inhibitor in either the adjuvant or advanced/metastatic setting. Part 2: Participants must have received 1-3 prior line(s) of treatment for advanced/metastatic disease, including prior endocrine therapy and CDK4/6 inhibitor in either the adjuvant or advanced/metastatic setting.
• Female participants will be required (either continue ongoing or initiate as soon as feasible) to have ovarian function suppression using gonadotropin-releasing hormone (GnRH) agonists (such as goserelin) or be postmenopausal.
• Male participants may be required to use GnRH agonists when being treated with fulvestrant at the discretion of the investigator.
• Participants must have a stable Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤ 1.
• Adequate organ function.
• Female participants of childbearing potential and nonsterile male participants with female partners of childbearing potential must be willing to use a highly effective method of birth control for the duration of the study and for 7 days after the last dose of BGB-21447, 6 months after the last dose of BGB-43395, and 2 years after the last dose of fulvestrant.
• Food effect substudy only: Participants who are able and willing to fast overnight (≥ 10 hours) and consume a high-fat meal.

Exclusion Criteria:

• Prior Bcl-2 inhibitor exposure. For triplet combination cohorts only: Prior therapy selectively targeting CDK4.
• Known leptomeningeal disease or uncontrolled, untreated brain metastases.
• Any malignancy ≤ 3 years before the first dose of study treatment(s) except for the specific cancer under investigation in this study and any locally recurring cancer that has been treated with curative intent (eg, treated papillary thyroid carcinoma, resected basal or squamous cell skin cancer, superficial bladder cancer, or carcinoma in situ of the cervix or breast).
• For Part 1B: Uncontrolled diabetes.
• History of hepatitis B or active Hepatitis C infection
• China Only: Untreated chronic hepatitis B or chronic hepatitis B virus (HBV) carriers with HBV DNA > 500 IU/ml (or > 2500 copies/ml) at screening.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.

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See plan description

See plan description