Study of Pembrolizumab Plus Chemotherapy Versus Placebo Plus Chemotherapy for Certain Types of Advanced Breast Cancer

Inclusion Criteria

• Have advanced breast cancer that cannot be removed by surgery and has not been treated with chemotherapy for this stage.
• Cancer has worsened after hormone therapy and is now suitable for chemotherapy.
• Meet specific criteria based on previous treatments, such as having used CDK4/6 inhibitors or other specific drugs.
• Showed cancer progression on scans or biopsy after the last hormone therapy.
• Suitable for chemotherapy as per study guidelines.
• Provide a recent biopsy sample from a tumor that has not been treated with radiation.
• Cancer must have a PD-L1 score of 1 or higher and be hormone receptor-positive and HER2-negative.
• Be in good general health with an ECOG performance status of 0 or 1.
• Have adequate organ function based on recent tests.
• Male participants must agree to use contraception and not donate sperm during and after treatment.
• Female participants must not be pregnant or breastfeeding and must use effective contraception if of childbearing potential.
• Have a negative pregnancy test before starting treatment.
• Have measurable cancer as per standard criteria.
• If taking certain bone-strengthening medications, must have stable doses before starting the study.
• Participants with Hepatitis B or C must have controlled viral loads.

Exclusion Criteria

• Cancer that can be treated with the intent to cure.
• Have certain medical conditions or treatments that could interfere with the study.
• Significant heart disease or recent heart events.
• Cancer that has spread to vital organs and is causing severe symptoms.
• Only have skin cancer.
• Have a BRCA mutation and have not tried PARP inhibitors.
• Have had chemotherapy for advanced breast cancer before.
• Have used certain immune therapies or investigational drugs recently.
• Have had recent radiation therapy or vaccines.
• Have immune system deficiencies or are on high-dose steroids.
• Have another active cancer needing treatment.
• Have active brain metastases or certain infections.
• Have severe allergies to study drugs.
• Have autoimmune diseases needing treatment.
• Have lung diseases needing steroids.
• Have active infections or a history of certain infections like HIV or TB.
• Have mental health or substance abuse issues affecting study participation.
• Are pregnant, breastfeeding, or planning to have children soon.
• Have had an organ transplant.