This clinical trial is focused on studying a type of breast cancer known as hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer. This type of cancer can be locally recurrent, meaning it has come back in the same area, or metastatic, meaning it has spread to other parts of the body. The study is investigating the effects of a treatment called pembrolizumab, also known by its code name MK-3475 or Keytruda, in combination with chemotherapy. Pembrolizumab is a medication that helps the immune system fight cancer cells. The chemotherapy drugs being used in this study include paclitaxel, doxorubicin, capecitabine, and their variations, such as paclitaxel albumin-bound and doxorubicin hydrochloride, liposomal. Some participants will receive a placebo instead of pembrolizumab to compare the effects.

The purpose of this study is to see if pembrolizumab combined with chemotherapy is more effective than chemotherapy alone in treating this type of breast cancer. Participants will be randomly assigned to receive either the pembrolizumab and chemotherapy combination or a placebo and chemotherapy. The study will monitor how long participants live without the cancer getting worse and overall survival rates. The study will also look at the quality of life and any side effects experienced by participants.

Participants will receive the treatment through an intravenous (IV) infusion, which means the medication is given directly into a vein. The study will last for a period of time, during which participants will have regular check-ups and assessments to monitor their health and the progress of the treatment. The study aims to provide valuable information on the effectiveness and safety of pembrolizumab in combination with chemotherapy for treating HR+/HER2- breast cancer.