An Open-label, Randomized Phase 3 Study of MK-2870 as a Single Agent and in Combination with Pembrolizumab Versus Treatment of Physician’s Choice in Participants with HR+/HER2- Unresectable Locally Advanced or Metastatic Breast Cancer

This trial is testing MK-2870 given alone and MK-2870 given with pembrolizumab (MK-2870 plus pembro) in people with advanced breast cancer that cannot be surgically removed.
This trial will compare: MK-2870 given alone and MK-2870 plus pembro to 4 standard chemotherapies that are used to treat breast cancer: paclitaxel, nab-paclitaxel, capecitabine, and liposomal doxorubicin, and MK-2870 given alone to MK-2870 plus pembro. This trial will: Test the safety of the trial drugs. See how well the trial drugs work in comparison to each other. See which trial drugs help participants live longer in comparison to each other. See which trial drugs help participants have a better quality of life in comparison to each other. MK-2870 is experimental. It has not been approved by the United States Food and Drug Administration (FDA) for use alone or in combination with pembro. Pembro is also called KEYTRUDA™. Pembro has been approved by certain health authorities for treating various cancers, including some types of breast cancer. Pembrolizumab is not approved for hormone receptor positive/human epidermal growth factor receptor 2 negative (HR+/HER2-) breast cancer.