Keynote-B49: This phase III trial is trying to understand how safe and effective immunotherapy (pembrolizumab) is when combined with chemotherapy in people with hormone receptor-positive, human epidermal growth factor receptor 2-negative breast cancerA Randomized, Double-blind, Placebo-controlled, Phase 3 Study of Pembrolizumab Plus Chemotherapy Versus Placebo Plus Chemotherapy for the Treatment of Chemotherapy-Candidate Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative (HR+/HER2-) Locally Recurrent Inoperable or Metastatic Breast Cancer (KEYNOTE-B49)

Inclusion

• Has locally recurrent inoperable or metastatic HR+/HER2- breast cancer, which has not been previously treated with cytotoxic chemotherapy in the noncurative setting
• Has progressed on prior endocrine therapy and is now a chemotherapy candidate, meeting the characteristics in regard to previous treatments of one of the following 4 groups:
• Group 1: Has progressed on 2 or more lines of endocrine therapy for advanced/metastatic HR+/HER2-disease, with at least given in combination with a Cyclin-dependent kinase 4/6 (CDK4/6) inhibitor. Prior treatment with mTOR and/or PI3-K inhibitors is allowed. OR
• GROUP 2a: Has progressed on 1 line of previous endocrine therapy for advanced/metastatic disease AND had a disease recurrence within 24 months of definitive surgery for the primary tumor and while on adjuvant endocrine therapy. Prior use of CDK4/6 inhibitors is required, either in the adjuvant and/or metastatic setting. Prior treatment with mTOR and/or PI3-K inhibitors is allowed. OR
• GROUP 2b: Has progressed within 12 months of starting 1 line of endocrine therapy with a CDK4/6 inhibitor for advanced/metastatic HR+/HER2- disease. OR
• GROUP 3: If no prior treatment with a CDK4/6 inhibitor, for advanced/metastatic disease and/or early stage disease (adjuvant), participants must have progressed within 6 months of starting 1 line of endocrine therapy with or without an mTOR or PI3-K inhibitor for metastatic disease AND had a relapse within 24 months of definitive surgery for primary tumor and while receiving adjuvant endocrine therapy.
• Has presented a documented radiographic disease progression (as assessed by the investigator and/or histology [biopsy or cytology] for participants presenting with new metastatic lesions) during or after the last administered endocrine therapy prior to entering the study.
• Is a chemotherapy candidate that meets the criteria specified in the protocol
• Provides a new or the last obtained core biopsy, preferably consisting of multiple cores, taken from a locally recurrent or a distant (metastatic) lesion not previously irradiated
• Has centrally confirmed PD-L1 CPS ≥1 and HR+ (estrogen receptor [ER] and/or progesterone receptor [PgR]) /HER2- breast cancer as defined by the most recent American Society of Clinical Oncology (ASCO)/(College of American Pathologists) CAP guidelines on most recent tumor biopsy
• Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1, as assessed within 7 days prior to the first dose of study treatment
• Has adequate organ function within 10 days prior to the start of study
• Male participants must agree to the following during the treatment period and for at least 6 months after the last dose of chemotherapy: refrain from donating sperm PLUS either be abstinent from heterosexual intercourse as their preferred and usual lifestyle or use contraception and agree to use a male condom plus partner use of an additional contraceptive
• A female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: is not a woman of childbearing potential (WOCBP) OR is a WOCBP and using a highly-effective contraceptive method during the treatment period and for at least 120 days after the last dose of pembrolizumab and 180 days after the last dose of chemotherapy (whichever occurs last), AND agrees not to donate eggs (ova, oocytes) to others or freeze/store for her own use for the purpose of reproduction during this period
• A WOCBP must have a negative highly sensitive pregnancy test (urine or serum) within 24 hours for urine or within 72 hours for serum before the first dose of study intervention
• Has measurable disease per RECIST 1.1 as assessed by the local site investigator/radiologist
• If receiving bisphosphonates or RANK ligand inhibitors, with stable doses for ≥4 weeks prior to the date of randomization, the participant may continue receiving this therapy during the study treatment. If participant needs to initiate these agents during the screening period, a bone scan to evaluate bone disease should be performed prior to randomization.
• Participants who are Hepatitis B surface antigen (HBsAg) positive are eligible if they have received Hepatitis B virus (HBV) antiviral therapy for at least 4 weeks prior to the first dose of study intervention and have undetectable HBV viral load prior to randomization
• Participants with history of Hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable at screening