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

Inclusion Criteria:

• Has unresectable locally advanced or metastatic centrally-confirmed hormone receptor positive (HR+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer
• Has radiographic disease progression on one or more lines of endocrine therapy for unresectable locally advanced/metastatic HR+/HER2- breast cancer, with one in combination with a CDK4/6 inhibitor
• Is a chemotherapy candidate
• Has an eastern cooperative oncology group (ECOG) performance status of 0 to 1 assessed within 7 days before randomization
• Has adequate organ function
• Human immunodeficiency virus (HIV)-infected participants must have well controlled HIV on antiretroviral therapy
• Participants who are Hepatitis B surface antigen (HBsAg) positive are eligible if they have received HBV antiviral therapy for at least 4 weeks, and have undetectable HBV viral load
• Participants with a history of Hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable

Exclusion Criteria:

• Has breast cancer amenable to treatment with curative intent
• Has experienced an early recurrence (<6 months after completing adjuvant/neoadjuvant chemotherapy) and therefore is eligible to receive second-line (2L) treatment
• Has symptomatic advanced/metastatic visceral spread at risk of rapidly evolving into life-threatening complications
• Has received prior chemotherapy for unresectable locally advanced or metastatic breast cancer
• Active autoimmune disease that has required systemic treatment in the past 2 years
• History of (noninfectious) pneumonitis/interstitial lung disease that requires steroids, or has current pneumonitis/interstitial lung disease
• Has an active infection requiring systemic therapy