A Study of Sacituzumab Tirumotecan (MK-2870) as a Single Agent and in Combination With Pembrolizumab (MK-3475) Versus Treatment of Physician's Choice in Participants With HR+/HER2- Unresectable Locally Advanced or Metastatic Breast Cancer (MK-2870-010)

Inclusion Criteria:

• Has unresectable locally advanced or metastatic centrally-confirmed hormone receptorpositive (HR+)/human epidermal growth factor receptor 2 negative (HER2-) breastcancer

• Has radiographic disease progression on one or more lines of endocrine therapy forunresectable locally advanced/metastatic HR+/HER2- breast cancer, with one incombination with a CDK4/6 inhibitor

• Is a chemotherapy candidate

• Has an eastern cooperative oncology group (ECOG) performance status of 0 to 1assessed within 7 days before randomization

• Has adequate organ function

• Human immunodeficiency virus (HIV)-infected participants must have well controlledHIV on antiretroviral therapy

• Participants who are Hepatitis B surface antigen (HBsAg) positive are eligible ifthey have received HBV antiviral therapy for at least 4 weeks, and have undetectableHBV viral load

• Participants with a history of Hepatitis C virus (HCV) infection are eligible if HCVviral load is undetectable

Exclusion Criteria:

• Has breast cancer amenable to treatment with curative intent

• Has experienced an early recurrence (<6 months after completing adjuvant/neoadjuvantchemotherapy) and therefore is eligible to receive second-line (2L) treatment

• Has symptomatic advanced/metastatic visceral spread at risk of rapidly evolving intolife-threatening complications

• Has received prior chemotherapy for unresectable locally advanced or metastaticbreast cancer

• Active autoimmune disease that has required systemic treatment in the past 2 years

• History of (noninfectious) pneumonitis/interstitial lung disease that requiressteroids, or has current pneumonitis/interstitial lung disease

• Has an active infection requiring systemic therapy