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)

Who May Qualify: - 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 your organs (liver, kidneys, etc.) are working well enough based on blood tests - Human weakened immune system 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 Who Should NOT Join This Trial: - 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 conditions (where your immune system attacks your own body) 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 Always talk to your doctor about whether this trial is right for you.