A Phase 3, Randomized, Open-label Study Comparing Efficacy and Safety of Sacituzumab Tirumotecan (Sac-TMT, MK-2870) as a Monotherapy and in Combination With Pembrolizumab (MK-3475) Versus Treatment of Physician's Choice in Participants With Previously Untreated Locally Recurrent Unresectable or Metastatic Triple-Negative Breast Cancer Expressing PD-L1 at CPS Less Than 10 (TroFuse-011)

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Has locally recurrent unresectable or metastatic TNBC that cannot be treated with curative intent * Has not received systemic treatment for locally recurrent unresectable or metastatic breast cancer * Participants previously treated for early-stage breast cancer must have completed all prior therapy for early-stage breast cancer with curative intent at least 6 months before the first disease recurrence * Is a candidate for treatment with pembrolizumab and one of the TPC options: paclitaxel or nab-paclitaxel or gemcitabine + carboplatin * Participants who have AEs due to previous anticancer therapies must have recovered to ≤Grade 1 or baseline with the exception of alopecia or vitiligo. Participants with endocrine-related AEs who are adequately treated with hormone replacement are eligible * 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, and have undetectable HBV viral load * Participants with history of hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Has breast cancer amenable to treatment with curative intent * Has TNBC with evaluable tumor programmed death ligand 1 (PD-L1) expression at combined positive score (CPS) ≥10 * Has received prior systemic therapy for treatment of locally recurrent unresectable or metastatic breast cancer * Has Grade ≥2 peripheral neuropathy * Has history of documented severe dry eye syndrome, severe Meibomian gland disease and/or blepharitis, or severe corneal disease that prevents/delays corneal healing * Has active inflammatory bowel disease requiring immunosuppressive medication or previous history of inflammatory bowel disease * Has uncontrolled, significant cardiovascular disease or cerebrovascular disease * Has skin only metastatic disease * Has advanced/metastatic, symptomatic visceral spread at risk of rapidly evolving into life-threatening complications * Human immunodeficiency virus (HIV)-infected participants with a history of Kaposi's sarcoma and/or Multicentric Castleman's Disease * Has known additional malignancy that is progressing or has required active treatment within the past 5 years * Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases may participate provided they are radiologically stable * Active autoimmune disease that has required systemic treatment in the past 2 years. Replacement therapy (eg, thyroxine, insulin, or physiologic corticosteroid) is allowed * Has a history of (noninfectious) pneumonitis/interstitial lung disease (ILD) that required steroids, has current pneumonitis/ILD, or has suspected ILD or pneumonitis that cannot be ruled out by standard diagnostic assessments * Concurrent active Hepatitis B (defined as HBsAg positive and/or detectable HBV deoxyribonucleic acid (DNA)) and Hepatitis C virus (HCV) (defined as anti-HCV antibody (Ab) positive and detectable HCV ribonucleic acid (RNA)) infection * History of stem cell/solid organ transplant * Has not adequately recovered from major surgery or has ongoing surgical complications