A Study of Sacituzumab Tirumotecan (Sac-TMT, MK-2870) as Monotherapy and in Combination With Pembrolizumab (MK-3475) in Participants With Triple-Negative Breast Cancer (MK-2870-011/TroFuse-011)

Researchers want to know if sacituzumab tirumotecan given alone or with pembrolizumab can treat triple negative breast cancer (TNBC). The main goal of this study is to learn if people treated with sacituzumab tirumotecan alone or with pembrolizumab live longer overall or without the cancer growing or spreading compared to people treated with chemotherapy.

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 * History of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease * 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