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)

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 withcurative intent

• Has not received systemic treatment for locally recurrent unresectable or metastaticbreast cancer

• Participants previously treated for early-stage breast cancer must have completedall prior therapy for early-stage breast cancer with curative intent at least 6months 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 recoveredto ≤Grade 1 or baseline with the exception of alopecia or vitiligo. Participantswith endocrine-related AEs who are adequately treated with hormone replacement areeligible

• Participants who are hepatitis B surface antigen (HBsAg) positive are eligible ifthey 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 HCVviral 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 atcombined positive score (CPS) ≥10

• Has received prior systemic therapy for treatment of locally recurrent unresectableor metastatic breast cancer

• Has Grade ≥2 peripheral neuropathy

• Has history of documented severe dry eye syndrome, severe Meibomian gland diseaseand/or blepharitis, or severe corneal disease that prevents/delays corneal healing

• Has active inflammatory bowel disease requiring immunosuppressive medication orprevious 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 evolvinginto life-threatening complications

• Human immunodeficiency virus (HIV)-infected participants with a history of Kaposi'ssarcoma and/or Multicentric Castleman's Disease

• Has known additional malignancy that is progressing or has required active treatmentwithin the past 5 years

• Has known active central nervous system (CNS) metastases and/or carcinomatousmeningitis. Participants with previously treated brain metastases may participateprovided 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) isallowed

• Has a history of (noninfectious) pneumonitis/interstitial lung disease (ILD) thatrequired steroids, has current pneumonitis/ILD, or has suspected ILD or pneumonitisthat cannot be ruled out by standard diagnostic assessments

• Concurrent active Hepatitis B (defined as HBsAg positive and/or detectable HBVdeoxyribonucleic acid (DNA)) and Hepatitis C virus (HCV) (defined as anti-HCVantibody (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 surgicalcomplications