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 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