A Phase 3, Randomized, Open-label, Study to Compare the Efficacy and Safety of Adjuvant MK-2870 in Combination With Pembrolizumab (MK-3475) Versus Treatment of Physician's Choice (TPC) in Participants With Triple-Negative Breast Cancer (TNBC) Who Received Neoadjuvant Therapy and Did Not Achieve a Pathological Complete Response (pCR) at Surgery

• Has centrally confirmed TNBC, as defined by the most recent American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines

• Has no evidence of locoregional or distant relapse, as assessed by the treating physician

• Had neoadjuvant treatment based on the KEYNOTE-522 regimen (pembrolizumab with carboplatin/taxanes and pembrolizumab with anthracycline-based chemotherapy) followed by surgery according to National Comprehensive Cancer Network (NCCN) treatment guidelines for TNBC

• Had adequate excision and surgical removal of all clinically evident disease in the breast and/or lymph nodes and have adequately recovered from surgery

• Has non-pathologic complete response at surgery

• Is able to continue on adjuvant pembrolizumab

• Randomization must be conducted within 12 weeks from surgical resection

• Completed adjuvant radiation therapy (if indicated) and recovered before randomization

• Has provided tissue from the surgical resection for central laboratory determination of trophoblast cell surface antigen 2 (TROP2) status

• If capable of producing sperm, the participant agrees to the following during the intervention period and for at least the time needed to eliminate each study intervention after the last dose of study intervention (120 days for sacituzumab tirumotecan and 95 days for capecitabine \[no restriction for pembrolizumab\]): agrees to refrain from donating sperm AND is either abstinent and agrees to remain abstinent or uses highly effective contraception

• For females (assigned at birth), is not pregnant or breastfeeding and ≥1 of the following applies: is not a participant of childbearing potential (POCBP) OR is a POCBP and uses highly effective contraception after the last dose of study intervention (210 days for sacituzumab tirumotecan, 120 days for pembrolizumab, and 185 days for capecitabine). Abstains from breastfeeding during the study intervention period and for at least 120 days after study intervention

• Participants who have AEs due to previous anticancer therapies must have recovered to ≤Grade 1 or baseline (except alopecia)

• Human immunodeficiency virus (HIV)-infected participants must have well controlled HIV on antiretroviral therapy (ART)

• An Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 assessed within 7 days before first dose of study treatment

• Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received hepatitis B birus (HBV) antiviral therapy for at least 4 weeks, and have undetectable HBV viral load prior to randomization