A Clinical Study of Sacituzumab Tirumotecan (Sac-TMT, MK-2870) in People With Breast Cancer (MK-2870-032)

Inclusion Criteria:

The main inclusion criteria include but are not limited to the following:

* Has previously untreated high-risk, early-stage, non-metastatic (M0) breast cancer (BC), defined as any of the following combined primary tumor (T) and regional lymph node (N) staging per AJCC 8th edition criteria as assessed by the physician investigator based on radiological and/or clinical assessment:

  * cT1c, N1-N2
  * cT2, N0-N2
  * cT3, N0-N2
  * cT4a-d, N0-N2
* The participant must have a centrally confirmed diagnosis of BC that is triple-negative or HR-low+/HER2- (defined as estrogen receptor (ER)-low+ expression in 1% to 10% cells and HER2- as by the most recent American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines.
* Provides a core needle biopsy from the primary breast tumor at screening to the central laboratory.
* Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 performed within 28 days before Cycle1 Day 1 (C1D1).
* Demonstrates adequate organ function.

Exclusion Criteria:

The main exclusion criteria include but are not limited to the following:

* Metastatic (Stage IV) breast cancer or clinical node stage 3 (cN3) nodal involvement
* Has received any prior treatment, including radiation, systemic therapy,and/or definitive surgery for currently diagnosed breast cancer
* Has undergone excisional biopsy of the primary tumor, axillary lymph node dissection, and/or axillary sentinel lymph node biopsy prior to study treatment.
* Received prior systemic anticancer therapy including investigational agents within 4 weeks before C1D1.
* Received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent, or with an agent directed to another stimulatory or coinhibitory T-cell receptor (eg, CTLA-4, OX- 40, CD137).
* Received prior treatment with a TROP2-targeted antibody-drug conjugate (ADC).
* Received prior treatment with a topoisomerase I inhibitor-containing ADC.
* Received a live or live-attenuated vaccine within 30 days before the first dose of study intervention.
* Known additional malignancy that is progressing or has required active treatment within the past 5 years.
* Uncontrolled systemic disease.
* History of (noninfectious) pneumonitis/interstitial lung disease that required steroids, has current pneumonitis/interstitial lung disease or has suspected interstitial lung disease (ILD) or pneumonitis that cannot be ruled out by standard diagnostic assessments at Screening..