The primary objectives of this trial are to:

* Characterize the safety and tolerability of TEV-56278
* Determine the Recommended Phase 2 Dose (RP2D)
* Evaluate antitumor activity of TEV-56278 (Part 2 only)
* Determine the safety and tolerability of TEV-56278 in combination with pembrolizumab
* Determine a RP2D of TEV-56278 in combination with pembrolizumab

The secondary objectives of this trial are to:

* Characterize the serum pharmacokinetics of TEV-56278
* Evaluate the antitumor activity of TEV-56278
* Determine the safety and tolerability of TEV-56278
* Evaluate other measures of antitumor activity of TEV-56278
* Evaluate anti-tumor activity

Participants will be treated up to 12 months with a follow-up period of up to 12 months after last infusion. The total duration of the trial will be up to 25 months for individual participants.

Participants who exhibit a favorable benefit risk profile at the end of the 12 month trial treatment period may be offered an opportunity for an extended treatment period in which they can be treated for a maximum of 12 additional months (up to 26 additional cycles of TEV-56278).

Researchers are looking for new ways to treat types of breast cancer that are both:

* High-risk, which means the cancer may have a higher chance of getting worse or coming back after treatment
* Early-stage, which means the cancer is in the breast or the lymph nodes around the breast The 2 types of breast cancer in this study are triple-negative breast cancer (TNBC) and hormone receptor (HR)-low positive/human epidermal growth factor receptor-2 (HER2) negative breast cancer. These cancers have zero or a low amount of a protein called HER2 and other proteins that attach to the hormones estrogen or progesterone.

Sacituzumab tirumotecan (also known as sac-TMT or MK-2870), the study medicine, is a type of targeted therapy. A targeted therapy is a treatment that works to control how specific types of cancer cells grow and spread.

The main goals of this study are to learn if people who receive sac-TMT, pembrolizumab, and chemotherapy:

* Have fewer cancer cells found in the tumors and lymph nodes removed during surgery compared to those who receive only pembrolizumab and chemotherapy
* Live longer without the cancer growing, spreading, or coming back compared to people who receive only pembrolizumab with chemotherapy