Study of Patritumab Deruxtecan Plus Pembrolizumab With Other Anticancer Agents in Participants With High-Risk Early-Stage Triple-Negative or Hormone Receptor-Low Positive/HER-2 Negative Breast Cancer (MK-1022-010, HERTHENA-Breast-03)

Part 1: Number of Participants Experiencing an Adverse Event (AE)

Timeframe: Up to ~43 weeks

An AE is defined as any unfavorable and unintended sign, symptom, disease, or worsening of preexisting condition temporally associated with study treatment and irrespective of causality to study treatment. The number of participants who experience an AE will be presented for Part 1.

Part 1: Number of Participants Who Experience One or More Dose-Limiting Toxicities (DLTs)

Timeframe: Up to 21 days

A DLT is defined by the National Cancer Institute Common Terminology for Adverse Events (NCI CTCAE) Version 5.0, assessed by investigator as drug-related: Grade (gr) 3 or 4 nonhematologic toxicity (with exceptions); gr 3 or gr 4 laboratory values (with exceptions); gr 3 or 4 febrile neutropenia; prolonged delay (\>2 weeks) in initiating Cycle 2 (cycle length = 3 weeks) due to intervention-related toxicity; any intervention-related toxicity that causes the participant to discontinue intervention during Cycle 1; interstitial lung disease as per investigator; any other gr ≥3 pulmonary toxicity; or gr 5 toxicity.

Part 1: Number of Participants who Discontinued Study Treatment Due to an AE

Timeframe: Up to ~30 weeks

An AE is defined as any unfavorable and unintended sign, symptom, disease, or worsening of preexisting condition temporally associated with study treatment and irrespective of causality to study treatment. The number of participants who discontinued study treatment due to an AE will be presented for Part 1.

Part 2: Pathological Complete Response (pCR) Rate Using the Definition of ypT0/Tis ypN0

Timeframe: Up to ~30 weeks

pCR (ypT0/Tis ypN0) is defined as the absence of residual invasive cancer on hematoxylin and eosin evaluation of the complete resected breast specimen and all sampled regional lymph nodes after completion of neoadjuvant systemic therapy at the time of definitive surgery.

Part 2: Number of Participants Experiencing an AE

Timeframe: Up to ~103 weeks

An AE is defined as any unfavorable and unintended sign, symptom, disease, or worsening of preexisting condition temporally associated with study treatment and irrespective of causality to study treatment. The number of participants who experience an AE will be presented for Part 2.

Part 2: Number of Participants who Discontinued Study Treatment Due to an AE

Timeframe: Up to ~90 weeks

An AE is defined as any unfavorable and unintended sign, symptom, disease, or worsening of preexisting condition temporally associated with study treatment and irrespective of causality to study treatment. The number of participants who discontinued study treatment due to an AE will be presented for Part 2.

Part 2: pCR-No Ductal Carcinoma in Situ (DCIS) Rate Using the Definition of ypT0 ypN0

Timeframe: Up to ~30 weeks

pCR-no DCIS (ypT0 ypN0) is defined as the absence of residual invasive and in situ cancer on hematoxylin and eosin evaluation of the complete resected breast specimen and all sampled regional lymph nodes after completion of neoadjuvant systemic therapy at the time of definitive surgery.

Part 2: Event-Free Survival (EFS)

Timeframe: Up to ~100 months

EFS is defined as the time from randomization to disease progression that precludes surgery, local or distant recurrence, or death due to any cause, whichever occurs first.

Part 2: Overall Survival (OS)

Timeframe: Up to ~100 months

OS is defined as the time from randomization to date of death due to any cause.

Part 2: Distant Progression or Distant Recurrence-Free Survival (DPDRFS)

Timeframe: Up to ~100 months

DPDRFS is defined as the time from randomization to first distant progression or distant recurrence event as assessed or death due to any cause, whichever occurs first.

Part 2: Residual Cancer Burden (RCB)

Timeframe: Up to ~30 weeks

RCB is defined as residual disease in either the breast or lymph node at the time of surgery. RCB score provides a continuous measurement of the extent of residual cancer. There are four RCB classes: RCB-0 (RCB score 0), RCB-1 (0\< RCB score \<1.36), RCB-2 (1.36 \<RCB score \<3.28), and RCB-3 (RCB score \>3.28).