Study of Patritumab Deruxtecan With Other Anticancer Agents in Participants With HER2 Positive Breast Cancer That Has Spread and Cannot Be Surgically Removed (MK-1022-009)

The following countries will be participating in the trial: Canada, United Kingdom, Israel, Japan, South Korea, and USA.

Inclusion criteria

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

• Has histologically confirmed HER2+ locally advanced unresectable breast cancer or metastatic breast cancer
• Human immunodeficiency virus (HIV)-infected participants must have well-controlled HIV on antiretroviral therapy (ART)
• Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received HBV antiviral therapy for at least 4 weeks, and have undetectable hepatitis B virus (HBV) viral load before allocation
• Participants with history of hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable
• Eastern Cooperative Oncology Group (ECOG) performance status 0 to 1 within 7 days before start of study intervention

Arm 1:

• Has received at least a minimum of 2 and a maximum of 5 prior lines of anti-HER2 therapy in the locally advanced or metastatic setting
• Had disease progression on or after any previous trastuzumab deruxtecan (T-DXd) treatment

Arm 2:

-Has received no more than 5 prior lines of anti-HER2 therapy in the locally advanced or metastatic setting

Arm 3:

-Has received and had disease progression from T-DXd treatment in any setting and a maximum of 3 prior lines of anti-HER2 therapy in the locally advanced or metastatic setting. T-DXd must be the most recent therapy received before enrollment.

Exclusion criteria

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

• Uncontrolled or significant cardiovascular disease
• History of (noninfectious) pneumonitis/interstitial lung disease (ILD) that required steroids or has current pneumonitis/interstitial lung disease
• Has clinically severe respiratory compromise
• Has any history of or evidence of any current leptomeningeal disease
• Has clinically significant corneal disease
• Evidence of ongoing uncontrolled systemic bacterial, fungal, or viral infection
• HIV infected participants with a history of Kaposi's sarcoma and/or Multicentric Castleman's Disease
• Known additional malignancy that is progressing or has required active treatment within the past 3 years
• Evidence of spinal cord compression or brain metastases
• Has an active infection requiring systemic therapy
• Concurrent active HBV and HCV infection
• Has had major surgical procedure (excluding placement of vascular access) less than 28 days

Arm 3 ONLY

• Has received prior treatment with tucatinib, lapatinib, or neratinib, or any investigational HER2-targeted tyrosine kinase inhibitors in the locally advanced or metastatic setting