A 48-year-old woman with newly diagnosed HER2-positive breast cancer and metastatic disease to the liver is being evaluated for systemic therapy. She has no prior chemotherapy or targeted therapy. What is the drug of choice for HER2-targeted therapy?

Explanation

## First-Line HER2-Targeted Therapy in Metastatic Breast Cancer **Key Point:** Trastuzumab (Herceptin) is the gold-standard first-line HER2-targeted monoclonal antibody for HER2-positive metastatic breast cancer and should be the backbone of any HER2-directed regimen. ### Mechanism and Role of Trastuzumab **High-Yield:** Trastuzumab is a recombinant humanized monoclonal antibody against HER2 that: - Blocks HER2 receptor signaling - Induces antibody-dependent cellular cytotoxicity (ADCC) - Improves overall survival in HER2+ metastatic disease by ~5 years when combined with chemotherapy [cite:Harrison 21e Ch 352] - Is used as backbone therapy in all HER2+ regimens ### HER2-Targeted Agent Hierarchy | Agent | Class | Role | Timing | | --- | --- | --- | --- | | **Trastuzumab** | mAb (anti-HER2) | **First-line backbone** | Initial therapy | | Pertuzumab | mAb (HER2 dimerization inhibitor) | First-line (with trastuzumab + chemo) | Combined with trastuzumab | | T-DM1 | Antibody-drug conjugate | Second-line (after progression on trastuzumab) | Post-trastuzumab progression | | Lapatinib | Tyrosine kinase inhibitor | Second-line or salvage | After trastuzumab/pertuzumab failure | **Clinical Pearl:** Pertuzumab is NOT used alone—it is always combined with trastuzumab and chemotherapy in first-line metastatic HER2+ disease. Trastuzumab remains the essential backbone. ### Treatment Algorithm for HER2+ Metastatic Breast Cancer ```mermaid flowchart TD A[HER2+ Metastatic BC]:::outcome --> B{Prior HER2-directed therapy?}:::decision B -->|No| C[Trastuzumab + Chemotherapy]:::action C --> D[Add Pertuzumab?]:::decision D -->|Yes| E[Trastuzumab + Pertuzumab + Chemo]:::action D -->|No| F[Trastuzumab + Chemo alone]:::action B -->|Yes, progressed| G[T-DM1 or Lapatinib-based regimen]:::action E --> H[Reassess at 6-8 weeks]:::decision F --> H H -->|Progression| I[Switch to T-DM1 or TKI]:::action ``` **Mnemonic:** **PERT** = **Pertuzumab** is an add-on; **TRAS** = **Trastuzumab** is the backbone. **Tip:** Trastuzumab is given intravenously (loading dose 8 mg/kg, then 6 mg/kg every 3 weeks) or subcutaneously (600 mg fixed dose). Monitor left ventricular ejection fraction (LVEF) at baseline and every 3 months due to cardiotoxicity risk.