Which finding best distinguishes HER2-positive breast cancer treated with trastuzumab from EGFR-mutant lung cancer treated with erlotinib?

Explanation

## Mechanism-Based Distinction: Trastuzumab vs. Erlotinib ### Mechanism of Action Comparison **Key Point:** Trastuzumab is a monoclonal antibody that binds HER2 extracellularly and triggers immune-mediated cell death (ADCC) and receptor internalization. Erlotinib is a small-molecule TKI that competitively inhibits the intracellular ATP-binding pocket of EGFR kinase. ### Detailed Mechanism Table | Aspect | Trastuzumab (mAb) | Erlotinib (TKI) | | --- | --- | --- | | **Drug class** | Monoclonal antibody (IgG1) | Small-molecule TKI | | **Target** | HER2 extracellular domain | EGFR intracellular kinase domain | | **Primary mechanism** | ADCC, CDC, receptor internalization | Competitive ATP-site inhibition | | **Route** | IV infusion | Oral | | **Resistance mechanism** | HER2 downregulation, PTEN loss | EGFR T790M mutation, MET amplification | | **CNS penetration** | Poor (large protein) | Good (lipophilic) | | **Cardiac toxicity** | Yes (HER2 on cardiomyocytes) | Rare | ### Mechanism Flowchart ```mermaid flowchart TD A[HER2-positive breast cancer]:::outcome --> B[Trastuzumab]:::action B --> C[Binds HER2 extracellularly]:::action C --> D[ADCC + Internalization]:::action D --> E[Cell death]:::outcome F[EGFR-mutant lung cancer]:::outcome --> G[Erlotinib]:::action G --> H[Penetrates cell membrane]:::action H --> I[Binds intracellular ATP pocket]:::action I --> J[Kinase inhibition]:::action J --> K[Apoptosis]:::outcome ``` ### Clinical Pearls **High-Yield:** Trastuzumab-induced cardiotoxicity (HER2 signaling in cardiomyocytes) is a major toxicity; erlotinib causes interstitial lung disease (ILD) and rash. These toxicity profiles reflect their different mechanisms and tissue distribution. **Clinical Pearl:** Erlotinib can achieve CNS concentrations sufficient to treat brain metastases; trastuzumab cannot reliably cross the BBB, necessitating alternative strategies (e.g., lapatinib, a dual EGFR/HER2 TKI) for CNS involvement in HER2+ disease. **Mnemonic:** **ADCC** = Antibody-Dependent Cellular Cytotoxicity (mAb mechanism); **ATP** = Adenosine Triphosphate (TKI target site). [cite:Harrison 21e Ch 107]