A 48-year-old woman is diagnosed with HER2-positive breast cancer with metastatic disease. Which drug targeting the HER2 oncogene is the first-line choice for her treatment?

Explanation

## HER2-Targeted Therapy in Breast Cancer **Key Point:** Trastuzumab (Herceptin) is the first-line monoclonal antibody targeting HER2 (human epidermal growth factor receptor 2) in HER2-positive breast cancer, including metastatic disease. ### HER2 Oncogene and Carcinogenesis - HER2 is a receptor tyrosine kinase overexpressed in ~15–20% of breast cancers due to gene amplification - Constitutive HER2 signaling drives proliferation, survival, and metastasis - HER2 positivity is determined by immunohistochemistry (IHC 3+) or fluorescence in situ hybridization (FISH) ### Trastuzumab Mechanism 1. Binds to extracellular domain of HER2 2. Blocks ligand-independent signaling 3. Induces antibody-dependent cellular cytotoxicity (ADCC) 4. Inhibits HER2 shedding and downstream PI3K/Akt and MAPK pathways **High-Yield:** Trastuzumab + chemotherapy (usually taxane-based) is the standard of care for HER2-positive metastatic breast cancer, improving overall survival by ~5 years compared to chemotherapy alone. ### Clinical Evidence - HER2-positive metastatic BC: median OS ~25–30 months with trastuzumab + chemotherapy - Without trastuzumab: median OS ~10–12 months - Trastuzumab can be combined with pertuzumab (dual HER2 blockade) for enhanced benefit **Clinical Pearl:** Cardiotoxicity (HF, LVEF decline) is a major side effect of trastuzumab; baseline echocardiography and periodic monitoring are mandatory. ### Why Not the Alternatives? | Option | Indication | Limitation in HER2+ Disease | |--------|-----------|-----------------------------| | Trastuzumab | HER2+ BC (all stages) | Gold standard; no limitation | | Tamoxifen | ER+ BC | Ineffective in HER2+ without ER; hormone-based, not targeted | | Paclitaxel alone | Chemotherapy backbone | Must combine with anti-HER2 agent for optimal outcome | | Letrozole | ER+ postmenopausal BC | Hormone therapy; not effective for HER2-driven disease | **Mnemonic:** **TRASH** = TRastuzumab for HER2-positive Aggressive Solid tumors (Breast, gastric, ovarian).