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?

Question

Accepted Answer

B. Trastuzumab. ## 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).

Answer

A. Paclitaxel alone

Answer

C. Letrozole

Answer

D. Tamoxifen

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

HER2 Oncogene and Carcinogenesis

Trastuzumab Mechanism

Clinical Evidence

Why Not the Alternatives?

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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