Understanding Targeted Therapies for Metastatic Breast Cancer: Mechanisms and Clinical Use

Targeted therapies attack specific molecular drivers of cancer—sparing more normal cells than chemotherapy. In metastatic breast cancer (MBC), they've transformed outcomes, especially for HR+ and HER2+ subtypes.

Major Targeted Therapy Classes

1. CDK4/6 Inhibitors (HR+/HER2–)

• Drugs: Palbociclib, Ribociclib, Abemaciclib
• Mechanism: Block cyclin-dependent kinases 4/6 → halt cell cycle at G1
• Use: + aromatase inhibitor (1st line) or fulvestrant (later)
• Key toxicity: Neutropenia (palbo/ribociclib), diarrhea/fatigue (abemaciclib), QT prolongation (ribociclib)

2. HER2-Targeted Agents

• Trastuzumab + Pertuzumab: Dual HER2 blockade (1st line + taxane)
• T-DM1 (ado-trastuzumab emtansine): ADC; 2nd line standard
• T-DXd (trastuzumab deruxtecan): Next-gen ADC; bystander effect; now preferred 2nd line (DESTINY-Breast03)
• Tucatinib + capecitabine + trastuzumab: Blood-brain barrier penetration; brain mets activity
• Margetuximab, ZW25, novel ADCs: In trials

3. PARP Inhibitors (BRCA1/2 mutated, HER2–)

• Olaparib, Talazoparib
• Mechanism: Synthetic lethality—PARP inhibition + HR deficiency = catastrophic DNA damage
• Use: Germline BRCA+ MBC (OlympiAD, EMBRACA)
• Toxicity: Anemia, nausea, fatigue; rare MDS/AML

4. PI3K/AKT/mTOR Pathway Inhibitors

• Alpelisib (PI3Kα): + fulvestrant for PIK3CA-mutated HR+ MBC (SOLAR-1)
• Capivasertib (AKT): + fulvestrant for PIK3CA/AKT1/PTEN altered (CAPItello-291)
• Everolimus (mTOR): + exemestane (BOLERO-2); stomatitis, pneumonitis, hyperglycemia

5. Antibody-Drug Conjugates (ADCs) – Broader

• Sacituzumab govitecan (Trop-2): TNBC, HR+ (TROPiCS-02)
• Dato-DXd (TROP-2): HR+ (TROPION-Breast01)
• Patritumab deruxtecan (HER3): Emerging

6. Immunotherapy

• Pembrolizumab + chemo for PD-L1+ TNBC (KEYNOTE-355/522)
• Limited role in HR+/HER2+ MBC currently

Clinical Decision Factors

• Subtype, prior treatments, mutation status, metastatic sites, performance status, patient preference
• Biomarker testing (tissue + liquid biopsy) at each progression
• Sequencing matters: maximize lines of therapy

Targeted therapy = precision. But resistance evolves. Trials are the pipeline for what's next.