Systemic Therapy

Targeted Therapy

Targeted therapy attacks specific features of the cancer cell while minimizing damage to normal cells. They work by:

• Blocking signals that tell cancer cells to grow
• Altering proteins in cancer cells to trigger cell death
• Preventing tumours from forming new blood vessels
• Helping the immune system recognize and attack cancer cells
• Delivering treatment directly to cancer cells

HER2-Targeted Therapies

For HER2 positive breast cancer, these therapies block HER2 receptors to slow or stop cancer growth.

• Monoclonal Antibodies: Lab-made proteins that bind to HER2 receptors, blocking tumour growth and helping the immune system destroy cancer cells. Examples:
  • Trastuzumab (Herceptin)
    Common side effects: Cardiotoxicity – every patient has a heart function test before and during treatment
  • Pertuzumab (Perjeta)
    Common side effects: Fatigue, nausea, vomiting, and mouth sores
     
• Tyrosine Kinase Inhibitors: Small drug molecules that block an enzyme called tyrosine kinase, which helps HER2 positive cancer cells grow. This enzyme works inside the cell and becomes overactive when HER2 is overexpressed.
  • Examples: lapatinib (Tykerb), neratinib (Nerlynx), tucatinib (Tukysa)
  • Common side effects: mouth sores, fatigue, cough, dyspnea, diarrhea, nausea, and vomiting

Antibody-Drug Conjugates (ADCs)

Antibody-drug conjugates are a type of targeted therapy that link an antibody (which targets the cancer cell) with a chemotherapy drug. The antibody helps deliver the chemotherapy directly into the cancer cell, helping reduce damage to healthy cells. Examples:

• Trastuzumab emtansine, T-DM1 (Kadcyla): Used for HER2 positive breast cancer
  • Common side effects: Fatigue, nausea, vomiting, bone and muscle pain, and low white blood cell counts
• Trastuzumab deruxtecan, T-DXd (Enhertu): Used for metastatic HER2 positive breast cancer
  • Common side effects: Nausea, vomiting, fatigue, and infection
• Sacituzumab govitecan, SG (Trodelvy): Used for metastatic HR positive and triple negative breast cancer (TNBC)
  • Common side effects: Diarrhea, nausea, vomiting, fatigue, low blood counts

ADCs are an evolving area of treatment and may be an option if cancer has progressed after other therapies.

CDK4/6 Inhibitors

For HR positive, HER2 negative breast cancer, these drugs slow cancer cell division and are used with hormone therapy to improve long-term disease control.

• Examples: palbociclib (Ibrance), ribociclib (Kisqali), abemaciclib (Verzenio)
  • Common side effects: Low white blood cell count, infection, fatigue, nausea and vomiting, and liver irritation vomiting, and mucositis

PI3K Inhibitors

These block part of the PI3K/AKT/PTEN pathway, which helps cancer cells survive and grow.

• Examples: Alpelisib (Piqray), and Inavolisib (Itovebi)
  • Common side effects: High blood sugar, diarrhea, rash, nausea and vomiting, fatigue, low appetite, and mouth sores

mTOR Inhibitors

Used with endocrine therapy for advanced HR positive, HER2 negative breast cancer after aromatase inhibitors stop working.

• Example: everolimus (Afinitor)
  • Common side effects: Mouth sores, fatigue, cough, diarrhea, rash, nausea, and vomiting

BRCA-Targeted Therapies

Used for HER2 negative metastatic breast cancer in BRCA1/2 mutation carriers.

• Example: olaparib (Lynparza)
  • Common side effects: Nausea, vomiting, fatigue, anemia, diarrhea, low white blood cell count, cough, and dizziness

Medical Review by Roochi Arora, MD, FRCPC, August 2025

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