Oncology-Anti-Tumor Antibiotics

I. Anthracyclines (Doxorubicin, Daunorubicin, Epirubicin, Idarubicin)

A. Mechanism of Action:

• Multiple Effects: Anthracyclines affect cell surfaces, signal transduction (activating protein kinase C), and, most importantly, DNA topoisomerase II (topo II). The exact contribution of each effect to cytotoxicity remains unclear.
• Free Radical Generation: Reduction to highly reactive compounds and free radical production contribute to their anti-cancer activity, but also cause cardiotoxicity (due to lower antioxidant defenses in the heart).
• Topoisomerase II Inhibition: Anthracyclines bind to the DNA-topo II complex, preventing DNA strand rejoining after temporary breaks, leading to DNA damage and cell death.

B. Drug Resistance:

• MDR1 Gene/P-glycoprotein (Pgp): This efflux pump, encoded by the MDR1 gene, actively removes anthracyclines from cells, limiting their effectiveness. Manipulating Pgp expression has had limited clinical success.
• MRP (Multidrug Resistance-associated Protein): Another efflux pump contributing to anthracycline resistance.

C. Pharmacokinetics & Metabolism:

• Rapid Initial Decline: After IV administration, plasma levels fall rapidly due to tissue distribution and DNA binding.
• Slow Elimination: Subsequent metabolism and elimination result in slow plasma concentration decline over several days.
• Liver Function: Dose reduction is recommended for patients with abnormal liver function to minimize toxicity.

D. Clinical Use:

• Doxorubicin & Epirubicin: IV administration for breast cancer, sarcoma, and hematological malignancies.
• Daunorubicin & Idarubicin: Primarily used in acute leukemia (idarubicin can be oral).

E. Toxicity:

• Acute Toxicities (5-10 days post-treatment): Myelosuppression (bone marrow suppression), mucositis (mouth sores), and alopecia (hair loss - reversible). Extravasation (leakage from IV site) is severe and lacks effective treatment.
• Cumulative Cardiotoxicity: Dose-dependent heart failure due to free radical accumulation. Risk is <5% below 450mg/m² doxorubicin, significantly increasing at higher doses. Pre-existing heart disease and radiation therapy increase risk. Liposomal doxorubicin reduces cardiotoxicity. Epirubicin, daunorubicin, and idarubicin are less cardiotoxic than doxorubicin.

II. Other Anti-Tumor Antibiotics

A. Mitoxantrone:

• Mechanism: Binds to DNA, interacts with topo II, but produces fewer free radicals than anthracyclines. Also a Pgp substrate.
• Clinical Use: Less cardiotoxic alternative to doxorubicin in advanced breast cancer, NHL, and non-lymphocytic leukemia, but less effective.

B. Dactinomycin (Actinomycin-D):

• Mechanism: Strong DNA intercalator, inhibiting RNA and protein synthesis. Pgp substrate.
• Clinical Use: Highly active against childhood cancers.

C. Mitomycin (MMC):

• Mechanism: Active against various solid tumors; also used as a radiosensitizer.
• Clinical Use: Used in combination chemotherapy for breast cancer, NSCLC, GI cancer, and as a radiosensitizer in anal cancer.
• Toxicity: Delayed and cumulative myelosuppression (especially thrombocytopenia). Administered every 6 weeks due to this toxicity. Uncommon side effects include hemolytic-uremic syndrome, pulmonary fibrosis, and cardiac complications.

III. Key Differences & Summary Table