Antitumor Antibiotics

Overview & MoA - Cycle's Hitmen

• Microbial-derived (e.g., Streptomyces). Disrupt DNA/RNA synthesis/function, halting cell cycle, inducing apoptosis.
• Key Mechanisms:
  • DNA Intercalation: Wedge between DNA base pairs, distorting structure, blocking replication/transcription (e.g., Dactinomycin, Doxorubicin).
  • Free Radical Generation: ROS cause DNA strand breaks (e.g., Doxorubicin, Bleomycin).
  • Topoisomerase II Inhibition: Stabilize enzyme-DNA complex, preventing DNA break re-ligation (e.g., Doxorubicin, Mitoxantrone).
  • DNA Alkylation: Covalently bind DNA, causing cross-links (e.g., Mitomycin C).
  • DNA Strand Scission: Directly cleave DNA strands (e.g., Bleomycin, needs $Fe^{2+}$).

⭐ Doxorubicin's major dose-limiting toxicity is cardiotoxicity (free radical mediated). Dexrazoxane can be used for cardioprotection. 📌 Bleomycin: Breaks DNA, specific for G2-M phase.

Anthracyclines - Ruby Red Risks

Derived from Streptomyces peucetius var. caesius. Potent broad-spectrum agents.

• Examples: Doxorubicin, Daunorubicin, Epirubicin, Idarubicin.
• 📌 "Ruby Red" drugs: Cause red urine.

Mechanism of Action (MoA):

• Intercalates DNA → blocks DNA/RNA synthesis.
• Inhibits Topoisomerase II → DNA strand breaks.
• Generates ROS (free radicals) → cardiotoxicity.

Clinical Uses:

• Leukemias (AML), lymphomas (Hodgkin's, NHL).
• Solid tumors: Breast, ovarian, lung, sarcomas.

Key Toxicities:

• Cardiotoxicity (Dose-Limiting):
  • Acute: Arrhythmias, pericarditis-myocarditis (reversible).
  • Chronic: Dose-dependent DCM (irreversible).
    • ROS-mediated myocyte damage.
    • Monitor: LVEF (ECHO/MUGA) baseline & periodic.
    • Prevention: Dexrazoxane (iron chelator).
• Myelosuppression (dose-limiting).
• Mucositis, stomatitis.
• Alopecia.
• Extravasation → tissue necrosis.
• Radiation recall.

⭐ Doxorubicin max cumulative dose: 450-550 mg/m² (Daunorubicin ~550 mg/m²) to limit irreversible heart failure. Dexrazoxane: cardioprotectant.

Bleomycin - Lung's Nemesis

• Mechanism: DNA strand scission via oxidative damage; forms complex with Fe²⁺ & O₂, generating free radicals. Cell cycle specific (G2 phase).
• Uses:
  • Testicular cancers (e.g., BEP regimen)
  • Hodgkin's lymphoma (e.g., ABVD regimen)
  • Squamous cell carcinomas (head & neck, skin, cervix)
  • Malignant pleural effusion (sclerosing agent)
• Adverse Effects:
  • Pulmonary fibrosis: Dose-dependent (cumulative dose > 400 units). 📌 "BLOW-mycin" for lung toxicity.
  • Skin: Hyperpigmentation (flagellate rash), hyperkeratosis.
  • Mucositis, fever, allergic reactions.
  • Minimal myelosuppression.

  ⭐ Bleomycin causes severe, potentially fatal pulmonary fibrosis, especially at cumulative doses exceeding 400 units; it is notably sparing of bone marrow.

• Key Feature: Low levels of inactivating enzyme (bleomycin hydrolase) in lungs & skin contribute to toxicity.

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Other Key Agents - Diverse Attackers

• Dactinomycin (Actinomycin D)

  • MoA: Intercalates between Guanine-Cytosine (G-C) base pairs in DNA; blocks DNA-dependent RNA polymerase. Halts transcription.
  • Uses: Key for pediatric cancers: Wilms' tumor, Ewing's sarcoma, rhabdomyosarcoma. Also gestational choriocarcinoma. 📌 Dactinomycin: "D" for DNA; "Actino-" for Acting on kids' tumors.
  • Toxicity: Severe myelosuppression, N/V, mucositis, alopecia.

    ⭐ Characteristic: Radiation recall phenomenon (inflammation at prior irradiated sites).

• Mitomycin C

  • MoA: Prodrug; bioreductively activated (esp. in hypoxic cells) to a bifunctional alkylating agent. Induces DNA cross-linking.
  • Uses: Hypoxic solid tumors (stomach, pancreas, bladder, anal). Intravesical instillation for superficial bladder cancer.
  • Toxicity: Delayed, cumulative myelosuppression (esp. thrombocytopenia). HUS (microangiopathic hemolytic anemia, thrombocytopenia, ARF). Interstitial pneumonitis.

High‑Yield Points - ⚡ Biggest Takeaways

• Anthracyclines (e.g., Doxorubicin): Dose-dependent cardiotoxicity (use Dexrazoxane), red urine.
• Bleomycin: Pulmonary fibrosis, skin changes, minimal myelosuppression. For testicular cancer, Hodgkin's.
• Dactinomycin (Actinomycin D): Key for pediatric tumors (Wilms', Ewing's, Rhabdomyosarcoma).
• Mitomycin C: Severe myelosuppression, risk of Hemolytic Uremic Syndrome (HUS).
• Plicamycin (Mithramycin): Treats hypercalcemia of malignancy; hepatotoxic.
• Anthracycline MOA: DNA intercalation, free radical formation, topoisomerase II inhibition.