Alkylating agents

Mechanism of Action - DNA's Sticky Situation

• Covalently adds an alkyl group ($R-CH_2$) to DNA.
• Primary target: N7 position of guanine.
• This alkylation causes:
  • Inter-strand & intra-strand cross-links: Prevents DNA helicase from unzipping DNA.
  • Abnormal base pairing (G-T instead of G-C).
  • DNA strand breaks.
• Net result: Halts DNA replication & transcription → triggers apoptosis.

⭐ Alkylating agents are cell-cycle non-specific, meaning they can damage DNA at any stage of the cell cycle.

The Alkylating Crew - Rogues' Gallery

• MOA: All agents covalently alkylate guanine (N7) in DNA, leading to DNA cross-linking, strand breaks, and apoptosis. They are cell cycle non-specific.

• Nitrogen Mustards (Cyclophosphamide, Ifosfamide)
  • Toxicity: Hemorrhagic cystitis (from acrolein metabolite).
  • Antidote: Mesna.
• Nitrosoureas (-mustines: Carmustine, Lomustine)
  • Highly lipophilic, cross the blood-brain barrier.
  • Use: Brain tumors (e.g., glioblastoma).
• Platinum Analogs (-platins: Cisplatin, Carboplatin)
  • Toxicity: Nephrotoxicity, ototoxicity, peripheral neuropathy.
  • 📌 Mnemonic: Cisplatin "splats" the kidneys and ears.
• Alkyl Sulfonates (Busulfan)
  • Toxicity: Severe myelosuppression, pulmonary fibrosis ("Busulfan lung").

⭐ Cisplatin-induced nephrotoxicity can be mitigated with Amifostine (a cytoprotective free-radical scavenger) and aggressive chloride diuresis.

Toxic Turf - The Body's Backlash

• Class-Wide Toxicities

  • Myelosuppression: Dose-limiting, causing pancytopenia.
  • Gonadal dysfunction: Infertility, premature menopause.
  • Alopecia & GI distress (nausea, vomiting).
  • Carcinogenesis: Risk of secondary cancers.

    ⭐ Long-term therapy with alkylating agents carries a significant risk of secondary malignancies, particularly Acute Myeloid Leukemia (AML) and Myelodysplastic Syndrome (MDS).

• Agent-Specific Adverse Effects

  • Cyclophosphamide & Ifosfamide:
    • Hemorrhagic Cystitis from acrolein metabolite.
    • Prevent with Mesna & vigorous hydration. 📌 Mesna for a messy bladder.
    • Can cause SIADH.
  • Platinum Analogs (Cisplatin, Carboplatin):
    • Cisplatin: Nephrotoxicity, peripheral Neuropathy, Ototoxicity.
    • Prevent nephrotoxicity with amifostine & chloride diuresis.
    • Carboplatin: Primarily myelosuppression (thrombocytopenia).
  • Busulfan:
    • Pulmonary fibrosis ("Busulfan Lung").
    • Skin hyperpigmentation.

Resistance Mechanisms - Cancer's Counter-Play

• ↑ DNA Repair: Enhanced activity of enzymes like O⁶-methylguanine-DNA methyltransferase (MGMT) reverses guanine alkylation before cross-links form.
• ↓ Drug Permeability: Decreased drug influx or increased efflux via membrane transporters reduces intracellular concentration.
• Drug Inactivation: ↑ levels of nucleophilic substances, especially glutathione (GSH), which "trap" and neutralize the drug. This is often mediated by glutathione-S-transferase (GST).

⭐ Amifostine is a cytoprotective agent used to mitigate cisplatin toxicity; it mimics glutathione's protective mechanism in normal cells.

High‑Yield Points - ⚡ Biggest Takeaways

• Alkylating agents are cell cycle-nonspecific and work by cross-linking DNA at the N7 of guanine, leading to apoptosis.
• Myelosuppression is the universal dose-limiting toxicity.
• Cyclophosphamide and ifosfamide cause hemorrhagic cystitis, preventable with mesna and hydration.
• Nitrosoureas (e.g., carmustine) cross the blood-brain barrier, making them effective for brain tumors.
• Busulfan is known for causing pulmonary fibrosis (“busulfan lung”).
• Long-term use carries a significant risk of secondary malignancies (e.g., AML/MDS).