What is the mechanism of action of cyclophosphamide?

Mechanism of Action of Cyclophosphamide

Cyclophosphamide is an alkylating agent that works by adding alkyl groups to DNA of rapidly dividing cells, cross-linking DNA strands, and thereby inhibiting DNA replication and cell proliferation. 1

Pharmacological Activation

Cyclophosphamide is a prodrug that requires metabolic activation to exert its therapeutic effects:

1. Hepatic Activation: Approximately 75% of cyclophosphamide is activated by hepatic microsomal cytochrome P450 enzymes, primarily:

   • CYP2B6 (highest 4-hydroxylase activity)
   • CYP2A6, CYP3A4, CYP3A5, CYP2C9, CYP2C18, and CYP2C19 2

2. Metabolic Pathway:

   • Cyclophosphamide → 4-hydroxycyclophosphamide (in equilibrium with aldophosphamide)
   • Aldophosphamide undergoes β-elimination to form the active metabolites:
     • Phosphoramide mustard (primary cytotoxic agent)
     • Acrolein (contributes to toxicity) 2

Mechanism of DNA Damage

The active metabolites, particularly phosphoramide mustard, exert cytotoxic effects through:

• DNA Alkylation: Adding alkyl groups to the N-7 position of guanine in DNA 3
• DNA Cross-linking: Formation of N,N-bis[2-(N7-guaninyl)ethyl]amine (G-NOR-G) cross-links between DNA strands 3
• Disruption of Cell Division: These cross-links prevent DNA replication and transcription, leading to cell death 1

Cellular Targets and Selectivity

Cyclophosphamide demonstrates selectivity in its effects:

• Rapidly Dividing Cells: Primarily affects cells with high proliferation rates (cancer cells)
• T-Cell Selectivity: Shows preferential effects on T cells, particularly at lower doses 4
• Regulatory T-Cell Suppression: A single low-dose administration can selectively suppress regulatory T cells (Tregs), which can counteract immunosuppression in cancer 4

Inactivation Pathways

Several pathways can inactivate cyclophosphamide metabolites:

• Aldehyde Dehydrogenase (ALDH): Converts aldophosphamide to inactive carboxyphosphamide
• Glutathione S-Transferase (GST): Conjugates metabolites for elimination 1
• Auto-induction: Cyclophosphamide appears to induce its own metabolism, resulting in increased clearance with repeated administration 2

Clinical Considerations

• Dosing Impact: The schedule and dose of administration significantly affect the immunological effects:

  • High doses: Complete eradication of hematopoietic cells
  • Lower doses: Relatively selective for T cells 4

• Cardiotoxicity Risk: High-dose cyclophosphamide-induced heart failure has been reported in up to 28% of patients, with higher risk in elderly patients and those previously exposed to anthracyclines or mediastinal irradiation 1

• Neurocognitive Effects: Cyclophosphamide can contribute to cognitive impairment through:

  • Oxidative stress in brain tissue
  • Generation of reactive oxygen species (ROS)
  • Neuroinflammation via pro-inflammatory cytokines 1

Monitoring Considerations

• Monitor for myelosuppression (dose-limiting toxicity)
• Watch for ECG alterations (low QRS voltage, non-specific T-wave or ST segment abnormalities) 1
• Assess for hemorrhagic cystitis and other urinary toxicities
• Consider genetic testing for enzymes involved in metabolism (CYP2B6) in patients with unexpected responses 1

Cyclophosphamide remains one of the most versatile and widely used anticancer and immunosuppressive agents despite being in clinical use for over 60 years, with applications ranging from cancer treatment to management of autoimmune disorders and transplant rejection.