What is the mechanism of action of Cyclophosphamide?

Mechanism of Action of Cyclophosphamide

Cyclophosphamide is a prodrug that works by cross-linking DNA through its active metabolites, particularly phosphoramide mustard, which alkylates DNA and prevents replication and transcription, ultimately leading to cell death. 1, 2

Metabolic Activation Process

Cyclophosphamide itself is inactive and requires biotransformation to exert its therapeutic effects:

1. Initial Activation: Cyclophosphamide is primarily activated in the liver by cytochrome P450 enzymes (especially CYP2B6, but also CYP2A6, 3A4, 3A5, 2C9, 2C18, and 2C19) 1, 2

2. Metabolic Pathway:

   • Hepatic metabolism converts cyclophosphamide to 4-hydroxycyclophosphamide 1
   • 4-hydroxycyclophosphamide exists in equilibrium with its ring-open tautomer, aldophosphamide 1, 3
   • Aldophosphamide undergoes spontaneous β-elimination to form the ultimate active metabolites: phosphoramide mustard and acrolein 1, 3

3. Inactivation Pathways:

   • 4-hydroxycyclophosphamide and aldophosphamide can be oxidized by aldehyde dehydrogenases to form inactive metabolites (4-ketocyclophosphamide and carboxyphosphamide) 1
   • Less than 5% of cyclophosphamide may be directly detoxified through side chain oxidation 1

Mechanism of Cytotoxicity

The therapeutic effect of cyclophosphamide occurs through several mechanisms:

• DNA Cross-linking: Phosphoramide mustard, the active metabolite, adds alkyl groups to DNA, particularly at the N-7 position of guanine, forming DNA-DNA cross-links (G-NOR-G adducts) 2, 4, 5

• Disruption of Cell Division: These cross-links prevent DNA replication and transcription, leading to inhibition of cell proliferation 2, 5

• Cell Death: The inability to repair extensive DNA damage triggers apoptosis, particularly in rapidly dividing cells 2

• Selective Toxicity: Cyclophosphamide shows relative selectivity for rapidly proliferating cells, including both malignant cells and certain immune cells 6, 2

Pharmacokinetic Considerations

• Auto-induction: Cyclophosphamide appears to induce its own metabolism, resulting in increased clearance and shortened half-life with repeated administration 1

• Elimination: 10-20% of cyclophosphamide is excreted unchanged in urine, with the majority eliminated as metabolites 1

• Tissue Selectivity: The relative sparing of some tissues occurs because aldehyde dehydrogenase (ALDH) is present at high levels in many tissues (e.g., bone marrow) but at lower levels in others (e.g., lymphocytes) 6

Clinical Applications

The mechanism of action explains cyclophosphamide's dual utility:

• Anticancer Agent: The DNA cross-linking activity makes it effective against rapidly dividing malignant cells 2, 5

• Immunosuppressant: Its ability to target lymphocytes makes it useful in autoimmune disorders and transplantation 6, 2

Understanding this mechanism helps explain both the therapeutic effects and common adverse reactions like myelosuppression, hemorrhagic cystitis (from acrolein), and potential for secondary malignancies (from DNA damage in non-target cells).