Nitrogen Mustards and Oxazaphosphorines: Cyclophosphamide and Ifosfamide

Mechanism

Pharmacological Class and Group

Bifunctional alkylating cytotoxic agents belonging to the oxazaphosphorine group.

Mechanism of Action

Cyclophosphamide and ifosfamide are prodrugs that require metabolic activation by the hepatic cytochrome P450 system. Its intermediate metabolite (4-hydroxycyclophosphamide) tautomerizes to aldophosphamide, which is spontaneously cleaved in target cells to release the phosphoramide mustard cation and acrolein.

Alkylation of DNA at the N7 Position of Guanine

1. Aziridinium Ion Formation: The beta-chloroethyl end of the active metabolite undergoes an intramolecular cyclization with elimination of the chloride ion, generating a highly reactive, positively charged intermediate: the aziridinium ion.

2. Nucleophilic Attack: This aziridinium ion is subjected to a nucleophilic attack by the nitrogen atom in position 7 (N7) of the guanine residues in the DNA double helix.

Aziridinium Ion Alkylation of N7 of Guanine Interstrand and Intrastrand Bridges

3. Bifunctional Cross Links: By having two chloroethyl reactive arms, the compound alkylates a second guanine adjacent or on the complementary strand, consolidating permanent interstrand cross-links. This prevents strand separation during cell replication in the S phase of the cell cycle, activating the p53-dependent apoptotic pathway.

Pharmacokinetics

High Resolution Pharmacokinetics

• Absorption and Bioavailability: Cyclophosphamide has excellent oral absorption (>90%), metabolizing rapidly in the liver. Ifosfamide is administered exclusively intravenously due to the need for close monitoring due to its neurotoxicity profile.
• Biotransformation: Cyclophosphamide is mainly activated by the isoenzymes CYP2B6 and CYP2C19. Ifosfamide is preferentially activated by the isoenzyme CYP3A4, which explains the high release of chloroacetaldehyde (a direct neurotoxic metabolite).
• Elimination: They are excreted by renal filtration in the form of active and inactive metabolites. Less than 15% of cyclophosphamide is cleared unchanged in the urine.
• Half-life (t_1/2): The plasma elimination half-life of cyclophosphamide varies between 4 and 8 hours; that of ifosfamide is approximately 6 to 15 hours (dose dependent).

Indicators and dose

Clinical Indications and Frequent Regimens

• Breast Cancer (AC Scheme): Doxorubicin (60 mg/m²) + Cyclophosphamide (600 mg/m²) administered every 21 days for 4 cycles.
• Non-Hodgkin lymphomas (CHOP Scheme): Cyclophosphamide, Doxorubicin, Vincristine (Oncovin) and Prednisone.
• Sarcomas of Soft Tissues and Bones: Ifosfamide at high doses (2-3 g/m²/d) combined with Doxorubicin or Etoposide.
• Serious Autoimmune Disorders (off-label): Refractory active lupus nephritis or Wegener's vasculitis, using low-dose monthly intravenous pulses.

Dosage and Clinical Adjustment

• Standard cyclophosphamide: 500 - 1000 mg/m² IV as a single infusion every 21 days.
• Standard ifosfamide: 1.5 - 2.5 g/m²/day IV as a 1-2 hour infusion for 3 to 5 consecutive days. Requires concomitant dosing of MESNA at 60% of the total ifosfamide dose (divided into doses at hours 0, 4, and 8 after starting the infusion or as a continuous infusion).
• Renal Adjustment: If creatinine clearance is < 30 mL/min, the dose of both compounds should be reduced by 25-30% to limit the retention of active metabolites.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Hypersensitivity to the active ingredient or oxazaphosphorines.
• Severe basal myelosuppression (Neutrophils < 1000/mm³, Platelets < 50,000/mm³).
• Active hemorrhagic cystitis or history of urothelial toxicity induced by previous chemotherapy.
• Lower urinary tract infection or active obstruction of the urinary outflow tract.

Relative Contraindications

• Moderate to severe renal dysfunction (risk of accumulation of urotoxic metabolites).
• Severe liver dysfunction (alters the metabolic activation of the prodrug).

Adverse Effects (ADR) and Specific Toxicity

• Myelosuppression: Dose-limiting neutropenia with nadir 10-14 days after infusion; Complete hematological recovery on day 21.
• Urothelial: Severe hemorrhagic cystitis secondary to bladder accumulation of acrolein.
• Gastrointestinal: Nausea and vomiting of moderate to high emetogenic potential (requires triple prophylaxis with 5-HT3 antagonist, dexamethasone and aprepitant).
• Neurological (Exclusive of Ifosfamide): Metabolic encephalopathy with confusion, hallucinations and lethargy mediated by the accumulation of chloroacetaldehyde in the central nervous system.

Critical Toxicity: Hemorrhagic Cystitis due to Acrolein and the Role of MESNA

The spontaneous decomposition of active oxazaphosphorine generates acrolein, an unsaturated aldehyde without antitumor activity but with a direct nephrotoxic profile on the cells of the urinary bladder epithelium:

Free Acrolein + Urothelium Depletion of Membrane Thiols Epithelial Detachment and Diffuse Hemorrhage

To counteract this effect, it is mandatory to administer MESNA (sodium 2-mercaptoethanesulfonate) in combination with aggressive saline hydration. The free thiol group of MESNA reacts chemically at the acidic pH of urine with the electrophilic group of acrolein, forming a non-toxic and stable soluble thioether, nullifying direct cellular damage to the bladder mucosa.

Drug Interactions of Clinical Relevance

• CYP3A4 and CYP2B6 inducers (Phenytoin, Carbamazepine, Rifampicin): They drastically accelerate the activation rate of the prodrug, acutely increasing the release of toxic metabolites and potentiating myelosuppression and urotoxicity.
• CYP inhibitors (Fluconazole, Clarithromycin): They reduce the rate of transformation to its active form, reducing the real antitumor efficacy.