Azathioprine

Mechanism

Mechanism of action

Azathioprine is a prodrug that, after absorption, is cleaved non-enzymatically in erythrocytes and tissues to release its initial active metabolite, 6-mercaptopurine (6-MP). Subsequently, 6-MP is metabolized cellularly and intracellularly through complex enzymatic cascades that result in the production of active thioguanine nucleotides (6-TGN):

1. Incorporation of False Nucleotides: The active metabolites 6-TGN are fraudulently incorporated into the replicating DNA and RNA chain of active immunological T and B lymphocytes. This disrupts de novo purine synthesis, blocks cellular gene transcription, and arrests the cell cycle.
2. Induction of T Lymphocyte Apoptosis: The active metabolite deoxy-GTP competitively blocks the activation of the intracellular G protein Rac1, selectively inducing the mediated apoptosis of stimulated T lymphocytes of the colonic mucosa, suppressing the inflammatory cellular cascade.

Metabolic Pathways and the Critical Role of the TPMT Enzyme

The metabolism of 6-mercaptopurine is regulated by three main competitive enzymatic pathways:

• Xanthine Oxidase (XO): Inactivates 6-MP to 6-thiouric acid in the liver in a harmless way.
• Thiopurine Methyltransferase (TPMT): Methylates and inactivates 6-MP to 6-methylmercaptopurine.
• Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT): Activating pathway that generates thioguanine nucleotides (6-TGN) directly responsible for immunosuppression and myelotoxicity.

Patients with autosomal recessive genetic polymorphisms that entail low or no TPMT enzymatic activity (≈ 0.3% of the population) massively divert metabolism towards the HGPRT pathway, accumulating lethal levels of 6-TGN. This triggers myelosuppression and severe and life-threatening bone marrow failure after normal doses of azathioprine. It is highly advisable to determine the phenotypic activity of TPMT or perform genotyping before starting treatment.

Pharmacokinetics

Pharmacokinetics

• Absorption: Rapid and incomplete absorption by oral route. Bioavailability of 30% - 47%.
• Distribution: Average volume of distribution of 0.8 L/kg. It easily crosses the placental barrier. Plasma protein binding of 30%.
• Metabolism: Extensive hepatic and intracellular, not dependent on cytochrome P450, but on cellular XO, TPMT and HGPRT.
• Excretion: Renal, being eliminated inertly as inactive methylated or oxidized metabolites in the urine.
• Half-life: Short. For azathioprine it is 10-20 minutes; of the 6-MP is 1 to 2 hours. However, the active intracellular 6-TGN metabolites have an extremely long half-life of several weeks, which explains why their full clinical therapeutic effect takes 8 to 12 weeks to manifest.

Indicators and dose

Approved indications

• Maintenance of clinical remission in moderate to severe active Crohn's disease and ulcerative colitis in patients dependent or refractory to corticosteroids.
• Complementary maintenance treatment post-organ transplant and in severe autoimmune hepatitis.

Dosage and Clinical Adjustment

Adults:

• 1.5 mg to 2.5 mg per kilogram of body weight (mg/kg/day) orally once daily with food.
• Close monitoring of complete blood count and liver transaminase profile should be performed: weekly during the first month of therapy, biweekly during the second month, and quarterly indefinitely during the remainder of active therapy.

Adjustment in Kidney Failure:

• eGFR 10 - 49 mL/min: Administer 75% of the dose calculated by weight.
• eGFR < 10 mL/min or hemodialysis: Administer 50% of the calculated dose, closely monitoring the hematological profile.

Security

Contraindications

• Absolute: Hypersensitivity to the drug or 6-MP. Active viral, bacterial or fungal infections of a severe nature. Homozygous deficiency of the TPMT enzyme. Coadministration with allopurinol or febuxostat. Pre-existing severe spinal cord failure. Active uncontrolled malignancies.

Adverse effects (ADRs)

• Severe myelosuppression (Dose-dependent): Extreme leukopenia (profound neutropenia with risk of opportunistic infections and sepsis), thrombocytopenia and anemia. It commonly occurs in subjects with a TPMT mutation or due to interactions.
• Hepatotoxicity: Elevation of transaminases, transient cholestasis or nodular regenerative hyperplasia of the liver due to deposition of methylated metabolites.
• Acute Immunoallergic Pancreatitis: It typically occurs in the first 4 weeks of starting treatment, is idiosyncratic in nature, not dose-dependent, and requires definitive and immediate suspension of the drug.
• Hepatosplenic T-Cell Lymphoma (HCLTHE): Malignancy with a very poor prognosis, described mainly in young men with IBD treated together long-term with azathioprine and anti-tumor necrosis factor (Anti-TNF) therapy.

Drug interactions

• Interaction with Allopurinol and Febuxostat (Extremely Critical): Allopurinol is a potent inhibitor of the enzyme xanthine oxidase (XO). Blocking this enzyme completely abolishes the main 6-MP inactivation pathway. This massively diverts metabolism towards the HGPRT activation pathway, raising blood 6-TGN levels and inducing life-threatening bone marrow failure with irreversible aplasia. Rule of use: Avoid association absolutely; If essential, the dose of azathioprine should be mandatory reduced to 25% - 33% of the standard dose, with weekly hematological counts being performed.
• Aminosalicylates (Mesalazine): Increases hematological toxicity due to competitive inhibition of TPMT.

Breastfeeding and Pregnancy

Use in Pregnancy: Traditionally controversial due to its proven placental passage. However, accumulated clinical data demonstrate that relapses of moderate course IBD are more harmful to pregnancy than therapy. For this reason, it is preferred to continue azathioprine during pregnancy under strict obstetric and fetal ultrasound surveillance.

Breastfeeding: Compatible. It is excreted in minute and harmless quantities in breast milk. No myelosuppressive effects or developmental delay have been reported in exposed infants.