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Azathioprine

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Anaesthetics and Critical Care

Obstetrics Gynaecology

Trauma and Orthopaedics

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Urology

Version Jan 2023

Screen for Thiopurine methyltransferase deficiency an enzyme that metabolises azathioprine. Patients should be warned to report immediately any signs or symptoms of bone marrow suppression e.g. inexplicable bruising or bleeding, infection

About : Always check the BNF link here

Drug used to suppress the immune system by its effect on purine metabolism
Used in autoinflammatory diseases and transplant medicine
About 1 in 300 have very low levels of TPMT which increases toxicity

Mechanism

Azathioprine is converted to 6-mercaptopurine
Formation of 6-ThioGTP can cause cell apoptosis
Effect mainly on T cell function. Blocks cell-mediated rejection

Indication/Dose

Suppression of transplant rejection, Azathioprine 1-2.5 mg/kg daily adjusted according to response
Autoimmune conditions, Azathioprine 1-3 mg/kg daily, adjusted according to response (consider withdrawal if no improvement within 3 months)
Inflammatory bowel disease (normal TPMT level) TARGET dose is Azathioprine 2mg to 2.5 mg/kg OD PO.
Drug should be started at half target dose for first week to minimise side effects. In patients with low TPMT levels take expert advice

Side effects

Bone marrow suppression, Suppresses red cell and platelet production
Hypersensitivity, malaise, dizziness, Arthralgia, Rash, alopecia
Pancreatitis, liver and renal impairment
IV dose is alkaline and very irritant and should be used only if the oral route not feasible

Dose range:You must check with BNF or drug datasheet

Indication	Dose	Frequency	Route
Transplant rejection	1-2.5 mg/kg/day	OD	PO
Autoimmune	1-3 mg/kg/day	OD	PO
Inflammatory bowel disease	2-2.5 mg/kg/day	OD	PO

Cautions

Monitor FBC weekly 4-8 weeks slowly to every 3 months
Thiopurine methyltransferase deficiency increases azathioprine toxicity

Interactions

**Allopurinol and Azathioprine/6 Mercaptopurine is a potentially lethal combination**
Requires oxidation by Xanthine Oxidase before renal excretion
Azathioprine toxicity with Allopurinol
Azathioprine toxicity with ACE Inhibitors, Angiotensin receptor blockers, Trimethoprim, Rifampicin

Contraindications

Thiopurine methyltransferase deficiency, Hypersensitivity
Severe active infections e.g. bacteria, varicella, HSV, Shingles

Dose

Use should be restricted to those with appropriate training and monitoring
Monitor FBC weekly 4-8 weeks slowly to every 3 months as well as LFT/U&E

References

Azathioprine in BNF

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