Azathioprine (Rheumatology)

How it works

It is a prodrug converted to 6-mercaptopurine, whose metabolites inhibit purine synthesis and suppress proliferation of T and B lymphocytes.

Prescribing in practice

• Thiopurine methyltransferase (TPMT) activity should be assessed before starting, because low or absent activity markedly increases the risk of severe, life-threatening myelosuppression.
• Allopurinol greatly increases azathioprine toxicity, so the combination is avoided or the azathioprine dose is substantially reduced under specialist guidance.
• Hepatotoxicity, pancreatitis and an increased long-term risk of infection and certain malignancies (including skin cancer) should be considered.

Monitoring

Monitor full blood count and liver function frequently after initiation and dose changes, then at regular intervals during maintenance.

Counselling the patient

• Report sore throat, fever, bruising, bleeding or jaundice without delay.
• Use sun protection and tell clinicians you take an immunosuppressant.
• Avoid live vaccines unless specifically advised.

Evidence & guidelines

Long-standing use in rheumatic disease is supported by clinical trial evidence and reflected in NICE and specialist society guidance.