Azathioprine

Azathioprine: Uses, Dosage and Side Effects

Overview

Azathioprine inhibits purine synthesis, which is necessary for cell proliferation especially of leucocytes and lymphocytes. Although the exact mechanism is unknown, it suppresses cell-mediated immunity, alters antibody production and inhibits cell growth.

Uses of Azathioprine

• Management of immune-mediated diseases such as immune-mediated hemolytic anemia and immune-mediated polyarthritis.
• Often used in conjunction with corticosteroids.

Dose of Azathioprine in Dogs and Cats

Dogs:

2 mg/kg p.o. q24h for a maximum of 2–3 weeks, then 0.5–2 mg/kg p.o. q48h.

Cats:

Not recommended.

Side Effects of Azathioprine in Dogs and Cats

• Bone marrow suppression is the most serious adverse effect.
• GI upset/anorexia, poor hair growth, acute pancreatitis and hepatotoxicity have been seen in dogs (the latter typically within 4 weeks of starting treatment).
• Cats in particular often develop a severe, non-responsive fatal leucopenia and thrombocytopenia.
• Avoid rapid withdrawal as this may cause a rebound hyperimmune response.

Contraindications of Azathioprine in Dogs and Cats

• Do not use in patients with bone marrow suppression
• Do not use in patients at high risk of infection or with hepatic disease.
• Use with caution in patients with a history of pancreatic disease.
• Not recommended for use in cats.

Some Notes:

• Routine haematology (including platelets) and biochemistry should be monitored closely: initially every 1–2 weeks; and every 1–2 months when on maintenance therapy.
•  In animals with renal impairment, dosing interval should be extended. Clinical responses can take up to 6 weeks.
• Azathioprine tablets should not be divided and should be stored at room temperature in well closed containers and protected from light.
• Enhanced effects and increased azathioprine toxicity when used with allopurinol.
• Increased risk of azathioprine toxicity with aminosalicylates and corticosteroids, avoid with ACE inhibitors as this may increase potential for haematological adverse events.