Drug Interactions Newsletter

Volume 1, Issue 31, 08/18/2009

Question

Why should the xanthine oxidase inhibitor, febuxostat (Uloric®) not be given to patients taking the immunosuppressant azathioprine (Azasan®, Imuran®)?

Answer

Febuxostat (Uloric®) is a xanthine oxidase (XO) inhibitor indicated for the chronic management of hyperuricemia in patients with gout.(1)  It differs from allopurinol in that it is a nonpurine analogue that specifically inhibits both the reduced and oxidized forms of XO and does not interfere with other metabolic pathways involved in purine and pyrimidine synthesis or degradation.(2-10)  The current product package insert indicates that while no pharmacokinetic drug interaction studies have been done, febuxostat is contraindicated in patients being treated with azathioprine or mercaptopurine.(1)  The use of azathioprine (Azasan®, Imuran®) is common for a number of conditions, some of which include the prevention of rejection of organ transplantation, inflammatory bowel disease (in particular Crohn's disease), rheumatoid arthritis, systemic lupus erythematosus, and several other immune mediated chronic diseases.(11-20) 

What happens if febuxostat and azathioprine are used together?
The simple answer is that the patient is at a high risk for potentially life threatening......

The complete answer to this question provides a figure that shows mechanism for the drug interaction between febuxostat and azathioprine. 

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