There is a known drug interaction (DI) between xanthine oxidase (XO) inhibitors and the thiopurine immunosuppressants, azathioprine (AZA) and mercaptopurine (6-MP). Xanthine oxidase inhibition increases concentrations of AZA and 6-MP active metabolites, possibly resulting in myelosuppression. When allopurinol is used with AZA or 6-MP, dose reduction of AZA or 6-MP is recommended. Febuxostat is a newer XO inhibitor approved for the treatment of gout.

To determine the clinical impact of the febuxostat-thiopurine DI.

Case series derived from the U.S. Food and Drug Administration Adverse Event Reporting System (FAERS) and published medical literature.

Nineteen patients who received concomitant febuxostat and either AZA or 6-MP.

Laboratory and clinical data.

Nineteen cases reporting myelosuppressive events were identified in patients receiving febuxostat with AZA or 6-MP. Eighteen cases were treated with the combination of AZA and febuxostat. A median of 1.6 months elapsed from initiation of the drug combination until discovery of the event. Sixteen cases required hospitalization; 15 reported administration of blood products. Thirteen cases reported resolution of the event with discontinuation of both drugs, two reported discontinuation of the thiopurine only, and one reported discontinuation of febuxostat only.

Thiopurine monotherapy may cause myelosuppression. Complications of immunosuppression that may contribute to the real-world morbidity and mortality associated with the febuxostat-thiopurine DI were not examined. Finally, FAERS data are limited by the voluntary nature of reporting.

Current febuxostat labeling contraindicates concomitant administration of febuxostat with either AZA or 6-MP. This case series demonstrates that the DI can result in clinically significant adverse events and is supportive of current febuxostat labeling.