Thank you for your inquiry.
I also was unable to find any definitive answer to your question on a literature search. However, in general, there would be no need for a washout period based on the information in the literature that we do have.
As you know, allopurinol is a xanthinoxidase inhibitor that is structurally similar to purine compounds. Febuxostat, on the other hand, is a non-purine inhibitor of xanthinoxidase. Although both contain ring structures, there are distinct structural differences which would make it unlikely that there is antigenic cross-reactivity between the two drugs. However, we do not, to my knowledge, have any definitive data regarding cross-reactivity in part because patients allergic to allopurinol were in general excluded from febuxostat clinical trials. But in those few patients who were entered, there was no adverse reaction to Uloric. But it is of note that 1.6% of patients developed rashes in both the febuxostat treated and allopurinol treated wings of these studies. In the placebo group, 0.7% developed rashes (1).
The only reason for a washout would be to make sure that continued manifestation of the adverse reaction was not due to cross-reactivity between the two drugs. Based on the above information, my opinion therefore would be that no washout period would be indicated.
Thank you again for your inquiry and we hope this response is helpful to you.
1. Fam AG, et al. Efficacy and safety of desensitization to allopurinol following cutaneous reactions. Arthritis Rheum 2001; 44(1):231-238.
Phil Lieberman, M.D.