Comparison of structures of allopurinol, oxypurinol, and febuxostat. Clinically utilized xanthine oxidase inhibitors. Allopurinol and its oxidation product oxypurinol are hydroxypyrazolopyrimidine analogues, respectively, of hypoxanthine and xanthine. As such, each can affect the activities of enzymes of purine and pyrimidine metabolism other than xanthine oxidase. Febuxostat is a thiazolecarboxylic acid derivative that does not resemble a purine or pyrimidine and has shown substantial specificity as a xanthine oxidase inhibitor .