Evaluation of the Effect of Aldehyde Oxidase Inhibitors on 6-Mercaptopurine Metabolism

Thiopurines, such as 6-mercaptopurine (6-MP) and azathioprine, are converted to the inactive metabolites 6-thioxanthin (6-TX) and 6-thiouric acid (6-TUA). Molybdenum-containing oxidoreductases, aldehyde oxidase (AOX) and xanthine oxidase (XO), are involved in the oxidation of 6-MP to 6-TX; XO inhibitors affect the therapeutic efficacy of thiopurines and the incidence of adverse effects, such as liver and blood disorders. However, the role of AOX in the pharmacokinetics of 6-MP remains unclear. To clarify the clinical importance of AOX-mediated drug-drug interactions, we evaluated whether drugs that inhibit AOX affect 6-MP metabolism. The metabolism of 6-MP to 6-TX was strongly inhibited by AOX inhibitors (amitriptyline, chlorpromazine, clomipramine, clozapine, hydralazine, quetiapine, and raloxifene) in a reaction mixture containing human liver cytosol. The inhibition of 6-TX production rate by each AOX inhibitor was 60-70% at high concentrations, although the XO inhibitor febuxostat showed an inhibition rate of 10-30%. Furthermore, the combination of febuxostat and each AOX inhibitor showed greater inhibition than when each compound was added alone. The AOX inhibitor did not alter 6-MP oxidation by recombinant XO. These results suggest that AOX inhibition may affect the pharmacokinetics of thiopurines. However, because of the lower activity of AOX in rats than that in humans, the contribution of AOX could not be assessed using in vivo experiments. Further studies are needed to evaluate the contribution of AOX to the therapeutic and adverse effects of thiopurines, both in clinical studies and in animal models of liver humanization.