Biol. Pharm. Bull. 28(2) 328—334 (2005)

diseases mainly mediated by a type I allergic reaction. Pharmacological studies have demonstrated that oxatomide acts as an antagonist for various chemical mediators such as histamine, leukotriene and platelet-activating factor, as well as inhibits the release of these substances, and that all these actions contribute to therapeutic effects of the drug. Oxatomide is widely applied to skin diseases, including chronic urticaria, skin itching, atopic dermatitis, allergic rhinitis and bronchial asthma in the clinical field. In a previous article, we showed that the P450 isoforms responsible for the metabolism of oxatomide were cytochrome P450 2D6 (CYP2D6) and CYP3A4, using microsome preparations of the in vitro expression systems derived from a human lymphoblastoid cell line. We also demonstrated that oxatomide inhibited the enzyme activity of CPY2D6-Val and CYP3A4 in a dose-dependent manner, using model substrates for these isoforms. A similar action has been reported for the antiallergic drugs, terfenadine and astemizole, which inhibit both CYP2D6-Val and CYP3A4. It has been shown that terfenadine and astemizole have side effects on the cardiovascular system, such as QT prolongation, ventricular arrhythmia and cardiac arrest. Since the principal metabolic pathway for the two drugs is mediated by CYP3A4, the blood concentration of the drugs is increased when CYP3A4 is strongly inhibited by concomitant drugs such as itraconazole and ketoconazole. On the other hand, there has been no report of side effects of oxatomide on the cardiovascular system, such as QT prolongation, but the drug has been shown to induce catalepsy at high doses in experimental animals, and infrequently to cause extrapyramidal disorder by clinical usage. Our results in the former article raised the possibility that oxatomide might inhibit the same enzymes as those inhibited by terfenadine and astemizole, which suggests we should investigate the mode of action of oxatomide in greater detail. Therefore, to clarify whether oxatomide brings about any drug interaction or is affected by the interaction, we examined kinetic parameters (Km and Vmax) for the oxatomide metabolism and inhibition constants (Ki) of oxatomide in metabolism of the model substrates for CYP2D6 and CYP3A4, and possible drug interaction is discussed.