Abstract
Oxatomide is an antiallergic drug used for the treatment of diseases mediated by type I allergy. Recently, terfenadine and astemizole, which have antiallergic actions similar to those of oxatomide, showed side effects on the cardiovascular system. This might be because concomitant drugs such as itraconazole inhibit cytochrome P450 3A4 (CYP3A4), the enzyme responsible for the degradation of terfenadine and astemizole, and thus the blood concentrations of the drugs are abnormally increased. In another article of this issue, we have reported that oxatomide is metabolized by CYP2D6-Val and CYP3A4, and simultaneously inhibits the metabolism of the model substrates for these enzymes. In this study, we performed the kinetic analysis of oxatomide metabolism using microsomes prepared from human liver, and found that the Km and Vmax values were 26.1 μM and 1254.4 pmol/mg protein/min, respectively. Ketoconazole, one of the representative inhibitors for CYP3A4, potently inhibited the metabolism of oxatomide, but other well-known CYP inhibitors did not show significant inhibition. These results suggest that the metabolism of oxatomide is principally catalyzed by CYP3A4. Furthermore, oxatomide inhibited the metabolism of (±) bufuralol and testosterone, model substrates for CYP2D6 and CYP3A4, respectively, in a dose-dependent manner with the Ki values of 57.4 and 24.3 μM, respectively. These observations, together with the finding that the putative highest concentration of oxatomide in blood was ≅40 ng/ml (≅93 nM) at 4 h after each dosage during consecutive 6-d administration, encouraged us to conclude that oxatomide won't inhibit CYP2D6 or CYP3A4 at clinical doses.
