Abstract

In vitro studies were conducted to identify the cytochromes P450 (CYP) involved in the oxidative metabolism of celecoxib. The hydroxylation of celecoxib conformed to monophasic Michaelis-Menten kinetics (mean ± S.D., n = 4 livers,K_m = 3.8 ± 0.95 μM,V_max = 0.70 ± 0.45 nmol/min/mg protein) in the presence of human liver microsomes, although substrate inhibition was significant at higher celecoxib concentrations. The treatment of a panel of human liver microsomal samples (n = 16 subjects) with antibodies against CYP2C9 and CYP3A4 inhibited the formation of hydroxy celecoxib by 72 to 92% and 0 to 27%, respectively. The presence of both antibodies in the incubation suppressed the activity by 90 to 94%. In addition, the formation of hydroxy celecoxib significantly correlated with CYP2C9-selective tolbutamide methyl hydroxylation (r = 0.92, P < .001) and CYP3A-selective testosterone 6β-hydroxylation (r= 0.55, P < .02). In contrast, correlation with activities selective for other forms of CYP was weak (r ≤ 0.46). Chemical inhibition studies showed that ketoconazole (selective for CYP3A4) and sulfaphenazole (selective for CYP2C9) inhibited the formation of hydroxy celecoxib in a concentration-dependent manner, whereas potent inhibitors selective for other forms of CYP did not show any significant effect over a range of 1 to 10 μM. In agreement, cDNA-expressed CYP2C9 catalyzed the formation of hydroxy celecoxib with an apparentK_m value (μM) and aV_max value (pmol/min/pmol recombinant CYP) of 5.9 and 21.7, whereas a higher K_m value (18.2) and a lower V_max value (1.42) were obtained with rCYP3A4. It is concluded that methyl hydroxylation of celecoxib is primarily catalyzed by human liver microsomal CYP2C9, although CYP3A4 also plays a role.