Abstract

Pancho Cuarón, Rubio Diego and Huerta de la Cruz

The kinetics of loratadine to de sloratadine was studied with human liver microsomes using substrate concentrations in the range of 0-50 µM. The specific cytochrome P450 (CYP) isoforms mediating the biotransformations were identified using microsome s containing specific recombinant CYP isozymes expressed in human lymphobla stoid cells, and by the use of CYP isoform -selective chemical inhibitors. In this study, the kinetic analysis of loratadine metabolism was performed by measuring the disappearance rate of parent compound (loratadine) and the production rate of its major metabolite (desloratadine), and it was found that the Michaelis-menten Km and Vmax values were 18.20 µM and 2169 pmol/min/mg for loratadine disappearance, and 25.20 µM and 486.98 pmol/min/mg for desloratadine formation, respectively. Among the recombinant CYPs, CYP3A4 and CYP2D6 exhibited the highest metabolic activity of loratadine, though no detectable activity was observed for CYPs 2C8, 2C9 and 2E1. Further kinetic analysis revealed first, that the clearance (Vmax/Km) values for CYP3A4 were 135.7 µl/min/mg protein for loratadine disappearance and 12.25 µl/min/mg protein for desloratadine formation, and secondly, that the clearance (Vmax/Km) values for CYP2D6 were 15.45 µl/min/mg protein for loratadine disappearance and 5 µl/min/mg protein for desloratadine formation. In addition, the formation of de sloratadine was inhibited by 2 µM ketoconazole (a CYP3A4 inhibitor) and 10 µM quinidine (a CYP2D6 inhibitor) by 66.43 and 33.03%, re spectively. Facts obtained in this study showed that this is the first time that kinetic parameters are described. In effect, this approach based on the disappearance rate of parent compound and production rate of major metabolite appears to be useful for detailing the kinetics of older drugs, for which detailed metabolic profiles have not been reported.