IDENTIFICATION OF HUMAN LIVER CYTOCHROME-P450 ENZYMES THAT METABOLIZETHE NONSEDATING ANTIHISTAMINE LORATADINE - FORMATION OF DESCARBOETHOXYLORATADINE BY CYP3A4 AND CYP2D6

[H-3]loratadine was incubated with human liver microsomes to determine which cytochrome P450 (CYP) enzymes are responsible for its oxidative metabolism. Specific enzymes were identified by correlation analysis, by inhibition studies (chemical and immunoinhibition), and by incubat ion with various cDNA-expressed human P450 enzymes. Descarboethoxylora tadine (DCL) was the major metabolite of loratadine detected following incubation with pooled human liver microsomes. Although DCL can theor etically form by hydrolysis, the conversion of loratadine to DCL by hu man liver microsomes was not inhibited by the esterase inhibitor pheny lmethylsulfonyl fluoride (PMSF), and was dependent on NADPH. A high co rrelation (r(2) = 0.96, N = 10) was noted between the rate of formatio n of DCL and testosterone 6 beta-hydroxylation, a CYP3A-mediated react ion. With the addition of ketoconazole (CYP3A4 inhibitor) to the incub ation mixtures, the residual rate of formation of DCL correlated (r(2) = 0.81) with that for dextromethorphan O-demethylation, a CYP2D6 reac tion. Rabbit polyclonal antibodies raised against the rat CYP3A1 enzym e (5 mg IgG/nmol P450) and troleandomycin (0.5 mu M), a specific inhib itor of CYP3A4, decreased the formation of DCL by 53 and 75%, respecti vely, when added to 1.42 mu M loratadine microsomal incubations. Quini dine (5 IJ-M), a CYP2D6 inhibitor, inhibited the formation of DCL appr oximately 20% when added to microsomal incubations of loratadine at co ncentrations of 7-35 mu M. Incubation of loratadine with cDNA-expresse d CYP3A4 and CYP2D6 microsomes catalyzed the formation of DCL with for mation rates of 135 and 633 pmol/min/nmol P450, respectively. The resu lts indicated that loratadine was metabolized to DCL primarily by the CYP3A4 and CYP2D6 enzymes in human liver microsomes. In the presence o f a CYP3A4 inhibitor, loratadine was metabolized to DCL by the CYP2D6 enzyme. Conformational and electrostatic analysis of loratadine indica ted that its structure is consistent with substrate models for the CYP 2D6 enzyme.