Cytochrome P450 isozymes involved in aromatic hydroxylation and side-chain N-desisopropylation of alprenolol in rat liver microsomes

Differences in cytochrome P450 and nuclear receptor mRNA levels in liver and small intestines between SD and DA rats

This study aimed to clarify the differences in mRNA levels of cytochrome P450 (CYP) isoforms and nuclear receptors between Dark Agouti (DA) and Sprague-Dawley (SD) rats which are animal models for poor metabolizers and extensive metabolizers for CYP2D6, respectively. Using liver and small intestine tissues of both rat strains, we investigated the mRNA levels of CYP1A, 2A, 2B, 2C, 2D, 2E, and 3A subfamilies and nuclear receptors which regulate the transcription of CYP isoforms. In the liver, male DA rats showed a low CYP2D2 mRNA level but high mRNA levels of CYP3A1, 3A2, and 1A1 compared to SD rats. No significant difference was noted in other CYP isoforms. The mRNA levels of CAR were higher in DA rats than those in SD rats. In small intestine, the mRNA levels of CYP isoforms and nuclear receptors exhibited no significant strain differences. In addition, the activity of CYP3A in small intestinal microsome did not differ between SD and DA rats. Female DA rats exhibited higher mRNA levels of CYP3A1, 3A2, and 2B1 in the liver than female SD rats. In conclusion, the mRNA levels of CYP3A1 and 3A2 isoforms and CAR in the liver but not in the small intestines were different between DA and SD rats in both sexes.

Pharmacokinetics and metabolism of metoprolol and propranolol in the female DA and female Wistar rat: the female DA rat is not always an animal model for poor metabolizers of CYP2D6

The purpose of this study was to clarify the pharmacokinetics of CYP2D6 substrates in female DA and Wistar rats, which are regarded as animal models of poor metabolizers and extensive metabolizers, respectively. In vivo pharmacokinetic and in vitro metabolic studies were conducted using metoprolol and propranolol, which show substantial and marginal polymorphisms in humans, respectively. After oral administration, the areas under the plasma concentration curves (AUC) for metoprolol and propranolol in DA rats were ca. 5- and 35-fold higher, respectively, than those in Wistar rats. There were no strain differences for serum protein binding or metabolism inhibition by quinine between the two compounds. Using a substrate depletion assay, the intrinsic clearances estimated for the two strains differed by 7.2-fold for metoprolol and 4.5-fold for propranolol. The discrepancy between the in vitro and in vivo profiles observed for propranolol, but not metoprolol, would be due to nonlinearity between the normalized AUC and the oral doses in DA rats, being associated with lower K(m) values. The larger strain difference in the AUCs of propranolol was proved by the in vitro kinetic parameters, implying that DA rats do not always reflect the polymorphic profiles in humans.

Enantioselectivity of bunitrolol 4-hydroxylation is reversed by the change of an amino acid residue from valine to methionine at position 374 of cytochrome P450-2D6

The enantioselectivity of 4-hydroxylation of bunitrolol (BTL), a beta-adrenoceptor blocking drug, was studied in microsomes from human liver, human hepatoma (Hep G2) cells expressing CYP2D6, and lymphoblastoid cells expressing CYP2D6. Kinetics in human liver microsomes showed that the Vmax value for (+)-BTL was 2.1-fold that of (-)-BTL, and that the Km value for (+)-BTL was lower than that for the (-)-antipode, resulting in the intrinsic clearance (Vmax/Km) of (+)-BTL being 2.1-fold over its (-)-antipode. CYP2D6 (CYP2D6-met) expressed in Hep G2 cells had a methionine residue at position 373 of the amino acid sequence and a rat-type N-terminal peptide (MELLNGTGLWSM) instead of the human-type (MGLEALVPLAVIV), and showed enantioselectivity of [(+)-BTL < (-)-BTL] for the rate of BTL 4-hydroxylation. In contrast, enantioselectivity [(+)-BTL > (-)-BTL] for Hep G2-CYP2D6 (CYP2D6-val) with a human-type N-terminal peptide that had a valine residue at 374, which corresponds to the methionine of the CYP2D6-met variant, was the same as that for human liver microsomes. We further confirmed that CYP2D6-met and CYP2D6-val expressed in human lymphoblastoid cells, both of which have methionine and valine, respectively, at position 374 and a human-type N-terminal peptide, exhibited the same enantioselectivities as those obtained from CYP2D6-met and CYP2D6-val expressed in the Hep G2 cell system. These results indicate that the amino acid at 374 of CYP2D6 is one of the key factors influencing the enantioselectivity of BTL 4-hydroxylation.