Modelling the loss of metabolic capacities of cultured hepatocytes: application to measurement of Michaelis-Menten kinetic parameters in in vitro systems

Impact of single nucleotide polymorphisms (R132Q and W120R) on the binding affinity and metabolic activity of CYP2C19 toward some therapeutically important substrates

Although CYP2C19 is minor human liver enzyme, it is responsible for the metabolism of many clinically important drugs. In this work, CYP2C19 wild type and its SNP mutants (R132Q and W120R) were prepared using over-expression system in E. coli, purified by column chromatography and their biological activities were compared. The enzyme activity toward certain drugs (amitriptyline, imipramine, lansoprazole and omeprazole) was investigated. Resonance Raman and UV-VIS spectroscopies revealed a minimal effect of SNP mutations on the heme structure. However, the mutation greatly affected the drug metabolism activities of CYP2C19. The degree of these effects was dependent on both the mutation and the chemical structure of the substrate. Surprisingly, the affected amino acid residue is located remotely from the heme center. Therefore, the direct effect of the mutation on the metabolic center is excluded. Alternatively, the significant impairment in the drug metabolism of these mutants could be attributed to a decrease in the electron flow to the iron center. Accordingly, understanding the effect of SNPs of CYP2C19, and the extents in which they participate in the drug metabolism, are important pillars that can enhance the therapeutic drugs efficacy and improve the patient outcomes toward the development of patient's tailored medicine.

Phenotypic and metabolic investigation of a CSF-1R kinase receptor inhibitor (BLZ945) and its pharmacologically active metabolite

1. 4-[2((1R,2R)-2-Hydroxycyclohexylamino)-benzothiazol-6-yloxyl]-pyridine-2-carboxylic acid methylamide (BLZ945) is a small molecule inhibitor of CSF-1R kinase activity within osteoclasts designed to prevent skeletal related events in metastatic disease. Key metabolites were enzymatically and structurally characterized to understand the metabolic fate of BLZ945 and pharmacological implications. The relative intrinsic clearances for metabolites were derived from in vitro studies using human hepatocytes, microsomes and phenotyped with recombinant P450 enzymes. 2. Formation of a pharmacologically active metabolite (M9) was observed in human hepatocytes. The M9 metabolite is a structural isomer (diastereomer) of BLZ945 and is about 4-fold less potent. This isomer was enzymatically formed via P450 oxidation of the BLZ945 hydroxyl group, followed by aldo-keto reduction to the alcohol (M9). 3. Two reaction phenotyping approaches based on fractional clearances were applied to BLZ945 using hepatocytes and liver microsomes. The fraction metabolized (fm) or contribution ratio was determined for each metabolic reaction type (oxidation, glucuronidation or isomerization) as well as for each metabolite. The results quantitatively illustrate contribution ratios of the involved enzymes and pathways, e.g. the isomerization to metabolite M9 accounted for 24% intrinsic clearance in human hepatocytes. In summary, contribution ratios for the Phase I and Phase II pathways can be determined in hepatocytes.

IN VITRO STUDIES ON QUETIAPINE METABOLISM USING THE SUBSTRATE DEPLETION APPROACH WITH FOCUS ON DRUG-DRUG INTERACTIONS

The metabolism of the atypical antipsychotic quetiapine was investigated by in vitro methods. Pharmacokinetic parameters were determined in human liver microsomes and recombinant cytochrome P450 measuring substrate depletion and product formation. The cytochrome P450 isozymes CYP3A4 and CYP2D6 displayed activity towards quetiapine. The isozyme CYP2D6 played a minor role in the metabolism of quetiapine as CYP3A4 contributed 89% to the overall metabolism. A Km value of 18 microM was determined by substrate depletion, suggesting linear kinetics under therapeutic conditions. Drugs known to inhibit CYP3A4, such as ketoconazole and nefazodone, displayed almost complete inhibition at low concentrations, whereas inhibitors of CYP2D6 do not seem to have a clinically relevant effect.

Impact of serum on clearance predictions obtained from suspensions and primary cultures of rat hepatocytes

The objective of the present study was to compare two configurations of the hepatocyte model namely suspensions (SH) and conventional primary cultures (CPC) for their ability to predict the hepatic clearance in vivo in the rat and, to investigate the impact of serum on the prediction accuracy. The metabolic competences of several cytochrome P450 isoenzymes were investigated both in CPC and SH in the presence or absence of serum. Under the same conditions, the in vitro intrinsic clearance of six test compounds metabolised by a variety of phase I and phase II enzymes (antipyrine, RO-X, mibefradil, midazolam, naloxone and oxazepam) were derived from Vmax/Km scaled up to the corresponding in vivo hepatic metabolic clearance. CYP activities were shown to be stable in both CPC and SH for up to 6 h of incubation, except for the CYP 3A1 activity that decreased in CPC even in the presence of serum. Moreover, the clearances predicted from SH in the presence of serum were closer to the in vivo values than those obtained from CPC. SH represent a convenient model to assess the hepatic metabolism of xenobiotics, the presence of serum in the incubation medium significantly improved in several instances the quality of the predictions.