DEVELOPMENT AND VALIDATION OF A HIGH-THROUGHPUT RADIOMETRIC CYP2C9 INHIBITION ASSAY USING TRITIATED DICLOFENAC

Study of atypical kinetic behaviour of cytochrome P450 2C9 isoform with diclofenac at low substrate concentrations by sweeping-MEKC combination

In view of the fact that several studies have shown that diclofenac hydroxylation by cytochrome P450 2C9 deviated from Michaelis-Menten kinetics at low substrate concentrations, sweeping combined with MEKC was applied for the kinetic study of this pharmacologically important reaction. A 50  μm fused silica capillary (56  cm effective length) was used to carry out all separations. 70  mM SDS in 20  mM phosphate 20  mM tetraborate buffer, pH 8.6, was used as the BGE. Injection was accomplished by the application of 50  mbar (5  kPa) pressure to the sample vial for 52  s. Separation was performed at 22  kV (positive polarity), with a capillary temperature of 25°C and detection at 200  nm. The higher sensitivity of the sweeping-MEKC combination compared with the simple MEKC method enabled this reaction to be fitted to a Hill kinetic model and confirmed the findings of other authors. A Michaelis constant of 2.91±0.10  μM, maximum reaction velocity of 9.16±0.16  nmol/min/nmol and Hill coefficient of 1.66±0.08 were determined. This value of Hill coefficient confirms the presence of a positive cooperativity at low diclofenac concentrations and supports the hypothesis of two substrates binding at or near the active site.

Preclinical pharmacokinetics and metabolism of a potent non-nucleoside inhibitor of the hepatitis C virus NS5B polymerase

The disposition of compound A, a potent inhibitor of the hepatitis C virus (HCV) NS5B polymerase, was characterized in animals in support of its selection for further development. Compound A exhibited marked species differences in pharmacokinetics. Plasma clearance was 44 ml min-1 kg-1 in rats, 9 ml min-1 kg-1 in dogs and 16 ml min-1 kg-1 in rhesus monkeys. Oral bioavailability was low in rats (10%) but significantly higher in dogs (52%) and monkeys (26%). Compound A was eliminated primarily by metabolism in rats, with biliary excretion accounting for 30% of its clearance. Metabolism was mainly mediated by cyclohexyl hydroxylation, with N-deethylation and acyl glucuronide formation constituting minor metabolic pathways. Qualitatively, the same metabolites were identified using in vitro systems from all species studied, including humans. The low oral bioavailability of compound A in rats was mostly due to poor intestinal absorption. This conclusion was borne out by the findings that hepatic extraction in the rat was only 30%, intraperitoneal bioavailability was good, and compound A was poorly absorbed from the rat isolated intestinal loop, with no detectable intestinal metabolism. Compound A was not an inhibitor of major human cytochrome P450 enzymes, indicating minimal potential for clinical drug-drug interactions. The metabolic clearance of compound A in rat, dog and monkey hepatocytes correlated with the systemic clearance observed in these species. Since compound A was very stable in human hepatocytes, the results suggest that it will be a low clearance drug in humans.

High-Throughput Radiometric CYP2C19 Inhibition Assay Using Tritiated (S)-Mephenytoin

A rapid and sensitive radiometric assay for assessing the potential of drugs to inhibit cytochrome P450 (P450) 2C19 in human liver microsomes is described. The new assay, which does not require high-performance liquid chromatography (HPLC) separation or mass spectrometric detection, is based on the release of tritium as tritiated water that occurs upon CYP2C19-mediated 4'-hydroxylation of (S)-mephenytoin labeled with tritium in the 4' position. Because this reaction is subject to an NIH shift, tritium was also introduced into the 3'- and 5'-positions of the tracer to enhance formation of a tritiated water product. Tritiated water was separated from the substrate using 96-well solid-phase extraction plates. The reaction is NADPH-dependent and sensitive to CYP2C19 inhibitors. IC(50) values for 15 diverse drugs differed less than 2.5-fold from those determined by quantification of the unlabeled 4'-hydroxy-(S)-mephenytoin product, using HPLC coupled to mass spectrometric detection. All of the steps of the new assay, namely incubation, product separation, and radioactivity counting, are performed in a 96-well format and can be automated. This assay represents a non-HPLC, high-throughput version of the classic (S)-mephenytoin 4'-hydroxylation assay, which is the most widely used method to assess the potential for CYP2C19 inhibition of new chemical entities.

Study of recombinant cytochrome P450 2C9 activity with diclofenac by MEKC

Cytochrome P450 2C9 (CYP2C9) is one of the most important isoforms in human liver involved in the metabolism of a large number of therapeutic agents. The aim of this paper is to demonstrate the applicability of CE for the determination of the enzymatic activity of CYP2C9 with diclofenac as a probe substrate. MEKC with SDS as a pseudostationary phase was used for this purpose. Compared to other assays, the MEKC-based method is rapid, can be automated and requires only a small quantity of enzymes and substrate. Moreover, the enzymatic reaction can be monitored with high sensitivity and repeatability even when the reaction mixture is used for the analysis without any pretreatment. The kinetic study on the given enzymatic reaction was also performed since the basic characterization of drug biotransformation generally begins with the enzyme kinetic analysis of metabolite formation. As a result, the Michaelis constant and maximum reaction velocity were evaluated, the values 3.44 +/- 0.45 microM and 19.78 +/- 0.76 nmol min(-1) nmol(-1), respectively, were in agreement with the literature data. On the other hand, a slight deviation from typical Michaelis-Menten kinetics with a weak positive cooperativity was found at diclofenac concentrations below 2 microM. The same atypical kinetic behavior of CYP2C9 was also observed by other authors.

DEVELOPMENT AND VALIDATION OF A HIGH-THROUGHPUT RADIOMETRIC CYP3A4/5 INHIBITION ASSAY USING TRITIATED TESTOSTERONE

A rapid and sensitive radiometric assay for assessing the potential of drugs to inhibit cytochrome P450 (P450) 3A4/5 in human liver microsomes is described. In contrast to the conventional testosterone 6beta-hydroxylation assay, the new method does not require high-performance liquid chromatography (HPLC) separation and mass spectrometry. The assay is based on the release of tritium as tritiated water that occurs upon CYP3A4/5-mediated 6beta-hydroxylation of testosterone labeled with tritium in the 6beta position. The radiolabeled product is separated from the substrate using 96-well solid-phase extraction plates. Using commercially available [1,2,6,7-(3)H]testosterone as substrate, we demonstrated that the reaction is NADPH-dependent, and sensitive to CYP3A4/5/5 inhibitors and a CYP3A4/5/5-specific inhibitory monoclonal antibody, but not to inhibitors of or antibodies against other P450 enzymes. The method was further improved by synthesis of testosterone specifically tritiated in the 6beta position, which displayed greatly improved conversion rate with an ensuing increase in assay sensitivity. Competition experiments using tritiated and unlabeled testosterone indicated that CYP3A4/5-mediated 6beta-hydroxylation exhibits positive cooperativity and a modest kinetic isotope effect. IC(50) values for more than 40 structurally diverse chemical inhibitors were not significantly different from those determined in the testosterone 6beta-hydroxylation assay, using HPLC-tandem mass spectrometry analysis. All the steps of the new assay, namely, incubation, product separation, and radioactivity counting, are performed in 96-well format and can be automated. This assay thus represents a high-throughput version of the classical testosterone 6beta-hydroxylation assay, which is the most widely used method to assess the potential for CYP3A4/5 inhibition of new chemical entities.