In vitroapproaches to investigate mechanism-based inactivation of CYP enzymes

Mechanism-based inactivation of cytochromes P450: implications in drug interactions and pharmacotherapy

Cytochrome P40 (CYP) enzymes dominate the metabolism of numerous endogenous and xenobiotic substances. While it is commonly believed that CYP-catalysed reactions result in the detoxication of foreign substances, these reactions can also yield reactive intermediates that can bind to cellular macromolecules to cause cytotoxicity or irreversibly inactivate CYPs that create them.Mechanism-based inactivation (MBI) produces either irreversible or quasi-irreversible inactivation and is commonly caused by CYP metabolic bioactivation to an electrophilic reactive intermediate. Many drugs that have been known to cause MBI in CYPs have been discovered as perpetrators in drug-drug interactions throughout the last 20-30 years.This review will highlight the key findings from the recent literature about the mechanisms of CYP enzyme inhibition, with a focus on the broad mechanistic elements of MBI for widely used drugs linked to the phenomenon. There will also be a brief discussion of the clinical or pharmacokinetic consequences of CYP inactivation with regard to drug interaction and toxicity risk.Gaining knowledge about the selective inactivation of CYPs by common therapeutic drugs helps with the assessment of factors that affect the systemic clearance of co-administered drugs and improves comprehension of anticipated interactions with other drugs or xenobiotics.

In vitro inhibition of human cytochrome P450 enzymes by licoisoflavone B from Glycyrrhiza uralensis Fisch. ex DC

Glycyrrhiza uralensis Fisch. ex DC, one of the 3 pharmacopeial species of licorice and widely used in dietary supplements, can inhibit certain cytochrome P450 (CYP) enzymes. Thereby, G. uralensis preparations have the potential to cause pharmacokinetic drug interactions when consumed along with prescription medicines. One compound (1.34 mg dry weight) responsible for inhibiting CYP2B6, CYP2C8, and CYP2C9 was isolated using bioactivity-guided fractionation from 250 g dried roots, stolons, and rhizomes. The enzyme kinetics and mechanisms of inhibition were determined using human liver microsomes, recombinant enzymes, and UHPLC-MS/MS-based assays. Identified as licoisoflavone B, this compound displayed reversible inhibition of CYP2C8 with an IC50 value of 7.4 ± 1.1 µM and reversible inhibition of CYP2C9 with an IC50 value of 4.9 ± 0.4 µM. The enzyme kinetics indicated that the mechanism of inhibition was competitive for recombinant CYP2C8, with a Ki value of 7.0 ± 0.7 μM, and mixed-type inhibition for recombinant CYP2C9, with a Ki value of 1.2 ± 0.2 μM. Licoisoflavone B moderately inhibited CYP2B6 through a combination of irreversible and reversible mechanisms with an IC50 value of 16.0 ± 3.9 µM.

Mechanism-Based Inactivation of Cytochrome P450 3A4 and 3A5 by the Fibroblast Growth Factor Receptor Inhibitor Erdafitinib

Erdafitinib (ERD) is a first-in-class pan inhibitor of fibroblast growth factor receptor 1-4 that has garnered global regulatory approval for the treatment of advanced or metastatic urothelial carcinoma. Although it has been previously reported that ERD elicits time-dependent inhibition (TDI) of cytochrome P450 (P450) 3A4 (CYP3A4), the exact biochemical nature underpinning this observation remains obfuscated. Moreover, it is also uninterrogated if CYP3A5-its highly homologous counterpart-could be susceptible to such interactions. Mechanism-based inactivation (MBI) of P450 is a unique subset of TDI that hinges on prior bioactivation of the drug to a reactive intermediate and possesses profound clinical and toxicological implications due to its irreversible nature. Here, we investigated and confirmed that ERD inactivated both CYP3A isoforms in a time-, concentration-, and NADPH-dependent manner with K_I, k_inact, and partition ratio of 4.01 and 10.04 μM, 0.120 and 0.045 min^-1, and 32 and 55 for both CYP3A4 and CYP3A5, respectively, when rivaroxaban was employed as the probe substrate. Co-incubation with an alternative substrate or direct inhibitor of CYP3A attenuated the rate of inactivation, whereas the addition of glutathione or catalase did not induce such protection. The lack of enzyme activity recovery following dialysis for 4 h and oxidation with potassium ferricyanide combined with the lack of a Soret peak in spectral scans collectively substantiated that ERD is an irreversible covalent MBI of CYP3A. Finally, glutathione trapping and high-resolution mass spectrometry experiments illuminated a plausible bioactivation mechanism of ERD by CYP3A arising from metabolic epoxidation of its quinoxaline ring.

The Application of Mass Spectrometry in Drug Metabolism and Pharmacokinetics

Drug metabolism and pharmacokinetics (DMPK) are fundamental in drug discovery. New chemical entities (NCEs) are typically evaluated with various in vitro and in vivo assays, which are time-consuming and labor intensive. These experiments are essential in identifying potential new drugs. Recently, mass spectrometry (MS) has played a key role in examining the drug-like properties of NCEs. Quantitative and qualitative mass spectrometry approaches are routinely utilized to obtain high-quality data in an efficient, timely, and cost-effective manner. Especially, liquid chromatography (LC) coupled with MS technology has been refined for metabolite identification (Met ID), which is critical for lead optimization. These qualitative and quantitative MS approaches and their specific utility in DMPK characterization will be described in this chapter.

Mechanism-Based Inactivation of Cytochrome P450 3A4 by Benzbromarone

Benzbromarone (BBR), a potent uricosuric agent for the management of gout, is known to cause fatal fulminant hepatitis. Although the mechanism of BBR-induced idiosyncratic hepatotoxicity remains unelucidated, cytochrome P450 enzyme-mediated bioactivation of BBR to electrophilic reactive metabolites is commonly regarded as a key molecular initiating event. However, apart from causing aberrant toxicities, reactive metabolites may result in mechanism-based inactivation (MBI) of cytochrome P450. Here, we investigated and confirmed that BBR inactivated CYP3A4 in a time-, concentration-, and NADPH-dependent manner with K _I, k _inact, and partition ratio of 11.61 µM, 0.10 minutes^-1, and 110, respectively. Coincubation with ketoconazole, a competitive inhibitor of CYP3A4, attenuated the MBI of CYP3A4 by BBR, whereas the presence of glutathione and catalase did not confer such protection. The lack of substantial recovery of enzyme activity postdialysis and after oxidation with potassium ferricyanide, combined with the absence of a Soret peak in spectral difference scans, implied that MBI of CYP3A4 by BBR did not occur through the formation of quasi-irreversible metabolite-intermediate complexes. Analysis of the reduced CO-difference spectrum revealed an ∼44% reduction in ferrous-CO binding and hinted that inactivation is mediated via irreversible covalent adduction to both the prosthetic heme moiety and the apoprotein. Finally, our in silico covalent docking analysis further suggested the modulation of substrate binding to CYP3A4 via the covalent adduction of epoxide-derived reactive intermediates of BBR to two key cysteine residues (Cys239 and Cys58) vicinal to the entrance of the orthosteric binding site. SIGNIFICANCE STATEMENT: Although the bioactivation of benzbromarone (BBR) to reactive metabolites has been well characterized, its potential to cause mechanism-based inactivation (MBI) of cytochrome P450 has not been fully investigated. This study reports the MBI of CYP3A4 by BBR via irreversible covalent adduction and develops a unique covalent docking methodology to predict the structural molecular determinants underpinning the inactivation for the first time. These findings lay the groundwork for future investigation of clinically relevant drug-drug interactions implicating BBR and mechanisms of BBR-induced idiosyncratic hepatotoxicity.

Evolution to low-dose NSAID therapy

All NSAIDs are to varying degrees associated with gastrointestinal, cardiovascular and renal adverse effects (AEs). Differences in selectivity for inhibition of the COX isozymes (COX-1/COX-2) have been used as an indicator of the likelihood of experiencing an AE, but the measure of 'selectivity' commonly used is less than desirable, and selectivity has not yielded unequivocal superior safety. Recent guidelines recommend that NSAIDs be used at the lowest effective dose and for the shortest period of time. In response, 'low-dose' NSAID formulations have been developed. Such formulations may help by reducing overall systemic exposure, thereby reducing the frequency or severity of AEs. It seems timely to review the need, rationale and application of such an approach.

Metabolic activation and drug-induced liver injury: in vitro approaches for the safety risk assessment of new drugs

Drug-induced liver injury (DILI) is a significant leading cause of hepatic dysfunction, drug failure during clinical trials and post-market withdrawal of approved drugs. Many cases of DILI are unexpected reactions of an idiosyncratic nature that occur in a small group of susceptible individuals. Intensive research efforts have been made to understand better the idiosyncratic DILI and to identify potential risk factors. Metabolic bioactivation of drugs to form reactive metabolites is considered an initiation mechanism for idiosyncratic DILI. Reactive species may interact irreversibly with cell macromolecules (covalent binding, oxidative damage), and alter their structure and activity. This review focuses on proposed in vitro screening strategies to predict and reduce idiosyncratic hepatotoxicity associated with drug bioactivation. Compound incubation with metabolically competent biological systems (liver-derived cells, subcellular fractions), in combination with methods to reveal the formation of reactive intermediates (e.g., formation of adducts with liver proteins, metabolite trapping or enzyme inhibition assays), are approaches commonly used to screen the reactivity of new molecules in early drug development. Several cell-based assays have also been proposed for the safety risk assessment of bioactivable compounds. Copyright © 2015 John Wiley & Sons, Ltd.

Influence of sesamin on CYP2C-mediated diclofenac metabolism: in vitro and in vivo analysis

Our previous studies revealed that sesamin caused a mechanism-based inhibition (MBI) of CYP2C9 in human liver microsomes. Additionally, we observed a similar MBI of CYP2C by sesamin in the rat liver microsomes. Sesamin-induced difference spectra of rat or human liver microsomes in the presence of NADPH showed a peak at 459 nm, suggesting the formation of a metabolic–intermediate (MI) complex of cytochrome P450 and the methylenedioxyphenyl group of sesamin. However, the peak disappeared in both liver microsomes within 30 min after the termination of the metabolism. These results suggest that the MI complex of cytochrome P450 and sesamin is unstable, and the effects of sesamin on human CYP2C9- or rat CYP2C-mediated drug metabolism may be small. To confirm this, in vivo studies using rats were performed. The pharmacokinetics of diclofenac, which is mainly metabolized by CYP2C11 in male rats, were investigated after a 3-days administration of sesamin (0, 10, and 100 mg/kg bw). No significant differences were observed among the three groups in the pharmacokinetic parameters, C_max, T_max, and AUC. Furthermore, administration of sesamin to rats for 7 days had no significant effects on diclofenac hydroxylation activity in rat liver microsomes. These results demonstrate that no significant interaction occurs between diclofenac and sesamin in rats. Moreover, the results of these in vitro and in vivo studies suggest that no significant interaction may occur between sesamin and diclofenac when sesamin is administered to humans as a supplement, since the standard sesamin dose in humans is much lower than that administered to rats in this study.

Irreversible enzyme inhibition kinetics and drug-drug interactions

This chapter describes the types of irreversible inhibition of drug-metabolizing enzymes and the methods commonly employed to quantify the irreversible inhibition and subsequently predict the extent and time course of clinically important drug-drug interactions.

Characterization of inhibition kinetics of (S)-warfarin hydroxylation by noscapine: implications in warfarin therapy

Noscapine is an antitussive and potential anticancer drug. Clinically significant interactions between warfarin and noscapine have been previously reported. In this study, to provide a basis for warfarin dosage adjustment, the inhibition kinetics of noscapine against warfarin metabolism was characterized. Our enzyme kinetics data obtained from human liver microsomes and recombinant CYP2C9 proteins indicate that noscapine is a competitive inhibitor of the (S)-warfarin 7-hydroxylation reaction by CYP2C9. Interestingly, noscapine also inhibited (S)-warfarin metabolism in a NADPH- and time-dependent manner, and removal of unbound noscapine and its metabolites by ultrafiltration did not reverse inhibition of (S)-warfarin metabolism by noscapine, suggesting mechanism-based inhibition of CYP2C9 by noscapine. Spectral scanning of the reaction between CYP2C9 and noscapine revealed the formation of an absorption spectrum at 458 nm, indicating the formation of a metabolite-intermediate complex. Surprisingly, noscapine is a 2- to 3-fold more efficient inactivator of CYP2C9.2 and CYP2C9.3 variants than it is of the wild type, by unknown mechanisms. Based on the inhibitory kinetic data, (S)-warfarin exposure is predicted to increase up to 7-fold (depending on CYP2C9 genotypes) upon noscapine coadministration, mainly due to mechanism-based inactivation of CYP2C9 by noscapine. Together, these results indicate that mechanism-based inhibition of CYP2C9 by noscapine may dramatically alter pharmacokinetics of warfarin and provide a basis for warfarin dosage adjustment when noscapine is coadministered.

Defining the COX inhibitor selectivity of NSAIDs: implications for understanding toxicity

The hypothesis that the anti-inflammatory activity of NSAIDs derives from COX inhibition is well established. It also underpins the accepted mechanism of the gastrointestinal and renal toxicity of NSAIDs. However, in terms of NSAID-induced cardiovascular toxicity, is COX inhibition then guilty by association? Multiple experimental models of COX-1/COX-2 inhibition have enabled ranking of the relative inhibitory activity of NSAIDs. Inhibition is expressed as an IC(50) value and the index of COX selectivity as the ratio of the IC(50) value for COX-2 and COX-1. These data informed the 'imbalance hypothesis' that the cardiovascular risk of NSAIDs results from an imbalance in the detrimental actions of COX-1-derived thromboxane A(2) and the beneficial actions of COX-2-derived prostacyclin (PGI(2)). Data derived from in vitro models used to generate NSAID IC(50) values are discussed in the context of the difficulties in defining COX selectivity and hence understanding the toxicity of NSAIDs in current clinical use.

Statistical methods for analysis of time-dependent inhibition of cytochrome p450 enzymes

Time-dependent inhibition (TDI) of cytochrome P450 (P450) enzymes, especially CYP3A4, is an important attribute of drugs in evaluating the potential for pharmacokinetic drug-drug interactions. The analysis of TDI data for P450 enzymes can be challenging, yet it is important to be able to reliably evaluate whether a drug is a TDI or not, and if so, how best to derive the inactivation kinetic parameters K(I) and k(inact). In the present investigation a two-step statistical evaluation was developed to evaluate CYP3A4 TDI data. In the first step, a two-sided two-sample z-test is used to compare the k(obs) values measured in the absence and presence of the test compound to answer the question of whether the test compound is a TDI or not. In the second step, k(obs) values are plotted versus both [I] and ln[I] to determine whether a significant correlation exists, which can then inform the investigator of whether the inactivation kinetic parameters, K(I) and k(inact), can be reliably estimated. Use of this two-step statistical evaluation is illustrated with the examination of five drugs of varying capabilities to inactivate CYP3A4: ketoconazole, erythromycin, raloxifene, rosiglitazone, and pioglitazone. The use of a set statistical algorithm offers a more robust and objective approach to the analysis of P450 TDI data than frequently employed empirically derived or heuristic approaches.

Interaction of lapatinib with cytochrome P450 3A5

Lapatinib, an oral tyrosine kinase inhibitor used for breast cancer, has been reported to cause idiosyncratic hepatotoxicity. Recently, it has been found that lapatinib forms a metabolite-inhibitor complex (MIC) with CYP3A4 via the formation of an alkylnitroso intermediate. Because CYP3A5 is highly polymorphic compared with CYP3A4 and also oxidizes lapatinib, we investigated the interactions of lapatinib with CYP3A5. Lapatinib inactivated CYP3A5 in a time-, concentration-, and NADPH-dependent manner using testosterone as a probe substrate with K(I) and k(inact) values of 0.0376 mM and 0.0226 min(-1), respectively. However, similar results were not obtained when midazolam was used as the probe substrate, suggesting that inactivation of CYP3A5 by lapatinib is site-specific. Poor recovery of CYP3A5 activity postdialysis and the lack of a Soret peak confirmed that lapatinib does not form a MIC with CYP3A5. The reduced CO difference spectrum further suggested that a large fraction of the reactive metabolite of lapatinib is covalently adducted to the apoprotein of CYP3A5. GSH trapping of a reactive metabolite of lapatinib formed by CYP3A5 confirmed the formation of a quinoneimine-GSH adduct derived from the O-dealkylated metabolite of lapatinib. In silico docking studies supported the preferential formation of an O-dealkylated metabolite of lapatinib by CYP3A5 compared with an N-hydroxylation reaction that is predominantly catalyzed by CYP3A4. In conclusion, lapatinib appears to be a mechanism-based inactivator of CYP3A5 via adduction of a quinoneimine metabolite.

Cyclosporine A- and tacrolimus-mediated inhibition of CYP3A4 and CYP3A5 in vitro

Cyclosporine A (CsA) and tacrolimus (Tac) are immunosuppressive drugs used in the majority of patients with solid organ transplants, generally in combination with a wide range of drugs. CsA and Tac seem not only to be substrates of CYP3A but have also been described as inhibitors of CYP3A. For CsA, in particular, inhibition of CYP3A has been suggested as the main mechanism of interactions seen clinically with various drugs. The aim of this study was to investigate the inhibitory effect and inhibition characteristics of CsA and Tac on CYP3A4 and CYP3A5 in vitro and to evaluate its clinical relevance. Inhibition by CsA and Tac was studied using midazolam as the probe substrate in coincubation and preincubation investigations using human liver microsomes (HLMs) as well as specific CYP3A4- and CYP3A5-expressing insect microsomes (Supersomes). In vitro-in vivo extrapolations (IVIVEs) were performed to evaluate the clinical relevance of the inhibition. Both CsA and Tac competitively inhibited CYP3A in HLMs, showing inhibition constants (K(i)) of 0.98 and 0.61 μM, respectively. Experiments in Supersomes revealed that Tac inhibited both CYP3A4 and CYP3A5, whereas CsA only inhibited CYP3A4. In contrast to the HLM experiments, studies in Supersomes showed inhibition by Tac to be NADPH- and time-dependent, with a 5-fold reduction in IC(50) after preincubation, supporting a time-dependent inhibition mechanism in recombinant microsomes. By application of HLM data, IVIVE estimated the area under the concentration versus time curve of midazolam to increase by 73 and 27% with CsA and Tac, respectively. The inhibitory effect was predominantly on the intestinal level, whereas hepatic intrinsic clearance seemed unaffected.

Assessment of reactive metabolites in drug-induced liver injury

The aim of the current review is to summarize present methods used for the determination of reactive metabolites, which can predict drug-induced liver injury (DILI) in drug discovery and development. DILI is one of the most frequent reasons for the withdrawal of an approved drug from the market, and it accounts for up to 50% of acute liver failure cases. This review is structured into three sections. The first section is a general overview of the relationship between drug metabolism and liver injury. The second section introduces in vitro methods for the assessment of reactive metabolites for drug discovery and development. In the third section, limitations and future directions for the development of methods for predicting DILI are described.

Identification of novel CYP2A6 inhibitors by virtual screening

The human CYP2A6 enzyme metabolises several xenobiotics including nicotine, the addictive component in tobacco. Reduced activity of CYP2A6, either for genetic reasons or by administering inhibitors of CYP2A6, reduces tobacco smoking. The aim was to design novel inhibitors of CYP2A6 using 3D-QSAR analysis combined with virtual screening. A 3D-QSAR model was utilised to identify the most important features of the inhibitors, and this knowledge was used to design inhibitors for CYP2A6. Chemical database screening yielded several potent inhibitor candidates such as alkylamine derivatives (compound no. 5, IC(50)=0.1 μM) and 1-benzothiophene-3-carbaldehyde that can be used as lead compounds in the development of drugs for smoking reduction therapy.

Bioactivation pathways of the cannabinoid receptor 1 antagonist rimonabant

In the present work, the characterization of the biotransformation and bioactivation pathways of the cannabinoid receptor 1 antagonist rimonabant (Acomplia) is described. Rimonabant was approved in Europe in 2006 for the treatment of obesity but was withdrawn in 2008 because of a significant drug-related risk of serious psychiatric disorders. The aim of the present work is to characterize the biotransformation and potential bioactivation pathways of rimonabant in vitro in human and rat liver microsomes. The observation of a major iminium ion metabolite led us to perform reactive metabolite trapping, covalent binding to proteins, and time-dependent inhibition of cytochrome P450 3A4 studies. The major biotransformation pathways were oxidative dehydrogenation of the piperidinyl ring to an iminium ion, hydroxylation of the 3 position of the piperidinyl ring, and cleavage of the amide linkage. In coincubations with potassium cyanide, three cyanide adducts were detected. A high level of covalent binding of rimonabant in human liver microsomes was observed (920 pmol equivalents/mg protein). In coincubations with potassium cyanide and methoxylamine, the covalent binding was reduced by approximately 40 and 30%, respectively, whereas GSH had no significant effect on covalent binding levels. Rimonabant was also found to inhibit cytochrome P450 3A4 irreversibly in a time-dependent manner. In view of these findings, it is noteworthy that, to date, no toxicity findings related to the formation of reactive metabolites from rimonabant have been reported.

Perpetrators of pharmacokinetic drug-drug interactions arising from altered cytochrome P450 activity: a criteria-based assessment

AIMS

To catalogue the perpetrators of CYP-mediated pharmacokinetic drug-drug interactions (PK-DDIs) using clinically relevant criteria, and to compare this with an analogous catalogue.

METHODS

Candidate inhibitors and inducers of CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A ('perpetrators') were evaluated using published clinical pharmacokinetic interaction studies. Studies were selected on the basis of ≥six human subjects, use of a validated in vivo probe substrate for the CYP enzyme, and clinically relevant dosing. Inhibitors were described according to the FDA classifications of strong, moderate or weak, whereas inducers were classified as major (≥twofold decrease in AUC) or weak (<twofold decrease in AUC). A catalogue of major perpetrators was constructed based on twofold changes in the clearance of probe substrates. Perpetrators in the clinical version of the Cytochromes P450 Drug Interaction Table (CDIT) were compared with the 'accepted' major perpetrators.

RESULTS

From a list of 216 candidate drugs (349 CYP-perpetrator pairs, CYP-PPs), 36 inhibitors and eight inducers were accepted as major perpetrators of PK-DDIs, resulting in 58 CYP-PPs. In comparison, the clinical version of the CDIT had a sensitivity of 33% and a positive predictive value of 68%. One hundred and ninety-nine CYP-PPs were rejected as major perpetrators, and 92 CYP-PPs had insufficient published human pharmacokinetic data for robust classification.

CONCLUSIONS

Using a criteria-based assessment, the number of drugs that are proven or likely major perpetrators of CYP-mediated PK-DDIs is relatively small. Current clinical decision support on PK-DDIs is inconsistent with the published evidence and can be improved using simple criteria.

Simple, direct, and informative method for the assessment of CYP2C19 enzyme inactivation kinetics

Many clinically relevant drug interactions involving cytochrome P450 inhibition are mediated by mechanism-based inactivation (MBI). Time-dependent inhibition is one of the major features distinguishing between reversible inhibition and MBI. It thus provides a useful screening approach for early drug interaction risk assessment. Accordingly, we developed an easy and informative fluorometric method for the assessment of CYP2C19 enzyme inactivation kinetics. Dibenzylfluorescein (DBF) is widely used as a profluorescent probe substrate for P450 activity and inhibition assays, but its use has been considered to be limited to traditional endpoint assays. We monitored CYP2C19-catalyzed metabolism of DBF using synthesized fluorescein benzyl ester and fluorescein benzyl ether along with commercially available fluorescein as intermediate standards. Furthermore, we demonstrated the use of DBF in a kinetic assay as a progress curve analysis for straightforward determination of whether a compound is a time-dependent inactivator of CYP2C19. The recombinant human CYP2C19 inactivation kinetics of isoniazid, ticlopidine, and tranylcypromine were evaluated, and their key kinetic parameters were measured from the same experiment. The known mechanism-based inactivators, isoniazid and ticlopidine, exhibited clear time-dependent inactivation with K(I) and k(inact) values of 250.5 ± 34 μM and 0.137 ± 0.006 min(-1) and 1.96 ± 0.5 μM and 0.135 ± 0.009 min(-1), respectively. Tranylcypromine did not display any time-dependent inhibition, which is consistent with its reported mechanism of competitive inhibition. In summary, DBF is suitable for use in the progress curve analysis approach and can be used as an initial screen to identify compounds that require more detailed investigations in drug interaction optimization.

The prediction of drug-glucuronidation parameters in humans: UDP-glucuronosyltransferase enzyme-selective substrate and inhibitor probes for reaction phenotyping and in vitro-in vivo extrapolation of drug clearance and drug-drug interaction potential

Major advances in the characterization of uridine diphosphate (UDP)-glucuronosyltransferase (UGT) enzyme substrate and inhibitor selectivities and the development of experimental paradigms to investigate xenobiotic glucuronidation in vitro now permit the prediction of a range of drug-glucuronidation parameters in humans. In particular, the availability of substrate and inhibitor "probes" for the major hepatic drug metabolizing UGTs together with batteries of recombinant enzymes allow the reaction phenotyping of drug glucuronidation reactions. Additionally, in vitro experimental approaches and scaling strategies have been successfully applied to the quantitative prediction of in vivo clearance via glucuronidation and drug-drug interaction potential.

Physiologically based pharmacokinetic model of mechanism-based inhibition of CYP3A by clarithromycin

The prediction of clinical drug-drug interactions (DDIs) due to mechanism-based inhibitors of CYP3A is complicated when the inhibitor itself is metabolized by CYP3Aas in the case of clarithromycin. Previous attempts to predict the effects of clarithromycin on CYP3A substrates, e.g., midazolam, failed to account for nonlinear metabolism of clarithromycin. A semiphysiologically based pharmacokinetic model was developed for clarithromycin and midazolam metabolism, incorporating hepatic and intestinal metabolism by CYP3A and non-CYP3A mechanisms. CYP3A inactivation by clarithromycin occurred at both sites. K(I) and k(inact) values for clarithromycin obtained from in vitro sources were unable to accurately predict the clinical effect of clarithromycin on CYP3A activity. An iterative approach determined the optimum values to predict in vivo effects of clarithromycin on midazolam to be 5.3 microM for K(i) and 0.4 and 4 h(-1) for k(inact) in the liver and intestines, respectively. The incorporation of CYP3A-dependent metabolism of clarithromycin enabled prediction of its nonlinear pharmacokinetics. The predicted 2.6-fold change in intravenous midazolam area under the plasma concentration-time curve (AUC) after 500 mg of clarithromycin orally twice daily was consistent with clinical observations. Although the mean predicted 5.3-fold change in the AUC of oral midazolam was lower than mean observed values, it was within the range of observations. Intestinal CYP3A activity was less sensitive to changes in K(I), k(inact), and CYP3A half-life than hepatic CYP3A. This semiphysiologically based pharmacokinetic model incorporating CYP3A inactivation in the intestine and liver accurately predicts the nonlinear pharmacokinetics of clarithromycin and the DDI observed between clarithromycin and midazolam. Furthermore, this model framework can be applied to other mechanism-based inhibitors.

In vitro and in silico identification and characterization of thiabendazole as a mechanism-based inhibitor of CYP1A2 and simulation of possible pharmacokinetic drug-drug interactions

Thiabendazole (TBZ) and its major metabolite 5-hydroxythiabendazole (5OH-TBZ) were screened for potential time-dependent inhibition (TDI) against CYP1A2. Screen assays were carried out in the absence and presence of NADPH. TDI was observed with both compounds, with k(inact) and K(I) values of 0.08 and 0.02 min(-1) and 1.4 and 63.3 microM for TBZ and 5OH-TBZ, respectively. Enzyme inactivation was time-, concentration-, and NADPH-dependent. Inactivation by TBZ was irreversible by dialysis and oxidation by potassium ferricyanide, and there was no protection by glutathione. 5OH-TBZ was a weak TDI of CYP1A2, and enzyme activity was recovered by dialysis. IC(50) determination of TBZ and 5OH-TBZ showed both compounds to be potent inhibitors, with IC(50) values of 0.83 and 13.05 microM, respectively. IC(50) shift studies also demonstrated that TBZ was a TDI of CYP1A2. In silico methods identified the thiazole group as a TDI fragment and predicted it as the site of metabolism. The observation pointed to epoxidation of the thiazole and the benzyl rings of TBZ as possible routes of metabolism and mechanisms of TDI. Drug-drug interaction (DDI) simulation studies using SimCyp showed good predictions for competitive inhibition. However, predictions for mechanism-based inhibition (MBI)-based DDI were not in agreement with clinical observations. There was no TBZ accumulation upon chronic administration of the drug. The in vitro MBI findings might therefore not be capturing the in vivo situation in which the proposed bioactivation route is minor. This might be the case for TBZ in which, in vivo, UDP glucuronosyltransferases and sulfanotransferase metabolize and eliminate the 5OH-TBZ.

In vitro evaluation of reversible and irreversible cytochrome P450 inhibition: current status on methodologies and their utility for predicting drug-drug interactions

It is widely accepted that today's practice of polypharmacy inevitably increases the incidence of drug-drug interactions (DDIs). Serious DDI is a major liability for any new chemical entity (NCE) entering the pharmaceutical market. As such, pharmaceutical companies employ various strategies to avoid problematic compounds for clinical development. A key cause for DDIs is the inhibition of cytochrome P450 enzymes (CYPs) that are responsible for metabolic clearance of many drugs. Screening for inhibition potency of CYPs by NCEs has therefore become a routine practice during the drug discovery stage. However, in order to make proper use of DDI data, an understanding of the strengths and weaknesses of the various experimental systems in current use is required. An illustrated review of experimental practices is presented with discussion of likely future developments. The combination of high quality in vitro data generation and the application of in vivo CYP inhibition modelling approaches should allow more informed decisions to be made in the search for drug molecules with acceptable DDI characteristics.