A comparison of relative abundance, activity factor and inhibitory monoclonal antibody approaches in the characterization of human CYP enzymology

In Vitro Metabolism and Transport Characteristics of Zastaprazan

Zastaprazan (JP-1366), a novel potassium-competitive acid blocker, is a new drug for the treatment of erosive esophagitis. JP-1366 is highly metabolized in human, mouse, and dog hepatocytes but moderately metabolized in rat and monkey hepatocytes when estimated from the metabolic stability of this compound in hepatocyte suspension and when 18 phase I metabolites and 5 phase II metabolites [i.e., N-dearylation (M6), hydroxylation (M1, M19, M21), dihydroxylation (M7, M8, M14, M22), trihydroxylation (M13, M18), hydroxylation and reduction (M20), dihydroxylation and reduction (M9, M16), hydrolysis (M23), hydroxylation and glucuronidation (M11, M15), hydroxylation and sulfation (M17), dihydroxylation and sulfation (M10, M12), N-dearylation and hydroxylation (M3, M4), N-dearylation and dihydroxylation (M5), and N-dearylation and trihydroxylation (M2)] were identified from JP-1366 incubation with the hepatocytes from humans, mice, rats, dogs, and monkeys. Based on the cytochrome P450 (CYP) screening test and immune-inhibition analysis with CYP antibodies, CYP3A4 and CYP3A5 played major roles in the metabolism of JP-1366 to M1, M3, M4, M6, M8, M9, M13, M14, M16, M18, M19, M21, and M22. CYP1A2, 2C8, 2C9, 2C19, and 2D6 played minor roles in the metabolism of JP-1366. UDP-glucuronosyltransferase (UGT) 2B7 and UGT2B17 were responsible for the glucuronidation of M1 to M15. However, JP-1366 and active metabolite M1 were not substrates for drug transporters such as organic cation transporter (OCT) 1/2, organic anion transporter (OAT) 1/3, organic anion transporting polypeptide (OATP)1B1/1B3, multidrug and toxic compound extrusion (MATE)1/2K, P-glycoprotein (P-gp), and breast cancer-resistant protein (BCRP). Only M1 showed substrate specificity for P-gp. The findings indicated that drug-metabolizing enzymes, particularly CYP3A4/3A5, may have a significant role in determining the pharmacokinetics of zastaprazan while drug transporters may only have a small impact on the absorption, distribution, and excretion of this compound.

Frequency of CYP2C9 Promoter Variable Number Tandem Repeat Polymorphism in a Spanish Population: Linkage Disequilibrium with CYP2C9*3 Allele

BACKGROUND

A promoter variable number tandem repeat polymorphism (pVNTR) of CYP2C9 is described with three types of fragments: short (pVNTR-S), medium (pVNTR-M) and long (pVNTR-L). The pVNTR-S allele reduces the CYP2C9 mRNA level in the human liver, and it was found to be in high linkage disequilibrium (LD) with the CYP2C9*3 allele in a White American population. The aim of the present study is to determine the presence and frequency of CYP2C9pVNTR in a Spanish population, as well as analyzing whether the pVNTR-S allele is in LD with the CYP2C9*3 allele in this population.

SUBJECTS AND METHODS

A total of 209 subjects from Spain participated in the study. The CYP2C9 promoter region was amplified and analyzed using capillary electrophoresis. Genotyping for CYP2C9*2 and *3 variants was performed using a fluorescence-based allele-specific TaqMan allelic discrimination assay.

RESULTS

The frequencies of CYP2C9pVNTR-L, M and S variant alleles are 0.10, 0.82 and 0.08, respectively. A high LD between CYP2C9pVNTR-S and CYP2C9*3 variant alleles is observed (D' = 0.929, r2 = 0.884).

CONCLUSION

The results from the present study show that both CYP2C9pVNTR and CYP2C9*3 are in a high LD, which could help to better understand the lower metabolic activity exhibited by CYP2C9*3 allele carriers. These data might be relevant for implementation in the diverse clinical guidelines for the pharmacogenetic analysis of the CYP2C9 gene before treatment with different drugs, such as non-steroidal anti-inflammatory drugs, warfarin, phenytoin and statins.

Prediction of Drug Clearance from Enzyme and Transporter Kinetics

Accurate estimation of in vivo clearance in human is pivotal to determine the dose and dosing regimen for drug development. In vitro-in vivo extrapolation (IVIVE) has been performed to predict drug clearance using empirical and physiological scalars. Multiple in vitro systems and mathematical modeling techniques have been employed to estimate in vivo clearance. The models for predicting clearance have significantly improved and have evolved to become more complex by integrating multiple processes such as drug metabolism and transport as well as passive diffusion. This chapter covers the use of conventional as well as recently developed methods to predict metabolic and transporter-mediated clearance along with the advantages and disadvantages of using these methods and the associated experimental considerations. The general approaches to improve IVIVE by use of appropriate scalars, incorporation of extrahepatic metabolism and transport and application of physiologically based pharmacokinetic (PBPK) models with proteomics data are also discussed. The chapter also provides an overview of the advantages of using such dynamic mechanistic models over static models for clearance predictions to improve IVIVE.

Physiologically Based Pharmacokinetic Modelling of Cytochrome P450 2C9-Related Tolbutamide Drug Interactions with Sulfaphenazole and Tasisulam

Background and Objectives

Cytochrome P450 2C9 (CYP2C9) is involved in the biotransformation of many commonly used drugs, and significant drug interactions have been reported for CYP2C9 substrates. Previously published physiologically based pharmacokinetic (PBPK) models of tolbutamide are based on an assumption that its metabolic clearance is exclusively through CYP2C9; however, many studies indicate that CYP2C9 metabolism is only responsible for 80–90% of the total clearance. Therefore, these models are not useful for predicting the magnitude of CYP2C9 drug–drug interactions (DDIs). This paper describes the development and verification of SimCYP^®-based PBPK models that accurately describe the human pharmacokinetics of tolbutamide when dosed alone or in combination with the CYP2C9 inhibitors sulfaphenazole and tasisulam.

Methods

A PBPK model was optimized in SimCYP^® for tolbutamide as a CYP2C9 substrate, based on published in vitro and clinical data. This model was verified to replicate the magnitude of DDI reported with sulfaphenazole and was further applied to simulate the DDI with tasisulam, a small molecule investigated for the treatment of cancer. A clinical study (CT registration # NCT01185548) was conducted in patients with cancer to assess the pharmacokinetic interaction of tasisulum with tolbutamide. A PBPK model was built for tasisulam, and the clinical study design was replicated using the optimized tolbutamide model.

Results

The optimized tolbutamide model accurately predicted the magnitude of tolbutamide AUC increase (5.3–6.2-fold) reported for sulfaphenazole. Furthermore, the PBPK simulations in a healthy volunteer population adequately predicted the increase in plasma exposure of tolbutamide in patients with cancer (predicted AUC ratio = 4.7–5.4; measured mean AUC ratio = 5.7).

Conclusions

This optimized tolbutamide PBPK model was verified with two strong CYP2C9 inhibitors and can be applied to the prediction of CYP2C9 interactions for novel inhibitors. Furthermore, this work highlights the utility of mechanistic models in navigating the challenges in conducting clinical pharmacology studies in cancer patients.

Predicted contributions of cytochrome P450s to drug metabolism in human liver microsomes using relative activity factor were dependent on probes

Contributions of cytochrome P450 (CYP450) isoforms to drug metabolism are often predicted using relative activity factor (RAF) method, assuming RAF values were independent of probe. We aimed to report probe-dependent characteristic of RAF values using CYP3A4 or CYP2C9 probes. Metabolism of four CYP3A4 probes (testosterone, midazolam, verapamil and atorvastatin) and three CYP2C9 probes (tolbutamide, diclofenac and S-warfarin) in human liver microsomes (HLM) and cDNA-expressed recombinant CYP450 (Rec-CYP450) systems were characterized and RAF_CL value was estimated as ratio of probe intrinsic clearance in HLM to that in Rec-CYP450. CYP450i contributions to metabolic reaction of a probe were predicted using other probes and compared with data from specific inhibitions. Contributions of CYP3A4 and CYP2C9 to metabolism of deoxypodophyllotoxin and nateglinide were also predicted. RAF values were dependent on probes, leading to probe-dependently predicted contributions. Predicted contributions of CYP3A4 to formations of 6β-hydroxytestosterone, 1'-hydroxymidazolam, norverapamil, ortho-hydroxyatorvastatin and para-hydroxyatorvastatin using other probes were 47.46-219.46%, 21.62-98.87%, 186.49-462.44%, 21.87-101.15% and 53.62-247.97%, respectively. Predicted contributions of CYP3A4 and CYP2C9 to nateglinide metabolism were 8.18-37.84% and 36.08-94.04%, separately. In conclusion, CYP450i contribution to drug metabolism in HLM estimated using RAF approach were probe-dependent. Therefore, contribution of each isoform must be confirmed by multiple probes.

Direct comparison of UDP-glucuronosyltransferase and cytochrome P450 activities in human liver microsomes, plated and suspended primary human hepatocytes from five liver donors

UDP-glucuronosyltransferases (UGTs) and cytochrome P450s (CYPs) are the major enzymes involved in hepatic metabolism of drugs. Hepatic drug metabolism is commonly investigated using human liver microsomes (HLM) or primary human hepatocytes (PHH). We describe the development of a sensitive assay to phenotype activities of six major hepatic UGT isoforms (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9 and UGT2B7) in intact PHH by analysis of glucuronidation of selective probe substrates. The non-selective, general substrate 7-hydroxycoumarin was included for comparison. For each liver donor preparation (five donors) UGT activities in cryopreserved suspended and plated PHH were compared to HLM prepared from the same donors. Standard CYP reaction phenotyping of seven major isoforms was performed in parallel. For all donors, CYP- and UGT-isoforms activity profiles were comparable in PHH and HLM, indicating that reaction phenotyping with selective probe substrates in intact cells primarily reflects respective CYP or UGT activity. System-dependent effects on UGT and CYP isoform activity were still found. While UGT activity of UGT1A1 was equivalent in plated and suspended PHH, UGT1A3, UGT1A6 and UGT2B7 activity was higher in suspended PHH and UGT1A9 and UGT1A4 activity was higher in plated PHH. The well-known decrease in activity of most CYP isoforms in plated compared to suspended PHH was confirmed. Importantly, we found a significant loss in CYP2C19 and CYP2B6 in HLM, activity being lower than in intact cells. Taken together, these findings implicate that, dependent on the UGT or CYP isoforms involved in the metabolism of a given compound, the outcome of metabolic assays is strongly dependent on the choice of the in vitro system. The currently described UGT- and CYP- activity profiling method can be used as a standard assay in intact cells and can especially aid in reaction phenotyping of in vitro systems for which a limited number of cells are available.

Impact of Probe Substrate Selection on Cytochrome P450 Reaction Phenotyping Using the Relative Activity Factor

Accurately assessing the contribution of cytochrome P450 (P450) isoforms to overall metabolic clearance is important for prediction of clinical drug-drug interactions (DDIs). The relative activity factor (RAF) approach in P450 reaction phenotyping assumes that the interaction between P450-selective probes and testing systems is the same as the interaction of drug candidate with those systems. To test this assumption, an intersystem clearance ratio (ICR) was created to evaluate the difference in values between RAF-scaled intrinsic clearance (CL_int) and measured CL_int in human liver microsomes (HLMs). The RAF value for CYP3A4 or CYP2C9 derived from a particular P450-selective probe reaction was applied to calculate RAF-scaled CL_int for other probe reactions of the same P450 isoform in a crossover manner and compared with the measured HLM CL_int When RAF derived from midazolam or nifedipine was used for CYP3A4, the ICR for testosterone 6β-hydroxylation was 31 and 25, respectively, suggesting significantly diverse interactions of CYP3A4 probes with the testing systems. Such ICR differences were less profound among probes for CYP2C9. In addition, these RAF values were applied to losartan and meloxicam, whose metabolism is mostly CYP2C9 mediated. Only using the RAF derived from testosterone for CYP3A4 produced the expected CYP2C9 contribution of 72%-87% and 47%-69% for metabolism of losartan and meloxicam, respectively. RAF derived from other CYP3A4 probes would have attributed predominantly to CYP3A4 and led to incorrect prediction of DDIs. Our study demonstrates a significant impact of probe substrate selection on P450 phenotyping using the RAF approach, and the ICR may provide a potential solution.

Derivatives of Procaspase-Activating Compound 1 (PAC-1) and their Anticancer Activities

PAC-1 induces the activation of procaspase-3 in vitro and in cell culture by chelation of inhibitory labile zinc ions via its ortho-hydroxy-N-acylhydrazone moiety. First reported in 2006, PAC-1 has shown promise in cell culture and animal models of cancer, and a Phase I clinical trial in cancer patients began in March 2015 (NCT02355535). Because of the considerable interest in this compound and a well-defined structure-activity relationship, over 1000 PAC-1 derivatives have been synthesized in an effort to vary pharmacological properties such as potency and pharmacokinetics. This article provides a comprehensive examination of all PAC-1 derivatives reported to date. A survey of PAC-1 derivative libraries is provided, with an indepth discussion of four derivatives on which extensive studies have been performed.

Mild hypothermia decreases the total clearance of glibenclamide after low dose administration in rats

BACKGROUND AND PURPOSE

Low dose glibenclamide exhibits pleiotropic protective effects in different central nervous system diseases. Previously, we have shown that mild hypothermia enhanced the efficacy of glibenclamide in the cultured cortical neuronal cells. This study aims to evaluate the impact of mild hypothermia on the pharmacokinetics of low dose glibenclamide in rats via its cytochrome P450 2C9 (CYP2C9) metabolic pathway.

METHODS

Male Sprague-Dawley rats were maintained at 37°C (normothermic group) or cooled to 33°C (hypothermic group). Glibenclamide (33μg/kg) or diclofenac (10mg/kg, a probe drug for assessing the activity of CYP2C9 which involves in glibenclamide and diclofenac metabolism in liver) were intravenously administered at 10min after stabilization of temperature. Plasma samples were collected at 9 different time points. Glibenclamide and diclofenac in sera were separated by liquid chromatography and quantified with tandem mass spectrometry.

RESULTS

Compared with normothermia, mild hypothermia significantly decreased the total clearance of glibenclamide (16.00±4.1-6.72±2.1mL/min/kg; p<0.01), and there was a non-significant trend in a slightly higher half-life, (1.64±0.34-2.71±1.7h, p=0.157). Area under the plasma concentration versus time curve (AUClast) in the hypothermic group was increased (33.2±11-77.8±18hng/mL, p<0.01). Moreover, mild hypothermia reduced the total clearance of diclofenac (10.33±1.53-7.20±1.66mL/min/kg, p<0.01), indicating that the CYP2C9 activity was compromised in reduced temperature.

CONCLUSION

Mild hypothermia reduced the total clearance of glibenclamide, probably via mediating the activity of CYP2C9. The impact of hypothermia in clinical application of glibenclamide should be considered.

Development of Safe and Effective Botanical Dietary Supplements

Regulated differently than drugs or foods, the market for botanical dietary supplements continues to grow worldwide. The recently implemented U.S. FDA regulation that all botanical dietary supplements must be produced using good manufacturing practice is an important step toward enhancing the safety of these products, but additional safeguards could be implemented, and unlike drugs, there are currently no efficacy requirements. To ensure a safe and effective product, botanical dietary supplements should be developed in a manner analogous to pharmaceuticals that involves identification of mechanisms of action and active constituents, chemical standardization based on the active compounds, biological standardization based on pharmacological activity, preclinical evaluation of toxicity and potential for drug-botanical interactions, metabolism of active compounds, and finally, clinical studies of safety and efficacy. Completing these steps will enable the translation of botanicals from the field to safe human use as dietary supplements.

In vitro identification of human cytochrome P450 isoforms involved in the metabolism of Geissoschizine methyl ether, an active component of the traditional Japanese medicine Yokukansan

1. Yokukansan (YKS) is a traditional Japanese medicine also called kampo, which has been used to treat neurosis, insomnia, and night crying and peevishness in children. Geissoschizine methyl ether (GM), a major indole alkaloid found in Uncaria hook, has been identified as a major active component of YKS with psychotropic effects. Recently, GM was reported to have a partial agonistic effect on serotonin 5-HT1A receptors. However, there is little published information on GM metabolism in humans, although several studies reported the blood kinetics of GM in rats and humans. In this study, we investigated the GM metabolic pathways and metabolizing enzymes in humans. 2. Using recombinant human cytochrome P450 (CYP) isoforms and polyclonal antibodies to CYP isoforms, we found that GM was metabolized into hydroxylated, dehydrogenated, hydroxylated+dehydrogenated, demethylated and water adduct forms by some CYP isoforms. 3. The relative activity factors in human liver microsomes were calculated to determine the relative contributions of individual CYP isoforms to GM metabolism in human liver microsomes (HLMs). We identified CYP3A4 as the CYP isoform primarily responsible for GM metabolism in human liver microsomes. 4. These findings provide an important basis for understanding the pharmacokinetics and pharmacodynamics of GM and YKS.

Removal of Metabolic Liabilities Enables Development of Derivatives of Procaspase-Activating Compound 1 (PAC-1) with Improved Pharmacokinetics

Procaspase-activating compound 1 (PAC-1) is an o-hydroxy-N-acylhydrazone that induces apoptosis in cancer cells by chelation of labile inhibitory zinc from procaspase-3. PAC-1 has been assessed in a wide variety of cell culture experiments and in vivo models of cancer, with promising results, and a phase 1 clinical trial in cancer patients has been initiated (NCT02355535). For certain applications, however, the in vivo half-life of PAC-1 could be limiting. Thus, with the goal of developing a compound with enhanced metabolic stability, a series of PAC-1 analogues were designed containing modifications that systematically block sites of metabolic vulnerability. Evaluation of the library of compounds identified four potentially superior candidates with comparable anticancer activity in cell culture, enhanced metabolic stability in liver microsomes, and improved tolerability in mice. In head-to-head experiments with PAC-1, pharmacokinetic evaluation in mice demonstrated extended elimination half-lives and greater area under the curve values for each of the four compounds, suggesting them as promising candidates for further development.

Monoclonal Antibodies and Multifunctional Cytochrome P450: Drug Metabolism as Paradigm

Monoclonal antibodies are reagents par excellence for analyzing the role of individual cytochrome P450 isoforms in multifunctional biological activities catalyzed by cytochrome P450 enzymes. The precision and utility of the monoclonal antibodies have heretofore been applied primarily to studies of human drug metabolism. The unique and precise specificity and high inhibitory activity toward individual cytochrome P450s make the monoclonal antibodies extraordinary tools for identifying and quantifying the role of each P450 isoform in the metabolism of a drug or nondrug xenobiotic. The monoclonal antibodies identify drugs metabolized by individual, several, or polymorphic P450s. A comprehensive collection of monoclonal antibodies has been isolated to human P450s: 1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C family, 2C19, 2D6, 2E1, 3A4/5, and 2J2. The monoclonal antibodies can also be used for identifying drugs and/or metabolites useful as markers for in vivo phenotyping. Clinical identification of a patient's phenotype, coupled with precise knowledge of a drug's metabolism, should lead to a reduction of adverse drug reactions and improved drug therapeutics, thereby promoting advances in drug discovery.

In vivo quantitative prediction of the effect of gene polymorphisms and drug interactions on drug exposure for CYP2C19 substrates

We present a unified quantitative approach to predict the in vivo alteration in drug exposure caused by either cytochrome P450 (CYP) gene polymorphisms or CYP-mediated drug-drug interactions (DDI). An application to drugs metabolized by CYP2C19 is presented. The metrics used is the ratio of altered drug area under the curve (AUC) to the AUC in extensive metabolizers with no mutation or no interaction. Data from 42 pharmacokinetic studies performed in CYP2C19 genetic subgroups and 18 DDI studies were used to estimate model parameters and predicted AUC ratios by using Bayesian approach. Pharmacogenetic information was used to estimate a parameter of the model which was then used to predict DDI. The method adequately predicted the AUC ratios published in the literature, with mean errors of -0.15 and -0.62 and mean absolute errors of 0.62 and 1.05 for genotype and DDI data, respectively. The approach provides quantitative prediction of the effect of five genotype variants and 10 inhibitors on the exposure to 25 CYP2C19 substrates, including a number of unobserved cases. A quantitative approach for predicting the effect of gene polymorphisms and drug interactions on drug exposure has been successfully applied for CYP2C19 substrates. This study shows that pharmacogenetic information can be used to predict DDI. This may have important implications for the development of personalized medicine and drug development.

Genetic variants in CYP (-1A2, -2C9, -2C19, -3A4 and -3A5), VKORC1 and ABCB1 genes in a black South African population: a window into diversity

AIM

The frequencies of variants of pharmacogenetic importance differ across populations. African populations exhibit the greatest genetic heterogeneity, cautioning against extrapolating results among African groups. The aim of this study was to genotype pharmacogenetically relevant variants in black South Africans, to expand the limited data set available for indigenous African populations.

SUBJECTS & METHODS

A total of 14 SNPs associated with seven genes known to influence drug metabolism or transport (CYP1A2, CYP2C19, CYP2C9, CYP3A4, CYP3A5, VKORC1 and ABCB1) were investigated in a South African black (SAB) population (n = 993) and allele frequencies were compared with populations of African, Asian and European origin.

RESULTS

The majority of SNPs in the SAB demonstrated significant allele frequency differences when compared with both Europeans and Asians. There was greater similarity between the SAB and the Luhya (Kenya) and the Yoruba (Nigeria), than with Maasai (Kenya) individuals. The CYP2C9 SNP (rs1799853) was not polymorphic in the SAB and two VKORC1 SNPs (rs17708472 and rs9934438) had low variant allele frequencies, limiting their relevance to warfarin dose in this population. Population differences are emphasized by the significant differences in ABCB1 and the CYP3A gene family allele frequencies, with implications for drug metabolism and transport.

CONCLUSION

This study highlights the importance of investigating and documenting genetic variation at loci of pharmacogenetic relevance among different populations since this information could be used to inform drug efficacy, safety and recommended dosage.

Genotoxicity of a freshwater cyanotoxin, cylindrospermopsin, in two human cell lines: Caco-2 and HepaRG

Cylindrospermopsin (CYN), a cyanotoxin produced by certain freshwater cyanobacteria, causes human intoxications and animal mortalities. CYN is a potent inhibitor of protein- and glutathione-synthesis. Preliminary evidence for in vivo tumor initiation has been found in mice but the mechanism remains unclear. Several in vitro and in vivo studies demonstrate that CYN is genotoxic and requires metabolic activation. In the present study, the genotoxicity of CYN was assessed in human hepatocyte and enterocyte cell lines, which are models for CYN target organs. The cytokinesis-block micronucleus assay was conducted on liver-derived HepaRG cells and colon-derived Caco-2 cells. Each cell-type was exposed to CYN in both the differentiated and the undifferentiated states, and both with and without the cytochrome P450 inhibitor, ketoconazole, to determine the involvement of metabolism in CYN genotoxicity. CYN increased the frequency of micronuclei in binucleated cells (MNBNC) in both Caco-2 and HepaRG cells. Moreover, ketoconazole reduced both the genotoxicity and cytotoxicity caused by CYN. Our results confirm the involvement of metabolic activation of CYN in mediating its toxicity and suggest that CYN is progenotoxic.

Approach to predict the contribution of cytochrome P450 enzymes to drug metabolism in the early drug-discovery stage: The effect of the expression of cytochromeb5with recombinant P450 enzymes

In order to evaluate the potential adverse effects due to genetic polymorphism and/or inter-individual variation, it is necessary to calculate the cytochrome P450 (CYP) contribution to the metabolism of new drugs. In the current study, the in vitro intrinsic clearance (CL(int)) values of marker substrates and drugs were determined by measuring metabolite formation and substrate depletion, respectively. Recombinant CYP microsomes expressing CYP2C9, CYP2C19 and CYP3A4 with co-expressed cytochrome b(5) were used, but those expressing CYP1A2 and CYP2D6 did not have co-expressed cytochrome b(5). The following prediction methods were compared to determine the CL(int) value using data from recombinant CYP enzymes: (1) relative CYP enzyme content in human liver microsomes; (2) relative activity factor (RAF) estimated from the V(max) value; and (3) RAF estimated from the CL(int) value. Estimating RAF from CL(int) proved the most accurate prediction method among the three tested, and differences in the CYP3A4 marker reactions did not affect its accuracy. The substrate depletion method will be useful in the early drug-discovery stage when the main metabolite and/or metabolic pathway has not been identified. In addition, recombinant CYP microsomes co-expressed with cytochrome b(5) might be suitable for the prediction of the CL(int) value.

Prediction of hepatic clearance in human from in vitro data for successful drug development

The in vivo metabolic clearance in human has been successfully predicted by using in vitro data of metabolic stability in cryopreserved preparations of human hepatocytes. In the predictions by human hepatocytes, the systematic underpredictions of in vivo clearance have been commonly observed among different datasets. The regression-based scaling factor for the in vitro-to-in vivo extrapolation has mitigated discrepancy between in vitro prediction and in vivo observation. In addition to the elimination by metabolic degradation, the important roles of transporter-mediated hepatic uptake and canalicular excretion have been increasingly recognized as a rate-determining step in the hepatic clearance. It has been, therefore, proposed that the in vitro assessment should allow the evaluation of clearances for both transporter(s)-mediated uptake/excretion and metabolic degradation. This review first outlines the limited ability of subcellular fractions such as liver microsomes to predict hepatic clearance in vivo. It highlights the advantages of cryopreserved human hepatocytes as one of the versatile in vitro systems for the prediction of in vivo metabolic clearance in human at the early development stage. The following section discusses the mechanisms underlying the systematic underprediction of in vivo intrinsic clearance by hepatocytes. It leads to the proposal for the assessment of hepatic uptake clearance as one of the kinetically important determinants for accurate predictions of hepatic clearance in human. The judicious combination of advanced technologies and understandings for the drug disposition allows us to rationally optimize new chemical entities to the drug candidate with higher probability of success during the clinical development.

Relative roles of CYP2C19 and CYP3A4/5 in midazolam 1′-hydroxylation

1. During the characterization of recombinant CYP2C19, it was observed that this enzyme metabolized midazolam, which is generally regarded as CYP3A4/5 substrate, and we therefore decided to pursue this observation further. 2. CYP2C19 showed a Michaelis-Menten pattern for midazolam 1'-hydroxylation and was inhibited by (+)-N-3-benzylnirvanol and S-mephenytoin, which are a standard potent inhibitor and a substrate of CYP2C19, respectively. 3. The inhibitory potency by CYP3A4/5 inhibitor on the midazolam 1'-hydroxylation in human liver microsomes (HLM) was correlated with the CYP3A4/5 specific catalytic activity, but such correlation was not observed in CYP2C19 enzyme. The in vitro intrinsic clearance value for midazolam 1'-hydroxylation was not changed by the addition of (+)-N-3-benzylnirvanol in four individual HLM preparations. 4. These results indicated that although CYP2C19 is capable of catalyzing midazolam 1'-hydroxylation, CYP3A4/5 play a more important role.

Approach to the prediction of the contribution of major cytochrome P450 enzymes to drug metabolism in the early drug-discovery stage

It is important to determine the cytochrome P450 (CYP) contribution of certain drugs by taking into consideration the attrition due to issues such as genetic polymorphism and inter-individual variation. In many cases in the early discovery stage, the metabolites of a new chemical have not been identified. Therefore, the present paper devised an approach in which the in vitro intrinsic clearance (CLint) value for new chemicals was determined by measuring substrate depletion. The following prediction methods were compared to calculate CLint using data from recombinant CYP enzymes: (1) the relative CYP content in human liver microsomes; (2) the relative activity factor (RAF) based on the Vmax value; and (3) the RAF value based on the CLint value. The most accurate prediction method was RAF based on CLint. This method would be useful in the early drug-discovery process in cases in which the main metabolite is not identified.

Comparative analysis of substrate and inhibitor interactions with CYP3A4 and CYP3A5

To evaluate the role that cytochrome (CYP) 3A5 plays in hepatic drug metabolism, the substrate selectivity and inhibitory potential of over 60 compounds towards CYP3A4 and CYP3A5 were assessed using Escherichia coli recombinant cell lines. CYP3A4-mediated metabolism predominated for many of the compounds studied. However, a number of drugs gave similar CL(int) estimates using CYP3A5 compared with CYP3A4 including midazolam (CL(int) = 3.4 versus 3.3 microl min(-1) pmol(-1)). Significant CYP3A5-mediated metabolism was also observed for several drugs including mifepristone (CL(int) = 10.3 versus 2.4 microl min(-1) pmol(-1)), and ritonavir (CL(int) = 0.76 versus 0.47 microl min(-1) pmol(-1)). The majority of compounds studied showed a greater inhibitory potential (IC(50)) towards CYP3A4 compared with CYP3A5 (eightfold lower on average). A greater degree of time-dependent inhibition was also observed with CYP3A4 compared with CYP3A5. The range of compounds investigated in the present study extends significantly previous work and suggests that CYP3A5 may have a significant role in drug metabolism particularly in populations expressing high levels of CYP3A5 and/or on co-medications known to inhibit CYP3A4.

Prediction of pharmacokinetic drug-drug interactions using human hepatocyte suspension in plasma and cytochrome P450 phenotypic data. II. In vitro-in vivo correlation with ketoconazole

Traditional cytochrome P450 (P450) based drug-drug interaction (DDI) predictions are based on the ratio of an inhibitor's physiological concentration [I] and its inhibition constant K(i). Determining [I] at the enzymatic site, although critical for predicting clinical DDIs, remains a technical challenge. In our previous study, a novel approach using cryopreserved human hepatocytes suspended in human plasma was investigated to mimic the in vivo concentration of ketoconazole at the enzymatic site (Lu et al., 2007), effectively eliminating the estimation of the elusive [I] value. P450 inhibition in this system appears to model that in vivo. Using the ketoconazole inhibition information in a human hepatocyte-plasma suspension together with quantitative P450 phenotypic information, we successfully predicted the pharmacokinetic DDIs for a small set of drugs, such as theophylline, tolbutamide, omeprazole, desipramine, midazolam, loratadine, cyclosporine, and alprazolam, as well as an investigational compound. For the applicability of this model on a wider scale the in vitro-in vivo correlation data set needed to be expanded. However, for most drugs in the literature there is not enough quantitative information on the involvement of individual P450s to predict DDIs retrospectively. To facilitate that, in this study we determined quantitative P450 phenotyping for seven marketed drugs: budesonide, buprenorphine, loratadine, sirolimus, tacrolimus, docetaxel, and methylprednisolone. Augmentation of the new data set with the one generated previously produced broader a database that provided further support for the wider applicability of this approach using ketoconazole as a potent CYP3A inhibitor. This application is predicted to be equally effective with other P450 inhibitors that are not substrates of efflux pumps.

In vitro-in vivo extrapolation of hepatic clearance involving active uptake: theoretical and experimental aspects

The importance of hepatic uptake transporters in drug clearance is well recognized. The subject is reviewed with the intention of providing an overview of the concepts in order to link the increasing knowledge of transporter-mediated uptake into established models of hepatic clearance. In order to understand and quantify their impact, models of hepatic elimination that incorporate permeability barriers are required. Models that include both active and passive uptake into hepatocytes are discussed and simulations of the influence of active uptake and passive diffusion on hepatic clearance are presented. The advantages and weaknesses of a number of in vitro assays of hepatic uptake are described, and their ability to predict hepatic clearance is reviewed.

The pivotal role of hepatocytes in drug discovery

This review promotes the value of isolated hepatocytes in modern Drug Discovery programmes and outlines how increased understanding, particularly in the area of in vitro-in vivo extrapolation (IVIVE), has led to more widespread use. The importance of in vitro metabolic intrinsic clearance data for predicting in vivo clearance has been acknowledged for several years and the greater utility of hepatocytes, compared with hepatic microsomes and liver slices, for this application is discussed. The application of hepatocytes in predicting drug-drug interactions (DDIs) resulting from reversible and irreversible (time-dependent) inhibition is relatively novel but affords the potential to study both phase I and phase II processes together with any impact of drug efflux and/or uptake (cellular accumulation). Progress in this area is reviewed along with current opinions on the comparative use of primary hepatocytes and higher throughput reporter gene-based systems for studying cytochrome P450 (CYP) induction. The appreciation of the role of transporter proteins in drug disposition continues to evolve. The study of hepatic uptake using isolated hepatocytes and the interplay between drug transport and metabolism with respect to both clearance and DDIs and subsequent IVIVE is also considered.

A Novel Model for the Prediction of Drug-Drug Interactions in Humans Based on in Vitro Cytochrome P450 Phenotypic Data

Ketoconazole has generally been used as a standard inhibitor for studying clinical pharmacokinetic drug-drug interactions (DDIs) of drugs that are primarily metabolized by CYP3A4/5. However, ketoconazole at therapeutic, high concentrations also inhibits cytochromes P450 (P450) other than CYP3A4/5, which has made the predictions of DDIs less accurate. Determining the in vivo inhibitor concentration at the enzymatic site is critical for predicting the clinical DDI, but it remains a technical challenge. Various approaches have been used in the literature to estimate the human hepatic free concentrations of this inhibitor, and application of those to predict DDIs has shown some success. In the present study, a novel approach using cryopreserved human hepatocytes suspended in human plasma was applied to mimic the in vivo concentration of ketoconazole at the enzymatic site. The involvement of various P450s in the metabolism of compounds of interest was quantitatively determined (reactive phenotyping). Likewise, the effect of ketoconazole on various P450s was quantitated. Using this information, P450-mediated change in the area under the curve has been predicted without the need of estimating the inhibitor concentrations at the enzyme active site or the K(i). This approach successfully estimated the magnitude of the clinical DDI of an investigational compound, MLX, which is cleared by multiple P450-mediated metabolism. It also successfully predicted the pharmacokinetic DDIs for several marketed drugs (theophylline, tolbutamide, omeprazole, desipramine, midazolam, alprazolam, cyclosporine, and loratadine) with a correlation coefficient (r(2)) of 0.992. Thus, this approach provides a simple method to more precisely predict the DDIs for P450 substrates when coadministered with ketoconazole or any other competitive P450 inhibitors in humans.

AUTOMATED ASSESSMENT OF TIME-DEPENDENT INHIBITION OF HUMAN CYTOCHROME P450 ENZYMES USING LIQUID CHROMATOGRAPHY-TANDEM MASS SPECTROMETRY ANALYSIS

Increasing reports of time-dependent inhibition of cytochrome P450 (P450) suggest further emphasis on interpreting the consequences, either from a pharmacokinetic or toxicological perspective. Two automated, time-dependent inhibition assays with a liquid chromatography-tandem mass spectrometric endpoint are presented. The initial assay utilizes human liver microsomes, a single concentration of inhibitor, and a single preincubation time of 30 min. Phenacetin, diclofenac, S-mephenytoin, bufuralol, and midazolam are used as substrates for CYP1A2, 2C9, 2C19, 2D6, and 3A4, and the assay differentiates between reversible and irreversible inhibition. The second assay uses individual recombinant human P450s, six inhibitor concentrations, and three time points to accurately define kinact and KI. A good correlation is demonstrated between kinact/KI and partition ratio, indicating that both terms are related in describing the efficiency of enzyme inactivation. Despite the single preincubation time point of 30 min used in the initial assay, a good relationship has been found to exist between the unbound IC50 estimated from this initial screen and the kinact/KI ratio derived from the more extensive subsequent single P450 assay. The higher throughput human liver microsomal assay can therefore generate IC50 values that can be used to predict the pharmacokinetic impact on cotherapies from the estimated kinact/KI ratio, predicted human dose, and pharmacokinetics.