Use of in vitro human metabolism studies in drug development. An industrial perspective

N-methylsansalvamide elicits antitumor effects in colon cancer cells in vitro and in vivo by regulating proliferation, apoptosis, and metastatic capacity

N-methylsansalvamide (MSSV), a cyclic pentadepsipeptide, was obtained from a strain of Fusarium solani f. radicicola. The current study investigated the anti-colorectal cancer effect of MSSV. MSSV exhibited the inhibition of the proliferation in HCT116 cells via induction of G0/G1 cell cycle arrest by downregulating CDK 2, CDK6, cyclin D, and cyclin E, and upregulating p21WAF1 and p27KIP1. Decreased phosphorylation of AKT was observed in MSSV-treated cells. Moreover, MSSV treatment induced caspase-mediated apoptosis through elevating the level of cleaved caspase 3, cleaved PARP, cleaved caspase 9, and pro-apoptotic Bax. MSSV revealed the declined MMP-9 level mediated by reduction in the binding activity of AP-1, Sp-1, and NF-κB motifs, which led to the migration and invasion of HCT116 cells. In vitro metabolism with rat liver S9 fractions was performed to examine the effect of MSSV metabolites. The metabolic process enhanced the inhibitory effect of MSSV on the HCT116 cell proliferation via decline of cyclin D1 expression and AKT phosphorylation. Finally, oral administration of MSSV inhibited the tumor growth of HCT116 xenograft mice. These results suggest that MSSV is a potential anti-tumor agent in colorectal cancer treatment.

Screening, molecular simulation & in silico kinetics of virtually designed covid-19 main protease inhibitors

Coronavirus (covid-19) infection is considered to be deadliest ever pandemic experienced by the human being. It has very badly affected the socio-economic health of human and stuck the scientific community to think and rethink about its complete eradication. But due to no effective treatment or unavailability of vaccine the health professional could not show any significant improvement to control the pandemic. The situation needs newer molecule, vaccine or effective treatment to control covid-19 infection. Different target in viruses has been explored and proteases enzymes were found to be therapeutically effective target for the design of potential anti-covid-19 molecule as it plays the vital role in viral replication and assembly. Structure-based drug design was employed to discover the small molecule of anti-covid-19. Here we considered the small library of naturally occurring polyphenolic compounds and molecular docking, Molecular dynamics (MD) simulations, free binding energy calculation and in-silico ADME calculations to identify the newer HITs. Based upon their score the two molecules were identified as promising candidate. The docking scores were found to be −7.643 and −7.065 for the HIT1 and HIT-2 respectively. In MD simulations study the RMSD values were found to be 4.3 Å & 4.9 Å respectively. To validate these results MM-GBSA was performed and their binding free energies were computationally determined. The prime energy values of identified HITs (−13412.45 & −13441.8 kJ/mole) were found to be very close proximity to reference molecule (−13493.05 kJ/mole). Then in-silico ADME calculations were performed to calculate the drug likeliness identified HITs. BY considering all the values comparative to reference molecule and obtained in-silico pharmacokinetic properties of identified HITs we can suggest that HIT-1 and HIT-2 would be the most promising molecules that can inhibit the main protease enzyme of covid-19. These two molecules would become the potential drug candidate for the treatment of covid-19 infections.

The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

In this report, metabolically competent in vitro systems have been reviewed, in the context of drug metabolising enzyme induction. Based on the experience of the scientists involved, a thorough survey of the literature on metabolically competent long-term culture models was performed. Following this, a prevalidation proposal for the use of the collagen gel sandwich hepatocyte culture system for drug metabolising enzyme induction was designed, focusing on the induction of the cytochrome P450 enzymes as the principal enzymes of interest. The ultimate goal of this prevalidation proposal is to provide industry and academia with a metabolically competent in vitro alternative for long-term studies. In an initial phase, the prevalidation study will be limited to the investigation of induction. However, proposals for other long-term applications of these systems should be forwarded to the European Centre for the Validation of Alternative Methods for consideration. The prevalidation proposal deals with several issues, including: a) species; b) practical prevalidation methodology; c) enzyme inducers; and d) advantages of working with independent expert laboratories. Since it is preferable to include other alternative tests for drug metabolising enzyme induction, when such tests arise, it is recommended that they meet the same level of development as for the collagen gel sandwich long-term hepatocyte system. Those tests which do so should begin the prevalidation and validation process.

Pharmacokinetic evaluation of C-3 modified 1,8-naphthyridine-3-carboxamide derivatives with potent anticancer activity: lead finding

To develop naphthyridine derivatives as anticancer candidates, pharmacokinetic (PK) evaluations of 10 novel derivatives of 1,4-dihydro-4-oxo-1-proparagyl-1,8-naphthyridine-3-carboxamide, with potent anticancer activity were done using in vitro ADME (absorption, distribution, metabolism, excretion) and pharmacokinetic--pharmcodynamic (PK/PD) assays. Only derivatives 5, 6, 9 and 10 showed better metabolic stability, solubility, permeability, partition coefficient and cytochrome P450 (CYP) inhibition values. PK of derivatives 5, 6, 9 and 10 in rat showed comparable PK profile for derivative 5 (C0 = 6.98 µg/mL) and 6 (C0 = 6.61 µg/mL) with no detectable plasma levels for derivatives 9 and 10 at 5.0 mg/kg i.v. dose. PK/PD assay of derivatives 5 and 6 in tumor-bearing mice (TBM) showed comparable PK but tumor plasma index (TPI) of derivative 6 (4.02) was better than derivative 5 (2.50), suggesting better tumor uptake of derivative 6. Derivative 6, as lead compound, showed highest tumor growth inhibition (TGI) value of 33.6% in human ovary cancer xenograft model.

Pharmacovigilance: effects of herbal components on human drugs interactions involving cytochrome P450

Cytochrome P450 (CYP P450) enzymes are a superfamily of mono-oxygenases that are found in all kingdoms of life. The CYP P450 enzymes constitute a large superfamily of haem-thiolate proteins involved in the metabolism of a wide variety of both exogenous and endogenous compounds. The CYP activities have been shown to be involved in numerous interactions especially between drugs and herbal constituents. The majority of serious cases of drug interactions are as a result of the interference of the metabolic clearance of one drug by yet another co-administered drug, food or natural product. Gaining mechanistic knowledge towards such interactions has been accepted as an approach to avoid adverse reactions. The inductions and inhibition of CYP enzymes by natural products in the presence of a prescribed drug has led to adverse effects. Herbal medicines such as St. John's wort (Hypericum perforatum), garlic (Allium sativa), piperine (from Piper sp.), ginseng (Ginseng sp.), gingko (Gingko biloba), soya beans (Glycine max), alfalfa (Medicago sativa) and grape fruit juice show clinical interactions when co-administered with medicines. This review documents the involvement of CYP enzymes in the metabolism of known available drugs and herbal products. We also document the interactions between herbal constituents & CYP enzymes showing potential drug-herb interactions. Data on CYP450 enzymes in activation (i.e. induction or inhibition) with natural constituents is also reviewed.

An automated liquid chromatography-mass spectrometry process to determine metabolic stability half-life and intrinsic clearance of drug candidates by substrate depletion

An automated process is described for the detailed assessment of the in vitro metabolic stability properties of drug candidates in support of pharmaceutical property profiling. Compounds are incubated with liver microsomes using a robotic liquid handler. Aliquots are taken at various time points, and the resulting samples are quantitatively analyzed by liquid chromatography-mass spectrometry utilizing ion trap mass spectrometers to determine the amount of compound remaining. From these data metabolism rates can be calculated. A high degree of automation is achieved through custom software, which is employed for instrument setup, data processing, and results reporting. The assay setup is highly configurable, allowing for any combination of up to six user-selected time points, variable substrate concentration, and microsomes or other biologically active media. The data, based on relative substrate depletion, affords an estimate of metabolic stability through the calculation of half-life (t(1/2)) and intrinsic clearance, which are used to differentiate and rank order drug leads. In general, t(1/2) is the time necessary for the metabolism, following first-order kinetics, of 50% of the initial compound. Intrinsic clearance is the proportionality constant between rate of metabolism of a compound and its concentration at the enzyme site. Described here is the setup of the assay, and data from assay test compounds are presented.

Reaction phenotyping: current industry efforts to identify enzymes responsible for metabolizing drug candidates

Reaction phenotyping studies to identify specific enzymes involved in the metabolism of drug candidates are increasingly important in drug discovery efforts. Experimental approaches used for CYP reaction phenotyping include incubations with cDNA expressed CYP enzyme systems and incubations containing specific CYP enzyme inhibitors. Since both types of experiments present specific advantages as well as known drawbacks, these studies are generally viewed as complementary approaches. Although glucuronidation pathways are also known to present potential drug-drug interaction issues as well as challenges related to their polymorphic expression, reaction phenotyping approaches for glucuronidation are generally limited to cDNA expressed systems due to lack of availability of specific UGT inhibitors. This article presents a limited review of current approaches to reaction phenotyping studies used within the pharmaceutical industry.

Pharmacological evaluation of C-3 modified Betulinic acid derivatives with potent anticancer activity

In vitro and in vivo pharmacological screening of Betulinic acid (BA) and five dihydro-BA derivatives modified at C-3 position [4-nitrobenzyl-oximino (1), 2-4-difluoro-benzoyloxy (2), 2-4-difluoro-benzylidene-amino (3), benzoyl-hydrazono (4), and 4-fluorophenyl-hydrazono (5)], having potent in vitro anti-cancer activity was carried out using ADME, animal PK and tumor studies. We found that BA and the derivatives had poor aqueous solubility (<0.1 microg/ml), low to moderate permeability (log Pe<-5.0) and high plasma protein binding (>70%). Although BA and 5 were metabolized by human liver microsomes, derivatives 1, 2, 3 and 4 possessed good in vitro metabolic stability. Except 3 which inhibited CYP1A2 isoform by more than 50% none of the other compounds inhibited key cytochrome P450 enzyme isoforms (CYP1A2, CYP2C9, CYP2D6 and CYP3A4) at 10 microM. Based on in vitro results one derivative 1 was tested in rodent PK and tumor studies. We found that 1 exhibited favorable pharmacokinetic characteristics of a systemically administered drug and showed better in vivo anti-tumor efficacy as compared to BA in a human colon cancer xenograft model. Our results show that BA derivatives are potential anti-cancer compounds which need to be explored in detail.

Mechanisms of venoocclusive disease resulting from the combination of cyclophosphamide and roxithromycin

BACKGROUND

High doses (>or=500 mg/m) of cyclophosphamide are known to cause venoocclusive disease (VOD). The authors recently observed a patient treated with immunosuppressive cyclophosphamide doses (100 mg/day) and roxithromycin who developed VOD. Because roxithromycin inhibits cytochrome P450 (CYP) 3A4 and P-glycoprotein, the patient may have been exposed to higher cyclophosphamide and/or cyclophosphamide metabolite concentrations.

METHODS

The effect of roxithromycin on the metabolism and toxicity of cyclophosphamide was studied using human hepatic microsomes and a human endothelial cell line.

RESULTS

Cyclophosphamide or roxithromycin at concentrations from 0.05 to 500 micromol/L were not toxic to endothelial cells as assessed by lactate dehydrogenase (LDH) leakage assay. However, the combination of roxithromycin (500 micromol/L) and cyclophosphamide was toxic for all the tested cyclophosphamide concentrations (0.05 to 500 micromol/L) without clear concentration dependence (LDH ratio 38.3 +/- 11.0 [mean +/- SEM] for the combination with cyclophosphamide 0.05 micromol/L and 50.2 +/- 10.2 for the combination with cyclophosphamide 500 micromol/L; P <or= 0.005 for all tested combinations vs. control). Although roxithromycin did not favor the generation of toxic metabolites from cyclophosphamide, it led to cyclophosphamide accumulation due to inhibition of both CYP3A4 and CYP2B6. Although roxithromycin inhibited P-glycoprotein, this was not the mechanism by which cyclophosphamide toxicity was increased because cyclophosphamide in combination with other P-glycoprotein inhibitors was not toxic to endothelial cells. In the presence of roxithromycin (500 micromol/L), cyclophosphamide (500 micromol/L) induced apoptosis in endothelial cells (34.3 +/- 10.4% apoptotic cells [in % of total cells] for the combination of cyclophosphamide and roxithromycin, 0.7 +/- 0.25% for cyclophosphamide alone, 0% for roxithromycin alone; P < 0.0001) most probably by mitochondrial membrane permeability transition and release of cytochrome c.

CONCLUSIONS

The combination cyclophosphamide and roxithromycin, but not the individual compounds, is toxic to endothelial cells by inducing apoptosis. Inhibition of P-glycoprotein and formation of toxic metabolites are unlikely causes.

Strategies to assess the drug interaction potential in translational medicine

Translational medicine is the drug development phase in which preclinical and clinical applied research is conducted to aid dose and disease selection with great financial impact. Thus, during this phase, early discontinuation of a drug that will later fail due to drug interactions is a must for a proper resource allocation. It is not only important to identify a potential interaction, but also to be able to differentiate between detectable interactions and clinically relevant interactions. Due to the scientific advancement, the prediction of drug interactions during translational medicine has shifted from empirical/observational to rational based. These investigations are thus in line with the FDA's Critical Path Initiative and are facilitated by the availability of mature technologies and by current European and US guidelines for both in vitro and in vivo studies. Because drug interactions must be evaluated in a multidisciplinary fashion, even if these studies are contracted externally, pharmaceutical companies should be directly involved in the conduction of such studies to fully exploit their potential and to allow a better and faster interpretation of the results.

Integrating in vitro ADMET data through generic physiologically based pharmacokinetic models

Early estimation of kinetics in man currently relies on extrapolation from experimental data generated in animals. Recent results from the application of a generic physiologically based model, Cloe PK) (Cyprotex), which is parameterised for human and rat physiology, to the estimation of plasma pharmacokinetics, are summarised in this paper. A comparison with predictive methods that involve scaling from in vivo animal data can also be made from recently published data. On average, the divergence of the predicted plasma concentrations from the observed data was 0.47 log units. For the external test set, > 70% of the predicted values of the AUC were within threefold of the observed values. Furthermore, the model was found to match or exceed the performance of three published interspecies scaling methods for estimating clearance, all of which showed a distinct bias towards overprediction. It is concluded that Cloe PK, as a means of integrating readily determined in vitro and/or in silico data, is a powerful, cost-effective tool for estimating exposure and kinetics in drug discovery and risk assessment that should, if widely adopted, lead to major reductions in the need for animal experimentation.

Automated QSPR through Competitive Workflow

This paper describes a novel software architecture, Competitive Workflow, which implements workflow as a distributed and competitive multi-agent system. The implementation of a competitive workflow architecture designed to model important computer-aided molecular design workflows, the Discovery Bus, is described. QSPR modelling results for three example ADME datasets, for solubility, human plasma protein binding and P-glycoprotein substrates using an autonomous QSPR modelling workflow implemented on the Discovery Bus are presented. The autonomous QSPR system allows exhaustive exploration of descriptor and model space, automated model validation and continuous updating as new data and methods are made available. Prediction of properties of novel structures by an ensemble of models is also a feature of the system.

Herbal interactions involving cytochrome p450 enzymes: a mini review

The metabolism of a drug can be altered by another drug or foreign chemical, and such interactions can often be clinically significant. Cytochrome P450 (CYP) enzymes, a superfamily of enzymes found mainly in the liver, are involved in the metabolism of a plethora of xenobiotics and have been shown to be involved in numerous interactions between drugs and food, herbs and other drugs. The observed induction and inhibition of CYP enzymes by natural products in the presence of a prescribed drug has (among other reasons) led to the general acceptance that natural therapies can have adverse effects, contrary to the popular beliefs in countries where there is an active practice of ethnomedicine. Herbal medicines such as St. John's wort, garlic, piperine, ginseng, and gingko, which are freely available over the counter, have given rise to serious clinical interactions when co-administered with prescription medicines. Such adversities have spurred various pre-clinical and in vitro investigations on a series of other herbal remedies, with their clinical relevance remaining to be established. Although the presence of numerous active ingredients in herbal medicines, foods and dietary supplements complicate experimentation, the observable interactions with CYP enzymes warrant systematic studies, so that metabolism-based interactions can be predicted and avoided more readily. This article highlights the involvement of CYP enzymes in metabolism-related drug-herb interactions and the importance of gaining a mechanism-based understanding to avoid potential adverse drug reactions, in addition to outlining other contributory factors, such as pharmacogenetics and recreational habits that may compound this important health issue.

Formation and antitumor activity of PNU-159682, a major metabolite of nemorubicin in human liver microsomes

PURPOSE

Nemorubicin (3'-deamino-3'-[2''(S)-methoxy-4''-morpholinyl]doxorubicin; MMDX) is an investigational drug currently in phase II/III clinical testing in hepatocellular carcinoma. A bioactivation product of MMDX, 3'-deamino-3'',4'-anhydro-[2''(S)-methoxy-3''(R)-oxy-4''-morpholinyl]doxorubicin (PNU-159682), has been recently identified in an incubate of the drug with NADPH-supplemented rat liver microsomes. The aims of this study were to obtain information about MMDX biotransformation to PNU-159682 in humans, and to explore the antitumor activity of PNU-159682.

EXPERIMENTAL DESIGN

Human liver microsomes (HLM) and microsomes from genetically engineered cell lines expressing individual human cytochrome P450s (CYP) were used to study MMDX biotransformation. We also examined the cytotoxicity and antitumor activity of PNU-159682 using a panel of in vitro-cultured human tumor cell lines and tumor-bearing mice, respectively.

RESULTS

HLMs converted MMDX to a major metabolite, whose retention time in liquid chromatography and ion fragmentation in tandem mass spectrometry were identical to those of synthetic PNU-159682. In a bank of HLMs from 10 donors, rates of PNU-159682 formation correlated significantly with three distinct CYP3A-mediated activities. Troleandomycin and ketoconazole, both inhibitors of CYP3A, markedly reduced PNU-159682 formation by HLMs; the reaction was also concentration-dependently inhibited by a monoclonal antibody to CYP3A4/5. Of the 10 cDNA-expressed CYPs examined, only CYP3A4 formed PNU-159682. In addition, PNU-159682 was remarkably more cytotoxic than MMDX and doxorubicin in vitro, and was effective in the two in vivo tumor models tested, i.e., disseminated murine L1210 leukemia and MX-1 human mammary carcinoma xenografts.

CONCLUSIONS

CYP3A4, the major CYP in human liver, converts MMDX to a more cytotoxic metabolite, PNU-159682, which retains antitumor activity in vivo.

Food-drug interactions via human cytochrome P450 3A (CYP3A)

Food-drug interactions have been reported to occur in various systems in the body. The causes of these interactions are mainly divided into pharmacodynamic and pharmacokinetic processes. Among these processes, drug metabolism plays a crucial role in drug interactions. Metabolic food-drug interactions occur when a certain food alters the activity of a drug-metabolizing enzyme, leading to a modulation of the pharmacokinetics of drugs metabolized by the enzyme. A variety of interactions have been documented so far. Foods consisting of complex chemical mixtures, such as fruits, alcoholic beverages, teas, and herbs, possess the ability to inhibit or induce the activity of drug-metabolizing enzymes. According to results obtained thus far, cytochrome P450 3A4 (CYP3A4) appears to be a key enzyme in food-drug interactions. For example, interactions of grapefruit juice with felodipine and cyclosporine, red wine with cyclosporine, and St John's wort with various medicines including cyclosporine, have been demonstrated. The results indicate the requirement of dosage adjustment to maintain drug concentrations within their therapeutic windows. The CYP3A4-related interaction by food components may be related to the high level of expression of CYP3A4 in the small intestine, as well as its broad substrate specificity, as CYP3A4 is responsible for the metabolism of more than 50% of clinical pharmaceuticals. This review article summarizes the findings obtained to date concerning food-drug interactions and their clinical implications. It seems likely that more information regarding such interactions will accumulate in the future, and awareness is necessary for achieving optimal drug therapy.

A novel incubation direct injection LC/MS/MS technique for in vitro drug metabolism screening studies involving the CYP 2D6 and the CYP 3A4 isozymes

A direct injection LC/MS/MS method involving a novel incubation technique was developed for the inhibition screening of CYP 2D6 and CYP 3A4 isoenzymes using dextromethorphan and midazolam as probe substrates. Both assays were performed using an electrospray ionization source in the positive ion mode. Direct injection was possible by using a short C 18, LC column (2 mm x 20 mm) with large particle diameter packing (10 microm). Analytical characteristics of the direct injection technique were studied by examining matrix effects, which showed suppression of the ESI signal between 0.20 and 0.65 min. The retention times for analytes were adjusted to approximately 0.8 min (k'>3), resulting in no matrix effect. Column lifetime was evaluated and determined to be approximately 160 direct injections of the matrix. The precision and accuracy of the control samples for the quantitation of dextromethorphan was between -0.53 and -12.80, and 3.73 and 6.69% respectively. Unlike conventional incubation techniques, incubations were carried out in an autosampler equipped with a heating accessory. This novel incubation method, which involved no stirring of the incubation mixture, estimated the Cl(int in vitro) for dextromethorphan and midazolam in human liver microsomes to be 1.65+/-0.22 ml/(hmg) and 0.861 ml/(min mg) respectively. The autosampler tray maintained uniform temperature and was sensitive to changes in temperature between 33 and 41 degrees C. High-throughput screening was performed using known inhibitors of the CYP 2D6 isozyme, and the system was evaluated for its ability to differentiate between these inhibitors. The strong inhibitor quinidine resulted in a 25.6% increase in t(1/2), the medium potency inhibitor chlorpromazine resulted in an increase of 6.14% and the weak inhibitor primaquine had no significant effect on half-life. This technique involves no sample preparation, demonstrated run times of 2 min per injection and can be fully automated. The method should therefore prove to be a valuable tool in the drug discovery process.

A simultaneous assessment of CYP3A4 metabolism and induction in the DPX-2 cell line

The DPX-2 cell line, a derivative of HepG2 cells, harbors human PXR and a luciferase-linked CYP3A4 promoter. These cells were used in a panel of cell-based assays for a parallel assessment of CYP3A4 induction, metabolism, and inhibition at the cellular level. CYP3A4 induction in the DPX-2 cell line by various agents was monitored in 96-well plates by a luciferase-based transcriptional activation assay. Of the prototypical CYP3A4 inducers examined, all exhibited elevated luciferase activity in DPX-2 cells. CYP3A4 enzyme activity in noninduced and rifampicin-induced DPX-2 cells was also assessed using Vivid fluorogenic substrates. Significantly elevated CYP3A4 activity levels (2.8-fold +/- 0.2-fold above DMSO-treated cells) were found in DPX-2 cells after 48 hours of exposure to rifampicin, but were undetectable in parental HepG2 cells. Rifampicin-induced activity levels were found to be suitable for assessing the inhibitory potential of new chemical entities in downstream CYP3A4 inhibition assays. The elevated CYP3A4 activity was inhibited 85% by 10 microM ketoconazole. In addition, a cytotoxicity assay to correct for possible toxic effects of compounds at the cellular level was applied. The comparative data obtained with a combination of the above assays suggests that the application of several independent in vitro technologies used in DPX-2 cells is the best possible strategy for the assessment of the complex phenomena of CYP3A4 induction and inhibition.

Drug metabolism by cultured human hepatocytes: how far are we from the in vivo reality?

The investigation of metabolism is an important milestone in the course of drug development. Drug metabolism is a determinant of drug pharmacokinetics variability in human beings. Fundamental to this are phenotypic differences, as well as genotypic differences, in the expression of the enzymes involved in drug metabolism. Genotypic variability is easy to identify by means of polymerase chain reaction-based or DNA chip-based methods, whereas phenotypic variability requires direct measurement of enzyme activities in liver, or, indirectly, measurement of the rate of metabolism of a given compound in vivo. There is a great deal of phenotypic variability in human beings, only a minor part being attributable to gene polymorphisms. Thus, enzyme activity measurements in a series of human livers, as well as in vivo studies with human volunteers, show that phenotypic variability is, by far, much greater than genotypic variability. In vitro models are currently used to investigate the hepatic metabolism of new compounds. Cultured human hepatocytes are considered to be the closest model to the human liver. However, the fact that hepatocytes are placed in a microenvironment that differs from that of the cells in the liver raises the question of to what extent drug metabolism variability observed in vitro actually reflects that in the liver in vivo. This issue has been examined by investigating the metabolism of the model compound, aceclofenac (an approved analgesic/anti-inflammatory drug), both in vitro and in vivo. Hepatocytes isolated from programmed liver biopsies were incubated with aceclofenac, and the metabolites formed were investigated by HPLC. The patients were given the drug during the course of clinical recovery, and the metabolites, largely present in urine, were analysed. In vitro and in vivo data from the same individual were compared. There was a good correlation between the in vitro and in vivo relative abundance of oxidised metabolites (4'-OH-aceclofenac + 4'-OH-diclofenac; Spearman's rho = 0.855), and the hydrolysis of aceclofenac (diclofenac + 4'-OH-aceclofenac + 4'-OH-diclofenac; rho = 0.691), while the conjugation of the drug in vitro was somewhat lower than in vivo. Globally, the metabolism of aceclofenac in vitro correlated with the amount of metabolites excreted in urine after 16 hours (rho = 0.95). Overall, although differing among assays, the in vitro/in vivo metabolism data for each patient were surprisingly similar. Thus, the variability observed in vitro appears to reflect genuine phenotypic variability among the donors.

Pharmacokinetic Drug Interactions of Gefitinib with Rifampicin, Itraconazole and Metoprolol

BACKGROUND AND OBJECTIVES

Gefitinib (IRESSA, ZD1839), an epidermal growth factor receptor tyrosine kinase inhibitor, has been approved in several countries for the treatment of advanced non-small-cell lung cancer. Preclinical studies were conducted to determine the cytochrome P450 (CYP) isoenzymes involved in the metabolism of gefitinib and to evaluate the potential of gefitinib to cause drug interactions through inhibition of CYP isoenzymes. Based on these findings, three clinical studies were carried out to investigate pharmacokinetic drug interactions in vivo with gefitinib.

METHODS

In preclinical studies radiolabelled gefitinib was incubated with: (i) hepatic microsomal protein in the presence of selective CYP inhibitors; and (ii) expressed CYP enzymes. Human hepatic microsomal protein was incubated with selective CYP substrates in the presence of gefitinib. Clinical studies were all phase I, open-label, single-centre studies; two had a randomised, two-way crossover design and the third was nonrandomised. The first and second studies investigated the pharmacokinetics of gefitinib in the presence of a potent CYP3A4 inducer (rifampicin [rifampin]) or inhibitor (itraconazole) in healthy male volunteers. The third study investigated the effects that gefitinib had on the pharmacokinetics of metoprolol, a CYP2D6 substrate, in patients with solid tumours.

RESULTS

The results of preclinical studies demonstrated that CYP3A4 is involved in the metabolism of gefitinib and that gefitinib is a weak inhibitor of CYP2D6 activity. In clinical studies when gefitinib was administered in the presence of rifampicin, geometric mean (gmean) maximum concentration and area under the plasma concentration-time curve (AUC) were reduced by 65% and 83%, respectively; these changes were statistically significant. When gefitinib was administered in the presence of itraconazole, gmean AUC increased by 78% and 61% at gefitinib doses of 250 and 500 mg, respectively; these changes also being statistically significant. Coadministration of metoprolol with gefitinib resulted in a 35% increase in the metoprolol area under plasma concentration-time curve from time zero to the time of the last quantifiable concentration; this change was not statistically significant. There was no apparent change in the safety profile of gefitinib as a result of coadministration with other agents.

CONCLUSIONS

Although CYP3A4 inducers may reduce exposure to gefitinib, further work is required to define any resultant effect on the efficacy of gefitinib. Exposure to gefitinib is increased by coadministration with CYP3A4 inhibitors, but since gefitinib is known to have a good tolerability profile, a dosage reduction is not recommended. Gefitinib is unlikely to exert a clinically relevant effect on the pharmacokinetics of drugs that are dependent on CYP2D6-mediated metabolism for their clearance, but the potential to increase plasma concentrations should be considered if gefitinib is coadministered with CYP2D6 substrates that have a narrow therapeutic index or are individually dose titrated.

In vitro metabolism of zolmitriptan in rat cytochromes induced with β-naphthoflavone and the interaction between six drugs and zolmitriptan

Zolmitriptan is a novel and highly selective 5-HT(1B/1D) receptor agonist used as an acute oral treatment for migraine. There are few reports regarding the in vitro metabolism of zolmitriptan. Previous studies indicated zolmitriptan was metabolized via CYP1A2 in human hepatic microsomes. In order to study the enzyme kinetics and drug interaction, the metabolism of zolmitriptan and possible drug-drug interactions were investigated in rat hepatic microsomes induced with different inducers. An active metabolite, N-demethylzolmitriptan, was detected and another minor, inactive metabolite that was reported in human hepatic microsomes was not detected in this study. The enzyme kinetics for the formation of N-demethylzolmitriptan from zolmitriptan in rat liver microsomes pretreated with BNF were 96+/-22 microM (K(m)), 11+/-3 pmol min(-1)mg protein(-1) (V(max)), and 0.12+/-0.02 microl min(-1)mg protein(-1) (CL(int)). Fluvoxamine and diphenytriazol inhibited zolmitriptan N-demethylase activity catalyzed by CYP1A2 (K(i)=3.8+/-0.3 and 3.2+/-0.1 microM, respectively). Diazepam and propranolol elicited a slight inhibitory effect on the metabolism of zolmitriptan (K(i)=70+/-11 and 90+/-18 microM, respectively). Cimetidine and moclobemide produced no significant effect on the metabolism of zolmitriptan. Fluvoxamine yielded a k(inactivation) value of 0.16 min(-1), and K(i) of 57 microM. The results suggest that rat hepatic microsomes are a reasonable model to study the metabolism of zolmitriptan, although there is a difference in the amount of minor, inactive metabolites between human hepatic microsomes and rat liver microsomes. The results of the inhibition experiments provided information for the interactions between zolmitriptan and drugs co-administrated in clinic, and it is helpful to explain the drug-drug interactions of clinical relevance on enzyme level. This study aso demonstrated that fluvoxamine may be a mechanism-based inactivator of CYP1A2.

Kiiv, an in vivo parameter for predicting the magnitude of a drug interaction arising from competitive enzyme inhibition

The goal of the study was to test the assumption that a competitive inhibition constant determined in vivo, Kiiv, like its corresponding in vitro counterpart, Ki, is independent of inhibitor concentration. Inhibition of the CYP2C9-dependent formation of (S)-7-hydroxy-warfarin from (S)-warfarin was measured in seven healthy subjects at three different doses of fluconazole. Prothrombin time measurements showed increasing anticoagulant activity with increasing fluconazole dose. The pharmacokinetic parameters calculated from the (S)- and (R)-warfarin plasma levels were consistent with previous studies. Fluconazole reduced the clearance of (S)-warfarin to a greater extent than that of (R)-warfarin. The decrease in clearance of both warfarin enantiomers was fluconazole dose-dependent. The formation of (S)-7-hydroxy-warfarin was inhibited by 31, 55, and 77% at the 100, 200, and 300 mg daily doses of fluconazole, respectively. Kiiv, values calculated from these data based on plasma fluconazole levels at each dose and data from earlier work at 400-mg daily doses of fluconazole were 30.7 +/- 23.7, 19.6 +/- 3.8, 17.9 +/- 7.5, and 19.8 +/- 3.5 microM, respectively. These results confirm the hypothesis that Kiiv is independent of inhibitor concentration.

Interactions of herbs with cytochrome P450

A resurgence in the use of medical herbs in the Western world, and the co-use of modern and traditional therapies is becoming more common. Thus there is the potential for both pharmacokinetic and pharmacodynamic herb-drug interactions. For example, systems such as the cytochrome P450 (CYP) may be particularly vulnerable to modulation by the multiple active constituents of herbs, as it is well known that the CYPs are subject to induction and inhibition by exposure to a wide variety of xenobiotics. Using in vitro, in silico, and in vivo approaches, many herbs and natural compounds isolated from herbs have been identified as substrates, inhibitors, and/or inducers of various CYP enzymes. For example, St. John's wort is a potent inducer of CYP3A4, which is mediated by activating the orphan pregnane X receptor. It also contains ingredients that inhibit CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. Many other common medicinal herbs also exhibited inducing or inhibiting effects on the CYP system, with the latter being competitive, noncompetitive, or mechanism-based. It appears that the regulation of CYPs by herbal products complex, depending on the herb type, their administration dose and route, the target organ and species. Due to the difficulties in identifying the active constituents responsible for the modulation of CYP enzymes, prediction of herb-drug metabolic interactions is difficult. However, herb-CYP interactions may have important clinical and toxicological consequences. For example, induction of CYP3A4 by St. John's wort may partly provide an explanation for the enhanced plasma clearance of a number of drugs, such as cyclosporine and innadivir, which are known substrates of CYP3A4, although other mechanisms including modulation of gastric absorption and drug transporters cannot be ruled out. In contrast, many organosulfur compounds, such as diallyl sulfide from garlic, are potent inhibitors of CYP2E1; this may provide an explanation for garlic's chemoproventive effects, as many mutagens require activation by CYP2E1. Therefore, known or potential herb-CYP interactions exist, and further studies on their clinical and toxicological roles are warranted. Given that increasing numbers of people are exposed to a number of herbal preparations that contain many constituents with potential of CYP modulation, high-throughput screening assays should be developed to explore herb-CYP interactions.

Simultaneous quantitative analysis of three drugs by high-performance liquid chromatography/electrospray ionization mass spectrometry and its application to cassette in vitro metabolic stability studies

A sensitive and selective liquid chromatography/electrospray mass spectrometry (LC/ESI-MS) method has been developed for the simultaneous quantitative determination of three new chemical entities (NCEs), of the class of aryloxy-substituted aryl piperazines, in rat liver S9 fraction. S9 fraction samples (0.1 mL) were simply extracted with 2% isopropanol in diethyl ether and the extracts analyzed by HPLC with the detection of the analytes in the selective ion recording (SIR) mode. The determination of the analytes was accurate and reproducible, with a limit of quantification of 50 ng/mL for all the analytes in rat liver S9 fraction. The standard calibration curve for the analytes was linear over the concentration range 50-4000 ng/mL. Analysis accuracy and precision over the concentration range were lower than +/-15%. This method offered significant increase in the analytical throughput, which is illustrated by the 'N-in-One' study of metabolic stability of the compounds in rat liver S9 fractions. The quantitative results from the 'N-in-One' procedure correlated well with those obtained from conventional discrete analyses. In addition, the samples were reanalyzed to allow for detection of the metabolites formed during the same incubation. The metabolites were first characterized by nominal mass measurement of the corresponding protonated molecules. Subsequent tandem mass spectrometry allowed confirmation of the detected metabolites.

Evaluation of human hepatocyte incubation as a new tool for metabolism study of androstenedione and norandrostenedione in a doping control perspective

Human hepatocyte incubations were used to study the metabolism of precursors of testosterone and nortestosterone and to evaluate qualitatively the correlation between in vitro and published in vivo urinary metabolic profiles. Both phase I and phase II biotransformations were observed in vitro: oxidoreduction at C-3 and C-17, reduction at C-4,5, hydroxylation at C-6 beta and C-16, glucuronidation and sulfation. All major metabolites detected in urine following oral administration of androstenedione and norandrostenedione were present in human hepatocyte incubations. The good correlation between in vitro and in vivo metabolic profiles indicates that hepatocyte incubations can be a useful tool to identify and characterize metabolites that could be potential urinary markers for detection of steroid abuse by athletes.

Predicting pharmacokinetics and drug interactions in patients from in vitro and in vivo models: the experience with 5,6-dimethylxanthenone-4-acetic acid (DMXAA), an anti-cancer drug eliminated mainly by conjugation

The novel anti-tumor agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) was developed in the Auckland Cancer Society Research Center. Its pharmacokinetic properties have been investigated using both in vitro and in vivo models, and the resulting data extrapolated to patients. The metabolism of DMXAA has been extensively studied mainly using hepatic microsomes, which indicated that UGT1A9 and UGT2B7-catalyzed glucuronidation on its acetic acid side chain and to a lesser extent CYP1A2-catalyzed hydroxylation of the 6-methyl group are the major metabolic pathways, resulting in DMXAA acyl glucuronide (DMXAA-G) and 6-hydroxymethyl-5-methylxanthenone-4-acetic acid. The predominant metabolite in human urine (up to 60% of total dose) was identified as DMXAA-G, which was chemically reactive, undergoing hydrolysis, intramolecular rearrangement, and covalent binding to plasma proteins. In vivo formation of DMXAA-protein adducts were also observed in cancer patients receiving DMXAA treatment. The comparison of the in vitro human hepatic microsomal metabolism and inhibition of DMXA by UGT and/or CYP substrates with animal species indicated species differences. Renal microsomes from all animal species examined had glucuronidation activity for DMXAA, but lower than the liver. In vitro-in vivo extrapolations based on human microsomal data indicated a 7-fold underestimation of plasma clearance in patients. In contrast, allometric scaling using in vivo data from the mouse, rat, and rabbit predicted a plasma clearance of 3.5 mL/min/kg, similar to that observed in patients (3.7 mL/min/kg). Based on in vitro metabolic inhibition studies, it appears possible to predict the effects on the plasma kinetic profile of DMXAA of drugs such as diclofenac, which are mainly metabolized by UGT2B7. However, it did not appear possible to predict the effect of thalidomide on the pharmacokinetics of DMXAA in patients based on in vitro inhibition and animal studies. These data indicate that preclincial pharmacokinetic studies using both in vitro and in vivo models play an important but different role in predicting pharmacokinetics and drug interactions in patients.

Flavin-containing monooxygenase activity in hepatocytes and microsomes: in vitro characterization and in vivo scaling of benzydamine clearance

Liver microsomes, and more recently cryopreserved hepatocytes, are commonly used in the in vitro characterization of the metabolism of new xenobiotics. The flavin-containing monooxygenases (FMO) are a major non p450 oxidase present in liver microsomes and hepatocytes. Since FMO is known to be thermally labile, and this enzyme may be involved in the metabolic clearance of some drugs, we sought to more completely characterize the metabolic competency of this enzyme in cryopreserved hepatocytes and in liver microsomes preincubated under various conditions using benzydamine as an in vitro and in vivo probe. The metabolism of benzydamine to its major metabolite, the N-oxide, is mediated by FMO3 in humans. We found that the in vitro microsomal t(1/2) was 70% longer when incubations were prewarmed at 37 degrees C in the absence of NADPH compared with prewarming in the presence of an NADPH-regenerating system, and N-oxide formation was inhibited >99%. Interestingly, the in vivo clearance predicted from these incubations and from human hepatocytes overpredicted the observed clearance of benzydamine in humans (>10.5 versus 2.4 ml/min/kg). In contrast, rat hepatocytes successfully predicted rat in vivo benzydamine clearance to within approximately 30% (>68 versus 48 ml/min/kg). Benzydamine N-oxidation in liver microsomes from all common preclinical species demonstrated heat sensitivity. This information should be considered when extrapolating metabolism data of xenobiotics from these in vitro systems.

Characterization of quinidine 3-hydroxylation as a probe for the CYP 3A enzyme using a novel capillary electrophoresis technique

Capillary electrophoresis (CE) with a direct injection technique was used to characterize the formation of (3S)-3-hydroxyquinidine (3-OHQ) as a probe for the CYP 3A isoenzymes in rat liver microsomes. Detection was performed either in the absorbance mode or by employing laser-induced fluorescence (LIF) detection. Michaelis-Menten parameters (mean values+/-S.D.) K(m) and V(max) for the formation of 3-OHQ from the probe drug quinidine sulfate (QS) in rat liver microsomes were 37+/-4.6 micro g/ml (47.1+/-5.9 micro M) and 321+/-4 ng/mg/h (942+/-11.7 pmol/mg/h), respectively. Inhibition studies were performed to evaluate the specificity of 3-OHQ as a probe for the CYP 3A enzyme. Ketoconazole and fluconazole were found to be inhibitors of 3-OHQ formation and exhibited K(i) values of 0.19 and 20.1 micro M, respectively. Inhibition with the weak inhibitor, erythromycin could only be estimated using LIF detection due to lack of sensitivity in the absorbance mode. The formation of 3-OHQ in rat liver microsomes can be used as a model for the screening of the CYP 3A enzyme. Direct injection, ensures faster analysis time due to the lack of sample preparation and the low volume capabilities of the technique makes it attractive for the screening of a large number of compounds.

5,6-dimethylxanthenone-4-acetic acid (DMXAA): a new biological response modifier for cancer therapy

The investigational anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid (DMXAA) was developed by the Auckland Cancer Society Research Centre (ACSRC). It has recently completed Phase I trials in New Zealand and UK under the direction of the Cancer Research Campaign's Phase I/II Clinical Trials Committee. As a biological response modifier, pharmacological and toxicological properties of DMXAA are remarkably different from most conventional chemotherapeutic agents. Induction of cytokines (particularly tumour necrosis factor (TNF-alpha), serotonin and nitric oxide (NO)), anti-vascular and anti-angiogenic effects are considered to be major mechanisms of action based on in vitro and animal studies. In cancer patients of Phase I study, DMXAA also exhibited various biological effects, including induction of TNF-alpha, serotonin and NO, which are consistent with those effects observed in in vitro and animal studies. Preclinical studies indicated that DMXAA had more potent anti-tumour activity compared to flavone-8-acetic acid (FAA). In contrast to FAA that did not show anti-tumour activity in cancer patients, DMXAA (22 mg/kg by intravenous infusion over 20 min) resulted in partial response in one patient with metastatic cervical squamous carcinoma in a Phase I study where 65 cancer patients were enrolled in New Zealand. The maximum tolerated dose (MTD) in mouse, rabbit, rat and human was 30, 99, 330, and 99 mg/kg respectively. The dose-limiting toxicity of DMXAA in cancer patients included acute reversible tremor, cognitive impairment, visual disturbance, dyspnoea and anxiety. The plasma protein binding and distribution into blood cells of DMXAA are dependent on species and drug concentration. DMXAA is extensively metabolised, mainly by glucuronidation of its acetic acid side chain and 6-methylhydroxylation, giving rise to DMXAA acyl glucuronide (DMXAA-G), and 6-hydroxymethyl-5-methylxanthenone-4-acetic acid (6-OH-MXAA), which are excreted into bile and urine. DMXAA-G has been shown to be chemically reactive, undergoing hydrolysis, intramolecular migration and covalent binding. Studies have indicated that DMXAA glucuronidation is catalysed by uridine diphosphate glucuronosyltransferases (UGT1A9 and UGT2B7), and 6-methylhydroxylation by cytochrome P450 (CYP1A2). Non-linear plasma pharmacokinetics of DMXAA has been observed in animals and patients, presumably due to saturation of the elimination process and plasma protein binding. Species differences in DMXAA plasma pharmacokinetics have been observed, with the rabbit having the greatest plasma clearance, followed by the human, rat and mouse. In vivo disposition studies in these species did not provide an explanation for the differences in MTD. Co-administration of DMXAA with other drugs has been shown to result in enhanced anti-tumour activity and alterations in pharmacokinetics, as reported for the combination of DMXAA with melphalan, thalidomide, cyproheptadine, and the bioreductive agent tirapazamine, in mouse models. Species-dependent DMXAA-thalidomide pharmacokinetic interactions have been observed. Co-administration of thalidomide significantly increased the plasma area of the plasma concentration-time curve (AUC) of DMXAA in mice, but had no effect on DMXAA's pharmacokinetics in the rat. It appears that the pharmacological and toxicological properties of DMXAA as a new biological response modifier are unlikely to be predicted based on preclinical studies. Similar to many biological response modifiers, DMXAA alone did not show striking anti-tumour activity in patients. However, preclinical studies of DMXAA-drug combinations indicate that DMXAA may have a potential role in cancer treatment when co-administered with other drugs. Further studies are required to explore the molecular targets of DMXAA and mechanisms for the interactions with other drugs co-administered during combination treatment, which may allow for the optimisation of DMXAA-based chemotherapy.

Contribution of CYP3A4, CYP2B6, and CYP2C9 isoforms to N-demethylation of ketamine in human liver microsomes

Ketamine is a widely used drug for its anesthetic and analgesic properties; it is also considered as a drug of abuse, as many cases of ketamine illegal consumption were reported. Ketamine is N-demethylated by liver microsomal cytochrome P450 into norketamine. The identification of the enzymes responsible for ketamine metabolism is of great importance in clinical practice. In the present study, we investigated the metabolism of ketamine in human liver microsomes at clinically relevant concentrations. Liver to plasma concentration ratio of ketamine was taken into consideration. Pooled human liver microsomes and human lymphoblast-expressed P450 isoforms were used. N-demethylation of ketamine was correlated with nifedipine oxidase activity (CYP3A4-specific marker reaction), and it was also correlated with S-mephenytoin N-demethylase activity (CYP2B6-specific marker reaction). Orphenadrine, a specific inhibitor to CYP2B6, and ketoconazole, a specific inhibitor to CYP3A4, inhibited the N-demethylation of ketamine in human liver microsomes. In human lymphoblast-expressed P450, the activities of CYP2B6 were higher than those of CYP3A4 and CYP2C9 at three concentrations of ketamine, 0.005, 0.05, and 0.5 mM. When these results were extrapolated using the average relative content of these P450 isoforms in human liver, CYP3A4 was the major enzyme involved in ketamine N-demethylation. The present study demonstrates that CYP3A4 is the principal enzyme responsible for ketamine N-demethylation in human liver microsomes and that CYP2B6 and CYP2C9 have a minor contribution to ketamine N-demethylation at therapeutic concentrations of the drug.

High-throughput screening of potential inhibitors for the metabolism of the investigational anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid

By screening potential inhibitors of drug metabolism using the in vitro models, potential drug-drug interactions in vivo may be predicted with the use of appropriate pharmacokinetic principles. This study aimed to develop a rapid screening system using human liver microsomes to efficiently identify the potential inhibitors of DMXAA metabolism. Initial IC50 was estimated by using a two-point method, and then Ki values were determined if required and compared with those initial IC50 values. More than 100 compounds including known substrates and inhibitors of human uridine diphosphate glucuronosyltransferases (UGTs) and cytochrome P450 (CYP), anti-cancer drugs and xanthenone analogues were screened for their inhibitory effect on DMXAA glucuronidation and 6-methylhydroxylation in human liver microsomes. Both metabolites of DMXAA, DMXAA acyl glucuronide (DMXAA-G) and 6-hydroxymethyl-5-methylxanthenone-4-acetic acid (6-OH-MXAA), formed in human liver microsomes were quantitated by validated HPLC methods. The results indicated that there was a significant relationship (r2 = 0.966, P < 0.001) between the two-point IC50 values and the apparent Ki values for 20 compounds showing significant inhibitory effects on DMXAA metabolism, suggesting the usefulness of the two-point determination for the initial screening of compounds. This study has been completed using a strategy for rapid HPLC analysis and thus provided early access to detailed information for potential inhibitors of DMXAA metabolism and allows for further DMXAA-drug interaction studies.

Cell suspensions, cell cultures, and tissue slices--important metabolic in vitro systems

In vitro subcellular and cellular systems have important and irreplaceable roles in the metabolic investigations that precede the development of new potential drugs. Of these model systems, tissue slices are probably the nearest to in vivo conditions. From the experimental and complexity points of view, perfused organs lie midway between tissue slices and whole organism. Preparation and working with liver slices is quick and easy, and, excess material can be cryopreserved and stored untill the next experiment. Slices can be prepared from a wide variety of organs and it is possible to co-incubate them. Another important feature is the possibility of interspecies comparison of slices. Different experiments can be run both in the short-term as well as long-term incubations. Each in vitro system has an important place for example, in the development of new medicaments. It is therefore important to compare and supplement experimental results from different in vitro systems when extrapolating to in vivo situations is done.

Integration of mass spectrometry into early-phase discovery and development of central nervous system agents

The early-phase discovery and development of useful central nervous system (CNS) agents present ample opportunities to exploit mass spectrometry and provide detailed compound/mixture characterization, or to make the process faster and/or more economic. Neuropeptide FF antagonists and centrally active thyrotropin-releasing hormone analogues were used as specific examples in this work. We evaluated the characterization of focused libraries of peptide derivatives by electrospray ionization, tandem mass spectrometry and liquid chromatography/tandem mass spectrometry on a quadrupole ion trap and nanoelectrospray on a Fourier transform ion cyclotron resonance mass spectrometer. Immobilized artificial-membrane chromatography was employed as a model to predict/rank new agents against lead compounds for their potential to reach the central nervous system in pharmacologically significant amounts. Measuring brain concentrations in rodents after the intravenous administration of test compounds was used as an in vivo approach, and we took advantage of microdialysis sampling that furnished samples without interfering tissue matrix and afforded the estimation of extracellular concentrations in a localized part of the brain. Overall, making atmospheric-pressure ionization mass spectrometry an integral part of the process has played a major role in increasing throughput, selectivity, specificity and detection sensitivity and thereby providing useful information about the extent or mechanism of transport and metabolic activation/inactivation in early-phase discovery and development of CNS agents.

Human cytochrome P450 2A6 is the major enzyme involved in the metabolism of three alkoxyethers used as oxyfuels

Methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), and t-amyl methyl ether (TAME) are three alkoxyethers added to gasoline to improve combustion and thereby to reduce the level of carbon monoxide and aromatic hydrocarbons in automobile exhaust. Oxidative demethylation of MTBE and TAME and deethylation of ETBE by CYP enzymes results in the formation of tertiary alcohols and aldehydes, both potentially toxic. The metabolism of these three alkoxyethers was studied in a panel of 12 human liver microsomes. The relatively low apparent Km(1) was 0.25+/-0.17 (mean+/-SD), 0.11+/-0.08 and 0.10+/-0.07 mM and the high apparent Km(2) was 2.9+/-1.8, 5.0+/-2.7 and 1.7+/-1.0 mM for MTBE, ETBE and TAME, respectively. Kinetic data, correlation studies, chemical inhibition and metabolism by heterologously expressed human CYPs support the assertion that the major enzyme involved in MTBE, ETBE and TAME metabolisms is CYP2A6, with a minor contribution of CYP3A4 at low substrate concentration.

A direct injection capillary electrophoretic technique for miniaturized high-throughput metabolic screening of the CYP 3A4 enzyme using quinidine as a probe

A capillary electrophoresis (CE) method has been developed for the determination of quinidine sulfate (QS) and (3S)-3-hydroxyquinidine (3-OHQ) by direct injection of microsomal incubation mixtures. 3-OHQ is the CYP 3A4 metabolite of QS and hence useful for metabolism screening studies. The method was validated analytically and tested for its effectiveness as a metabolic inhibition model. A linear calibration was found to provide the best fit for the standard curve with an r of 0.9966 and all residuals less than 12%. The percent relative standard deviations (RSDs) of the two controls, 2 and 8 microg/ml were 5.27 and 2.90% and the percent difference from normal (% DFN) were -12.58 and -0.31% respectively. The limit of quantitation (LOQ) in the incubation matrix was 0.5 microg/ml. 3-OHQ formation complied with Michaelis-Menten kinetics and the mean values+/-S.D. of Km and Vmax were 36.98+/-4.62 microg/ml and 321.39+/-3.88 ng/mg/h respectively. Preliminary inhibition studies suggest that the method has adequate sensitivity to screen for high and medium inhibitors of the CYP 3A4 isozyme. The lack of sample preparation coupled with the small sample size capability of CE would enable the direct injection technique to aid in miniaturized high-throughput screening.

Carbamazepine: a 'blind' assessment of CVP-associated metabolism and interactions in human liver-derived in vitro systems

1. The ability of various in vitro systems for CYP enzymes (computer modelling, human liver microsomes, precision-cut liver slices, hepatocytes in culture, recombinant enzymes) to predict various aspects of in vivo metabolism and kinetics of carbamazepine (CBZ) was investigated. 2. The study was part of the EUROCYP project that aimed to evaluate relevant human in vitro systems to study drug metabolism. 3. CBZ was given to the participating laboratories without disclosing its chemical nature. 4. The most important enzyme (CYP3A4) and metabolic route (10,11-epoxidation) were predicted by all the systems studied. 5. Minor enzymes and routes were predicted to a different extent by various systems. 6. Prediction of a clearance class, i.e. slow clearance, was correctly predicted by microsomes, slices, hepatocytes and recombinant enzymes (CYP3A4). 7. The 10,11-epoxidation of CBZ by the recombinant CYP3A4 was enhanced by the addition of exogenous cytochrome-b5, leading to a considerable over-prediction. 8. Induction potency of CBZ was predicted in cultured hepatocytes in which 7-ethoxycoumarin O-deethylase was used as an index activity. 9. It seems that for a principally CYP-metabolized substance such as CBZ, all liver-derived systems provide useful information for prediction of metabolic routes, rates and interactions.

Use of in vitro drug metabolism data to evaluate metabolic drug-drug interactions in man: the need for quantitative databases

It has become widely accepted that metabolic drug-drug interactions can be forecast using in vitro cytochrome P450 (CYP) data. For any CYP form-inhibitor pair, the magnitude of the interaction will depend on the potency of the inhibitor (inhibition constant, Ki) the concentration of the inhibitor available for inhibition ([I]), the fraction of the substrate dose metabolized by CYP (fm), and the fraction of the CYP-dependent metabolism catalyzed by the inhibited CYP form (e.g., fm,CYP3A4). While progress is being made toward our understanding of the factors necessary for predictions of [I]/Ki in vivo, it is evident that there is a need for quantitative databases that contain in vitro (e.g., Ki, fm,CYP3A4) and in vivo pharmacokinetic/absorption-distribution-metabolism-excretion (PK/ADME) data (e.g., fm) for a large number of marketed drugs. Ultimately, such databases would allow one to integrate all of the data necessary for the prediction of drug-drug interactions and permit the rational evaluation of new drug entities.

High-performance liquid chromatography determination of N- and O-demethylase activities of chemicals in human liver microsomes: application of postcolumn fluorescence derivatization using Nash reagent

Formaldehyde is liberated in the process of cytochrome P450 (CYP) mediated demethylation of a wide variety of compounds containing the CH(3)N or CH(3)O functionality. A highly sensitive method using a high-performance liquid chromatography (HPLC) system with postcolumn derivatization was developed to measure the liberated formaldehyde as N- and O-demethylase activity of drugs in human liver microsomes. Following the chromatographic separation of formaldehyde on a C18 column, the formaldehyde was reacted with the Nash reagent in the postcolumn reactor at 100 degrees C and detected by the fluorescence method. The results showed that the present method has excellent precision and accuracy. The intra- and interassay variances of this method were less than 10%. The newly developed HPLC method was found to be about 80-fold more sensitive than the colorimetric method in detection of formaldehyde. The N-demethylase activity of sertraline in rat liver microsomes determined by the present method did not differ from those detected by previous methods quantifying produced desmethyl metabolite. The present method has been successfully applied to determine the N-demethylase activities of several drugs, including aminopyrine, erythromycin, fluoxetine, S-mephenytoin, and sertraline, in human liver microsomes. This assay should be useful for generic analysis of N- and O-demethylase activities of xenobiotic and endobiotic chemicals by CYP enzymes.

Expression and induction of CYP1A1/1A2, CYP2A6 and CYP3A4 in primary cultures of human hepatocytes: a 10-year follow-up

1. The aims were to refine experimental conditions (using 76 human hepatocyte preparations) in terms of the selection of enzyme inducers and their optimal concentration, the treatment duration with inducers and the choice of specific cytochrome P450 isoform(s) probes to optimize the use of primary hepatocytes for predicting the potential induction by new chemical entities of cytochrome P450 isoforms in vivo in man. 2. In the absence of any inducer, basal cytochrome P450 isoform(s)-mediated activities decreased to 20% of their initial activity (end of the seeding period) by 72-96 h. In contrast, UGT-dependent enzyme activities remained at a constant level (+/- 20%) up to the fifth day of culture. 3. Beta-naphthoflavone, at an optimal concentration of 50 microM and after a 3-day treatment, specifically and potently induced 7-ethoxyresorufin (10.4 +/- 10.4-fold, n = 74) and phenacetin (6.6 +/- 6.4-fold, n = 60) O-deethylation processes, markers for CYP1A1 and CYP1A2 isoforms respectively. Only a 2-fold increase was noted following treatment with 2 mM phenobarbitone, whereas dexamethasone and rifampicin had no effect at all. 4. A 3-day treatment of human hepatocytes with 50 microM dexamethasone was associated with a major induction of both coumarin 7-hydroxylation (9.4 +/- 11.4-fold, n = 49) and nifedipine dehydrogenation (4.7 +/- 3.8-fold, n = 61), markers for CYP2A6 and CYP3A4 respectively. Phenobarbitone, however, exhibited a broad but moderate inducing effect on 7-ethoxyresorufin (2.2 +/- 1.5-fold, n = 55) and phenacetin (1.7 +/- 0.9-fold, n = 54) O-deethylation, coumarin 7-hydroxylation (3.9 +/- 9.2-fold, n = 50) and nifedipine dehydrogenation (2.1 +/- 2.0-fold, n = 47). 5. Km obtained for the different cytochrome P450 isoform substrates in untreated hepatocytes were in the same range of magnitude that those determined on human hepatic microsomal fractions. Enzyme induction processes were characterized by a large increase in apparent Vmax whereas apparent Km were not affected. 6. These studies demonstrate that human hepatocytes in primary culture can respond specifically and quantitatively to model inducers. This in vitro system offers a useful approach to study the regulation of human hepatic biotransformation activities and should facilitate the demand for a reproducible method for addressing cytochrome P450 induction.

CYP2A6: a human coumarin 7-hydroxylase

Coumarin 7-hydroxylation is catalysed by a high-affinity CYP2A6 enzyme in human liver microsomes. CYP2A6 is the only enzyme catalysing this reaction and consequently the formation of 7-hydroxycoumarin can be used as 'an in vitro and in vivo probe' for CYP2A6. CYP2A6 is a major contributor to the oxidative metabolism of nicotine and cotinine, and it also contributes, to a larger or smaller extent, to the metabolism of a few pharmaceuticals (e.g. fadrozole), nitrosamines, other carcinogens (e.g. aflatoxin B1) and a number of coumarin-type alkaloids. CYP2A6 may be inducible by antiepileptic drugs and it is decreased in alcohol-induced severe liver cirrhosis. Several mutated or deleted CYP2A6 alleles have been characterized. Although CYP2A6 represent up to 15% of human microsomes P450 proteins, it is still one of the less well characterised cytochrome P450 enzymes.

Relative contribution of cytochromes P-450 and flavin-containing monoxygenases to the metabolism of albendazole by human liver microsomes

AIMS

Albendazole (ABZ; methyl 5-propylthio-1H-benzimidazol-2-yl carbamate) is a broad spectrum anthelmintic whose activity resides both in the parent compound and its sulphoxide metabolite (ABS). There are numerous reports of ABZ metabolism in animals but relatively few in humans. We have investigated the sulphoxidation of ABZ in human liver microsomes and recombinant systems.

METHODS

The specific enzymes involved in the sulphoxidation of ABZ were determined by a combination of approaches; inhibition with an antiserum directed against cytochrome P450 reductase, the effect of selective chemical inhibitors on ABZ sulphoxidation in human liver microsomes, the capability of expressed CYP and FMO to mediate the formation of ABS, regression analysis of the rate of metabolism of ABZ to ABS in human liver microsomes against selective P450 substrates and regression analysis of the rate of ABS sulphoxidation against CYP expression measured by Western blotting.

RESULTS

Comparison of Vmax values obtained following heat inactivation (3min at 45 degrees C) of flavin monoxygenases (FMO), chemical inhibition of FMO with methimazole and addition of an antiserum directed against cytochrome P450 reductase indicate that FMO and CYP contribute approximately 30% and 70%, respectively, to ABS production in vitro. Comparison of CLint values suggests CYP is a major contributor in vivo. A significant reduction in ABZ sulphoxidation (n = 3) was seen with ketoconazole (CYP3 A4; 32-37%), ritonavir (CYP3 A4: 34-42%), methimazole (FMO: 28-49%) and thioacetamide (FMO; 32-35%). Additive inhibition with ketoconazole and methimazole was 69 +/- 8% (n = 3). ABS production in heat - treated microsomes (3 min at 45 degrees C) correlated significantly with testosterone 6beta-hydroxylation (CYP3A4; P < 0.05) and band intensities on Western blots probed with an antibody selective for 3A4 (P < 0.05). Recombinant human CYP3 A4, CYP1A2 and FMO3 produced ABS in greater quantities than control microsomes, with those expressing CYP3A4 producing threefold more ABS than those expressing CYP1A2. Kinetic studies showed the Km values obtained with both CYP3A4 and FMO3 were similar.

CONCLUSIONS

We conclude that the production of ABS in human liver is mediated via both FMO and CYP, principally CYP3A4, with the CYP component being the major contributor.

Identification of the major human liver cytochrome P450 isoform(s) responsible for the formation of the primary metabolites of ziprasidone and prediction of possible drug interactions

AIMS

To identify the cytochrome P450 (CYP) isoform(s) responsible for the formation of the primary metabolite of ziprasidone (ziprasidone sulphoxide), to determine the kinetics of its formation and to predict possible drug interactions by investigating CYP isoform inhibition in an in vitro study.

METHODS

In vitro metabolism of [14C]-ziprasidone was studied using human liver microsomes. The metabolites were identified using mass spectrometry. The kinetics of metabolite formation were determined using [14C]-ziprasidone (10-200 microM) over 5 min, and Km and Vmax were estimated from Lineweaver-Burk plots. IC50 values for the inhibition of specific probe substrates for CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4, by ziprasidone, risperidone and 9-hydroxyrisperidone were also determined using human liver microsomes from three subjects. Mean Ki values were calculated.

RESULTS

Three CYP-mediated metabolites - ziprasidone sulphoxide, ziprasidone sulphone and oxindole acetic acid - were identified. The apparent Km and Vmax values for the formation of the major metabolite, ziprasidone sulphoxide (measured as the sum of sulphoxide and sulphone) were 235 microM and 1.14 nmol mg(-1) protein min(-1), respectively. Isoform-selective inhibitors and recombinant enzymes indicated that CYP3A4 is responsible for the formation of ziprasidone metabolites. Ziprasidone was not a substrate for the other isoforms studied. Similar in vitro inhibition of CYP2D6 (Ki 6.9-16 microM) and CYP3A4 (Ki 64-80 microM) was obtained with ziprasidone, risperidone and 9-hydroxyrisperidone. The in vivo free drug concentrations associated with clinically effective doses of ziprasidone are at least 1500-fold lower than the mean Ki for either CYP2D6 inhibition or CYP3A4 inhibition.

CONCLUSIONS

Ziprasidone is predominantly metabolized by CYP3A4 in human liver microsomes and is not expected to mediate drug interactions with coadministered CYP substrates, at clinically effective doses.

Cytochrome P450 enzyme activity and protein expression in primary porcine enterocyte and hepatocyte cultures

1. A method for the isolation and cultivation of porcine hepatocytes and porcine duodenal enterocytes for the investigation of drug oxidation reactions has been established. 2. Hepatocytes as well as enterocytes metabolized ethoxyresorufin (EROD) and ethoxycoumarin (ECOD) effectively, the rate being 31+/-17 pmol/h x dish (EROD) and 9530+/-4062 pmol/h x dish (ECOD) in the case of hepatocytes, and 9+/-4 pmol/h x dish (EROD) and 510+/-467 pmol/h x dish (ECOD) in the case of enterocytes. Diazepam, another CYP monooxygenase substrate, was also metabolized by porcine hepatocytes but not with porcine enterocytes, thus indicating differences in the metabolic competence of the liver and the gut. 3. The ability to induce enzymes responsible for the metabolism of ethoxyresorufin and ethoxycoumarin was investigated in vitro on treatment of the cell cultures with either 50 microM 3-methylcholanthrene (3-MC) or 50 microM beta-naphthoflavone (beta-NF). With enterocyte cultures, ECOD activity was inducible up to 20-fold, whereas EROD remained unchanged following treatment with either 3-MC or beta-NF. 4. Western blotting provided additional evidence for the expression of CYP1A1 and CYP3A4 at the protein level and treatment of cultured enterocytes with 30 microM Aroclor 1254 or 50 microM beta-NF resulted in enhanced expression of the CYP1A protein, and CYP3A4 protein expression was induced following treatment with 50 microM DEX, 2 mM PB, 30 microM Aroclor 1254 or 50 microM beta-NF. 5. The metabolism of diazepam was also investigated with baculovirus-expressed human CYP enzymes (2C8, 2C9-ARG, 2C9-CYS, 2C19, 3A4, 3A4+cytochrome b5 and 3A5) and evidence was obtained to suggest the formation of temazepam and oxazepam by enzymes of the CYP3A subfamily. Small amounts (32+/-12 ng/ml) of desmethyldiazepam were additionally recovered in microsomal preparations of all CYP-transfected cell lines. 6. In conclusion, porcine duodenal enterocytes can successfully be cultured for a short period and may be used as a tool for studying intestinal metabolism, whereas porcine hepatocytes can be cultured for prolonged periods (>10 days) reliably to investigate hepatic drug oxidation reactions.

Characterization of drug metabolizing activities in pig hepatocytes for use in bioartificial liver devices: comparison with other hepatic cellular models

BACKGROUND/AIMS

The pig is considered the best donor of hepatocytes for bioartificial liver devices, but little is known about the metabolic capability of pig hepatocytes. Therefore, we have evaluated drug metabolizing activities in pig hepatocytes and liver microsomes and compared the results with those of man and other animal hepatic cellular models that are potential sources of cells for bioreactors, such as rat, rabbit and dog hepatocytes and hepatoma cell lines.

METHODS

Total cytochrome P450 levels, six phase 1 activities representative of the most relevant cytochrome P450 enzymes (7-ethoxycoumarin O-deethylase, 7-ethoxy-, 7-methoxy- and 7-benzoxyresorufin O-dealkylases, coumarin 7-hydroxylase and p-nitrophenol hydroxylase), two phase 2 activities (glutathione S-transferase and UDP-glucuronyltransferase) and CYP-dependent regioselective testosterone metabolism were evaluated in in vitro models of different species.

RESULTS

The pattern of specific cytochrome P450 activities and the metabolic profile of testosterone in intact hepatocytes were essentially the same as those measured in liver microsomes. Relatively low ethoxy-, methoxy-, and benzoxyresorufin O-dealkylation rates were found in pig liver microsomes and hepatocytes as compared to hepatic in vitro human models. However, in contrast with the other species studied, stereoselective testosterone oxidation profiles were practically identical in human and pig models. Finally, the metabolic capability of hepatoma cell lines was very limited in comparison with that of hepatocytes.

CONCLUSIONS

Pig hepatocytes are able to maintain in culture the phase 1 and phase 2 activities found in liver microsomes. The high metabolic similarities found between pig and human hepatocytes lend support to the use of pig hepatocytes in bioartificial liver devices.

Application of oral bioavailability surrogates in the design of orally active inhibitors of rhinovirus replication

Previous studies in rats and humans demonstrated poor oral bioavailability of potent in vitro 2-aminobenzimidazole inhibitors of rhinovirus replication due to significant first-pass elimination and possibly also to poor aqueous solubility. Estimations of aqueous solubility, as well as measurements of caco-2 permeability and NADPH dependent compound loss in rat liver microsomal incubations were employed alongside traditional in vivo experiments in rats to guide subsequent chemistry efforts. Retention of activity upon replacement of the metabolically labile vinyl oxime in the lead molecule with a vinyl carboxamide was a major breakthrough; however, oral bioavailability among the latter compounds was variable. Based on the ability to independently measure solubility, permeability, and metabolic stability of new compounds, variable solubility across the series (ranging from approximately 1 to 10 microg/mL) was identified as the cause of the inconsistent performance. Subsequent efforts to improve solubility led to the discovery of highly soluble (>10 mg/mL) and potent dessulfonyl vinyl carboxamide benzimidazoles. Determination of the metabolic stability of these compounds as a surrogate of the extent of their first-pass elimination supported a prediction of excellent oral bioavailability. In comparison to the sulfonyl-containing vinyl carboxamides, caco-2 permeabilities were reduced 5 to 10-fold; however, these were considered to be in the range of well-absorbed compounds based on comparison to a series of reference compounds of known percentage absorption in humans. Subsequent experiments in the rat verified the oral bioavailability of these N-alkyl compounds, with one compound (368177) having an absolute oral bioavailability of 89.4%. The application of solubility and caco-2 permeability as surrogates for oral absorption potential, in conjunction with the use of microsomal incubations as a surrogate for first-pass metabolism, was shown to augment a rational chemistry approach to discover orally bioavailable inhibitors of rhinovirus replication. Future expanded use of these surrogates is planned.