In Vivo and in Vitro Characterization of Chlorzoxazone Metabolism and Hepatic CYP2E1 Levels in African Green Monkeys: Induction by Chronic Nicotine Treatment

Functioning of drug-metabolizing microsomal cytochrome P450s: In silico probing of proteins suggests that the distal heme 'active site' pocket plays a relatively 'passive role' in some enzyme-substrate interactions

PURPOSE

The currently held mechanistic understanding of microsomal cytochrome P450s (CYPs) seeks that diverse drug molecules bind within the deep-seated distal heme pocket and subsequently react at the heme centre. To explain a bevy of experimental observations and meta-analyses, we indulge a hypothesis that involves a "diffusible radical mediated" mechanism. This new hypothesis posits that many substrates could also bind at alternate loci on/within the enzyme and be reacted without the pertinent moiety accessing a bonding proximity to the purported catalytic Fe-O enzyme intermediate.

METHODS

Through blind and heme-distal pocket centered dockings of various substrates and non-substrates (drug molecules of diverse sizes, classes, topographies etc.) of microsomal CYPs, we explored the possibility of access of substrates via the distal channels, its binding energies, docking orientations, distance of reactive moieties (or molecule per se) to/from the heme centre, etc. We investigated specific cases like- (a) large drug molecules as substrates, (b) classical marker drug substrates, (c) class of drugs as substrates (Sartans, Statins etc.), (d) substrate preferences between related and unrelated CYPs, (e) man-made site-directed mutants' and naturally occurring mutants' reactivity and metabolic disposition, (f) drug-drug interactions, (g) overall affinities of drug substrate versus oxidized product, (h) meta-analysis of in silico versus experimental binding constants and reaction/residence times etc.

RESULTS

It was found that heme-centered dockings of the substrate/modulator drug molecules with the available CYP crystal structures gave poor docking geometries and distances from Fe-heme centre. In conjunction with several other arguments, the findings discount the relevance of erstwhile hypothesis in many CYP systems. Consequently, the newly proposed hypothesis is deemed a viable alternate, as it satisfies Occam's razor.

CONCLUSIONS

The new proposal affords expanded scope for explaining the mechanism, kinetics and overall phenomenology of CYP mediated drug metabolism. It is now understood that the heme-iron and the hydrophobic distal pocket of CYPs serve primarily to stabilize the reactive intermediate (diffusible radical) and the surface or crypts of the apoprotein bind to the xenobiotic substrate (and in some cases, the heme distal pocket could also serve the latter function). Thus, CYPs enhance reaction rates and selectivity/specificity via a hitherto unrecognized modality.

Influence of tobacco smoke exposure on pharmacokinetics of ethyl alcohol in alcohol preferring and non-preferring rats

BACKGROUND

A vast majority of people who abuse alcohol are also defined as "heavy smokers". Tobacco smokes induces CYP1A1, CYP1A2, CYP2A6 isoenzymes, but on the other hand, ethanol activates CYP2E1, which can be important during combined, chronic use of both of them. The aim of the study was to evaluate the influence of tobacco smoke xenobiotics on ethanol pharmacokinetics and the level of its metabolites in alcohol preferring and non-preferring rats.

METHODS

Ethanol, acetaldehyde, methanol, n-propanol and n-butanol were determined in whole blood by means of gas chromatography. Cotinine in serum was determined by LC-MS/MS. A non-compartmental analysis (cotinine, acetaldehyde) and Widmark equation (ethanol) were used for pharmacokinetic parameters calculation.

RESULTS

Ethanol levels were lower in animals exposed to tobacco smoke compared to rats receiving this xenobiotic, without a prior exposure to tobacco smoke. Lower values of the studied pharmacokinetic parameters were observed in the alcohol preferring males compared to the non-alcohol preferring rats. Both n-propanol and n-butanol had higher values of the pharmacokinetic parameters analyzed in the animals exposed to tobacco smoke and ethanol compared to those, which ethanol was administered only once.

CONCLUSIONS

An increase in maximum concentration and the area under concentration-time curve for ethanol after its administration to rats preferring alcohol and exposed to tobacco smoke are accompanied by a decrease in the volume of distribution. The changes in the volume of distribution may be caused by an increase in the first-pass effect, in the intestinal tract and/or in the liver. The acetaldehyde elimination rate constant was significantly higher in alcohol-preferring animals.

Effects of cigarette smoking on metabolism and effectiveness of systemic therapy for lung cancer

INTRODUCTION

Cigarette smoke associated polycyclic aromatic hydrocarbons can induce key drug-metabolizing enzymes of cytochrome P450 and isoforms of the glucuronyl transferases families. These enzymes metabolize several systemic therapies for lung cancer. Induction of these enzymes may lead to accelerated clearance with resultant impact on systemic therapy efficacy and toxicity in smokers compared with nonsmokers. This article reviews published literature regarding the influence of smoking as it relates to alteration of metabolism of systemic therapy in lung cancer.

METHODS

A structured search of the National Library of Medicine's PubMed/MEDLINE identified relevant articles. Data were abstracted and analyzed to summarize the findings.

RESULTS

Studies that analyzed pharmacokinetic data were prospective. Smokers receiving erlotinib exhibited rapid clearance, requiring a higher dose to reach equivalent systemic exposure compared with nonsmokers. Smokers receiving irinotecan also demonstrated increased clearance and lower systemic exposure. There was no difference in clearance of paclitaxel or docetaxel in smokers. Chemotherapy-associated neutropenia was worse in nonsmokers compared with smokers in patients treated with paclitaxel, docetaxel, irinotecan, and gemcitabine.

CONCLUSIONS

Systemic therapy for lung cancer has a narrow therapeutic index such that small changes in plasma concentrations or exposure in smokers may result in suboptimal therapy and poor outcomes. Smoking cessation must be emphasized at each clinical visit. However, prospective trials should take into consideration the effects of smoking history on drug pharmacokinetics and efficacy. The metabolizing enzyme phenotype in smokers may require individualized dose algorithms for specific agents.

Cytochrome P450 2E1: its clinical aspects and a brief perspective on the current research scenario

Research on Cytochrome P450 2E1 (CYP2E1), a key enzyme in alcohol metabolism has been very well documented in literature. Besides the involvement of CYP2E1 in alcohol metabolism as illustrated through the studies discussed in the chapter, recent studies have thrown light on several other aspects of CYP2E1 i.e. its extrahepatic expression, its involvement in several diseases and pathophysiological conditions; and CYP2E1 mediated carcinogenesis and modulation of drug efficacy. Studies involving these interesting facets of CYP2E1 have been discussed in the chapter focusing on the recent observations or ongoing studies illustrating the crucial role of CYP2E1 in disease development and drug metabolism.

Significantly reduced cytochrome P450 3A4 expression and activity in liver from humans with diabetes mellitus

BACKGROUND AND PURPOSE

Patients with diabetes mellitus require pharmacotherapy with numerous medications. However, the effect of diabetes on drug biotransformation is not well understood. Our goal was to investigate the effect of diabetes on liver cytochrome P450 3As, the most abundant phase I drug-metabolizing enzymes in humans.

EXPERIMENTAL APPROACH

Human liver microsomal fractions (HLMs) were prepared from diabetic (n= 12) and demographically matched nondiabetic (n= 12) donors, genotyped for CYP3A4*1B and CYP3A5*3 polymorphisms. Cytochrome P450 3A4, 3A5 and 2E1 mRNA expression, protein level and enzymatic activity were compared between the two groups.

KEY RESULTS

Midazolam 1'- or 4-hydroxylation and testosterone 6β-hydroxylation, catalyzed by P450 3A, were markedly reduced in diabetic HLMs, irrespective of genotype. Significantly lower P450 3A4 protein and comparable mRNA levels were observed in diabetic HLMs. In contrast, neither P450 3A5 protein level nor mRNA expression differed significantly between the two groups. Concurrently, we have observed increased P450 2E1 protein level and higher chlorzoxazone 6-hydroxylation activity in diabetic HLMs.

CONCLUSIONS AND IMPLICATIONS

These studies indicate that diabetes is associated with a significant decrease in hepatic P450 3A4 enzymatic activity and protein level. This finding could be clinically relevant for diabetic patients who have additional comorbidities and are receiving multiple medications. To further characterize the effect of diabetes on P450 3A4 activity, a well-controlled clinical study in diabetic patients is warranted.

Independent and combined effects of ethanol self-administration and nicotine treatment on hepatic CYP2E1 in African green monkeys

Cytochrome P450 2E1 metabolizes ethanol and also bioactivates many toxins and procarcinogens. Elevated levels of hepatic CYP2E1 are associated with an increased susceptibility to chemical toxicity and carcinogenesis. This study investigated the induction of hepatic CYP2E1 by ethanol and nicotine, alone and in combination, in a nonhuman primate model. Monkeys that self-administered ethanol and that received subcutaneous injections of nicotine (0.5 mg/kg b.i.d.), alone and in combination, were compared with control animals (four groups, n = 10/group). Chlorzoxazone (CZN) was used as a probe drug to phenotype in vivo CYP2E1 activity before and after chronic ethanol and/or nicotine exposure. CYP2E1 protein levels and in vitro chlorzoxazone metabolism were assessed in liver microsomes. Average daily ethanol consumption was ≈3.0 g/kg (blood ethanol levels ≈24 mM) and was unaffected by nicotine treatment. Ethanol self-administration and nicotine treatment, alone and in combination, significantly increased in vivo CZN disposition compared with that in control animals. The effect of ethanol was only observed at higher levels of intake. Ethanol and nicotine increased CYP2E1 protein levels and in vitro CZN metabolism, with combined exposure to both drugs resulting in the greatest increase. The effect of ethanol was also dependent on level of intake. Chronic exposure to ethanol and nicotine induced hepatic CYP2E1 activity and protein levels, particularly when both drugs were used in combination and when ethanol intake was high. These results have important implications for public health, given the association between elevated CYP2E1 and disease, and the large proportion of individuals who are exposed to ethanol and nicotine, often in combination.

Effects of nicotine on cytochrome P450 2A6 and 2E1 activities

AIMS

Smoking slows the metabolism of nicotine and accelerates the metabolism of chlorzoxazone, which are probe reactions for cytochrome P450 2A6 (CYP2A6) and CYP2E1 activities, respectively. We aimed to determine the role of nicotine in these metabolic effects of cigarette smoking.

METHODS

The study had a single-blind, randomized, crossover two-arm design. Twelve healthy smokers were given two transdermal patches with 42-mg nicotine a day or placebo patches, each for 10 days. The subjects abstained from smoking during the study arms. Oral chlorzoxazone was given on day 7 and deuterium-labelled nicotine-d(2) and cotinine-d(4) infusion on day 8.

RESULTS

There was no significant influence of transdermal nicotine administration on pharmacokinetic parameters of nicotine-d(2) or on the formation of cotinine-d(2). Nicotine decreased the volume of distribution (62.6 vs. 67.7 l, 95% confidence interval of the difference -9.7, -0.6, P= 0.047) of infused cotinine-d(4). There were no significant differences in disposition kinetics of chlorzoxazone between the treatments.

CONCLUSIONS

CYP2A6 and CYP2E1 activities are not affected by nicotine. The tobacco smoke constituents responsible for the reduced CYP2A6 and increased CYP2E1 activities remain unknown.

Cynomolgus monkey CYPs: a comparison with human CYPs

Characteristics of twelve cytochromes P450 (CYPs) from cynomolgus monkeys were compared with those of human CYPs that play an important role in drug metabolism. Eleven members of CYP1A, CYP2A, CYP2C, CYP2D, CYP2E, and CYP3A subfamilies from cynomolgus monkeys exhibited a high degree of homologies (more than 90%) in cDNA and amino acid sequences with corresponding human CYPs, and catalysed typical reactions of corresponding human CYPs. One member of the cynomolgus monkey CYP2C subfamily, CYP2C76, exhibited a lower homology (around 70%) in amino acid sequences with other cynomolgus monkey and human CYP2C subfamilies. CYP2C76 catalysed typical CYP2C substrates with low activities, and has not been found in humans. CYPs identified in cynomolgus monkeys were similar to CYP1A1, CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP3A5 in humans. These results indicate that cynomolgus monkeys express CYPs similar to human CYPs that are important in drug metabolism.

Genetic polymorphism in CYP2E1: Population distribution of CYP2E1 activity

Cytochrome P-450 2E1 (CYP2E1) is a key enzyme in the metabolic activation of a variety of toxicants including nitrosamines, benzene, vinyl chloride, and halogenated solvents such as trichloroethylene. CYP2E1 is also one of the enzymes that metabolizes ethanol to acetaldehyde, and is induced by recent ethanol ingestion. There is evidence that interindividual variability in the expression and functional activity of this cytochrome (CYP) may be considerable. Genetic polymorphisms in CYP2E1 were identified and linked to altered susceptibility to hepatic cirrhosis induced by ethanol and esophageal and other cancers in some epidemiological studies. Therefore, it is important to evaluate how such polymorphisms affect CYP2E1 function and whether it is possible to construct a population distribution of CYP2E1 activity based upon the known effects of these polymorphisms and their frequency in the population. This analysis is part of the genetic polymorphism database project described in the lead article in this series and followed the approach described in that article (Ginsberg et al., 2009, this issue). Review of the literature found that there are a variety of CYP2E1 variant alleles but the functional significance of these variants is still unclear. Some, but not all, studies suggest that several upstream 5' flanking mutations affect gene expression and response to inducers such as ethanol or obesity. None of the coding-region variants consistently affects enzyme function. Part of the reason for conflicting evidence regarding genotype effect on phenotype may be due to the wide variety of exposures such as ethanol or dietary factors and physiological factors including body weight or diabetes that modulate CYP2E1 expression. In conclusion, evidence is too limited to support the development of a population distribution of CYP2E1 enzyme activity based upon genotypes. Health risk assessments may best rely upon data reporting interindividual variability in CYP2E1 function for input into physiologically based pharmacokinetic (PBPK) models involving CYP2E1 substrates.

Diclofenac hydroxylation in monkeys: Efficiency, regioselectivity, and response to inhibitors

The catalytic efficiency, regioselectivity, and response to chemical inhibitors of diclofenac (DF) hydroxylation in three Old World monkey liver microsomes (rhesus, cynomolgus, and African green monkey) are different from those determined with human liver microsomes. In contrast to the high affinity-high capacity (low Km-high Vmax) characteristics of DF 4'-hydroxylation in humans, this reaction proceeded in all monkey species with catalytic efficiencies >20-fold lower. However, DF 5-hydroxylation, a negligible reaction in human liver microsomes, was kinetically favored in monkeys mainly due to the increased Vmax values. Chemical inhibitors (reversible or mechanism-based) selective to human CYP3A4 and CYP2C9 failed to differentiate monkey orthologs involved in DF hydroxylation. Immunoinhibition studies with monoclonal antibodies against human CYPs revealed the major contribution of CYP2C and CYP3A to 4'- and to 5-hydroxylation, respectively, in rhesus and cynomolgus liver microsomes. However, in African green monkeys, in addition to CYP2C, CYP3A also appeared to be involved in 4'-hydroxylation. Further studies with recombinant rhesus and African green monkey CYP2C and CYP3A enzymes (rhesus CYP2C75, 2C74, and 3A64; African green monkey CYP2C9agm and CYP3A4agm) confirmed the major role of CYP enzymes of these two subfamilies in DF 4'- and 5-hydroxylation. Clearly, while monkey CYP2C and 3A enzymes retain the same substrate selectivity towards DF hydroxylation as their human orthologs, their altered catalytic efficiency and response to chemical inhibitors may indicate different structural features of active sites as opposed to human orthologs.

Phenobarbital induces monkey brain CYP2E1 protein but not hepatic CYP2E1, in vitro or in vivo chlorzoxazone metabolism

Cytochrome P450 2E1 (CYP2E1) is expressed in the brain and liver, and can metabolize clinical drugs and activate toxins. The effect of phenobarbital on hepatic and brain CYP2E1 is unclear. We investigated the effect of chronic phenobarbital treatment on in vivo chlorzoxazone disposition (a CYP2E1 probe drug), in vitro chlorzoxazone metabolism, and hepatic and brain CYP2E1 protein levels in African Green monkeys (Cercopithecus aethiops). Monkeys were given oral saccharine or saccharine supplemented with 20 mg/kg phenobarbital (N = 6/group) for 22 days. Phenobarbital did not induce in vivo chlorzoxazone disposition, in vitro chlorzoxazone metabolism or hepatic CYP2E1 protein levels (all P > 0.05). However, phenobarbital induced brain CYP2E1 protein levels, using immunoblotting, by 1.26-fold in the cerebellum (P = 0.01) and 1.46-fold in the putamen (P = 0.04). Phenobarbital also increased cell-specific CYP2E1 expression, for example in the frontal cortical pyramidal neurons and cerebellar Purkinje cells. This data indicates that phenobarbital does not alter hepatic metabolism, but may alter metabolism of CYP2E1 substrates within the brain.