Purification and characterization of the human liver cytochromes P-450 involved in debrisoquine 4-hydroxylation and phenacetin O-deethylation, two prototypes for genetic polymorphism in oxidative drug metabolism

Roles of Individual Human Cytochrome P450 Enzymes in Drug Metabolism

Our knowledge of the roles of individual cytochrome P450 (P450) enzymes in drug metabolism has developed considerably in the past 30 years, and this base has been of considerable use in avoiding serious issues with drug interactions and issues due to variations. Some newer approaches are being considered for "phenotyping" metabolism reactions with new drug candidates. Endogenous biomarkers are being used for noninvasive estimation of levels of individual P450 enzymes. There is also the matter of some remaining "orphan" P450s, which have yet to be assigned reactions. Practical problems that continue in drug development include predicting drug-drug interactions, predicting the effects of polymorphic and other P450 variations, and evaluating interspecies differences in drug metabolism, particularly in the context of "metabolism in safety testing" regulatory issues ["disproportionate (human) metabolites"]. SIGNIFICANCE STATEMENT: Cytochrome P450 enzymes are the major catalysts involved in drug metabolism. The characterization of their individual roles has major implications in drug development and clinical practice.

Cardiovascular Pharmacogenetics: From Discovery of Genetic Association to Clinical Adoption of Derived Test

Recent breakthroughs in human genetics and in information technologies have markedly expanded our understanding at the molecular level of the response to drugs, i.e., pharmacogenetics (PGx), across therapy areas. This review is restricted to PGx for cardiovascular (CV) drugs. First, we examined the PGx information in the labels approved by regulatory agencies in Europe, Japan, and North America and related recommendations from expert panels. Out of 221 marketed CV drugs, 36 had PGx information in their labels approved by one or more agencies. The level of annotations and recommendations varied markedly between agencies and expert panels. Clopidogrel is the only CV drug with consistent PGx recommendation (i.e., "actionable"). This situation prompted us to dissect the steps from discovery of a PGx association to clinical translation. We found 101 genome-wide association studies that investigated the response to CV drugs or drug classes. These studies reported significant associations for 48 PGx traits mapping to 306 genes. Six of these 306 genes are mentioned in the corresponding PGx labels or recommendations for CV drugs. Genomic analyses also highlighted the wide between-population differences in risk allele frequencies and the individual load of actionable PGx variants. Given the high attrition rate and the long road to clinical translation, additional work is warranted to identify and validate PGx variants for more CV drugs across diverse populations and to demonstrate the utility of PGx testing. To that end, pre-emptive PGx combining genomic profiling with electronic medical records opens unprecedented opportunities to improve healthcare, for CV diseases and beyond. SIGNIFICANCE STATEMENT: Despite spectacular breakthroughs in human molecular genetics and information technologies, consistent evidence supporting PGx testing in the cardiovascular area is limited to a few drugs. Additional work is warranted to discover and validate new PGx markers and demonstrate their utility. Pre-emptive PGx combining genomic profiling with electronic medical records opens unprecedented opportunities to improve healthcare, for CV diseases and beyond.

Ninety-eight semesters of cytochrome P450 enzymes and related topics-What have I taught and learned?

This Reflection article begins with my family background and traces my career through elementary and high school, followed by time at the University of Illinois, Vanderbilt University, the University of Michigan, and then for 98 semesters as a Vanderbilt University faculty member. My research career has dealt with aspects of cytochrome P450 enzymes, and the basic biochemistry has had applications in fields as diverse as drug metabolism, toxicology, medicinal chemistry, pharmacogenetics, biological engineering, and bioremediation. I am grateful for the opportunity to work with the Journal of Biological Chemistry not only as an author but also for 34 years as an Editorial Board Member, Associate Editor, Deputy Editor, and interim Editor-in-Chief. Thanks are extended to my family and my mentors, particularly Profs. Harry Broquist and Minor J. Coon, and the more than 170 people who have trained with me. I have never lost the enthusiasm for research that I learned in the summer of 1968 with Harry Broquist, and I have tried to instill this in the many trainees I have worked with. A sentence I use on closing slides is "It's not just a laboratory-it's a fraternity."

Mutual Modulation of the Activities of Human CYP2D6 and Four UGTs during the Metabolism of Propranolol

Cytochromes P450 (CYP) and UDP-glucuronosyltransferases (UGT) are two enzyme families that play an important role in drug metabolism, catalyzing either the functionalization or glucuronidation of xenobiotics. However, their mutual interactions are poorly understood. In this study, the functional interactions of human CYP2D6 with four human UGTs (UGT1A7, UGT1A8, UGT1A9, and UGT2A1) were investigated using our previously established co-expression model system in the fission yeast Schizosaccharomyces pombe. The substrate employed was propranolol because it is well metabolized by CYP2D6. Moreover, the CYP2D6 metabolite 4-hydroxypropranolol is a known substrate for the four UGTs included in this study. Co-expression of either UGT1A7, UGT1A8, or UGT1A9 was found to increase the activity of CYP2D6 by a factor of 3.3, 2.1 or 2.8, respectively, for the conversion of propranolol to 4-hydroxypropranolol. In contrast, UGT2A1 co-expression did not change CYP2D6 activity. On the other hand, the activities of all four UGTs were completely suppressed by co-expression of CYP2D6. This data corroborates our previous report that CYP2D6 is involved in functional CYP-UGT interactions and suggest that such interactions can contribute to both adverse drug reactions and changes in drug efficacy.

Drug Metabolism: A Half-Century Plus of Progress, Continued Needs, and New Opportunities

The systematic study of drug metabolism began in the 19th Century, but most of what we know now has been learned in the last 50 years. Drug metabolism continues to play a critical role in pharmaceutical development and clinical practice, as well as contributing to toxicology, chemical carcinogenesis, endocrinology, and drug abuse. The importance of the field will continue, but its nature will continue to develop with changes in analytical chemistry, structural biology, and artificial intelligence. Challenges and opportunities include toxicology, defining roles of genetic variations, and application to clinical issues. Although the focus of this Minireview is cytochrome P450, the same principles apply to other enzymes and transporters involved in drug metabolism. SIGNIFICANCE STATEMENT: Progress in the field of drug metabolism over the past 50 years has helped make the pharmaceutical enterprise what it is today. Drug metabolism will continue to be important. Challenges and opportunities for the future are discussed.

Pharmacogenetics of the cytochromes P450: Selected pharmacological and toxicological aspects

With the availability of detailed genomic data on all 57 human cytochrome P450 genes, it is clear that there is substantial variability in gene product activity with functionally significant polymorphisms reported across almost all isoforms. This article is concerned mainly with 13 P450 isoforms of particular relevance to xenobiotic metabolism. After brief review of the extent of polymorphism in each, the relevance of selected P450 isoforms to both adverse drug reaction and disease susceptibility is considered in detail. Bleeding due to warfarin and other coumarin anticoagulants is considered as an example of a type A reaction with idiosyncratic adverse drug reactions affecting the liver and skin as type B. It is clear that CYP2C9 variants contribute significantly to warfarin dose requirement and also risk of bleeding, with a minor contribution from CYP4F2. In the case of idiosyncratic adverse drug reactions, CYP2B6 variants appear relevant to both liver and skin reactions to several drugs with CYP2C9 variants also relevant to phenytoin-related skin rash. The relevance of P450 genotype to disease susceptibility is also considered but detailed genetic studies now suggest that CYP2A6 is the only P450 relevant to risk of lung cancer with alleles associated with low or absent activity clearly protective against disease. Other cytochrome P450 genotypes are generally not predictors for risk of cancer or other complex disease development.

Roles of cytochrome P450 enzymes in pharmacology and toxicology: Past, present, and future

The development of the cytochrome P450 (P450) field has been remarkable in the areas of pharmacology and toxicology, particularly in drug development. Today it is possible to use the knowledge base and relatively straightforward assays to make intelligent predictions about drug disposition prior to human dosing. Much is known about the structures, regulation, chemistry of catalysis, and the substrate and inhibitor specificity of human P450s. Many aspects of drug-drug interactions and side effects can be understood in terms of P450s. This knowledge has also been useful in pharmacy practice, as well as in the pharmaceutical industry and medical practice. However, there are still basic and practical questions to address regarding P450s and their roles in pharmacology and toxicology. Another aspect is the discovery of drugs that inhibit P450 to treat diseases.

From pharmacogenetics to pharmaco-omics: Milestones and future directions

The origins of pharmacogenetics date back to the 1950s, when it was established that inter-individual differences in drug response are partially determined by genetic factors. Since then, pharmacogenetics has grown into its own field, motivated by the translation of identified gene-drug interactions into therapeutic applications. Despite numerous challenges ahead, our understanding of the human pharmacogenetic landscape has greatly improved thanks to the integration of tools originating from disciplines as diverse as biochemistry, molecular biology, statistics, and computer sciences. In this review, we discuss past, present, and future developments of pharmacogenetics methodology, focusing on three milestones: how early research established the genetic basis of drug responses, how technological progress made it possible to assess the full extent of pharmacological variants, and how multi-dimensional omics datasets can improve the identification, functional validation, and mechanistic understanding of the interplay between genes and drugs. We outline novel strategies to repurpose and integrate molecular and clinical data originating from biobanks to gain insights analogous to those obtained from randomized controlled trials. Emphasizing the importance of increased diversity, we envision future directions for the field that should pave the way to the clinical implementation of pharmacogenetics.

In Vitro and In Vivo Assessment of Atemoya Fruit (Annona atemoya) for Food-Drug Interactions

BACKGROUND AND OBJECTIVES

Atemoya (Annona atemoya) is increasingly being consumed worldwide because of its pleasant taste. However, only limited information is available concerning possible atemoya-drug interactions. In the present study, the issue of whether atemoya shows food-drug interactions with substrate drugs of the major drug-metabolizing cytochrome P450s (i.e., CYP1A2, CYP2C9, and CYP3A) is addressed.

METHODS

The ability of atemoya juice to inhibit the activities of phenacetin O-deethylase (CYP1A2), diclofenac 4'-hydroxylase (CYP2C9), and midazolam 1'-hydroxylase (CYP3A) was examined in vitro using human and rat liver microsomes. The in vivo pharmacokinetics of phenacetin and metabolites derived from it in rats when atemoya juice or fluvoxamine (a CYP1A2 inhibitor) was preadministered were also investigated.

RESULTS

Atemoya juice significantly inhibited CYP1A2 activity in human liver microsomes, but not the activities of CYP2C9 and CYP3A. In spite of this inhibition, preadministration of atemoya had no effect on the pharmacokinetics of phenacetin, a CYP1A2 substrate, in rats. Meanwhile, preadministration of fluvoxamine significantly extended the time needed for the elimination of phenacetin, possibly due to the inhibition of CYP1A2. This suggests that the intake of an excess amount of atemoya juice is necessary to cause a change in the pharmacokinetics of phenacetin when the IC₅₀ values for CYP1A2 inhibition by atemoya and fluvoxamine are taken into account.

CONCLUSION

The results indicate that a daily intake of atemoya would not change the pharmacokinetics of CYP1A2 substrates such as phenacetin as well as CYP2C9- and CYP3A-substrate drugs.

The Central Role of Cytochrome P450 in Xenobiotic Metabolism-A Brief Review on a Fascinating Enzyme Family

Human Cytochrome P450 (CYP) enzymes constitute a superfamily of membrane-bound hemoproteins that are responsible for the metabolism of a wide variety of clinically, physiologically, and toxicologically important compounds. These heme-thiolate monooxygenases play a pivotal role in the detoxification of xenobiotics, participating in the metabolism of many structurally diverge compounds. This short-review is intended to provide a summary on the major roles of CYPs in Phase I xenobiotic metabolism. The manuscript is focused on eight main topics that include the most relevant aspects of past and current CYP research. Initially, (I) a general overview of the main aspects of absorption, distribution, metabolism, and excretion (ADME) of xenobiotics are presented. This is followed by (II) a background overview on major achievements in the past of the CYP research field. (III) Classification and nomenclature of CYPs is briefly reviewed, followed by (IV) a summary description on CYP's location and function in mammals. Subsequently, (V) the physiological relevance of CYP as the cornerstone of Phase I xenobiotic metabolism is highlighted, followed by (VI) reviewing both genetic determinants and (VI) nongenetic factors in CYP function and activity. The last topic of the review (VIII) is focused on the current challenges of the CYP research field.

A history of the roles of cytochrome P450 enzymes in the toxicity of drugs

The history of drug metabolism began in the 19th Century and developed slowly. In the mid-20th Century the relationship between drug metabolism and toxicity became appreciated, and the roles of cytochrome P450 (P450) enzymes began to be defined in the 1960s. Today we understand much about the metabolism of drugs and many aspects of safety assessment in the context of a relatively small number of human P450s. P450s affect drug toxicity mainly by either reducing exposure to the parent molecule or, in some cases, by converting the drug into a toxic entity. Some of the factors involved are enzyme induction, enzyme inhibition (both reversible and irreversible), and pharmacogenetics. Issues related to drug toxicity include drug-drug interactions, drug-food interactions, and the roles of chemical moieties of drug candidates in drug discovery and development. The maturation of the field of P450 and drug toxicity has been facilitated by advances in analytical chemistry, computational capability, biochemistry and enzymology, and molecular and cell biology. Problems still arise with P450s and drug toxicity in drug discovery and development, and in the pharmaceutical industry the interaction of scientists in medicinal chemistry, drug metabolism, and safety assessment is critical for success.

Phe-125 and Phe-226 of pig cytochrome P450 1A2 stabilize the binding of aflatoxin B1 and 7-ethoxyresorufin through the key CH/π interactions

Cytochrome P450 1A2 (CYP1A2) plays important roles in the metabolism of many planar and aromatic drugs and also contributes to the bioactivation of aflatoxin B₁ (AFB₁) in vivo. To date, the structural basis for CYP1A2's preference to the planar substrates remains unclear. Herein, we investigated the structure-activity relationships for pig CYP1A2 catalyzing AFB₁ and 7-ethoxyresorufin (7-ER). A molecular docking study was performed based on a constructed model of pig CYP1A2, which predicted the contributions of Thr-118, Thr-124, Phe-125, Phe-226, Leu-260, and Asp-313 to the substrate catalysis. Site-directed mutagenesis and kinetic analyses exhibited the common grounds: Phe-125, Phe-226 and Asp-313 were vital to AFB₁ oxidation (including exo-epoxidation and 9A-hydroxylation) and ethoxyresorufin O-deethylation. Meanwhile, Phe-125 and Phe-226 formed CH/π interactions with AFB₁/7-ER, and Asp-313 formed hydrogen bonds with them. Based on other published reports, this study further emphasizes the critical roles of Phe-125 and Phe-226 in recognizing the planar substrates. Our findings highlight the structural basis of pig CYP1A2 specifically catalyzing AFB₁ and 7-ER, and may help to elucidate the underlying mechanism of CYP1A2's metabolic preference to the planar and aromatic substrates.

Cytochrome P450 research and The Journal of Biological Chemistry

In honor of the 100th birthday of Dr. Herbert Tabor, JBC's Editor-in-Chief for 40 years, I will review here JBC's extensive coverage of the field of cytochrome P450 (P450) research. Research on the reactions catalyzed by these enzymes was published in JBC before it was even realized that they were P450s, i.e. they have a "pigment" with an absorption maximum at 450 nm. After the P450 pigment discovery, reported in JBC in 1962, the journal proceeded to publish the methods for measuring P450 activities and many seminal findings. Since then, the P450 field has grown extensively, with significant progress in characterizing these enzymes, including structural features, catalytic mechanisms, regulation, and many other aspects of P450 biochemistry. JBC has been the most influential journal in the P450 field. As with many other research areas, Dr. Tabor deserves a great deal of the credit for significantly advancing this burgeoning and important topic of research.

Increased Phenacetin Oxidation upon the L382V Substitution in Cytochrome P450 1A2 is Associated with Altered Substrate Binding Orientation

Leucine382 of cytochrome P450 1A2 (CYP1A2) plays an important role in binding and O-dealkylation of phenacetin, with the L382V mutation increasing substrate oxidation (Huang and Szklarz, 2010, Drug Metab. Dispos. 38:1039–1045). This was attributed to altered substrate binding orientation, but no direct experimental evidence had been available. Therefore, in the current studies, we employed nuclear magnetic resonance (NMR) longitudinal (T₁) relaxation measurements to investigate phenacetin binding orientations within the active site of CYP1A2 wild type (WT) and mutants. Paramagnetic relaxation time (T_1P) for each proton of phenacetin was calculated from the T₁ value obtained from the enzymes in ferric and ferrous-CO state in the presence of phenacetin, and used to model the orientation of phenacetin in the active site. All aromatic protons of phenacetin were nearly equidistant from the heme iron (6.34–8.03 Å). In contrast, the distance between the proton of the –OCH₂– group, which is abstracted during phenacetin oxidation, and the heme iron, was much shorter in the L382V (5.93 Å) and L382V/N312L (5.96 Å) mutants compared to the N312L mutant (7.84 Å) and the wild type enzyme (6.55 Å), consistent with modeling results. These studies provide direct evidence for the molecular mechanism underlying increased oxidation of phenacetin upon the L382V mutation.

The challenges of implementing pharmacogenomic testing in the clinic

INTRODUCTION

Pharmacogenomic testing has the potential to greatly benefit patients by enabling personalization of medication management, ensuring better efficacy and decreasing the risk of side effects. However, to fully realize the potential of pharmacogenomic testing, there are several important issues that must be addressed. Areas covered: In this expert review we discuss current challenges impacting the implementation of pharmacogenomic testing in the clinical practice. We emphasize issues related to testing methods, reporting of the results, test selection, clinical interpretation of the results, cost-effectiveness, and the long-term use of pharmacogenomic results in clinical practice. We identify opportunities and future directions to facilitate clinical implementation. Expert commentary: Several key elements are necessary to optimally integrate pharmacogenomic testing into clinical practice. Collaborative efforts among laboratories are needed to improve standardization of testing and reporting of the results. Clinicians need educational opportunities to improve understanding of which test to order and how to interpret the results. The electronic health records and other clinical systems need to improve their storage of the pharmacogenomics test results and interoperability to facilitate the use of clinically actionable results to improve patient care.

Life and Times in Biochemical Toxicology

The biochemical facets of toxicology have always had a major role in providing insight into mechanisms. Some of the history of the development of this area is summarized, including metabolism, enzymology, and the chemistry of reactive intermediates. Knowledge in these fields has had a major impact in the areas of drug metabolism and safety assessment, which are both critical steps in the development of pharmaceuticals and the rational use of commodity chemicals. The science of toxicology has developed considerably with input from other disciplines and today is poised to emerge as a predictive science with even more dramatic impact. The challenges ahead are considerable but there is renewed excitement in the potential of the field. As in the past, further advances in the field of toxicology will require the input of knowledge from many disciplines.

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.

Reduction of aromatic and heterocyclic aromatic N-hydroxylamines by human cytochrome P450 2S1

Many aromatic amines and heterocyclic aromatic amines (HAAs) are known carcinogens for animals, and there is also strong evidence of some in human cancer. The activation of these compounds, including some arylamine drugs, involves N-hydroxylation, usually by cytochrome P450 enzymes (P450) in Family 1 (1A2, 1A1, and 1B1). We previously demonstrated that the bioactivation product of the anticancer agent 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (5F 203), an N-hydroxylamine, can be reduced by P450 2S1 to its amine precursor under anaerobic conditions and, to a lesser extent, under aerobic conditions [Wang, K., and Guengerich, F. P. (2012) Chem. Res. Toxicol. 25, 1740-1751]. In the study presented here, we tested the hypothesis that P450 2S1 is involved in the reductive biotransformation of known carcinogenic aromatic amines and HAAs. The N-hydroxylamines of 4-aminobiphenyl (4-ABP), 2-naphthylamine (2-NA), and 2-aminofluorene (2-AF) were synthesized and found to be reduced by P450 2S1 under both anaerobic and aerobic conditions. The formation of amines due to P450 2S1 reduction also occurred under aerobic conditions but was less apparent because the competitive disproportionation reactions (of the N-hydroxylamines) also yielded amines. Further, some nitroso and nitro derivatives of the arylamines could also be reduced by P450 2S1. None of the amines tested were oxidized by P450 2S1. These results suggest that P450 2S1 may be involved in the reductive detoxication of several of the activated products of carcinogenic aromatic amines and HAAs.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Polymorphic metabolism by functional alterations of human cytochrome P450 enzymes

The study of cytochrome P450 pharmacogenomics is of particular interest because of its promise in the development of rational means to optimize drug therapy with respect to patient's genotype to ensure maximum efficacy with minimal adverse effects. Drug metabolizing P450 enzymes are polymorphic and are the main phase I enzymes responsible for the metabolism of clinical drugs. Therefore, polymorphisms in the P450s have the most impact on the fate of clinical drugs in phase I metabolism since almost 80% of drugs in use today are metabolized by these enzymes. Predictive genotyping for P450 enzymes for a more effective therapy will be routine for specific drugs in the future. In this review, we discuss the current knowledge of polymorphic metabolism by functional alterations in nonsynonymous SNPs of P450 1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 enzymes.

Hydroxylation of Debrisoquine Using Perfused Liver Isolated from Sprague Dawley and DA Rats: Comparison With In-vivo Results

The hydroxylation of debrisoquine was investigated in Sprague-Dawley (SD) and Dark-Agouti (DA) rats. Female and male rats were phenotyped in-vivo with debrisoquine six times during their growth. The ratios debrisoquine/4-hydroxydebrisoquine of the female DA rats increased until the 15th week and then decreased; but the values of the metabolic ratios never exceeded 2. Female DA rats cannot be considered as genetically deficient for hydroxylation of debrisoquine in regard to the metabolic ratio, but the percentage of debrisoquine excretion is up to ten fold higher than that in the other strains. Therefore SD and DA rat livers were perfused for 2 h when the clearance of debrisoquine was significantly lower in the female DA group than in the other groups. 4-Hydroxydebrisoquine in the perfusate increased with time, but the amount after 120 min was 12 fold lower in the female DA rat group than in the female SD rat group. The results of the male DA group fell between. This study confirms that female DA rats present a lower debrisoquine 4-hydroxylating capacity than other rats but shows that urinary metabolic ratio is not sufficient to assess the deficiency of debrisoquine hydroxylation.

Pharmacogenetics and human genetic polymorphisms

The term pharmacogenetics was first used in the late 1950s and can be defined as the study of genetic factors affecting drug response. Prior to formal use of this term, there was already clinical data available in relation to variable patient responses to the drugs isoniazid, primaquine and succinylcholine. The subject area developed rapidly, particularly with regard to genetic factors affecting drug disposition. There is now comprehensive understanding of the molecular basis for variable drug metabolism by the cytochromes P450 and also for variable glucuronidation, acetylation and methylation of certain drugs. Some of this knowledge has already been translated to the clinic. The molecular basis of variation in drug targets, such as receptors and enzymes, is generally less well understood, although there is consistent evidence that polymorphisms in the genes encoding the β-adrenergic receptors and the enzyme vitamin K epoxide reductase is of clinical importance. The genetic basis of rare idiosyncratic adverse drug reactions had also been examined. Susceptibility to reactions affecting skin and liver appears to be determined in part by the HLA (human leucocyte antigen) genotype, whereas reactions affecting the heart and muscle may be determined by polymorphisms in genes encoding ion channels and transporters respectively. Genome-wide association studies are increasingly being used to study drug response and susceptibility to adverse drug reactions, resulting in identification of some novel pharmacogenetic associations.

Significant increase in phenacetin oxidation on L382V substitution in human cytochrome P450 1A2

Human CYP1A2 is an important drug-metabolizing enzyme, similar in sequence to CYP1A1 but with distinct substrate specificity. We have previously shown that residue 382 affected CYP1A1 and CYP1A2 specificities with alkoxyresorufins. To determine whether this residue is also important for the metabolism of other substrates, we have investigated phenacetin oxidation by single (T124S, T223N, V227G, N312L, and L382V) and multiple (L382V/T223N, L382V/N312L, L382V/T223N/N312L, and L382V/T124S/N312L) mutants of CYP1A2. The enzymes were expressed in Escherichia coli and purified. All the CYP1A2 mutants that contained the L382V substitution displayed much higher activities than the wild-type enzyme, with k(cat) values 3-fold higher, in contrast to other mutants, for which k(cat) decreased. Likewise, a significant increase in specificity, expressed as the k(cat)/K(m) ratio, was observed for the mutants containing the L382V substitution. The efficiency of coupling of reducing equivalents to acetaminophen formation was decreased for all the single mutants except L382V, for which the coupling increased. This effect was also observed with multiple CYP1A2 mutants containing the L382V substitution. Low activities of the four other single mutants were likely caused by dramatically increased uncoupling to water. In contrast, the increase in activity of the L382V-containing mutants resulted from decreased water formation. This finding is consistent with molecular dynamics results, which showed decreased phenacetin mobility leading to increased product formation. The results of these studies confirm the importance of residue 382 in CYP1A2-catalyzed oxidations and show that a single residue substitution can dramatically affect enzymatic activity.

Pharmacogenomics: a new paradigm to personalize treatments in nephrology patients

Although notable progress has been made in the therapeutic management of patients with chronic kidney disease in both conservative and renal replacement treatments (dialysis and transplantation), the occurrence of medication-related problems (lack of efficacy, adverse drug reactions) still represents a key clinical issue. Recent evidence suggests that adverse drug reactions are major causes of death and hospital admission in Europe and the United States. The reasons for these conditions are represented by environmental/non-genetic and genetic factors responsible for the great inter-patient variability in drugs metabolism, disposition and therapeutic targets. Over the years several genetic settings have been linked, using pharmacogenetic approaches, to the effects and toxicity of many agents used in clinical nephrology. However, these strategies, analysing single gene or candidate pathways, do not represent the gold standard, being the overall pharmacological effects of medications and not typically monogenic traits. Therefore, to identify multi-genetic influence on drug response, researchers and clinicians from different fields of medicine and pharmacology have started to perform pharmacogenomic studies employing innovative whole genomic high-throughput technologies. However, to date, only few pharmacogenomics reports have been published in nephrology underlying the need to enhance the number of projects and to increase the research budget for this important research field. In the future we would expect that, applying the knowledge about an individual's inherited response to drugs, nephrologists will be able to prescribe medications based on each person's genetic make-up, to monitor carefully the efficacy/toxicity of a given drug and to modify the dosage or number of medications to obtain predefined clinical outcomes.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

Quantitation of human cytochrome P450 2D6 protein with immunoblot and mass spectrometry analysis

Accurate quantification of cytochrome P450 (P450) protein contents is essential for reliable assessment of drug safety, including the prediction of in vivo clearance from in vitro metabolism data, which may be hampered by the use of uncharacterized standards and existence of unknown allelic isozymes. Therefore, this study aimed to delineate the variability in absolute quantification of polymorphic CYP2D6 drug-metabolizing enzyme and compare immunoblot and nano liquid chromatography coupled to mass spectrometry (nano-LC/MS) methods in identification and relative quantification of CYP2D6.1 and CYP2D6.2 allelic isozymes. Holoprotein content of in-house purified CYP2D6 isozymes was determined according to carbon monoxide difference spectrum, and total protein was quantified with bicinchoninic acid protein assay. Holoprotein/total CYP2D6 protein ratio was markedly higher for purified CYP2D6.1 (71.0%) than that calculated for CYP2D6.1 Supersomes (35.5%), resulting in distinct linear calibration range (0.05-0.50 versus 0.025-0.25 pmol) that was determined by densitometric analysis of immunoblot bands. Likewise, purified CYP2D6.2 and CYP2D6.10 and the CYP2D6.10 Supersomes all showed different holoprotein/total CYP2D6 protein ratios and distinct immunoblot linear calibration ranges. In contrast to immunoblot, nano-LC/MS readily distinguished CYP2D6.2 (R296C and S486T) from CYP2D6.1 by isoform-specific proteolytic peptides that contain the altered amino acid residues. In addition, relative quantitation of the two allelic isozymes was successfully achieved with label-free protein quantification, consistent with the nominated ratio. Because immunoblot and nano-LC/MS analyses measure total P450 protein (holoprotein and apoprotein) in a sample, complete understanding of holoprotein and apoprotein contents in P450 standards is desired toward reliable quantification. Our data also suggest that nano-LC/MS not only facilitates P450 quantitation but also provides genotypic information.

An NH2-terminal truncated cytochrome P450 CYP3A4 showing catalytic activity is present in the cytoplasm of human liver cells

Cytochrome P450 3A4 (CYP3A4), is the dominant human liver hemoprotein enzyme localized in the endoplasmic reticulum (ER), and is responsible for the metabolism of more than 50% of clinically relevant drugs. While we were studying CYP3A4 expression and activity in human liver, we found that anti-CYP3A4 antibody cross-reacted with a lower band in liver cytoplasmic fraction. We assessed the activities of CYP3A4 and its truncated form in the microsomal and cytoplasmic fraction, respectively. In the cytoplasmic fraction, truncated CYP3A4 showed catalytic activity when reconstituted with NADPH-cytochrome P-450 reductase and cytochrome b5. In order to determine which site was deleted in the truncated form in vitro, we transfected cells with N-terminal tagged or C-terminal tagged human CYP3A4 cDNA. The truncated CYP3A4 is the N-terminal deleted form and was present in the soluble cytoplasmic fraction. Our result shows, for the first time, that N-terminal truncated, catalytically active CYP3A4 is present principally in the cytoplasm of human liver cells.

The development of drug metabolism research as expressed in the publications of ASPET: Part 3, 1984-2008

The dramatic changes in drug metabolism research in the last 25 years are well documented in the publications of the American Society for Pharmacology and Experimental Therapeutics (ASPET). New analytical tools combined with modern molecular biological techniques have provided unprecedented access to the workings of the cell. A field that concentrated on only a handful of primary enzymes now has a list of hundreds in its purview. Genetic variation, environmental impact, and molecular diversity have all come under study in attempts to follow the fate of drugs and chemicals. Examples from ASPET journals will be used to illustrate the dramatic advancements in the field.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Investigations into the analysis and modeling of the cytochrome P450 cycle

The main focus of our research is to explore the fundamental dynamics of the mechanism of the cytochrome P450 (CYP450) cycle. For this purpose we propose a system-theoretical approach, a time-dependent metabolic control analysis (tdMCA), to the analysis and quantitative modeling of the CYP450 catalytic pathway. This provides theoretical enlightenment for us to assess the transient response of the system to perturbations. In addition, the robustness of the cycle has also been observed, where perturbations elicit very weak responses and the system quickly recovers to the steady state (in an average of 10(-5) s). The tdMCA also shows that the two electron transfers to the cycle have different impacts on the system, and the cycle is more sensitive to the first electron than to the second one. Knowing the dynamics of transient fluctuations, the robustness of the cycle, and the effects from the key interim steps, one has a deeper understanding of the catalytic mechanism of cytochrome P450.

Tolfenamic acid is a potent CYP1A2 inhibitor in vitro but does not interact in vivo: correction for protein binding is needed for data interpretation

OBJECTIVE

Our aim was to correlate the in vitro and in vivo CYP1A2 inhibition potential of tolfenamic acid, an NSAID highly (99.7%) bound to plasma proteins, to study the significance of protein binding of inhibitor in metabolic drug interactions.

METHODS

The effect of tolfenamic acid on CYP1A2 (phenacetin O-deethylation) was studied using human liver microsomes, with and without albumin (0-10 mg/ml). In a randomized, crossover study, 10 volunteers took 200 mg tolfenamic acid or placebo t.i.d. for 3 days. On day 2, a caffeine test was performed. On day 3, each ingested 4 mg of the CYP1A2 substrate tizanidine. Plasma tizanidine, its metabolites (M) and tolfenamic acid, and pharmacodynamic variables were measured.

RESULTS

Tolfenamic acid strongly inhibited phenacetin-O-deethylation in vitro (IC(50) 1.8 microM without albumin). Albumin decreased its inhibitory effect in a concentration-dependent manner; the IC(50) exceeded 100 microM with 10 mg/ml of albumin. Tolfenamic acid had no effect on the area under the concentration-time curve (AUC(0-oo)), peak concentration, time of peak concentration or half-life of tizanidine or M-3; only the AUC(0-oo) of secondary metabolite M-4 was slightly decreased (13%, P = 0.004). The caffeine test and the pharmacodynamic effects of tizanidine were unchanged.

CONCLUSIONS

Tolfenamic acid potently inhibits CYP1A2 in vitro when studied without albumin, but not in vivo. This apparent discrepancy is due to the high protein binding of tolfenamic acid. To avoid overestimation of the interaction potential, the inhibitory effect of highly albumin-bound compounds should also be studied in vitro with albumin, or their exact unbound plasma concentration should be used in predictions.

Disparity in holoprotein/apoprotein ratios of different standards used for immunoquantification of hepatic cytochrome P450 enzymes

An analysis of reported hepatic abundances of CYP3A4 and 3A5 indicated that values determined by immunoquantification using commercially available, unpurified recombinant enzymes as standards are significantly lower than those determined using purified enzymes or human liver microsomes characterized with lysosomal peptides (CYP3A4: mean 45 versus 121 pmol/mg protein, p < 0.01; CYP3A5: mean 28 versus 83 pmol/mg protein, p < 0.05). When immunoquantifying cytochromes P450 (P450s), it is assumed that the holoprotein (holo)/apoprotein ratio is the same in the samples and the standard. Estimates of holo/apoprotein ratios from data reported for a range of P450s purified from human liver and non-commercial recombinant systems indicated less than complete and variable heme coupling dependent on enzyme and system.

Molecular and functional characterization of drug-metabolizing enzymes and transporter expression in the novel spontaneously immortalized human hepatocyte line HC-04

In toxicological research, immortalized human hepatocytes provide a useful alternative to primary hepatocytes because interindividual variability in the expression of drug-metabolizing enzymes and drug transporters can largely be eliminated. However, it is essential that the cell line retain the original phenotype. The purpose of this study was to characterize a novel spontaneously immortalized human hepatocyte cell line, HC-04, with respect to the transcript and functional protein expression profile for the major drug-metabolizing enzymes and transmembrane transporters. HC-04 cells retained hepatocyte-specific function including albumin production and ornithine transcarbamoylase and glucose-6-phosphatase activity. Most of the major CYP forms were expressed at basal levels and responsive to inducing agents. In particular, CYP3A4 was expressed abundantly, and HC-04 cells were able to metabolize the CYP3A4 probe, midazolam, at a rate similar to primary human hepatocytes. Furthermore, the major human sulfotransferase and UDP-glucuronosyltransferase forms, as well as members of the ABC and SLC transporter superfamilies, nuclear receptors, and hepatic transcription factors were also expressed. HC-04 cells readily responded to standard hepatotoxicants that are dependent on CYP-mediated bioactivation, while another, tumor-derived cell line remained refractory to the drug challenge. Collectively, HC-04 cells provide a reliable, stable, and reproducible model for biomechanistic studies in drug toxicology.

Extending the capabilities of nature's most versatile catalysts: directed evolution of mammalian xenobiotic-metabolizing P450s

Cytochrome P450 enzymes are amongst the most versatile enzymatic catalysts known. The ability to introduce a single atom of oxygen into an organic substrate has led to the diversification and exploitation of these enzymes throughout nature. Nowhere is this versatility more apparent than in the mammalian liver, where P450 monooxygenases catalyze the metabolic clearance of innumerate drugs and other environmental chemicals. In addition to the aromatic and aliphatic hydroxylations, N- and O-dealkylations, and heteroatom oxidations that are common in drug metabolism, many more unusual reactions catalyzed by P450s have been discovered, including reductions, group transfers and other biotransformations not typically associated with monooxygenases. A research area that shows great potential for development over the next few decades is the directed evolution of P450s as biocatalysts. Mammalian xenobiotic-metabolizing P450s are especially well suited to such protein engineering due to their ability to interact with relatively wide ranges of substrates with marked differences in structure and physicochemical properties. Typical characteristics, such as the low turnover rates and poor coupling seen during the metabolism of xenobiotics, as well as the enzyme specificity towards particular substrates and reactions, can be improved by directed evolution. This mini-review will cover the fundamental enabling technologies required to successfully engineer P450s, examine the work done to date on the directed evolution of mammalian forms, and provide a perspective on what will be required for the successful implementation of engineered enzymes.