Functional Characterization of Single Nucleotide Polymorphisms with Amino Acid Substitution in CYP1A2, CYP2A6, and CYP2B6 Found in the Japanese Population

A modeling approach to evaluate the balance between bioactivation and detoxification of MeIQx in human hepatocytes

BACKGROUND

Heterocyclic aromatic amines (HAA) are environmental and food contaminants that are potentially carcinogenic for humans. 2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) is one of the most abundant HAA formed in cooked meat. MeIQx is metabolized by cytochrome P450 1A2 in the human liver into detoxificated and bioactivated products. Once bioactivated, MeIQx metabolites can lead to DNA adduct formation responsible for further genome instability.

METHODS

Using a computational approach, we developed a numerical model for MeIQx metabolism in the liver that predicts the MeIQx biotransformation into detoxification or bioactivation pathways according to the concentration of MeIQx.

RESULTS

Our results demonstrate that (1) the detoxification pathway predominates, (2) the ratio between detoxification and bioactivation pathways is not linear and shows a maximum at 10 µM of MeIQx in hepatocyte cell models, and (3) CYP1A2 is a key enzyme in the system that regulates the balance between bioactivation and detoxification. Our analysis suggests that such a ratio could be considered as an indicator of MeIQx genotoxicity at a low concentration of MeIQx.

CONCLUSIONS

Our model permits the investigation of the balance between bioactivation (i.e., DNA adduct formation pathway through the prediction of potential genotoxic compounds) and detoxification of MeIQx in order to predict the behaviour of this environmental contaminant in the human liver. It highlights the importance of complex regulations of enzyme competitions that should be taken into account in any further multi-organ models.

Prevalence of poor and rapid metabolizers of drugs metabolized by CYP2B6 in North Indian population residing in Indian national capital territory

Identification of poor and rapid metabolizers for the category of drugs metabolized by cytochrome P450 2B6 (CYP2B6) is important for understanding the differences in clinical responses of drugs metabolized by this enzyme. This study reports the prevalence of poor and rapid metabolizers in North Indian population residing in the National Capital Territory. The prevalence of poor and rapid metabolizers was determined in the target population for the category of drugs metabolized by CYP2B6 by measuring plasma bupropion, a drug metabolized by CYP2B6, and its metabolite. Bupropion (75 mg) was administered to 107 volunteers, and the drug (bupropion) and its metabolite (hydroxybupropion) were determined simultaneously by LCMS/MS in the plasma. CYP2B6 activity was measured as hydroxybupropion/bupropion ratio, and volunteers were categorized as rapid or poor metabolizers on the basis of cutoff value of log (hydroxybupropion/bupropion). Significant differences were observed between the mean metabolite/drug ratio of rapid metabolizers (Mean = 0.59) and poor metabolizers (Mean = 0.26) with p<0.0001. Results indicate that 20.56% individuals in the target population were poor metabolizers for the category of drugs metabolized by CYP2B6. Cutoff value defined in this study can be used as a tool for evaluating the status of CYP2B6 using bupropion as a probe drug. The baseline information would be clinically useful before administering the drugs metabolized by this isoform.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

PhRMA white paper on ADME pharmacogenomics

Pharmacogenomic (PGx) research on the absorption, distribution, metabolism, and excretion (ADME) properties of drugs has begun to have impact for both drug development and utilization. To provide a cross-industry perspective on the utility of ADME PGx, the Pharmaceutical Research and Manufacturers of America (PhRMA) conducted a survey of major pharmaceutical companies on their PGx practices and applications during 2003-2005. This white paper summarizes and interprets the results of the survey, highlights the contributions and applications of PGx by industrial scientists as reflected by original research publications, and discusses changes in drug labels that improve drug utilization by inclusion of PGx information. In addition, the paper includes a brief review on the clinically relevant genetic variants of drug-metabolizing enzymes and transporters most relevant to the pharmaceutical industry.

The Evolving Role of Drug Metabolism in Drug Discovery and Development

Drug metabolism in pharmaceutical research has traditionally focused on the well-defined aspects of absorption, distribution, metabolism and excretion, commonly-referred to ADME properties of a compound, particularly in the areas of metabolite identification, identification of drug metabolizing enzymes (DMEs) and associated metabolic pathways, and reaction mechanisms. This traditional emphasis was in part due to the limited scope of understanding and the unavailability of in vitro and in vivo tools with which to evaluate more complex properties and processes. However, advances over the past decade in separate but related fields such as pharmacogenetics, pharmacogenomics and drug transporters, have dramatically shifted the drug metabolism paradigm. For example, knowledge of the genetics and genomics of DMEs allows us to better understand and predict enzyme regulation and its effects on exogenous (pharmacokinetics) and endogenous pathways as well as biochemical processes (pharmacology). Advances in the transporter area have provided unprecedented insights into the role of transporter proteins in absorption, distribution, metabolism and excretion of drugs and their consequences with respect to clinical drug-drug and drug-endogenous substance interactions, toxicity and interindividual variability in pharmacokinetics. It is therefore essential that individuals involved in modern pharmaceutical research embrace a fully integrated approach and understanding of drug metabolism as is currently practiced. The intent of this review is to reexamine drug metabolism with respect to the traditional as well as current practices, with particular emphasis on the critical aspects of integrating chemistry and biology in the interpretation and application of metabolism data in pharmaceutical research.

Structure-function analysis of cytochromes P450 2B

In the last 4 years, breakthroughs were made in the field of P450 2B (CYP2B) structure-function through determination of one ligand-free and two inhibitor-bound X-ray crystal structures of CYP2B4, which revealed many of the structural features required for binding ligands of different size and shape. Large conformational changes of several plastic regions of CYP2B4 can dramatically reshape the active site of the enzyme to fit the size and shape of the bound ligand without perturbing the overall P450 fold. Solution biophysical studies using isothermal titration calorimetry (ITC) have revealed the large difference in the thermodynamic parameters of CYP2B4 in binding inhibitors of different ring chemistry and side chains. Other studies have revealed that the effects of site-specific mutations on steady-state kinetic parameters and mechanism-based inactivation are often substrate dependent. These findings agree with the structural data that the enzymes adopt different conformations to bind various ligands. Thus, the substrate specificity of an individual enzyme is determined not only by active site residues but also non-active site residues that modulate conformational changes that are important for substrate access and rearrangement of the active site to accommodate the bound substrate.

Prevalence of CYP2B6 alleles in malaria-endemic populations of West Africa and Papua New Guinea

OBJECTIVE

Cytochrome P450 2B6 (CYP2B6) is involved in the metabolism of artemisinin drugs, a novel series of antimalarials. Our aim was to analyze the prevalence of the most commonly observed CYP2B6 alleles in malaria-endemic populations of West Africa (WA) and Papua New Guinea (PNG).

METHODS

Using a post-PCR ligation detection reaction-fluorescent microsphere assay, frequencies of CYP2B6*1A, *2, *3, *4, *5, *6, *7, and *9 were determined in WA (n=166) and PNG (n=174). To compare with the results of previous studies, we also determined the allele frequencies in 291 North Americans of various ethnic groups.

RESULTS

Significant differences were observed between WA and PNG for the frequencies of alleles CYP2B6*1A (45% vs 33%, P = 0.003), *2 (4% vs. 0%, P<0.001), *6 (42% vs 62%, P<0.001), and *9 (8% vs 1%, P<0.001), and genotypes *1A/*9 (9% vs 0%, P<0.001) and *6/*6 (17% vs 43%, P<0.001). The frequencies of CYP2B6 genotypes in the populations were in Hardy-Weinberg equilibrium, except for PNG where an overall significant deficit of heterozygosity was observed (H (O)=0.431, H (E)=0.505, P=0.004). The allele frequencies in Asian-Americans and Caucasians-Americans were comparable to those documented for Japanese and Caucasian populations.

CONCLUSIONS

CYP2B6 variants, previously shown to affect metabolism of a variety of drugs, occur in WA and PNG, and there are significant genetic differences at the CYP2B6 locus in these populations. It may be important to determine if these differences alter the efficacy of artemisinin drugs.