CYP450 and Ethnicity

Chapter 16

CYP450 and Ethnicity Joseph McGraw Metabolism Laboratory, Department of Pharmaceutical and Administrative Sciences, Concordia University School of Pharmacy, USA

Chapter Outline 16.1 Overview 16.2  Variation: Importance of Race/Ethnicity 16.3  Variation: CYP450 Phenotyping 16.4 CYP450 Variation in Different Racial/Ethnic Populations—CYP450 Families 16.4.1  CYP 1 16.4.2 CYP2C8/9/19

323 326 327 329 329 330

16.1 OVERVIEW Racial/ethnic differences in drug response lay at the foundation of pharmacogenetics and pharmacogenomics. “Pharmacogenetics” refers to research involving heredity, drug pharmacokinetics, and pharmacologic response. It began with epidemiologic investigations of the underlying reasons for differences in response to, or inherent toxicity of, drug therapy. As with many medical studies performed in this early era, careful observation guided much of the research. Early studies implicated demographic factors such as race/ethnicity and metabolic factors such as drug clearance as important variables related to response and/or toxicity. In the 1950s, Clayman and Hockwald et al. observed different toxicity rates in the antimalarial primaquine among African Americans versus Caucasians [1,2]. In close chronologic proximity, Hughes and Vogel et al. showed different rates of isoniazid elimination due to slow and rapid individual acetylation phenotypes [3,4]. These early studies pointed to the fact that genetic factors were likely responsible for the observed differences in drug response or toxicity among different racial/ethnic groups. In 1959, Vogel coined the term pharmacogenetics [5]. Pharmacologic investigation is often categorized as pharmacokinetic or pharmacodynamic in nature. Much early

16.4.3 CYP2D6 333 16.4.4  CYP3A4 and CYP3A5 333 16.4.5  Other CYP450 Isoforms 335 16.5  Future Perspectives 337 References 337

work acknowledged genetic drivers of pharmacologic response, but focused on pharmacokinetics (also known as metabolic studies) instead of pharmacodynamics. This is likely due, in part, to the lack of molecular biology tools for pharmacodynamic study at the time. Additionally, there was a plethora of pharmacokinetic knowledge based on robust analytical techniques already in place by the 1950s. Indeed, receptor-mediated response was difficult to assess in the early days of pharmacology while robust chemical assessment tools were commonly available. The more recent revolution in molecular biology over the last few decades has made receptor isolation and functional characterization less of a monumental task. Other more recent advances in human genetics have spurred growth in research linking human genetics and pharmacology so that now we are attempting to achieve the goal of “personalized medicine.” In addition to researchers, the discipline of pharmacogenetics has garnered interest from the mainstream. The goal of the U.S. Food and Drug Administration (FDA) Critical Path Initiative 2010 is to “continue improving risk–benefit balance of approved drugs by enhancing drug product label language to include pharmacogenetics, where appropriate” [6]. Hepatic CYP450 enzymes were identified as major determinants of variability in drug metabolism early in pharmacogenetics, and their importance has not waivered [7].

Handbook of Pharmacogenomics and Stratified Medicine. http://dx.doi.org/10.1016/B978-0-12-386882-4.00016-5 © 2014 Elsevier Inc. All rights reserved.

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Genetic analyses of CYP450s began with identification of single-nucleotide polymorphisms (SNPs) in exons; they have expanded to identify other genetic variations such as promoter region polymorphisms and gene duplications. Pharmacokinetic differences are important determinants of variability. They were the focus of early studies and continue to be studied today. In pharmacokinetics, it is not surprising that one of the first families of oxidative enzymes characterized was the hepatic CYP450 enzyme family. Approximately 75% of commonly used drugs are CYP450 substrates, with CYP450-mediated oxidation responsible for approximately 80% of oxidative drug metabolism [8,9]. The variability of human CYP450 is complicated by the induction and inhibition of CYP450s by other drugs and xenobiotic exposures in addition to the target drug. Host variability in enzyme activity, combined with induction and inhibition, can lead to significant drug–drug interactions [10,11]. Many drug labels contain CYP450 related pharmacogenetic information. Tables are presented later in the chapter with regard to specific CYP450 isoforms and their respective substrates and/or inhibitors for which pharmacogenetic information is available in the drug package insert [12]. Hepatic CYP450s are oxidative enzymes responsible for most human oxidative xenobiotic metabolism. This metabolism is known as phase I metabolism and is often followed by conjugation of polar adducts in a process known as phase II metabolism. CYP450s participate in biosynthesis of endogenous compounds, such as steroids. However, these biosynthetic CYP450s share a specific relationship with their respective substrates and do not participate in xenobiotic metabolism. CYP450 isoforms with sequence homologies of 40% or more are categorized into families, with subfamilies based on homologies of 55% or more. Human CYP450 isoforms fall into 18 family groups with 44 subfamilies. There exist 57 sequenced human genes and 58 pseudogenes [13]. Pseudogenes are CYP450 genes that lack functionality. Xenobiotic-metabolizing CYP450 isoforms belong almost exclusively to CYP families 1, 2, and 3. They are more promiscuous in their substrate specificity compared to the other human CYPs that have specific endogenous ligands. The primary xenobiotic-metabolizing hepatic CYP450 isoforms are CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, and 3A5. These isoforms account for approximately 90% of CYP450-mediated drug metabolism [14]. CYP450s are heme-containing enzymes that catalyze different reactions, most commonly hydroxylation of their substrates. CYP450-mediated metabolism requires the cofactor NADPH and CYP450 reductase, and cytochrome b5 is required for maximal activity for several hepatic CYP450s. The CYP450 catalytic steps are still under investigation but are believed to proceed as follows: 1. Substrate binding near the heme iron in the ferric form. 2. Substrate binding displaces water from the heme iron.

PART | VI  Fundamental Pharmacogenomics

3. The heme is reduced to its ferrous form via an electron transferred from CYP450 reductase. 4. Oxygen binds the heme iron. 5. A second electron is transferred to the bound oxygen. 6. An unstable peroxy intermediate forms. 7. A water molecule is released, generating compound I, which is a reactive iron (IV) oxo (or ferryl) radical that abstracts a proton and electron from the substrate. 8. An iron (IV) hydroxide complex is formed (known as Compound II). 9. Compound II rapidly recombines with the substrate, yielding the hydroxylated product [15]. After fully cycling through the oxidative reaction, there is a net input of one oxygen molecule, one substrate molecule, two protons, and two electrons, accompanied by the net release of a hydroxylated product and one water molecule (Figure 16.1). Although CYP450s are expressed in every tissue of the body, hepatic CYP450s are the dominant CYP450 in regard to impact on the systemic pharmacokinetics of drugs and xenobiotics. CYP450 may be inducible, constitutively expressed, or both, and although much of its transcriptional apparatus has been delineated it is still an active area of research. The transcriptional apparatus of CYP450 varies by tissue along with expression. In the last few decades, research has shown that intestinal CYP450 is also a major determinant of xenobiotic pharmacokinetics, although to a lesser degree compared to hepatic enzymes. Genetic polymorphisms in human xenobiotic-metabolizing CYP450 isoforms provide a major source of variability in CYP450 activity. Polymorphisms may exist that result in alteration of the physical CYP450’s protein structure, gene copy number, or inducibility. An alteration in CYP450 protein structure may alter the protein’s substrate affinity and catalytic activity. Often, polymorphisms result in alteration in functional phenotype. CYP450 2D6 is a CYP450 for which an individual may harbor multiple copies of CYP450 alleles. If the allele copies code for functional enzymes, multiple copies confer an enhanced metabolic phenotype. Newer polymorphisms have been identified in the promoter region of various CYP450 isoforms which alter their inducibility. They may enhance or interrupt transcription-factor binding, resulting in either enhanced or diminished inducibility, respectively (Figure 16.2). Drug–drug interactions provide another source of variability through CYP450. Inhibition or induction of CYP450-mediated metabolism may occur because of concomitant exposure to multiple drugs or xenobiotics. Several mechanisms of CYP450 inhibition exist, but they all result in elevated exposure to drugs that would normally have been metabolized by the inhibited CYP450 isoform. Figure 16.3 illustrates the most common CYP450 inhibition mechanism via competitive inhibition. In this scenario, another

Chapter | 16 CYP450 and Ethnicity

325

R-OH

R-H

R-OH 9. Fe III

H-O-H

R*

1. Fe III

H-O-H

8. Fe IV

NADP+ -

R H

R-H

CYP450 catalytic cycle

=O

2. Fe III

7. Fe IV*

R-H 3. Fe II

6. Fe III (1 )

R-H

R-H

5.Fe III

-

4.Fe III

(2 )

O O

=

O O

-

+

H

POR NADPH

O O H

-

R H

e- -

H-O-H

O2

-

(1 )

NADP+

+

H

e-

POR or Cyt b5

NADPH

FIGURE 16.1  CYP450 catalytic cycle.

Translation of CYP450 protein

ID

Cytosol

TF

ID - Inducer TF - Transcription Factor

Nucleus Transcription of CYP450 mRNA

FIGURE 16.2  CYP450 induction.

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D

D

OH

D - Drug/Substrate

CYP450

D I

I - Inhibitor

CYP450

FIGURE 16.3  CYP450 inhibition.

drug prevents a CYP450 substrate from being metabolized by occupying the CYP450 isoform active site and blocking substrate access. Effects due to CYP450 inhibition may be further complicated by shunting the bulk of substrate metabolism toward another less efficient CYP450 isoform. Although such scenarios are currently being experienced by patients today, they likely go unnoticed because of their complexity. These types of scenarios will also increase in the future as the prevalence of polypharmacy increases. Of the approximately 75% of drugs that are CYP450 substrates, about 40% are metabolized by highly polymorphic CYP450 isoforms [9,16]. As drug manufacturers continue to develop new drugs, the percentage contribution of polymorphic enzymes involved in metabolism is expected to decline. CYP isoform-mediated metabolism characterization is a prerequisite for approval, and most manufacturers avoid new chemical entities that are polymorphic CYP450 substrates. The reported percentage of CYP450 in liver varies likely because of population differences such as racial/ethnic makeup. Also, researchers have reported different specificities of antibodies used in Western blots [17,18]. Many literature reports about CYP450 activity are quite different from one another. Often apoprotein variability is reported in terms of fold activity for many CYPs, such as CYP3A4; however, in vivo activity is probably less variable. Even the highly polymorphic CYP2C19 showed only 21-fold in vivo interindividual variability [19]. A report by Galetin et al. showed CYP3A4 that fold activity differences in healthy individuals was only 4–10-fold compared to a 40–50-fold variability in apoprotein expression shown in other studies [18]. Table 16.1 summarizes the percentage

contribution of hepatic CYP isoforms to total CYP450, the percentage of drugs metabolized by each isoform, and the in vivo (when reported) fold variability in isoform activity.

16.2  VARIATION: IMPORTANCE OF RACE/ETHNICITY Race is a social construct. However, individuals tend to choose mates and reproduce within perceived racial/ethnic groups, and thus allele frequencies between groups may differ. In addition, haplotype blocks may vary by racial ethnic group because of differences in recombination sites. Racial/ ethnic stratification may be necessary for the specific population genetic statistical approaches used by researchers. Hardy-Weinberg Equilibrium (HWE) calculations are a common way to assess whether a genotype is distributed as expected in a cohort. Genes not in HWE may be associated with a specific aberrant phenotype. Since HWE calculations assume random mating of individuals in a group, calculations of HWE require stratification of the different racial/ ethnic groups being studied. These groups are often stratified in investigations where racial/ethnic differences might confound the disease–genetic relationship. However, self-reporting of race/ethnicity can be inaccurate in terms of genetic composition. This has led some investigators to identify ancestral genotype when working with genetically heterogeneous groups [24]. It should be kept in mind that the total impact of race/ethnicity on overall genetic variation is quite minimal. Only 5–15% of genetic variation is attributable to ancestral populations living on different continents [25].

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TABLE 16.1 Variability in and Proportions of CYP450 in Human Liver CYP

Isoform Contribution to Total Liver CYP450 (%)

Drugs Metabolized by CYP450 Isoform (%)

Isoform Fold Interindividual ­Variability

1A2

8–13

10

40–130

2A6

4–13

3

20–>100

2B6

2–10

4

20–250

2C8

7–12

5

2–9

2C9

10–20

10–20

3–27

2C19

4–13

5–13

21–28

2D6

1–2

15–25

>1000

2E1

2–9

3

20

3A4

20–50

36–50

40–50 but 4–10-fold in healthy ­populations

3A5

0.2–2

Expected to be similar to 3A4

ND

ND = not reported. Source: Based on data reported in a number of references. See [14,16,18–23].

Regardless of type, investigators usually set up studies in similar ways. They attempt to create an “all things being equal” scenario except for the exposure, intervention, or genetic differences of interest. Since the genetic variation attributable to racial/ethnic groups is minimal, the genetic differences responsible for phenotypic differences among these groups become easier to identify. Small genetic differences may be harnessed to identify loci and genes of interest. Several researchers have used these differences in disease frequency to identify disease gene loci involved in complex diseases such as cancer [26]. Not surprisingly, genetic variability among racial/ethnic groups is not equal for all genes. Genes linked to environmental responses are more likely to vary by geography and thus differ by race/ethnicity. A major driver of such genetic variation is selection pressure. Genes that confer a survival advantage will be selected in a population residing in a particular environment. Xenobiotic-metabolizing CYP450s fall into this category because different CYP450 phenotypes can confer a survival advantage [27]. An example is a CYP450 metabolic phenotype that confers resistance to toxicity from ingestion of a plant endemic to a certain region [28]. Another example is a CYP450 phenotype that resists bioactivation of a fungal carcinogen endemic to tropical regions [29]. The observed racial/ethnic differences in both genotype and phenotype for xenobiotic-metabolizing CYP450s are likely due to ancestral environment–related positive selection. In the case of xenobiotic-metabolizing CYP450 genes, there are many examples of allele and haplotype frequency differences in racial/ethnic groups. Many reports have

thoroughly described such observations, which lie at the foundation of pharmacogenetics and pharmacogenomics. One recent example is given by the Pedersen et al., who investigated the frequency of the newly discovered CYP2C19 hyperfunctional allele CYP2C19*17 in Scandinavians and Ethiopians [30]. CYP2C19*17 appears to have higher frequency in Caucasians and some Eastern African groups. However, it was also found to be in linkage disequilibrium with the other fully functional CYP2C isoform alleles for 2C8 and 2C9. In other words, the hyperfunctional CYP2C19 allele is more likely to be found on the same chromosome as the wild-type CYP2C8 and CYP2C9 alleles. In haplotype predictions, 99.7% of Scandinavian CYP 2C19*17 carriers also possess CYP2C8*1 and CYP2C9*1 on the same haplotype. Out of 10 modeled haplotypes, the aforementioned haplotype was the second most frequent (19% frequency), following wild-type CYP2C8/9/19. The selective advantage of enhanced CYP2C function is unknown, but is surely not linked to modern drug exposures.

16.3  VARIATION: CYP450 PHENOTYPING Assessment of human hepatic CYP450 activity involves both invasive and noninvasive methods. However, each method has inherent benefits and drawbacks. Invasive methods usually involve biopsy of hepatic tissue followed by in vitro reconstitution-system assays to assess drug-metabolizing capacity. Reconstituted metabolism systems used to identify CYP450 activity include harvested hepatocytes, hepatic tissue slices, or microsomal preparations from liver biopsy tissue. Besides being highly invasive, these approaches are

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subject to other problems in interpretation. Cultured hepatocytes begin to lose their ability to express CYP450 soon after being extracted. Microsomes maintain their metabolic activity for long periods of time, but require significant amounts of tissue for testing. Many reconstitution systems do not reflect variability due to other physiologic parameters that affect metabolism, such as hepatic blood flow. In vitro-to-in vivo extrapolation models are emerging that incorporate in vitro data on drug-metabolism reconstitution systems into highly predictive pharmacokinetic models. These models also incorporate physiologically based pharmacokinetic model parameters, such as hepatic blood flow, thus compensating for some of the limitations of reconstitution systems. Substrate probes are drugs that undergo hepatic CYP450–mediated metabolism selectively via one CYP450 isoform. Once the probe is administered, elimination of specific metabolites may be monitored to characterize CYP isoform activity. Substrate probes can be used in both in vitro and in vivo reconstitution systems. Avoidance of the need for invasive techniques on human subjects has been achieved by the use of animals in pharmacokinetic models. However, many animal species have dramatically different CYP450 activity at the isoform level. Exceptions include primates with robust CYP450-isoform sequence homology to humans such as cynomolgus monkeys. However, even these models do not replicate human pharmacokinetics perfectly for all drugs metabolized by certain isoform combinations. Currently, pooled human hepatic microsomes are readily available from vendors such as BD Biosciences; these pools are created from large numbers of individuals (e.g., n = 150), so they are reflective of population-level metabolism. Recombinant technologies allow transfection of human genes into other cell systems, such as BD Biosciences’ baculovirus-transfected insect cells. These transfections allow for investigation of metabolism at the individual isoform level, providing a valuable tool for investigations of individual isoform–mediated metabolism and its contribution to overall metabolism. Other tools for identifying individual-isoform metabolism are CYP450 isoform–specific monoclonal antibodies, drug phenotyping probes, specific inhibitors, and specific inducers. Drug-phenotyping probes were the first tools used to identify isoform-specific metabolism in vivo, and they are still effective tools. Before our current knowledge of isoform-specific CYP450 metabolism, pharmacokinetic studies showed differential metabolism among individuals due to genetic differences. Since these differences were difficult to observe if the metabolic clearance was attributable to multiple enzymes, investigators suspected a specific relationship between the substrate and the liver enzyme that metabolized it. Examples include Smith, Echelbaum, as well as Kupfer and Wendlunds’ discovery of variability in the respective metabolic phenotypes of debrisoquine, sparteine,

PART | VI  Fundamental Pharmacogenomics

and mephenytoin due to polymorphisms in drug oxidative metabolic enzymes [3]. Variable activity for both debrisoquine and sparteine would later be attributed to CYP2D6 polymorphisms, while variability in mephenytoin activity is now known to be due to polymorphisms in CYP2C19. Not surprisingly, CYP2D6 and CYP2C19 are two of the most variable CYP450 isoforms expressed in human liver. Early studies showed that drug clearance serves as a noninvasive mechanism to probe enzyme function. Hence, these drugs were among the first drug “probes” of CYP450 activity. In 2004, Zhou et al. created a list of recommended CYP450 substrate probes after reviewing the literature and recommendations from the Committee for Proprietary Medicinal Products (CPMP), the FDA, the online drug interaction table by Flockhart, the European Federation for Pharmaceutical Sciences (EUFEPS), and the American Association of Pharmaceutical Sciences (AAPS) [16]. Establishing in vivo metabolic phenotypes is not straightforward. There is no strict agreement on phenotype nomenclature, nor are there clearly defined phenotype definitions. Phenotype is often assessed by administering different substrate probes and reporting metabolic ratios of different metabolites in different matrices at different times, using different sampling methods (extraction and detection). For example, one study might assess CYP1A2 phenotype by administering caffeine, detecting it and its major metabolite, paraxanthine, in urine obtained 12 h after dosing, extracting the parent and metabolite using solid-phase extraction, and quantifying them using HPLC with UV detection. However, another study might administer caffeine and then identify it, along with paraxanthine and other metabolites in plasma, 6 h after dosing, using liquid/liquid extraction and detecting it using HPLC MS/MS. Table 16.2 shows the major parameters that may vary when performing in vivo substrate-probe CYP450-phenotyping studies. The FDA has provided recommendations to help manufacturers with pharmacogenomic submissions accompanying new drug applications, but it does not define a systematic approach for all CYP450 and it focuses on in vitro approaches [32]. Two reviews propose steps for validation of probe assays and cocktail assays [33,34]. In some studies, phenotypic groups are designated by different names or the number of phenotypic categories varies. According to several researchers, the distribution of metabolic phenotype must exhibit multiple modes if variant alleles that impact activity are present. Also, a phenotype directly determined by alleles that code for enzymes with different activity (such as wildtype and loss-of-function phenotype) should result in a histogram with at least two modes (a.k.a bimodal distribution). However, one could argue that phenotype is determined by a mixture of epigenetic, environmental, and multigenic factors. In this setting, the impact of known CYP450 variant alleles on metabolic phenotype may not drive a population into a clearly defined multi-modal distribution.

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TABLE 16.2 Major Parameters in Phenotype Measurement Substrate Probe

Metabolite(s) Measured

Matrix

Time (h)

Extraction Method

Sample Detection Method

Multiple probes for each CYP450 Isoform (e.g., caffeine or phenacetin for CYP1A2)

Most abundant metabolite (e.g., paraxanthine for caffeine)

Urine

Often varies by study (e.g., 4, 6, 8, 10, 12)

Solid-phase

HPLC with different detection (ex. UV)

Several metabolites (e.g. paraxanthine, and 1,7 dimethyluracil)

Saliva

Liquid/liquid extraction

GC with different detection (ex. FID)

All identified ­metabolites

Serum

On-column cleanup

GC MS or GC MS/ MS

Plasma

Direct injection

HPLC MS or HPLC MS/MS

Source: Based on data reported in Carillo et al. [31].

Many studies performed on the highly variable CYP2D6 and CYP2C19 describe four different phenotypic groups. However, it must be kept in mind that these groups are predicted genotypes based on known polymorphic alleles with altered function. The currently used names for the phenotypic groups for CYP2D6 and CYP2C19 are: Poor Metabolizer, Intermediate Metabolizer, Extensive Metabolizer, and Rapid Metabolizer. Other genotype category names used include Intermediate Metabolizer, Poor/ Rapid Heterozygotes, Rapid Heterozygotes, and UltraRapid Metabolizer. Pharmacokinetic studies with various substrates have revealed either an additive or dominant mechanism of inheritance for the metabolizer traits. Phenotype is thus determined by the sum of allele functions or by carriage of an allele with profoundly altered activity, such as a lack of non-functional allele or a hyperfunctional allele. Initial pharmacokinetic and platelet-response studies with clopidogrel suggested that mixed carriers of the CYP2C19*2 (a loss of function allele) and CYP2C19*17 allele (a gain of function allele) have a metabolizer phenotype similar to homozygous wild type (*1) carriers [35]. However, later studies show that the phenotype manifests as lower activity than homozygous wild type carriers [36]. Figure 16.4 gives an example of the frequency of different genotypes for CYP2C19 along with respective predicted metabolizer phenotype for each. Readers should keep in mind that there is still significant variability of true phenotype in each predicted phenotype category. This is due to other factors such as induction, drug-drug interactions, or other genes that may impact pharmacokinetics such as transporters.

16.4  CYP450 VARIATION IN DIFFERENT RACIAL/ETHNIC POPULATIONS—CYP450 FAMILIES Cytochrome P450 variations occur in different ethnic ­populations for most CYP450 isoforms. Clinically ­relevant variants are discussed to assist in identification of the impact of variants on the metabolism of specific drugs or ­xenobiotics when applied across many ethnic groups. Tables of drugs follow discussions of each CYP isoform. The drugs have either U.S. or European labeling comments and/or dosing ­recommendations because of pharmacogenetic-related impacts on drug pharmacokinetics (see Tables 16.3–16.6).

16.4.1  CYP 1 The human CYP 1 family includes CYP1A1, 1A2, and 1B1. CYP1A1 and CYP1B1 are important enzymes involved in xenobiotic metabolism but do not participate in much drug metabolism. Both are highly induced by Aryl Hydrocarbon receptor (AHR) binding ligands, such as dioxin, while also acting as metabolic enzymes for many of these substrates. CYP1A1 and CYP1B1 are expressed mostly in nonhepatic tissues which is why they are seldom studied in regards to drug metabolism [39]. The primary hepatic CYP 1 family enzyme is CYP1A2, which metabolizes approximately 10% of drugs including caffeine, ropinirole, theophylline, and tizanidine along with several important psychiatric medications including clozapine, haloperidol, olanzapine, and trazodone [40,41]. CYP1A2 is constitutively expressed, inducible, and subject to epigenetic regulation [42]. Drug inducers of CYP1A2 include albendazole, lansoprazole, omeprazole,

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25

Extensive (EM)

20 Poor metabolizers Intermediate (IM) (PM)

Percent

15 Rapid (RM) / Ultrarapid (UM) 10

5

0

OMP log ratio FIGURE 16.4  Histogram of omeprazole metabolic ratios and proposed phenotype groups. The histogram is based on a log [omeprazole]/[5-OH omeprazole] ratio measured 4 hr after a 30 mg CR omeprazole dose in saliva was extracted via molecular sieve filtration followed by HPLC MS/MS analysis (unpublished data, McGraw and Bichler 2013). CYP2C19 ultrarapid metabolizers possess two gain-of-function alleles (*17/*17); rapid heterozygotes harbor one *1 allele and one gain-of-function allele (for example, *1/*17); poor/rapid heterozygotes harbor one reduced-function and one gain-of-function allele (for example, *2/*17); extensive metabolizers are homozygous for the nonvariant *1 allele (*1/*1); intermediate metabolizers possess one *1 allele and one reduced-function allele (for example, *1/*2); and poor metabolizers harbor two reduced-function alleles (for example, *2/*2). Source: Adapted from Scott et al. [36].

TABLE 16.3 US and/or European Drugs with CYP1A2 Related Pharmacogenetic Labeling Comments or Dosing Recommendations CYP1A2-US Fluvoxaminea

Olanzapinea

CYP1A2-Europe None

None

a

U.S. and/or European labeling recommendation. Source: Based on data reported in Whirl-Carrillo et al. [50].

and primaquine while contraceptives, fluvoxamine, and several quinolones are important inhibitors [40,41]. CYP1A2 activity varies widely (40–130-fold), due in part to its induction via several commonly consumed chemical exposures such as caffeine, charcoal barbecued meats, cigarette smoke, omeprazole, and oral contraceptives [43]. Genetics appear to be responsible for approximately 35% of variability in CYP1A2 activity [44]. CYP1A2 ­phenotypes are often categorized as extensive and poor while some studies report an ultrarapid phenotype. Metabolic ratio ­distributions have been described as bimodal

or ­trimodal [45]. However, most studies do not report ­phenotypic groups beyond poor and normal (i.e., extensive) ­metabolizers. The frequency of poor metabolizer-phenotype status varies in different racial/ethnic groups. The two major haplotypes associated with it are known as CYP1A2*1C (rs2069514) and CYP1A2*1K (rs2069526, rs12720461, and rs762551). The CYP1A2*1C allele is commonly found in Asians (frequency ∼25%) and is associated with decreased function, while the CYP1A2*1K allele (Asian frequency 0–4%) is associated with lower ­ inducibility. Examples of poor metabolizer frequency in different populations are Australians (5%), Chinese (5%), and ­ Japanese (14%). Overall, Asian and African populations have lower CYP1A2 activity compared to Caucasians [18]. The CYP1A2*1F allele (rs762551) appears to enhance the inducer function of CYP1A2 activity in carriers exposed to inducers such as cigarette smoke or omeprazole [46,47]. It is highly prevalent with a similar distribution across different racial/ethnic groups (frequency 60–66%) [47–49].

16.4.2 CYP2C8/9/19 Human CYP 2C-family enzymes share more than 80% sequence homology and exhibit both overlapping and

Chapter | 16 CYP450 and Ethnicity

331

TABLE 16.4 US and/or European Drugs with CYP2C Family Related Pharmacogenetic Labeling Comments or Dosing Recommendations CYP2C8-US None

None

CYP2C8-Europe None

None

CYP2C9-US Celecoxiba

Flurbiprofena

Fluvoxaminea

Glimepirideb

Phenytoinb

Tolbutamideb

Warfarinc

Gliclazideb

Glibenclamideb

Phenprocoumonb

Amitriptyllineb

Carisoprodola

Citalopramc

Clobazama

Clomipramineb

Clopidogrelc

Diazepama

Doxepinb

Drospirenonea & ethinyl estradiol

Escitalopramb

Esomeprazolec

Fluvoxaminea

Imipramineb

Lansoprazolec

Modafinila

Nelfinavira

Omeprazolec

Pantoprazolec

Prasugrela

Rabeprazolec

Sertralineb

Voriconazolec

CYP2C9-Europe Acenocoumarolb

CYP2C19-US

CYP2C19-Europe Moclobemideb a

U.S. and/or European labeling recommendation. bDosing recommendation. cLabeling and Dosing recommendations. Source: Based on data reported in Whirl-Carrillo et al. [50].

distinctive substrate specificity. They represent about 20% of liver CYP450 and metabolize about 25% of clinical drugs. The least significant drug-metabolizing CYP2C isoform in terms of expression, CYP2C8, represents about 7% of total hepatic CYP450 and metabolizes around 5% of clinical drugs, including cerivastatin, several NSAIDs, paclitaxel, repaglinide, and thiazolidinediones. It is inhibited by gemfibrozil [51]. Several CYP2C8 polymorphisms believed to be clinically relevant include CYP2C8*1B, *2, *3, *4, and *5. Carriage of CYP2C8*2, a fairly prevalent allele, is believed to result clinically in impaired metabolic activity [52]. Conflicting results in the identification of CYP2C8 phenotype may be due to confounding factors such as participation of other enzymes (e.g., overlapping substrate specificity with CYP2C9) and linkage disequilibrium. Haplotype analyses may help alleviate these problems. The CYP2C8*3 allele exhibits lower activity in vitro, but has been associated with higher clearance of CYP2C8

substrates in vivo. Rodriguez-Antona et al. showed increased clearance in the CYP2C8*3 allele containing haplotype carriers (haplotype B or haplotype D). Paclitaxel clearance was higher in haplotype B carriers, and repaglinide (a thiazolidinedione) clearance was higher in haplotype B or haplotype D carriers who were also heterozygous SLCO1B1 521T/T carriers (SLCO1B1 carriers have increased repaglinide uptake) [53]. Both drugs were metabolized to a lesser extent in the CYP2C8*1B-containing haplotype C carriers. The activity of the CYP2C8*4 allele is unknown, and the CYP2C8*5, *7, and *11 alleles are low frequency loss of function alleles. The CYP2C9 alleles with impaired function observed clinically are 2C9*2, 2C9*3, and 2C9*5. CYP2C9 and 2C19 share 92% sequence homology [54]. CYP2C9 substrates include the angiotensin receptor antagonist losartan, the anticoagulant warfarin, the anticonvulsant phenytoin, the antidepressant fluvoxamine, the anti-inflammatories (celecoxib, diclofenac, flurbiprofen, and ibuprofen), fluvastatin,

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TABLE 16.5 US and/or European Drugs with CYP 2D6 Related Pharmacogenetic Labeling Comments or Dosing Recommendations CYP2D6-US Amitriptylinec

Aripiprazolec

Atomoxetinec

Carvedilolc

Citaloprama

Clomipraminec

Clozapinec

Codeinec

Desipraminec

Dextromethorphana

Doxepinc

Duloxetineb

Flecainideb

Fluoxetine and Olanzapinea

Galantaminea

Geftiniba

Haloperidolb

Iloperidonea

Imipraminec

Metoprololc

Mirtazapineb

Modafinila

Nortriptylinec

Olanzapineb

Oxycodoneb

Paroxetinec

Perphenazinea

Pimozidea

Propafenonec

Propranolola

Protriptylinea

Quinidinea

Risperidonec

Tamoxifenb

Terbinafinea

Tetrabenazinea

Thioridazinea

Timolola

Tiotropiuma

Tolterodinea

Tramadolb

Tramadol & Acetaminophena

Trimipraminec

Venlafaxinec

Fluvoxaminea

CYP2D6-Europe Cevimelinea

Flupenthixolb

Zuclopenthixolb

a

U.S. and/or European labeling recommendation. bDosing recommendation. cLabeling and Dosing recommendations.

Source: Based on data reported in Whirl-Carrillo et al. [50].

TABLE 16.6 US and/or European Drugs with CYP3A Family Related Pharmacogenetic Labeling Comments or Dosing Recommendations CYP3A4-US Aripiprazolea

Fluvoxaminea

Ticagrelora CYP3A4-Europe See Above

CYP3A5-US Tacrolimusa CYP3A5-Europe See above a

U.S. and/or European labeling recommendation. Source: Based on data reported in Whirl-Carrillo et al. [50].

Gefitiniba

Nelfinavira

Chapter | 16 CYP450 and Ethnicity

nateglinide, and the sulfonylureas (glyburide, glibenclamide, glimepiride, and tolbutamide). Inhibitors of CYP2C9 include amiodarone, benzbromarone, bucolome, fluconazole, miconazole, and sulphaphenazole; it is induced by rifampicin [51]. Significant interest has focused on the effect of CYP2C9 polymorphism on warfarin activity and dosing. Limdi et al. found an association between CYP2C9 variant allele carriers and lower warfarin dosing in Caucasian Americans, but not African Americans. They also found a significantly higher prevalence of variant genotypes in Caucasians versus African Americans (29.82% vs. 9.73%) despite the *5, *6, and *11 variants observed only in African Americans [55]. Scott et al. recently found a CYP2C9 allele labeled *8 with possibly lower functionality which may have a prevalence close to 9% in African Americans. They noted that addition of this allele to screening panels makes warfarin dose prediction more accurate in African Americans [56]. CYP2C19 is highly polymorphic, with at least 24 variant alleles of which many have no enzymatic activity. Important substrates for CYP2C19 include several benzodiazepines (diazepam, etizolam), the antiplatelet agent clopidogrel, proton pump inhibitors (lansoprazole, omeprazole, and rabeprazole), phenytoin, sertraline, and voriconazole. Inhibitors include ticlopidine, fluvoxamine, and voriconazole, while ritonavir, rifampicin, and rifabutin act as inducers [51]. The variants *2–*8 have been shown to be inactive [57]. They also distribute differently across racial/ethnic groups. Unlike other CYP450s, Caucasians and Africans share similar overall frequencies of poor metabolizer (PM) phenotype (1–8%), while Asians show a higher prevalence of PMs (13–23%) [21]. Phenotypes have been designated as extensive (two functional alleles), intermediate (one functional/one dysfunctional), poor (two loss-of-function alleles) and rapid (one or two gain-of-function alleles) (Table 16.7). The lack of functional CYP2C19 is a risk factor for adverse cardiovascular outcomes in patients taking clopidogrel because clopidogrel must be bioactivated by CYP2C19 for activity [35–37].

16.4.3 CYP2D6 CYP2D6 is responsible for metabolizing a number of important drugs containing amine functional groups, including members of the following psychotropic classes: anticholinergics/parasympathomimetics, antidepressants and monoamine modulating drugs (for example, serotonin 5-HT3 receptor antagonists, monoamine oxidase inhibitors (MAOIs), serotonin reuptake inhibitors (SSRIs), and tricyclic antidepressants); antipsychotics (typical and atypical); opiates; and synthetic opiate derivatives. CYP2D6 also metabolizes several cardiac drugs from antiarrhythmic classes as well as beta blockers; some antifungals; and the antiestrogen tamoxifen. It is inhibited by celecoxib,

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cinacalcet, quinidine, several SSRIs (paroxetine and fluoxetine), and terbinafine [51]. A noteworthy attribute of CYP2D6 is that it is not inducible; rather, it is highly polymorphic with more than 100 variant alleles and ∼200-fold variability in the metabolism of at least 100 drugs [58–61]. An attribute specific to CYP2D6 in comparison to other CYP450 enzymes is the presence of gene duplications that may confer an ultrarapid metabolizer phenotype. These polymorphisms are designated CYP2D6*(gene variant)XN, where XN refers to the number of gene copies. For example, CYP2D6*1X2 represents two copies of CYP2D6*1. The CYP2D6*1XN, *2XN, and *35XN alleles confer enhanced metabolic phenotype, while CYP2D6*17XN and CYP2D6*41XN show decreased activity and CYP2D6*4XN alleles show none. Four potential CYP2D6 phenotypic subgroups exist. These groups are usually defined by the respective number of their functional alleles: ultrarapid (3), extensive (2), intermediate (1), and poor metabolizers (0). Most CYP2D6 polymorphisms result in an allele that lacks metabolic activity. However, the prevalence of poor metabolizer phenotypes varies by racial/ethnic group: Asians (∼1%), Caucasians (5–10%), and Africans (0–19%) [58].

16.4.4  CYP3A4 and CYP3A5 The human CYP3A family includes CYP3A4, 3A5, 3A7, and 3A43, with CYP3A4 being the most abundant CYP450 isoform, representing up to 50% of liver CYP450. CYP3A4 represents approximately 20–50% of CYP450 content in liver and is responsible for 36–50% of drug metabolism. CYP3A5 is a closely related homologue that represents only 2% of CYP3A in Caucasian livers [9,22]. Examples of CYP3A4 substrates include many members of several drug classes, including benzodiazepines, calcium channel blockers, corticosteroids, ergot alkaloids, statins, transplant medications, and vinca alkaloids. Major inhibitors include azole antifungals, calcium channel blockers (diltiazem, verapamil), cimetidine, grapefruit juice, several macrolide antibiotics (erythromycin, clarithromycin, telithromycin, troleandomycin), nefazodone, and protease inhibitors. Inducers include rifampicin, rifabutin, ­phenobarbital, phenytoin, carbamazepine, efavirenz, and St. John’s Wort [41]. Intestinal CYP3A4 also makes a major contribution to the metabolism of orally administered drugs because it contributes to first-pass metabolism [8]. Constitutive CYP3A4 variability is estimated at about 5-fold; however, illness, inhibition, and induction-related interactions can enhance variability up to 400-fold [8,18]. Other estimates across different populations project 40–50-fold variability [16]. Some estimates characterize variability in terms of apoprotein expression, which may overestimate in vivo variability for healthy subjects not experiencing drug

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PART | VI  Fundamental Pharmacogenomics

TABLE 16.7 CYP2C19 - Example of Genotypes, Frequency, and Predicted Phenotype CYP 2C19 Genotype Normal Allele (*1)

Major Phenotypic Categories

Metabolizer Phenotype

Prevalence Range (%)

Poor

Poor

7(±8)

Intermediate

Intermediate

34(±17)

Poor/Rapid ­Heterozygotes

4(±4)

Extensive

Extensive

51(±15)

X,X

Rapid

Rapid ­Heterozygotes

18(±17)

X

Ultra-rapid

2(±4)

Loss of Function (*2, *3, …)

Gain of Function (*17)

X,X X

X X

X

X X,X

Source: Based on data reported in the following references [30,36–38]. Note: 2C19 East Asian prevalence of poor metabolizer phenotype may be as high as 24%.

interactions. Population CYP3A4 metabolic phenotypes follow a unimodal distribution in most studies, which has been presented as evidence that CYP3A4 does not have a genetically determined poor or rapid metabolizer phenotype [45]. However, genetic contributions to variability may be obscured when other factors, such as enzyme induction via inducer exposure, are accounted for. CYP3A4 and CYP3A5 share significant sequence homology and almost identical substrate specificity, but exhibit somewhat differing metabolic rates. CYP3A enzymes have a large active site and accommodate a great variety of substrates. Because of the polygomous nature of the active sites, substrates, including substrate probes, are often metabolized by both CYP3A4 and CYP3A5, but a few substrate probes appear to undergo selective metabolism. Erythromycin and quinidine are more specific CYP3A4 probes, while CYP3A5 preferentially metabolizes alprazolam alpha-hydroxylation, tacrolimus, and vincristine [62]. Caucasians exhibit lower CYP3A5 activity as a group because of a high prevalence of the splice variant, CYP3A5*3 [62], which is a highly prevalent polymorphism in all ethnic groups and causes the absence of functional CYP3A5 protein. CYP3A5*3 prevalence estimates in different groups are as follows: African Americans (32%), Caucasians (90–93%), East Asians (73%), Hispanics (65%), and South Asians (60%) [63–65]. Caucasians show enhanced CYP3A4-mediated metabolism in comparison to those of African ancestry [66]. Conversely, most Caucasians carry the loss-of-function CYP3A5*3 polymorphism. In most individuals, CYP3A5 has a minimal impact, as it represents only 2% of hepatic

CYP450 in Caucasians. However, it may represent up to 50% of hepatic and intestinal CYP450 in some individuals [67]. A proposed cause for the differences in CYP450 activity may be the differential activity of CYP450 cofactors such as NADPH-dependent CYP450 reductase and cytochrome b5 (see Figure 16.1). These cofactors are variable in different groups and may contribute to different CYP3A4 and CYP3A5 metabolic activities [68–70]. However, Elens et al. found carriage of the CYP450 oxidoreductase variant POR*28 was associated with lower in vivo CYP3A5 activity but had no impact on CYP3A4 activity [63]. They assessed CYP3A4/5 activity by administering midazolam, a nonspecific CYP3A4/5 probe, while assessing CYP3A4 specific activity with erythromycin, a specific CYP3A4 probe. They concluded that differences in CYP3A-mediated metabolism in Caucasians versus other racial/ethnic groups are not likely explained via CYP3A5-mediated metabolism or cofactor variability. Another major source of variability is interaction between CYP3A4/5 and drug transporters. Ogasawar et al. found that CYP3A5 expressors that are also in the MRP2 (transporter) high-activity group showed up to 2.3-fold lower dose– normalized trough tacrolimus concentrations. However, the researchers found no relationship with CYP3A4 variants [71]. As with most proteins and enzymes studied in pharmacogenetics, new functionally relevant polymorphisms and haplotypes of CYP450 enzymes are being identified. CYP3A4*1B is a CYP3A4 polymorphism that may alter CYP3A4 expression. It is an A→G transition at the −293 position in the gene promoter region that is highly prevalent in African Americans but not in Caucasians. The

Chapter | 16 CYP450 and Ethnicity

CYP3A4*1B prevalence among different ethnic groups is Africans (76%), Caucasians (2–9.6%), Chinese and Taiwanese (0%), Hispanic Americans (9.3–11%), and African Americans (35–67%). Although it may provide much needed answers, the clinical impact of CYP3A4*1B is still under debate [72]. Other, yet to be discovered, candidate polymorphisms may explain differential CYP450 activity in different racial/ethnic groups as they are identified. A newly identified SNP, CYP3A4*20, results in a premature stop codon, thus coding for a truncated CYP3A with no activity [22]. This SNP has a relatively high prevalence among differing ethnic groups: Caucasian (6%), African American (26%), and Chinese (22%). The CYP3A4*22 variant allele is an intron 6 SNP found to influence RNA expression and statin, tacrolimus, and cyclosporin dose requirements [63,64,72]. Several other novel variants were also recently identified in a South African cohort, including CYP3A4*24, which is predicted to affect function; it was found at prevalences of 10.3%, 3.1%, and 3.2%, respectively, in Khoisan, Xhosa, and mixed-ancestry individuals [72]. More research is necessary before a clear understanding of the mechanism behind racial/ethnic differences in CYP3A4/5 is achieved. In addition to racial/ethnic metabolism differences, it should be noted that women have higher CYP3A activity than men, but this is offset by mens’ larger hepatic mass [73,74].

16.4.5  Other CYP450 Isoforms The CYP 2 gene cluster encodes several different CYP450 isoforms (CYP2A6, 2A7, 2A13, 2B6, 2F1, and 2S1) that are not often the focus of drug metabolism studies. Several factors explain this: they are minor contributors to hepatic CYP450 content, they have few known drug substrates, and/or they are expressed only in extrahepatic tissues. Although these human xenobiotic CYP450 enzymes are not highly expressed in liver, most are distributed in other tissues throughout the body. Pavek and Dvorak provide an excellent review of the distribution of these enzymes in extra-hepatic tissues [75]. Although CYP450s are not major sources of drug metabolism, they are involved in metabolism of other xenobiotics. Phenotypic differences in xenobiotic metabolism and differential metabolism of environmental xenobiotics, such as occupational and environmental contaminants, have also been reported among different racial/ethnic groups. Research regarding extrahepatic CYP450 metabolism has led to the belief that these isoforms may play a role in susceptibility and risk for diseases such as cancer [39]. CYP2A6 is an important human xenobiotic-metabolizing enzyme. However, it metabolizes only ∼3% of drugs and is not the primary metabolic enzyme for most of these, likely because of its small active site and low overall

335

hepatic expression. CYP2A6 is primarily expressed in liver (1–10% of liver CYP450) and nasal mucosa. It bioactivates several known procarcinogens such as aflatoxin B1 and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (i.e., nicotine-derived nitrosamine ketone, NNK), while also acting as the major CYP isoform responsible for nicotine and tegafur metabolism. CYP2A6 inhibitors include isoniazid and methoxsalen as well as several endogenous catecholamines and steroids, while it is induced by phenobarbital and rifampicin [76]. CYP2A6*1A and *1B alleles are the most common CYP2A6 variants. The prevalence of CYP2A6*1A is 40% in Asians and 66% in Caucasians, whereas CYP2A6*1B is found in 40% of Asians and 30% of Caucasians [77,78]. More recently, four new dysfunctional alleles (*4G, *4H, *1B4, and *1L) have been discovered in individuals of African ancestry, with a combined prevalence of 7.3% [79,80]. African ancestry–specific polymorphisms are important because they help to explain the lower nicotine clearance observed in those of African descent when compared to those of Caucasian descent [81–83]. Nicotine metabolism differences are also important because African Americans suffer higher rates of smoking-related diseases [80]. As previously mentioned, CYP2A6 is important in the metabolism of environmental xenobiotics. Polychlorinated biphenyls (PCBs) are environmental contaminants that were banned from use in the 1970s but are still found in human tissues today. CYP2A6 was found to specifically metabolize one PCB congener, known as PCB congener 101, which is highly prevalent in the environment but should be rapidly metabolized in most humans [84]. PCB 101 was found at higher serum concentrations in African American women reporting exclusive African ancestry versus African American women of mixed race/ethnicity [85]. It was believed that higher PCB 101 levels may have been a marker of poor metabolism linked to those of African descent. This may be an environmental correlate to the lower nicotine metabolism in carriers of loss-of-function CYP2A6 polymorphisms. CYP2B6 is highly polymorphic and highly inducible (20–250-fold variation), with 53 allelic variants described thus far [13,86]. Basal CYP2B6 expression in human liver is low, but is highly induced with relatively low hepatic expression compared to other CYP isoforms (∼1–10% of liver CYP450) [87]. CYP2B6 variability among racial/ethnic groups is noteworthy for several reasons. Its phenotypic activity does not appear to vary by racial/ethnic group in men but is 3.6- and 5.0-fold higher in Hispanic women compared to Caucasian and AfricanAmerican women, respectively [88]. CYP450 allele frequencies also vary highly among large racial/ethnic groupings such as Asians. For this reason, it is important to be more specific in terms of racial/ethnic stratification regarding it.

336

Important substrates for CYP2B6 include aflatoxin, bupropion, chlorpyrifos, cyclophosphamide, efavirenz, nevirapine, and nicotine [86,89]. It is inhibited by clopidogrel, mifepristone (RU486), selegiline, methadone, tamoxifen, ticlopidine, and thioTEPA, while it is induced by artemisinin antimalarials, carbamazepine, cyclophosphamide, efavirenz, hyperforin, metamizole, N,N-diethyl-m-toluamide (DEET), phenytoin, phenobarbital, rifampicin, ritonavir, and statins [87]. Several CYP2B6 SNPs may be present on one allele because of its high rate of polymorphism. The most common SNP present in multiple alleles is c.516G→T; this SNP is responsible for a loss-of-function phenotype and is often found in combination with c.785A→G [90]. As mentioned previously, there is a large amount of heterogeneity in the frequency of the c.516G→T allele in different Asian populations: Han Chinese (21%), Hong Kong Chinese (43%), Indian (39%), Japanese (14–20%), Korean (15%), Southern Chinese (35%), Taiwanese (14%), Thai (32%), Uygur Chinese (28%), and Vietnamese (27%) [91]. The activity phenotype of many CYP2B6 variants has not been characterized, and some variants appear to behave differently toward different substrates. CYP2B6*6 is a common variant (15–60% frequency) that confers significantly reduced activity in vivo attributable to aberrant splicing [92]. Many loss-of-function alleles have been identified; they include CYP2B6*2, *6, *7, *11, *15, *16, *18, *26, *27, and *28. Dysfunctional alleles have been shown to be present in more than 45% of individuals, but this figure varies widely by population. CYP2B6*18 is a loss-of-function allele prevalent in those of African ancestry (4–12%). A variant CYP2B6 genotype has been linked to altered pharmacokinetics of bupropion, cyclophosphamide, efavirenz, methadone, and neviripine [87]. CYP2E1 is a low-abundance hepatic CYP450 isoform (∼5% hepatic CYP450) for which only two polymorphisms have been associated with altered in vivo activity [57,93,94]. Drugs metabolized primarily by CYP2E1 include chlorzoxazone, dacarbazine, enflurane, ethanol, halothane, isoniazid, sevoflurane, and theophylline; CYP2E1 is involved in toxicity of acetaminophen, ethanol, and styrene [95,96]. The CYP2E1*1D allele is associated with increased 2E1 activity in the presence of chronic ethanol ingestion and obesity [97]. CYP2E1*6 allele carriers who were alcoholics had significantly lower CYP2E1 activity [98]. CYP2E1 is located in the endoplasmic reticulum membrane, as are all CYP450s, but it is also located in the mitochondrial membrane. Induction of mitochondrial CYP2E1 increases reactive oxygen species generation and is thought to participate in the toxic effects of ethanol [99]. Table 16.8 lists helpful websites that provide more information regarding CYP450 enzymes, drug interactions, and

PART | VI  Fundamental Pharmacogenomics

TABLE 16.8 CYP450, Drug Interaction, and Pharmacogenetics Websites CYP450 Websites

Topics

www.fda.gov/Drugs/DevelopmentApprovalProcess/ DevelopmentResources/ DrugInteractionsLabeling/ ucm080499.htm#background

Drug development and interactions

http://Pubchem.ncbi.nlm. nih.gov

Drug information and ­interactions

http://drnelson.uthsc.edu/ CytochromeP450.html

General

http://cpd.ibmh.msk.su/

General

www.icgeb.org/~p450srv/

General

http://bioinformatics.charite. de/supercyp/

Interactions and general

www.medicine.iupui.edu/ Flockhart/table.htm

Interactions

www.umm.edu/adam/drug_ checker.htm

Interactions

www.drug-interactions.com

Interactions

www.druginteractioninfo.org/

Interactions

www.interspeciesinfo.com/ Interspecies

Interspecies metabolism

http://catalog2.corning.com/ Lifesciences/media/pdf/HighLow_P450_Single_Donor_ HLM_Panel_031_5_1_13.pdf

Metabolism

www.fda.gov/Drugs/DevelopmentApprovalProcess/ DevelopmentResources/ DrugInteractionsLabeling/ ucm093664.htm

Metabolism

www.cypalleles.ki.se/

Pharmacogenomics

http://pharmgkb.org/

Pharmacogenomics

www.fda.gov/Drugs/ScienceResearch/ResearchAreas/Pharmacogenetics/ ucm083378.htm

Pharmacogenetic information in drug package inserts

Subscription Information Sources www.themedicalletter.com/ adi

Interactions

www.lexi.com/institutions/ products/pda/lexi-complete/

Drug information/Pharmacogenomics database

Chapter | 16 CYP450 and Ethnicity

337

CYP450 pharmacogenetics. The listed descriptions give a brief idea of content, but most of the websites offer in-depth information beyond this.

data will allow prescribers to maximize drug efficacy for their patients while minimizing the risks of toxicity and side effects.

16.5  FUTURE PERSPECTIVES

REFERENCES

As mentioned previously, CYP450s are the primary phase I enzymes responsible for metabolizing approximately 75% of all drugs. Most of this metabolism is mediated by the xenobiotic-metabolizing, hepatically expressed CYP450 isoforms CYP1A2, CYP2C8/9/19, CYP2D6, CYP2E1, and CYP3A4/5. Early researchers identified differences in CYP450-mediated metabolism among racial/ethnic groups. Identification of the genetic determinants of these differences spawned the field of pharmacogenetics. Today we continue to identify the genetic and environmental determinants of differences in drug response at the individual and group levels. Although race is a social construct, racial stratification is often a necessity in population genetic studies involving CYP450 genes. The contribution of interethnic genetic variation is minimal compared to interindividual variation. However, unstable genes linked to environmental circumstances, such as CYP450, vary dramatically among different racial/ethnic groups. Many examples of racial/ethnic differences in CYP450 allele frequencies and haplotype blocks have been identified. Not surprisingly, metabolic phenotype prevalences vary by race/ethnicity for most of the important CYP450 isoforms. The wealth of pharmacogenetic data accumulated thus far regarding the aforementioned CYP450 isoforms has led to many drug-labeling notices and/or dosing recommendations for individuals with variant metabolic phenotypes who may be taking specific drugs. A barrier to their use is the lack of information regarding individual patients’ metabolic phenotype. Scott et al. have provided an up-to-date list of providers of CYP2C19 genotyping, and the number of vendors offering such services is likely to increase [36]. Indeed, whole exome sequencing should become fairly common with an ever growing number of next-generation sequencing providers. However, in the case of CYP450 phenotype, exon sequencing may not provide complete answers because new clinically significant polymorphisms in nonexon regions such as the promoter region are being discovered. Increased use of haplotype approaches in pharmacogenetics research should also prove to be beneficial. This is especially true for CYP450 isoforms that share loci, among them the CYP2C family. As more data are gathered, and as knowledge of individual pharmacogenetic information (e.g., metabolic phenotype) becomes more widespread, the goal of personalized medicine will become a reality. Enhanced knowledge of drug- and patient-specific pharmacogenetic

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PART | VI  Fundamental Pharmacogenomics

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Chapter | 16 CYP450 and Ethnicity

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