Principles of pharmacogenetics--implications for the anaesthetist

British Journal of Anaesthesia Page 1 of 11

DOI: 10.1093/bja/aeh200

Principles of pharmacogenetics—implications for theanaesthetist

G. Iohom1, D. Fitzgerald2 and A. J. Cunningham1 2*

1Department of Anaesthesia and Intensive Care Medicine, Beaumont Hospital, Dublin 9, Ireland.2Royal College of Surgeons in Ireland, Dublin 2, Ireland

*Corresponding author. E-mail: [email protected]

Br J Anaesth 2004

Keywords: anaesthesia, general; genetic factors, polymorphism; metabolism, poor metabolizers;

metabolism, ultrarapid metabolizers; pharmacogenetics

‘If it were not for the great variability among individuals,

medicine might as well be a science and not an art.’ Sir

William Osler’s observations in 189260 reflect a perception

of medicine over 100 years ago, highlighting the lack of

available objective data to make decisions that are tailored

to individual patients. One hundred years later, scientists are

on the verge of being able to identify inherited differences

between individuals, which may predict each patient’s

response to a particular drug. This ability will have undoubted

benefits in the discovery, development, and delivery of new

medicines. Sir William Osler, if he were alive today, would be

forced to reconsider his view of medicine as an art not

a science.

Pharmacogenetics emerged as a discipline that attempted

to understand the hereditary basis for differences in respons-

iveness or inter-individual variation to therapeutic agents.85

Variation in a drug effect may vary from 2- to 10-fold or 100-

fold, even among members of the same family.35 90 Similar

inter-patient variability is observed in the risk of adverse

effects of a drug or a chemical.85

Pharmacogenetics has been defined as the study of vari-

ability in drug response as a result of heredity factors.52 More

recently, the term ‘pharmacogenomics’ has been introduced.

While the former term is largely used in relation to variants in

genes that influence drug response, the latter refers to changes

in gene expression as a consequence of drug exposure.14 The

value of an understanding of pharmacogenetics for the cli-

nician is to enable optimum therapeutic efficacy; to avoid

toxicity of those drugs whose metabolism is catalysed by

polymorphic isoenzymes; and to contribute to the rational

design of new drugs. Pharmacogenetics and pharmacogeno-

mics cannot be understood without a grasp of basic medical

genetics.

Medical genetics

Anaesthetists traditionally possess a basic knowledge of

medical genetics to understand conditions such as malignant

hyperthermia (MH) and atypical plasma cholinesterase. To

optimally use anaesthetic agents, including opioids, a more

comprehensive understanding of medical genetics will be

required.15

Genes and alleles

A gene has its specific locus (from Latin, place) on a given

chromosome. The gene for eye colour thus has its position in

the human DNA determined by chromosome, and position on

that chromosome. The gene, however, comes in various

forms or alleles. For eye colour, the allele for blue eyes

and the allele for brown eyes are two different alleles for

the same gene. Although the expression is sometimes

used, strictly speaking there is no such thing as a disease

gene or a disease locus, only disease alleles.

Phenotype and genotype

The classical definition of phenotype is ‘the way you look’. If

you have genes for blue eyes, that is your genotype. Your

phenotype is ‘being a blue-eyed person’. A phenotype could

also be an enzyme activity above or below a certain value, or

being able to metabolize a certain substance.

Markers

Genetic research necessitates distinction between individuals

at the DNA level. Different as we may be, we are identical for

long stretches of DNA. Researchers usually attempt to iden-

tify gene markers, a short piece of DNA that can easily be

detected. Two separate forms of markers exist, and the dif-

ferent forms can be used to tell the difference between indi-

viduals (or chromosomes, or parts of DNA). Finding a marker

is like spotting the lanterns of a ship in the night. If you see one

of the lanterns, you know you are not actually seeing the ship,

but you can have a good guess as to where the ship is. When

investigators find a marker that may be located close to a gene

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of interest, while the gene per se may not be located, sub-

stantial progress has been made. Some markers are single

base mutations, others consist of repeats of short sequences

where individuals differ in how many repeats they have,

termed microsatellite markers.

Polymorphisms

The word polymorphism comes from the Greek poly, several,

and morphe, form. Polymorphism, thus, is something that can

take one of several forms. A DNA polymorphism exists when

individuals differ in their DNA sequence at a certain point in

their genome. A normal form and a mutated form may repres-

ent such a difference. The mutated form can be a single base

mutation, or variation over a short stretch of DNA. The term

polymorphism is a general one. It is most often used to

describe a marker that occurs in several forms—a marker

polymorphism. Only 3% of DNA consists of coding

sequences and in most regions of the genome, a polymorph-

ism is of no clinical consequence. However, the term poly-

morphism is also used about a mutation inside a coding

sequence, where the mutation might be causing disease.

Single nucleotide polymorphisms (SNPs) are changes in a

single base at a specific position in the genome, in most cases

with two alleles. SNPs are found at a frequency of about

1:1000 bases in humans. By definition, the more rare allele

should be more abundant than 1% in the general population.40

The relative simplicity of SNP genotyping technologies and

the abundance of SNPs in the human genome have made them

very popular in recent years.21 Yet, there still is some debate

about the usefulness of SNP markers compared with micro-

satellite markers for linkage studies, and how many SNP

markers will have to be analysed for meaningful association

studies.41

Genotyping

Several genotyping technologies have reached maturity in the

last few years and are being integrated into large sale geno-

typing operations supported by automation. The choice of a

technology for genotyping depends on whether a few differ-

ent SNPs are to be genotyped in many individuals, or many

different SNPs are to be genotyped in a few individuals.64

Although genotyping methods are very diverse, broadly, each

method can be separated into two elements. The first element

is a method for interrogating an SNP. This is a sequence of

molecular biological, physical, and chemical procedures for

the distinction of the alleles of an SNP, that is hybridization,79

primer extension,72 oligonucleotide ligation,54 and nuclease

cleavage.55 The second element is the actual analysis or mea-

surement of the allele-specific products, that is by gel separa-

tion,63 microarrays,7 mass spectrometry,22 flow cytometry,9

etc. Often, very different methods share elements, like read-

ing out a fluorescent tag in a plate reader, or the method of

generating allele-specific products (i.e. by primer extension

or oligonucleotide ligation), which can be analysed in differ-

ent analysis formats.21

Linkage studies

A gene and a marker are said to be linked if they reside close to

each other on the same chromosome. In linkage studies, a set

of markers, with a known location on the genetic map, is used

to track down a gene of unknown location. In a linkage study,

the disease allele is not known, but merely manifests itself as

disease in the person who carries it. The basic assumption is

that if a certain disease allele and a certain marker allele are

found together in a family, the two are physically close on the

same chromosome. The statistical methods of linkage ana-

lysis calculate just how unlikely it would be to consistently

find a marker allele and a disease allele together. If this turns

out to be very unlikely to have occurred by chance, the alter-

native hypothesis of linkage of marker and disease gene is

accepted. If a marker allele and a disease allele occur together

consistently in a population, they are said to be in linkage

disequilibrium. Genetic linkage studies have identified

various loci on causative genes for malignant hyperthermia

susceptibility (MHS).49

Association studies

Association studies exploit the fact that there may be linkage

disequilibrium in the population. A study may start out with a

large number of markers with a known location. A number of

patients with a disease are examined for these markers. If a

substantial majority of patients have the same markers, it is

likely that the gene responsible for the disease is located close

to the markers that the patients have in common. The apo-

lipoprotein (apo) E4 genotype, for example, is strongly asso-

ciated with Alzheimer’s disease, representing a susceptibility

gene, but is not necessarily causative of the disease, meaning

that having this genotype is not generally sufficient to cause

Alzheimer’s.57

Pharmacogenetics—focusing on drug targets

Classically, pharmacologists have concentrated on genetic

variability that alters drug metabolizing enzymes to explain

variation in pharmacokinetic responses to drug therapy.

However, it is now apparent that genetic variability can affect

many other important proteins such as transporter proteins

and receptors. Thus, pharmacogenetics is best defined as the

study of genetic variations that cause a variable drug response

and includes the genetic polymorphism of drug transporters,

drug metabolizing enzymes, and drug receptors.

Enzymes

Genetic determinants of drug response can be divided

into two types: (i) those characterized by alteration in drug

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metabolism, such as those as a result of differences in levels of

N-acetyltransferase (NAT) or atypical plasma cholinesterase

and (ii) those characterized by alteration in pharmaco-

dynamics. Inter-individual variation in therapeutic drug

response and toxicity is most often a result of variability in

drug metabolism rather than pharmacodynamics.34

Observed genetic variability in drug metabolism can be

either monogenic or polygenic. When the variability is a

result of single genes, the term monogenic is used. When

genes, which individually produce small effects but collec-

tively lead to significant effects are involved, the term poly-

genic is used.30 Much pharmacogenetic research focuses on

the monogenic variants of drug-metabolizing enzymes and

on polymorphisms (i.e. variants that exist in at least 1% of the

population).1 Such genes generally affect drug biotransfor-

mation by altering the amount or function of an enzyme. The

existence of such polymorphisms explains why drug meta-

bolism shows a polymodal distribution. In other words,

patient populations can be divided into two groups (or pheno-

types) according to their abilities to metabolize specific probe

drugs. Poor or slow metabolizers have deficient metabo-

lizing ability; in contrast, extensive metabolizers metabolize

drugs more rapidly and may need higher doses to produce a

therapeutic response.18

Drug metabolism is divided into phase I and phase II reac-

tions. Phase I reactions, including oxidation, reduction, and

hydrolysis, introduce a polar group into the molecule, whereas

phase II reactions conjugate an endogenous hydrophilic

substance with the drug, resulting in more water-soluble

compounds.

Phase I enzymes

Oxidation, a major route of metabolism for many drugs,

is catalysed by the mixed function oxidase system, which

comprises cytochrome P450 (CYP) enzymes.

P450 enzymes are found in virtually all tissues with the

highest concentration in the endoplasmic reticulum of the

liver.66 73 The recommended nomenclature of cytochrome

P450 isoenzymes is based upon grouping enzymes and genes

into families and subfamilies with the prefix CYP denoting

cytochrome P450. Families are characterized by an Arabic

number (i.e. CYP2) and subfamilies are indicated by a letter

(i.e. CYP2D). The individual genes coding for one specific

isoenzyme are denoted by a second Arabic number after

the letter in italics, that is CYP2D6. Members of the same

enzyme=gene family may exhibit more than 40% identity in

amino acid sequences, while a subfamily consists of those

sharing greater than 55% sequence identity (Fig. 1).67

Several P450 isoenzymes are involved in human hepatic

drug metabolism. Their activities may be inhibited or induced

by drugs or environmental xenobiotics. Their activity is

also determined genetically as a consequence of common

polymorphisms.

The activity of cytochrome P450 enzymes can be measured

by administration of a probe drug, known to be selectively

metabolized by the CYP enzyme under study, followed by

measurement of the metabolic ratio (the ratio of the drug

dosage or unchanged drug to metabolite in serum or

urine). However, such phenotyping takes into account all

factors influencing the activity of the enzyme, such as the

presence of a competing substrate, and is sensitive to the

overall process of drug metabolism.

Genotyping involves identification of defined genetic

mutations on the CYP genes that give rise to the specific

drug metabolism phenotype. These mutations include genetic

alterations that lead to over-expression (gene duplication),

absenceofanactiveproteinproduct(nullallele),orproduction

of a mutant protein with diminished catalytic capacity (inac-

tivatingallele).Genotypingmethods requiresmallamountsof

blood or tissue, are not affected by underlying disease or by

drugs taken by the patient, and need to be done only once in a

lifetime. By screening for genetic variants, an individual’s

drug metabolism phenotype can be characterized.84

Besides the P450 genes, other phase I enzymes are

polymorphic, such as alcohol dehydrogenases (ADH) and

acetaldehyde dehydrogenase (ALDH), as well as dihydro-

pyrimidine dehydrogenase (DPD). With respect to the first

two enzymes, the clearance of ethanol is significantly

affected, ADHB2 giving a higher rate of ethanol metabolism

and ALDH2 polymorphism influencing acetaldehyde

metabolism. Poor metabolizers for ALDH2 develop flush

reactions and anti-abuse like side-effects when drinking

ethanol and the number of alcoholics with this genotype is

lower. A polymorphism relevant to treatment with anticancer

drugs is present in DPD. 5-Fluorouracil is metabolized by this

enzyme. Subjects with impaired enzyme activity caused by

inactivating gene mutations suffer from a severely increased

risk of adverse reactions, including myelotoxicity and

neurotoxicity following 5-fluorouracil administration.31

Phase II enzymes

Several enzyme families directly conjugate drugs or their

oxidative metabolites. There are 15 human uridine diphos-

phate glucuronosyltransferases (UGTs), broadly classified

into the UGT1 (phenol=bilirubin) and UGT2 (steroid=bile)

families.78 87 Considerable polymorphism in glutathione

S-transferase (GST) expression has been described and asso-

ciated with susceptibility to disease, particularly cancer and

asthma (both as disease-causing and disease-modifying

Fig 1 Example of cytochrome P450 (CYP) nomenclature system.

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factors).13 23 NAT was the first drug metabolizing enzyme for

which a genetic polymorphism was discovered (slow and fast

acetylators). Slow acetylators show a greater therapeutic

response than fast acetylators to several drugs (i.e. isoniazid,

hydralazine) but may be more susceptible to side-effects.

There are two human NATs, NAT1 and NAT2, with discrete

but overlapping substrate specificities. Although there

are polymorphisms in NAT1, it is the genetic variability

in NAT2 that is responsible for the slow-acetylator pheno-

type.24 Sulfotransferases (STs) catalyse the elimination of

acetaminophen and morphine in neonates.53

The clinical implications of polymorphism of drug meta-

bolizing enzymes are drug toxicity and therapeutic failure.

The clinical relevance, however, depends on the therapeutic

ratio of the drug.77

Transporter proteins

The influence of the genetic make-up of an individual is not

limited to drug metabolism. Genetic variability influences

drug absorption and this forms the basis for slow and rapid

drug absorption.

Most drugs or drug metabolites enter the cells by passive

diffusion. Some drugs are actively transported by transporter

proteins, of which membrane transporters may play a key

role. These transmembrane transporters are members of the

large protein family known as ABC (adenosine triphosphate

binding cassette) proteins.39 Although they do not catalyse

biotransformation per se, they nonetheless markedly affect

drug bioavailability and can act in conjunction with intracel-

lular drug metabolizing enzymes.

P-Glycoprotein (also called multi-drug resistant P-

glycoprotein, MDR1) is the first cloned and best-characte-

rized ABC protein.61 92 At the blood–brain barrier,

P-glycoprotein may influence the uptake of substrates into

the brain: high P-glycoprotein levels may limit the uptake of

sufficient amounts of the desired drug into the brain, and

reduced P-glycoprotein activity could lead to abnormally

increased accumulation in the brain and undesired side-

effects of a drug.6

A second subfamily of ABC proteins is the multi-drug

resistance-associated proteins, also known as the multi-

specific organic anion transporter.5 The first protein to be dis-

covered in this category was MRP1, whose over-expression is

responsible for the majority of non-P-glycoprotein-mediated

multi-drug resistance. There are seven currently known

MRPs with uncertain clinical significance. Rifampicin is

known to induce human MRP2.17

Receptors

When examining the response to a drug, the most obvious

target for genetic studies of drug response is the receptor.

Genetic variability influences interactions with receptors and

this forms the basis for poor or efficient receptor interactions.

The polymorphisms in genes encoding receptors relevant to

drug treatment of different diseases cause widespread vari-

ation in sensitivity to many drugs. For example, individuals

with a mutation in the gene encoding prothrombin may have

increased risk of cerebral vein thrombosis when using oral

contraceptives. Other examples of the impact of genetic

polymorphisms include: angiotensin converting enzyme

(ACE) and its sensitivity to ACE inhibitors; b-adrenergic

receptors and their sensitivity to b-agonists in asthmatics;

and 5 hydroxytryptamine receptors and the response to

certain neuroleptics.15

Mutations in cardiac potassium channel genes such as

HERG (human ether-a-go-go-related gene) and KvLQT1

(chromosome 11 linked LQT gene) may both give suscept-

ibility to drug-induced long QT syndrome, or KCNE2

(a potassium gene encoding MinK-related peptide-1,

MiRP1) may give susceptibility to drug-induced arrhyth-

mias. All are of clinical relevance to an anaesthetist.15

Pharmacogenomics is the research activity, which at the

genome level, aims to identify disease genes and new drug

response markers.31 The role of pharmacogenetics is increas-

ingly recognized by the pharmaceutical industry with

research programmes directed at drug discovery and devel-

opment.93 Candidate drugs whose metabolism may involve

polymorphic pathways may be screened out. Pharmacoge-

netic data may be used either to design better compounds or to

help plan clinical studies. Screening volunteers and patients

included in clinical trials may become necessary to minimize

adverse events and optimize efficacy. Clinical investigations

in various populations will help clarify inter-ethnic differ-

ences in drug disposition and response to a given drug.

Knowledge of pharmacogenetics should help reduce the

time and cost associated with new drug development.

Anaesthetic implications

Clinical experience suggests that there is great heterogeneity

in anaesthetic requirements in the way patients recover from

uncomplicated anaesthesia, as well as their requirements for

postoperative analgesia. Some of these differences can be

explained by genetically determined differences in transport

proteins, in drug targets and in enzyme functions. It is also

important to know to what extent environmental factors (such

as smoking, diet, and other drugs) interact with genetic fac-

tors to modulate drug effects.

Recovery from general anaesthesia is dependent on factors

governing drug sensitivity and drug disposition. Recovery

from a single dose of i.v. anaesthetic agent is dependent

on redistribution, whereas recovery after a prolonged infu-

sion is progressively more dependent on metabolism and

elimination of the drugs.28 Aging as well as environmental

factors may influence drug dynamics. Both alcohol and

tobacco play an important role in determining the degree

of liver enzyme induction, which determines the rate of meta-

bolism of some medications, including volatile anaesthetic

agents, thus influencing outcome from anaesthesia.71

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Enzymes

Genetic polymorphisms in metabolizing enzymes become

relevant if: they are responsible for 50% or more of the clear-

ance of a drug; when using drugs with a steep dose–response

curve and a narrow therapeutic window; and when using

drugs whose activity depends upon a metabolite formed by

a polymorphic enzyme.

Plasma cholinesterase

Inherited deficiency=reduced effect of plasma cholinesterase

will result in prolonged muscle relaxation after succinylcho-

line.68 This was the first documented example of inherited

variations in anaesthetic drug effects. The level and quality of

plasma cholinesterase activity (acylcholine-acylhydrolase

E.C.1.1.8, butyrylcholinesterase (BChE)) in a patient deter-

mines the duration of action of succinylcholine and mivacur-

ium.33 Genetic variation is one of several factors determining

the activity of cholinesterase in plasma. The expansion from

only four known forms of human serum BChE a few years ago

to over 20 variants identifiable at DNA level at present has

greatly increased the complexity of diagnosis and interpreta-

tion of these genetic traits. Although the presence of a single

genetic variant allele does not usually cause an increased

duration of action of succinylcholine or mivacurium, it

may do so if it occurs in heterozygous combination with

otherwise induced low BChE activity. Therefore, it is impor-

tant to be able to diagnose not only the well-known atypical

variant but also the low-activity variants such as the H, J, K,

and S variants. Using standard enzymatic and inhibition ana-

lysis, it is not possible to distinguish between the usual geno-

type (UU) and genotypes in which one of the quantitative

variants, H, J, K, or S, is present in heterozygous combination

with the usual gene (UH, UJ, UK, or US).33 Recent advances

in molecular biology techniques have allowed analysis of the

detailed structure of the human BCHE gene. For qualitative

variants, a portion of the structural gene responsible for the

amino acid sequence of the protein (BChE) is altered. This

structural gene mutation accounts for the abnormal kinetic

properties of the variant BChE protein. Variants in which

there is a marked quantitative reduction in the level of enzy-

matic activity could result from: (i) a structural modification

that causes little or no active enzyme being preserved (such

as a mutation to a ‘stop’ codon, or the production of a very

unstable enzyme), or (ii) a regulatory defect affecting pri-

marily the rate of enzyme synthesis. The latter type mutations

are much more difficult to diagnose, as they can occur over

a much larger region of the BCHE gene. Structural gene

differences should be within the 1722 nucleotide bases

located in exons 2, 3, or 4, as these contain all the codons

representing the 574 amino acids making up the monomeric

BChE enzyme protein sequence.42

In addition, plasma cholinesterase availability could be

decreased, and thus neuromuscular block after succinyl-

choline lengthened, by competitive interaction with (i) anti-

cholinesterases, including neostigmine, edrophonium, and

ecothiopate (an organophosphorus compound once used

topically as a miotic in ophthalmology) and=or (ii) other

drugs metabolized by plasma cholinesterase, such as etomi-

date, propanidid, ester local anaesthetics, methotrexate,

monoamine oxidase inhibitors, and esmolol.68

CYP enzymes

CYP families 1–3 are responsible for phase I metabolism of

most drugs. Enzymes in the CYP2C, CYP2D, and CYP3A

subfamilies are most active in metabolizing clinically used

drugs.18

CYP2C enzymes eliminate oral hypoglycaemics, war-

farin, some antiepileptics, non-steroidal anti-inflammatory

drugs,amitriptyline,barbiturates,diazepam,andomeprazole.

The most important substrates for CYP2D6 are a number of

psychoactive drugs such as antidepressants and neuroleptics,

and cardiovascular drugs such as beta-blockers and antiar-

rhythmics. Drugs in this class relating to anaesthetic practice

include codeine, tramadol, ondansetron, granisteron, and

metaraminol.10 It has been postulated that CYP2D6 poor

metabolizers are more susceptible to pain than extensive

metabolizers because of a defect in synthesizing endogenous

opioids.15 Codeine is ineffective as an analgesic in 6–7%

of a Caucasian population as a result of homozygosity for

non-functional CYP2D6 mutant alleles. CYP2D6 deficient

patients will not convert codeine to morphine. Postoperative

pain treatment with codeine-containing drugs will therefore

have limited effect in patients with this trait, whose request

for larger doses of codeine could easily be misinterpreted as

drug addiction. This genetic variation makes it not surprising

that a standardized prescription of codeine for pain relief will

result in remarkable variation in the adequacy of pain relief.

CYP2E1 is more of toxicological interest as it has been

reported to have a unique capacity to activate many xeno-

biotics to hepatotoxic (among them acetaminophen) or

carcinogenic products.47 CYP2E1 is the principal, if not

sole human liver microsomal enzyme catalysing defluorina-

tion of sevoflurane. It is also the principal, but not exclusive

enzyme responsible for the metabolism of methoxyflurane,

and is responsible for a significant fraction of isoflurane and

enflurane metabolism (Table 1). Identification of CYP2E1 as

the major anaesthetic metabolizing enzyme in humans pro-

vides a mechanistic understanding of fluorinated ether anaes-

thetic metabolism and toxicity.37 75

CYP3A4 handles drugs such as local anaesthetics, a

number of antiepileptics, steroid hormones, systemic anti-

fungals, midazolam, erythromycin, alfentanil, and possibly

also fentanyl and sufentanil (Table 1).19 As human alfentanil

metabolism is catalysed predominantly, if not exclusively by

CYP3A3=4 (electrophoretically inseparable and immuno-

chemically indistinguishable), inter-individual variability

in human alfentanil disposition and alfentanil drug inter-

actions may be attributable to individual differences in

CYP3A3=4 activity. Variability in CYP3A3=4 expression

may have genetic and environmental (age, sex, disease

state, concomitant drug administration) components, and

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the degree of variability is sufficient to explain the 10-fold

range in alfentanil clearance observed clinically.36

Enzymes of phase II of drug metabolism show extensive

polymorphism (Table 2). Despite this, investigations of geno-

type–phenotype correlations have been confounded by lim-

ited substrate specificity and a less pronounced hereditary

contribution to differences in bioactivity.31 87 Genetic poly-

morphism in genes encoding enzymes such as UGTs, STs,

NAT 1 and 2, and GSTs may cause significant variation in

rendering drugs water soluble and then suitable for renal

excretion.85

Several mutations in UGT1 genes have been reported.

Inheritance of two defective alleles is associated with reduced

bilirubin-conjugating activity, giving rise to clinical condi-

tions such as Crigler-Najjar and Gilbert syndromes.78

Lorazepam is metabolized to a 3-O-phenolic glucuronide

in man and excreted in urine, a reaction possibly catalysed

by UGT2B7. Lorazepam clearances were 20–40% lower in

Gilbert patients when compared with controls.78 Oxazepam

administered as a racemic mixture was shown to be prefer-

entially excreted as the (S) glucuronide, and a low S=R

glucuronide ratio was used to assess poor glucuronidation

of oxazepam. Ten per cent of the whole population was

determined to be poor glucuronidators of (S) oxazepam,

which suggested a genetic relationship to the UGT2B7

isoform.56 Inter-ethnic differences in codeine glucuroni-

dation, reduced in Han Chinese men when compared with

a Swedish population, are known to exist.91

Angiotensin converting enzyme

Most recently, Lasocki and colleagues provided the first evi-

dence for an in vivo association between the pressure–flow

relationship and the insertion=deletion polymorphism of the

ACE gene. Multivariate analysis showed that homozygosity

for the D allele was the only predictive variable of the slope

of the curve. These findings indicate a modified vascular

response toflowinDDpatients.The investigatorsalsoshowed

an increased vascular reactivity to phenylephrine associated

with the D allele of the ACE gene.43

Transporter proteins

P-Glycoprotein, a member of the adenosine triphosphate-

binding cassette superfamily of cellular efflux drug

transporters, is expressed in the capillary endothelium of

the blood–brain barrier and in many other cell membranes

such as intestinal enterocytes and biliary and renal epithelial

cells.82 Block of P-glycoprotein allows enhanced central ner-

vous system (CNS) entry of some drugs, offering new pos-

sibilities to explain CNS-related adverse effects during the

administration of drugs that are substrates of P-glycoprotein

and, furthermore, to manipulate the CNS entry of drugs

whose target is located in the brain.89

The P-glycoprotein substrate and inhibitor cyclosporin

was shown to increase fentanyl-induced analgesia in

mice.11 More recently, morphine has been shown to increase

analgesia in P-glycoprotein knockout mice compared with

wild-type mice.74 Thus, P-glycoprotein may limit morphine

entry into the brain. Loperamide, a widely used anti-

diarrhoeal agent, although a potent opioid in vitro, produces

only gastrointestinal opioid effects and lacks CNS effects.

This apparent tissue selectivity is probably a result of

loperamide being a P-glycoprotein substrate, so that P-

glycoprotein in the CNS effectively prevents access of loper-

amide to the CNS. Supportive of this hypothesis is the finding

that in mice with MDR1 gene disruption, brain loperamide

Table 1 Clinically relevant genetic polymorphisms affecting phase I metabolism and effect of a drug15 84

Gene=enzyme Drug Drug effect linked to polymorphism

CYP1A2 Caffeine, haloperidol, acetaminophen, theophylline

CYP2C8-9 Ibuprofen, phenytoin, warfarin, tolbutamide, diazepam Anticoagulant effect of warfarin

CYP2C19 Amitriptyline, barbiturates, diazepam, omeprazole

CYP2D6 Antiarrhythmics, antihypertensives, beta-blockers,

tricyclic antidepressants, neuroleptics, selective

serotonin reuptake inhibitors, morphine derivatives

Tardive dyskinesia from chlorpromazine,

reduced analgesic effect of codeine, altered

beta-blocker effect

CYP2E1 Acetone, ethanol, acetaminophen, volatile anaesthetics Hepatotoxicity

CYP3A4 Carbamazepine, quinidine, alfentanil, fentanyl? Sufentanil?

Local anaesthetics, steroids, midazolam, erythromycin,

systemic antifungals

Table 2 Clinically relevant genetic polymorphisms affecting phase II metabolism and effect of a drug15 44

Gene=enzyme Drug Drug effect linked to polymorphism

DPD Fluorouracil 5-Fluorouracil neurotoxicity

NAT (NAT 2) Isoniazide, hydralazine, sulphonamides,

procainamide

Hypersensitivity to sulfonamides,

hydralazine-induced lupus

Uridine diphosphate glycosyl transferase

(UGT1As, UGT2Bs)

Acetaminophen, propofol, opioids, naproxen,

ibuprofen

Not yet elucidated

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concentrations were 8-fold higher than those observed in

normal mice, and lethal opioid effects were produced.62

In contrast to loperamide, opioids in widespread clinical

use as i.v. anaesthetic agents or adjuvants, such as fentanyl,

sufentanil, and alfentanil, are not in vitro substrates of P-

glycoprotein. Morphine is a P-glycoprotein substrate with

clearly less clinical relevance than loperamide. Inhibitory

effects of the opioids fentanyl, sufentanil, and alfentanil on

P-glycoprotein activity in vitro are reached only with

relatively high concentrations.82 Thus, the wide spectrum

of P-glycoprotein activity may partly explain the varying

CNS-related effects of opioids.

As new drugs are introduced into clinical practice, it will be

important to assess whether they are P-glycoprotein sub-

strates or inhibitors to assess their potential for drug interac-

tion. Inter-individual variability in P-glycoprotein activity

is now recognized, which may at least partially depend on

genetic polymorphism. Homozygosity for an allele asso-

ciated with deficient P-glycoprotein activity occurs in 24%

of white people.26

Receptors

Polymorphisms in genes encoding receptors (drug targets)

may explain some of the variation in sensitivity to drugs.

Ryanodine receptor (RYR1)

MH is a classic example of a dramatic interaction

between a drug and a mutated receptor. In patients with a

mutation of the skeletal muscle RYR1 gene, exposure to

halothane and=or succinylcholine produces uncontrolled

release of calcium from skeletal muscle cells, leading to

hyperthermia, myolysis, and ultimately multi-organ failure.

To date, almost 50 different mutations in the RYR1 gene on

chromosome 19 are known to cause susceptibility to malig-

nant hyperthermia (MHS phenotype).49 Genetic linkage stu-

dies indicate that the RYR1 locus (MHS1) on chromosome

19q13.1 accounts for at least 50% of MH families. A second

locus, MHS2, was assigned tentatively to chromosome 17q in

North American families,45 but could not be replicated in

European families despite extensive efforts. However, it is

quite possible that the differences in the protocols result in

detection of different phenotypes and are weighted differ-

ently with respect to identification of modulating gene

effects.16 Markers linked to the CACNL2A gene on chromo-

some 7q have been tentatively linked to MHS in a single

European family (MHS3).29 A systematic linkage study

using a set of polymorphic microsatellite markers covering

the entire human genome in a small number of large, appar-

ently non-chromosome 19 linked European MH families,

identified a locus on chromosome 3q13.1 (MHS4),70 a

locus on chromosome 1q (MHS5),50 59 and a tentative

locus on chromosome 5q (MHS6).59 A causative gene at

the MHS3 and MHS4 locus has yet to be identified.

The CACNL1A3 gene encodes the a1-subunit of the DHP

receptor maps to the MHS5 locus and functions as a voltage

gated channel. Evidence for the MHS6 locus is weak and its

validity remains to be confirmed.49

m-Opioid receptor (MOR)

Variation in the expression of this receptor determines the

analgesic potency of morphine. It also explains the difference

in the response to painful stimuli and response to opioid

drugs, probably a result of a genetic polymorphism in the

transcription-regulating region of this gene.77 This makes the

MOR gene a candidate for susceptibility or resistance to pain.

A SNP in the human MOR gene at position 118 (A118G

transition) results in a receptor variant that bindsb-endorphin

nearly three times more tightly than the most common allelic

form of the receptor.4 This makes b-endorphin nearly three

times more potent than in individuals without the mutation. It

is unclear whether this variant has direct or indirect implica-

tions for opioid addiction.15

More recently, three SNPs in the hMOR gene that cause

amino acid substitutions in the third intracellular (i3) loop of

MOR have been identified (R260H, R265H, and S268P).

Each of the three SNPs caused substantial changes in basal

G protein coupling, calmodulin binding, or both. Carriers of

the mutant alleles might display altered responses to narcotic

analgesics.83

GABAA receptor

A new class of human GABAA receptor subunit (e), that

confers insensitivity to the potentiating effects of i.v. anaes-

thetic agents on gabaminergic transmission, has been identi-

fied. Wilke and colleagues identified the gene, symbolized

GABRE, coding for class epsilon of the GABAA receptor

(gene map locus Xq28).88 Genetic variation in the gene

encoding for this subunit of a GABA-receptor may be of

importance for the sensitivity to diazepam, barbiturates

and propofol, or susceptibility to alcohol addiction.8 32 Vola-

tile anaesthetics act through a different site on the GABAA

receptor molecule from the i.v. anaesthetics, although the

nature and location of that site remains unclear (Fig. 2).

Volatile anaesthetics and propofol show no significant selec-

tivity between any receptor subtype. In contrast, etomidate

acts preferentially through receptors containingb2 orb3 sub-

units. This selectivity is determined by a single amino acid (an

asparagine at amino acid number 265 in b2 and b3 subunits;

a serine at the equivalent position in b1.2 Extrapolating

the results of animal studies to humans, it could be predicted

that an anaesthetic that is selective for b3 containing

GABAA receptors would enable faster recovery, perhaps

without the ‘hangover’ effect.86 Gene targeting in mice

may be valuable for elucidating the mechanism of action

for some drugs. A variety of manipulations are possible,

including introducing a gene not present normally (transgenic

mice), removing an endogenous gene (‘knockout mice’), or

replacing an endogenous gene with an altered copy (‘knockin

mice’).27 Knockout of the GABAA g2 receptor subunit gene

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resulted in mice that were insensitive to the sedative=hypnotic actions of benzodiazepines.20

The effects and side-effects of any drug will significantly

depend on the genetic impact on its pharmacokinetics and

pharmacodynamics. An active drug which is metabolized

slowly by a person with increased receptor sensitivity may

cause toxic effects, while an active drug which is metabolized

extensively by a person with reduced receptor sensitivity,

may have a reduced effect.15

An understanding of the CYP system and its substrates is

also a key factor in the prevention of important drug–drug

interactions, either as a result of enzyme induction or inhibi-

tion. The former may take some time to develop and usually

reduces the effect of the drug involved, while the latter takes

place instantly with side-effects as a common result.12 The

numberofpossibilitiesareoverwhelming,butmaybereduced

somewhat by a few rules of thumb. It is usually required that

these interactions occur when the involved drugs are sub-

strates for the same CYPs. Susceptibility to, for instance,

inhibition interactions usually requires that one metabolic

product accounts for 30–40% of the effect of a drug, and

that its metabolic pathway is catalysed by a single enzyme.46

Future developments

What is the role of genotyping in current anaesthesia prac-

tice? Should genotyping be performed routinely in each

instance when the drug of choice is substrate for a poly-

morphic enzyme, especially when a close relationship

between serum drug concentration and effect is demon-

strated?48 Alternatively, should genotyping be indicated

when individuals demonstrate a suboptimal response to

drugs that are substrates for polymorphic enzymes?

Routine screening of patients before starting pharma-

cotherapy would have significant cost implications. Cost

savings associated with toxic episodes or therapeutic failure

and subsequent intervention could be expected in most

specialties. But in anaesthesia, we administer drugs to a

large number of patients, often once only, and frequently

only briefly after the patient has presented for treatment. In

this setting, a genetic screening programme is unlikely to

represent a cost-effective method for reducing morbidity.

However, once conditions such as MH are documented,

family screening becomes a logical follow-up. The quest for a

simple non-invasive diagnostic test for MH susceptibility has

moved forward dramatically since the first descriptions of

linkage of the RYR1 gene to MHS. However, resolution of

issues confounding the genetics of MH and its associated

disorders will be necessary before genetic diagnosis can

widely replace the in vitro contraction test.49

Similarly, screening for abnormal BChE genotypes is

neither practical nor cost effective in daily clinical practice,

but it could be used to supplement equivocal results obtained

by biochemical methods or in situations where establishing

the genotype may be clinically important.81

Furthermore, recent findings in molecular research suggest

that the outcome of cardiovascular surgery is at least partly

determined by the individual patient’s genetic predisposition

to react to surgical trauma and extracorporeal circulation.

Genomic variations may prove to serve as future diagnostic

tools for the risk stratification of patients undergoing cardio-

vascular surgery. The evaluation of possible genomic

markers for risk stratification of patients at high risk of

developing adverse outcomes has begun.69

Pharmacogenetics will have its application in clinical

research.76 When designing clinical trials, genotype can be

used a priori, as an exclusion criterion. With this methodo-

logy, the study group can be smaller and more homogeneous,

though less representative. Phase I trials can thus be designed

for representative populations of the principal metabolic

patterns.51 Alternatively, the genotype can be used a

Fig 2 Cartoon representation of the g-aminobutyric acid (GABA)-A receptor sitting in a cell membrane, indicating some of the drug binding sites. The spots

represent chloride ions flowing through the channel. Reprinted from Drug Discovery Today, Vol. 8, Whiting PJ. GABA-A receptor subtypes in the brain: a

paradigm for CNS drug discovery? p. 448, Copyright 2003, with permission from Elsevier.

Iohom et al.

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posteriori, as a stratification factor. It should also be recalled

that ethnic origin has an effect on allelic polymorphism, and

that trials conducted on Caucasians may not necessarily be

extrapolated to other ethnic groups.58

Conclusions

Pharmacogenetics is only at the very beginning of its use in

clinical practice. It is very likely that its contribution to new

drug development will become reality. Common polymorph-

isms in drug targets dictate that DNA sequence variations will

be taken into account in the genomic screening processes

aimed at new drug development. This will provide new

insights for the development of medications that target criti-

cal pathways in disease pathogenesis, and medications that

can be used to prevent diseases in individuals who are gene-

tically predisposed to them. This represents a migration from

the traditional strategy of trying to develop medications that

are safe and effective for every member of the population, but

one that is a pharmacological long shot because of highly

potent medications, genetically diverse patients, and diseases

that have heterogeneous subtypes. It is anticipated that, over

the next decade, the Human Genome Project, coupled with

DNA array technology, high-throughput screening systems

and advanced bioinformatics, will permit rapid elucidation of

complex genetic components of human health and disease.

Despite its promise, genomics has yet to make a major impact

on drug development processes. With tens of thousands of

genes now at our disposal, trying to identify the best drug

targets is difficult. By 2005, it has been predicted that the

average pharmaceutical company will be assessing 200 tar-

gets per year and 70% of these will be novel.57

Although pharmacogenetics is unlikely to change the way

anaesthesia is practised today it may help to elucidate inter-

patient variability in drug response. We will, undoubtedly,

see its impact on other specialties, on new drug development,

and in drug delivery systems.

AcknowledgementSupported by the Higher Education Authority in Ireland, Programme forResearch in Third Level Institutions.

References1 Arias TD, Jorge LF, Barrantes R. Uses and misuses of definitions of

genetic polymorphism. A perspective from population pharmaco-genetics. Br J Clin Pharmacol 1991; 31: 117–8

2 Belelli D, Lambert JJ, Peters JA, Wafford K, Whiting PJ. The inter-action of the general anesthetic etomidate with the gamma-

aminobutyric acid type A receptors is influenced by a singleamino acid. Proc Natl Acad Sci USA 1997; 94: 11031–6

3 Bertz RJ, Granneman GR. Use of in vitro and in vivo data to estimatethe likelihood of metabolic pharmacokinetic interactions. Clin

Pharmacokinet 1997; 32: 210–584 Bond C, LaForge KS, Tian M, et al. Single-nucleotide polymorphism

in the human mu opioid receptor gene alters beta-endorphin

binding and activity: possible implications for opiate addiction.Proc Natl Acad Sci USA 1998; 95: 9608–13

5 Borst P, Evers R, Kool M, et al. A family of drug transporters: themultidrug resistance-associated proteins. J Natl Cancer Inst 2000;

92: 1295–3026 Brinkmann U. Functional polymorphism of the human multidrug

resistance (MDR1) gene: correlation with P glycoprotein expres-sion and activity in vivo. Novartis Found Symp 2002; 243: 207–10

7 Brown PO, Botstein D. Exploring the new world of the genomewith DNA microarrays. Nature Genet 1999; 21: 33–7

8 Buggy DJ, Nicol B, Rowbotham DJ, Lambert DG. Effects of intra-venous anaesthetic agents on glutamate release: a role for GABAA

receptor-mediated inhibition. Anesthesiology 2000; 92: 1067–739 Cai H, White PS, Torney D, et al. Flow cytometry-base minise-

quencing: a new platform for high-throughput single-nucleotidepolymorphism scoring. Genomics 2000; 66: 135–43

10 Cholerton S, Daly AK, Idle JR. The role of individual humancytochromes P450 in drug metabolism and clinical response.

Trends Pharmacol Sci 1992; 13: 434–911 Cirella VN, Pantuck CB, Lee YJ, Pantuck EJ. Effects of cyclosporin

on anesthetic action. Anesth Analg 1987; 66: 703–6

12 Dale O, Olkkola KT. Cytochrome P450, molecular biology andanaesthesia. Acta Anaesthesiol Scand 1998; 42: 1025–7

13 Eaton DL, Bammler TK. Concise review of the glutathionS-transferases and their significance to toxicology. Toxicol Sci

1999; 49: 156–6414 Evans WE, Relling MV. Pharmacogenomics: translating functional

genomics into rational therapeutics. Science 1999; 286: 487–9115 Fagerlund TH, Braaten O. No pain relief from codeine . . .? An

introduction to pharmacogenomics. Acta Anaesthesiol Scand2001; 45: 140–9

16 Fletcher JE, Rosenberg H, Aggarwal M. Comparison of Europeanand North American malignant hyperthermia diagnostic protocol

outcomes for use in genetic studies. Anesthesiology 1999; 90:654–61

17 Fromm MF, Kauffmann HM, Fritz P, et al. The effect of rifampintreatment on intestinal expression of human MRP transporters.

Am J Pathol 2000; 157: 1575–8018 Gonzalez FJ, Idle JR. Pharmacogenetic phenotyping andgenotyping.

Present status and future potential. Clin Pharmacokinetics 1994; 26:59–70

19 Guitton J, Buronfosse T, Desage M, Lepape A, Brazier JL, Beaune P.Possible involvement of multiple cytochrome P450s in fentanyl and

sufentanil metabolism as opposed to alfentanil. Biochem Pharmacol1997; 53: 1613–9

20 Gunther U, Benson J, Benke D, et al. Benzodiazepine-insensitivemice generated by targeted disruption of the g2 subunit of

g-aminobutyric acid type A receptors. Proc Natl Acad Sci USA1995; 92: 7749–53

21 Gut IG. Automation in genotyping of single nucleotide polymorph-isms. Hum Mutat 2001; 17: 475–92

22 Haff L, Smirnov IP. Single-nucleotide polymorphism identificationassays using a thermostable DNA polymerase and delayed extrac-

tion MALDI-TOF mass spectrometry. Genome Res 1997; 7: 378–88

23 Hayes JD, Strange RC. Glutathion S-transferase polymorphismsand their biological consequences. Pharmacology 2000; 61:

154–6624 Hein DW, Doll MA, Fretland AJ, et al. Molecular genetics and

epidemiology of the NAT1 and NAT2 acetylation polymorphisms.Cancer Epidemiol Biomarkers Prev 2000; 9: 29–42

25 Herman RJ, Chaudhary A, Szakacs CB. Disposition of Lorazepam inGilbert’s syndrome: effects of fasting, feeding and enterohepatic

circulation. J Clin Pharmacol 1994; 34: 978–84

Pharmacogenetics and the anaesthetist

Page 9 of 11

by guest on October 5, 2015

http://bja.oxfordjournals.org/D

ownloaded from

26 Hoffmeyer S, Burk O, von Richter O, et al. Functional polymorph-isms of the human multiresistance gene: multiple sequence varia-

tions and correlation of one allele with P-glycoprotein expressionand activity in vivo. Proc Natl Acad Sci USA 2000; 97: 3473–8

27 Homanics GE, Quinlan JJ, Mihalek RM, Firestone LL. Alcohol andanesthetic mechanisms in genetically engineered mice. Front Biosci

1998; 3: D548–5828 Hughes MA, Glass PS, Jacobs JR. Context-sensitive half-time

in multicompartment pharmacokinetic models for intravenousanesthetic drugs. Anesthesiology 1992; 76: 334–41

29 Iles DE, Lehman-Horn F, Scherer SW, et al. Localisation of the geneencoding the a2=d subunits of the L-type voltage-dependent cal-

cium channel to chromosome 7q and analysis of the segregation offlanking markers in malignant hyperthermia susceptible families.

Hum Mol Genet 1994; 3: 969–7530 Inaba T, Nebert DW, Burchell B, et al. Pharmacogenetics in clinical

pharmacology and toxicology. Can J Physiol Pharmacol 1995; 73:331–8

31 Ingelman-Sundberg M. Pharmacogenetics: an opportunity for asafer and more efficient pharmacotherapy. J Intern Med 2001;

250: 186–200

32 Iwata N, Cowley DS, Radel M, Roy-Byrne PP, Goldman D. Rela-tionship between a GABAA alpha 6 Pro385Ser substitution and

benzodiazepine sensitivity. Am J Psychiatry 1999; 156: 1447–933 Jensen FS, Schwartz M, Viby-Mogensen J. Identification of human

plasma cholinesterase variants using molecular biological tech-niques. Acta Anaesthesiol Scand 1995; 39: 142–9

34 Kalow W. Pharmacogenetics: its biological roots and the medicalchallenge. Clin Pharmacol Ther 1993; 54: 235–41

35 Keown PA, Stiller CR, Laupacis AL, et al. The effects and side effectsof cyclosporine: relationship to drug pharmacokinetics. Transplant

Proc 1982; 14: 659–6136 Kharasch ED, Thummel KE. Human alfentanil metabolism by

cytochrome P450 3A3=4. An explanation for the interindivi-dual variability in alfentanil clearance? Anesth Analg 1993; 76:

1033–937 Kharasch ED, Thummel KE. Identification of cytochrome P450 2E1

as the predominant enzyme catalyzing human liver microsomaldefluorination of sevoflurane, isoflurane and methoxyflurane.

Anesthesiology 1993; 79: 795–80738 van der Kleij FG, Schmidt A, Navis GJ, et al. Angiotensin converting

enzyme insertion=deletion polymorphism and short-term renalresponse to ACE inhibition: role of sodium status. Kidney Int

1997; 63: 23–639 Klein I, Sarkadi B, Varadi A. An inventory of the human ABC

proteins. Biochim Biophys Acta 1999; 1461: 237–6240 Kruglyak L. The use of a genetic map of biallelic markers in linkage

studies. Nature Genet 1997; 17: 21–441 Kruglyak L. Prospect for whole-genome linkage disequilibrium

mapping of common disease genes. Nature Genet 1999; 22:139–44

42 La Du BN. Butyrylcholinesterase variants and the new methods ofmolecular biology. Acta Anaesthesiol Scand 1995; 39: 139–41

43 Lasocki S, Iglarz M, Seince PF, et al. Involvement of renin-

angiotensin system in pressure-flow relationship. Anesthesiology2002; 96: 271–5

44 Leeder JS. Pharmacogenetics and pharmacogenomics. Pediatric ClinN Am 2001; 48: 765–81

45 Levitt RC, Olckers A, Meyers S, et al. Evidence for the localizationof a malignant hyperthermia susceptibility locus (MHS2) to human

chromosome 17q. Genomics 1992; 14: 562–646 Levy RH. Cytochrome P450 isoenzymes and antiepileptic drug

interactions. Epilepsia 1995; 36: S8–13

47 Lieber CS. Cytochrome P-4502E1: its physiological and patholo-gical role. Physiol Rev 1997; 77: 517–44

48 Linder MW, Prough RA, Valdes R. Pharmacogenetics: a laboratorytool for optimizing therapeutic efficiency. Clin Chem 1997; 43:

254–6649 McCarthy TV, Quane KA, Lynch PJ. Ryanodine receptor mutations

in malignant hyperthermia and central core disease. Hum Mutat2000; 15: 410–7

50 Monnier N, Procaccio V, Stieglitz, et al. Malignant-hyperthermiasusceptibility is associated with a mutation of the alpha 1-subunit of

the human dihydropyridine-sensitive L-type voltage dependentcalcium-channel receptor in skeletal muscle. Am J Hum Genet

1997; 60: 1316–2551 Murphy MP, Beaman ME, Clark LS, et al. Prospective CYP2D6

genotyping as an exclusion criterion for enrollment of a phaseIII clinical trial. Pharmacogenomics 2000; 10: 583–90

52 Nebert DW. Pharmacogenetics and pharmacogenomics: why isthis relevant to the clinical geneticist? Clin Genet 1999; 56:

247–5853 Negishi M, Pedersen LG, Petrotchenko E, et al. Structure and

function of sulfotransferases. Arch Biochem Biophys 2001; 390:

149–5754 Nilsson M, Malmgren H, Samiotaki M, et al. Padlock probes: cir-

cularizing oligonucleotides for localized DNA detection. Science1994; 265: 2085–8

55 Parsons BL, Heflich RH. Genotypic selection methods for thedirect analysis of point mutations. Mutat Res 1997; 387:

97–12156 Patel M, Tang BK, Grant DM, Kalow W. Interindividual variability in

the glucuronidation of (S) oxazepam contrasted with that of (R)oxazepam. Pharmacogenetics 1995; 5: 287–97

57 Pimstone S. Old-fashioned genetics remains genomics’ best friend.PharmaTech 2002; 2: 88–90

58 Poolsup N, Li Wan PO A, Knight TL. Pharmacogenetics and psy-chopharmacotherapy. J Clin Pharm Ther 2000; 25: 197–220

59 Robinson RL, Monnier N, Wolz W, et al. A genome wide search forsusceptibility loci in three European malignant hyperthermia pedi-

grees. Hum Mol Genet 1997; 6: 953–6160 Roses AD. Pharmacogenetics and the practice of medicine. Nature

2000; 405: 857–6561 Schinkel AH. The physiological function of drug-transporting

P-glycoproteins. Semin Cancer Biol 1997; 3: 161–7062 Schinkel AH, Wagenaar E, Mol CA, van Deemter L. P-glycoprotein

in the blood-brain barrier of mice influences the brain penetrationand pharmacological activity of many drugs. J Clin Invest 1996; 97:

2517–2463 Schuber AP, Michalowsky LA, Nass GS, et al. High throughput

parallel analysis of hundreds of patient samples for more than100 mutations in multiple disease genes. Hum Mol Genet 1997;

6: 337–4764 Shi MM, Bleavins MR, de la Iglesia FA. Technologies for detecting

genetic polymorphisms in pharmacogenomics. Mol Diagn 1999; 4:343–54

65 Shimada T, Yamazaki H, Mimura M, Inui Y, Guengerich FP. Inter-

individual variations in humancytochrome P-450enzymes involvedin the oxidation of drugs, carcinogens and toxic chemicals: studies

with liver chromosomes of 30 Japanese and 30 Caucasians.J Pharmacol Exp Ther 1994; 270: 414–23

66 Slaughter RL, Edwards DJ. Recent advances: the cytochrome P450enzymes. Ann Pharmacother 1995; 29: 619–24

67 Spatzeneger M, Jaeger W. Clinical importance of hepatic cyto-chrome P450 in drug metabolism. Drug Metabolism Rev 1995;

27: 397–417

Iohom et al.

Page 10 of 11

by guest on October 5, 2015

http://bja.oxfordjournals.org/D

ownloaded from

68 Stockley IH. Neuromuscular blocker and anaesthetic drug inter-actions. In: Stockley IH, ed. Drug Interactions. London: Pharmaceu-

tical Press, 1998; 722–5669 Stuber F, Hoeft A. The influence of genomics on outcome after

cardiovascular surgery. Curr Opin Anaesthesiol 2002; 15: 3–870 Sudbrak R, Procaccio V, Klausnitzer M, et al. Mapping of a further

malignant hyperthermia susceptibility locus to chromosome3q13.1 Am J Hum Genet 1995; 56: 684–91

71 Sweeney BP. Why does smoking protect against PONV? Br JAnaesth 2002; 89: 1–4

72 Syvanen AC. Fromgels to chips: ‘minisequencing’ primer extensionfor analysis of point mutations and single nucleotide polymorph-

isms. Hum Mutat 1999; 13: 1–1073 Tanaka E. Update: genetic polymorphism of drug metabolizing

enzymes in humans. J Clin Pharmacy Therapeut 1999; 24: 323–974 Thompson SJ, Koszdin K, Bernards CM. Opiate-induced analgesia

is increased and prolonged in mice lacking P-glycoprotein. Anesthe-siology 2000; 92: 1392–9

75 Thummel KE, Kharasch ED, Podoll T, Kunze K. Human liver micro-somal enflurane defluorination catalyzed by cytochrome P-450

2E1. Drug Metab Dispos 1993; 21: 350–7

76 Tribut O, Lessard Y, Reymann JM, Allain H, Bentue-Ferrer D.Pharmacogenomics. Med Sci Monit 2002; 8: 152–63

77 Tucker GT. Clinical implications of genetic polymorphism in drugmetabolism. J Pharmacy Pharmacol 1994; 46: 417–24

78 Tukey RH, Strassburg CP. Human UDP-glucuronosyltransferases:metabolism, expression, and disease. Annu Rev Pharmacol Toxicol

2000; 40: 581–61679 Tyagi S, Bratu DP, Kramer FR. Multicolor molecular beacons for

allele discrimination. Nat Biotechnol 1998; 16: 49–5380 Uhl GR, Soar I, Wang Z. The mu opiate receptor as a candidate

gene forpain: polymorphism, variations inexpression, nociception,and opiate responses. Proc Natl Acad Sci USA 1999; 96: 7752–5

81 Viby-Mogensen J, Ostergaard D, Donati F, et al. Pharma-cokinetic studies of neuromuscular blocking agents: good

clinical research practice (GCRP). Acta Anaesthesiol Scand 2000;44: 1169–90

82 Wandel C, Kim R, Wood M, Wood A. Interaction of morphine,fentanyl, sufentanil, alfentanil and loperamide with the efflux drug

transporter P-glycoprotein. Anesthesiology 2002; 96: 913–2083 Wang D, Quillan JM, Winans K, Lucas JL, Sadee W. Single nucleo-

tide polymorphisms in the human m opioid receptor gene alterbasal G protein coupling and calmodulin binding. J Biol Chem 2001;

276: 34624–3084 van der Weide J, Steijns LSW. Cytochrome P450 enzyme system:

genetic polymorphisms and impact on clinical pharmacology. AnnClin Biochem 1999; 36: 722–9

85 West WL, Knight EM, Pradhan S, Hinds TS. Interpatient variability:genetic predisposition and other genetic factors. J Clin Pharmacol

1997; 37: 635–4886 Whiting P. GABA-A receptor subtypes in the brain: a paradigm for

CNS drug discovery? Drug Discov Today 2003; 8: 445–5087 de Wildt SN, Kearns GL, Leeder JS, et al. Glucuronidation in

humans. Clin Pharmacokinet 1999; 36: 439–5288 Wilke K, Gaul R, Klauck SM, Poustka A. A gene in human chromo-

some band Xq28 (GABRE) defines a putative new subunit class of

the GABAA neurotransmitter receptor. Genomics 1997; 45: 1–1089 Wood M. Drug distribution: Less passive, more active? Anesthesiol-

ogy 1997; 87: 1274–690 Wu CY, Benet LZ, Herbert MF, et al. Differentiation of absorption

and first-pass gut and hepatic metabolism in humans: studies withcyclosporine. Clin Pharmacol Ther 1995; 58: 492–7

91 Yue QY, Svensson JO, Sawe J, Bertilsson L. Codeine metabolism inthree oriental populations: a pilot study in Chinese, Japanese and

Koreans. Pharmacogenetics 1995; 5: 173–7792 Zhang Y, Benet LZ. The gut as a barrier to drug absorption:

combined role of cytochrome P450 3A and P-glycoprotein. ClinPharmacokinet 2001; 40: 159–68

93 Zuhlsdorf MT. Relevance of pheno- and genotyping in clinical drugdevelopment. Int J Clin Pharmacol Ther 1998; 36: 607–12

Pharmacogenetics and the anaesthetist

Page 11 of 11

by guest on October 5, 2015

http://bja.oxfordjournals.org/D

ownloaded from