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Current Drug Metabolism, 2003, 4, 33-44
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Drug Metabolism and Individualized Medicine
Pratima Srivastava*
Division of Pharmacokinetics and Drug Metabolism Central Drug Research Institute Lucknow-1 India
Abstract: Drug metabolism refers to the biochemical transformation of a compound into another more polar chemical
form. Absorption, distribution, metabolism and excretion comprise an integral part in understanding the safety and
efficacy of a potential new drug. Detailed in-depth knowledge of the Pharmacokinetics and Drug Metabolism of a new
drug entity is considered a prerequisite to know the appropriate route of administration, correct dose etc. Sometimes there
is (are) different/unwanted effect(s) of certain drugs in different populations. This is particularly true for the drug having
narrow therapeutic index. Often these different effects are detrimental to an individual, thus termed as adverse drug
reactions. After the raw draft of human genome has evolved, it has become increasingly clear that change(s) in the drug
response between individuals, is due to the occurrence of genetic polymorphisms in the Phase I and II drug metabolizing
enzymes, due to which distinct subgroups in the population differ in their ability to perform certain drug
biotransformation reactions. The study about the occurrence of genetic polymorphisms in drug metabolizing enzymes is
termed as Pharmacogenetics/ Pharmacogenomics. Pharmacogenetic characterization of particular drug can be both
phenotypically or genotypically conducted in population groups. The study is very important to check the post-marketed
drug withdrawal, if a particular percentage of population suffers from adverse drug reactions, and thus a lot of revenue be
saved. The study also helps to find out Right Medicine for Right Individual or Individualized Medicine.
HUMAN GENOME
The mission impossible has been possible, thanks and
credentials to the scientists involved in the project. The $ 3
billion Human Genome Project to map and read the
complete instruction manual of humanity had its origins in
the meeting at ski resort town of Alta in Utah, US. Scientists
agreed that it would have to be one immense, complex and
expensive program: transcribing the BOOK OF LIFE. On
June 26Th 2000, officially the book of life was open for all
after the completion of the first rough draft of the Human
Genome Project [1]. Near completion of the Human Genome
Project may not be taken as an end to the genetic
characterization of the human genetic element but rather a
beginning in which we can be benefited by the previous
present knowledge in the field. Real information, which
could be gathered from the study, is:
#
Human genes are actually much less than estimated
earlier.
#
It is actually a gene network and not a gene, which is
responsible for translation.
#
Much correlation exists between the bacterial and the
human genome.
#
The difference in genomes between different races is
minuscule (0.1%).
This difference in the genome is responsible for altered
enzymatic/receptor activities in the human, and when it is in
*Address correspondence to this author at the Dept. of Pharmacology and
Therapeutics Roswell Park Cancer Institute Buffalo NY-14263 USA; Email: [email protected]; [email protected] and
[email protected]
1389-2002/03 $41.00+.00
the drug metabolism enzymes, then it is sure that the individual will suffer from alteration in the drug metabolizing
capacity. Also it may make the individual prone to the
infection and the toxins generated therein, because of the
change in the endobiotics metabolizing capacity.
During our increasing efforts to know the uncanny occult of
nature we come across different diseases, which affect the
human population and to harness them there are myriad of
drugs. The effect of the drugs is to curb the disease but
sometimes a quite contrary picture emerges i.e. the
occurrence of the unnatural side effects of the drugs
popularly known as the adverse drug reactions or ADRs [2].
This is due to the vulnerable alteration(s) in the genetic
makeup of an individual, and sometimes it can be life
threatening also [3].
The therapeutic as well as side effects of a drug may vary
according to genetic make-up of an individual [4]. This is
particularly important for drugs with narrow therapeutic
index, or a wide dose response curve. Because of genetic
variation in the drug metabolizing capacity, a predisposed
individual may show one of the following variant responses:
a). Lack of efficacy at normal drug dose, requiring higher
dose to achieve the expected therapeutic response.
b). Much higher effect at the normal dose leading to
development of significant side effects which are
otherwise expected only at the higher dose.
For these individuals, a lower dose of the drug may be
effective and acceptable. If the number of such individuals in
the population is large (which may vary from population to
population) then even an otherwise good drug may be
discarded as ineffective or too toxic.
© 2003 Bentham Science Publishers Ltd.
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Current Drug Metabolism, 2003, Vol. 4, No. 1
In the genetically variable individuals the drug has a
qualitatively different response, therefore, unexpected
(idiosyncratic) adverse drug reactions may be seen; eg.
Primaquine induced haemolytic anemia in G-6PD deficient
individual [5]. It is well evident that as a result of the genetic
differences, the rate of metabolism of nortriptyline in adults
ranges from 18 to 93 hr. Please refer to http://www.
pharmabiz.com/archives and F.D.C. Reports 1997 [6].
DRUG METABOLISM
Historical Perspectives
The leading early workers in the field of Drug
Metabolism were chemists. Woehler was the pioneer person
who identified potential chemical transformations occurring
in the body and conducted experiments in dogs [7-8]. He
speculated the conversion of benzoic acid to hippuric acid,
but his initial experiments in dogs led to the conclusion that
only benzoic acid was excreted in the urine. This was due to
the fact that hippuric acid was not fully characterized until
1829 by Liebig.
Alexander Ure performed the first human metabolism
study in 1841, he observed the conversion of benzoic acid to
hippuric acid and proposed the use of benzoic acid for the
treatment of gout [9]. W.Keller, in Woehler's laboratory
provided the confirmation of Ure's experiments. Thus, not
only had Keller and Woehler delineated the first biochemical
study they unveiled a ready source for new compounds.
Since then a lot many novel and innovative works have been
conducted in the field of Drug Metabolism.
Drug metabolizing enzymes are responsible for
degradation of drugs and environmental pollutants to aid
their excretion and are important determinants of drug action
[10]. Williams (1959) [11] found out that drug metabolism is
carried out in two phases: Phase I and Phase II. In the first
phase the hydrophobic moieties are converted into more
hydrophilic ones by oxidation, reduction, hydrolysis,
hydroxylation or deamination reactions, and then the
conjugation/synthesis/addition starts in which there can be
sulphation, glucuronidation, methylation, acetylation (to
name the few) to eliminate the hydrophilic toxic material
outside the body.
CYTOCHROME P-450
Cytochrome P-450s are the pivot part of the Phase I drug
metabolism. It is an unusual carbonmonoxide binding
pigment in its reduced state giving rise to absorption maxima
of 450nm. The pigment was characterized by Omura & Sato
(1964) [12]. Cytochrome P-450 monoxygenase system of
enzymes is responsible for the major portion of drug
metabolism in humans. They are ubiquitous in nature.
Although commonly serving to detoxify xenobiotics, these
enzymes are also involved for the activation of the
procarcinogens and promutagens in the human body. It is of
importance for the lipophilic drugs such as the CNS- active
drugs, which generally must be lipophilic to penetrate the
blood brain barrier. There are about 215 different families of
Pratima Srivastava
cytochrome P-450. Bacteria class has 72 families, lower
eukaryotes have 29 families, and plants have 50 families and
animal kingdom have 67 families. The human liver
microsomal cytochrome P-450 (CYP EC 1.14.14.1) has been
shown to play a major role in the metabolism of many drugs,
some of which exhibit a narrow therapeutic index or a steep
dose-pharmacological response profile [13-14]. Among the
numerous P450 subtypes, CYP2D6, 3A4/3A5, 1A2, 2E1,
2C9 and 2C19 play important part in genetically determined
responses to abroad spectrum of drugs (Fig. 1). Currently,
the Cytochrome P450 (CYP) family of enzymes is the
subject of much research [15]. The best known of the
cytochrome P-450 enzymes is CYP2D6, 4-6% of the total,
which plays an important part in the metabolism of betablockers, antidepressants and other drugs [16]. Another
important cytochrome P-450 is 3A4, which is involved in the
first pass metabolism as well as the systemic clearance of the
drugs. Drugs interactions with this isoform have high clinical
potential in cardiovascular diseases and cancer as well as the
mutation/polymorphism in it can lead to adverse drug
reactions [17]. Other cytochrome P-450 subtypes also result
in delayed metabolism of certain medicines [18-19]. Slow Nacetyltransferase forms are found in majority of the
populations.
The metabolism of the drug may take longer than
anticipated, thereby increasing the risk and intensity of sideeffects [20]. In case of high metabolic rates the therapeutic
effect may be diminished or absent. Metabolic rates depend
mainly on cytochrome P-450 and N-acetyltransferase
enzymes [21]. Patients may be classified as fast or slow
metabolizers [22], depending on the activity levels of these
enzymes. Some variants are slow metabolizers while others
are ultra-rapid metabolizers. Slow metabolizers may be
exposed to the active product for longer than ultra-rapid
metabolizers, or have greatly diminished exposure to the
active metabolite. Conversely, ultra-rapid metabolizers risk
longer exposure to the metabolite and much shorter exposure
to the administered medication. Usually drug is inactivated
by the P450 [23] and poor metabolizers have higher blood
level of drug and enhanced drug toxicity. It can result in loss
of action of prodrug or toxicity due to the increase in
metabolism of another pathway leading to toxic product as
well as can have the important interactions leading to toxic
response if combination of drugs metabolized by P450 is
used [24].
If the in vitro clearance of a drug is largely (fm>
CYP40%) mediated by a single polymorphically expressed
or allelic variant form of CYP, it is anticipated that poor
metabolizers will be characterized by disparate
pharmacokinetics (e.g. elevated plasma AUCs and /or
increased T1/2) [25]. In addition, drugs are often
metabolized to pharmacologically active metabolites via
CYP- mediated oxidations, which implies that the
pharmacodynamics of many drugs can be modulated by
induction/inhibition of certain CYPs [26]. Approximately
40% of human cytochrome P-450 (CYP) − dependent drug
metabolism is carried out by polymorphic enzymes [27-28].
These enzymes are known to show inter-individual and interethnic variation in the activities and are governed mainly by
genetic factors [29]. Also, drug metabolism is affected by
many external and internal factors some of them include:
Drug Metabolism and Individualized Medicine
Current Drug Metabolism, 2003, Vol. 4, No. 1
35
Fig. (1). There are a negligible number of drugs, which do not undergo CYP450 dependent biotransformation prior to elimination.
age, sex, environment, diet, effect of one drug on another
(drug-drug interactions) as well as the genetic makeup of an
individual [30].
POLYMORPHISMS IN DRUG METABOLISM/PHARMACOGENETICS/PHARMACOGENOMICS/SNPS
Interindividual variations in the drug metabolism are due
to the presence of the alterations in the nucleotide sequence
of the drug metabolizing enzymes [31] and when these
variations are stabilized in generation they are termed as
polymorphisms.The
N-acetyl
transferase
(NAT)
polymorphism, discovered in the 1950s demonstrated
remarkable variability in allele frequency among different
ethnic species [32]. A high proportion of slow acetylators is
present in populations around the equator, and higher and
higher proportions of rapid acetylators as one moves north.
Large number of polymorphisms also occurs in GST
(glutathione-S-transferase), an enzyme of Phase II drug
metabolism involved in detoxification of environmental
toxins. The GST null allele has been shown to affect
individuals’ susceptibility to various forms of cancer. The
presence of combined allele M1 and T1 deficiencies in
glutathione-S-transferase genes increases the susceptibility
of white patients to tarcrine (an antialzheimer drug)
hepatotoxicity [33].
CYP2D6 metabolizes more than 30 or 40 commonly
used drugs. This enzyme (debrisoquine hydroxylase) was the
first drug-metabolizing enzyme found to be polymorphically
expressed. It has >100-fold variability in its function, there
are more than 70 variant alleles of CYP2D6. Alteration in
the CYP2D6 activity may lead to the decreased first pass
metabolism and excretion (metoprolol), decreased
metabolism of the drugs, thereby causing the increased
retention in the body (perhexilene). The variation in this
gene can make some individuals being poor metabolizers
(null allele) with impaired degradation and excretion of
many drugs like debrisoquine, metoprolol, nortriptyline and
propafone and others being very rapid metabolizers. Also
during the null variation there is no effect of the opium
analgesics on the patients, because they cannot be converted
into their active metabolites [34].
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Current Drug Metabolism, 2003, Vol. 4, No. 1
CYP2C9 metabolizes the S-enantiomer of warfarin,
which is majorily responsible for anticoagulant effect.
Therefore, in the CYP2C9 deficient patients, less amount of
warfarin is required; the normal dosage of the drug will lead
to excessive bleeding in these patients [35]. CYP2C19 also
exist in the genetic variant forms. The null allele of this is
highly sensitive to the drugs including omeprazole,
diazepam, propranolol, mephenytoin, amitriptyline, and
hexobarbital.
TPMT (thiopurine methyltransferase) is responsible for
the metabolism of 6-mepcaptopurine and its pro-drug,
azathiopurine into thioguanine nucleotide, which is required
for the cytotoxic effect of the formers. TPMT has 8 allelic
variants, which exhibit low activity. This effect is associated
with more amounts of the drug requirement and hence more
toxic side −effects [36].
UGT (UDP-glucuronosyl transferase) helps in the
glucuronidation of bilirubin, decreased activity of the UGT1
is linked with the patients of Gilbert syndrome. Toxicity was
observed with the antitumor drug Irinotecan in the patients
having heterozygous or homozygous genotype for UGT1
[37].
Transporters are important to determine the rate of
absorption, distribution and excretion of the drugs. Amongst
the transporters P-glycoproteins has been found to exhibit
polymorphism. P-glycoprotein is a multidrug resistance
efflux pump, any alteration in it is definitely going to affect
the occurrence of resistance as well as its therapeutic aspects
[38].
Receptors like Beta 2-adrenoceptor, dopamine D3
receptor (toxicity towards dopamine), ryanodine receptor
(defect leads to the enhanced rates of Ca2+ release from the
sarcoplasmic reticulum during anaesthesia treatment) have
been found to possess the polymorphisms [39].
Ion channels responsible for the normal ventricular
repolarization are known to exhibit polymorphic characters,
resulting in the prolonged QT interval time following
administration of sulfamethoxazole-trimethoprim [40].
The study about the genetic behavior of drugs is
Pharmacogenetics. Pharmacogenetics may be defined as
study of genetically controlled variation in response to
xenobiotics [41]. The later includes drugs as well as various
environmental chemicals. The variation in drug response can
be either pharmacokinetic or pharmacodynamic in nature.
Pharmacokinetic variation determines the level of the active
substance at the site of its action while pharmacodynamic
effect refers to quantitative/qualitative variation in response
of the target [42]. Pharmacological and toxicological effects
often vary extensively not only in different strains /species
but also in a similar strain/species. It may be the result of
either only a single amino acid alteration in a protein/enzyme
(pharmacogenetics) or involving more than one amino acid
in more than one protein/enzyme (pharmacogenomics),
controlling the xenobiotic receptor binding/ disposition [43].
Different responses of drug/ exogenous chemicals by
different species/ strain serves as a sensitive probe to explore
basic/altered mechanism of xenobiotic action.
Pratima Srivastava
Historical Perspectives
It has long been known that the organisms behave
differently to the environmental factors and that it is
governed basically by the genetical makeup. Also in mid
nineteen it was established that the chemicals are excreted in
different forms. Garrod (1902) [44] while studying the
alcaptonuria in humans hypothesized the connection between
the enzymes and the genes. On the basis of this he suggested
that the genetic material could play an important role in the
biotransformation of chemicals. Pharmacogenetics was
evolved in the 1950’s to understand the genetic differences
that cause people to metabolize drugs differently. Early
pharmacogenetic studies were based on gross ethnic
variation. There were three major ethnic groups: the
Negroid, Mongoloid and Caucasoid. Ethnicity can be further
narrowed to geography, anthropology, language, and race
[45]. These studies of populations and incidence were first
carried out mainly by chance or just because investigators
were curious about how one race compared to another in
response to a drug. The collected data showed that this
should be a standard part of any drug development process.
The first demonstrated pharmacogenetic trait was
chemical insensitivity to phenylthiourea (PTU). Individuals
with PTU insensitivity could not taste the chemical. It was
the first chemical insensitivity shown to be heritable [46].
Another early pharmacogenetic trait shown to have variation
by ethnic group was drug-induced hemolysis due to glucose6-phosphate dehydrogenase deficiency [47]. From the 1950s
to the present, there has been a rush of new technologies
combined with a new genetic approach so that variations
could be isolated on a person-to-person level. One of the first
genetic level discoveries was population variation of the Nacetyl transferase polymorphism, a polymorphism with a
connection to geography [48]. The allelic frequency–the
frequency of the gene within that population–of the slow
acetylator polymorphism decreases as geographic latitude
increases.
Friedrich Vogel coined the word Pharmacogenetics [49]
and the first book on the subject was published in 1962 [50].
There are more than 150 Pharmacogenetic differences (some
of the important ones are mentioned in the table 1), basically
due to the alterations in the receptor, enzymes, transporters
or channels. With advancement of technology, it is important
to evaluate the pharmacogenomic profile of the selected
drug(s), such as the antipsychotics, antidepressants and
antihypertensives, which require careful dose titration, as
well as the drugs having narrow therapeutic index [41].
While pharmacogenetics has to do with individuals’
response to certain drugs, pharmacogenomics is a broader
term used to "describe the commercial application of
genomic technology in drug development and
therapy…[S]cientists will identify or design therapeutic
agents that interact with these targets in a way that achieves
positive clinical outcome and minimal toxicity [51]." Infact
Pharmacogenomics can be regarded as the best blend of
combinatorials- chemistry and biology which will sort out
the relationship between the genotype and phenotype effects
of a pharmacophore on the patient. Adam (2001) has given
the essential prerequisites for the development of a
Drug Metabolism and Individualized Medicine
Table 1.
Current Drug Metabolism, 2003, Vol. 4, No. 1
37
Percentage of Incidence of Different Pharmacogenetic Traits in Various Population Groups
Pharmacogenetic
Traits
% of Incidence In
African
blacks
American
Blacks
American
Whites
Chinese
Eastern
eskimos
Mediterra
nean
Japanese
Caucasian
Asians
1. Phenylthiourea or
taste blindness
65
2-23
30
6
40
nd
nd
nd
nd
2. G6PD deficiency
10-15
nd
nd
nd
nd
10-15(nil)
nd
nd
nd
3. Alcohol Sensitivity
nd
nd
nd
nd
nd
nd
15-20
10-15
nd
4. N-acetyltransferase
nd
36-42
nd
nd
nd
nd
68
55-60
nd
5. Glutathione-Stransferase null allele
0.31
nd
nd
nd
nd
nd
nd
0.4-0.5
nd
6. Cyp2D6
5-10
nd
nd
nd
nd
nd
nd
5-10
1-2
7. Cyp2C19
nd
nd
nd
nd
nd
nd
10-20
3-15
18-23
8. TPMT null
nd
nd
nd
nd
nd
nd
nd
0.3%
nd
9. Cyp 3A4 –V
nd
nd
53
nd
nd
nd
nd
9
nd
10. UGT
heterogenous allele
nd
nd
nd
nd
nd
1-2
nd
nd
nd
11. Cyp 2C9 deficiency
nd
nd
nd
nd
nd
nd
nd
25
nd
TPMT: thiopurine methyltransferase.
UGT: UDP glucuronosyl transferase.
successful pharmacogenomic-model, which includes the
definite genotype and phenotype characterization happening
in response to the drug treatment (Genetic Signatures) and its
use to predict the drug response of an individual [52].
Classification of Pharmacogenomic Phenomenon
Pharmacogenomic phenomenon can be classified by
combining two criteria: pharmacological and genetic.
Genetic point of view: single gene effect (including
polymorphisms) and polygenic effects and pharmacological
point of view: time − course of drug action (pharmacokinetics) including the influence of biotransformation, and
effects of drugs on tissues and organs (pharmacodynamics).
The study of pharmacogenetic differences holds the
potential to improve therapeutic effectiveness and limit
toxicity of available drugs. Pharmacogenetics can provide
substantial efficiency in clinical research by facilitating
smaller clinical trials in individuals with similar genetic
background [53]. The approach of rigorous determination of
genotype/phenotype relationships in individual drug
responses provide physicians and researchers with the key
information that allows them to precisely prescribe or design
the right drug, at the right dose, for the right patient. This
singular individualized approach to therapeutics is enabled
by highthroughput genotyping and provides significant
public health benefits to the population at large.
Pharmacogenetic conditions are of great practical interest
in understanding the causes of large differences among
subjects in response to drugs/ environmental chemicals [54].
That explains why research in Pharmacogenetics has become
both diverse and complex. Genetic differences can result in
considerable variation in the rate at which a given medicine
is broken down within a patient’s body [55]. Due to the
recent advances made in Pharmacogenomics, it is interesting
to explore and define the receptor binding/ drug disposition
in terms of pharmacogenetic characterization of individuals.
Pharmacogenetic effect can be caused by difference not
only in enzymatic conversion rates, but also by interindividual variations in the proteins to which the drugs are
targeted (target proteins), or by genetically determined
unintended interference with normal physiological processes
[56]. As pharmacogenetic knowledge develops, it can be
increasingly used for the development of new medicines.
This may result, on the one hand, in drugs, that exhibit less
variation in their metabolic conversion rates and, on the
other, in drugs for which pre-prescription DNA or enzyme
testing is desirable. Most recently released drugs have been
developed with metabolic variations involving the
cytochrome P-450 enzymes in mind [57]. In the years ahead,
the number of potential pharmaceutical research topics will
grow rapidly because of increasing (pharmaco-) genetic
knowledge. Pharmacogenetics provides a perspective on
individuality of drug response.
The genome in human vary in terms of single base
changes or Single nucleotide polymorphisms (SNPs, pronounced “snips) [58]. Such variants are frequent throughout the
genome, in the ratio of about one SNP for every 1000 base
pairs. Hundreds of thousands of polymorphisms can now be
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Current Drug Metabolism, 2003, Vol. 4, No. 1
identified and precisely ordered in a high-density SNP map
[59]. The SNP consortium is a joint venture of Wellcome
Trust with 10 major Pharmaceutical Companies please refer
to http:// www.snp.cshl.org. About 300,000 SNPs are expected
to be complete by Apr 2002.
If in clinical trials only patients having pharmacogenetic
efficacy profiles are enrolled, regulatory authorities could
consider a significantly enhanced surveillance system with
provisional marketing approval for patients with “efficacy”
pharmacogenomic profiles [60]. As ADRs are documented,
DNA could be extracted for patients with a particular drugrelated adverse event and compared with well-matched
patients who took the drug but did not experience the ADR.
The abbreviated SNP profile for these ADRs could then be
added to the abbreviated SNP profile for knowing new
drug’s efficacy [61]. Search of predictive SNPs in genes
relies on the promoters (transcription), transcript processing
and amino acid coding regions. There must be a
comprehensive correlation between the advanced genotypephenotype data as well as the statistical analysis and eresearch of the same to gain important information from the
same and to implement them in finding out the adverse drug
reactions.
Pratima Srivastava
Also due to the drug-drug interactions there can be adverse
drug reactions; the best example can be the coadministration
of Phentermine and Fenfluramine, which result in the
development of valvular fibrosis in a small subset of healthy
obese women, may be occurrence of polymorphism in a
particular gene, thus there was a withdrawal of Fenfluramine
from the market. There are myriads of ADRs which can be
life threatening too (please refer to table 2).
We can already identify gene variants that encode for
drug metabolising enzymes, which may affect the effective
dose or ADR pattern. These gene variants can be due to the
variation in the nucleotide sequences and the reason [69]
behind the occurrence of this polymorphism can be
1. Single nucleotide polymorphism or SNP
2. Insertion or deletions in the nucleotide sequence or Indel
3. Random Insertion or deletions in the nucleotide sequence
or Tandem indel
4. Amplification/ rearrangement of genes [70].
Amongst these the single nucleotide polymorphism is the
most occurring process.
ADVERSE DRUG REACTIONS
Adverse drug reactions (ADRs) are often unnoticed or
well characterized, but then too more than 1 million deaths
occurs globally per annum, particularly in the developing
countries [62]. Recently ADR has been defined as “an
appreciate harmful or unpleasant reaction, resulting from an
intervention related to the use of the medicinal products,
which predicts hazards from the future administration and
warrants prevention or specified treatment or alteration of
the dosage regimen or withdrawal of the product” [63].
Adverse events are more likely to be missed in smaller
clinical trials and that no safety information will be
generated outside the efficacy population, which could
present a risk for “off-label” use or in circumstances where
the response profile is not used. However, characterization of
rare ADRs through conventional clinical trials and current
post-marketing surveillance systems based on the voluntary
reporting of ADRs following registered and off- label use of
medicines already presents a challenge [64].
Pharmacogenetics has the promise of removing much of
the uncertainty of the reliability of the drugs. Physicians will
be able to use a medicine response profile to predict an
individual’s likely response before a medicine is prescribed
[65]. Although not immediately obvious, the pharmaceutical
industry and the public should also benefit by faster and
more efficient clinical trials, more treatments for more
patients, reduced costs of drug development, expansion of
research to cover more diseases, and improved drug
surveillance [66-68].
It is important to mention here that due to lack of prior
pharmacogenetic studies six drugs (Redux, Posicor, Duract,
Rizulin, Trovan and Lotronex) had been recently
postmarketedly withdrawn by the FDA because they
produced idiosyncratic drug reactions in interethnic groups.
TECHNOLOGIES USED IN PHARMACOGENETIC
STUDIES
Various technologies have aided the advancement of
pharmacogenetics. The major include development of gene
and protein sequencers − for determination of the nucleotide
and amino acid sequence of genes and proteins respectively.
There have been two main strategies used in screening for
polymorphism in an individual: phenotyping and genotyping
[71].
Phenotyping is an observable biochemical measure.
Phenotyping would determine the presence and activity of a
particular metabolic enzyme in a tissue biopsy often referred
to as functional phenotyping. Metabolic phenotyping
measures the level of metabolites in a person after
administration of a drug, such as administering caffeine and
measuring the concentration of caffeine metabolites in a
blood sample. Phenotyping is usually very straightforward,
but also more invasive and potentially dangerous, due to the
administration of drugs and their resulting side effects.
Genotyping determines the specific genetic code of an
individual. Genotyping is safer because it can be done on an
easily obtainable sample of tissue (cheek swath, blood, etc.)
The results, though, are often harder to interpret. Phenotyping gives the end result of pharmacogenetic differences
between people, while genotyping gives the root cause of the
different responses. There are various technologies being
used for genotyping. Cutting edge biotechnological analyses
have helped to advance the discovery of new genetic
variation that could improve the use of drugs.
Genotyping Technologies used in the pharmacogenetical
assays include:
Drug Metabolism and Individualized Medicine
Table 2.
Current Drug Metabolism, 2003, Vol. 4, No. 1
39
List of the Known Drugs Causing Adverse Drug Reactions
Drugs
Adverse Reactions
Genetic Trait Involved
Primaquine/
Hemolytic Anemia
G6PDh Deficiency
Methemoglobinemia
Methemoglobin reductase Deficiency
5-flurouracil
Death
Dihydropyrimidine Deficiency
CPT-11
Diarrhea
Glucuronidation UDPGA Deficiency
Propafenone/
Central Nervous
Cyp 2D6 poor metabolizers
Rhythmol
System Affects
Warfarin/
Death
Cyp 2C9 poor metabolizers
Tacrine
Liver toxicity, vomiting nausea
APOE-4 positive subjects
Isoniazid
Peripheral Neuropathy
Slow Acetylators
Dapsone/
Sulphonamides/
Nitrofuratoin
Dapsone/
Nitrites/
Barbiturates/
Estrogens/
Alcohol
Coumadin
Polymerase chain reaction (PCR)/Restriction Fragment
Length Polymorphism (RFLP): Amplifies specific
nucleotide sequences to measurable levels. Determines
genetic variation of an individual, but is often time
consuming and cumbersome.
Oligonucleotide Ligation Assay (OLA): High throughput
screening using short sequences of radioactive or
fluorescently labeled DNA that is complementary to specific
DNA sequences.
DNA Chips: Various cutting edge technologies to either
rapidly determine nucleotide sequences (Gene Chips) or
determine the global genetic expression or subset thereof of
all genes in a tissue or individual (Microarrays). Affymetrix
is developing a silicon based gene chip to rapidly determine
the sequence of certain important genes (p53, HIV virus,
etc.). Microarrays give rapid profile of what genes are being
highly expressed and those that are not [72-73].
Single-stranded conformation polymorphism (SSCP):
Detects single nucleotide differences between the genes of
any two samples. It helps to detect polymorphism that might
be linked to certain pharmacogenetic responses.
POTENTIAL GAINS / HAZARDS OF PHARMACOGENETICS
To understand the potential of Pharmacogenomics, the
first step is to acknowledge shortfalls in current drug
regulatory systems and the power of pharmacogenetics to
meet those shortfalls [74]. Pharmacogenetics is not really
about disease diagnosis; instead, it is about drug efficacy and
safety, i.e. THE RIGHT DRUG FOR THE RIGHT
PATIENT or individualized medicine. A patient’s
response to a drug may depend on one or more factors that
can vary according to the alleles/genes that an individual
carries. These factors include the absorption, distribution,
metabolism and elimination profile of drug, drug
concentration at the target site and the number and
morphology of target receptors. Advances in genetics will
lead to profiles that take account of these variable factors.
Specific pharmacogenetic profiles, in the form of a medicine
response profile, will differentiate those who have a greater
chance of responding to a particular medicine. As experience
with a medicine accumulates, pharmacogenetic profiles
associated with ADRs could also be identified and added to
efficacy profiles to create a comprehensive medicine
response profile. Pharmacogenetics has the potential to
increase the speed and amount of data collected from a
clinical trial that can ultimately increase the efficacy of drug
treatment.
Pharmacogenomics is strengthened much more by the
upcoming bioinformatics, which will sort out the relationship
between the phenotype and the genotype characters which
occur as a result of drug supplementation. Also the
upcoming e-research in the global electronic environment is
likely to help in great aspect to achieve the goals set by the
Pharmacogenomicist. Pharmacogenetic data can aid in the
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Current Drug Metabolism, 2003, Vol. 4, No. 1
selection of a compound for future clinical development and
offer a powerful tool for optimizing therapeutic efficacy.
Pharmacogenetics may also help design therapeutic agents
targeting specific groups of patients with a set of genotypic
characteristics that otherwise would deprive them of a cure;
in other words pathway towards individualized medicine,
which can be termed as the combined effects of the
individual’s molecular and clinical aspect towards a drug.
Pharmacogenomics will yield drugs targeted to act at or
near the cause of a disease. Genetically defining patient
populations will help improve outcomes and genetic
prognostics will revolutionize treatment and improve costeffectiveness. Pharmacogenomics is already making an
impact in a wide array of disease states and drug therapy; it
will eventually become part of standard patient management
in selecting and monitoring drug therapy towards
Individualized Medicine.
However, there are chances of potential risks too from
pharmacogenetic research applied through drug therapy. A
drug that works wonderfully on 75% of the population could
not only be ineffective but extremely harmful to the other
25% of the population. Because of the specificity of
pharmacogenetic therapy trials, this drug would have a
greater chance of approval through the trial process than a
drug tested without taking into account genomic differences.
But then the chances for the occurrence of ADRs in 25% of
the population will increase to a much greater extent.
There should be proper guidelines on the genetics and the
drug interaction. The Food and Drug Administration and the
European Medicines Evaluation Agency view genetic
variation as just one of the many factors that contribute to
drug response. FDA may also require pharmacogenetic data
pertaining to drug toxicity for a drug with a narrow
therapeutic index. The Center for Biologics Evaluation and
Research (CBER) states that drugs associated with serious
toxicities may be approvable, particularly if indicated for
life-threatening disease without existing treatments.
Although in practice the drug approval is always carried out
on the basis of risk-benefit ratio. Many cancer drugs, for
example, are quite toxic, but they are approvable because
cancer is a fatal illness with no known cure.
But it is certain that the use of pharmacogenetic data will
dramatically increase the individualization of drug therapy;
therefore, the pharmacist’s role is extremely important.
However, it will also have an impact on the health insurance
and responsibility towards the higher treatment costs,
because now the drug companies would like to fetch the
same benefit from the less population. There is also the need
to strict policies and procedures to guard against
unauthorized disclosure of genetic information. At present it
is difficult to analyze the pharmacogenomic profile of the
new chemical entity whose metabolic profile is poorly
understood.
Thus pharmacogenetic research has the potential to
subdivide each disease according to genetics, not symptoms.
Specific diagnoses may be based on the molecular
mechanisms involved rather than clinical presentation.
Molecular mechanism differences will subdivide patient
Pratima Srivastava
groups with common diseases like hypertension, diabetes,
and cancer. Health care professionals will use genetic tests to
predict how a disease will progress and the therapeutic
response to anticipate. Drug development will be based on
understanding of molecular pathogenesis. The role of genes
in determining disease susceptibility, progression,
complications, and response to treatment could all be
potentially mapped.
CURRENT AND FUTURE APPLICATIONS
Pharmacogenetics and its application through
pharmacogenomics has broad potential to affect almost every
drug treatment schedule. Listed below are just some of the
few current and upcoming applications.
Cardiology
Long QT syndrome is a rare condition in which people
have a slower repolarization of the myocardium after
depolarization. This rare disorder resulting in various cardiac
pathologies is associated with five genes. A mutation in
LQT2 affects potassium channels, while a mutation in LQT3
affects sodium channels. Though both mutations result in the
same symptom but they have vastly different causes and
treatment−requiring a different drug for each mutation. A
genetic test for each mutation would be highly valuable since
the disease is so rare. This also points to another potential
use of pharmacogenetics i.e. identification of potential
targets for pharmaceutical development.
Another potential application is in the CYP2D6 drug
metabolizer enzyme, responsible for metabolizing a large
number of cardiac drugs, including beta-blockers. Poor
metabolizers have two to three fold higher drug
concentrations in their plasma, which can lead to dizziness
and contribute to non-compliance, resulting in greater
incidence of other side effects and drug-drug interactions.
Rhythmol, an antiarrhythmic drug, is also affected by the
CYP2D6 genotype. A poor metabolizer will have a higher
concentration of the drug in their plasma and low
concentration of the active metabolite. Therefore, a poor
metabolizer need a higher dosage of this drug, and has a
higher rate of nervous system side effects. Another drug
might be better for this population subgroup [75].
CYP2C9 enzyme is involved in warfarin metabolism.
One percent of the U.S. population is poor metabolizers of
warfarin—and thus need only one seventh the normal dose
of warfarin. Without identification and proper treatment
adjustment, poor metabolizers risk overdose and possibly
death [76].
Neurology
Alzheimer’s disease is the fourth leading cause of death
and costs the United States $93 billion per year in direct and
indirect costs. Of the two forms of Alzheimer’s disease,
familial and sporadic, the latter has been linked to 85% of all
cases worldwide, and 50% of those have been linked to the
apolipoprotein gene.
Drug Metabolism and Individualized Medicine
Apolipoprotein E (ApoE) has been implicated in synaptic
remodeling and regeneration and amyloid metabolism, and
appears to modify the pathology of Alzheimer’s disease.
Every individual has two copies of the ApoE gene. The
ApoE isoform 4 (ApoE-4) has been shown to be associated
with Alzheimer’s disease. As the number of copies of ApoE4 in an individuals’ two gene complement increase from no
copies to one copy and then to two copies, the age of onset
of Alzheimer’s disease decreases from above 85 to 75 to 65
years old. ApoE-4 is also distinctly involved in drug
treatment for Alzheimer’s disease. Non-ApoE-4 subjects
respond well to Tacrine, while ApoE-4 subjects do not. In
Alzheimer’s drug treatment trials, the participants are all
stratified by ApoE genotype [77].
Current Drug Metabolism, 2003, Vol. 4, No. 1
41
group based on genotype. This could speed up the clinical
trial process and drug approval because no longer would a
clinical trial be abandoned if a drug showed minimal
effectiveness in a large population. Effect in a smaller subset
of the trial−−linked to a specific genotype−could yield some
salvageable component of the trial and yield valuable
therapy for a small population likely to be overlooked in
large trials [80].
Pain Management
Celebrex, a pain relief drug, acts only on the COX-2
enzyme and not COX-1 enzyme. Provides pain relief without
damaging stomach lining.
Oncology
Environmental Medicine
Cancer treatment often involves the use of toxic
chemicals to kill all the cancerous cells while trying to harm
the least number of healthy cells. Pharmacogenetic research
has the potential to isolate specific chemotherapeutic agents
that have limited toxic side effects to an individual, but still
attack and destroy the cancerous tumor [78].
One example where pharmacogenetics can be applied is
in individuals with dihydropyrimidine dehydrogenase (DPD)
deficiency. Patients with this rare genetic disorder develop
neurotoxicity when treated with common chemotherapy such
as 5-fluorouracil. Understanding the relationship between
chemotherapy medications and certain genotypes can help
isolate the least toxic chemotherapy for each individual.
Alcohol, isoniazid, fasting and diabetes induce CYP2E1,
several polymorphisms in it are related to different types of
cancerous growth. Herceptin is a drug for breast cancer, a
monoclonal antibody that inhibits the action of the HER2
gene. Twenty to thirty percent of all breast cancer patients
overexpress HER2; these are the patients who will benefit
from treatment with this drug. Diagnostic screening tests to
determine potential candidates for treatment with Herceptin
are under development [79].
Asthma
Certain genetic variations may determine who responds
to asthma drugs. Recent clinical trials have shown that
certain asthmatics do not respond to treatment with
investigational agent ABT-761. This drug interferes with the
5-lipoxygenase (ALOX5) pathway. People with the one or
two alleles of the mutant genotype have decreased or no
response to ABT-761, respectively. About 6% of asthmatics
do not carry a wild-type allele at the ALOX5 promoter,
raising the issue that while most pharmacogenetics focuses
on the gene that codes for a particular enzyme, it is also
important to focus on the regions that control expression of
proteins.
Clinical Trials
Pharmacogenetics can be used in analyzing clinical trial
data to determine if a drug is more efficacious for a selected
As most of the environmental carcinogens are
metabolically activated or inactivated by CYP isoforms,
profiling an individual genome to predetermine what
potential risks one should avoid is very important [81].
Individuals with a certain p53 mutation are highly
susceptible to cervical cancer when exposed to human
papilloma virus [82].
Depression
Metabolism of tricyclic antidepressants has been linked
to certain forms of the CYP2D6 enzyme. 5% of people lack
CYP2D6 and are "poor metabolizers". These individuals
need greater attention to adverse effects and clinical
response. American Blacks have a greater percentage of poor
metabolizers then American Caucasians.
An example of pharmacogenetic applications assistance
in drug development and clinical trials is in case of the drug
Amonafide. Initial clinical safety trials identified two
different safe doses. There were two dosage recommendation
of 250 mg/m2 and 400 mg/m2which later finalized to 300
mg/m2. This resulted in patients either receiving too low a
dose to be effective or a dose that resulted in withdrawal
from the trial. If the patients had been phenotyped or
genotyped, two major sub-populations based on NAT2
polymorphisms would have been identified: one group that
metabolized the drug well and developed toxic levels of the
metabolite, and one group that did not. It was later
determined that the population could be phenotyped using
caffeine as a metabolic substitute to determine NAT2
phenotype [83].
CONCLUSION
We can conclude essence of Pharmacogenomics in the
form of the chart as mentioned below.
The study of pharmacogenetic differences holds the
potential to improve therapeutic effectiveness and limit
toxicities of available drugs. Pharmacogenetics can provide
substantial efficiency in clinical research by facilitating the
42
Current Drug Metabolism, 2003, Vol. 4, No. 1
Chemistry
Pratima Srivastava
Pharmacology
Biochemistry
Pharmacokinetics
Drug Metabolism
Pharmacogenetics
Bioinformatics
Pharmacogenomics
conduct of smaller clinical trials by targeting groups of
patients with similar genetic background. The approach of
rigorous determination of genotype/phenotype relationships
in individual drug responses will provide physicians and
researchers with the key information that allows them to
precisely prescribe or design the right drug, at the right dose,
for the right patient. This singular individualized approach to
therapeutics is enabled by highthroughput genotyping and
will provide significant public health benefits to the
population at large. Pharmacogenomics is rather in true
sense the implementation of the research data from
laboratory bench to humankind. It will give clinicians the
tool to predetermine the response of pharmacotherapy by
looking for specific polymorphisms in the drug metabolizing
enzymes. Individualized Medicine (the promise of the
revolution in the field of medicine) is thus a combined effect
of the person’s molecular and clinical aspect towards a drug.
However, prepharmacogenomic studies should be a must
for the development of new chemical entities, so that least
number of ADRs as well as the postmarketedly withdrawal
of the drug comes into picture. Also heed should be paid in
relation to metabolite(s) library produced by inter-ethnic
variations in drug metabolizing enzymes as well as in vitro
and in vivo correlation of the pharmacokinetic/
pharmacogenomic vis-a –vis different genetically engineered
cell-lines and human.
Drug Metabolism and Individualized Medicine
Thus, the benefits of the Drug Metabolism +Genetics
(Pharmacogenomics), which are in the pipeline of modern
era of drug discovery and develoment, will be something
called INDIVIDUALIZED MEDICINE.
ACKNOWLEDGEMENTS
The author is thankful to Drs. C.M. Gupta, Director, R.C.
Gupta, Dy. Director and V.C. Pandey, ex Dy Director.
Central Drug Research Institute, Lucknow, India for their
encouragement in writing the review.
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