Pharmacogenetics

1. Pharmacogenetics
Dr. Ashishkumar Baheti
MD Pharmacology

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Overview
 Introduction
 Genomic basis of Pharmacogenetics
 Pharmacogenetics in drug development
 Benefits of pharmacogenetics
 Concerns about pharmacogenetic approach
 Summary

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Differential drug efficacy
At a recommended dosage –
 A drug is efficacious in most
 Not efficacious in others → Lack of efficacy
 Harmful in few → Unexpected side effects
Same symptoms
Same signs → Different patients
Same disease ↓ Same drug at
same dose
Different effects

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Why does drug response vary
Genetic
variation
Environment
al factors
Drug response is considered to be gene - by - environment
phenotype

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Exogenous & Endogenous factors
contribute to variation in drug response

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Study of genetic basis for variation in
drug response
Goal is to understand how someone's
genetic makeup determines how well
medicine works in an individual body as
well as what side effects are likely to
occur
Pharmacogenetics

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Pharmacogenetics vs Pharmacogenomics
VS
Pharmacogenetics
Study of variability
in drug response
determined by
single gene
Pharmacogenomi
cs
Study of
variability in drug
response
determined by
multiple genes
within the genome

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History
 1959 - Freidrich Vogel coined the term “Pharmacogenetics”
 In first half of 20th century,
 Prolonged neuromuscular blockade following normal doses of
succinylcholine
 Neurotoxicity due to Isoniazide
 Methaemoglobinemia in G6PD deficiency

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Types of genetic variants -
 Two types of sequence variation have been associated
with variation in human phenotype –
1. Single nucleotide polymorphism(SNPs)
2. Insertions/ Deletions (Indels)
Polymorphism - a variation in the DNA sequence
that is present at an allele frequencyof 1% or
greater in population

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Single nucleotide polymorphism –
(SNPs)
 Single base pair positions in genomic DNA at
which different sequence alternatives(alleles)
exist wherein the least frequent allele has an
abundance of 1% or greater
 Most common form of genetic variation
Insertion/Deletion polymorphism
(Indels)
 Less frequent in genome & are of particularly
low frequency in coding regions of genes

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Molecular mechanism of genetic polymorphism
1. Coding non-synonymous SNPs
↓
Changes the amino acid codon
↓
Change protein structure, stability, substrate affinities, or introduce a stop
codon.
2. Coding synonymous SNPs
↓
No change in the amino acid codon (functional consequences -transcript
stability/ splicing)
3. Noncoding SNPs ( in promoters, introns, or other regulatory regions)
↓
Affect transcription factor binding, enhancers, transcript stability, or splicing.

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 Indels can have any of the same effects as SNP substitutions:
short repeats in the promoter (which can affect transcript
amount), or insertions/deletions that add or subtract amino
acids
 Copy number variations involve large segments of genomic
DNA -
Gene duplications ( increased protein expression and
activity)
Gene deletion ( complete lack of protein production, or
inversions of genes → disrupt gene function)
eg.TPMT,thiopurine methyltransferase; ABCB1, the multidrug
resistance transporter (P-glycoprotein); CYP, cytochrome P450 .

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Pharmacogenetic study design
consideration
 Pharmacogenetic trait – measurable/discernible
trait associated with drug (eg.enzyme activity, blood
pressure, drug metabolite in plasma or urine )
 Most pharmacogenetic traits are multigenic rather
than monogenic so considerable effort is being made
to identify the important genes and their
polymorphisms influencing variability in drug
response
1. Pharmacogenetic measures
Pharmacogenetic
measures
Candidate gene
association study
Pharmacogenetic
phenotypes

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2.Candidate gene association study
 Pathways involved in drug response are known /at least
partially known, pharmacogenetic studies are highly
amenable to candidate gene association studies
 After genes in drug response are identified,pharmacogenetic
study is to identify the genetic polymorphisms responsible for
therapeutic / adverse responses to the drug

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Databases that contain information on polymorphisms and
mutations in human genes

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3. Pharmacogenetic phenotypes
Pharmacogenetics
is relevant to
variation in drug
response
Variation in
Pharmacokinetic
Variation in
Pharmacodynamic
s

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Variation in Pharmacokinetic
response -
 Variation in phase I drug metabolism : CYP450
 Variation in phase II drug metabolism

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Variation in Phase I metabolism -
 CYP1A1, CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4 –
pharmacogenetically important as they are responsible
for phase I metabolism of > 90% commonly used drugs
 Many CYP genes are highly polymorphic with alleles that
have functional consequences for how individuals
respond to drug therapy
 CYP alleles result in absent, decreased or increased
enzyme activity & thereby affecting rate of drug
metabolism

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Phase II metabolism
Genes encoding phase II metabolism are also functionally
polymorphic and responsible for individual variability in drug
response
N- acetylpolymorphism and Isoniazide
eg. Cholinestarase polymorphism & prolonged postoperative
paralysis
Succinylcholine – normally hydrolysed by butyrcholinesterase

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Contd..
BCHE gene encoding butyrylcholinesterase enzyme
Two alleles Usual (U) & Atypical(A) - Major determitants of
cholinesterase activity in plasma
Homozygotes respond abnormally with prolonged muscle
paralysis after succinylcholine administration
Cholinesterase deficiency is due to homozygosity for A allele &
has lower activity than usual type

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Variation in pharmacodynamics response
 Glucose 6 phosphate dehydrogenase deficiency
 Malignant hyperthermia

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Genetic variation in both
pharmacokinetic & pharmacodynamics –
Warfarin therapy
 Anticoagulant
 Narrow therapeutic index
 Wide interindividual variability in dose requirement

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Contd…
 Warfarin is metabolized by CYP2C9 to inactive
metabolites and exerts its anticoagulant effect partly via
inhibition of VKORC1
 Common polymorphisms in both genes, CYP2C9 and
VKORC1, impact on warfarin pharmacokinetics and
pharmacodynamics, respectively, to affect the population
mean therapeutic doses of warfarin necessary to maintain
the desired degree of anticoagulation and minimize the
risk of thrombosis or bleeding

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Pharmacogenetics & drug development
eg. Warfarin , Dapsone , Rasburicase, Azathioprine ,
Mercaptopurine , Irinotecan .(FDA has changed these drugs label as
these have pharmcogenetic issue)
Genetic/ genomic
interindividual
variability
Development of
genotype sp.drug
&
sp. drug dosing

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Pharmacogenetics research in
antineoplastic drugs -
It is an active field of research - ↓ life threatening
toxicity & improve therapeutic efficacy of
antineoplastic drugs
Nearly 20 pharmacogenetics biomarkers & 30
chemotherapeutic agents have been included in drug
package inserts & recommended by FDA & some of
these biomarkers improve T/t efficacy , ↓ toxicity ,
lower health care costs

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6-MP & TPMT
Antimetabolite – T/t of lymphoma & leukemia
Clearance – TPMT
↓ TPMT activity better therapeutic effects & low side effects
More than 20 genetic variants of TPMT
{ rs 1800462 (G>C),rs 1142345 (A>G),rs 1800460 (G>A) } missense
variation - ↓ TPMT activity
FDA has recomonded genotyping of these three TPMT SNPs prior to
usage of 6-MP
10% suggested dose in homozygous TPMT & 50% suggested dose in
heterozygous TPMT

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Cetuximab/ Panitumumab & KRAS
 Monoclonal antibodies inhibiting growth & survival of tumour cells
with overexpressed EGFR in colon & head neck cancers
 These drugs found to be ineffective in some patients with EGFR
mutation
 Association between resistance to these drugs and KRAS mutation
 KRAS – membrane GTPase that activates proteins in EGFR signalling
pathway
 Stimulation of these proteins lad to cancer development
(independent of EGFR signalling)
 KRAS mutated, inhibiting EGFR by these drugs have no effect KRAS
induced cancer
contd…

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 KRAS mutated, inhibiting EGFR by these drugs have no
beneficial effect in KRAS induced cancer
 40% colon cancer patients have this mutation
 Pharmacogenetic research found that mutation in exon 2 at
G12 & G13 results in stimulation of KRAS and cancer
development.
 FDA recommend pharmacogenetic test at these positions
 AS per drug label only patient with EGFR expressing colon
cancer & KRAS mutant negative are supposed to use these
drugs

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Why is pharmacogenetics a good
approach
 Many people have severe adverse reactions to drugs
 Many people respond to drugs at different doses
 Ineffective drugs are a waste of money to take
 Genetics don’t change
 Genetics can point to the cause not just the symptom

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Pharmacogenetics benefits
Improve drug
choices
Decreases ADR
Safer dosing options
Improvement in
drug development
Decreases health
care cost

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Personalised drug
 Emerging goal to make medical practice more
personalised
 Pharmacogenetics is an important step towards this
goal

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Concerns about pharmacogenetics
approach
 How reliable are the tests?
 Are health care providers prepared to use this
information?
 Will tailor made medicine lead to discrimination?
 Will this affect people’s privacy?