Pharmacogenetics and individual variation of drug

response G Vijay Narasimha Kumar

Asst. Professor, Dept.of. Pharmacology

Sri Padmavathi school of Pharmacy

The Proposition of the Usual Dose

Paracelsus

(1493-1541)

“The dose makes the poison”

Background Three discoveries in the 1950s gave rise to the discipline of Pharmacogenetics.

Primaquine (G-6-PD deficiency)

Isoniazid metabolism of (acetylation polymorphism)

Atypical plasma cholinesterase: succinylcholine

Differential drug efficacy

Same symptoms,Same findings,Same disease? Same drug

Same dose

Different Effects

Different patients

At a recommended prescribed dosage—

a drug is efficacious in most.

not efficacious in others.

harmful in a few.

Lack of efficacy

Unexpected side-effects

Patient population with same disease phenotype Patients with normal response

to drug therapy

Patients with non-response to drug therapy

Patients with drug toxicity

Genotyping

Toxic responders

Non-responders

Responders

People react differently to drugs

“One size does not fit all …”

Why does drug response vary?

Same symptoms,Same findings,Same disease? Same drug

Same dose

Different Effects

Possible Reasons: Individual variation By chance…

Different patients

EthnicityAgePregnancyGenetic factorsDiseaseDrug interactions

……

Genetic Differences

A

G SNP

The Problem of Variability

“Variability is the law of life, and

as no two faces are the same,

so no two bodies are alike,

and no two individuals react alike,

and behave alike under the

abnormal conditions which we

know as disease.”Sir William Osler

(1849-1919)

Why does drug response vary? Genetic variation Primarily two types of genetic mutation events create all forms of variations:Single base mutation which substitutes one nucleotide for another

--Single nucleotide polymorphisms (SNPs)

Insertion or deletion of one or more nucleotide(s) --Tandem Repeat Polymorphisms

--Insertion/Deletion Polymorphisms

Polymorphism: A genetic variation that is observed at a frequency of >1% in a population

Genetic polymorphisms

Pharmacokinetic

-Transporter -Plasma protein binding -Metabolism

Pharmacodynamic

-Receptors -Ion channels -Enzymes -Immune molecules

Single nucleotide polymorphisms (SNPs) SNPs are 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.

For example a SNP might change the DNA sequence AAGCTTACto ATGCTTAC

SNPs are the most commonly occurring genetic differences.

Single nucleotide polymorphisms (SNPs)

SNPs are very common in the human population.

Between any two people, there is an average of one SNP every ~1250 bases.

Most of these have no phenotypic effect

◦ Venter et al. estimate that only <1% of all human SNPs impact protein

function (lots of in “non-coding regions”)

Some are alleles of genes.

Due to individual variation…20-40% of patients benefit from an approved drug70-80% of drug candidates fail in clinical trialsMany approved drugs removed from the market due to adverse drug effects

The use of DNA sequence information to measure and predict the reaction of individuals to drugs.Personalized drugs

Faster clinical trials

Less drug side effectsPharmacogenetics

Inter individual Variability in Drug Response

Disease Drug Class Rate of Poor Response

Asthma Beta-agonists 40-75%

Hypertension Various 30%

Solid Cancers Various 70%

Depression SSRIs, tricyclics 20-40%

Diabetes Sulfonylureas, others 50%

Arthritis NSAIDs, COX-2 inhibitors 30-60%

Schizophrenia Various 25-75%

How Can The Causes of Variability be Unraveled?

Pharmacogenetics: Study of interindividual variation in DNA

sequence related to drug absorption and disposition (Pharmacokinetics)

and/or drug action (Pharmacodynamics) including polymorphic

variation in genes that encode the functions of transporters,

metabolizing enzymes, receptors and other proteins.”

Pharmacogenomics: Systemic genomic analysis in populations of

treated subjects to identify variants that predict drug response

including the occurrence of adverse reactions.

“The study of how people respond differently to medicines due

to their genetic inheritance is called pharmacogenetics.”

“Correlating heritable genetic variation to drug response”

An ultimate goal of pharmacogenetics is to understand how someone's

genetic make-up determines, how well a medicine works in his or her

body, as well as what side effects are likely to occur.

“Right medicine for the right patient”

Pharmacogenetics VS. Pharmacogenomics

Pharmacogenetics: Study of variability in drug response determined by single genes.

Pharmacogenomics: Study of variability in drug response determined by multiple genes within the genome.

Pharmacogenetics The study of variations in genes that determine an individual’s response to drug therapy.

Common variation in DNA sequence (i.e. in >1% of population)

Genetic Polymorphism: SNPs; INDEL; VNTRs

Potential Target Genes are those that encode:Drug-metabolizing enzymesTransportersDrug targets

Current Concept of Pharmacogenomics

Paradox of Modern Drug Development

1. Clinical trials provide evidence of efficacy and

safety at usual doses in populations.

2. Physicians treat individual patients who can vary

widely in their response to drug therapy.

Modern Rendition of Paracelsus

Paracelsus(1493-1541)

“The dose makes the poison, but

differently for genetically different

individuals.”

Pharmacologic effect

Clinical response

Toxicity Efficacy

DISTRIBUTION

ABSORPTION

ELIMINATION

Pharmacokinetics

Pharmacodynamics

dose administered

drug in tissuesof distribution

concentration insystemic circulation

concentration atsite of action

metabolism and/or excretion

Pharmacokinetic factors - Absorption- Distribution- Metabolism- Elimination

Pharmacodynamic factors- Target proteins- Downstream messengers

Determinants of Drug Efficacy and Toxicity

A patient’s response to a drug may depend on factors that can vary according to the alleles that an individual carries, including :

Single mutant gene

inherited

“drug metabolism”

“drug response”

Qualitative differences in

Genetic polymorphism

1. PHARMACOKINETIC GENETIC VARIATIONS:

The extent to which an individual metabolises a drug is genetically determined

Monozygotic twins

•Identical twins•Metabolise drugs similarly

Dizygotic twins•Non – identical twins•Variation in drug metabolism

PHARMACOGENETIC VARIATIONS IN PHASE – 1 DRUG METABOLISM:

Example 1: Presence of atypical pseudo choline esterase:incidence of presence of atypical pseudo choline esterase is 1:2500 in population.

“Autosomal recessive inheritance” “Trait”

Acts on

Succinyl choline

Action terminated in “5 minutes”

“Atypical” pseudo choline esterase

Succinyl choline

“normal” pseudo choline esterase

Requires 1- 2 hours for termination of action

Respiratory failure

Cannot/slowly hydrolyses

Example 2: Hydroxylase polymorphism: Autosomal recessive trait.

Phenytoin

metabolite

In slow hydroxylators

Phenytoin

Slow hydroxylation

Phenytoin toxicity

Hydroxylation

PHASE – 2 ACETYLATOR STATUS : Many drugs are metabolised by “hepatic N – acetylase enzyme”. This enzyme is non – inducible.

Two phenotypes are present in the population

Rapid acetylators(Autosomal dominant)

Contains high amount of hepatic N- acetylaseEg: Eskimos and Japanese

Slow acetylators(Autosomal recessive)

Contains low amount of hepatic N - acetylase

Eg: Egyptians, Swedes and Mediterranean Jews

Rapid acetylators

Isoniazid

Increased levels of acetyl hydrazine

Hepato toxicity

Metabolised fast

Slow acetyators

Isoniazid

Accumulation of drug

Pyridoxine kinase

Pyridoxine

Pyridoxyl phosphate

“peripheral neuritis”

So vitamin B6 is added to patients on Isoniazid therapy

inhibits

Dapsone Dapsone Haemolysis

2. PORPHYRIA: Some types are autosomal dominant, some are autosomal recessive and

some are X – linked.Mechanisms of drug induced attacks of Porphyria:

Succinyl CO A+

glycine

ALA synthase 8 – Amino laevulinic

acid(ALA)

PorphobilinogenPorphyrins

Heame

Cytokines

Reduction in haem synthesis

Stimulate ALA synthase

Increased ALA synthesis

Increase haem synthesis

will

Leading to

Enzyme inducing drugs

Increased levels of cytochromes

Increased heam demand

Stimulation of ALA synthase

Increased ALA production

But due to defect in enzymes involved in porphyrin synthesis

Continued..

due to defect in enzymes involved in porphyrin synthesis

Increased accumulation of porphyrins

Porphyria

So persons who are deficient in enzymes involved in porphyrin and heam

synthesis, if enzyme inducing drugs are given, it will lead to “porphyria”.

Eg; Alcohol, Barbiturates, Carbamazepine, Nitrous oxide.

• Management of acute attack of porphyria:

No specific measures

High intake of carbohydrates inhibits ALA synthase activity and a high

carbohydrate diet will not do any harm.

Haematin has been used as a specific therapy, which increases the free pool of

heam in the liver.

Examples of genetic polymorphism influencing pharmacokineticsGene product Drugs Response affected

CYP2C9 Tolbutamide,warfarin,phenytoin,NSAIDs Anticoagulant effect of warfarin

CYP2C19 Mephenytoin, omeprazole, hexobarbital, mephobarbital, propranolol, proguanil, phenytoin,clopidogrel

Peptic ulcer response to omeprazole, CVS events after clopidogrel

CYP2D6 B-blockers, antidepressants, antipsychotics, codeine, debrisoquine etc

Tardive dyskinesia from antipsychotics, narcotic side effects, codeine efficacy,

imipramine dose requirement,B- blocker effect

CYP3A4/3A5/3A7 Macrolides, cyclosporine, tacrolimus, Ca2+ channel blockers, midazolam, steroids.etc

Efficacy of immunosuppressive effects of tacrolimus

PHARMACOGENETIC VARIATION IN DRUG RESPONSE DUE TO ENZYME DEFICIENCY:

Glucose – 6 – phosphate dehydrogenase deficiency(G-6-PD) : Deficiency in RBC’s Sex – linked recessive trait( X – linked) Africans, American negroes, Mediterranean Jews, middle east and south east

races. Drugs having oxidising properties can cause haemolytic anaemia in persons

having G-6-PD deficiency.Eg: Primaquine, Sulphonamides, Dapsone, Nitrofurantoin, Quinine, Chloroquine,

Quinidine

Pentose phosphate pathway in RBC:

glucose Glucose 6 phosphate 6 – phosphoglucolactone

NADP+ NADPH + H+

Gluta thione reductase

Oxidised glutathione

Reduced glutathione

(GSH)

Hydrogen peroxides H2O

Only source of NADPH in RBC is pentose phosphate path way

Decreased oxidative stress

So new RBC’s have reduced glutathione but as they become older the enzyme level decreases

Normal G- 6 – P – D deficiency patients

Hemolytic anemia is self limited when G6PD deficiency is very mild.

Because older RBC’s are destroyed and the young RBC’s have

normal or nearly normal enzymatic activity

Infection Chemicals (oxidising agents)

Oxidising stress

Increased oxidative stress

As NADPH is not produced in G6PD deficiency patients

The enzyme reduced glutathione will quench free radicals until its levels are present

But when reduced glutathione is fully consumed

Continued..

When reduced glutathione is fully consumed

Haemoglobin( Fe2+)

Methaemoglobin(fe3+)

Improper functioning

Enzymes and other proteins

Damaged by oxidants

Denatured

Cross binding and protein deposition in cell membranes

oxidised

Continued..

Cross biding and protein deposition in cell membranes

Forms Heinz bodies which are attached to cell membrane

RBC’s having Heinz bodies are sequestered by macrophages in the spleen

Increased haemoglobin destruction

Increased Bilirubin levels

Jaundice

Haemolysis

FAVISM :Illness that occurs in G 6 PD deficiency patients with acute

hemolysis by eating raw beans.( Vicia fabu)

G 6 – P – D deficiencyAlso causes

The build up of glucose and thus there is an increase of advanced glycation end products( AGE)

Cell damage

As RBC will be carrying more oxygen, they are at high risk of haemolysis

Examples of genetic polymorphism influencing drug responseGene Drugs Response

Dihydropyridine dehydrogenase Fluorouracil 5-flurouracil neurotoxicity

N-acetyl-transferase ( NAT2)

INHHydralazine

Sulfonamides

INH-neurotoxicityHydralazine-induced lupus

Hypersensitivity to Sulfonamides

Glutathione transferase Several anticancer drugs Decrease response

Thiopurine methyltransferase MercaptopurineAzathioprine

Increased toxicity & risk of second cancer

• An ultimate goal of pharmacogenetics is to understand how someone's genetic make-up determines, how well a medicine works in his or her body, as well as what side effects are likely to occur.

“Right Medicine for the Right Patient”

Personalized medicine

Develop drugs that target Persons of specific genotypes

Prescribe existing drugs tailored to specific genotypes

Pharmacogenetics in clinical trials

• Identifying Patients – Enrichment Designs

• Exclusion/ Inlcusion Criteria

Safety /Efficacy/Drug Interaction

• Drug Label implications

Potential Benefits of Pharmacogenetics

• Improve Drug Choices:– Each year, ~100,0000 people die of adverse reactions to medicine &

~2 million are hospitalized– Pharmacogenitics will predict who's likely to have a negative or

positive reaction to a drug

• Safer Dosing Options– Testing of Genomic Variation Improve Determination of Correct Dose

for Each Individual

• Improvement in Drug Development:– Permit pharmaceutical companies to determine in which populations new

drugs will be effective

• Decrease Health Care Costs– Reduce number of deaths & hospitalizations due to adverse drug

reactions– Reduce purchase of drugs which are ineffective in certain individuals due

to genetic variations

• Speed Up Clinical Trials for New Drugs

Potential Benefits of Pharmacogenetics

"Here's my sequence..."

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