“The right dose of the right drug to the right person” is one of the goals of pharmacogenomics and personalized medicine.

WHAT IS PHARMACOGENOMICS?

Pharmacogenomics is the study of how an individual's genetic inheritance affects the body's response to drugs.

The term ‘Pharmacogenomics’ comes from the words ‘pharmacology’ (the science of drugs) and ‘genomics’ (the study of genes and their functions) and is thus the intersection of pharmaceuticals and genetics.

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PRINCIPLES OF PHARMACOGENETICS A prerequisite for pharmacogenetics is

heterogeneity in drug response. The definitions of drug response are varied and can include surrogate measurements measured in the laboratory (e.g., international normalized ratio [INR] for warfarin) or clinical endpoints (e.g., stent thrombosis for clopidogrel).

A genetic basis for drug response is suggested when responses are similar within family members (and therefore are heritable) or significantly different in across ethnic backgrounds.

THREE BROAD CLASSES OF GENETIC VARIANTS INFLUENCE DRUG RESPONSE: 1) PHARMACOKINETIC; 2) PHARMACODYNAMIC; AND 3) THOSE ASSOCIATED WITH THE UNDERLYING DISEASE MECHANISM

Genetic polymorphisms in drug-metabolizing enzymes, transporters, receptors, and other drug targets have been linked to inter individual differences in the efficacy and toxicity of many medications.

Pharmacogenomic studies are rapidly elucidating the inherited nature of these differences in drug disposition and effects, thereby enhancing drug discovery and providing a stronger scientific basis for optimizing drug therapy on the basis of each patient’s genetic constitution.

Pharmacogenomic Studies

Pharmacogenomic Studies

Pharmacogenomics uses genome-wide approaches to elucidate the inherited basis of differences between persons in the response to drugs.More than 1.4 million single-nucleotide polymorphisms were identified in the initial sequencing of the human genome,with over 60,000 of them in the coding region of genes.

A QUICK LOOK ON GENETICSCentral dogma theory of molecular biology 1. DNA is transcribed into RNA which is translated into a protein 2. Three nucleotides form a codon 3. A series of codons constitutes a gene (a) Genes encode proteins which may affect drug response: (i) Metabolizing enzyme (ii) Transporter (iii) Receptor

Human DNA sequence 1. 99.9% identical from person to person 2. 3 billion total nucleotides (0.1% difference is larger than it seems) (a) Differences can predict pharmacokinetic and pharmacodynamic

response to drugs

Examples of gene mutations (source of genetic differences):

1. Single nucleotide polymorphism – one nucleotide base pair replaces another

2. Insertion/deletions – nucleotide or nucleotide sequence is added or deleted

3. Tandem repeats – nucleotide sequence repeats in tandem (e.g. AGAGAGAG)

4. Frameshift mutation – an insertion/deletion mutation in which the change in number of nucleotides is not a multiple of three

5. Defective splicing – internal polypeptide segment is abnormally removed and remaining ends are joined

6. Premature stop codon – premature termination of the polypeptide chain 7. Copy number variations – an abnormal number of copies of a gene

Polymorphisms – variation (mutation) in at least 1% of population

Eg. Eye color , Hair color , Blood type and Drug metabolizing enzymes

SINGLE-NUCLEOTIDE POLYMORPHISMS (SNPS)

Single Nucleotide Polymorphism (SNP):

GAATTTAAG GAATTCAAG SNPs are defined as Single base-pair

positions in genomic DNA that vary among individuals in one or several populations.

SNPs are believed to underlie susceptibility to such common diseases as cancer, diabetes, and heart disease and to contribute to the traits that make individuals unique.

SNPs are used as genomic biomarkers. Hence SNP analysis can be used to

enhance drug discovery and development.DNA molecule 1

differs from DNA molecule 2 at a single base-pair location (a C/T polymorphism)

PHARMACOGENETICS AND CYP ENZYMES a) Over 50 cytochrome P450 isoenzymes Three families – CYP1, CYP2, CYP3 Fifteen known to metabolize drugs At least seven with documented polymorphisms

– CYP2A6, 2C9, 2C19, 2D6, 3A4/5, 1A2 b) Two copies of each gene encode for a CYP

enzyme Each copy is referred to as an allele c) Example of a polymorphic CYP enzyme

NORMAL GENE SNP VARIANT GENE

TODAY’S DRUG

PHARMACOGENOMIC DRUG

SUCCESS

FAILURE

SUCCESS

60% 40%

Principle of Pharmacogenomics:

PHARMACOGENOMICS IN CARDIOLOGY

THIENOPYRIDINES

CLOPIDOGREL

CLOPIDOGREL Activation and mechanism . Clopidogrel is a pro-drug that requires hepatic bioactivation

85% of the dose is hydrolyzed by ubiquitous esterases, which leaves only 15% to be converted to the active form.

The activation of clopidogrel is a two step process: (a) Clopidogrel is converted to 2-oxoclopidogrel via CYP2C19,

CYP1A2, and CYP2B6 with each enzyme contributing 45%, 36%, and 19%, respectively.

(b) 2-oxoclopidogrel is converted to the thiol active metabolite

via CYP3A4/5, CYP2B6, CYP2C19, and CYP2C9 with a contribution reported to be 40%, 33%, 21%, and 7%, respectively.

The thiol active metabolite irreversibly forms a disulfide bridge with a cysteine residue within the P2Y12 receptor

This action prevents activation of the GPIIb/IIIa receptor complex, thereby inhibiting aggregation for the platelet’s lifespan (about 10 days)

RESPONSE VARIABILITY

There is high Interindividual variability in platelet response to clopidogrel after stenting. Clopidogrel “resistance” is defined as an absolute change in platelet aggregation <10% before and after clopidogrel administration in response to 5μmol/L ADP.

PROPOSED CAUSES OF RESPONSE VARIABILITY

CLOPIDOGREL PHARMACOGENETICS AND CYP2C19

Clinical response to clopidogrel : In parallel with the platelet function data, the

CYP2C19*2 allele is associated with a graded risk of death, MI, or stroke. Carriers of 1 allele (intermediate metabolizers) have a 1.5-fold increased risk, and carriers of 2 alleles (poor metabolizers) experience a 1.8-fold increase. This pattern also extends to stent thrombosis as well with a 2.6- and 4-fold increased risk in those with 1 and 2 *2 alleles, respectively . Therefore, the CYP2C19 genetic associations with platelet function are mirrored in the clinical response to clopidogrel in the setting of PCI. These observations formed the foundation for updating the clopidogrel label by the Food and Drug Administration to include pharmacogenetic information.

BLACK BOX WARNING

. Similarly, the gain of function variant, CYP2C19*17,

is associated with increased risk of bleeding , and protection from ischemic events CYP2C19*2 carriers treated with prasugrel or ticagrelor do not show a heightened risk of cardiovascular death, MI, stroke, or stent thrombosis

Genetic testingAvailable tests to analyze CYP2C19 genotype i. TaqMan® assay ii. AmpliChip® CYP450 iii. INFINITITM Analyzer assay

CLINICAL PHARMACOGENETICS IMPLEMENTATION CONSORTIUM GUIDELINES

Alternative strategies a. Increased clopidogrel dose A loading dose of 900 mg or a maintenance dose of 225 mg

have been shown to overcome resistance in carriers of one CYP2C19 reduced-function allele but not two reduced-function alleles

b. Prasugrel Rapidly hydrolyzed to active metabolite. CYP variants not

shown to affect PK/PD or clinical outcomes c. Ticlopidine Not shown to be dependent on CYP2C19 status d. Ticagrelor Directly binds to platelets without need of activation. Not

shown to be affected by CYP2C19 status e. Cilostazol + clopidogrel Reduces platelet reactivity in CYP2C19 reduced-function

allele carriers but not noncarriers

Consensus statements currently do not recommend routine

testing. However, there is sufficient evidence to support physicians who may choose to pursue CYP2C19*2 testing in selected patients:

1)for diagnosis in patients with complications of clopidogrel therapy such as stent thrombosis in compliant clopidogrel users; or

2) for the choice of dual antiplatelet therapy in the ACS/PCI setting where the physician believes that additional information regarding the risk/benefit profile for clopidogrel will influence the choice of drug therapy .

Outside of these scenarios, there is minimal rationale to support CYP2C19 testing

STATINS

Genetic variations effect 4 types (at least) of “responses” elicited by statins:

low-density lipoprotein cholesterol (LDLc) lowering

protection from cardiovascular events musculoskeletal side effects; and statin adherence.

WARFARINMETABOLISM AND SITE OF ACTION

CYP2C9 SNPs alter warfarin metabolism:

CYP2C9*1 (WT) – normal activity CYP2C9*2 (Arg144Cys) - low/intermediate activity CYP2C9*3 (Ile359Leu) - low activity

Two relatively common variants, CYP2C9*2 and

CYP2C9*3, encode an enzyme with reduced activity, requiring lower maintenance doses of warfarin.

Approximately 25% of whites have at least one variant

allele of CYP2C9*2 or CYP2C9*3, whereas these variant alleles are less common in blacks and Asians.

Warfarin dose reduction requires as follows :

Heterozygosity for CYP2C9*2 or CYP2C9*3 allele : 20%-30%

Homozygosity for the CYP2C9*2 or CYP2C9*3 allele : 50%-70%

Effect of CYP2C9 Genotype on Anticoagulation

EFFECT OF VKORC1 GENOTYPE ON ANTICOAGULATION Three polymorphic variants of VKORC1

Non-A,Non-A : wild type – Requiring more warfarin dose

Non-A/A : Heterozygous – Requiring 25% dose reduction

A/A : Homozygous - Requiring 50% dose reduction Asians have the highest prevalence of VKORC1 variants,

followed by whites and blacks. Polymorphisms in VKORC1 likely explain 30% of the variability

in warfarin dose requirements. VKORC1 variants are more prevalent than variants of CYP2C9.

Genotype Freq in Asians (%)

Dose reduction

Non-A,Non-A : wild type

7 --

Non-A/A : Heterozygous

30 26

A/A : Homozygous 63 50

BETA BLOCKERS Beta-adrenergic receptor antagonists (or beta-blockers) are a diverse class of agents that primarily antagonize the beta-

1 adrenergic receptor, encoded by ADBR1. Variation in CYP2D6 (pharmacokinetic) and ADRB1, ADRB2,

and GRK5 (all pharmacodynamic) have received the most attention.

Two variants in ADBR1, the Ser49Gly and Arg389Gly , lead to

impaired down-regulation and higher signal transduction,respectively Therefore, carriers of either variant have enhanced, beta-1-receptor activity and more betablocker sensitivity. Healthy volunteers and patients with hypertension who carry 2 Arg389 variants have a greater HR or BP reduction mainly with beta-blockers.

Clinical implications. In general, carriers of the

Arg389 variant have: 1) enhanced reduction in HR and BP; 2) larger improvements in LVEF; and 3) longer survival when treated with chronic beta-blocker therapy compared to persons with the Gly389 variant. Although it is unlikely that beta-blocker therapy will ever be withheld for carriers of the Gly389 variant, a potential application of these findings would be to consider advanced heart failure therapies (e.g., left ventricular assist devices, biventricular pacing, or transplantation) at an earlier stage in patients with the Gly389 variant.

Because certain beta-blockers such as atenolol and

carvedilol are minimally handled by CYP2D6 (131), these may be reasonable alternates for carriers of CYP2D6*4 with metoprolol-induced bradycardia.

RENIN ANGIOTENSIN ALDOSTERONE SYSTEM

ACE GENE 287 bp insertion (I) or deletion (D) in intron 16

DD genotype with increased ACE activity and worse clinical outcome

Use of b blockers and ACEI attenuate adverse outcome of DD genotype with no effect on II and ID.

ANGIOTENSINOGEN

Methionine to Threonine switch at AA 235

Increased angiotensinogen levels with HTN

Modest risk of HTN in whites

Aldosterone synthase

C to T transition at position 344

344 C allele ass with higher aldosterone levels

TT genotype has greater impact of ISDN+ HYDZ combination

POTENTIAL OF PHARMACOGENOMICS

BARRIERS Complexity of finding gene variations that affect drug

response

Millions of SNPs must be identified and analyzed to determine their involvement (if any) in drug response.

Many genes are likely to influence responses

Limited knowledge of which genes are involved with each drug response

Disincentives for drug companies to make multiple pharmacogenomic products

Most pharmaceutical companies have been successful with their "one size fits all" approach to drug development

For small market- Pharmaceutical companies has to spend hundreds of millions of dollars on pharmacogenomics based drug development!----- “US Orphan Drug law”

EDUCATING HEALTHCARE PROVIDERS & PATIENTS

Introducing multiple pharmacogenomic products to treat the same condition for different population subsets complicates the process of prescribing and dispensing drugs

Physicians must execute an extra diagnostic step to determine which drug is best suited to each patient

Need for a better understanding of genetics by all physicians

Thank you