Dr. Adithi S Raghavan presented on the topic of pharmacogenetics. The presentation defined pharmacogenetics and outlined several key points: 1. Pharmacogenetics is the study of genetic basis for variation in drug response. Variation can be due to differences in pharmacokinetics, pharmacodynamics, or idiosyncratic reactions. 2. Twin and family studies show drug metabolism and response are highly heritable. Genetic factors account for much of the interindividual variability in drug response. 3. Important examples of single gene disorders influencing drug response include atypical plasma cholinesterase and malignant hyperthermia. 4. Pharmacogenetic testing is becoming more clinically available

1. Presented by:
Dr.Adithi S Raghavan
Moderated by:
Dr.Anuradha H.V

2. Outline
1. Definition – Pharmacogenetics
2. Variation in drug response
3. Pharmacogenetic importance
4. Elementary genetics
5. Single gene PK disorders
6. Therapeutic drugs & clinically available PG
tests
7. Conclusion

3. Introduction
Pharmacogenetics is the study of the genetic basis
for variation in drug response.
Encompasses Pharmacogenomics which employs
tools for surveying the entire genome to assess
multigenic determinants in drug response.

4. Variation In drug response
Interindividual variation in response to drugs –
Serious problem
Results in Lack of efficacy/Unexpected side
effects
Variation:
 Pharmacokinetic
 Pharmacodynamic
 Idiosyncratic

5. Pharmacokinetic variation
• Too much/not enough drug @site of action
• Genes influence PK by altering expression
of Proteins involved in ADME

6. Pharmacodynamic variation
PD->/<effect of a drug @ a given conc @ site
of action
Interindividual variation in
• Drug targets
• G-proteins
• Other downstream events

7. Idiosyncratic
 Qualitatively abnormal reaction that occurs
only in a few exposed individuals
 Results from differences in enzymes or
immune mechanisms

8. Importance of Pharmacogenetics to
variability in Drug response

9. Pharmacogenetic variations
May be due to :
A.Single mutant gene-Genetic polymorphism
B.Polygenic influences
 Polygenic influences & environmental factors
are responsible for normal biological variations.
 Clinical practice-not significant

10.  Classical family studies provide information
on drug response & genetics
 Data limited
 Twin studies show that drug metabolism is
highly heritable, with genetic factors
accounting for variation

11. Figure 7–2.
Pharmacogenetic contribution to pharmacokinetic parameters.
t1/2 of antipyrine is more concordant in identical in
comparison to fraternal twin pairs. Bars show the t 1/2
of antipyrine in identical (monozygotic) and fraternal
(dizygotic) twin pairs. (Redrawn from data in Vesell
and Page, 1968.)

12. Genetic
Fraternal
twins-Wide
Variation in
Drug
responses
Environmental
Identical
Only
Environmental

13.  Comparison of Intra twin vs. Inter pair variability
75-80% variability in PK t 1/2 s of drugs eliminated
by metabolism is heritable
 Heritability estimated by
 Comparing intra-subject vs. inter subject
variability in drug response
 or disposition in unrelated individuals
Assuming high intra subject reproducibility
translates into high heritably.

15. Genes
 Fundamental units
of heredity
 Consist of ordered
sequence of
nucleotides(Adenin
e, Guanine
,Thymidine &
Cytosine-A,G,T,C.

16.  Written in italics
•
CYP2D6-protein
•
CYP2D6-Gene
 Most DNA-Chromosome
 Small amountMitochondria.(Maternal
Ovum)

17. DNA
Transcribed
Complementary
mRNA
Translated in
Rough
Endoplasmic
reticulum
Protein
product
Post
translational
modification
Sequence
of Amino
Acids

18.  Rate of transcription Promoter region
 RNA polymerase binds to
initiate transcription

19. Mutations
Heritable changes in
base sequence of DNA
Redundant
Polymorphism
Eliminated
by natural
selection
Persist in
several
generations
Silent
mutation
Confers
advantage

20. Polymorphisms
 Different alternative sequences at a locus within
DNA strand(alleles) that persist in a population
through several generations.
 Arise due to mutation.
 Stable-non functional.- Die out-disadvantageous
 Increase in frequency over generations-selective
advantage

21. Balanced polymorphism
 Situation where several
functionally distinct
forms of a gene are
common in a population
 Ambiguity-preserves the
gene
Susceptibility
to Hemolysis
Linked gene
for G6PD
Deficiency
Partial
resistance to
Malaria

22. Single Nucleotide Polymorphism
SNPs are DNA sequence variations that occur
when a single nucleotide in the genome
sequence is altered.
May entail substitution of one nucleotide to
another(C for T)
Result in ‘frame shift’ in translation

23. SNPs
 Result can be loss of protein synthesis, abnormal
protein synthesis or an abnormal rate of protein
synthesis.
 Individuals differ from each other approx. every 300-
1000 nucleotides with an estimated total of 30 million
SNP.
 Can occur in coding & non coding regions
 Important determinant of disease-e.g. Inherited
Thrombophilia

24. Inherited Thrombophilia
SNP in
Factor V
Leiden
Prolonged
immobility
In Case of
Hemorrhage
than
thrombosis
Increased
risk of
Venous
Thrombosis
Advantage?

25. Combination
of SNPs
• In or near a gene
Known as
Haplotype
• Inherited
from each
parent
Predisposition
to a Disease

26. Figure 7–10.
Types of genetic variants that have been significantly
associated with complex human traits and disease in 208
genome-wide association studies. See
www.genome.gov/gwastudies/.

28. Mutation
Disrupts Gene
function
‘Single
gene disorder’
Inherted;Mendelian
fashion

29. Atypical Plasma cholinesterase
Walter Kalow ;Suxamethonium sensitivity ;rate
of metabolism
Mendelian Autosomal Recessive trait
Short acting NM blocker
Plasma cholinesterase

30. 1:3000
fail to
inactivate
SXM
rapidly
Prolonged
NM Block
Recessive
gene
Abnormal
plasma
cholinesterase

32. Blood
test
Dibucaine
Homozygoteswhen exposed
HeterozygotesIntermediate
Inhibits Abnormal
enzyme less

33. Malignant hyperthermia
 Autosomal dominant
inherited.
 Idiosyncratic ADR due to
SXM on Ryanodine receptor
 Also due to halogenated
 Rapid rise of body
temperature,
inhalational agents
(Halothane)
 Incidence 1:20000
 muscle rigidity,
 tachycardia & cyanosis.

34.  Mechanism: Sudden rise in
release Ca2+ from sarcoplasmic
stores leading to muscle
contraction & hyper metabolic
rate.
 Potentially fatal .
 Important test family members
of affected.
 Impractical to screen routinely

35. Treatment:
Dantrolene 1 mg/kg i.v repeated up to 10mg/kg.
(prevents release of Ca2+ from sarcoplasmic
reticulum)
Symptomatic Rx of Hyperthermia
Rx of Cardiac arrhythmias

36. Acute intermittent porphyria
 Commonest & most severe form of hepatic
porphyria
 Autosomal dominant
 Mutation in gene coding Porphobilinogen
deaminase(PBGD)

37. Haem
Biosynthesis
PBGD key
enzyme
Red cell
precursors
Hepatocytes &
other cells

38. Mutation
Reduces
activity
Haem
precursors
(incl.Porphyrin)
PBGD
Build
up

39.  Strong interplay with environment through
exposure to drugs ,hormones& other chemicals
 Use of sedative, anticonvulsant or other drugs in
patients undiagnosed-can be Lethal
 Most drugs(not just CYP inducers) can precipitate
acute attacks in susceptible individuals

40. ALA
synthase
in liver
Induced by
drugs like
Barbiturates
ALA(delta amino
laevulanic acid)
Porphyrins
Increased ALA
production
Acute
Attack

41. Frank disease 5 times
more common in Women
Hormonal
Fluctuations
Precipitate
Acute
Attacks

42. Drug acetylation status

43. Acetyltransferase
• Single recessive gene
associated with low enzyme
activity
Acetylation
Elimination
of Isoniazid

44.  American population; equal no's ‘fast’
&’slow’ acetylators
 Other ethnic groups ;different proportions
 Slow>Egyptians,british swedish
 Rapid> Canadians,Asians,Latin americans

45. Isoniazid;2 different forms toxicity
Peripheral
Neuropathy
Isoniazid
Slow acetylators
Hepatotoxicity
Acetylhydrazine
Fast acetylators

46. Hydralazine
Acetyltransferase
Metabolism
of
Procainamide
Dapsone
Other
sulfonamides

47. Acetylator status
Influences
Drug induced lupus
Skin, joints &
Autoimmune
kidneys
Idiopathic ADR
Caused
By many agents

48. Aminoglycoside
ototoxicity
•
Mitochondrial gene
•
Most common predisposing
mutation
m.1555A>G,mitochondrial
DNA mutation
•
30-60% ototoxicity in
china(Aminoglycosides-cheap)

49. Bind to Bacterial ribosomes
Mutation of human
mitochondrial ribosomes
is similar
Aminoglycosides
For a single dose in
susceptible individuals.
Screening for this variant
appropriate in children
Increased affinity to
ribosomes in hair cells in
ear for several months

50. Defect in Ethanol metabolism
• Rate of Metabolism differs with
race
• Oriental races-
accumulation
of acetaldehyde.
• Due to slower rate of oxidation
of acetaldehyde as a result of
genetic polymorphism
Especially in
Japanese

51.  Around 80% of Asians have a variant gene ADH1B
 Almost all Chinese and Koreans- ADH1C
 coding alcohol dehydrogenase -toxic acetaldehyde at
a much higher efficiency
 50% of Asians, the increased acetaldehyde
accumulation, the mitochondrial ALDH2 allele,
 less functional acetaldehyde dehydrogenase enzyme,

53. Personalized Medicine
Understanding
human genome
Simpler methods
identify genetic
information
Genetic information
specific to individual
No
toxicity
No trial
& error
Preselect
effective drug

54. US FDA has approved PG labeling info to
package inserts of over 50 drugs
Use
patchy

55. Clinical Pharmacogenetic tests
 Anticipated to be one of the first applications of
human genome sequencing.
 Development slowed by various scientific ,
commercial, political and educational barriers.
 Cost effectiveness?
 Evidence in support of a test is less convincing
than the ideal of an RCT of PG informed
prescribing strategy versus current best practice

56. Tests increasingly used
1. Variants of different HLA strongly linked to
susceptibility to severe idiosyncratic
reactions
2. Genes controlling aspects of drug
metabolism
3. Genes encoding drug targets

57. Methodology
Mutations in
Germline
All cells of the
Next generation
Venous blood
samples(Chromosomal &
Mitochondrial DNA in
WBCs

58. Somatic cell mutations
• Genomic tests done on DNA
from samples of tumor
Presence or absence
guides drug selection
obtained surgically.
• Tests involve amplification of
relevant sequences and
Tumours
Pathogenesis
molecular biological methods
often utilizing chip technology
to identify various
polymorphisms

60. ABACAVIR & HLA-B*5701
 Abacavir-Reverse transcriptase inhibitor
 Highly effective - HIV Infection
 Severe Rashes
 Susceptibility linked to HLA variant
HLAB*5701

62. Anticonvulsants & HLAB*1502
Carbamazepine
 Severe life threatening rashes
 Stevens Johnson Syndrome
 Toxic epidermal Necrolysis
 Almost only in Asians
 FDA recommends Chinese patients to be screened for this
allele
 Similar problem with Phenytoin for same allele

63. Clozapine and HLA-DQB1*0201
 Effective antipsychotic drug
 Agranulocytosis 1% of patients
 Studies-small
 Specificity and sensitivity yet to be
established

65. Thiopurines and TPMT
 Thioguanine,Mercaptopurine & its
prodrug Azathioprine
Low TPMT
activity
High
TPMT
High Conc of
active TGN in
blood
Lower
conc
TGN
 Treat Leukemia's(ALL),Inflammatory
Bowel disease & Immunosuppression
 Cause Bone marrow & Liver toxicity
 Detoxified by Thiopurine S
methyltransferase(TPMT) present in
blood cells & by Xanthine oxidase
Bone marrow
Toxicity
Reduced
efficacy

66. Before treatment
 Phenotyping (by a blood test for TPMT activity)
 Genotyping TPMT Alleles
TPMT*3A,TPMT*3C,TPMT*2 is recommended.
 Careful monitoring of WBC count & drug
interaction with allopurinol(due its effect on
Xanthine Oxidase)

67. 5-FLUOROURACIL(5-FU) &DPYD
Extensively used to treat solid Tumours.
Unpredictable mucocutaneous toxicity.
Detoxified by dihydropyrimidine
dehydrogenase(DPYD)-clinically identifiable
multiple genetic variants
FDA recommends no to be given to those with
DPYD deficiency

68. TAMOXIFEN & CYP2D6
TAMOXIFEN
CYP2D6
Polymorphic
variation
Estrogen
antagonist
ENDOXIFEN
 Suggested link between CYP2D6 genotype&
efficacy.
 Genotyping tests available.
 Tetrabenzaine used to Huntington's disease may
also be influenced by cyp2d6

69. IRINOTECAN & UGTA1*28
 Topoisomerase I inhibitor.
 Marked activity against colorectal & lung
cancers(minority)
 Toxicity(Diarrhoea & BM suppression very severe
 Active metabolite SN-38
 UDP glucuronyltransferase
 Reduced activity
syndrome
Hyberbilirubinemia
Gilberts

71. TRASTUZUMAB & HER2
• Herceptin is mAB that antagonizes
epidermal growth factor(EGF) by binding to
one of its receptors(human epidermal
growth factor receptor 2-HER2)
• Somatic mutation
tissue
HER2 in tumour

72. DASATINIB,IMATINIB & BCR-ABL1
o DASATINIB –dual BCR/ABL & Src tyrosine
kinase inhibitor
o Used in hematological
malignancies(Philadelphia chromosome)
o CML ALL
o Mutation (T3151) in BCR/ABL confers
resistance to inhibitory effect of dasatinib.

73. o IMATINIB-TYROSINE kinase inhibitor
o CML & other myelodysplastic disorders.

75. WARFARIN & CYP2C9 +VKORC1
GENOTYPING
 WARFARIN
 Dosing individualized by measuring
INR(International normalized ratio)
 Thrombotic effects(lack of efficacy)
 Adverse effects(bleeding) common
 PG testing proposed based on polymorphism in its
key target, vitamin K epoxide reductase(VKOR)
&CYP2C9 GENOTYPE involved in its metabolism

76. Conclusion
Pharmacogenetics
 proves that concept of susceptibility to ADR can
be genetically determined
 Offers possibility of a more precise ‘Personalised ‘
Medicine for several drugs & disorders.
 Field of intense research, rapid progress.
Challenge remains about its feasibility in Clinical
setup