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
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
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.)
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.
14.
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)
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/.
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)
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
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,
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
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
59.
60. ABACAVIR & HLA-B*5701
Abacavir-Reverse transcriptase inhibitor
Highly effective - HIV Infection
Severe Rashes
Susceptibility linked to HLA variant
HLAB*5701
61.
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
64.
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
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
70.
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.
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
Notes de l'éditeur
Prior to genomics forward genetic phenotype to genotypeLater ulta
DRUG METABOLISM – SUX lnsoprazole
Malig hyperthermia ryanodine receptors
antibiotics
Drug response –Gene by environment phenotypeIndividual’s response to a drug depends on a -complex interplay between environmental & genetic factors
What proportion of drug response variability is genetically determined?
Antipyrine a pyrazolone analgesic ,eliminated exclusively by metabolism & is substrate for multiple CYPs
suggests that about Validity needs to be established
Dna sequence codes for protein-ExonIntrons interruptIntrons transcribed to mRNA,not protein
Dna sequence codes for protein-ExonIntrons interruptIntrons transcribed to mRNA,not protein
Rate of transcription controlled by Promoter region to which RNA polymerase binds to initiate transcription
Silent mutation may not lead to change in a sequence.
Balanced-disadvantage ,for e.g. in homozygote is balanced by an advantage, for e.g. in a heterozygoteSusceptibility to hemolysis is an idiosyncratic reaction in response to oxidative stress in form of exposure to various dietary constituents' include several drugs(e.g. primaquine))
Occur every 100-300 bases along the 3 billion base human genome2/3rd C for T
Factor V-Coagulation factor
Approximately 500 SNPs were associated with human disease and complex traits. Intergenic and intronic SNPs comprise the largest fraction of associated variants.
Recognized for albinism by Sir. Lack gene-melanin; inborn errors of metabolismBritish physician early part 20th century initiated study of biochemical genetics
In 1950s Walter Kalow discovered Suxamethonium sensitivity was due to genetic variation in rate of metabolism
DO@:typical-6min.atypical 1-2hrs
Dibucanine no.N->80Hetero-40-60Homo-<20Detected by a blood test that measures dibucaine (plasma cholinesterase inhibitor)that measure
42 degrees
Skeletal muscle relaxant
1960 Price Evans demonstrated rate of drug acetylation varied in different populations as a result of balance polymorphism.Figure of Gaussian distribution which contrasts plasma con s achieved 3 hrs after a dose of salicylate with bimodal distri after a dose of Isoniazid
Increased susceptibility inherited exclusively from mother to children
Increased conc of aminoglycoside in endolymph
After a modest intake-palpitation&flushing of face
less common in Thailand and Indiathat converts alcohol to than other gene variants. is worsened by another gene variantresponsible for the breakdown of acetaldehyde.
No trial & error as supported by physiological clues
Human leucocyte Antigen
SJS-multiform rashes with blistering & other lesions extends in GITTEN-outer layer of skin peels away from dermis as though it has been scalded
TGN-6 thioguanine nucleotides
Was observed
FDA recommends poor metabolizers should not be prescribed more than 50 mg daily because of risk of severe depression
Philadelphia chromosome results from translocation defect parts of 2 chromosomes (9&22) swap placesPart of bcr(breakpoint cluster region) in chromosome 22 links to abelson(abl) region of chromosome 9
Figure 7–13.Pharmacogenetics of warfarin dosing. Warfarin is metabolized by CYP2C9 to inactive metabolites, and exerts its anticoagulant effect partly via inhibition of VKORC1 (vitamin K epoxide hydrolase), an enzyme necessary for reduction of inactive to active vitamin K. Common polymorphisms in both genes, CYP2C9 and VKORC1, impact on wFigure 7–13.Pharmacogenetics of warfarin dosing. Warfarin is metabolized by CYP2C9 to inactive metabolites, and exerts its anticoagulant effect partly via inhibition of VKORC1 (vitamin K epoxide hydrolase), an enzyme necessary for reduction of inactive to active vitamin K. 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 (often measured by the international normalized ratio [INR] blood test) and minimize the risk of too little anticoagulation (thrombosis) or too much anticoagulation (bleeding)..,