PHARMACOGENOMICSSheba Medical Center
Some examples of drug response rates
How do we know if a patient will respond (or have an adverse response) to a drug?
(we don’t)
• 20 to 80 healthy volunteers • Determine max tolerable dose
– Phase II clinical trials (efficacy)
• Several hundred patients • estimate therapeutic dose range
– Drugs approved with a specific indication and dose range
– Phase III monitoring.
– Dose that patients actually receive depends on…
• Prior treatment history of patient (drug naïve?) • Physicians experience with prescribing drug • Empirical knowledge of appropriate dose • In practice – start low, ramp to ~80% of recommended dose before trying
different drug • Liability issues
– Therapeutic drug monitoring
Potential causes of variability in drug effects: • Pathogenesis and the severity of the disease being
treated.
• Individual’s age, gender, lifestyle (including environmental factors), behavior, nutritional state, renal and liver function, and concomitant illnesses.
• Genetic variation
A patient’s response to a drug may depend on factors that can vary according to the alleles that an individual carries, including
Pharmacokinetic factors • Absorption • Distribution • Metabolism • Elimination
Pharmacodynamic factors • target proteins • downstream messengers
G6PD deficiency Story • Hemolysis occurs in G6PD deficient individuals due to
the consumption of certain foods or drugs. • NADPH and GSH are necessary for the reduction of
hydrogen peroxide. • RBC membrane is unstable in the presence of free
radicals and hydrogen peroxide.
Presentation Notes
G6PD . : ,
G6PD Gene • Location: q28 locus of X-chromosome. • gene: 13 exons, 12 introns. 18,5 Kbp. • 400 variant alleles – point mutations,
deletions and replacements. • The G6PD enzyme is not fully active
when it is mutated. • Instability of the mutated enzymes due
to its susceptibility to proteolytic enzymes.
Succinylcholine Story
Mode of action: depolarizing muscle relaxant. Time of onset: 1.0-1.5 minutes. Clinical duration: 5-8 minutes. Mode of elimination: hydrolysis by plasma ChEst.
Prolonged muscle relaxation Due to
Inherited deficiency of plasma cholinesterase
Presenter
Presentation Notes
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Pharmacogenetics of Plasma Cholinesterase
Gene location: E1 locus on q of chromosome 3. Allelic variants: Eu (wt), Ea, Ef, Es.
Ea: atipical dibucaine-resistant variant A209G Asp70Gly
% of population Polymorphism in genotyping
Polymorphism in phenotyping
3.9 Eu/Ea; Eu/Ef; Eu/Es
60 > ED > 8 min
1:105 Es/Es ED > 8 hr
ED = effect duration
• Daily dose: 5mg/Kg PO
• Neurotoxicity (2% of population at 5mg/Kg
and 10-20% at higher doses): peripheral neuritis, optic neuritis, dizzinesss, ataxia, paresthesias, muscle twitching, stupor, toxic encephalopathy.
300 pts. Isoniazide PO 5 mg/Kg. serum levels after 3hrs.
Heterogeneity in the way individuals respond to
medications in terms of: Efficacy Toxicity
Pharmacogenomics The ultimate goal of pharmacogenomics is to define the contribution of the genetics differences in drug disposition or drug targets to drug response, thereby to improve the safety and the efficacy of drug therapy through use of genetically guided, individualized treatment.
Genotype vs Phenotype Polymorphism: drug deactivation
Presenter
PM = pure metabolizer EM = extensive metabolizer UM = ultrarapid metabolizer
WARFARIN Oral anticoagulant. S-warfarin is hydroxylated to inactive 7-OH W by CYP2C9.
CYP2C9*1:100%; *2(SNP in exon 3): 12%: * 3(SNP in exon 7): 5% activity
CYP2C9 mutations are associated with adverse clinical outcomes
Higher incidence of serious bleeding: 10.92 vs 4.89 pt-yr. Higher incidence of life-threatening bleeding: 1.56 vs 0.7 pt-yr. Increased risk of above-range INRs: (hazard ratio: 1.4). More time to achieve a stable warfarin dose (95 days; p<0.004). Increased risk of bleeding during initiation of therapy (hazard ratio 3.94). Increased risk of bleeding at any time during therapy (hazard ratio 2.39).
Presenter
Genotype vs Phenotype Polymorphism drug activation (1)
Codeine and breastfeeding
• Pharmacogenetics of morphine poisoning in a breastfed neonate of a codeine- prescribed mother
• Did the drug cause death? Codeine and breastfeeding
Codeine and breastfeeding
CYP2D6 – effect of gene duplication: inadequate therapeutic response to
standard doses of nortriptyline
NEJM 6 feb 2003
Protein: 413 AA
Distribution: arterioles, veins, bronchi: vaso-/ bronchodilation
Genotype: 9 single nucleotide polymorphisms, 5 silent 4 missense SNPs (at AA 16, 27, 34, 164)
ß2- Receptor-SNPs and Drug Response Bronchodilator responsiveness
Among mild asthmatics, Arg16-homozygotes (WT) have a faster and greater bronchodilator response to oral albuterol (after a 12 hour drug holiday) when compared to Arg16Gly- homozygotes or heterozygotes
PK PD CPT 1999
•Epidermal Growth Factor Receptor over-expressed in 40-80% of Non-Small-Cell Lung Carcinomas •EGFR signaling triggered by binding of growth factors, resulting in dimerization of receptor, auto/trans phosphorylation via tyrosine kinase domain, recruitment of downstream effectors and activation of cell prolioferation/survival programs
•Gefitinib inhibits EGFR by blocking ATP cleft, thereby preventing activation by autophosphorylation
•Tumor responses seen in only ~10% of patients with chemotherapy resistant advanced NSCLC
•A subgroup of patients with NSCLC have specific mutations in the EGFR gene that constitutively activate the receptor, and confer sensitivity ot Gefitinib
Lynch TJ et al., NEJM 350,2129; Paez JG et al., Science, 304,1497
Gefitinib •25/275 patients at Mass General identified as being responsive. EGFR sequenced in 9 responsive patients.
Gefitinib
•All mutations in kinase domain •Matched tissue showed wild type sequence. No mutations seen in 7 non- responsive cases analysed •Heterozygous mutations seen in 2 cases. Dominant gain of function mutation •Heterologous expression in Cos-7 cells shows mutant receptors (IC50 = 0.015uM) more sensitive to inhibition by gefitinib than wild type (IC50 = 0.1uM).
Gefitinib
Conclusions •Only a subgroup of NSCLC tumors harbour EGFR mutations.
•This subtype of NSCLC sensitive to genfitinib
•Patients should be screened for EGFR mutations, and if they have them, be given gefitinib as first line therapy.
P-Glycoprotein
NEJM 6 feb 2003 p. 538-549
Treatment modifications and patient genotypes
8(6):300-304,2002
Coagulation factor
Coagulation factor
Vitamin K1H2 Vitamin K 2,3 epoxide
VKOR
Carboxylase
-
-
VKORC1
An 18 kD warfarin sensitive protein. Missense mutations in its encoding gene associated with inherited warfarin resistance and VKDCF deficiencies
Glutathione-S- Transferase
High homology between GST A1 and purified microsomes that reconstituted VKOR activity
Microsomal Epoxide
Hydrolase
Anchored to the ER membrane and harbors vitamin K 2,3 epoxide binding Sites.
0 1 2 3 4 5 6 7 8
mu/muwt/muwt/wt CYP2C9 & VKORC1 genotype
CYP2C9 dose
Warfarin daily dose by genotype combinations
Warfarin daily dose (mg/day) mean ± SE CALU VKORC1 CYP2C9 N Compound
genotype
The relative effects of selective variables on warfarin dose requirements (mg/day)
Significance Partial r2 β-coefficient Variable
0.002 0.109 -0.33 Age
0.0004 0.142 0.38 Weight
0.04 0.047 -0.22 K1H2
Total r2=0.714
Control patients (n=99)
Potential causes of variability in drug effects:
A patient’s response to a drug may depend on factors that can vary according to the alleles that an individual carries, including
G6PD deficiency Story
300 pts.IsoniazidePO 5 mg/Kg.serum levels after 3hrs.
Heterogeneity in the way individuals respond to medications in terms of:
Pharmacogenomics
WARFARIN
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Codeine and breastfeeding
Codeine and breastfeeding
Tamoxifen
CYP2D6 – effect of gene duplication:inadequate therapeutic response to standard doses of nortriptyline
Polymorphisms of the ß2- receptor (1)
ß2- Receptor-SNPs and Drug Response Bronchodilator responsiveness
Gefitinib (Iressa)
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VKOR-a multicomponent enzyme complex
The relative effects of selective variables on warfarin dose requirements (mg/day)
A novel coding VKORC1 polymorphism – Asp36tyr
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