Lecture 3 - Pharmacogenetics Flashcards
What is pharmacogenetics?
Study of interindividual variation in DNA sequence related to drug response. For a large number of clinically relevant drugs, genetic variation plays a large role in PK/PD differences between patients.
Taste blindness?
1932 - 1st pharmacogenetics study
Some detected bitter taste of PTC when crystals placed on tongue, others could only detect a very slight taste. Study of 800 families - autosomal recessive inheritance.
Frequency of non-tasters varied in populations of different ethnicity. Mechanism unknown but was prototype for future studies.
Genetic control of drug levels in man?
1968 - pharmacokinetic studies in twins.
Dicumarol (vit K antag) and antipyrine (analgesic and antipyretic) given to identical/non-identical twins and plasma levels detected to determine half life.
Monozygotic twins had less variability therefore variability in metabolism seemed to be controlled genetically.
Goals of pharmacogenetics?
Clinical = avoid adverse drug reactions, maximise efficacy, select responsive patients. Scientific = link variation in genotype to phenotype, determine mechanisms responsible for link, translate link into enhanced understanding, treatment and prevention.
What do SNPs have implications for?
Pharmacokinetics (ADME) and pharmacodynamics (receptor expression, drug binding).
JUPITER TRIAL?
RCT to see if statins reduce CV risk in patients with raised CRP, randomised to rosuvastatin or placebo. Genomewide association study of 1.1m SNPs = genetic substudy.
ATP-binding Cassette Subfamily G Member 2 (ABCG2) or Breast Cancer Resistance Protein (BCRP) = xenobiotic trransporter, may play role in multi-drug resistance to chemotherapeutic agents. Large disequilibrium with SNP of BRCP gene (C421A), causes decreased transporter activity.
After 20mg dose, area under plasma conc.-time curve (AUC) of statin was 80% greater in individuals in 421CA and 421AA mutant group. Due to increased BioA due to decreased intestinal efflux of drug by variant BCRP.
SEARCH study?
2008 - genomewide association study of people with myopathy taking 80mg simvastatin.
Association of myopathy with SNP in SLCO1B1 on chromosome 12. Encodes organic anion transporting polyP OATP1B1, known to regulate hepatic uptake of statins. In nearly complete linkage disequilibrium with another noncoding SNP linked to statin metabolism.
Resting accumulation of simvastatin acidin plasma –> myopathy, 60% cases explained by allele. After 40mg dose, AUC of active simvastatin acid 22% greater, more frequent in asians and african americans.
JUSTIFICATION FOR INDIVIDUALIZATION OF LIPID LOWERING THERAPY.
Clopidogrel?
Prodrug, P450, inhibits P2y12 ADP on platelets.
- Interindividual variation in metabolism due to changes in CYP2C19 gene. Most SNPs, including CYP2C19*2 allele, encode inactive enzymes –> good, intermediate or poor metabolisers.
- Patients with SNP - less active mets, decreased inhibition of platelets, 3.58x risk of cardio events. Alternative drug = PRASUGREL. Virtually all metabolised to active drug and SNPs have little effect. Increased risk of bleeding and more expensive though.
Warfarin?
50% of interindividual variation due to CYP2C9 and VKOR1 SNPs.
- CYP2C9 metabolises warfarin to inactive metabolites. Common variants have very little activity so poor metabolism and require lower therapeutic dose.
- VKOR1 SNPs result in increased levels of expression so need increased doses of drug to attain therapeutic effect.
Describe HER family?
HER1-4, tyrosine kinase receptors. Receptor-specific ligands for HER1,3 and 4. Extracellular domain of HER2 can adopt a fixed conformation resembling ligand activated state, permitting it to dimerize in absence of ligand.
Kinase activation stimulated by ligand binding –> homo/heterodimers –> signal transduction cascades promote cellular proliferation and survival.
HER pathway? (2)
Phosphorylated Tyr on intracellular HER2 activated PI3-K –> phosphorylates a phosphatidylinositol –> binds and phosphorylates AK transforming factor (AKt) –> drives cell survival.
In parallel, guanine nucleotide exchange factor activates receptor activation factor (RAF) and then MEK, which phosphorylates MAPK. –> drives proliferation and downstream effect = VEGF production –> angiogensis.
Overexpression of HER2?
- Causes enhanced spontaneous dimerization of HER2 with other epidermal growth factor receptors –> increased cell proliferation.
- Anti HER2 only indicated in breast cancer. HER2 amplification in 20-30%, more aggressive and worse prognosis.
Assays of HER2?
IHC - can show how much HER2 protein in sample. Graded 0-3+. When over 2+, UK guidelines recommend further test (FISH)
FISH - measures amount of gene in each cell, can be used to visualise specific genes or portions of genes (extra copies of HER2 gene). Gene amplification determined from ratio between no. of signals from HER2 gene probe and CEN-17 reference chromosome probe. Result either positive or negative - correlates with herceptin response but more expensive than IHC.
Tyrosine kinase inhibitors?
HER overexpressed in brain, lung, bladder, head and neck, stomach cancers.
- TK inhibitors competitively inhibit binding of ATP to ATP binding cleft of TK domain of receptor, required for autophosphorylation.
- GEFITINIB was first, against NSCLCs. Average effiacy not great but was noticed that majority of patients with highly responsive tumours had somatic mutations of HER1 gene.
- Small deletions or point mutations. Net result = repositioning of critical residues surrounding ATP-binding cleft of TK domain or receptor –> increased affinity for drug –> more competitive.
- BUT all cases eventually progress despite treatment, which arises from new tumour cells expressing second EGFR mutation - steric hindrance, makes EGFR unable to bind drug. Development of secondary mutation that confers resistance suggests tumour cells remain dependant on active EFGR pathway for proliferation.
HER1 assays?
FISH correlates with response. Adavantageous to determine whether HER1 gene expressed in tumour tissue contains somatic mutations around ATP binding site in order to tailor therapy with appropriate TK inhibitor.
