Pharmacogenetics Lecture Notes PDF

Royal College of Surgeons in Ireland Medical University of Bahrain Pharmacogenetics Salim Fredericks  2 Learning Outcomes Illustrate, using examples, the clinical relevance of genetic variation in drug metabolising enzymes (i.e. p...

Royal College of Surgeons in Ireland Medical University of Bahrain Pharmacogenetics Salim Fredericks  2 Learning Outcomes Illustrate, using examples, the clinical relevance of genetic variation in drug metabolising enzymes (i.e. pharmacokinetics). Illustrate, using examples, the clinical relevance of genetic variation in the target of drugs (i.e. pharmacodynamics). Illustrate, using an example, how pharmacogenomics effects can be polygenic. Illustrate, using examples, the clinical relevance of genetic variation in ‘off target’ genes.  3 Individuals will show a range of responses to a specific dose of a particular drug… Can we use genetics to help select the correct drug and dose for safer, more efficacious treatment? Challenge is predicting response..  4..but these responses can be “complex” - influenced by both genetic and environmental factors.. Environmental (nurture) – Age – Diet – Organ function; underlying disease – Concomitant therapy; drug interactions Genetic (nature) – As coded for by DNA Each drug trait will have unique profile Not all genetic components will be clinically relevant Administration Elimination DMEs DMEs Pro- Active Drug- Drug Drug Metabolite CYP2D6 No benefit in CODEINE 10% patients MORPHINE Approx. 10% null for CYP2D6 P450 genes contain clinical relevant 6 genetic variation.. Take from: Genetics in Medicine Thompson & Thompson Codine → Morphine Conversion carried out by CYP2D6. Poor metabolisers = ↓ Morphine = little therapeutic effect for standard dose Ultrafast metabolisers = ↑ Morphine = intoxication Tamoxifen is inactive until it is converted in the body into endoxifen by CYP2D6, which has several alleles. Depending on which allele combination a woman has, she may metabolize tamoxifen: slowly, intermediately or rapidly to endoxifen. Metabolism of foreign compounds (xenobiotics) Elimination in urine Phase I Phase II (conjugation) Functional groups such as -OH, -NH2, Available functional groups in a -COOH introduced into drug molecule drug molecule (which may or may Main function of phase I metabolism not result from phase I) are is to prepare drugs for phase II conjugated with hydrophilic groups. metabolism Increases water solubility and ease of excretion CYP3A4/5 and UGT, are involved in the metabolism of more than 75% of drugs in use Relative contributions of various cytochrome P450 isoforms (A ) and different phase II pathways (B) to metabolism of drugs in clinical use. Many drugs are metabolized by two or more of these pathways. Some drugs interfere with tamoxifen conversion by competing for the CYP2D6 enzyme Which drugs are the most important in this group?  11 Candidate gene example 1: Mercaptopurine (Purinethol) & azathioprine Inhibits purine (A/G) nucleotide synthesis Therefore inhibits DNA replication Used to treat : – leukemia & inflammatory bowel disease, organ transplants, rheumatoid arthritis Causes myelosuppression – Infection due to ↓ WBC – Anemia due to ↓ RBC  12 Azathioprine Metabolism Prodrug AZATHIOPRINE (thiopurine methyl transferase) Inactive… TPMT deficiency Inactive… XO TPMT OXIDIZED 6-MERCAPTOPURINE 6-METHYL METABOLITES 6-MP MERCAPTOPURINE XO: xanthine oxidase 6-MMP HPRT Good candidate gene GMPDEHYDROGENASE IMP SYNTHETASE Major Activemetabolite metabolite 6-THIOGUANINE Toxicity NUCLEOTIDES responsible for activity 6-TGN Incorporation in to DNA..  13 TMPT genotype influences enzyme activity Homozygote (v/v) Heterozygote (v/wt) Homozygote (wt/wt) Low/slow activity Mid activity High/fast activity Image taken from: Wang & Weinshilboum Oncogene (2006) 25, 1629–1638  14 Dosing by TPMT genotype greatly reduces the incidence of toxicity 90-95% reduction in dose! Cheok & Evans – Nature Reviews Cancer – 2006, Vol 6 – 117-128 FDA have recommended genetic testing for mercaptopurine since 2003  Drug target 15 (calcineurin) Drug transport (ABC1/ Pgp) Tacrolimus Drug metabolism and (CYP3A4/5) ciclosporin Protein targets Ion channels Carrier molecules Enzymes Receptors  Cystic Fibrosis is caused by a variety of different mutations in the CFTR genes These mutations are damaging in different ways: DeltaF508 most common CFTR mutation in Ireland. G551D is next most common. but other mutations are also be present, at very low frequencies  Treatment of cystic fibrosis with kalydeco/ivacaftor Class III & IV mutations respond well to /ivacaftor. kalydeco/ivacaftor Image from: http://www.cftr.info/  19 Warfarin An anticoagulant Used in prevention of thrombosis and embolism 21.2m prescriptions in 2003 in US alone.. Narrow therapeutic range ↑ dose = risk of bleeding and hemorrhage ↓ dose = thrombosis/embolism – Further complicated by diet, disease state, drug interactions Dosing monitored via international normalised ratio (INR) standardized version of “prothrombin time” test  20 VKORC1 polymorphisms and warfarin Vitamin K epoxide reductase lowers levels of active vitamin K, which is essential for the activation of factors II, VII, IX, X, and proteins C and S. VKORC1 polymorphisms usually result in greater responsiveness to the warfarin, and therefore, lower dose requirements Lippincott’s Pharmacology  21 CYP2C9 & VKORC1 polymorphisms and warfarin Active drug is biotransformed CYP2C9 to inactive daughter. Thus a loss-of-function CYP2C9 polymorphisms lead to higher active drug concentration. + VKORC1 polymorphisms: result in greater respons to European J of Clin Pharmac 2012 CYP2C9 and VKORC1 polymorphisms influence warfarin dose v the warfarin, and therefore, ariability in patients on long-term anticoagulation lower dose requirements  22 Results from candidate gene studies.. 1999: CYP2C9 (metaboliser) Correlation reported between CYP2C9 dose requirements. Genotype explaining 6% of dose 2006: VKORC1 (target) Report of correlation between VKORC1 variation and dose Explains approx 25% of dose variability For original reports see: N Engl J Med. 2005 Jun 2;352(22):2285-93. Lancet (1999) 353: 717-9) VKORC1 genotype Resulting web tool.. 23 FDA have recommended genetic testing for warfarin since 2007  24 Stevens Johnson Syndrome.. SJS Severe adverse reaction involving separation of epidermis from dermis. Early stage of toxic epidermal necrolysis TEN Incidence: 1-5 / million population/year Major cause is adverse effect of pharmaceuticals..  25 HLA example 1: Carbamazepine induced SJS.. An anti-epileptic drug. Target: sodium channels SJS = 1-6 cases per 10,000 new users.. (x10 in Asia) 2004: Strong link between HLA-B*1502 and SJS (100% sensitivity, 97% specificity). 2008 FDA makes labelling change for CBZ Advises all Asian descent individuals be typed by HLA-B*1502 prior to exposure to the drug Global frequency of HLA-B*1502 BUT – Test is only effective in Asian descent populations. B*1502 is absent from many regions but still see SJS due to CBZ.. Search for additional tests ongoing See: Nature (2004) 428: page 486 for original report