Pharmacogenetics

Pharmacogenetics

1. Pharmacogenetics Dr. Ashishkumar Baheti MD Pharmacology

2. Overview Introduction Genomic basis of Pharmacogenetics Pharmacogenetics in drug development Benefits of pharmacogenetics Concerns about pharmacogenetic approach Summary

3. Differential drug efficacy At a recommended dosage – A drug is efficacious in most N ot efficacious in others → Lack of efficacy Harmful in few → Unexpected side effects Same symptoms Same signs → Different patients Same disease ↓ Same drug at same dose Different effects

4. Why does drug response vary Environmental factors Drug response is considered to be gene - by - environment phenotype

5. Exogenous & Endogenous factors contribute to variation in drug response

6. Study of genetic basis for variation in drug response Goal is to understand how someone's genetic makeup determines how well medicine works in an individual body as well as what side effects are likely to occur Pharmacogenetics

7. Pharmacogenetics vs Pharmacogenomics VS Pharmacogenetics Study of variability in drug response determined by single gene Pharmacogenomics Study of variability in drug response determined by multiple genes within the genome

8. History 1959 - Freidrich Vogel coined the term “ Pharmacogenetics ” In first half of 20 th century, Prolonged neuromuscular blockade following normal doses of succinylcholine Neurotoxicity due to Isoniazide Methaemoglobinemia in G6PD deficiency

9. Types of genetic variants - Two types of sequence variation have been associated with variation in human phenotype – 1 . Single nucleotide polymorphism(SNPs) 2 . Insertions/ Deletions ( Indels ) Polymorphism - a variation in the DNA sequence that is present at an allele frequencyof 1% or greater in population

10. Single nucleotide polymorphism – (SNPs) Single base pair positions in genomic DNA at which different sequence alternatives(alleles) exist wherein the least frequent allele has an abundance of 1% or greater Most common form of genetic variation Insertion/Deletion polymorphism ( Indels ) Less frequent in genome & are of particularly low frequency in coding regions of genes

11. Molecular mechanism of genetic polymorphism 1. Coding non-synonymous SNPs ↓ Changes the amino acid codon ↓ Change protein structure, stability, substrate affinities, or introduce a stop codon . Coding synonymous SNPs ↓ No change in the amino acid codon (functional consequences - transcript stability/ splicing) Noncoding SNPs ( in promoters, introns, or other regulatory regions) ↓ Affect transcription factor binding, enhancers , transcript stability, or splicing.

12. Indels can have any of the same effects as SNP substitutions: short repeats in the promoter (which can affect transcript amount), or insertions/deletions that add or subtract amino acids Copy number variations involve large segments of genomic DNA - Gene duplications ( increased protein expression and activity) Gene deletion ( complete lack of protein production, or inversions of genes → disrupt gene function) e g.TPMT,thiopurine methyltransferase ; ABCB1, the multidrug resistance transporter (P-glycoprotein); CYP, cytochrome P450 .

13. Pharmacogenetic study design consideration Pharmacogenetic trait – measurable/discernible trait associated with drug ( eg.enzyme activity, blood pressure, drug metabolite in plasma or urine ) Most pharmacogenetic traits are multigenic rather than monogenic so considerable effort is being made to identify the important genes and their polymorphisms influencing variability in drug response 1. Pharmacogenetic measures Pharmacogenetic measures Candidate gene association study Pharmacogenetic phenotypes

14. 2.Candidate gene association study P athways involved in drug response are known /at least partially known, pharmacogenetic studies are highly amenable to candidate gene association studies After g enes in drug response are identified,pharmacogenetic study is to identify the genetic polymorphisms responsible for therapeutic / adverse responses to the drug

15. Databases that contain information on polymorphisms and mutations in human genes

16. 3. Pharmacogenetic phenotypes

17. Variation in Pharmacokinetic response - Variation in phase I drug metabolism : CYP450 Variation in phase II drug metabolism

18. Variation in Phase I metabolism - CYP1A1, CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4 – pharmacogenetically important as they are responsible for phase I metabolism of > 90% commonly used drugs Many CYP genes are highly polymorphic with alleles that have functional consequences for how individuals respond to drug therapy CYP alleles result in absent, decreased or increased enzyme activity & thereby affecting rate of drug metabolism

19. Phase II metabolism Genes encoding phase II metabolism are also functionally polymorphic and responsible for individual variability in drug response N- acetylpolymorphism and Isoniazide eg. Cholinestarase polymorphism & prolonged postoperative paralysis Succinylcholine – normally hydrolysed by butyrcholinesterase

20. Contd.. BCHE gene encoding butyrylcholinesterase enzyme Two alleles Usual (U) & Atypical(A) - Major determitants of cholinesterase activity in plasma Homozygotes respond abnormally with prolonged muscle paralysis after succinylcholine administration Cholinesterase deficiency is due to homozygosity for A allele & has lower activity than usual type

21. Variation in pharmacodynamics response Glucose 6 phosphate dehydrogenase deficiency Malignant hyperthermia

22. Genetic variation in both pharmacokinetic & pharmacodynamics – Warfarin therapy Anticoagulant Narrow t herapeutic index Wide interindividual variability in dose requirement

23. Contd … Warfarin is metabolized by CYP2C9 to inactive metabolites and exerts its anticoagulant effect partly via inhibition of VKORC1 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 and minimize the risk of thrombosis or bleeding

24. Pharmacogenetics & drug development eg . Warfarin , Dapsone , Rasburicase , Azathioprine , Mercaptopurine , Irinotecan . (FDA has changed these drugs label as these have pharmcogenetic issue)

25. Pharmacogenetics research in antineoplastic drugs - It is an active field of research - ↓ life threatening toxicity & improve therapeutic efficacy of antineoplastic drugs N early 20 pharmacogenetics biomarkers & 30 chemotherapeutic agents have been included in drug package inserts & recommended by FDA & some of these biomarkers improve T/t efficacy , ↓ toxicity , lower health care costs

26. 6-MP & TPMT

27. Cetuximab / Panitumumab & KRAS Monoclonal antibodies inhibiting growth & survival of tumour cells with overexpressed EGFR in colon & head neck cancers These drugs found to be ineffective in some patients with EGFR mutation Association between resistance to these drugs and KRAS mutation KRAS – membrane GTPase that activates proteins in EGFR signalling pathway Stimulation of these proteins lad to cancer development (independent of EGFR signalling) KRAS mutated, inhibiting EGFR by these drugs have no effect KRAS induced cancer contd …

28. KRAS mutated, inhibiting EGFR by these drugs have no beneficial effect in KRAS induced cancer 40% colon cancer patients have this mutation Pharmacogenetic research found that mutation in exon 2 at G12 & G13 results in stimulation of KRAS and cancer development . FDA recommend pharmacogenetic test at these positions AS per drug label only patient with EGFR expressing colon cancer & KRAS mutant negative are supposed to use these drugs

29. Why is pharmacogenetics a good approach Many people have severe adverse reactions to drugs Many people respond to drugs at different doses Ineffective drugs are a waste of money to take Genetics don’t change Genetics can point to the cause not just the symptom

30. Pharmacogenetics benefits

31. Personalised drug Emerging goal to make medical practice more personalised Pharmacogenetics is an important step towards this goal

32. Concerns about pharmacogenetics approach How reliable are the tests ? Are health care providers prepared to use this information ? Will tailor made medicine lead to discrimination ? Will this affect people’s privacy?