7. pharmacogenetics

Genetic polymorphisms are variations in gene sequences that occur in at least 1% of the general population, resulting in multiple alleles or variants of a gene sequence.
The most commonly occurring form of genetic variability is the single nucleotide polymorphism (SNP, often called “snip”)

1. Dr. S P Srinivas Nayak , PharmD ., MSc ., PGDND., (PhD) Assistant Professor, PIPR, PU PHARMACOGENETICS

2. Lecture Objectives After completion of this lecture, student will be able to: Define pharmacogenetics and pharmacogenomics. Define genetic polymorphism and explain the difference between genotype and phenotype. Explain with relevant examples how genetic variability influences drug response, pharmacokinetics, and dosing regimen design. Describe the relevance of CYP enzymes and their genetic variability to pharmacokinetics and dosing . List the major drug transporters and describe how their genetic variability can impact pharmacokinetics. Discuss the main issues in applying genomic data to patient care, for example, clinical interpretation of data from various laboratories and accuracy of record keeping of large amounts of genomic data.

3. INTRODUCTION Pharmacogenetics is the study of how genes affect the way people respond to drug therapy The goal of pharmacogenetics is to individualize drug therapy to a person's unique genetic makeup An understanding of an individual's genetic makeup is thought to be the key to creating personalized drugs with greater efficacy and safety

4. Introduction Therefore, Pharmacogenetics is an established discipline that studies the genetic basis of interindividual variability in the response to drug therapy, and allows for individualization of drug therapy A closely related, and considered by some to be an equivalent or overlapping field, is pharmacogenomics

5. Introduction Pharmacogenomics involves study of the role of genes and their genetic variations (DNA, RNA level) in the molecular basis of disease, and therefore, the resulting pharmacologic impact of drugs on that disease In general  pharmacogenetics  usually refers to how variation in one single gene influences the response to a single drug Pharmacogenomics  is a broader term, which studies how all of the genes (the genome) can influence responses to drugs

6. Introduction Genes are made of DNA, and so  is  the  genome  itself. A  gene  consists of enough DNA to code for one protein, and a  genome is  simply the sum total of an organism's DNA

7. Introduction

8. Introduction

9. Introduction Application of pharmacogenetics to pharmacokinetics and pharmacodynamics helps the development of models that predict an individual's risk to an adverse drug event and therapeutic response For example, the monoclonal antibody Herceptin was designed to treat a subset of breast cancer patients who overexpress the HER-2 (human epidermal growth factor receptor-2) gene. Patients who lack HER-2 overexpression are considered to be nonresponders to Herceptin therapy

10. Introduction The outcome of disease, resistance to treatment, and adverse reactions are increasingly recognized as an interaction of the individual's genes and the environment Dominic Kwiatkowski, recently reviewed the role of genes in human susceptibility to infection

11. Introduction He predicted that recent advances in genetics and genotyping technology will make it feasible within the next decade to screen the whole genome for genetic factors that determine susceptibility to HIV and AIDS, malaria, and tuberculosis Kwiatkowski listed many genes and their encoded proteins that play roles in immunity and disease fighting

12. Introduction

13. GENETIC POLYMORPHISMS Genetic polymorphisms are variations in gene sequences that occur in at least 1% of the general population , resulting in multiple alleles or variants of a gene sequence . The most commonly occurring form of genetic variability is the single nucleotide polymorphism (SNP, often called “snip ”) An example of such an effect occurs if nucleotide position 2935 of the CYP2D6 gene has a C instead of an A (c.2935A>C). During translation this results in the insertion of a proline instead of histidine at amino acid position 324 generating the CYP2D6*7 allele, with no drug metabolizing activity

14. GENETIC POLYMORPHISM IN DRUG METABOLISM Drug metabolism is responsible for the chemical modification of drugs or other xenobiotics that usually results in increased polarity to enhance elimination from the body. The enzymes that perform drug metabolism are classified as either phase I or phase II enzymes. Phase I enzymes perform oxidation, reduction, and hydrolysis reactions while phase II enzymes perform conjugation reactions . Polymorphisms have been reported in both phases of drug-metabolizing enzymes and can affect the pharmacokinetic profile of a drug

15. CYTOCHROME P-450 ISOZYMES The CYP450s are divided into families identified with numbers (CYP1, CYP2, CYP3, etc ) and subfamilies identified with letters (CYP2A, CYP2B, etc ) The major drug metabolizing CYP450 families are CYP1, CYP2, and CYP3

16. CYP2D6 CYP2D6 is the most highly polymorphic CYP with more than 70 allelic variants reported. It is responsible for the metabolism of antidepressants, antiarrhythmics , beta-adrenergic antagonists, and opioids , which frequently have narrow therapeutic indices . It is estimated that approximately 10% of the Caucasian population, 1% of the Asian population, and between 0% and 19% of the African population have a PM phenotype of CYP2D6, resulting in increased plasma concentration of the parent drug due to decreased metabolic clearance

17. DRUG EFFECT FDA INFORMATION

18. This has been reported with the breast cancer agent tamoxifen . Tamoxifen has an active metabolite ( endoxifen ) produced by CYP2D6 that is thought to be responsible for much of its antiestrogenic activities. The patient with the PM phenotype would not metabolize tamoxifen to the active metabolite and, therefore, does not benefit from clinically relevant endoxifen concentrations

19. CYP1A2 CYP1A2 is responsible for the metabolism of about 5% of marketed drugs including fluvoxamine, clozapine, olanzapine, and theophylline. Approximately 15% of the Japanese, 5% of the Chinese, and 5% of the Australian populations are classified as CYP1A2 poor metabolizers. The most frequent allelic variant is CYP1A2*1F, which results in an increased expression caused by an SNP in the upstream promoter region. Enhanced enzyme levels are thought to cause faster substrate clearance, which has been associated with treatment failures for clozapine in smokers with the *1F allele. CYP1A2*1C is also an SNP in the upstream promoter region that results in decreased enzyme expression and has a prevalence up to 25% in Asian populations

20. CYP2C9 CYP2C9 has at least 30 different allelic variants with the two most common being CYP2C9*2 and *3. Both of these variants result in reduced CYP2C9 activity and are carried by about 35% of the Caucasian population. CYP2C9 helps in metabolism of warfarin, if is not appropriately lowered, then there is an increased risk of bleeding. It also metabolises nonsteroidal anti-inflammatory drugs, sulfonylureas, angiotensin II receptor antagonists, and phenytoin( because of their high therapeutic indices (except phenytoin), do not usually result in adverse effects)

21. CYP2C19 The CYP2C19 PM phenotype results in a lack of efficacy for the antiplatelet prodrug clopidogrel . For activation, clopidogrel requires a two-step metabolism by several different CYP450 with CYP2C19 being a significant contributor.

22. CYP3A4 CYP3A4 is the most abundant CYP450 in the liver and metabolizes over 50% of the clinically used drugs. SNP that is found in about 2.7% of the Caucasian population and has some decreased clearance for the calcium channel blocker nifedipine but not for testosterone 6β-hydroxylation.

23. Plasma pseudocholinesterase or serum butyrylcholinesterase Patients with slowed metabolism of succinylcholine have elevated blood levels, prolonged duration of action, and prolonged apnea compared to patients with fully functional pseudocholinesterase . Dihydropyrimidine dehydrogenase (DPD ) Polymorphisms in DPD result in a loss of enzymatic activity leading to the accumulation of the chemotherapeutic agent 5-flourouracil (5-FU), which leads to significant toxicity including leukopenia, thrombocytopenia, and stomatitis .

24. Thiopurine S- methyltransferase (TPMT ) Thiopurine drugs including 6-mercaptopurine (MP) and azathioprine are used for their anticancer and immunosuppressive properties but can have significant adverse effects including myelosuppression . The loss of TPMT function is present in about 5% of the Caucasian population and results in accumulation of MP leading to an increased risk for adverse effects like leukopenia.

25. Uridine Diphosphate (UDP)- glucuronosyltransferase Irinotecan is a prodrug topisomerase-1 inhibitor that is approved to treat metastatic colon or rectal cancer. The active metabolite of irinotecan , SN-38, is produced by ester hydrolysis and is primarily cleared through biliary excretion after inactivation by UGT (Rothenberg, 1998). The accumulation of SN-38 is associated with dose- and treatment-limiting adverse effects including bone marrow toxicity and diarrhea

26. N- Acetyltransferase N- acetyltransferase (NAT) was identified as a polymorphic enzyme through phenotypic observations of fast or slow acetylators of the anti-tuberculosis drug, isoniazid

27. GENETIC POLYMORPHISM IN DRUG METABOLISM : CYP P-450 ISOENZYMES

28. Genetic Polymorphism in Drug Transport and Drug Targets Several membrane transporter proteins are involved in drug absorption from the intestinal tract and distribution through the body. An increased appreciation of the influence of these transporters on the uptake and efflux of drugs into or out of tissues has enhanced interest in the pharmacogenetics of these transporters

29. MDR1 (P-Glycoprotein) The MDR1 or ABCB1 gene codes for the efflux protein P-glycoprotein (P- gp ) that is frequently associated with drug resistance to antineoplastic agents including vincristine and doxorubicin.

30. ABC Transporters The multidrug resistance-associated proteins (MRPs) are members of the ATP-binding cassette (ABC) superfamily with six members currently, of which MRP1 (ABCC1), MRP2 (ABCC2), and MRP3 (ABCC3) are commonly known to effect drug disposition. Like MDR, these transporters can also be expressed in cancer cells, which confer resistance to the chemotherapeutic agent tamoxifen

31. Solute Carrier Transporters Another important class of drug transporters is the solute carriers (SLCs) such as the organic anion transporter protein (OATP) and organic cation transporter (OCT ). These transporters are located throughout the body and have various roles in the transport of many different drugs. OATP1B1 (coded by the SLCO1B1 gene) is a hepatic influx transporter with at least 40 non-synonymous SNPs identified that result in either an altered expression or activity of OATP1B1

32. Summary Pharmacogenetics is the study of how genes affect the way people respond to drug therapy Pharmacogenetics usually refers to how variation in one single gene influences the response to a single drug Pharmacogenomics is a broader term, which studies how all of the genes (the genome) can influence responses to drugs