PHAR 751 Pharmacogenomics Sarah Brown, Pharm.D. Pharmacy Practice Resident Asante Health System

Definitions Pharmacology + Genomics = Pharmacogenomics The study of how an individual’s genetic inheritance affects the body’s response to drugs

More definitions Genotype –Genetic constitution of an individual –Gene combination at one specific locus or any specified combination of loci Phenotype –Observable trait –Manifestation of a genotype

Accessed 3/5/07

Human Genome Project Sequenced the human genome = 3 billion base pairs 30,000 genes in human DNA –Human DNA (4 nucleotides) 3 nucleotides = 1 codon 1 codon = 1 amino acid (aa) Many codons = 1 gene Thousands of genes = 1 chromosome

Polymorphisms A mutation in genetic code that occurs in >1% of the population Discontinuous genetic variation resulting in the occurrence of several different forms or types of individuals w/in a single species.

Pharmacogenomics 20 – 95% of variability in drug disposition and effects Sequence variants in genes encoding: –Drug-metabolizing enzymes –Drug transporters –Drug targets

Genetic polymorphisms Transporters Drug transporters Enzymes, Monooxygenases –Phase I Oxidative reactions –Phase II Acetylation Glucuronidation Methylation

Types of polymorphisms Single Nucleotide Polymorphism (SNP): single base difference in DNA sequence Insertion/deletion polymorphism Synonymous SNP: does not result in change in aa (CCA and CCG = proline) Non-synonymous SNP: results in change in aa (AGC and GGC = serine, glycine)

SNPs Responsible for ~90% of all genetic variation  Predispose person to a disease  Influence response to a drug Human Genome Project Information website. Accessed 3/4/07http://

Polymorphism examples Higher organisms: male and female sexes Humans: different blood types –A polymorphism that persists over many generations is usually maintained b/c no single form has an overall advantage or disadvantage  Some polymorphisms have no visible manifestation

Polymorphism of: Drug metabolism Drug targets Disease-modifying genes

Drug target polymorphism: β2-adrenoreceptor A: ↑ venodilation B: ↑ airway response C: ↑ desensitization

Evans WE, McLeod HL. Pharmacogenomics – Drug Disposition, Drug Targets, and Side Effects. N Engl J Med 2003;348(6):

P-gp polymorphism MDR1 is polymorphic –25 mutations identified –The C3435T polymorphism = p-gp expression in the intestine Homozygous for the T allele  ↓ lower intestinal p- gp  variations in drug absorption

Differences in allele frequency: Ethnicity C3435T polymorphism Homozygous for C allele –West Africans: 83% –African Americans: 61% –Whites: 26% –Japanese 34% More or less p-gp? Schaeffeler, et al. Frequency of C3435T polymorphism of MDR1 gene in African people. The Lancet. 2001; 358:383-4.

Study: MDR1 alleles To identify SNPs in coding region of MDR1 gene To assess prevalence in European American and African American populations To investigate the possible functional significance of polymorphisms –fexofenadine Kim, et al. Identification of functionally variant MDR1 alleles among European Americans and African Americans. Clin Pharmacol Ther 2001;70(2):

Study: MDR1 alleles 60 patients –10 SNPs 6 nonsynonymous 4 synonymous MDR1 Kim, et al. Identification of functionally variant MDR1 alleles among European Americans and African Americans. Clin Pharmacol Ther 2001;70(2):

 Statistically significant interethnic difference

PK: p-gp & sex, racial background ∆ Males ■ Females ○ African Americans ▼European Americans  No difference between groups

Genotype vs. Phenotype: exon mg fexofenadine po MDR1 exon 26, C3435T □ CT ■ CC ○ TT P=0.036 CC vs. TT

Genotype vs. Phenotype: exon 21 MDR1 exon 21, G2677T □ GT ■ GG ○ TT P= GG vs. TT

Genotype vs. Phenotype MDR1*1 or MDR1*2 alleles □ *1*2 ■ *1*1 ○ *2*2

Study conclusions Multiple SNPs present in the human MDR1 gene Polymorphism alters p-gp activity Genetic variation differs d/t racial background

Polymorphisms: Enzymes Frequently polymorphic Phenotypic consequence –Leads to inter-individual variability in drug response? –Other factors: molecular basis, expression of other drug-metabolizing enzymes, concurrent medications or illnesses

Consequences of enzyme polymorphisms: Drug toxicities Thiopurine methyltransferase-deficiency –Hematopoietic toxicity when treated w/ standard doses of azathioprine or mercaptopurine Slow acetylator phenotype –Hydralazine-induced lupus –Isoniazid-induced neuropathies –Dye-associated bladder cancer –Sulfonamide-induced hypersensitivity rxns

Consequences of enzyme polymorphisms ↑ CYP1A activity + slow acetylation = ↓ myelosuppression from active metabolites of amonafide ↓ drug-metabolizing enzyme  ↓ pro-drug activation –CYP2D6, opioid analgesics

PK: Ethnic differences Unlikely: –No gut or hepatic first-pass effect –Low plasma protein-binding (<70-80%) –No/minimal hepatic metabolism –No/minimal renal tubular secretion Likely: –Gut or hepatic metabolism –High plasma protein-binding –Hepatic metabolism as major route

Ethnic differences: hepatic metabolism Chinese vs. Caucasians –Higher metabolism Propranolol Morphine –No difference Triazolam Cerivastatin –Lower metabolism Desipramine Alprazolam Diazepam Omeprazole Nifedipine Codeine

Ethnic differences: hepatic metabolism African descent vs. Caucasians –Higher metabolism Propranolol –Lower metabolism Nifedipine Methyprednisolone Phenytoin –No difference Metoprolol/labetolol Albuterol Terbutaline Trimazosin Procainamide Etoposide

Ethnic variations Passive absorption, filtration at the glomerulus, and passive tubular reabsorption will not differ between ethnic groups For many drugs, PK prediction is difficult Wide therapeutic window? Narrow therapeutic window?