PHAR 751 Pharmacogenomics

Sarah Brown, Pharm.D.Pharmacy Practice Resident

Asante Health Systemsbrown@asante.org

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

www.globecartoon.com/neweconomy/13.html 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

http://www.genique.com/images/codage_genes.gif

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. http://www.ornl.gov/sci/techresources/Human_Genome/faq/snps.shtml Accessed 3/4/07

http://www.theonion.com/content/files/images/Infographic-Human-Genome-C.article.jpg

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):538-549.

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):189-199.

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):189-199.

Statistically significant interethnic difference

PK: p-gp & sex, racial background

∆ Males

■ Females

○ African Americans

▼European Americans

No difference between groups

Genotype vs. Phenotype: exon 26

180 mg fexofenadine po

MDR1 exon 26, C3435T□ CT■ CC○ TTP=0.036 CC vs. TT

Genotype vs. Phenotype: exon 21

MDR1 exon 21, G2677T□ GT■ GG○ TTP= 0.054 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?