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Drug Metabolism and Pharmacogenetics

Drug Metabolism and Pharmacogenetics. Brendan Stamper University of Washington Dept. of Medicinal Chemistry. What is Medicinal Chemistry?. Medicinal Chemistry is a scientific discipline involved with designing, synthesizing and developing pharmaceuticals suitable for therapeutic use

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Drug Metabolism and Pharmacogenetics

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  1. Drug Metabolism and Pharmacogenetics Brendan Stamper University of Washington Dept. of Medicinal Chemistry

  2. What is Medicinal Chemistry? • Medicinal Chemistry is a scientific discipline involved with designing, synthesizing and developing pharmaceuticals suitable for therapeutic use • Highly interdisciplinary science combining genetics, molecular biology, biochemistry, organic chemistry, pharmacology, toxicology

  3. Outline • Background • Drug Metabolism • Pharmacogenetics • Examples • CYP2D6 • Codeine • DDI Scenario (fluoxetine) • ALDH2 • Ethanol • DDI Scenario (acetaminophen)

  4. Basic Vocabulary • Lipophilicity vs Hydrophilicity • Lipophile: “Fat-lover” • Hydrophile: “Water-lover” • Xenobiotic: a foreign chemical substance

  5. More Basic Vocabulary • Metabolism: chemical reactions that occur in living organisms • Enzyme: a biomolecule that catalyzes a chemical reaction Inducer Enzyme Xenobiotic Metabolite Inhibitor

  6. Today’s Focus • We will focus on the enzymatic conversion of lipophilic xenobiotics to more water soluble metabolites . . . . . . and how these processes are influenced by genetic predisposition WHY DO WE CARE?

  7. Lipophilic xenobiotics can be potentially dangerous because they can easily permeate lipid cell membranes and accumulate within cells • By converting lipophilic xenobiotics to hydrophilic metabolites we can facilitate elimination

  8. Today’s Focus • We will focus on the enzymatic conversion of lipophilic xenobiotics to more water soluble metabolites . . . . . . and how these processes are influenced by genetic predisposition (i.e. Can we expect xenobiotic metabolism to be consistent from person-to-person?) NO

  9. Variability in Dose-Response “If it were not for the great variability among individuals, medicine might as well be a science and not an art” William Osler 1849-1919 What factors are responsible for this variability?

  10. Variability in Dose-Response Genetics Age Race Drug Response Stress Disease Gender Diet Occupational Exposure

  11. Pharmacogenetics • Definition: The study of genetic variation that gives rise to variability in drug response (Optimize efficacy and limit toxicity) Genetics Age Race Drug Response Stress Disease Gender Diet Occupational Exposure

  12. How do we predict optimal dose for most efficacious response • Step 1: Understand the mechanism of drug action at the molecular level • Step 2: Understand how genetic variations affect drug action • Step 3: Rational choice of drug and dosage

  13. DNA ATGC Codon Gene Chromosome Genome English Abcdef . . . Word Sentence Chapter Book DNA Is Like a Language Like language, DNA changes over time

  14. Polymorphism • Polymorphism: Change in DNA sequence that occurs in more than 1% of the population • Allele: An alternative form of a gene (i.e. site of sequence variation) • SNP (Single Nucleotide Polymorphism) Gene: ATG-GGA-TGC-TAA met-gly-cys-STOP SNP: ATG-GCA-TGC-TAA met-ala-cys-STOP • Impact of new allele • Alter protein function • Alter protein structure or stability • No consequence

  15. CYP2D6 and Codeine Example #1

  16. Codeine • Analgesic • Prodrug • CYP2D6-mediated bioactivation critical for analgesic effect • 200mg codeine is equivalent to 30mg morphine (~10%) CYP2D6 More Lipophilic More Hydrophilic

  17. CYP2D6 • Drug metabolizing enzyme • Member of the P450 family (Cytochrome P450 2D6) • Common substrates • Beta-blockers (Metoprolol) • SSRIs (Fluoxetine) • Opiods (Codeine) • SERMs (Tamoxifen) • Highly polymorphic enzyme • Over 100 reported Rowland et al, JBC (2006) 281:7614-7622

  18. Allele Kinetic Data Kinetic plot of product formation versus substrate concentration for metabolic turnover of codeine catalyzed by highly purified recombinant CYP2D6 isoforms in vitro. Yu et al, JPET (2002) 303:1291-1300

  19. CYP2D6 Genotypes • Ultra Metabolizers • Extensive Metabolizers • Intermediate Metabolizers • Poor Metabolizers

  20. Poor vs Extensive Metabolizers Individual plasma concentration of codeine and morphine in 14 extensive (filled) and 14 poor (open) metabolizers after an oral dose of codeine. Poulsen et al, Eur J Clin Pharmacol (1996) 51:289-295

  21. How can we sort the population into the different CYP2D6 metabolizing groups?

  22. Urinary Metabolic Ratio of CYP2D6 Substrate • Dose patients with CYP2D6 substrate (codeine) • Collect urine sample that contains substrate and metabolite • Calculate ratio of substrate over metabolite Substrate (codeine) Metabolite (morphine) High (poor metabolizer) Low (extensitve metabolizer) =

  23. Urinary Metabolic Ratio of CYP2D6 Substrate Intermediate Metabolizers Roden et al, Ann Intern Med 2006; 145:749-757

  24. Outcomes of CYP2D6 Allelic Variations No analgesic effect Slight analgesic effect Pain relief ‘Overdose’ effect Null allele Expected plasma concentration-time curve with therepeutic window indicated by the boxed area Decreased function allele Fully functional allele Zanger et al, Naunyn-Schiedeberg’s Arch Pharmacol (2004) 369: 23-37

  25. Codeine Pharmacogenetics Green: Intermediate and Extensive Metabolizers Purple: Ultra Metabolizers Orange: Poor Metabolizers

  26. Who are most affected by CYP2D6 polymorphisms?

  27. Ethnic Variation in CYP2D6 Mutation Frequencies CYP450 allele nomenclature committee database: http://www.imm.ki.se/cypalleles

  28. DDI Scenario:Codeine and Fluoxetine +

  29. Drug-Drug Interactions:Codeine + Fluoxetine CYP2D6 X Codeine Morphine Fluoxetine (inhibitor) Zanger et al, Naunyn-Schiedeberg’s Arch Pharmacol (2004) 369: 23-37

  30. Summary: CYP2D6 & Codeine • Codeine is a prodrug (requires metabolism) • CYP2D6 metabolizes codeine to morphine • CYP2D6 is a highly polymorphic enzyme • Populations can be separated into different metabolic sub-groups • UMs: ‘Overdose’ analgesic effect • EMs/IMs: Predicted analgesic effect • PMs: No analgesic effect • Urinary sampling can enable the pre-sorting of different metabolizers • Co-treatment with fluoxetine: EMs to PMs

  31. ALDH2 and Ethanol Example #2

  32. Ethanol • Low dose: Muscle relaxant, euphoria impaired judgment • High dose: CNS depressant, impaired sensory/motor function • Toxic when BAC > 400mg/dL (0.4%)

  33. Ethanol Metabolism ADH ALDH • Alcohol to aldehyde to carboxylic acid • Ethanol: CNS depressant • Acetaldehyde: Vasodilator • Flushing • Hangover effects • Acetic Acid: Relatively harmless More Lipophilic More Hydrophilic

  34. ALDH2 • Aldehyde dehydrogenase 2 • Mitochondrial enzyme • Homotetramer • Substrates: aldehydes • Cofactor: NAD+ • Catalyzes the oxidation of aldehydes • Polymorphic enzyme Larson et al, JBC (2005) 280:30550-30556

  35. ALDH2 Genotypes • ALDH2*1/*1 Wild-type Homozygous • ALDH2*1/*2 Heterozygous • ALDH2*2/*2 Mutant Homozygous • *2 allele = E487K mutation

  36. ALDH Crystal Structure *2 Mutation: E487K Violet&Blue: NAD+-bound ALDH2*1 Red: NAD+-bound ALDH2*2 Larson et al, JBC (2005) 280:30550-30556

  37. ALDH2 Activity Among Differing Genotypes MALD AALD BALD Comparison of substrate specific activities of human liver ALDH2 derived from three ALDH genotypes PALD Kitagawa et al, FEBS Letters (2000) 476:306-311

  38. ALDH2 Influence on AALD Blood Levels Following Ethanol Ingestion Ginsberg et al, Reg Toxicol Pharmacol (2002) 36, 297-309 ADH ALDH2*1/*1 ALDH2*1/*2 ALDH2*2/*2 X No Flushing No Hangover Effects Some Flushing Some Hangover Effects Flushing Hangover Effects X X

  39. Who is Affected? ALDH2 polymorphism by ethnic group Ginsberg et al, Reg Toxicol Pharmacol (2002) 36, 297-309

  40. How do you treat an acetaldehyde overdose?

  41. DDI Scenario:(Ethanol and Acetaminophen) + CNS Depressant Analgesic/Antpyretic

  42. Drug-Drug Interaction:Ethanol and Acetaminophen Glutathione adducts (Detoxification) X CYP2E1 Protein adducts (Toxicity) Ethanol (inducer) NAPQI Cell Death Liver Damage

  43. Toxic Scenario • Chronic alcohol abuser induces CYP2E1 • ‘Activated’ CYP2E1 forms more toxic metabolite (NAPQI) • Increased levels of NAPQI can lead to glutathione depletion • Increased protein adduct formation leading to cell death and liver damage If a you are a chronic alcohol abuser, use ibuprofen instead of acetaminophen to treat your hangover.

  44. Summary: ALDH2 & Ethanol • Ethanol metabolism occurs in two steps • ADH: Ethanol to acetaldehyde (toxic metabolite) • ALDH2: Acetaldehyde to acetic acid (detoxification step) • Three common ALDH2 genotypes • ALDH2*1/*1: No flushing or hangover effects • ALDH2*1/*2: Some flushing and hangover effects • ALDH2*2/*2: Flushing and hangover effects • ALDH2*2 most prevalent in Asian populations • Chronic alcohol abusers should not take acetaminophen to treat their hangovers

  45. Things to Think About • Drug metabolizing enzymes tend to metabolize lipophilic compounds into hydrophilic compounds (absorption to excretion) • Your genotype impacts how you metabolize drugs • Pharmacogenetics can be used to optimize therapy • Science is interdisciplinary Genetics - Molecular Biology - Biochemistry - Organic Chemistry - Pharmacology - Toxicology

  46. Thank you!

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