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Drug Digestion system

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  1. Drug Metabolism S.P. Markey Laboratory of Neurotoxicology NIMH, NIH Nov. 16, 2006

  2. Evolution of Drug Metabolism As a SciencePost WWII Pioneers • Richard Tecwyn Williams – Great Britain • 1942, worked on the metabolism on TNT with regard to toxicity in munitions workers; due to the war he assembled teams to work on metabolism of sulfonamides, benzene, aniline, acetanilide, phenacetin, and stilbesterol • Developed concept of Phase 1 & Phase 2 Reactions. • Biotransformation involves metabolic oxygenation, reduction, or hydrolysis; result in changes in biological activity (increased or decreased) • Second phase, conjugation, in almost all cases resulted in detoxication.

  3. Evolution of Drug Metabolism As a SciencePost WWII Pioneers • Bernard B. Brodie, U.S. • NYU and Laboratory of Industrial Hygiene, NYC 1949 – Metabolic fate of acetanilide and phenacetin in man (with Julius Axelrod) • 1950s, NIH – pioneering studies on all aspects of drug metabolism; esp. reserpine, serotonin;hexobarbital tolerance • 1952 – R.T. Williams spent 6 months at NIH; subsequently many students went between both labs (Richard Adamson, James Gillette, and Sidney Udenfriend) • 1950s, Brodie lab developed the spectrophotofluorimeter (Robert Bowman)

  4. Drug Metabolism Extrahepatic microsomal enzymes (oxidation, conjugation) Hepatic microsomal enzymes (oxidation, conjugation) Hepatic non-microsomal enzymes (acetylation, sulfation,GSH, alcohol/aldehyde dehydrogenase, hydrolysis, ox/red)

  5. Oxidative Reactions: Cytochrome P450 mediated Examples Formation of an inactive polar metabolite Phenobarbital Formation of an active metabolite By Design: Purine & pyrimidine chemotherapy prodrugs Inadvertent: terfenadine – fexofenadine Formation of a toxic metabolite Acetaminophen – NAPQI Liver Microsomal System

  6. Drug NADP+ Fe+3 CYP CYP R-Ase Drug PC OH NADPH Fe+3 CYP CO CO Fe+2 Drug CYP CYP-Fe+2 OH hu Drug Drug O2 Fe+2 CYP H2O O2 Drug 2H+ Electron flow in microsomal drug oxidizing system e- Drug e-

  7. Cytochrome P450 Isoforms (CYPs) - An Overview • NADPH + H+ + O2 + Drug ® NADP+ + H2O + Oxidized Drug • Carbon monoxide binds to the reduced Fe(II) heme and absorbs at 450 nm (origin of enzyme family name) • CYP monooxygenase enzyme family is major catalyst of drug and endogenous compound oxidations in liver, kidney, G.I. tract, skin, lungs • Oxidative reactions require the CYP heme protein, the reductase, NADPH, phosphatidylcholine and molecular oxygen • CYPs are in smooth endoplasmic reticulum in close association with NADPH-CYP reductase in 10/1 ratio • The reductase serves as the electron source for the oxidative reaction cycle

  8. CYP Families • Multiple CYP gene families have been identified in humans, and the categories are based upon protein sequence homology • Most of the drug metabolizing enzymes are in CYP 1, 2, & 3 families . • CYPs have molecular weights of 45-60 kDa. • Frequently, two or more enzymes can catalyze the same type of oxidation, indicating redundant and broad substrate specificity. • CYP3A4 is very common to the metabolism of many drugs; its presence in the GI tract is responsible for poor oral availabilty of many drugs

  9. CYP Nomenclature • Families - CYP plus arabic numeral (>40% homology of amino acid sequence, eg. CYP1) • Subfamily - 40-55% homology of amino acid sequence; eg. CYP1A • Subfamily - additional arabic numeral when more than 1 subfamily has been identified; eg. CYP1A2 • Italics indicate gene (CYP1A2); regular font for enzyme • Comprehensive guide to human Cyps http://drnelson.utmem.edu/human.P450.table.html

  10. CYP Tables • Human CYPs - variability and importance in drug metabolism • Isoforms in metabolism of clinically important drugs • Factors that influence CYP activity • Non-Nitrogenous CYP inhibitors • Extrahepatic CYPs


  12. Human Liver Drug CYPs S. Rendic & F.J. DiCarlo, Drug Metab Rev 29:413-80, 1997 *L. Wojnowski, Ther Drug Monit 26: 192-199, 2004

  13. Participation of the CYP Enzymes in Metabolism of Some Clinically Important Drugs Adapted from: S. Rendic Drug Metab Rev 34: 83-448, 2002

  14. Factors Influencing Activity and Level of CYP Enzymes Red indicates enzymes important in drug metabolism Adapted from: S. Rendic Drug Metab Rev 34: 83-448, 2002

  15. Non-nitrogenous Substances that Affect Drug Metabolism • Grapefruit juice - CYP 3A4 inhibitor; highly variable effects; fucocoumarins • Bailey, D.G. et al.; Br J Clin Pharmacol 1998, 46:101-110 • Bailey, D.G et al.; Am J Cardiovasc Drugs 2004, 4:281-97. • St John’s wort, other herbal products • Tirona, R.G and Bailey, D.G. ;Br J Clin Pharmacol. 2006,61: 677-81 • Isosafrole, safrole • CYP1A1, CYP1A2 inhibitor; found in root beer, perfume

  16. Overheard Conversation • At a B&B breakfast table, after grapefruit juice was served, someone remarked “A friend read the package insert with her prescription and the fine print warned against drinking grapefruit juice…is this true? Should it be avoided with all medications? How about grapefruit itself? How about orange juice?”

  17. Effect of Grapefruit Juice on Felodipine Plasma Concentration 5mg tablet with juice without • Review- D.G. Bailey, et al.; Br J Clin Pharmacol 1998, 46:101-110

  18. Grapefruit Juice Facts • GJ or G, lime, or Sun Drop Citrus soda, Seville OJ(not most OJ) elevates plasma peak drug concentration, not elimination t1/2 • GJ reduced metabolite/parent drug AUC ratio • GJ caused 62% reduction in small bowel enterocyte 3A4 and 3A5 protein; liver not as markedly affected (i.v. pharmacokinetics unchanged) • GJ effects last ~4 h, require new enzyme synthesis • Effect cumulative (up to 5x Cmax) and highly variable among individuals depending upon 3A4 small bowel basal levels

  19. Human Drug Metabolizing CYPs Located in Extrahepatic Tissues S. Rendic & F.J. DiCarlo, Drug Metab Rev 29:413-80, 1997

  20. Human Drug Metabolizing CYPs Located in Extrahepatic Tissues (cont’d) S. Rendic & F.J. DiCarlo, Drug Metab Rev 29:413-80, 1997

  21. CYP Biotransformations • Chemically diverse small molecules are converted, generally to more polar compounds • Reactions include (see text): • Aliphatic hydroxylation, aromatic hydroxylation • Dealkylation (N-,O-, S-) • N-oxidation, S-oxidation • Deamination • Dehalogenation

  22. Non-CYP Drug Biotransformations • Oxidations • Hydrolyses • Conjugation (Phase 2 Rxs) • Major Conjugation Reactions • Glucuronidation (high capacity) • Sulfation (low capacity) • Acetylation (variable capacity) • Examples:Procainamide, Isoniazid • Other Conjugation Reactions: O-Methylation, S-Methylation, Amino Acid Conjugation (glycine, taurine, glutathione) • Many conjugation enzymes exhibit polymorphism

  23. Non-CYP drug oxidations (1) • Monoamine Oxidase (MAO),Diamine Oxidase (DAO)- MAO (mitochondrial) oxidatively deaminates endogenous substrates including neurotransmitters (dopamine, serotonin, norepinephrine, epinephrine); drugs designed to inhibit MAO used to affect balance of CNS neurotransmitters (L-DOPA); MPTP converted to toxin MPP+ through MAO-B. DAO substrates include histamine and polyamines. • Alcohol & Aldehyde Dehydrogenase- non-specific enzymes found in soluble fraction of liver; ethanol metabolism • Xanthine Oxidase- converts hypoxanthine to xanthine, and then to uric acid. Drug substrates include theophylline, 6-mercaptopurine. Allopurinol is substrate and inhibitor of xanthine oxidase; delays metabolism of other substrates; effective for treatment of gout.

  24. Non-CYP drug oxidations (2) • Flavin Monooxygenases • Family of enzymes that catalyze oxygenation of nitrogen, phosphorus, sulfur – particularly facile formation of N-oxides • Different FMO isoforms have been isolated from liver, lung (S.K. Krueger, et al. Drug Metab Rev 2002; 34:523-32) • Complete structures defined (Review: J. Cashman, 1995, Chem Res Toxicol 8:165-181; Pharmacogenomics 2002; 3:325-39) • Require molecular oxygen, NADPH, flavin adenosine dinucleotide (FAD) • Single point (loose) enzyme-substrate contact with reactive hydroperoxyflavin monoxoygenating agent • FMOs are heat labile and metal-free, unlike CYPs • Factors affecting FMOs (diet, drugs, sex) not as highly studied as CYPs

  25. Hydrolysis secondary to Cyp Metabolism Involvement of Liver Carboxylesterases in the In Vitro Metabolism of Lidocaine S. E. H. Alexson, M. Diczfalusy, M. Halldin, S Swedmark Drug Metab Disp 30: 643-647, 2002

  26. Conjugation Reactions Glucuronidation Liver has several soluble UDP-Gluc-transferases

  27. Glucuronic acid conjugation to phenols, 3°-amines, aromatic amines

  28. Conjugation Reactions Sulfation + (PAPS, 3’-phosphoadenosine- 5’-phosphosulfate) Examples: ethanol, p-hydroxyacetanilide, 3-hydroxycoumarin

  29. Sulfation may produce active metabolite

  30. Conjugation ReactionsAcetylation Examples: Procainamide,isoniazid, sulfanilimide, histamine NAT enzyme is found in many tissues, including liver

  31. Procainamide Unchanged in Urine, 59% 24% Fast 17% Slow 3% Unchanged in Urine, 85% 1% NAPA 0.3%

  32. Procainamide trace metabolite non-enzymatic Lupus?

  33. Additional Effects on Drug Metabolism • Species Differences • Major differences in different species have been recognized for many years (R.T. Williams). • Phenylbutazone half-life is 3 h in rabbit, ~6 h in rat, guinea pig, and dog and 3 days in humans. • Induction • Two major categories of CYP inducers • Phenobarbital is prototype of one group - enhances metabolism of wide variety of substrates by causing proliferation of SER and CYP in liver cells. • Polycylic aromatic hydrocarbons are second type of inducer (ex: benzo[a]pyrene). • Induction appears to be environmental adaptive response of organism • Orphan Nuclear Receptors (PXR, CAR) are regulators of drug metabolizing gene expression

  34. PBP CAR PXR PXR and CAR Protect Against Xenobiotics co-activator target genes xenobiotics xenoprotection RXR cytoplasm nucleus S.A. Kliewer

  35. CYP3A Regulation • Diverse drugs activate through heterodimer complex • Protect against xenobiotics • Cause drug-drug interactions T.M. Wilson, S. A. Kliewer 2002:1, 259-266

  36. 1 3 5 7 9 11 13 15 17 19 CYP3A Inducers ActivateHuman,Rabbit, andRatPXR rifampicin PCN Cell-based reporter assay dexamethasone RU486 clotrimazole troglitazone tamoxifen Reporter activity (fold) S.A. Kliewer

  37. human PXR Ligand DNA 82% 94% rabbit PXR 77% 96% mouse PXR 76% 96% rat PXR Pregnane X Receptor (PXR) • PXR is one of Nuclear Receptor (NR) family of ligand-activated transcription factors. • Named on basis of activation by natural and synthetic C21 steroids (pregnanes), including pregnenolone 16a-carbonitrile (PCN) • Cloned due to homology with other nuclear receptors • Highly active in liver and intestine • Binds as heterodimer with retinoic acid receptor (RXR) S.A. Kliewer

  38. CAR PXR Constitutive Androstane Receptor (CAR) S.A. Kliewer • Highly expressed in liver and intestine • Sequestered in cytoplasm • Co-factor complex required for activation; anchored by PPAR-binding protein (PBP) • Binds response elements as RXR heterodimer • High basal transcriptional activity without ligand • Activated by xenobiotics • phenobarbital, TCPOBOP (1,4-bis[2-(3,5-dichloropyridyloxy)]benzene)

  39. PXR and CAR Regulate Overlapping Genes PCN (PXR) TCPOBOP (CAR) • Phase I enzymes • Cyp3a11 • Cyp2b10 • Aldh1a1 • Aldh1a7 • Phase II enzymes • Ugt1a1 • Gst-a1 • Transporters • Mrp2 • Mrp3 • Oatp2 (3.5x) (12x) (2.1x) (1.6x) (3.4x) (110x) (1.9x) (1.9x) (2.8x) (16x) (15x) Liver RNA (2.0x) (1.9x) (3.0x) (9.2x) S.A. Kliewer

  40. Acetaminophen (Paracetamol) • Acetanilide – 1886 – accidentally discovered antipyretic; excessively toxic (methemoglobinemia); para-aminophenol and derivatives were tested. • Phenacetin introduced in 1887, and extensively used in analgesic mixtures until implicated in analgesic abuse nephropathy • Acetaminophen recognized as metabolite in 1899 • 1948-49 Brodie and Axelrod recognized methemoglobinemia due to acetanilide and analgesia to acetaminophen • 1955 acetaminophen introduced in US

  41. Acetaminophen and p-Aminophenols Acetanilide, 1886 (accidental discovery of antipyretic activity; high toxicity) Phenacetin or acetophenetidin,1887 (nephrotoxic, methemoglobinemia) 75-80% 70-90% Metabolic pathway quantified; (Brodie &Axelrod, 1948) popular in US since 1955 Acetaminophen,1893

  42. Acetaminophen Toxicity • Acetaminophen overdose results in more calls to poison control centers in the United States than overdose with any other pharmacologic substance. • The American Liver Foundation reports that 35% of cases of severe liver failure are caused by acetaminophen poisoning which may require organ transplantation. • N-acetyl cysteine is an effective antidote, especially if administered within 10 h of ingestion [NEJM 319:1557-1562, 1988] • Management of acetaminophen overdose [Trends Pharm Sci 24:154-157, 2003

  43. Acetominophen Metabolism ~60% ~35% CYP2E1* CYP1A2 CYP3A11 *induced by ethanol, isoniazid Protein adducts, Oxidative stress Toxicity NAPQI N-acetyl-p-benzoquinone imine

  44. Acetaminophen Protein Adducts CYPs HS-Protein H2N-Protein S.D. Nelson, Drug Metab. Rev. 27: 147-177 (1995) K.D. Welch et al., Chem Res Toxicol 18:924-33 (2005)

  45. Acetaminophen toxicity mechanism • N-acetyl cysteine is an effective agent to block GSH depletion and rescue from liver damaging toxicity • CAR and PXR modulate acetaminophen toxicity (2002, 2004) • CAR-null mice are resistant to acetaminophen toxicity • hepatic GSH lowered in wild type (but not in KO) after acetaminophen • CAR-humanized mice demonstrate same toxicity response • Activation of PXR induces CYP3A11 and markedly enhances acetaminophen toxicity in wild type mice • CAR transcription co-activator KO blocks toxicity (2005)

  46. toxicity NAPQI toxicity linked to PXR activationG. Guo et al. 2004, Toxicol Sci 82(2):374-80 CAR CYP2E1* CYP3A11 toxicity PXR Xenobiotics oxidative stress mechanism ?

  47. Drug Metabolism - WWW Information Resources • http://www.icgeb.trieste.it/p450/ • Directory of P450 Containing Systems; comprehensive web site regarding all aspects of chemical structure (sequence and 3D) of P450 proteins from all species; steroid ligands; links to related sites including leading researchers on P450 • http://www.fda.gov/cder/guidance/ • Site contains many useful documents regarding drug metabolism and FDA recommendations including "Drug Metabolism/Drug Interaction Studies in the Drug Development Process: Studies in Vitro", FDA Guidance for Industry • http://www.sigmaaldrich.com/Area_of_Interest/Biochemicals/Enzyme_Explorer.html • Site has many commercially available drug metabolizing enzymes and useful links to multiple drug metabolism resources • http://www.biocatalytics.com/p450.html • Six freeze dried human CYPs (1A2, 2C9, 2C19, 2D6, 2E1, 3A4) available for drug metabolism studies