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Comprehensive Overview of Drug Metabolism Phase II Pathways

Explore the intricate pathways of Phase II drug metabolism including UGTs, GSTs, NATs, and more. Learn about substrate specificity, diseases related to metabolism, and regulatory mechanisms.

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Comprehensive Overview of Drug Metabolism Phase II Pathways

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  1. Lectures • Wednesday, October 8th • Drug Metabolism Overview • Phase I Drug Metabolism • Monday, October 13th • Phase II Drug Metabolism • Intrinsic Clearance • Wednesday, October 15th • In vitro Experimental Methods • In vivo Experimental Methods

  2. Drug Metabolism: Phase II Dr. Arthur G. Roberts

  3. Outline • Introduction • UDP-glucuronosyltransferases (UGTs) • Sulfotransferases (SULTs) • Glutathione-S-Transferases (GSTs) • N-acetyltransferases (NATs) • Methyltransferases • Catechol O-Methyltransferases (COMTs) • Histamine Methyltransferases (HMTs) • ThiopurineMethyltransferases (TPMTs)

  4. Pathways of Drug Metabolism Drugs Oxidation and Reduction Conjugation

  5. UGTs TPMT COMT HMT STs GST-A GST-P GST-T GST-M NAT2 NAT1 others Fractional Contribution HMT COMT STs TPMT GST-A GST-P UGTs GST-T GST-M NAT2 NAT1 others HMT: histamine methyltransferase; TPMT: thiopurine methyltransferase; COMT: catechol O-methyltransferase; UGT: Uridine Glucuronosyl-S-Transferases; ST: Sulfotransferase; GST: Glutathione-S-Transferases Evans and Relling, Science (1999)

  6. UDP-glucuronosyltransferases (UGTs) Membrane UDP = N-terminal C-terminal glucuronic acid (GA)

  7. Conjugates to nucleophilic groups such as amines, amides, thiols, carboxylic acids and alcohols/phenols Includes some unusual substrates (e.g. b di-ketone)

  8. Tissue Location (Shipkova, 2005 Clin. Chim. Acta)

  9. Subcellular Location UDP-N-acetylglucosamine UDP Glucuronic Acid Smooth ER Smooth ER, rough ER, golgi apparatus and nuclear membrane (Clark, 1994)

  10. Substrate Specificity • Promiscuous • Drug-drug interaction • UGT-UGT Interaction

  11. Anticancer Drugs Other Drugs UGT1A1 Inhibitors

  12. Endogenous Substrates steroids

  13. Drug-Drug Interaction in UGTs Substrate not metabolized by UGT1A1 Zidovudine (a.k.a AZT) Estradiol (hormone) Ethinylestradiol (contraceptive) site of glucuronidation

  14. UGT-UGT Interaction Serotonin Morphine Kurkela, M., Patana, A. S., Mackenzie, P. I., Court, M. H., Tate, C. G., Hirvonen, J., Goldman, A., & Finel, M. (2007) Pharmacogenetics and genomics17, 115-126.

  15. Families • UGT1A • Planar and bulky phenols (e.g. estrogens), amines and unsaturated carboxylic acids • UGT1A1: bilirubin and morphine • UGT2B • drugs, steroids, bile acids and unsaturated carboxylic acids

  16. Morphine

  17. Regulation: Induction AhR=Aryl Hydrocarbon Receptor CAR=ConstituitiveAndrostane Receptor PXR=Pregnane-X-Receptor XRE=Xenobiotic Response Elements PBREM=Phenobarbital Response Elements PXRRE=PXR Response Elements PXR CAR AhR PXRRE PBREM XRE UGT1A/2B UGT1A/2B UGT1A (Xie, 2004)

  18. Diseases • Hyperbilirubinemia • Gilbert’s syndrome (20-30% UGT1A1 Activity) • Crigler-Najjar syndrome • Type I (0% UGT1A1 Activity) • Type II (10% UGT1A1 Activity) • Gray Baby Syndrome (Chloramphenicol) • Neonatal Jaundice • UGT1A1*28/*28 • light therapy • Autoimmune Hepatitis

  19. D C B A E Chloramphenicol (Broad Spectrum Antibiotic)

  20. Metabolic Switching Drugs Toxic Oxidation Conjugation

  21. Sulfotranferases (SULTs) PAPS (Petrotchenko, 2001 FEBS Letters)

  22. Reaction/Mechanism PAPS synthase

  23. Common Substrates

  24. Resulting Products OSO3 NSO3 OSO3 NOSO3 OSO3 *Note: This molecule is hydroxylated first.

  25. Subcellular Location • Cytosolic • Homodimers/Heterodimers • Membrane-bound • Golgi Apparatus

  26. Families and General Substrate Specificities • SULT1 • Phenols, Catechols, Benzylic Alcohols, Amines, Estrogens • SULT2 • Steroids

  27. SO3 SULT1 or SULT2? 1-hydroxymethylpyrene (Environmental pollutant)

  28. Glutathione-S-Transferases GST A1-1 GST A4-4 % exchanged >50% 35-50% 20-35% <20% Hou, L., Honaker, M. T., Shireman, L. M., Balogh, L. M., Roberts, A. G., Ng, K. C., Nath, A., & Atkins, W. M. (2007) J. Biol. Chem.282, 23264-23274.

  29. General Reaction (GSH) (GS-Conjugate) (Townsend, 2003 Oncogene)

  30. Reactions epoxides R R R R halogenated alkanes GS GS isothiocyanates isocyanates O quinones o-quinones

  31. More Reactions X X EWG EWG EWG X- Nucleophilic Aromatic Substitution EWG EWG EWG EWG = strong electron withdrawing group; X= halide or other good leaving group GS GS Michael Addition spontaneous thiolysis H2O GS GSH + HO spontaneous EWG EWG

  32. SG OH Carbamazepine Epoxide Metabolite

  33. GS Sulforaphane (From vegetables- Anticancer, Antimicrobial)

  34. SG HCl chlorobutane

  35. SG NAPQI (n-acetyl-p-benzoquinone imine)

  36. Families and Tissue Locations • alpha (A) - Liver • kappa (K) • mu (M) – brain, lung , testes, lymphoblasts, skeletal muscle • omega (O) • pi (P) – all tissues except Liver • sigma (S) – macrophages, placenta and fat • prostaglandin metabolism • theta (T) • zeta (Z)

  37. Location • Tissue • Everywhere • Subcellular • Cytosolic, Mitochondrial and Microsomal

  38. Regulation • GST A, M and P • electrophiles that produce reactive oxygen • antioxidant response element

  39. Genetics (Often Missing) • GST M1 • <50% Caucasians and East Asians • >90% Polynesians and Micronesians • >50% Africans • absence increases risk for lung and bladder cancer • GST T1 • 20% missing in Caucasians • 50% missing in Asians • 40% missing in Africans

  40. Glutathione conjugate metabolism Urine

  41. N-acetyltransferases (NAT) Acetyl-CoA

  42. General Reaction NAT R R

  43. Reactions (Sim, 2008 Toxicology)

  44. Families, Location and General Substrate Specificities • NAT1 • everywhere • folate degradation • breast cancer • NAT2 • Liver and Gut • sulfamethazine and arylhydrazine • isoniazid induced neurotoxicity and hydralazine-induced lupus (slow metabolizers) sulfamethazine arylhydrazine (Sim, 2008 Toxicology)

  45. NAT1 and Folate Degradation folate folatecatabolite NAT1 acetylated folatecatabolite

  46. Substrate Specificities p-aminobenzoic acid (PABA) 2-aminofluorene (2-AF) hydralazine (HDZ) (Sim, 2008 Toxicology)

  47. Sulfamethoxazole (antibiotic)

  48. hydralazine (smooth muscle relaxant)

  49. Genetics: Polymorphic Variants NAT2 NAT1 (Sim, 2008 Toxicology)

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