Drug metabolism lecture

• Drug metabolism is required to convert lipophilic compounds into more hydrophilic compounds so they can be excreted
• If lipid soluble non-polar compounds (drugs) are not metabolised, they will remain in the blood and tissues and maintain their pharmacological effects for much longer
• Pharmacokinetics refers to the movement of drugs throughout the body and involves the drug's:

  • Absorption
  • Metabolism
  • Distribution
  • Elimination
• Orally administered drugs dissolve in the GI tract and are absorbed through the gut, where they then enter the liver and then the bloodstream, where they are circulated
• Successful absorption of a drug is influenced by its ability to cross membranes
• Chemical stability of a drug is influenced by, e.g. its stability in the stomach - resistance to extremes of pH and protease degradation
• Metabolic stability requires the drug to, e.g. have no interaction with other metabolites
• Phase I transformations introduce or unmask a functional group of a drug, e.g. by oxygenation or hydrolysis
• Examples of functional groups introduced to drugs during Phase I transformation:

  • Hydroxyl (OH)
  • Thiol (SH)
  • Amine (NH2)
  • Carboxylic acid (COOH)
• Phase II transformations generate highly polar derivates that can be excreted, by introducing:

  • Glucuronide
  • Sulfate
  • Acetate
• Phase I metabolism consists of either:

  • Oxidation - addition of oxygen or removal of hydrogen.
  • Reduction - removal of oxygen or addition of hydrogen.

  Reduction is less common than oxidation
• Cytochrome P450 system in liver is always responsible for phase I oxidation but not always for phase II reduction; this can take place via reductases in different sites in the body
• Oxidation of metabolites results in increased polarity, causing increased water solubility and therefore causing the metabolite to be excreted in urine
• Phase I oxidation reactions include:

  • Aliphatic or aromatic hydroxylation
  • N-, or S-oxidation
  • N-, O-, S-dealkylation
• Phase I reduction reactions include:

  • Nitro reduction to hydroxylamine/ amine
  • Carbonyl reduction to alcohol
• Hydrolysis of metabolites produces more polar metabolites due to the addition of water
• Esterase and amidase enzymes can hydrolyse drugs
• Summary of metabolite oxidation enzymes:

  • Cytochrome P450 monooxygenase system
  • Alcohol dehydrogenase
  • Aldehyde dehydrogenase
  • Flavin-containing monooxygenase system
  • Monoamine oxidase
• Summary of metabolite reduction enzymes:

  • NADPH-cytochrome P450 reductase
  • Reduced (ferrous) cytochrome P450
• Summary of metabolite hydrolysis enzymes:

  • Esterases and amidases
  • Epoxide hydrolase
• Phase I metabolism takes place via a microsomal mixed-function oxidase system that is cytochrome P450, oxygen and NADPH dependent
• Oxidation takes place in the binding site of cytochrome P450; NADPH transfers electrons to P450
• The mixed-function P450 oxidase is found in microsomes within the endoplasmic reticulum of many cell types (liver, kidney, lung and intestine)
• Cytochrome P450 is anchored to the endoplasmic reticulum by an amphipathic helix
• NADPH-cytochrome P450 reductase is a flavoenzyme containing FAD and FMN and is responsible for transferring electrons to P450
• A haem group is co-ordinated in the binding site of P450. Fe3+ ions are reduced to Fe2+ to oxidise the substrate and vice versa
• The cytochrome P450 binding site is described as promiscuous because it can accommodate a wide range of functional groups and isoforms
• The promiscuity of the P450 binding site comes from the large binding site - successful binding is reliant on conducive orientation with S-warfarin and the haem group, which is accommodated by Cys435
• When the products of Phase I metabolism are not sufficiently hydrophilic or inactive to be eliminated, they must undergo Phase II metabolism as well.
• Phase I reactions provide a functional group to undergo Phase II reactions; Phase II reactions modify functional groups by a conjugation reaction.
• Phase II reactions require coenzymes (i.e. enzymes to operate alongside P450) due to the handling of large polar compounds
• Conjugation reactions in Phase II are catalysed by transferases
• Reactions that can happen in Phase II metabolism:

  • Glucuronidation
  • Sulfation
  • Acetylation
  • Amino acid conjugation
  • Glutathione conjugation
  • Fatty acid conjugation
  • Condensation reactions
• Glucuronidation is the most important Phase II pathway for drugs.
• Products of glucuronidation are often excreted in the bile
• General pathway of glucuronidation:

  1. Addition of UDP via phosphorylase (UTP -> PPi)
  2. Oxidation via UDP dehydrogenase (2 NAD+ -> 2 NADH)
• N-glucuronidation:

  • Occurs with aromatic amines
  • Occurs with amides and sulfonamides
• O-glucuronidation:

  • Occurs by ester linkages with carboxylic acids
  • Occurs by ether linkages with phenols and alcohols
• Sulfation is the major Phase II pathway for phenols
• Sulfation takes place at low substrate concentrations, whereas glucuronidation takes place at high substrate concentrations