W8.2_Drug Metabolism

Question 1

Define metabolism. What are the key sites of first-pass metabolism and why? Relate the production of metabolites to excretion. Define first-pass effect and bioavailability.

Answer 1

• Enzyme-mediated chemical reactions (biotransformation) of drug into another chemical species
• Metabolism is separated into phase 1 and 2 metabolism
• Liver and intestine as key sites for first-pass drug metabolism
• Liver as key site of metabolic elimination (others: brain, lung, kidney, blood, etc.)
• Metabolism predominantly occurs in liver as blood from the gut flows directly to liver after oral administration before reaching systemic circulation
• Metabolites: lower lipophilicity by increasing polarity -> reduce permeability and increase aqueous solubility -> more likely be excreted (opposite mechanism for prodrugs)
• First-pass effect: metabolism directly following absorption from the gut
• Bioavailability (F): accounts for both absorption and removal from first pass in liver

Question 2

Define phase 1 metabolism. What are the different types of phase 1 metabolism (3) and explain them by giving some examples.

Answer 2

• Phase 1 metabolism: changes in molecular structure of drug
• Oxidation (very common)
  -> Aliphatic/aromatic hydroxylation (C-H into C-OH environments)
  -> Sometimes N-/S- oxidation
  -> Sometimes N-/O-/S- dealkylation can occur
• Reduction (very rare)
  -> Nitro reduction to hydroxylamine/amine
  -> Carbonyl reduction to alcohol
• Hydrolysis (common)
  -> Ester/amide/phosphate to acid and alcohol/amine
  -> Hydrazides to acid and substituted hydrazine

Question 3

Define phase 2 metabolism. What are the different types of phase 2 metabolism (3) and explain them by giving some examples.

Answer 3

• Phase 2 metabolism: conjugation of polar species to drug/metabolites
• Glucuronidation
  -> attachment of glucuronic acid to carboxylic acid/alcohol/phenol/amine
• Sulfation
  -> attachment of sulphate group to alcohol/phenol/amine
• Glutathione conjugation (gly-cys-glu)
  -> attachment of tripeptide into reactive sites of halo-cpds/epoxides/arene oxides/quinone-imine groups to reduce toxicity

Question 4

Explain the possibility of multiple metabolism routes in a drug and their roles. Can a drug have multiple phase 1 metabolisms? Describe the variability in metabolic pathways. Define active metabolites and describe how metabolites can be identified.

Answer 4

• Multiple phase 1 and 2 metabolism routes: can occur in single drug
• Phase 1 metabolism install functionality -> facilitates subsequent phase 2 metabolism (especially -OH group as it provides a site)
• Products from multiple phase 1 metabolism: uncommon
• Reason why multiple phase 1 metabolisms are uncommon: molecules become more hydrophilic-> less likely to be metabolised again (but if hydrophobic enough, can go through multiple phase 1)
• Individual variability of metabolic pathways exists
• Does not always allow conclusive elicitation of specific structures
• ex. the hydroxyl group can be in any substitutable positions
• Active metabolites: still have biological activity
• Initial identification of metabolites: undertaken by mass spectrometry

Question 5

Explain the properties of cytochrome p450 mediated oxidation. Describe the structure of CYP450 enzyme.

Answer 5

• Primary metabolic route for most drug compounds
• > 100 isoforms in humans, some utilised in phase 1 metabolism (others: hormone biosynthesis…)
• Membrane-bound, haem-containing proteins coordinating FeII/III at active site

Question 6

Describe the process of CYP450 phase 1 metabolism. What is its ultimate goal?

Answer 6

• Haem react with oxygen and an electron derived from cofactor NADPH to form iron oxide species -> further reduced to FeV double bonded to oxygen (super reactive) -> radical transfer process (FeV reduced to FeIV to bond with -OH -> radical recombines with -OH instantly) to convert back to FeIII
• ∴ hydroxylation of molecule by converting C-H to C-OH environments

Question 7

How can the relative rate of oxidation be determined? Suggest some susceptible groups prone to CYP450 mediated phase 1 metabolism and give an example.

Answer 7

• Relative rate of oxidation correlates with stability of radical
• Hydrogen atoms next to aromatic groups/C-H environments next to heteroatoms favour hydroxylation/dealkylation (same mechanism)
• Oxidative nature of dealkylation in heteroatoms: collapse of unstable hemiaminal compounds -> remove alkyl groups
• ex. codeine into morphine by CYP2D6 enzymes

Question 8

Explain the dilemma in predicting metabolic rates in terms of chemical structure of drug compounds.

Answer 8

• Chemical structure of the drug compounds can be used to predict sites of metabolisms in compound
• Enantiomers can interact differently with homo-chiral CYPs
• Yet different compounds have different metabolic rates/profiles