Updated: 8/19/2016

Phase I vs. Phase II Metabolism

Review Topic
  • Phase I metabolism 
    • oxidation (via cytochrome P450), reduction, and hydrolysis reactions
    • phase I reactions convert a parent drug to more polar (water soluble) active metabolites by unmasking or inserting a polar functional group (-OH, -SH, -NH2)
    • geriatric patients have decreased phase I metabolism
      • drugs metabolized via phase I reactions have longer half-lives
      • geriatric patients metabolism drugs by phase II reactions
  • Phase II metabolism
    • glucuronidation, acetylation, and sulfation reactions
      • "conjugation reactions" that increase water solubility of drug with a polar moiety
        • glucuronate, acetate, and sulfate, respectively
      • phase II reactions convert a parent drug to more polar (water soluble) inactive metabolites by conjugation of subgroups to -OH, -SH, -NH2 functional groups on drug
        • drugs metabolized via phase II reactions are renally excreted
      • patients deficient in acetylation capacity (slow acetylators) may have prolonged or toxic responses to normal doses of certain drugs because of decreased rates of metabolism 

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