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Updated: Oct 7 2022

Phase I vs. Phase II Metabolism

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https://upload.medbullets.com/topic/107006/images/350px-xenobiotic_metabolism.jpg
  • Overview
    • 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
    • First pass metabolism
      • oral intake of medications result in processing by the liver prior to reaching the systemic circulation
      • systemic concentrations can be far lower than that initially absorbed
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