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Pharmacokinetics | Drug Metabolism

Pharmacokinetics | Drug Metabolism

Ninja Nerd

28:06

Overview

This video explains drug metabolism, a crucial part of pharmacokinetics, focusing on how the body processes drugs for excretion. It details the two main phases of drug metabolism: Phase I and Phase II biotransformation. Phase I reactions, primarily involving the Cytochrome P450 (CYP450) system, modify drugs through oxidation, reduction, or hydrolysis, often converting active drugs to inactive metabolites, prodrugs to active forms, or toxic substances to non-toxic ones. Phase II reactions, also known as conjugation, involve adding polar molecules (like glucuronate, sulfate, or glutathione) to the drug, making it more water-soluble and easier to excrete. The video highlights the significance of CYP450 enzymes, particularly CYP3A4 and CYP2D6, and discusses factors influencing metabolism, including genetic polymorphisms, drug interactions (inducers and inhibitors), liver disease, and age.

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Chapters

  • •Drug metabolism is essential for drug excretion from the body.
  • •Metabolism converts drugs into forms that are easier to excrete (e.g., via urine or feces).
  • •Common goals of metabolism include converting toxic substances to non-toxic ones, activating prodrugs, and inactivating active drugs.
  • •Drug metabolism occurs in two phases: Phase I and Phase II biotransformation.
  • •Drugs may undergo Phase I, Phase II, both, or bypass one phase.
  • •Phase I reactions often involve oxidation, reduction, or hydrolysis.
  • •Phase II reactions involve conjugation, adding polar molecules to the drug.
  • •The CYP450 system, a group of heme-containing enzymes primarily in the liver's smooth endoplasmic reticulum, is central to Phase I metabolism.
  • •Key CYP450 enzymes include CYP3A4 (metabolizes ~70-75% of drugs) and CYP2D6.
  • •CYP450 enzymes convert non-polar, lipid-soluble drugs into more polar, water-soluble substances.
  • •These reactions include oxidation, reduction, and hydrolysis.
  • •Genetic variations (polymorphisms) in CYP450 enzymes, especially CYP2D6, affect drug metabolism rates.
  • •Rapid metabolizers quickly convert active drugs to inactive forms, potentially leading to reduced therapeutic effect.
  • •Slow metabolizers convert drugs slowly, increasing the risk of active drug accumulation and toxicity.
  • •Other drugs can interact with the CYP450 system, altering metabolism.
  • •CYP450 inducers increase enzyme activity, speeding up drug metabolism (decreasing active drug levels).
  • •CYP450 inhibitors decrease enzyme activity, slowing drug metabolism (increasing active drug levels and potential toxicity).
  • •These interactions can lead to sub-therapeutic or toxic effects, especially with drugs like warfarin.
  • •Phase II reactions, also called conjugation, add polar molecules to drugs already modified by Phase I or directly.
  • •These reactions are catalyzed by transferase enzymes.
  • •Common conjugation reactions include glucuronidation, sulfation, methylation, acetylation, and glutathione conjugation.
  • •The goal is to further increase the drug's polarity and water solubility for efficient excretion.
  • •Liver disease significantly impairs CYP450 function, increasing the risk of drug toxicity.
  • •Age affects metabolism: infants and the elderly have reduced CYP450 activity.
  • •The primary routes of excretion are renal (urine) and biliary (feces), with minor roles for lungs and intestines.
  • •Clopidogrel, a prodrug metabolized by CYP450 to its active form, can have reduced efficacy when co-administered with CYP450 inhibitors like ameprazole, increasing MI risk.
  • •Phase II reactions include sulfation, glucuronidation, etc., while Phase I reactions include oxidation, reduction, and hydrolysis.

Key Takeaways

  1. 1Drug metabolism transforms drugs into excretable forms, primarily through Phase I (modification) and Phase II (conjugation) reactions.
  2. 2The Cytochrome P450 (CYP450) enzyme system is the main driver of Phase I metabolism, making drugs more polar.
  3. 3Genetic variations (polymorphisms) in CYP450 enzymes can lead to significant differences in drug response (rapid vs. slow metabolizers).
  4. 4Drug interactions are critical: inducers increase metabolism (reducing drug effect), while inhibitors decrease metabolism (increasing drug levels and toxicity).
  5. 5Phase II conjugation reactions add polar groups (e.g., glucuronate, sulfate) to enhance water solubility and facilitate excretion.
  6. 6Liver function, age, and disease states significantly impact a patient's ability to metabolize drugs.
  7. 7Understanding metabolism is vital for predicting drug efficacy, toxicity, and managing polypharmacy.
  8. 8Prodrugs require metabolism to become active, making CYP450 activity crucial for their therapeutic effect.
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