Makindo Medical Notes.com

Overview of Pharmacokinetics

Pharmacokinetics is the study of how drugs move through the body over time. It involves the processes of absorption, distribution, metabolism, and excretion (ADME).

Key Concepts

• Absorption:
  • Process by which a drug enters the bloodstream from its site of administration.
  • Affected by factors such as drug formulation, route of administration, and blood flow to the absorption site.
  • Bioavailability: The fraction of an administered dose that reaches the systemic circulation in an unchanged form.
• Distribution:
  • Dispersion of a drug throughout the body fluids and tissues.
  • Affected by factors such as blood flow, tissue permeability, and binding to plasma proteins.
  • Volume of Distribution (Vd): A theoretical volume that a drug would occupy if it were uniformly distributed throughout the body at the same concentration as in the blood.
• Metabolism:
  • Biotransformation of a drug into metabolites, primarily occurring in the liver.
  • Involves Phase I (functionalization) and Phase II (conjugation) reactions.
  • Affects the duration and intensity of a drug's pharmacological action.
• Excretion:
  • Removal of a drug and its metabolites from the body.
  • Occurs primarily through the kidneys (urine) but also through bile, feces, lungs, and sweat.
  • Half-life (t1/2): The time it takes for the plasma concentration of a drug to reduce by half.

Detailed Processes

• Absorption:
  • Oral Absorption:
    • Drug passes through the gastrointestinal tract and is absorbed into the bloodstream.
    • Affected by gastric emptying time, pH, and presence of food.
  • Parenteral Absorption:
    • Includes intravenous, intramuscular, and subcutaneous routes.
    • Bypasses the gastrointestinal tract, leading to faster absorption.
  • Transdermal Absorption:
    • Drug is absorbed through the skin into the bloodstream.
    • Used for sustained-release medications.
• Distribution:
  • Plasma Protein Binding:
    • Drugs can bind to plasma proteins like albumin, affecting their distribution and availability.
  • Tissue Distribution:
    • Some drugs accumulate in specific tissues, influencing their therapeutic and toxic effects.
  • Blood-Brain Barrier:
    • Protects the brain from potentially harmful substances, influencing drug distribution to the central nervous system.
• Metabolism:
  • Phase I Reactions:
    • Include oxidation, reduction, and hydrolysis reactions.
    • Introduce or expose functional groups to increase drug polarity.
  • Phase II Reactions:
    • Conjugation reactions that increase water solubility for easier excretion.
  • First-Pass Effect:
    • Oral drugs may be significantly metabolized in the liver before reaching systemic circulation.
• Excretion:
  • Renal Excretion:
    • Glomerular filtration, tubular secretion, and reabsorption determine drug elimination via urine.
  • Biliary Excretion:
    • Drugs are excreted into bile and eliminated in feces.
    • Enterohepatic recycling can prolong drug action.

Clinical Relevance

• Dosing Regimens:
  • Understanding pharmacokinetics helps in designing appropriate dosing schedules to maintain therapeutic drug levels.
• Drug Interactions:
  • Knowledge of pharmacokinetic properties helps predict and manage drug interactions.
• Individual Variability:
  • Factors such as age, genetics, liver and kidney function affect pharmacokinetics and drug response.
• Toxicology:
  • Pharmacokinetics is essential for understanding drug overdose, toxicity, and management strategies.

Summary

Pharmacokinetics is crucial for understanding how drugs move through the body, encompassing absorption, distribution, metabolism, and excretion. It informs drug dosing, efficacy, and safety, helping healthcare providers optimize therapeutic outcomes and minimize adverse effects.