Pharmacokinetics Overview

Introduction to Pharmacokinetics

• Focus on what happens to drugs once they enter the body.
• Key questions:
  • How do drugs move around in the body?
  • Do they stay indefinitely or are they removed?
• Four main processes in pharmacokinetics:
  1. Absorption
  2. Distribution
  3. Metabolism
  4. Excretion

Key Processes

1. Absorption

• Movement of the drug from its administration site across membranes into the bloodstream.
• Routes of Administration:
  • Topical (through skin or mucous membranes)
  • Oral (through stomach or intestines)
• Factors Affecting Absorption:
  • Presence of food can slow down oral absorption.
• Bioavailability:
  • Proportion of drug absorbed into systemic circulation.

2. Distribution

• Movement of the drug through the bloodstream to target cells.
• Influencing Factors:
  • Interaction with blood components (e.g., plasma proteins).
  • Binding to proteins can hinder reaching target cells.
  • Other drugs may displace primary drugs from protein binding.
• Anatomical Barriers:
  • Blood-Brain Barrier: Limits certain substances from entering brain tissue.
  • Blood-Placental Barrier: Regulates substances crossing to the fetus; some harmful substances can pass (e.g., alcohol).
  • Blood-Testicular Barrier: Prevents many substances from reaching male testes.

3. Metabolism

• Chemical reactions that modify drugs, often aimed at inactivation.
• First-Pass Effect:
  • Occurs when drugs are metabolized in the liver after absorption from the intestines.
  • Can greatly reduce drug bioavailability (over 90% can be inactivated).
• Enzymes in the body detoxify small molecules since the immune system targets larger particles.

4. Excretion

• Removal of drugs or their remnants from the body.
• Methods of excretion:
  • Exhalation (gaseous drugs)
  • Urination (kidneys play a major role)
  • Defecation (via bile and feces)
  • Sweating and saliva (less effective)
• Entrohepatic Recirculation:
  • Bile recirculation aids in drug excretion.

Conclusion

• Summary of drug's journey:
  • Ingestion ➔ Absorption ➔ Distribution ➔ Metabolism ➔ Excretion.
• Basic understanding of pharmacokinetics and its importance in drug administration and effects.