Distribution of Drugs within Body Compartments

Introduction

• The focus is on drug distribution within body compartments.
• Previous tutorial was on drug metabolism; distribution is discussed first for better understanding.

Body Compartments

• Major Compartments:
  • Blood (plasma)
  • Fat
  • Extracellular fluid
  • Intracellular fluid
• Minor Compartments:
  • Cerebral spinal fluid
  • Peritoneum
  • Synovial fluid in joints
  • Fetus in pregnancy

Drug Distribution Process

• Drugs often placed into the blood (intravenous administration).
• Binding Molecules:
  • Drugs associate with proteins like albumin, increasing storage capacity.
  • Creates an equilibrium between bound and unbound drug.
• Movement Between Compartments:
  • Unbound drug moves to new compartments, maintaining equilibrium.
  • New compartment sequesters some of the drug, continuing the balance.

Factors Affecting Drug Distribution

• Equilibrium Constant:
  • Shows balance direction between compartments.
  • Affected by:
    • Permeability of barriers
    • pH levels
    • Binding capacity
    • Fat solubility (important for fat compartments)

Volume of Distribution (Vd)

• Indicates the amount of drug needed for a desired plasma concentration.
• Formula:
  • Vd = Total amount of drug in the body / Concentration in plasma
• Example:
  • Desired plasma concentration of morphine: ( \frac{3}{70} ) mg/L
  • Vd for morphine: 5 L/kg
  • Calculation shows a 70 kg person needs a dose of 15 mg.

Conclusion

• Next tutorial will cover drug metabolism and the cytochrome P450 system.
• Encouragement to support content creation.