Pharmacokinetics Overview

Pharmacokinetics refers to the movement and modification of a medication inside the body, essentially describing what the body does to a medication and how it happens.

Routes of Administration

• Oral: Swallowed by the mouth.
• Intravenous (IV): Injected into a vein.
• Intramuscular (IM): Injected into a muscle.
• Inhalational: Inhaled into the lungs.
• Nasal: Sprayed into the nose.
• Cutaneous: Applied onto the skin.

ADME Process

• Absorption: Movement from administration site into circulation.
• Distribution: Movement from circulation into body tissues.
• Metabolism: Breakdown of the medication.
• Excretion: Removal via urine or feces.

Elimination

• Metabolism + Excretion: Together referred to as elimination.

Absorption

• Types:
  • Passive Transport: No energy required.
    • Facilitated Diffusion: Uses transport proteins.
    • Passive Diffusion: Lipid-soluble, non-polar medications move from high to low concentration.
  • Active Transport: Energy in the form of ATP is used.
  • Bulk Transport (Endocytosis): For large molecules.
• Factors Affecting Absorption:
  • pH of the environment: Weak acids absorbed in acidic environments, weak bases in alkaline.
  • Surface area: Larger surface area increases absorption (e.g., small intestine).
  • Blood supply, presence of food, or other materials.

First-Pass Metabolism

• Medications taken orally are metabolized in the liver before reaching systemic circulation.
• Alternative Routes: IV, IM, transdermal, sublingual, inhalational to bypass the first-pass effect.

Bioavailability (F)

• Fraction of the medication reaching systemic circulation unchanged.
• IV Administration: Bioavailability of 1 or 100%.

Distribution

• Factors Affecting Distribution:
  • Blood Supply: Rapid distribution to areas with high blood flow (brain, liver, kidneys).
  • Blood-Brain Barrier: Allows only small, fat-soluble medications to enter the brain.
  • Size and Polarity: Smaller, lipid-soluble medications distribute more easily.
  • Plasma Protein Binding: Only unbound medications can diffuse into tissues.
• Apparent Volume of Distribution (VD):
  • Represents how extensively a medication is distributed.
  • Smaller VD indicates a medication remains mainly in the plasma.
  • Larger VD indicates distribution throughout body compartments.
  • Conditions like liver cirrhosis or chronic kidney disease alter VD.

Recap

• Absorption: Affected by route, pH, chemical properties, surface area, and contact time.
• Bioavailability: Measure of absorption.
• Distribution: Influenced by blood flow, size, polarity, and plasma protein binding.
• VD: Indicates extent of distribution.