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LessonPharmacokinetics

Pharmacokinetics

Pharmacokinetics is the study of how a drug moves through the body—from the moment it enters until it is eliminated.

Think: "What the body does to the drug"

(Contrast: pharmacodynamics = what the drug does to the body)

Pharmacokinetics: ADME Pathway

Key Components of Pharmacokinetics

1. Absorption

How a drug enters the bloodstream.

Mechanisms:

  • Passive diffusion → moves along concentration gradients
  • Facilitated diffusion → uses carrier proteins
  • Active transport → requires energy
  • Pinocytosis → cell "engulfing" of substances

Bioavailability:

Percentage of drug that reaches systemic circulation.

IV / IO = 100% bioavailability

Other routes may lose drug due to breakdown or incomplete absorption.

2. Distribution

How the drug spreads throughout the body.

Volume of Distribution (Vd)
  • Theoretical space the drug occupies in the body
  • Helps determine drug concentration in plasma
Protein Binding

Drugs travel in two forms:

  • Bound (inactive reservoir)
  • Unbound (active form)

👉 Only unbound drug can leave the bloodstream, reach tissues, and produce effects.

3. Biotransformation

Chemical changes.

Chemical changes made to a drug after it enters the body.

Purpose:

  • Convert inactive drugs into active forms (prodrugs)
  • Make drugs more water-soluble for elimination
4. Metabolism

Processing.

How the body chemically processes a drug.

  • Primarily occurs in the liver
  • Uses enzymes to modify drugs

The First-Pass Effect

Drugs taken orally face an obstacle course before reaching the entire body:

GI Tract → Liver → Systemic Circulation

👉 The liver may break down a large portion before it reaches circulation, reducing drug availability.

Bypassing First-Pass Metabolism:

IV IM Sublingual Transdermal

5. Excretion

How the body eliminates drugs.

Primary Routes

  • Kidneys (most important) → urine
  • Liver/bile → feces
  • Lungs → gases (e.g., anesthetics)

Minor Routes

  • Sweat
  • Saliva
  • Breast milk

⚠️ Drugs in breast milk can affect infants.

Big Picture Summary

Every drug follows this path:

Absorption ↓ Distribution ↓ Metabolism ↓ Excretion
  • • Liver metabolism increases water solubility → easier excretion
  • • Kidneys remove water-soluble substances
  • • Biliary system prevents reabsorption by sending drugs to stool