Pharmacokinetics Series: Absorption and Routes of Drug Administration

Introduction

• Second tutorial in the pharmacokinetics series.
• Focus on absorption and routes of drug administration.
• Recommended to watch the first tutorial for terminology understanding.

Simplified Body Plan

• Blood flow path:
  • From gut to liver via hepatic portal system.
  • From liver through venous circulation to the heart.
  • Through pulmonary arteries to the lungs.
  • From lungs back to heart and then to arterial circulation.
• Aim: Place drugs in different parts of this system and measure arterial blood concentration over time.

Graph Explanation

• Y-axis: Blood concentration of the drug.
• X-axis: Time.

Routes of Drug Administration

Oral Administration

• Most common route.
• Drug path: Digestive system → Gut absorption → Liver → Venous circulation → Heart → Lungs → Heart → Arterial circulation.
• Results in a slow rise and fall of drug concentration.
  • Cmax: Quite low.
  • Tmax: Quite high.

Intravenous (IV) Administration

• Direct route, quicker journey to arterial circulation.
• Path: Heart → Lungs → Heart → Arteries.
• Quicker rise in drug concentration.
  • Cmax: Larger.
  • Tmax: Smaller.

Inhalation

• Path: Heart → Arterial circulation.
• Fastest rise in drug concentration.
  • Cmax: Much larger.
  • Tmax: Even smaller.

Importance of Routes

• Different routes cause different Cmax and Tmax despite identical doses.
• Applications:
  • IV analgesic for quick pain relief.
  • Oral administration for drugs with side effects at high concentrations.

Key Points

• Area Under Curve (AUC): Same for all routes due to same drug dose.
• Excretion Rates: Vary due to first-order kinetics.
  • Rate of excretion proportional to drug concentration.
• Half-Life: Consistent across routes, a property of the drug.

Future Tutorial

• Focus on drug metabolism and cytochrome P450 in the liver.

Conclusion

• Encouragement to donate for more tutorials at www.handwritten-tutorials.com