Bioavailability and Drug Absorption Quiz

Bioavailability

• Bioavailability is the fraction of administered drug that gains access to the systemic circulation in a chemically unchanged form.
• Example: if 100 mg of a drug are administered orally and 70 mg are absorbed unchanged, the bioavailability is 0.7 or 70%.

Determination of Bioavailability

• Bioavailability is determined by comparing plasma levels of a drug after a particular route of administration (e.g., oral administration) with plasma drug levels achieved by IV injection.
• The area under the curve (AUC) is measured by plotting plasma concentrations of the drug versus time.
• Bioavailability of a drug administered orally is the ratio of the AUC calculated for oral administration compared with the AUC calculated for IV injection.

Factors that Influence Bioavailability

• First-pass hepatic metabolism: When a drug is absorbed across the GI tract, it enters the portal circulation before entering the systemic circulation.
• Rapid metabolism by the liver decreases the amount of unchanged drug that gains access to the systemic circulation.

Therapeutic Equivalence

• Two similar drugs are therapeutically equivalent if they have comparable efficacy and safety.
• Clinical effectiveness depends on both the maximum serum drug concentrations and the time required to reach peak concentration.

Drug Distribution

• Drug distribution is the process by which a drug reversibly leaves the bloodstream and enters the interstitium (extracellular fluid) and/or the cells of the tissues.
• Factors that influence drug distribution include blood flow, capillary permeability, binding of the drug to plasma and tissue proteins, and the relative hydrophobicity of the drug.

Factors Affecting Drug Absorption

• Total surface area available for absorption: The intestine has a surface rich in microvilli, increasing the surface area for absorption.
• Contact time at the absorption surface: Factors such as diarrhea or presence of food in the stomach can affect the rate of absorption.
• Other factors: Parasympathetic input, sympathetic input, anticholinergics, and exercise can influence gastric emptying and absorption.

Test your knowledge on bioavailability, which is the fraction of an administered drug that reaches the systemic circulation unchanged. Understand how to calculate bioavailability and determine it by comparing drug plasma levels after different routes of administration.