Pharmacokinetics — Bioavailability

DEFINITION

The fraction of drug that reaches the circulation compared with the same dose given intravenously. (%)

(or)

The ratio of the area under the stated concentration–time curve (AUC) divided by the area under the i.v. concentration–time curve. (%)

FACTORS AFFECTING BIOAVAILABILITY

• Pharmaceutical factors
  • Preparation: Decreasing order: Solutions > Suspensions > Capsule > Tablet > Coated tablet
  • Particle Size
  • Salt form
  • Crystal forms have better availability compared to amorphous forms
  • Degree of ionization: – Non-ionised, lipid soluble drugs – higher bioavailability

• Pharmacological factors
  • Gastric Emptying and GI Motility
  • GI Diseases: Coeliac, Crohn’s
  • Timing in relation to food intake
  • First pass metabolism: The degree of metabolic breakdown of an orally administered drug that occurs in the intestine or liver before it reaches the systemic circulation.
  • Drug-Drug interactions
  • Pharmacogenetic factors

• Other factors:
  • Area of absorptive surface
  • State of circulation (shock, tissue perfusion)
  • Hepatic Insufficiency, Poor renal function
  • Route of administration

Sources: Cross and Plunkett. Physics, Pharmacology, Physiology for Anaesthetists. Goodman & Gilman’s Pharmacological Basis of Therapeutics. Katzung Clinical Pharmacology

JC 2020

Answer by JC