Spring Intro 2023

Mike Kelly

Basic Principles: Pharmacokinetics - Bioavailability

 Bioavailability is closely related to absorption and exposure  Definition - This is the fraction of a dose that reaches the systemic circulation following extravascular administration  Bioavailability is affected by – How well the tablet/formulation releases the drug – How well the drug dissolves i.e. solubility – How well a drug passes across the gastrointestinal membrane i.e. permeability – Whether it is metabolised in the gut and/or by the liver – Whether it is a substrate for efflux proteins in the gastrointestinal cell membranes

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Pre-systemic metabolism

Ingestion of drug

Hepatic first pass

GUT

Hepatic portal vein

LIVER

Poor solubility, Possible metabolism by CYP3A4/gut flora, Luminal degradation

SYSTEMIC CIRCULATION

Drug passes across the GIT membrane

Unabsorbed drug into faeces

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