Define bioavailability (10% of marks). Outline the factors which affect it (90% of marks).
Many candidates spent time defining and describing aspects of pharmacokinetics which were not relevant to the question. E.g. clearance, volume of distribution and half-life. Candidates who scored well utilised a structure which incorporated the headings of the factors which affect the bioavailability of medications with a simple description as to the nature of the effect. These factors included: the physical properties of the drug, the preparation, patient factors, the route of administration and metabolism amongst others.
- Bioavailability is the fraction of the dose which reaches systemic circulation intact
- IV bioavailability is by definition 100%
- "Absolute" bioavailability compares one non-IV route with IV administration.
- "Relative" bioavailability compares one non-IV route or formulation with another (instead of using IV route as a reference).
- Bioavailability is measured using the area under the concentration-time curve (Dost's Law). The ratio of AUCs is the bioavailability value.
Generic influences on drug bioavailability
Factors affecting first pass metabolism
Factors affecting gastrointestinal absorption
Bioavailability via transdermal and mucosal routes of administration
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