Define bioavailability. Outline the factors which affect it.
College Answer
Many candidates did not specify that bioavailability describes the proportion/fraction of drug
reaching the systemic circulation (to differentiate from the portal circulation). Some candidates
considered only factors impacting absorption from the GI tract or stated that bioavailability
related only to orally administered drugs. Candidates failed to provide an equation, or got
equations or graphs wrong. Nearly all candidates failed to provide a comprehensive list of
factors affecting bioavailability.
Discussion
This question is similar to Question 5(p.2) from the first paper of 2010, which specifically asked for factors which influence oral bioavailability. This time, the question was broader in scope, and the exam candidates (clearly doing the past papers for preparation) were taken aback by this.
Definition of bioavailability:
 Bioavailability is the fraction of the dose which reaches systemic circulation intact.
It is described by this equation:
 Bioavailability is measured using the area under the concentrationtime 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

References
Wesch, Roland. "Absolute and relative bioavailability." Drug Discovery and Evaluation: Methods in Clinical Pharmacology. Springer Berlin Heidelberg, 2011. 173180.
Vaughan, D. P. "A modelindependent proof of Dost's law of corresponding areas." Journal of pharmacokinetics and biopharmaceutics 5.3 (1977): 271276.
Balant, L. P. "Is there a need for more precise definitions of bioavailability?." European Journal of Clinical Pharmacology 40.2 (1991): 123126.
KochWeser, Jan. "Bioavailability of drugs." New England Journal of Medicine 291.10 (1974): 503506.