Discuss the pharmacokinetic factors that affect drug half-life.
Half-life (t½) is the time required to change the amount of a drug in the body by one-half
during elimination. Candidates were expected to discuss the two main factors which affect
drug half-life, namely volume of distribution and clearance. Marks were awarded for the
formula (t½ = 0.693 x Vd /CL), the factors which affect the volume of distribution and drug
clearance but not for a discussion of factors affecting drug absorption.
The college did not ask the candidates to define half-life in the question, but they defined it in their answer. According to the official text (Birkett et al, 2009), "Half-life is the time take for the amount of the drug in the body (or the plasma concentration) to fall by half".
Half-life is described by the equation:
Half-life of drugs is therefore influenced by the following factors
- Volume of distribution, which is in turn influenced by:
- Properties of the drug, including molecule size, charge, pKa, protein binding, tissue binding and lipid-water partition coefficient
- Properties of the patient's body fluids, including volume status, protein content, body fluid pH and presence of drugs which compete for binding sites
- Pathophysiological states, including age, gender, obesity, pregnancy, and oedema
- Presence of extracorporeal sites of distribution, eg. circuit fluid content and drug-adsorbing circuit components
- Clearance, which is in turn influenced by:
- Concentration of the drug
- Susceptibility of the drug to biotransformation (i.e. clearance by metabolism, usually hepatic)
- Susceptibility of the drug to removal by filtration or diffusion (i.e. clearance by the kidneys, lungs etc)
- Active excretion of the drug (its removal by active transort mechanisms)
- Functional status of clearance organs (eg. liver and kidneys)
- Blood delivery to organs of clearance
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