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CLINICAL PHARMACOKINETICS –
TIME COURSE OF DRUG ACTION
6.1 Clinical Pharmacokinetics
• The underlying principle of clinical pharmacokinetics is that a relationship exists between
the effects of a drug and the concentration of drug in the body.
• In clinical pharmacokinetics we try to provide:
1) a quantitative relationship between drug dose and effect
2) a framework to interpret measurements of drug concentrations in biological fluids to
benefit patient drug therapy.
• The most important parameters determining drug disposition in humans are:
1) Clearance – the body’s efficiency in drug elimination.
2) Volume of Distribution – the apparent space in the body available to contain the drug.
3) Elimination Half Life (T1/2) – a measure of the rate of removal of the drug from the
body.
4) Bioavailability – the fraction of drug that reaches the systemic circulation unchanged.
6.2 Plasma Drug Concentrations
Measuring Drug Concentrations
• Ideally drug concentrations would be measured from the site of
action.
• In reality, this is not feasible.
• Let’s take the example of drugs used to treat schizophrenia.
These drugs act in the brain.
• Clearly taking a sample from a patient’s brain to measure drug
concentrations is invasive and would likely do more harm than
good.
• In reality drug concentrations are usually measured in
plasma.
• Plasma is a good site to measure drug concentrations
because:
1) It is relatively non-invasive.
2) For most drugs there is a good correlation between
plasma concentration and therapeutic and toxic drug
effects.
