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Abstract
Variations in response to drugs may be pharmacodynamic, implying inter-individual
differences in the response of receptors in equal concentrations of drug, or pharmacokinetic,
implying that individuals receiving the same dose of drug will have different concentrations
of drug in their body fluids. Either type of variation can be inherited or acquired.
Variations in receptor sensitivity do occur but few instances, inherited or acquired,
have well documented clinical relevance. If the dose response relationship for the
drug in question is not steep, or if the therapeutic index is low, drug concentration
in the region of the receptor will not be critical and causes of kinetic variation
are unlikely to be clinically significant. However, it is the many causes of kinetic
variation which are best described. These include effects due to drug formulation
and changes in the absorption, distribution, metabolism and excretion of drugs. If
a consideration of dynamics suggests that drug concentration will determine therapeutic
efficacy, analysis and prediction of variability due to these factors is desirable.
Prediction requires an accurate description of the system but commonly used pharmacokinetic
models may fail when prediction is a goal. The variables, volume of distribution (Vd) and rate constant of elimination (Ke) are hybrid in that they arise from the interaction of patient and drug characteristics.
Important events including macromolecular binding and altered blood flow may not be
represented. More data is required to determine the clinical significance of pharmacodynamic
variation but better analytical tools are required to deal with kinetic variation
when this is important. Specifically, pharmacokinetic models should represent physiological
variables and levels of unbound drug in body fluids should receive greater emphasis.
Keywords
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© 1983 Published by Elsevier Inc.