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Regarding dose-independent pharmacokinetic parameters in nonlinear pharmacokinetics.

There seems to be an increasing trend in pharmacokinetics and pharmacodynamics to deal with parameters that require minimal kinetic structure assumptions. Such parameters, which often inappropriately are called "model-independent parameters", seem more rational to work with than parameters that depend on the particular structure of specific models. The structure of specific models may be impossible to verify experimentally or to justify from physiological and/or anatomical considerations. Various mean time (MT) parameters such as mean residence time (MRT) and mean transit time (MTT) parameters were introduced in the early tracer kinetic and physiology literature.1-3 The generality and conceptual value of MT parameters have created a strong interest in such parameters, attested by the more than 100 recent publications specifically dealing with these parameters.4 There has been a similar interest in volume of distribution at steady state (Vda8)56 and clearance (CL) parameters. The above parameters have predominantly been applied in linear disposition kinetics. Although it appears to be true for many drugs in their therapeutic concentration range that the linear disposition assumption is a reasonably accurate assumption, significant deviations are seen for several drugs. The nonlinearity creates the problem that many of the popular pharmacokinetic parameters become quite meaningless as disposition parameters because they become dependent on the rate and extent of drug input (absorption). Essentially, they cannot be classified as disposition parameters in such cases, and should not be called parameters when they become time-dependent functions of the input. The purposes of this letter are threefold.