Formulation-dependent pharmacokinetics and pharmacodynamics of propofol in rats

Propofol, a highly lipophilic anaesthetic, is commercially formulated as a lipid emulsion (diprivan) for intravenous use. This formulation is characterized by rapid onset and offset of effect after rapid intravenous administration and an effect-site equilibration half-life (t1/2kE0) of 1.7 min in rats. Paradoxically these characteristics are usually associated with relatively water-soluble anaesthetics. To test the influence of the formulation on propofol pharmacokinetics, effect-site equilibration kinetics and pharmacodynamics we performed a pharmacokinetic-pharmacodynamic study of propofol in chronically instrumented rats after administration in a lipid-free formulation. In this report we present the results of this study and compare these results with previous data obtained with rats receiving propofol in the emulsion formulation. Compared with the emulsion formulation the distribution volumes (VdC and VdSS) were significantly higher but the t1/2kE0 (2.0 min) was similar for the lipid-free formulation. The concentration-effect relationship was biphasic. Propofol effect-site concentrations required to achieve 50% activation, peak activation, 50% inhibition of peak activation effect and maximum inhibition were significantly lower, indicating a higher apparent steady-state potency for the lipid-free formulation compared with the emulsion formulation. The evanescent characteristics of propofol's effect-time-course disappeared when the anaesthetic was administered in the lipid-free formulation. These results suggest that the nature of the formulation can profoundly influence the clinical characteristics of intravenously administered drugs by modifying the pharmacokinetics or pharmacodynamics or both.