ESTIMATION OF AMOBARBITAL PLASMA-EFFECT SITE EQUILIBRATION KINETICS - RELEVANCE OF POLYEXPONENTIAL CONDUCTANCE FUNCTIONS

The time delay between drug plasma concentrations and effect has been modeled most commonly by the effect compartment approach, assuming first-order monoexponential equilibrium kinetics between plasma and effect site. So far this assumption has not been rigorously probed. The purpose of the present investigation was to model the delay between amobarbital plasma, concentrations and EEG effect using a new approach based on system analysis principles. This approach models the equilibrium between plasma and effect site without assuming a specific kinetic structure. Assuming linear distribution kinetics between plasma and effect site, the relationship between the two variables may be described by a convolution type of linear operation, involving a conductance function phi(t), which is approximated by a sum of exponentials. Six male Wistar-derived rats received an iv infusion of amobarbital at a rate of 10 mg/kg per min until isoelectric periods of 5 sec or longer appeared on the EEG. Frequent arterial blood samples were obtained and EEG was continuously quantified using aperiodic analysis. The amplitudes in the 2.5-30 Hz frequency band were used as EEG effect measure. The delay between plasma concentrations and EEG effect was best modeled by a biexponential conductance function. The use of a biexponential conductance function resulted in a significant further reduction (41 +/- 10%) in hysteresis when compared to a monoexponential function, indicating that the assumption of simple first-order monoexponential equilibration kinetics is inadequate. The use of a biexponential conductance function also resulted in a significantly different shape of the effect site concentration-EEG effect relationship and hence the estimated pharmacodynamic parameters, when compared with a monoexponential function. This relationship showed a biphasic behavior, with EEG effects being maximal at amobarbital concentrations of 29.6 +/- 1.3 mg/L. At 80.2 +/- 2.0 mg/L the EEG effect was reduced 50% below baseline values. A comparison was made with the equilibration between amobarbital plasma and cerebrospinal fluid (CSF) concentrations. Six male Wistar-derived rats received an iv infusion of amobarbital, 10 mg per min for 15 min. Arterial blood and CSF samples were taken simultaneously at regular intervals. The equilibrium between plasma and CSF concentrations was best fitted by a monoexponential conductance function. Significant differences in equilibration profiles of CSF and effect site with the plasma site were observed. To reach 50% equilibrium the effect site requires 2.5 +/- 0.3 min and the CSF 3.5 +/- 0.2 min, to reach 95% the values were, respectively, 90 +/- 27 and 15 +/- 1 min. This suggests that CSF is kinetically distinguishable from the effect site.