The proposed population PK model allowed for simultaneous estimation of B (max) and K (D) for a group of animals using single injection PET experiments per animal.

SUMMARYPurpose: The purpose of the present investigation was to quantify alterations in GABA A receptor density in vivo in rats subjected to amygdala kindling. Methods: The GABA A receptor density was quantified by conducting a [ 11 C]flumazenil (FMZ) positron emission tomography (PET) study according to the full saturation method, in which each animal received a single injection of FMZ to fully saturate the GABA A receptors. Subsequently, the concentration-time curves of FMZ in blood [using highpressure liquid chromatography with UV detector (HPLC-UV) or high-performance liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS)] and brain (with PET-scanning) were analyzed by population modeling using a pharmacokinetic model, containing expressions to describe the time course of FMZ in blood and brain. Results: The GABA A receptor density (B max ) in kindled rats was decreased by 36% compared with controls. This is consistent with a reduction of 28% in electroencephalography (EEG) effect of midazolam in the same animal model, suggesting that a reduced number of GABA A receptors underlies the decreased efficacy of midazolam. Furthermore, receptor affinity (K D ) was not changed, but the total volume of distribution in the brain (V Br ), is increased to 178% of control after kindling, which might indicate an alteration in the transport of FMZ across the blood-brain barrier. Conclusions: Both the GABA A receptor density (B max ), and possibly also the blood-brain barrier transport of FMZ (V Br ) are altered after kindling. Furthermore, this study indicates the feasibility of conducting PET studies for quantifying moderate changes in GABA A receptor density in a rat model of epilepsy in vivo.

Summary:Purpose: The objective of this investigation was to characterize quantitatively the time-dependent changes in midazolam (MDL) efficacy in the silent period after induction of status epilepticus (SE) in rats. The changes in MDL efficacy were correlated to changes in ex vivo GABA A -receptor expression.Methods: MDL efficacy was quantified by pharmacokineticpharmacodynamic (PK-PD) modeling by using the β-frequency of the EEG as PD end point. Two PK-PD experiments were performed in each animal: the first experiment before and the second experiment at either day 4 or day 14 after SE. SE was induced by repetitive intraperitoneal injections with kainate. GABA Areceptor expression was determined by ex vivo autoradiography with [3 H]flumazenil. Results:The concentration versus EEG effect relation of midazolam was successfully described by the sigmoidal E max model. The maximal effect on the β-frequency of the EEG (E max ) was reduced to 51.6 ± 35.6% and 25.8 ± 33.7% of the original value at 4 and 14 days after induction of SE. The ex vivo study with [ 3 H]flumazenil showed that the observed reductions in E max were paralleled by a reduction in GABA A -receptor density.Conclusions: The efficacy of MDL is decreased in the silent period after SE, which can be partly accounted for by a reduction in GABA A -receptor density.

A one-compartment disposition model with two input functions and first-order elimination adequately described the pharmacokinetics of tapentadol ER. The dose-dependency in the pharmacokinetics of tapentadol ER is adequately described by the absorption model. None of the covariates were considered as clinically relevant factors that warrant dose adjustments.

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