Introduction
The GABAA-receptor plays an important role in epileptogenesis. A key question is to what extent down regulation or changes in properties of the GABAA-receptor can explain pharmacoresistance to antiepileptic drugs. Several methods for assessing GABAA-receptor properties using [11C]flumazenil (FMZ) and PET have been described. These methods usually provide quantitative measures of either the total volume of distribution in tissue or the binding potential (BP). The BP is defined as the ratio of Bmax and KD. To derive Bmax and KD separately, multiple scans need to be performed. In the present study a full saturation approach was developed, in which the whole range of receptor occupancies, obtained in a single experiment, was used to calculate Bmax and KD.
Methods
After injection of an excess amount of FMZ, fully saturating the receptors, the concentration time curves of FMZ in brain (using a single LSO layer HRRT PET scanner) and arterial blood (using HPLC-UV) were measured. From these data, Bmax and KD were estimated using population pharmacokinetic (PK) modelling. A 4-compartment PK model was used, comprising 1 blood, 1 tissue (body) and 2 brain (free + non-specific binding and specific binding) compartments. Population PK modelling allows for simultaneous analysis of the data from all animals, whilst still taking inter-individual parameter variability into account.
Results
24 rats were scanned, injecting either 2000 ug (n=2), 1000 ug (n=1), 500 ug (n=7), 100 ug (n=3), 50 ug (n=3), 25 ug (n=2), or 1 ug (n=6) FMZ. Following a dose of 1 ug FMZ no blood concentrations could be measured, because of the detection limit of the HPLC-UV method. Population PK-modelling, however, permits analysis of incomplete individual datasets. Using this approach, it was possible to derive separate (population) values for Bmax (14.5+/−3.7 ng/ml) and KD (4.7+/−1.5 ng/ml) with a satisfactory degree of precision.
Conclusions
A novel full saturation approach is reported, which allows for simultaneous estimation of both Bmax and KD in a single experiment per individual animal. This method is now available for analysis of an ongoing study in an experimental animal model for epilepsy (amygdala kindling). Moreover, this approach can also be used in human studies.