Application of Population Pharmacokinetic Analysis for Quantification of in vivo Binding Properties in the Rat Brain by Positron Emission Tomography
C. Lia Liefaard (1), Bart A. Ploeger (1,2), Carla F.M. Molthoff (3), Ronald Boellaard (3), Adriaan A. Lammertsma (3), Meindert Danhof (1,2), Rob A. Voskuyl (1,4)
(1)Division of Pharmacology, LACDR, Leiden University, Leiden, The Netherlands; (2)LAP&P Consultants BV, Leiden, The Netherlands; (3)PET Center, VU University Medical Center, Amsterdam, The Netherlands; (4)Epilepsy Institute of The Netherlands, Heemstede, The Netherlands
Objectives: Changes in GABAA receptor density and affinity play an important role in many forms of epilepsy. Using Positron Emission Tomography (PET) receptor binding properties, maximum binding capacity (Bmax) and receptor affinity (KD) of specific GABAA ligands (e.g. Flumazenil, FMZ), can be determined non-invasively. A novel full saturation approach is presented, which, in contrast with presently available methods, allows simultaneous estimation of Bmax and KD from a single PET-study, using population analysis.
Methods: Following a saturating intravenous injection of [11C]FMZ, the concentration time curves of FMZ in brain (using PET) and blood (using HPLC-UV) were measured. The data were analysed with NONMEM. The structural PK model consisted of a 4-compartment model, containing a blood, a tissue and 2 brain compartments: Brain Free and Brain Bound. In the model the total FMZ concentration in the brain as measured by PET, reflects the sum of the concentrations in the Brain Free and the Brain Bound compartment. All exchanges between compartments are described using first order rates. Specific binding is dependent on both the concentration of free ligand and the concentration of receptors available for binding. The PK model includes inter-individual (IIV) and inter-occasion (IOV) variability in the volume of distribution of the brain (VBr). The residual error was assumed to be proportional to the concentration in blood and brain. Finally, a residual error was added to take into consideration the greater uncertainty in blood concentrations that are close to the detection limit. This residual variance was fixed to the square of half of the detection limit.
Results: Analysis of all data with the proposed model resulted in the precise estimation of all parameters, including the specific binding of FMZ in the brain, as characterised by Bmax (32.7 ± 7.95 ng/ml) and KD (10.1 ± 2.61 ng/ml). Variability in the positioning of the animals in the scanner resulted in variation in the location of the region of interest in the brain. This variation was successfully characterised by a significant variability (IIV and IOV, CV = 28.7%) in VBr.
Conclusions: In conclusion a novel full saturation approach is reported, which allows simultaneous estimation of both Bmax and KD in a single experiment. This model will be used to assess changes in GABAA-receptor properties in relation to progression of epilepsy.