In vivo and in vitro validation of reference tissue models for the mGluR5 ligand [11C]ABP688

The primary objective of this study was to verify the suitability of reference tissue-based quantification methods of the metabotropic glutamate receptor type 5 (mGluR₅) with [¹¹C]ABP688. This study presents in vivo (Positron Emission Tomography (PET)) and in vitro (autoradiography) measurements of mGluR₅ densities in the same rats and evaluates both noninvasive and blood-dependent pharmacokinetic models for the quantification of [¹¹C]ABP688 binding. Eleven rats underwent [¹¹C]ABP688 PET scans. In five animals, baseline scans were compared with blockade experiments with the antagonist 1,2-methyl-6-(phenylethynyl)-pyridine (MPEP), and arterial blood samples were drawn and corrected for metabolites. Afterward, saturation-binding autoradiography was performed. Blocking with MPEP resulted in an average decrease of the total distribution volume (V_T) between 43% and 58% (thalamus and caudate-putamen, respectively) but had no significant effect on cerebellar V_T (mean reduction: −0.01%). Comparing binding potential (BP_ND) based on the V_T with noninvasively determined BP_ND revealed an average negative bias of 0.7% in the caudate-putamen and an average positive bias of 3.1% in the low-binding regions. Scan duration of 50 minutes is required. The cerebellum is a suitable reference region for the quantification of mGluR₅ availability as measured with [¹¹C]ABP688 PET in rats. Blood-based and reference region-based PET quantification shows a significant linear relationship to autoradiographic determinations.