Quantification of [11C]ABP688 Binding to mGluR5 in Human Brain using Cerebellum as Reference Region: Biological Interpretation and Limitations

INTRODUCTION: In vitro data from primates provide conflicting evidence about the suitability of the cerebellum as a reference region for quantifying type 5 metabotropic glutamate receptor (mGluR5) binding parameters with positron emission tomography (PET). METHODS: We first measured mGluR5 density in postmortem human cerebellum using [³H]ABP688 autoradiography (n=5) and immunohistochemistry (n=6). Next, in vivo experiments were conducted in healthy volunteers (n=6) using a high-resolution PET scanner (HRRT) to compare [¹¹C]ABP688 binding potential (BPND) values obtained with reference tissue methods and the two-tissue compartment model vs. metabolite-corrected arterial input function. RESULTS: The postmortem data showed that, relative to the hippocampus, the cerebellum had 35% less mGluR5 immunoreactivity and 94% fewer [³H]ABP688 binding sites. In vivo brain regional [¹¹C]ABP688 BP_ND values using the cerebellum as a reference region were highly correlated with BPND values and distribution volumes derived by arterial input methods (R² > 0.9). CONCLUSION: The scarce availability of cerebellar allosteric binding sites at autoradiography, compared to immunohistochemistry results, might reflect the presence of distinct mGluR5 isoforms or conformational state. Together with our PET data, these data support the proposition that [¹¹C]ABP688 BPND using the cerebellum as a reference region provides accurate quantification of mGluR5 allosteric binding in vivo. Studies relying on this method could, therefore, be used in clinical populations, providing that stronger initial assumptions are met.