Quantification of [11C]ABP688 binding in human brain using cerebellum as reference region: biological interpretation and limitations

In vitro data from primates provide conflicting evidence about the cerebellum suitability as a reference region for quantifying type 5 metabotropic glutamate receptor (mGluR5) binding parameters with positron emission tomography (PET). To address this, 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. The postmortem data showed that, relative to the hippocampus, the cerebellum had 26% less mGluR5 immunoreactivity and 94% fewer [3H]ABP688 binding sites. In vivo brain regional [11C]ABP688 BPND 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). The absence of cerebellar allosteric binding sites might reflect the presence of distinct mGluR5 isoforms or conformational state. Together with our PET data, these results support the proposition that [11C]ABP688 BPND using cerebellum as a reference region provides accurate quantification of mGluR5 allosteric binding in vivo. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was supported in part by Alzheimer's Association [NIRG-08-92090 to PR-N], Nussia & Andre Aisenstadt Foundation [PR-N], Fonds de la recherche en sante du Quebec [16326, PR-N], and the Canadian Institutes of Health Research [MOP-115131 to PR-N and MOP-36429 to ML]. The study benefited from the financial support of Health Canada, through the Canada Brain Research Fund, an innovative partnership between the Government of Canada (through Health Canada) and Brain Canada, and the Montreal Neurological Institute. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: All in vitro experiments were carried out in accordance with the guidelines provided by the Douglas Brain Bank research board, approved by the Research and Ethics Board of the Douglas Research Institute, McGill University. The in vivo study was carried out in accordance with the Declaration of Helsinki and was approved by the Research and Ethics Board of the Montreal Neurological Institute/McGill University. Written informed consent with details of the experimental procedures and approved by our institution Research Ethics Board was obtained from all subjects before scan sessions I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors