Feasibility Evaluation of Myocardial Cannabinoid Type 1 Receptor Imaging in Obesity: A Translational Approach

Objectives: The aim of this study was to evaluate the feasibility of targeted imaging of myocardial cannabinoid type 1 receptor (CB1-R) and its potential up-regulation in obese mice with translation to humans using [¹¹C]-OMAR and positron emission tomography (PET)/computed tomography (CT). Background: Activation of myocardial CB1-R by endocannabinoids has been implicated in cardiac dysfunction in diabetic mice. Obesity may lead to an up-regulation of myocardial CB1-R, potentially providing a mechanistic link between obesity and the initiation and/or progression of cardiomyopathy. Methods: Binding specificity of [¹¹C]-OMAR to CB1-R was investigated by blocking studies with rimonabant in mice. The heart was harvested from each mouse, and its radioactivity was determined by γ-counter. Furthermore, [¹¹C]-OMAR dynamic micro-PET/CT was carried out in obese and normal-weight mice. Ex vivo validation was performed by droplet digital polymerase chain reaction (absolute quantification) and RNAscope Technology (an in situ ribonucleic acid analysis platform). Subsequently, myocardial CB1-R expression was probed noninvasively with intravenous injection of CB1-R ligand [¹¹C]-OMAR and PET/CT in humans with advanced obesity and normal-weight human control subjects, respectively. Results: Rimonabant significantly blocked OMAR uptake in the heart muscle compared with vehicle, signifying specific binding of OMAR to the CB1-R in the myocardium. The myocardial OMAR retention quantified by micro-PET/CT in mice was significantly higher in obese compared with normal-weight mice. Absolute quantification of CB1-R gene expression with droplet digital polymerase chain reaction and in situ hybridization confirmed CB1-R up-regulation in all major myocardial cell types (e.g., cardiomyocytes, endothelium, vascular smooth muscle cells, and fibroblasts) of obese mice. Obese mice also had elevated myocardial levels of endocannabinoids anandamide and 2-arachidonoylglycerol compared with lean mice. Translation to humans revealed higher myocardial OMAR retention in advanced obesity compared with normal-weight subjects. Conclusions: Noninvasive imaging of cardiac CB1-R expression in obesity is feasible applying [¹¹C]-OMAR and PET/CT. These results may provide a rationale for further clinical testing of CB1-R-targeted molecular imaging in cardiometabolic diseases.