Introduction: Monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) are the two primary enzymes that regulate the tone of endocannabinoid signaling. Although new PET radiotracers have been discovered for imaging FAAH in vivo, no such radiotracer exists for imaging MAGL. Here we report the radiosynthesis of five candidate MAGL radiotracers and their ex vivo evaluations in mice and rats.
Methods: Candidate carbamate and urea MAGL inhibitors were radiolabeled at the carbonyl position by [(11)C]CO2 fixation. Radiotracers were administered (tail-vein injection) to rodents and brain uptake of radioactivity measured at early and late time points ex vivo. Specificity of uptake was explored by pretreatment with unlabeled inhibitors (2 mg/kg, ip) 30 min prior to radiotracer administration.
Results: All five candidate MAGL radiotracers were prepared in high specific activity (>65 GBq/μmol) and radiochemical purity (>98%). Moderate brain uptake (0.2-0.8 SUV) was observed for each candidate while pretreatment did not reduce uptake for four of the five tested. For two candidates ([(11)C]12 and [(11)C]14), high retention of radioactivity was observed in the blood (ca. 10 and 4 SUV at 40 min) which was blocked by pretreatment with unlabeled inhibitors. The most promising candidate, [(11)C]18, demonstrated moderate brain uptake (ca. 0.8 SUV) which showed circa 50% blockade by pretreatment with unlabeled 18.
Conclusion: One putative and four reported potent and selective MAGL inhibitors have been radiolabeled via [(11)C]CO2 fixation as radiotracers for this enzyme. Despite the promising in vitro pharmacological profile, none of the five candidate radiotracers exhibited in vivo behavior suitable for PET neuroimaging.
Keywords: CO(2) fixation; Carbon-11; Monoacylglycerol lipase; Positron emission tomography; Radiotracer development.
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