Novel reversible-binding PET ligands for imaging monoacylglycerol lipase

Purpose/Background: Monoacylglycerol lipase (MAGL) is a 33 kDa serine protease that hydrolyses 2-arachidonoylglycerol (2-AG) to arachidonic acid in the mammalian central nervous system (CNS). Given that arachidonic acid and its eicosanoid metabolites induce inflammation and promote neurodegeneration, selective inhibition of MAGL constitutes a promising therapeutic strategy for neurodegenerative diseases. To date, several 11C- or 18F-labeled MAGL PET agents have been developed. While most of them bind to MAGL via an irreversible mechanism, only a limited number of reversible PET ligands have been disclosed. Herein, we report the design and synthesis of multiple reversible MAGL inhibitors based on a piperazinyl azetidine scaffold. Based on a structure-activity relationship study, the best performing reversible MAGL inhibitors, namely, target compounds 10 and 15 were labeled with fluorine-18 for further evaluations by autoradiography and PET imaging in rodents and non-human primates. Both [18F]10 and [18F]15 ([18F]MAGL-2102) exhibited favorable brain uptakes, heterogeneous radioactivity distribution, good specific binding, and adequate brain kinetics. Methods: A series of piperazinyl azetidine based MAGL inhibitor candidates were designed and synthesized (Fig. 1A). An initial screening was carried out with all inhibitor candidates in human MAGL inhibitor screening assay (Fig. 1B), and the potencies of the most promising irreversible inhibitor 10 and 15 were determined (Fig. 1C). The selectivity for MAGL over other serine hydrolases was assessed by activity-based protein profiling assays (ABPP) (Fig. 1D). The radiosyntheses of [18F]10 and [18F]15 were achieved via the Cu-mediated 18F-fluorination of the corresponding borate precursor with [18F]Et4NF in DMA/nBuOH at 110 ℃ for 20 min (Fig. 1E). In vitro autoradiography studies with were performed with [18F]10 (Fig. 1F) and [18F]15 on sagittal rat brain sections (Fig. 1G). Dynamic PET imaging studies were conducted with [18F]10 (Fig. 1H) and [18F]15 in rodents (Fig. 1I) and non-human primates (Fig. 1J). Results: All Candidates were obtained in 21-61% yields (Fig. 1A). Only 10 and 15 exhibited above 50% inhibition of MAGL at 9.26 nM (Fig. 1B) and their potencies (IC50: 4.2 and 4.6 nM, respectively) were measured (Fig. 1C). Both 10 and 15 exhibited excellent selectivity for MAGL over other lipid hydrolases (Fig. 1D). Ligands [18F]10 and [18F]15 were obtained in high radiochemical yields (57% and 53%, respectively, decay-corrected) and high molar activities (37 and 63 GBq/µmol, respectively) (Fig. 1E). Ligands [18F]10 and [18F]15 exhibited high radioactivity accumulation in the hippocampus, cortex, thalamus, striatum and cerebellum in both autoradiography and PET studies on rats and rhesus (Fig. 1F-1J). The reversible MAGL inhibitor, JNJ-42226314, remarkably diminished the radioactivity in MAGL-rich regions and abolished the regional heterogeneity (Fig. 1F-1J). Conclusion: We have designed and synthesized a novel panel of reversible candidate MAGL inhibitors based on a piperazinyl azetidine scaffold. Compounds 10 and 15 were identified as best-in-class MAGL inhibitors with excellent potencies and MAGL selectivities by in vitro pharmacological evaluations. A robust radiolabeling procedure was established for [18F]10 and [18F]15. Further evaluations of [18F]10 and [18F]15 in autoradiography and dynamic PET imaging studies in rodents and non-human primates demonstrated that both [18F]10 and [18F]15 had heterogenous regional brain distribution, which was consistent with known MAGL-expression patterns, and reversible binding in vivo. Specific MAGL binding was demonstrated for [18F]10 and [18F]15 on the rodent brain in vitro as well as in vivo. Notably, [18F]15 exhibited superior performance characteristics by means of a higher general brain uptake and enhanced in vivo specificity. In conclusion, [18F]15 ([18F]MAGL-2102) constitutes a promising reversible PET ligand for visualization of MAGL, which holds promise for clinical translation.
SNMMI2022