Radiosynthesis and Evaluation of Novel 18F Labeled PET Ligands for Imaging Monoacylglycerol Lipase

Monoacylglycerol lipase (MAGL) is a 33 kDa cytosolic serine hydrolase that is widely distributed in the central nervous system and peripheral tissues. MAGL hydrolyzes monoacylglycerols into fatty acids and glycerol, playing a crucial role in endocannabinoid degradation. Inhibition of MAGL in the brain elevates levels of 2-arachidonoylglycerol and leads to decreased pro-inflammatory prostaglandin and thromboxane production. As such, MAGL is considered a potential target for treating neuropsychiatric disorders, metabolic syndromes, and cancer. Based on a novel spirocyclic system, we synthesized two fluorinated carbamate scaffolds as reversible MAGL inhibitors (epimers: (R)-6, IC₅₀ = 18.6 nM and (S)-6, IC₅₀ = 1.6 nM). In vitro autoradiography studies of [¹⁸F](R)-6 (codenamed [¹⁸F]MAGL-2304) and [¹⁸F](S)-6 (codenamed [¹⁸F]MAGL-2305) demonstrated heterogeneous distribution and specific binding affinity to MAGL-rich brain regions. Autoradiography with MAGL knockout mouse brain tissues confirmed the binding specificity of [¹⁸F](S)-6. Dynamic PET imaging studies revealed that [¹⁸F](S)-6 exhibited limited brain uptake and homogenous distribution in rat brains. In vivo P-gp inhibition enhanced [¹⁸F](S)-6 uptake in the brain, suggesting that [¹⁸F](S)-6 constitutes a P-gp efflux substrate. This research could provide new directions in the design of MAGL PET ligands that are based on spirocyclic scaffolds.