Development of two new PET radiotracers with different brain kinetics for imaging monoacylglycerol lipase in brain

Objectives: Monoacylglycerol lipase (MAGL) is a serine hydrolase that hydrolyzes 2-arachidonoylglycerol (2-AG) into arachidonic acid (AA) and glycerol in brain. Because 2-AG and AA are endogenous biologically active ligands in brain, inhibition of MAGL is an attractive therapeutic target for brain diseases, such as neuroinflammation. Recently some therapeutic drug candidates have been developed by irreversibly inhibiting MAGL in brain. In this study, to visualize the brain MAGL and provide proof-of-concept for the drug candidates, we developed two new irreversible PET tracers ¹¹C-1 and ¹¹C-2 and evaluated their in vivo potentials in the brain.

Methods: The unlabeled compounds 1 and 2 were synthesized by 5-step reactions starting from commercially-available chemicals, respectively. The radiosynthesis of ¹¹C-1 or ¹¹C-2 was performed by reacting alcohol with ¹¹C-phosgene, followed by reaction with the corresponding amine. Small-animal PET scans for rats were conducted to examine the in vivo brain regional distribution of radioactivity. In vitro and in vivo stability tests were also conducted.

Results: Compounds 1 and 2 were obtained in total yields of 20-30%, respectively. Radiotracers ¹¹C-1 and ¹¹C-2 were achieved in around 10% isolated radiochemical yields (relative to ¹¹CO₂, non-decay corrected). Their radiochemical purity exceeded 98%, and the molar activity was higher than 74 GBq/μmol at the end of synthesis. PET imaging studies showed that the two radiotracers could pass through blood-brain-barrier and the initial uptakes reached 1.5 SUV in the rat brain. Their wide distribution pattern of radioactivity was consistent with the pattern of MAGL in the brain. Pretreatment with MAGL-selective inhibitor JW642 significantly reduced the uptake of radioactivity in the brain. While the radioactivity level for ¹¹C-1 decreased in the brain after the radiotracer injection, the radioactivity for ¹¹C-2 retained a definite level until the end of PET. In vitro stability tests showed that ¹¹C-CO₂ was continuously released from the incubated mixture of ¹¹C-1 with brain homogenate, whereas no gaseous radioactivity was formed in the mixture of ¹¹C-2 with the homogenate.

Conclusions: Two novel PET tracers ¹¹C-1 and ¹¹C-2 for imaging MAGL were synthesized and obtained with applicable radioactivity yield and enough quality for the evaluation studies. PET imaging studies with ¹¹C-1 and¹¹C-2 showed high brain uptakes and in vivo specific bindings for MAGL in the rat brain. ¹¹C-2 displayed the brain kinetics as a typical irreversible radiotracer while ¹¹C-1 showed a different kinetics from ¹¹C-2. The radiotracers could be used to visualize MAGL in diseased brain models for different purposes.