Synthesis and evaluation of PET ligands for monoacylglycerol lipase in brain

Objectives: Monoacylglycerol lipase (MAGL) is an attractive target in development of pharmaceuticals for central nervous system disorders and neuroinflammation. So far, to visualize MAGL, we have developed several irreversible-type PET ligands [1,2]. Of these, PET ligands containing azetidine moieties have showed unexpected brain kinetics different from that of the typical irreversible-type PET ligand. Here, to understand relationship between the chemical structure and brain kinetics, we synthesized three new ligands including the azetidine ([11C]1), pyrrolidine ([11C]2) and piperidine ([11C]3) moieties and compared their kinetics in the brain. Methods: Compounds 1–3 and their precursors 5–7 for radiolabeling were synthesized from various pyrazole heterocyclic compounds in 6–7 steps. Synthesis of [11C]1–3 was performed using a home-made automated synthesizer equipped with [11C]COCl2 that was prepared from cyclotron-produced [11C]CO2[3]. As a general procedure exemplified for [11C]1 (Scheme 1), [11C]COCl2 was trapped into a solution of 1,1,1,3,3,3-hexafluoro-2-propanol (4; 3.1 μL) and 1,2,2,6,6-pentamethylpiperidine (PMP; 5.4 μL) in THF (200 μL) at 0 °C. The reaction mixture was heated at 30 °C for 3 min. A solution of corresponding pyrazole 5–7 (1.0 mg) and PMP (2.2 μL) in THF (200 μL) was added into the mixture and this mixture was heated at 30 °C for 3 min and then the solvent was removed at 80 °C for 5 min. The residue dissolved in the preparative HPLC solvent was applied to the semi-preparative HPLC column. Brain kinetics of these radioligands were evaluated by small-animal PET imaging using rats. Results: Starting from 22–24 GBq of [11C]CO2, [11C]1–3 were synthesized with 16–24% radiochemical yields (decay-corrected), 49–60 GBq/μmol molar activity, and ≥98% radiochemical purity. The average total synthesis time from the end of bombardment was 44 min. PET imaging with [11C]1 showed high radioactive uptake with a moderate clearance in MAGL-rich brain regions. In contrast, although radioactive uptake of [11C]3 in the brain indicated similar distribution to that of [11C]1, radioactive clearance from brain was not detected during PET scan. Conclusions: We successfully synthesized several 11C-labeled pyrazole heterocyclic compounds and compared their brain kinetics by PET imaging using rats. The result of present work suggested that the big heterocycle PET ligand for MAGL was more stable in vivo.