Abstract
1044
Objectives Inhibition of monoacylglycerol lipase (MAGL) has been proposed as means of elevating 2-arachidinoylglycerol to treat pain, inflammation, and neurological disorders while avoiding undesired side effects encountered through direct cannabinoid receptor modulation. A PET radiotracer for MAGL would allow non-invasive quantification of this enzyme’s density and distribution in vivo and thus facilitate drug discovery for this target. In the present study, we identified a novel MAGL inhibitor and performed PET imaging studies with its 11C-isotopologue.
Methods The lead compound, (4-(1-(4'-chloro-2-methoxy-3'-(trifluoromethyl)-[1,1'-biphenyl]-4-carbonyl)azetidin-3-yl)piperazin-1-yl)(thiazol-2-yl)methanone (PADA) was identified in patent literature with IC50 value of 0.4 nM on a MAGL inhibition enzyme assay. [11C]PADA was labeled by reaction of the corresponding phenolic precursor in the presence of NaOH with 11CH3I in DMF. Dynamic PET studies in Sprague-Dawley rats and Pgp/Bcrp KO mice under anaesthesia were carried out for 90 min. Results: The standard compound, PADA, was synthesized in eight steps with 3% overall yield from commercially available materials. [11C]PADA was synthesized in >5% uncorrected radiochemical yield, relative to 11CO2, with specific activity >2 Ci/μmol and > 99% radiochemical purity. PET imaging in normal rats showed low-to-moderate (0.5 SUV) brain penetration. Radioactivity in brain tissues increased rapidly and showed fairly homogeneous distribution, peaked at 3 min (0.5 SUV in cerebellum and 0.4 SUV in cerebral cortex), and gradually washed out to 0.3 SUV after 10 min and remained at this level over 90 min. Whole brain uptake of [11C]PADA in PgP/Bcrp KO (ABCB1a/1b-/-ABCG2-/-) mice were 1.4 SUV, representing a significant four-fold increase in comparison to that of wild-type mice. Regional brain analysis indicated brain uptake in cerebral cortex was initially 1.4 SUV at 2 min, then slowly increased to 1.6 SUV over 90 min scan time. Brain uptake in the cerebellum was 1.0 SUV at 2 min and gradually decreased to 0.8 SUV at the end of scan. The distribution pattern in the brain also changed from homogenous (wild type) to heterogeneous (PgP/Bcrp KO mice). These results indicate [11C]PADA is a substrate for PgP/Bcrp efflux pump in the murine brain.
Conclusions We have efficiently synthesized a new piperidinyl azetidine MAGL radioligand, [11C]PADA, in good radiochemical yield, purity and specific activity. The brain penetration and efflux pump studies indicate [11C]PADA is not suitable as a neuroligand for MAGL. Potential uses as a radiotracer for imaging MAGL in the peripheral system are currently being evaluated.