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内容記述 |
Intracellular pH (pHi) is a valuable index for predicting neuronal damage and injury. However, no PET probe is currently available for monitoring pHi in vivo. In this study, we developed a new approach for visualizing the hydrolysis rate of monoacylglycerol lipase (MAGL), which is widely distributed in neurons and astrocytes throughout the brain. This approach uses PET with the new radioprobe [11C]QST-0837 (1,1,1,3,3,3-hexafluoropropan-2-yl-3-(1-phenyl-1Hpyrazol-3-yl)azetidine-1-[11C]carboxylate), a covalent inhibitor containing an azetidine carbamate skeleton for MAGL. The uptake and residence of this new radioprobe depends on the pHi gradient, and we evaluated this with in silico, in vitro, and in vivo assessments. Molecular dynamics simulations predicted that because the azetidine carbamate moiety is close to that of water molecules, the compound containing azetidine carbamate would be more easily hydrolyzed following binding to MAGL than would its analog containing a piperidine carbamate skeleton. Interestingly, it was difficult for MAGL to hydrolyze the azetidine carbamate compound under weak-acidic (pH 6) conditions because of a change in the interactions with water molecules on the carbamate moiety of their complex. Subsequently, an in vitro assessment using rat brain homogenate to confirm the MD simulation-predicted behavior of the azetidine carbamate compound showed that [11C]QST-0837 reacted with MAGL to yield an [11C]complex, which was hydrolyzed to liberate 11CO2 as a final product.
Additionally, the 11CO2 liberation rate was slower at lower pH. Finally, to indicate the feasibility of estimating how the hydrolysis rate depends on pHi in vivo, we performed a PET study with [11C]QST-0837 using ischemic rats. In our proposed in vivo compartment model, the clearance rate of radioactivity from the brain reflected the rate of [11C]QST-0837 hydrolysis (clearance through the production of 11CO2) in the brain, which was lower in a remarkably hypoxic area
than in the contralateral region. In conclusion, we indicated the potential for visualization of the pHi gradient in the brain using PET imaging, although some limitations remain. This approach should permit further elucidation of the pathological mechanisms involved under acidic conditions in multiple CNS disorders. |
