In vivovisualization of intracellular pH gradient in brain using PET imaging

Intracellular pH (pHi) is a valuable index for predicting hypoxic brain damage. However, no positron emission tomography (PET) probe is currently available for monitoring pHi in vivo . In this study, we developed a new approach for visualizing monoacylglycerol lipase (MAGL) activity in the brain. This approach used PET with a new probe [ 11 C]HC-A, an azetidine carbamate inhibitor, whose uptake and residence depended on the pHi gradient was evaluated with in silico , in vitro , and in vivo assessments. Molecular dynamics simulations predicted that complex (complex-A) between HC-A and MAGL would be difficult to hydrolyze under acidic conditions. In vitro assessment using rat brain homogenate showed that [ 11 C]HC-A reacted with MAGL to yield [ 11 C]complex-A, which was rapidly hydrolyzed to liberate 11 CO 2 . The 11 CO 2 liberation rate was slower at lower pH. In PET with [ 11 C]HC-A using ischemic rats, the radioactivity clearance rate, which reflects the production rate of 11 CO 2 in the brain, was lower in a remarkably hypoxic area than in the contralateral region. In conclusion, we successfully visualized the pHi gradient in the brain using PET imaging.