Brain and whole-body imaging in nonhuman primates with [11C]MeS-IMPY, a candidate radioligand for β-amyloid plaques

[11C]MeS-IMPY ([S-methyl-11C]N,N-dimethyl-4-(6-(methylthio)imidazo[1,2-a]pyridine-2-yl)aniline) is a potential radioligand for imaging β-amyloid plaques with positron emission tomography (PET). The aims of this study were to evaluate [11C]MeS-IMPY uptake in nonhuman primate brain and to estimate radiation exposure from serial whole-body images. Eight PET studies were performed in rhesus monkeys to measure the brain uptake and washout of [11C]MeS-IMPY. Time-activity data were analyzed with one-tissue and two-tissue compartmental models using radiometabolite-corrected plasma input function. In addition, two whole-body PET scans were acquired for 120 min to determine the biodistribution of [11C]MeS-IMPY. Tomographic PET images were compressed into a single planar image to identify organs with the highest radiation exposures. Estimates of the absorbed dose of radiation were calculated using OLINDA 1.0. Injection of [11C]MeS-IMPY caused little change in pulse rate, blood pressure, respiratory rate and temperature. [11C]MeS-IMPY showed high standardized brain uptake values of ∼500% and 600% between 2 and 3 min in cortical regions and the cerebellum, respectively. The brain uptake of [11C]MeS-IMPY was widespread and quite uniform across all cortical regions. Activity rapidly washed out of the brain, with 20% of peak activity remaining at 40 min. Thus, all brain regions showed minimal retention of radioactivity, consistent with these healthy young animals having negligible amyloid plaques. Regional brain activity fitted well into a one-tissue compartment model. The average volume of distribution in all brain regions was 7.66±2.14 ml/cm3 (n=4). The organs with the highest radiation exposure (μSv/MBq) were the gallbladder wall (33.4), urinary bladder (17) and lungs (12.9), with a resulting effective dose of 4.9 μSv/MBq (18 mrem/mCi). The high brain uptake, rapid washout and quantifiable volume of distribution in nonhuman primate brain further support the evaluation of [11C]MeS-IMPY. Calculated dosimetry results are comparable with those for other 11C-labeled brain imaging radioligands.