Synthesis And Biological Evaluation Of [¹¹C] MP4A, A Tracer For Brain Acetylcholinesterase Activity, And Its Application In PET Neuroimaging

Partâ… The Preparation of[¹¹C]MP4A,a PET tracer for Brain Acetylcholinesterase ActivityAim:To synthesize 4-piperidyl acetate and label it as N-[¹¹C]methyl-4-piperidyl acetate([¹¹C]MP4A),a PET tracer for measuring acetylcholinesterase(AchE) activity in brain.Method:After the synthesis of P4A.[¹¹C]methyl iodide was synthesized by using automatic synthesis module for[¹¹C]methyl iodide,then reacted directly with P4A or indirectly with P4A after converting to[¹¹C]—Triflate—CH₃ to develop [¹¹C]MP4A.Results:The purity of P4A was higher than 99%,and the radiochemical purity of[¹¹C]MP4A was more than 98%by means of[¹¹C]—Triflate—CH3.The synthesis time after EOB was 20min,and the decay-corrected yield ranges from 40%to 45%.Conclusion:The chemical purity and the yields of[¹¹C]MP4A prepared by using P4A was high enough to be applied in the further study.Partâ…¡The distribution of[¹¹C]MP4A in ratAim:To investigate the uptake and distribution of[¹¹C]MP4A in rat brain by using PET imaging.Method:[¹¹C]MP4A were administered intravenously in SD rats.At the time point of 5,15,25,35,45,60min after injection,rats were decapitated and the blood,cerebral cortex, cerebellum and brainstem were collected.The samples were weighted and the radioactivity was measured and expressed as%ID/g.Results:In blood, there was a rapid and high uptake of[¹¹C]MP4A after injection,and a rapid elimination of radioactivity as well.The radioactivity was(13.44± 1.88)%ID/g at 5 min and almost returned to baseline at the time point of 60min after injection.The uptakes in all organs including brain almost remained constant at the time points between 15 min and 25 min.The average radioactivity in kidney was the highest among all the organs.The radioactivity in cerebral cortex was(2.26±0.29)%ID/g at 5min after injection of[¹¹C]MP4A.In Brain,the average radio-uptake in cortex was higher than that in cerebellum.The radioactivity ratio of cortex/cerebellum was 1.13-2.08.At 60min after injection,the radioactivity in cerebellum,brainstem and cerebral cortex significantly decreased,with the reduction being 77%,80%and 40%respectively compared with the highest radioactivity in corresponding regions.Conclusion:The uptake of[¹¹C]MP4A in blood is rapid,and reachs the highest level immediately after the injection.The uptake of subregions in rat brain maintains constant within 15-25 minutes.These results indicate that[¹¹C]MP4A is liposoluble,which make it easy cross blood-brain barrier.The uptake of[¹¹C]MP4A in cortex is higher and its elimination rate is relatively slower compared with other subregions in rat brain.Thus,the[¹¹C]MP4A uptake is able to represent the activity of AchE in cortex,suggesting that[¹¹C]MP4A can be served as a powerful biomarker for in vivo measurement of AchE activity in brain in the further study.Partâ…¢Pre-clinical evaluation of[¹¹C]MP4A with PET:In vivo imaging of acetylcholinesterase in the brains of transgenic mice,aged mice and rhesus monkeyAim:To in vivo show the activity of acetylcholinesterase in the brain of transgenic mice,aged mice and adult rhesus monkey by using[¹¹C]MP4A and PET.Method:Mice were anesthetized after i.v.injection of anesthetic in veins on tail,while the rhesus monkey being anesthetized through ulnar vein at one side.[¹¹C]MP4A was then injected through ulnar vein at another side in the monkey or the veins on tail in the mice.Those experimental animals were kept at a fixed position and sequential PET/CT scans were performed.Radioactivity in regions of interest(ROI) were measured and expressed as a percentage of that in whole body(%ID).The differences of%ID in ROIs were compared between groups.After PET scans,mice were decapitated immediately,and cerebrum,cerebellum and brainstem were collected.Each sample was weighted after radioactivity was measured.The radioactivity was expressed as%ID/g,and the distribution of radioactivity in brains of transgenic mice and aged mice was compared.Results:PET imaging show that the radioligand accumulated symmetrically in the brains of both mice groups after the injection of[¹¹C]MP4A.There was significant decrease of[¹¹C]MP4A in the brains of transgenic mice compared with that in aged mice.The average%ID/g values of cerebra in transgenic mice and aged mice were 1.49 and 2.62 respectively(p=0.025).[¹¹C]MP4A PET imaging in the brain of rhesus monkey clearly showed that radioactive uptakes in brain were symmetrically distributed,mostly in subcortex and moderately in cortex.Conclusions:[¹¹C]MP4A PET imaging shows good quality in the brain of both mice and monkey.The radioactivity gathers more in subcortex than that in cortex,with the radio distributions being similar as the histochemistry distribution of AchE,indicating that [¹¹C]MP4A is able to be used for the in vivo evaluatation of the acetylcholinesterase activity in brain by PET.