Evaluation of 18F-labeled acetylcholinesterase substrates as PET radiotracers

Four 18F-labeled acetylcholinesterase (AChE) substrates, (S)-N-[18F]fluoroethyl-2-piperidinemethyl acetate (1), (R)-N-[18F]fluoroethyl-3-pyrrolidinyl acetate (2), N-[18F]fluoroethyl-4-piperidinyl acetate (3), and (R)-N-[18F]fluoroethyl-3-piperidinyl acetate (4), were evaluated for in vivo blood and brain metabolism in mice, brain pharmacokinetics in rats monkeys (M. nemistrina) using PET imaging. All 18F-labeled compounds were compared to N-[11C]methyl-4-piperidinyl propionate (PMP). Compound 1 was completely metabolized within 1 min in mouse blood and brain. This compound had relatively fast regional brain pharmacokinetics and poor discrimination between brain regions with different AChE concentration. Compound 4 showed relatively slower blood metabolism and slower pharmacokinetics than compound 1 but again poor discrimination between brain regions. Both compounds 1 and 4 showed different kinetic profiles than PMP in PET studies. Compound 3 had the slowest blood metabolism and slower pharmacokinetics than PMP. Compound 2 showed highly encouraging characteristics with an in vivo metabolism rate, primate brain uptake, and regional brain pharmacokinetics similar to [11C]PMP. The apparent hydrolysis rate constant k3 in primate cortex was very close to that of [11C]PMP. This compound has potential to be a good PET radiotracer for measuring brain AChE activity. The longer lifetime of 18F would permit longer imaging times and allows preparation of radiotracer batches for multiple patients and delivery of the tracer to other facilities, making the technique more widely available to clinical investigators.