[¹¹C]Paraoxon: Radiosynthesis, Biodistribution and In Vivo Positron Emission Tomography (PET) Imaging in Rat

Synthesis of the acetylcholinesterase inhibitor paraoxon (POX) as a carbon-11 positron emission tomography tracer ([¹¹C]POX) and profiling in live rats is reported. Naïve rats intravenously injected with [¹¹C]POX showed a rapid decrease in parent tracer to ~ 1% with an increase in radiolabeled serum proteins to 87%, and red blood cells (RBCs) to 9%. Protein and RBC leveled over 60 min reflecting covalent modification of proteins by [¹¹C]POX. Ex vivo biodistribution and imaging profiles in naïve rats had the highest radioactivity levels in lung followed by heart and kidney, and brain and liver the lowest. Brain radioactivity levels were low but observed immediately after injection, and persisted over the 60 min experiment. This showed for the first-time that even low POX exposures (~200 ng of tracer) can rapidly enter brain. Rats given an LD₅₀ dose of non-radioactive paraoxon at the LD₅₀ 20 or 60 min prior to [¹¹C]POX tracer revealed protein pools were blocked. Blood radioactivity at 20 min were markedly lower than naive levels due to rapid protein modification by non-radioactive POX, however by 60 min, the blood radioactivity returned to near naïve levels. Live rat tissue imaging-derived radioactivity values were 10-37% of naïve levels in non-radioactive POX pretreated rats at 20 min, but by 60 min the AUC values had recovered to 25-80% of naive. The live rat imaging supported blockade by non-radioactive POX pretreatment at 20 min and recovery of proteins by 60-min. Significance Statement Paraoxon (POX) is an organophosphorus (OP) compound and a powerful prototype and substitute for OP chemical warfare agents (CWAs) such as sarin, VX etc. To study the distribution and penetration of POX into the CNS and other tissues, a positron emission tomography (PET) tracer analog, [¹¹C]POX, was prepared. Blood and tissue radioactivity levels in live rats demonstrated immediate penetration into the CNS and persistent radioactivity levels in tissues indicative of covalent target modification.