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

Abstract

Synthesis of the acetylcholinesterase inhibitor paraoxon (POX) as a carbon-11 positron emission tomography tracer ([ 11 C]POX) and profiling in live rats is reported. Naïve rats intravenously injected with [ 11 C]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 minutes, reflecting covalent modification of proteins by [ 11 C]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-minute experiment. This showed for the first time that even low POX exposures (∼200 ng tracer) can rapidly enter brain. Rats given an LD 50 dose of nonradioactive paraoxon at the LD 50 20 or 60 minutes prior to [ 11 C]POX tracer revealed that protein pools were blocked. Blood radioactivity at 20 minutes was markedly lower than naïve levels due to rapid protein modification by nonradioactive POX; however, by 60 minutes the blood radioactivity returned to near naïve levels. Live rat tissue imaging-derived radioactivity values were 10%–37% of naïve levels in nonradioactive POX pretreated rats at 20 minutes, but by 60 minutes the area under the curve (AUC) values had recovered to 25%–80% of naïve. The live rat imaging supported blockade by nonradioactive POX pretreatment at 20 minutes and recovery of proteins by 60 minutes. 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 central nervous system (CNS) and other tissues, a positron emission tomography (PET) tracer analog, carbon-11–labeled paraoxon ([ 11 C]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.

Progress and Challenges in Developing Medical Countermeasures for Chemical, Biological, Radiological, and Nuclear Threat Agents

“…These outcomes are consistent with hallmark features of chronic epilepsy, such as spontaneous recurrent seizures, hippocampal sclerosis, and mossy fiber sprouting. Thompson and colleagues prepared and demonstrated the brain distribution of paraoxon as a carbon-11 positron emission tomography tracer ([11C] paraoxon), indicating rapid entry into the brain ( Hayes et al, 2024 ). This tracer could be a valuable diagnostic tool for unraveling OP neurotoxicity.…”

Special Section on Medical Countermeasures, From Bench to Battlefield: Translating Experimental Therapies for Effective Combat Against Chemical Threats–Editorial

“…A comparative study reveals chronic epileptic seizures and pathology following exposure to different OP compounds, highlighting long-term consequences (Blair et al, 2024). [ 11 C]POX was introduced as a diagnostic tool for OP neurotoxicity via positron emission tomography (Hayes et al, 2024). Enhanced efficacy of lacosamide and rufinamide was observed when combined with ketamine and midazolam for controlling soman-induced seizures (Lumley et al, 2024).…”