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Oxime antidotes regenerate organophosphate‐inhibited acetylcholinesterase (AChE). Although they share a common mechanism of AChE reactivation, the rate and amount of oxime that enters the brain are critical to the efficacy, a process linked to the oxime structure and charge. Using a platform based on the organophosphate [18F]‐VXS as a positron emission tomography tracer for active AChE, the in vivo distribution of [18F]‐VXS was evaluated after an LD50 dose (250 μg/kg) of the organophosphate paraoxon (POX) and following oximes as antidotes. Rats given [18F]‐VXS tracer alone had significantly higher radioactivity (two‐ to threefold) in the heart and lung than rats given LD50 POX at 20 or 60 min prior to [18F]‐VXS. When rats were given LD50 POX followed by 2‐PAM (cationic), RS194b (ionizable), or monoisonitrosoacetone (MINA) (neutral), central nervous system (CNS) radioactivity returned to levels at or above untreated naive rats (no POX), whereas CNS radioactivity did not increase in rats given the dication oximes HI‐6 or MMB‐4. MINA showed a significant, pairwise increase in CNS brain radioactivity compared with POX‐treated rats. This new in vivo dynamic platform using [18F]‐VXS tracer measures and quantifies peripheral and CNS relative changes in AChE availability after POX exposure and is suitable for comparing oxime delivery and AChE reactivation in rats. [ABSTRACT FROM AUTHOR]

O‐(1‐Fluoropropan‐2‐yl)‐O‐(4‐nitrophenyl) methylphosphonate is a reactive organophosphate ester (OP) developed as a surrogate of the chemical warfare agent sarin that forms a similar covalent adduct at the active site serine of acetylcholinesterase. The radiolabeled O‐(1‐[18F]fluoropropan‐2‐yl)‐O‐(4‐nitrophenyl) methylphosphonate ([18F] fluorosarin surrogate) has not been previously prepared. In this paper, we report the first radiosynthesis of this tracer from the reaction of bis‐(4‐nitrophenyl) methylphosphonate with 1‐[18F]fluoro‐2‐propanol in the presence of DBU. The 1‐[18F]fluoro‐2‐propanol was prepared by reaction of propylene sulfite with Kryptofix 2.2.2 and [18F] fluoride ion. The desired tracer O‐(1‐[18F]fluoropropan‐2‐yl)‐O‐(4‐nitrophenyl) methylphosphonate was obtained in a >98% radiochemical purity with a 2.4% ± 0.6% yield (n = 5, 65 minutes from start of synthesis) based on starting [18F] fluoride ion and a molar activity of 49.9 GBq/μmol (1.349 ± 0.329 Ci/μmol, n = 3). This new facile radiosynthesis routinely affords sufficient quantities of [18F] fluorosarin surrogate in high radiochemical purity, which will further enable the tracer development as a novel radiolabeled OP acetylcholinesterase inhibitor for assessment of OP modes of action with PET imaging in vivo. Synthesis of a fluorine‐18‐labeled PET tracer surrogate of sarin to evaluate the exposure mechanism and reactivation of acetylcholinesterase. [ABSTRACT FROM AUTHOR]

O-(2-Fluoroethyl)-O-( p-nitrophenyl) methylphosphonate 1 is an organophosphate cholinesterase inhibitor that creates a phosphonyl-serine covalent adduct at the enzyme active site blocking cholinesterase activity in vivo. The corresponding radiolabeled O-(2-[18F]fluoroethyl)-O-(p-nitrophenyl) methylphosphonate, [ 18 F]1, has been previously prepared and found to be an excellent positron emission tomography imaging tracer for assessment of cholinesterases in live brain, peripheral tissues, and blood. However, the previously reported [ 18 F]1 tracer synthesis was slow even with microwave acceleration, required high-performance liquid chromatography separation of the tracer from impurities, and gave less optimal radiochemical yields. In this paper, we report a new synthetic approach to circumvent these shortcomings that is reliant on the facile reactivity of bis-(O,O-p-nitrophenyl) methylphosphonate, 2, with 2-fluoroethanol in the presence of DBU. The cold synthesis was successfully translated to provide a more robust radiosynthesis. Using this new strategy, the desired tracer, [ 18 F]1, was obtained in a non-decay-corrected radiochemical yield of 8 ± 2% (n = 7) in >99% radiochemical and >95% chemical purity with a specific activity of 3174 ± 345 Ci/mmol (EOS). This new facile radiosynthesis routinely affords highly pure quantities of [ 18 F]1, which will further enable tracer development of OP cholinesterase inhibitors and their evaluation in vivo. [ABSTRACT FROM AUTHOR]