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The highly toxic nerve agent sarin (o-isopropyl methyl-phosphonofluoridate, GB) has been used in several armed conflicts and terror attacks in recent decades. Due to its inherent high sensitivity, liquid chromatography-mass spectrometry (LC-MS/MS) has the potential to detect ultratrace levels of fluoride-regenerated G and V agents after appropriate chemical derivatization. A new method for the retrospective determination of exposure to sarin was developed. The method is based on sarin regeneration from blood using the fluoride-induced technique followed by derivatization with 2-[(dimethylamino)methyl]phenol (2-DMAMP) and LC-ESI-MS/MS (MRM) analysis. The validated method presents good linear response in the concentration range of 5-1000 pg/mL with a limit of quantitation (LOQ) of 5 pg/mL, 13.8% accuracy, 16.7% precision and a total recovery of 62% ± 9%. This new analytical approach has several advantages over existing GC/GC-MS-based methods in terms of sensitivity, specificity and simplicity, in addition to a short LC-MS cycle time of 12 min. The method was successfully applied in an in vivo experiment for retrospective determination of sarin in a rabbit exposed to 0.1 LD 50 sarin (1.5 µg/kg, i.v.). GB-2-DMAMP was easily determined in samples drawn up to 11 days after exposure. The high S/N ratio (500) observed for the GB-2-DMAMP signal in the 11day sample poses the potential for an extended time frame of months for analysis with this new method for the retrospective detection of sarin exposure. To the best of our knowledge, this is the first report on LC-MS/MS trace analysis of regenerated GB from biological matrices.

The hydroxyl oxygen of the catalytic triad serine in the active center of serine hydrolase acetylcholinesterase (AChE) attacks organophosphorus compounds (OPs) at the phosphorus atom to displace the primary leaving group and to form a covalent bond. Inhibited AChE can be reactivated by cleavage of the Ser-phosphorus bond either spontaneously or through a reaction with nucleophilic agents, such as oximes. At the same time, the inhibited AChE adduct can lose part of the molecule by progressive dealkylation over time in a process called aging. Reactivation of the aged enzyme has not yet been demonstrated. Here, our goal was to study oxime reactivation and aging reactions of human AChE inhibited by mipafox or a sarin analog (Flu-MPs, fluorescent methylphosphonate). Progressive reactivation was observed after Flu-MPs inhibition using oxime 2-PAM. However, no reactivation was observed after mipafox inhibition with 2-PAM or the more potent oximes used. A peptide fingerprinted mass spectrometry (MS) method, which clearly distinguished the peptide with the active serine (active center peptide, ACP) of the human AChE adducted with OPs, was developed by MALDI-TOF and MALDI-TOF/TOF. The ACP was detected with a diethyl-phosphorylated adduct after paraoxon inhibition, and with an isopropylmethyl-phosphonylated and a methyl-phosphonylated adduct after Flu-MPs inhibition and subsequent aging. Nevertheless, nonaged nonreactivated complexes were seen after mipafox inhibition and incubation with oximes, where MS data showed an ACP with an NN diisopropyl phosphoryl adduct. The kinetic experiments showed no reactivation of activity. The computational molecular model analysis of the mipafox-inhibited hAChE plots of energy versus distance between the atoms separated by dealkylation showed a high energy demand, thus little aging probability. However, with Flu-MPs and DFP, where aging was observed in our MS data and in previously published crystal structures, the energy demand calculated in modeling was lower and, consequently, aging appeared as a more likely reaction. We document here direct evidence for a phosphorylated hAChE refractory to oxime reactivation, although we observed no aging.

The direct reaction of seven pyridinium oximes with the organophosphorus compounds (OPCs) crotylsarin, cyclosarin, and VX was studied by spectrophotometry. This method allows to quantify different parameters: (a) the half-life times (t (1/2)) of the oxime-OPC reactions on the basis of the changes in the absorption at the zwitterion (betaine) peak maximum, (b) the first- and second-order rate constants (k (1), k (2)), and (c) the maximum reaction velocities (v (max)). The results of the study show that the reaction velocity of the nerve agents with any of the oximes investigated decreased in the order crotylsarin > cyclosarin > VX. The comparison of the reaction rates of the three therapeutically used oximes (2-PAM, obidoxime, HI 6) with the respective OPC gave the highest rate for crotylsarin and cyclosarin with obidoxime and to a similar degree with HI 6, while in the case of VX the most reactive oxime was HI 6. The reaction velocity of the nerve agents with the monopyridinium oxime 2-PAM was lower as compared to the bispyridinium oximes (obidoxime, HI 6). The results obtained with the two sarin analogues indicate that the direct reaction with 2-PAM, obidoxime, or HI 6 could be used for non-corrosive decontamination purposes, especially, if sensitive biological surfaces like skin, mucous membranes, or wounds are considered. However, in view of the concentrations of nerve agents and oximes, which could be expected during OPC poisoning in man, the maximum reaction velocities would not be high enough to contribute markedly to the detoxication of nerve agents in vivo.

The direct reaction of seven pyridinium oximes with the nerve agents sarin, soman, and tabun was followed by a spectrophotometric method. The half-lives (t1/2) of the oximes, the first- and second-order rate constants (k1, k2), and the maximal reaction velocity (vmax) were calculated according to changes in the absorbance of the zwitterion (betaine) peak. In all cases the reaction velocity of the nerve agents with any of the oximes was highest with tabun, followed by sarin and then soman. Comparing the reaction rates of three therapeutically used oximes with the same nerve agent, the highest rate was obtained for soman with obidoxime, for sarin with 2-PAM, and for tabun with HI 6. The maximal reaction velocities reveal that the detoxification of the nerve agents by direct reaction with oximes and the subsequent decomposition of the phosphonyl oxime in vivo do not substantially contribute to the therapeutic effect of these antidotes.

The interaction of obidoxime (Toxogonin) with sarin was shown by different analytical methods. The UV spectrum of obidoxime at pH 7.4 yields two absorption maxima, lambda 1 = 284 nm and lambda 2 = 353 nm. The peak at lambda 2 = 353 nm is representative for the amount of zwitter-ionic obidoxime, i.e. the active form of obidoxime. By addition of sarin, lambda 1 shifts immediately to 278 nm and the intensity at lambda 2 decreases, thus indicating an interaction. TLC and 31P-NMR evidence shows that both mono-phosphonylated and diphosphonylated obidoximes are present. Decomposition of phosphonylated obidoxime in MOPS (3-[N-morpholino] propanesulfonic acid) buffered D2O at pH 7.4 occurs with t1/2 = 13.3 min at 24 degrees C. Decomposition of di-phosphonylated obidoxime is faster. It is suggested that decomposition of di-phosphonylated obidoxime occurs through the mono-phosphonylated form. Formation and decomposition of mono- and di-phosphonylated obidoxime is pH dependent. We conclude that obidoxime exerts a detoxifying effect by capturing free sarin molecules and thus increasing its polarity. Thereby the transition of sarin through the blood-brain barrier is restricted and its renal elimination facilitated.