The R-(+)isomer of O-n-hexyl S-methyl phosphorothioamidate causes delayed neuropathy in hens after generation of a form of inhibited neuropathy target esterase (NTE) which can be reactivated ex vivo

An alternative in vitro method for detecting neuropathic compounds based on acetylcholinesterase inhibition and on inhibition and aging of neuropathy target esterase (NTE)

Organophosphorus-induced delayed polyneuropathy (OPIDP) is a syndrome induced by certain organophosphorus compounds (OPs) through a mechanism based on the inhibition and further modification (aging) of neuropathy target esterase (NTE). OECD guidelines for testing the capability of OPs to trigger OPIDP include two in vivo tests with hens. Activities of acetylcholinesterase and NTE found in SH-SY5Y human neuroblastoma cells were inhibited by 10 different OPs with kinetics similar to those found with chicken brain enzymes (model system for in vivo and in vitro-ex vivo assays). NTE in SH-SY5Y cells inhibited by these OPs aged and reactivated similarly to that described for hen brain NTE ex vivo. In short, we have developed an alternative methodology for predicting the capability of OPs to induce OPIDP based on the inhibition kinetics of acetylcholinesterase and NTE and on the capability of OPs to age the inhibited NTE from SH-SY5Y cell line. The results obtained always agreed with the previously reported ex vivo results with hen brain. The developed methodology correctly predicted the neuropathic potential of the tested OPs in eight cases. The in vivo-in vitro discrepancies with two of the tested compounds can be explained on the basis of differences between in vivo and in vitro biotransformation.

Screening for peripheral neuropathy in chemical warfare victims

We aimed to assess the prevalence of peripheral neuropathy in chemical warfare victims. We speculated that peripheral neuropathy is a late complication of exposure to chemical warfare agents. Late complications of exposure to chemical warfare agents are not well known and are poorly discussed in the existing literature. Scientific data regarding delayed complications are sparse, but this warrants recognition, especially when the clinician has to treat chemical warfare victims. The hazards of organophosphate pesticides and several toxins, although recognized to some extent, are, however, different from the hazards of chemical warfare agents which are far more serious. In this study, 100 chemical warfare patients, with varying degrees of exposure and an average age of 37.2+/-9.0 years, were examined clinically and studied electrodiagnostically from January 2002 to January 2003. Five of these patients proved to be suffering from axonal neuropathy. This rate was significantly higher than that found in the normal population. Our data indicate that chemical warfare agents may cause peripheral neuropathy in chemical warfare victims. In conclusion, organophosphorous agents used against Iranian troops during the war on Iran correlate with delayed neuropathy in these victims.

Stereospecific hydrolysis of a phosphoramidate as a model to understand the role of biotransformation in the neurotoxicity of chiral organophosphorus compounds

Calcium-dependent and EDTA-resistant hydrolyses of R and S isomers of O-hexyl O-2,5-dicholorophenyl phosphoramidate (HDCP) were observed in serum and subcellular fractions of liver, kidney and brain from hen, rat and rabbit. In serum, the Ca(2+)-dependent hydrolysis was much higher in rabbit than in other species. Liver showed a higher activity than kidney and brain. The S-HDCP isomer was hydrolysed to a higher extent than the other isomer. The fact that this stereospecificity favours the S-isomer is more clearly observed in rabbit serum, and in rat and rabbit liver particulate fractions. In such tissues and species, the EDTA-resistant hydrolysis was not stereospecific. Soluble fractions of rat brain and of hen liver, kidney and brain, showed a lower total activity but with a higher proportion of EDTA-resistant activity and a higher hydrolysis of the R-HDCP isomer. The Ca(2+)-dependent stereoselective biodegradation of S-HDCP is dominant in the most active tissues in rabbit and rat. It can therefore be concluded that S-HDCP would be biodegraded faster than R-HDCP. Furthermore, R-HDCP is the isomer that will remain at a higher proportion to be available for interaction with the target of neurotoxicity.

Inhibition and aging of neuropathy target esterase by the stereoisomers of a phosphoramidate related to methamidophos

Discrepancies in the aging reaction between neuropathy target esterase (NTE) inhibited in vitro and in vivo by racemic mixtures of O-alkyl O-2,5-dichlorophenyl phosphoramidates have been observed. It suggested the existence of differences in the interactions (inhibition and aging) between NTE and each stereoisomers of the above mentioned compounds. In order to verify this hypothesis, stereoisomers of O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) were isolated by chiral column chromatography, followed by the evaluation of NTE inhibition and aging for each stereoisomers. The loss of reactivation capacity by KF was used as criterion of aging. The stereoisomer S-(-)-HDCP inhibited hen brain NTE with an I50 of 7.6 nM for 30 min of incubation, this being similar to the value obtained for the racemic mixture (I50 = 6.2 nM), and much lower than that recorded for R-(+)-HDCP (I50 = 191 nM). NTE inhibited by HDCP racemic mixture and the stereoisomer S-(-)-HDCP was reactivated by KF after 20 h of incubation at 37 degrees C. The NTE inhibited by R-(+)-HDCP could not be fully reactivated after inhibition.

Assessment of the neurotoxic potential of chlorpyrifos relative to other organophosphorus compounds: a critical review of the literature

Chlorpyrifos (diethyl 3,5,6-trichloro-2-pyridyl phosphorothionate) is a broad-spectrum organophosphorus (OP) insecticide. Anticipated increases in the already extensive use of this compound have prompted this reassessment of its neurotoxicity. Because chlorpyrifos and other OP insecticides are designed to produce acute cholinergic effects through inhibition of acetylcholinesterase (AChE) and some OP compounds can cause OP compound-induced delayed neurotoxicity (OPIDN) via chemical modification of neurotoxic esterase (neuropathy target esterase, NTE), this review focuses on the capacity of chlorpyrifos to precipitate these and other adverse neurological consequences. Chlorpyrifos exhibits only moderate acute toxicity in many mammalian species, due largely to detoxification of the active metabolite, chlorpyrifos oxon, by A-esterases. Rats given large doses of chlorpyrifos (sc in oil) have prolonged inhibition of brain AChE, possibly due to slow release of the parent compound from a depot. Associated cognitive and motor deficits return to normal well before recovery of AChE activity and muscarinic receptor down-regulation, as expected from classic tolerance. Controlled studies of OP compound exposures in humans also indicate that cognitive dysfunction requires substantial AChE inhibition. Information is relatively sparse on neurological dysfunction that is secondary to theoretical reproductive, developmental, or immunological effects, but the best available data indicate that such effects are unlikely to result from exposures to chlorpyrifos. In accord with the much greater inhibitory potency of chlorpyrifos oxon for AChE than for NTE, clinical reports and experimental studies indicate that OPIDN from acute exposures to chlorpyrifos requires doses well in excess of the LD50, even when followed by repeated doses of the OPIDN potentiator phenylmethanesulfonyl fluoride (PMSF). Likewise, studies in hens show that subchronic exposures at the maximum tolerated daily dose do not result in OPIDN. Although exposure to chlorpyrifos as a result of normal use is unlikely to produce classical OPIDN, a recent report stated that mild reversible sensory neuropathy had occurred in eight patients who had been exposed subchronically to unknown amounts of chlorpyrifos. It is not clear whether these cases represent an incorrect linkage of cause and effect, a newly disclosed reversible sensory component of OPIDN, or an entirely new phenomenon. The question of the potential for chlorpyrifos to cause this mild sensory neuropathy could be resolved by the use of quantitative tests of sensory function in animal experiments and/or prospective studies of humans with known exposures to chlorpyrifos.

Stereo-specific degradation of the R-(+) isomer of O-n-hexyl-S-methylphosphorothioamidate catalysed by rabbit serum

Resolved isomers of O-n-hexyl-S-methylphosphorothioamidate (HXM) which had been synthesised by separate stereospecific routes were analysed by chiral glc: about 2-3% of R-(+) isomer was found in the S-(-) sample and accounted for nearly all the inhibitory power against neuropathy target esterase. Incubation of racemic HXM with rabbit serum led to slow but very specific disposal of R-(+) isomer to undetectable levels with very slight loss of S-(-): the rate of disposal was roughly estimated to be about 1% of the published rate of hydrolysis of paraoxon. Incubation with crystalline chymotrypsin caused a preferential but not totally selective disposal of S-(-) isomer.

Symposium introduction: retrospect and prospects for neuropathy target esterase (NTE) and the delayed polyneuropathy (OPIDP) induced by some organophosphorus esters

This article introduces a Symposium devoted to Neuropathy Target Esterase (NTE). The characteristics of the disorder known as OPIDP are described and the steps by which NTE was identified as the target are summarised. Studies with many organophosphates, phosphinates and chiral phosphonates are entirely consistent with a 2-step process of initiation referred to as 'NTE (70-80%) aging': about 70-80% of available nervous system NTE is first covalently phosphylated causing inhibition of esterase activity, and then the molecules of inhibited NTE undergo a covalent bond-cleavage leaving a negative charge in the region of the still-bound phosphorus. This understanding has clarified structure/activity studies of neuropathic potential of OP esters and is now routinely applied in toxicological evaluations for regulatory purposes. However, the biological function of NTE has remained a mystery. Prospective views of the role of NTE are presented by different authors. Attempts to isolate catalytically active or radiolabelled inhibited NTE are near to success. Since the Symposium, complete isolation of NTE affinity-purified from hen brain has been reported (see M.K. Johnson & P. Glynn, Toxicologist, 13 (1993) 211, Abstr. 773). Some minor, but possibly significant, differences in properties of a soluble and a membrane-bound form of NTE in sciatic nerve has been identified. The nature of the disturbance brought about by covalent binding of organophosphoramidates at the active site of NTE and the discovery that 'non-aging' inhibitors of NTE can promote neuropathy in hens given a sub-neuropathic dose of neuropathic OPs has led to a concept of NTE inhibitors having a range of 'partial agonist' effects at the covalent binding site. Evidence is emerging that the promotion target may be 'cousin-of NTE' with very similar inhibition characteristics and a function in the processes of response to or repair of axonal damage.