Organophosphate pesticides (OPPs; e.g. chlorpyrifos, diazinon, paraoxon) are a wide and heterogeneous group of organophosphorus compounds. Their biological activity of inhibiting acetylcholinesterase (AChE) or butyrylcholinesterase (BChE) ranks them as life endangering agents. The necessary treatment after OPP exposure involves the use of parasympatolytics (e.g. atropine), oxime reactivators (e.g. obidoxime), and anticonvulsive drugs (e.g. diazepam). Therefore, the reactivators of AChE are essential compounds in the treatment of OPP intoxications. Commercial AChE reactivators (e.g. pralidoxime, HI-6, obidoxime, trimedoxime, methoxime) were originally developed for other members of the organophosphate family, such as nerve agents (e.g. sarin, soman, tabun, VX). Pralidoxime, HI-6, and methoxime were found to be weak reactivators of OPP-inhibited AChE. Obidoxime and trimedoxime showed satisfactory reactivation against various OPPs with minor toxicity issues. During the last two decades, the treatment of OPP exposure has become more widely discussed because of growing agricultural production, industrialization, and harmful social issues (e.g. suicides). In this review is the summarized design, evaluation, and structure-activity relationship studies of recently produced AChE reactivators. Since pralidoxime, over 300 oximes have been produced or tested against OPP poisoning, and several novel compounds show very promising abilities as comparable (or higher) to commercial oximes. Some of these are highlighted for their further testing of OPP exposure and, additionally, the main structure-activity relationship of AChE reactivators against OPP is discussed.

Design, evaluation and structure-activity relationship studies of the AChE reactivators against organophosphorus pesticides.

Limitation of the Ellman method: cholinesterase activity measurement in the presence of oximes.

Organophosphates are one of the most common causes of poisoning, especially in the Third world, with high morbidity and mortality. The treatment of this type of poisoning involves the use of atropine and oximes. Atropine has been used successfully in large doses to counteract the muscarinic effects of organophosphate poisoning, but the efficacy of oximes in the management of this poisoning remains under question. In this study, we undertook a meta-analysis by reviewing all clinical trials to evaluate the efficacy of oximes in the management of organophosphate poisoning. The databases of PUBMED, EMBASE, Cochrane, SCOPUS, and the search engine of Google were searched for all clinical trials on the use of oximes in organophosphate poisoning. The inclusion criteria were death, development of intermediate syndrome, and need for ventilation. Six clinical trials met the inclusion criteria and were included in the metaanalysis. The chi2 tests for heterogeneity (P = 0.25, 0.16, and 0.33, respectively) indicated that the included studies were not significantly heterogeneous and could be combined. A significant relative risk (P = 0.0017) for death among oxime-exposed was 2.17 (95% CI of 1.34-3.51). The 'need for ventilation' in patients who received oxime was higher (P = 0.03) than those who did not receive oxime with a relative risk of 1.53 (1.16-2.02). The incidence of 'intermediate syndrome' for oxime-exposed patients was significantly higher (P = 0.01) than oxime non-exposed patients with a relative risk of 1.57 (95% CI 1.11-2.11). It can be concluded that oximes are not effective in the management of organophosphate-poisoned patients and, surprisingly, they can be dangerous and worsen the patient's clinical situation.

Increased morbidity and mortality in acute human organophosphate-poisoned patients treated by oximes: a meta-analysis of clinical trials.

Oximes are cholinesterase reactivators of use in poisoning with organophosphorus compounds. Pralidoxime (PRX) is used clinically as an adjunct to atropine in such exposure. Clinical experience with PRX (and other oximes) is, however, disappointing and routine use has been questioned. In addition it is known that oximes are not equally effective against all existing organophosphorus compounds. There is a clear demand for ‘broad spectrum’ cholinesterase reactivators with a higher efficacy than PRX.



Over the years new reactivators of cholinesterase of potential clinical utility have been developed. Their chemical structures were derived from those of existing esterase reactivators, especially pralidoxime, obidoxime and HI-6.



The purpose of the study was to quantify in vitro the extent of oxime (pralidoxime, K-27, K-33, K-48, methoxime and BI-6) conferred protection, using paraoxon as an inhibitor. Paraoxon (POX), the active metabolite of parathion (O,O-diethyl-O-p-nitro-phenyl phosphorothioate) is a non-neuropathic organophosphate.



Red blood cell (RBC) acetylcholinesterase (AChE) activities in whole blood were measured photometrically in the presence of different POX concentrations and the IC50 was calculated. Determinations were repeated in the presence of increasing oxime concentrations. The IC50 of POX increases with the oxime concentration in a linear manner. The calculated IC50 values were plotted against the oxime concentrations to obtain an IC50 shift curve. The slope of the shift curve (tg α) was used to quantify the magnitude of the protective effect (nm IC50 increase per µm reactivator). Based on our determinations the new K series of reactivators is far superior to pralidoxime, methoxime and BI-6, K-27 being the outstanding compound with a tg α value of 3.7 (nm IC50 increase per µm reactivator) which is ≈ 13 times the reactivator ability of PRX.



In general there is an (expected) inverse relationship between the binding constant K and the slope of the IC50 shift curve (tg α) for all oximes examined. K-27 (the most protective substance judging by the tg α) has the lowest K value (highest affinity).



In vivo testing of the new oximes as an organophosphate protective agent is necessary. Copyright © 2005 John Wiley & Sons, Ltd.

Five oximes (K-27, K-33, K-48, BI-6 and methoxime) in comparison with pralidoxime: in vitro reactivation of red blood cell acetylcholinesterase inhibitied by paraoxon

Swine models in the design of more effective medical countermeasures against organophosphorus poisoning.

Organophosphate compounds are responsible for a large number of accidental and/or suicidal exposures and have been used also for warfare and terrorism. The mechanism of toxicity is by inhibition of cholinesterase. Oximes are the only enzyme reactivators clinically available but clinical experience with oximes is disappointing. There is a gap between laboratory data and clinical impression concerning the efficacy of oxime compounds. Oximes are responsible for thiocholinesteratic activity, a spurious signal caused by interaction between pralidoxime and the thiocholine substrate used for photometric enzyme activity determinations. In a prospective, controlled, non-randomized study performed in anaesthetized miniature pigs, we quantified the extent of pralidoxime-induced cholinesteratic pseudo-activity ex vivo (human blood) and in vivo (minipig) in order to be able to correct values obtained by photometric methods. Plasma cholinesteratic activity using two substrates (acetylthiocholine and butyrylthiocholine) was determined in vitro and in vivo in the presence of pralidoxime. Pralidoxime reacts with the substrate (acetyl- and butyrylthiocholine) used for enzyme activity determinations, producing a spurious signal implying cholinesterase activity (even in the absence of plasma and thus of any enzyme). Cholinesterase activities determined photometrically after pralidoxime therapy can be erroneously high. Although in theory this could mislead clinicians into assuming an efficacious therapy, this is unlikely to occur in vivo under normal pralidoxime dosing conditions. To avoid any ambiguity it is recommended that blood be drawn for enzyme activity determinations prior to reactivator use and no less than 1 h after its administration.

Enzyme reactivator treatment in organophosphate exposure: clinical relevance of thiocholinesteratic activity of pralidoxime.

In vitroprotection of plasma cholinesterases by metoclopramide from inhibition by paraoxon: METOCLOPRAMIDE IN ORGANOPHOSPHATE (PARAOXON) POISONING

Intoxications with organophosphorus compounds such as paraoxon (POX) are frequent. Oximes are the only enzyme reactivators clinically available. In vitro and in vivo studies have shown that l-lactate reduces the inhibition of plasma acetylcholine-esteratic activity (AChEA) (in vitro and in vivo) and plasma butyrylcholine-esteratic activity (BChEA) (at least in vitro and possibly in vivo) by POX. However, a short infusion of 10 g of lactate was unable to elevate the plasma lactate level for >3 h. In this study we tested a substance related to l-lactate, i.e. pyruvic acid. The purpose of this animal experimental study (female minipigs with historical control group) was to determine in vivo whether intravenous (i.v.) pyruvic acid application under normoxic/normocapnic/normohydrogenaemic conditions is able to elevate blood lactate levels and whether it is able to protect AChEA and BChEA from POX inhibition.



Animals were anaesthetized, intubated and mechanically ventilated. Each received 1 mg kg−1 body wt. of POX in 50 ml of saline over 50 min and 10 g (ca. 0.5 g kg−1 body wt.) of i.v. pyruvic acid in 50 ml of saline over 50 min. They were compared with a historical control group of six animals that received only 1 mg kg−1 body wt. of POX in 50 ml of saline over 50 min. In central venous blood measurements of plasma AChEA and BChEA, the measurements were performed before (baseline), immediately after POX (50 min after start) and 110, 170, 230, 290, 530 and 1010 min after the start of infusion. A 10 g aliquot of i.v. pyruvic acid had a statistically significant protective effect in vivo on AChEA but not on BChE activity. Further study of the in vivo effects of pyruvic acid and l-lactate after paraoxon intoxication and a formal comparison with standard oxime therapy seems warranted. Also, a combination therapy with l-lactate and pyruvic acid in vivo should be investigated. Copyright © 2003 John Wiley & Sons, Ltd.

Intravenous pyruvic acid application in minipigs partially protects acetylcholine-esteratic but not butyrylcholine-esteratic activity in plasma from inhibition by paraoxon.

Vanessa Ewald

Papers

Enzyme reactivator treatment in organophosphate exposure: clinical relevance of thiocholinesteratic activity of pralidoxime.

In vitroprotection of plasma cholinesterases by metoclopramide from inhibition by paraoxon: METOCLOPRAMIDE IN ORGANOPHOSPHATE (PARAOXON) POISONING

Intravenous pyruvic acid application in minipigs partially protects acetylcholine-esteratic but not butyrylcholine-esteratic activity in plasma from inhibition by paraoxon.