A-series agent A-234: initial in vitro and in vivo characterization.

A-series agent A-234 belongs to a new generation of nerve agents. The poisoning of a former Russian spy Sergei Skripal and his daughter in Salisbury, England, in March 2018 led to the inclusion of A-234 and other A-series agents into the Chemical Weapons Convention. Even though five years have already passed, there is still very little information on its chemical properties, biological activities, and treatment options with established antidotes. In this article, we first assessed A-234 stability in neutral pH for subsequent experiments. Then, we determined its inhibitory potential towards human recombinant acetylcholinesterase (HssAChE; EC 3.1.1.7) and butyrylcholinesterase (HssBChE; EC 3.1.1.8), the ability of HI-6, obidoxime, pralidoxime, methoxime, and trimedoxime to reactivate inhibited cholinesterases (ChEs), its toxicity in rats and therapeutic effects of different antidotal approaches. Finally, we utilized molecular dynamics to explain our findings. The results of spontaneous A-234 hydrolysis showed a slow process with a reaction rate displaying a triphasic course during the first 72 h (the residual concentration 86.2%). A-234 was found to be a potent inhibitor of both human ChEs (HssAChE IC50 = 0.101 ± 0.003 µM and HssBChE IC50 = 0.036 ± 0.002 µM), whereas the five marketed oximes have negligible reactivation ability toward A-234-inhibited HssAChE and HssBChE. The acute toxicity of A-234 is comparable to that of VX and in the context of therapy, atropine and diazepam effectively mitigate A-234 lethality. Even though oxime administration may induce minor improvements, selected oximes (HI-6 and methoxime) do not reactivate ChEs in vivo. Molecular dynamics implies that all marketed oximes are weak nucleophiles, which may explain the failure to reactivate the A-234 phosphorus-serine oxygen bond characterized by low partial charge, in particular, HI-6 and trimedoxime oxime oxygen may not be able to effectively approach the A-234 phosphorus, while pralidoxime displayed low interaction energy. This study is the first to provide essential experimental preclinical data on the A-234 compound.

A delayed treatment model for the evaluation of scopolamine for VX nerve agent intoxication.

Most research on medical countermeasures for nerve agent exposure assumes a military scenario, in which (autoinjector) treatment is envisaged to be available immediately. In a civilian setting however, treatment is delayed until arrival of first-aid responders. This may significantly affect treatment efficacy and the requirements for secondary intensive care. The aim of the current study was to develop a guinea pig model to evaluate the efficacy of delayed treatment following nerve agent exposure. We identified a trigger-to-treat based on a progressive stage of the toxidrome following VX exposure, which was associated with the subsiding of clonic movements. This paradigm resulted in treatment consistently being administered between 15 and 25 min post-exposure. Using the model, we investigated the potential for the anticholinergic scopolamine to act as a delayed treatment either as a standalone treatment, or as an adjunct to delayed treatment with Standard of Care (SOC), containing atropine, 2-PAM, and midazolam. The study provides a framework for a small animal model for evaluating the efficacy of treatment administered at a specific stage of the toxidrome, when immediate treatment is absent. As an adjunct, scopolamine treatment did not result in improved survival, but did show a beneficial effect on recovery, in terms of general posture. As a standalone treatment, scopolamine showed a significant, dose-responsive, beneficial effect on survival and recovery. These promising results warrant additional studies to investigate which observed physiological improvements are relevant for the recovery process and residual injury.

The tertiary oxime monoisonitrosoacetone penetrates the brain, reactivates inhibited acetylcholinesterase, and reduces mortality and morbidity following lethal sarin intoxication in guinea pigs.

The brain is a critical target for the toxic action of organophosphorus (OP) inhibitors of acetylcholinesterase (AChE) such as the nerve agent sarin. However, the available oxime antidote 2-PAM only reactivates OP-inhibited AChE in peripheral tissues. Monoisonitrosoacetone (MINA), a tertiary oxime, reportedly reactivates AChE in the central nervous system (CNS). The current study investigated whether MINA would be beneficial as a supplemental oxime treatment in preventing lethality and reducing morbidity following lethal sarin exposure, MINA supplement would improve AChE recovery in the body, and MINA would be detectable in the CNS. Guinea pigs were exposed to sarin and treated with atropine sulfate and 2-PAM at one minute. Additional 2-PAM or MINA was administered at 3, 5, 15, or 30 min after sarin exposure. Survival and morbidity were assessed at 2 and 24 h. AChE activity in brain and peripheral tissues was evaluated one hour after MINA and 2-PAM treatment. An in vivo microdialysis technique was used to determine partitioning of MINA into the brain. A liquid chromatography-tandem mass spectrometry method was developed for the analysis of MINA in microdialysates. MINA-treated animals exhibited significantly higher survival and lower morbidity compared to 2-PAM-treated animals. 2-PAM was significantly more effective in reactivating AChE in peripheral tissues, but only MINA reactivated AChE in the CNS. MINA was found in guinea pig brain microdialysate samples beginning at ~10 min after administration in a dose-related manner. The data strongly suggest that a centrally penetrating oxime could provide significant benefit as an adjunct to atropine and 2-PAM therapy for OP intoxication.

A novel high-performance liquid chromatography with diode array detector method for the simultaneous quantification of the enzyme-reactivating oximes obidoxime, pralidoxime, and HI-6 in human plasma.

Oximes such as pralidoxime (2-PAM), obidoxime (Obi), and HI-6 are the only currently available therapeutic agents to reactivate inhibited acetylcholinesterase (AChE) in case of intoxications with organophosphorus (OP) compounds. However, each oxime has characteristic agent-dependent reactivating efficacy, and therefore the combined administration of complementary oximes might be a promising approach to improve therapy. Accordingly, a new high-performance liquid chromatography method with diode-array detection (HPLC-DAD) was developed and validated allowing for simultaneous or single quantification of 2-PAM, Obi, and HI-6 in human plasma. Plasma was precipitated using 5% w/v aqueous zinc sulfate solution and subsequently acetonitrile yielding high recoveries of 94.2%-101.0%. An Atlantis T3 column (150 × 2.1mm I.D., 3 µm) was used for chromatographic separation with a total run time of 15 min. Quantification was possible without interferences within a linear range from 0.12 to 120 µg/mL for all oximes. Excellent intra-day (accuracy 91.7%-98.6%, precision 0.5%-4.4%) and inter-day characteristics (accuracy 89.4%-97.4%, precision 0.4%-2.2%) as well as good ruggedness were found. Oximes in processed samples were stable for at least 12 h in the autosampler at 15°C as well as in human plasma for at least four freeze-thaw cycles. Finally, the method was applied to plasma samples of a clinical case of pesticide poisoning.

Reactivation of VX-Inhibited Human Acetylcholinesterase by Deprotonated Pralidoxime. A Complementary Quantum Mechanical Study.

In the present work, we performed a complementary quantum mechanical (QM) study to describe the mechanism by which deprotonated pralidoxime (2-PAM) could reactivate human (Homo sapiens sapiens) acetylcholinesterase (HssAChE) inhibited by the nerve agent VX. Such a reaction is proposed to occur in subsequent addition-elimination steps, starting with a nucleophile bimolecular substitution (SN2) mechanism through the formation of a trigonal bipyramidal transition state (TS). A near attack conformation (NAC), obtained in a former study using molecular mechanics (MM) calculations, was taken as a starting point for this project, where we described the possible formation of the TS. Together, this combined QM/MM study on AChE reactivation shows the feasibility of the reactivation occurring via attack of the deprotonated form of 2-PAM against the Ser203-VX adduct of HssAChE.

In Vitro Cysteine Reactivates Organophosphate Insecticide Dichlorvos-Inhibited Human Cholinesterases.

OBJECTIVES: Organophosphate (OP) pesticides inhibit both red blood cell (RBC) and plasma cholinesterases (ChEs). Oximes, especially pralidoxime (2-PAM), are widely used as antidotes to treat OP poisoning. In addition, N-acetylcysteine (NAC) is sometimes used as an adjuvant antidote. The current study aimed to assess the feasibility of using NAC as a single therapeutic agent for OP poisoning in comparison to in vitro 2-PAM. METHODS: This study was carried out at the Razi Drug Research Center of Iran University of Medical Sciences, Tehran, Iran, between April and September 2014. A total of 22 healthy human subjects were recruited and 8 mL citrated blood samples were drawn from each subject. Dichlorvos-inhibited blood samples were separately exposed to low and high doses (final concentrations of 300 and 600 µmol.L-1, respectively) of 2-PAM, NAC and cysteine. Plasma and RBCs were then separated by centrifugation and their ChE activity was measured using spectrophotometry. RESULTS: Although cysteine-and not NAC-increased the ChE activity of both plasma and RBCs over those of dichlorvos, it did not increase them over those of a high dose of 2-PAM. CONCLUSION: These results suggest that the direct reactions of 2-PAM and cysteine with dichlorvos and the reactivation of phosphorylated ChEs occurr via an associative stepwise addition-elimination process. High therapeutic blood concentrations of cysteine are needed for the elevation of ChE activity in plasma and RBCs; however, both this agent and NAC may still be effective in the reactivation of plasma and RBC ChEs.

Efficacy of fresh packed red blood transfusion in organophosphate poisoning.

The mortality rate caused by organophosphate (OP) poisoning is still high, even the standard treatment such as atropine and oxime improves a lot. To search for alternative therapies, this study was aimed to investigate the effects of packed red blood cell (RBC) transfusion in acute OP poisoning, and compare the therapeutic effects of RBCs at different storage times.Patients diagnosed with OP poisoning were included in this prospective study. Fresh RBCs (packed RBCs stored less than 10 days) and longer-storage RBCs (stored more than 10 days but less than 35 days) were randomly transfused or not into OP poisoning patients. Cholinesterase (ChE) levels in blood, atropine usage and durations, pralidoxime durations were measured.We found that both fresh and longer-storage RBCs (200-400 mL) significantly increased blood ChE levels 6 hours after transfusion, shortened the duration for ChE recovery and length of hospital stay, and reduced the usage of atropine and pralidoxime. In addition, fresh RBCs demonstrated stronger therapeutic effects than longer-storage RBCs.Packed RBCs might be an alternative approach in patients with OP poisoning, especially during early stages.

Can anisodamine be a potential substitute for high-dose atropine in cases of organophosphate poisoning?

A case of organophosphate (OP) poisoning was admitted to the emergency room. The patient accepted treatment with pralidoxime (PAM), atropine, and supporting therapy. It was observed that even after 22 h after treatment, 960 mg of atropine was not enough for the patient to be atropinized. However, a 160-mg follow-up treatment of anisodamine was quite enough for atropinization after 4 h. As a case report, more studies are required before any definite conclusion can be reached regarding the use of anisodamine as a potential substitute for high-dose atropine in cases of OP poisoning.

Fresh frozen plasma as a successful antidotal supplement in acute organophosphate poisoning.

Despite improvements to intensive care management and specific pharmacological treatments (atropine, oxime, diazepam), the mortality associated with organophosphate (OP) poisoning has not substantially decreased. The objective of this examination was to describe the role of fresh frozen plasma (FFP) in acute OP poisoning. After a deliberate ingestion of malathion, a 55-year-old male suffering from miosis, somnolence, bradycardia, muscular fasciculations, rales on auscultation, respiratory insufficiency, as well as from an inhibition of red blood cell acetylcholinesterase (AChE) and plasma butyrylcholinesterase (BuChE), was admitted to hospital. Malathion was confirmed in a concentration of 18.01 mg L(-1). Apart from supportive measures (including mechanical ventilation for four days), antidotal treatment with atropine, oxime-pralidoxime methylsulphate (Contrathion(R)), and diazepam was administered, along with FFP. The potentially beneficial effects of FFP therapy included a prompt increase of BuChE activity (from 926 IU L(-1) to 3277 IU L(-1); reference range from 7000 IU L(-1) to 19000 IU L(-1)) and a reduction in the malathion concentration, followed by clinical recovery. Due to BuChE replacement, albumin content, and volume restitution, FFP treatment may be used as an alternative approach in patients with acute OP poisoning, especially when oximes are not available.

Acute dermal irritation, sensitization, and acute toxicity studies of a transdermal patch for prophylaxis against ({+/-}) anatoxin-a poisoning.

The skin irritating, sensitizing, and acute dermal toxicity potential of a novel combinational prophylactic transdermal patch, mainly composed of eserine and pralidoxime chloride as active pharmaceutical ingredients, against (±) anatoxin-a poisoning were investigated in rabbits, guinea pigs, and rats in compliance with the Organisation for Economic Cooperation and Development guidelines. In primary skin irritation test, rabbits were dermally attached with the therapeutically active transdermal patch or with a placebo patch for 72 hours. The transdermal patches did not induce any adverse reactions such as erythema and edema on intact skin sites. The active patch was classified as a practically nonirritating material based on the score in the primary irritation index. In the Buehler test, guinea pigs were sensitized by the active or placebo transdermal patches attached for 24 hours. The patches did not induce any sensitization reactions in contrast to a severe sensitization reaction that occurred in the positive control. Therefore, the active patch and placebo patch were both graded as weak in sensitization score and rate. Acute dermal toxicity test in rats did not produce any overt signs of toxicity following a 14-day treatment period. Taken together, these findings suggest that the transdermal patch does not cause skin irritation, skin sensitization, or dermal toxic effects following dermal application.

Applied clinical pharmacology and public health in rural Asia--preventing deaths from organophosphorus pesticide and yellow oleander poisoning.

Self-poisoning with pesticides or plants is a major clinical problem in rural Asia, killing several hundred thousand people every year. Over the last 17 years, our clinical toxicology and pharmacology group has carried out clinical studies in the North Central Province of Sri Lanka to improve treatment and reduce deaths. Studies have looked at the effectiveness of anti-digoxin Fab in cardiac glycoside plant poisoning, multiple dose activated charcoal in all poisoning, and pralidoxime in moderate toxicity organophosphorus insecticide poisoning. More recently, using a Haddon matrix as a guide, we have started conducting public health and animal studies to find strategies that may work outside of the hospital. Based on the 2009 GSK Research in Clinical Pharmacology prize lecture, this review shows the evolution of the group's research from a clinical pharmacology approach to one that studies possible interventions at multiple levels, including the patient, the community and government legislation.

Syntheses and in vitro evaluations of uncharged reactivators for human acetylcholinesterase inhibited by organophosphorus nerve agents.

Organophosphorus nerve agents (OPNAs) are highly toxic compounds that represent a threat to both military and civilian populations. They cause an irreversible inhibition of acetylcholinesterase (AChE), by the formation of a covalent P-O bond with the catalytic serine. Among the present treatment of nerve agents poisoning, pyridinium and bis-pyridinium aldoximes are used to reactivate this inhibited enzyme but these compounds do not readily cross the blood brain barrier (BBB) due to their permanent cationic charge and thus cannot efficiently reactivate cholinesterases in the central nervous system (CNS). In this study, a series of seven new uncharged oximes reactivators have been synthesized and their in vitro ability to reactivate VX and tabun-inhibited human acetylcholinesterase (hAChE) has been evaluated. The dissociation constant K(D) of inhibited enzyme-oxime complex, the reactivity rate constant kr and the second order reactivation rate constant k(r2) have been determined and have been compared to reference oximes HI-6, Obidoxime and 2-Pralidoxime (2-PAM). Regarding the reactivation of VX-inhibited hAChE, all compounds show a better reactivation potency than those of 2-PAM, nevertheless they are less efficient than obidoxime and HI-6. Moreover, one of seven described compounds presents an ability to reactivate tabun-inhibited hAChE equivalent to those of 2-PAM.

An in vivo zebrafish screen identifies organophosphate antidotes with diverse mechanisms of action.

Organophosphates are a class of highly toxic chemicals that includes many pesticides and chemical weapons. Exposure to organophosphates, either through accidents or acts of terrorism, poses a significant risk to human health and safety. Existing antidotes, in use for over 50 years, have modest efficacy and undesirable toxicities. Therefore, discovering new organophosphate antidotes is a high priority. Early life stage zebrafish exposed to organophosphates exhibit several phenotypes that parallel the human response to organophosphates, including behavioral deficits, paralysis, and eventual death. Here, we have developed a high-throughput zebrafish screen in a 96-well plate format to find new antidotes that counteract organophosphate-induced lethality. In a pilot screen of 1200 known drugs, we identified 16 compounds that suppress organophosphate toxicity in zebrafish. Several in vitro assays coupled with liquid chromatography/tandem mass spectrometry-based metabolite profiling enabled determination of mechanisms of action for several of the antidotes, including reversible acetylcholinesterase inhibition, cholinergic receptor antagonism, and inhibition of bioactivation. Therefore, the in vivo screen is capable of discovering organophosphate antidotes that intervene in distinct pathways. These findings suggest that zebrafish screens might be a broadly applicable approach for discovering compounds that counteract the toxic effects of accidental or malicious poisonous exposures.

Antidote for organophosphate insecticide poisoning: atropine and pralidoxime

Acute organophosphate (OP) poisoning produces cholinergic symptoms resulting from the inhibition of cholinesterase, and the overstimulation of muscarinic and nicotinic receptors in the synapses. The dominant clinical features of acute cholinergic toxicity include bradycardia, miosis, lacrimation, salivation, bronchorrhea, and bronchospasm. All symptomatic patients should receive therapy with oxygen, atropine, and pralidoxime. Atropine works as a physiologic antidote by competitively occupying muscarinic receptor sites, reducing the effects of excessive acetylcholine. Atropine should be immediately administered, and the dose can be titrated according to the severity of OP poisoning. A large dose may be necessary to overcome the excessive cholinergic state in case of severe poisoning. Pralidoxime is a biochemical antidote that reactivates acetylcholinesterase by removing OP from it. It is effective in treating both muscarinic and nicotinic symptoms. After some period of time, the acetylcholinesterase-OP compound undergoes a conformational change, known as aging, which renders the enzyme irreversibly resistant to reactivation by a pralidoxime. There has been a great deal of controversy over the effectiveness of pralidoxime in acute OP poisoning. However, it may be beneficial to administer pralidoxime for a sufficient period in case of severe poisoning with a large quantity of OP, which is common in Korea.

Acute renal failure alters the kinetics of pralidoxime in rats.

There is a trend towards increasing doses of pralidoxime to treat human organophosphate poisonings that may have relevance in subpopulations. Indeed, pralidoxime is eliminated unchanged by the renal route. This study assesses the effect of renal failure on the kinetics of pralidoxime in a rat model of acute renal failure induced by potassium dichromate administration. On the first day, Sprague-Dawley rats received subcutaneously potassium dichromate (study) or saline (control). Forty-eight hours post-injection, animals received pralidoxime methylsulfate (50mg/kg of pralidoxime base) intramuscularly. Blood specimens were sampled during 180min after the injection. Urine was collected daily during the 3 days of the study. Plasma pralidoxime concentrations were measured by liquid chromatography with electrochemical detection. There was a 2-fold increase in mean elimination half-life and a 2.5-fold increase in mean area under the curve in the study compared to the control group. The mean total body clearance was halved in the study compared to the control group. Our study showed acute renal failure does not modify the distribution of pralidoxime but significantly alters its elimination from plasma. These results suggest that dosages of pralidoxime should be adjusted in organophosphate-poisoned humans with renal failure when using high dosage regimen of pralidoxime.

Pralidoxime in carbaryl poisoning: an animal model.

INTRODUCTION: Poisoning from organophosphates and carbamates is a significant cause of morbidity and mortality worldwide. Concerns have been expressed over the safety and efficacy of the use of oximes such as pralidoxime (2-PAM) in patients with carbamate poisoning in general, and more so with carbaryl poisoning specifically. The goal of the present study was to evaluate the role of 2-PAM in a mouse model of lethal carbaryl poisoning. METHODS: Female ICR Swiss Albino mice weighing 25-30 g were acclimated to the laboratory and housed in standard conditions. One hundred and ten mice received an LD50 dose of carbaryl subcutaneously. Ten minutes later, they were randomized by block randomization to one of eight treatment groups: normal saline control, atropine alone, 100 mg/kg 2-PAM with and without atropine, 50 mg/kg 2-PAM with and without atropine, and 25 mg/kg 2-PAM with and without atropine. All medications were given intraperitoneally and the atropine dose was constant at 4 mg/kg. The single objective endpoint was defined as survival to 24 hours. Fatalities were compared using a Chi squared or Fisher's exact test. RESULTS: Following an LD50 of carbaryl, 60% of the animals died. Atropine alone statistically improved survival (15% lethality). High dose 2-PAM with and without atropine was numerically worse, but not statistically different from control. While the middle dose of 2-PAM was no different than control, the addition of atropine improved survival (10% fatality). Low-dose 2-PAM statistically improved survival (25% lethality). Atropine further reduced lethality to 10%. CONCLUSION: When appropriately dosed, 2-PAM alone protects against carbaryl poisoning in mice. Failure to demonstrate this benefit in other models may be the result of oxime overdose.

Protective drugs in acute large-dose exposure to organophosphates: a comparison of metoclopramide and tiapride with pralidoxime in rats.

Weak and reversible inhibitors of cholinesterase(s), when coadministered in excess with a more potent inhibitor such as organophosphates, can act in a protective manner. The benzamide compound, metoclopramide, confers some protection (putatively via this mechanism) for cholinesterases against inhibition by paraoxon both in vitro and in vivo, after chronic small-dose exposure. Tiapride is a related benzamide. In this study, we compared the protection by metoclopramide and tiapride in rats acutely exposed to large doses of paraoxon with the therapeutic "gold standard," pralidoxime. Group 1 received 1 micromol paraoxon (approximately 75% lethal dose), Group 2 received 50 micromol metoclopramide, Group 3 received 50 micromol tiapride, Group 4 received 50 micromol pralidoxime, Group 5 received 1 micromol paraoxon + 50 micromol metoclopramide, Group 6 1 micromol paraoxon + 50 micromol tiapride, and Group 7 1 micromol paraoxon + 50 micromol pralidoxime. All substances were administered intraperitoneally. The animals were monitored for 48 h and mortality was recorded at 30 min, 1, 2, 3, 4, 24, and 48 h. Blood was taken for red blood cell acetylcholinesterase measurements at baseline, 30 min, 24, and 48 h. With the exception of Group 7, in which some late mortality was observed, mortality occurred mainly in the first 30 min after paraoxon administration with minimal changes occurring thereafter. Mortality at 30 min was 0% in the metoclopramide, tiapride, and pralidoxime groups and 73 +/- 20 (paraoxon), 65 +/- 15 (paraoxon + metoclopramide), 38 +/- 14 (paraoxon + tiapride), and 13 +/- 19 (paraoxon + pralidoxime). Mortality at 48 h was 75 +/- 18 (paraoxon), 67 +/- 17 (paraoxon + metoclopramide), 42 +/- 16 (paraoxon + tiapride), and 27 +/- 24 (paraoxon + pralidoxime). Metoclopramide does not significantly influence mortality after acute large-dose paraoxon exposure. Both tiapride and pralidoxime significantly decreased mortality in our model. The protection conferred by tiapride was significantly less than that conferred by pralidoxime at 30 min, but was not significantly different at 24 and 48 h.

Dimethylphosphoryl-inhibited human cholinesterases: inhibition, reactivation, and aging kinetics.

Human poisoning by organophosphates bearing two methoxy groups, e.g. by malathion, paraoxon-methyl, dimethoate and oxydemeton-methyl, is generally considered to be rather resistant to oxime therapy. Since the oxime effectiveness is influenced not only by its reactivating potential but also by inhibition, aging and spontaneous reactivation kinetics, experiments were performed with human acetyl- (AChE) and butyrylcholinesterase (BChE) to determine the respective kinetic constants. The efficacy of obidoxime in reactivating dimethylphosphoryl-AChE was 40, 9 and 3 times higher than of HI 6, pralidoxime and HLö 7, respectively. Aging (t1/2 3.7 h) and spontaneous reactivation (t1/2 0.7 h) occurred concomitantly, with the portion of the aged enzyme being dependent on the presence of excess inhibitor. Calculation of steady-state AChE activity in the presence of inhibitor and oxime revealed that obidoxime was superior to pralidoxime. In addition, organophosphate concentrations up to 10(-6) M (paraoxon-methyl) and 10(-4) M (oxydemeton-methyl) could be counteracted at clinically relevant oxime concentrations (10 microM). These data indicate that oximes may effectively reactivate human dimethylphosphoryl-AChE. Failure of oximes may be attributed to megadose intoxications and to prolonged time intervals between poison uptake and oxime administration. The potency of the oximes to reactivate dimethylphosphoryl-BChE was much lower and the spontaneous reactivation slower (t1/2 9 h), while aging proceeded at a comparable rate. Thus, BChE activity determination for diagnosis and therapeutic monitoring may give no reliable information on AChE status.

Reappraisal of indications and limitations of oxime therapy in organophosphate poisoning.

1 In vitro studies with human erythrocyte acetylcholinesterase (AChE) and the mouse diaphragm model were performed to unravel the various microscopic reaction parameters that contribute to the dynamic equilibrium of AChE inhibition, ageing and reactivation. These data may help to define more precisely the indications and limitations of oxime therapy in organophosphate (OP) poisoning. 2 Diethylphosphoryl-AChE resulting from intoxications with parathion, chlorpyrifos, chlorfenvinphos, diazinon and other OPs is characterized by slow spontaneous reactivation and low propensity for ageing. This kind of phosphorylated enzyme is particularly susceptible to reactivation by oximes. 3 None of the oximes tested (pralidoxime, obidoxime, HI 6 and HLö 7) can be regarded as a universally suitable reactivator. Obidoxime turned out to be the most potent and most efficacious oxime in reactivating AChE inhibited by various classes of OP insecticides and tabun. Obidoxime, however, was inferior to HI 6 against soman, sarin, cyclosarin and VX. Pralidoxime was generally less potent. 4 The kinetic data of reactivation established for diethylphosphoryl-AChE of human red cells indicate that the usually recommended dosage to attain a plasma concentration of 4 micrograms/ml does not permit exploitation of the full therapeutic potential of the oximes, in particular of pralidoxime. However, in suicidal mega-dose poisoning, oximes, even at optimal plasma concentrations, may be unable to cope with the fast re-inhibition of reactivated AChE in the first days following intoxication. 5 It is suggested that oximes be administered by continuous infusion following an initial bolus dose as long as reactivation can be expected and until permanent clinical improvement is achieved.

Acute toxicity of cyclohexylmethylphosphonofluoridate (CMPF) in rhesus monkeys: serum biochemical and hematologic changes.

Changes in serum biochemical and hematological parameters were studied in 20 male rhesus monkeys following acute poisoning by the organophosphate nerve agent cyclohexylmethylphosphonofluoridate (CMPF or GF). Animals were challenged with 5 x LD50 GF (233 micrograms/kg, IM) following pretreatment with pyridostigmine (0.3-0.7 mg/kg per 24 h) and treated with atropine (0.4 mg/kg, IM) and either 2-PAM (25.7 mg/kg, IM) or H16 (37.8 mg/kg, IM) at the onset of clinical signs or at 1 min after exposure. Muscle fasciculations, tremors, or convulsions occurred in 19 of 20 animals. Serum biochemical and hematologic parameters were analyzed 2 days and 7 days after exposure and compared to pre-exposure baseline values. Significant increases in creatine kinase (CK), lactate dehydrogenase (LD), aspartate transaminase (AST), alanine transaminase (ALT) and potassium ion (K+), associated with damage to striated muscle and metabolic acidosis, occurred in both oxime-treated groups 2 days after exposure. Total protein, albumin, red blood cell (RBC) count, hemoglobin concentration (Hb) and hematocrit (Hct), were decreased in both oxime-treated groups at 7 days. The results demonstrate that animals exposed to a single high dose of GF and treated with standard therapy exhibit changes in serum biochemical and hematological indices directly and indirectly associated with their clinical presentations.