Studies of the amplification of carbaryl toxicity by various oximes

Acute organophosphate and carbamate pesticide poisonings - a five-year survey from the National Poison Control Center Of Serbia

Pesticide poisonings, intentional as well as accidental, are common, especially in undeveloped and developing countries. The goal of this study was to analyze the clinical presentation of patients hospitalized due to acute organophosphate (OPP) or carbamate pesticide (CP) poisoning as well as to analyze the factors that potentially influenced the severity and outcome of the poisonings. A retrospective cross-sectional study was performed. The age and gender of each patient were recorded, the type of ingested pesticide, whether the poisoning was intentional or accidental, number of days of hospitalization, the severity of the poisoning, and the outcome of the treatment (i.e., whether the patient survived or not). Clinical aspects of poisonings were analyzed, as well as the therapeutic measures performed. 60 patients were hospitalized due to acute OPP or CP poisoning, out of 51 (85.00%) were cases of intentional self-poisoning. The majority of patients were poisoned by OPPs (76.67%), in one-third the causative agent was malathion, followed in frequency by chlorpyrifos and diazinon. Dimethoate poisonings were manifested with the most severe clinical picture. A 70% or lower activity of reference values of acetylcholinesterase and butyrylcholinesterase was found in 50% and 58% of patients, respectively. The most common symptom was miosis (58.33%), followed by nausea and vomiting. Pralidoxime reactivated acetylcholinesterase inhibited by chlorpyrifos or diazinon, but not with malathion or dimethoate. Impairment of consciousness and respiratory failure, as well as the degree of acetylcholinesterase and butyrylcholinesterase inhibition, were prognostic signs of the severity of poisoning. The lethal outcome was more often found in older patients (t = 2.41, p = 0.019). The type of ingested pesticide significantly affects the severity and outcome of poisoning as well as the effectiveness of antidotes.

Recognition and Assessment of Antidotal Effects of Diphenhydramine against Acute Carbaryl Insecticide Poisoning in a Chick Model

Diphenhydramine antagonizes poisoning produced by cholinesterase (ChE) inhibiting insecticides. This study examines the effects of diphenhydramine against acute poisoning induced by the carbamate insecticide carbaryl in a chick model. The effects of diphenhydramine on the 24 h median Lethal Dose (LD50), and acute toxicity of carbaryl were assessed in chicks (7-15 days old). The plasma and whole brain ChE activities were measured electrometrically in vitro and in vivo. Diphenhydramine at 10mg/Kg Body wt. administered intramuscularly 15 min before carbaryl dosing increased the oral LD50 value of carbaryl (207 mg/Kg Body wt.) by 62%. Carbaryl at 250 mg/Kg Body wt. has orally produced toxidrome of cholinergic poisoning with 100% lethality in 24 h. Diphenhydramine (10mg/ Kg Body wt.) used 15 min before carbaryl (250mg/Kg Body wt., orally) was the most effective dose (vs 5 and 20mg/Kg Body wt.) in delaying carbaryl-toxicity and increasing survivals in chicks. The intramuscular median effective dose (ED50) of diphenhydramine which prevented 24 h carbaryl-death in chicks was 8.6mg/ Kg Body wt. The antidotal response to diphenhydramine was similar to that of the standard antidote atropine sulfate. Diphenhydramine at 10mg/Kg Body wt., given immediately after carbaryl (200mg/Kg Body wt.), reduced the percentages of plasma and whole brain ChE inhibitions in vivo by 12- and 13%, respectively. Carbaryl (10μmol/L) in vitro inhibited ChE activities in the plasma and brain by 53 and 77%, respectively; these inhibitions were reduced by 13- and 14%, respectively, when diphenhydramine (10μmol/L) was added to in vitro reactions. Diphenhydramine exerted antidotal action against a model of acute and lethal carbaryl intoxication in chicks.

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 LD 50 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. Human & Experimental Toxicology (2007) 26, 125-129

Efficacy of Recommended Prehospital Human Equivalent Doses of Atropine and Pralidoxime Against the Toxic Effects of Carbamate Poisoning in the Hartley Guinea Pig

PURPOSE

Aldicarb and methomyl are carbamate pesticides commonly implicated in human poisonings. The primary toxic mechanism of action for carbamate poisoning is cholinesterase (ChE) inhibition. As such, it is logical to assume that the currently accepted therapies for organophosphate poisoning (muscarinic antagonist atropine and the oxime acetylcholinesterase reactivator pralidoxime chloride [2-PAM Cl]) could afford therapeutic protection. However, oximes have been shown to be contraindicated for poisoning by some carbamates.

METHODS

A protective ratio study was conducted in guinea pigs to evaluate the efficacy of atropine and 2-PAM Cl. The ChE activity was determined in both the blood and the cerebral cortex.

RESULTS

Coadministration of atropine free base (0.4 mg/kg) and 2-PAM Cl (25.7 mg/kg) demonstrated protective ratios of 2 and 3 against aldicarb and methomyl, respectively, relative to saline. The data reported here show that this protection was primarily mediated by the action of atropine. The reactivator 2-PAM Cl had neither positive nor negative effects on survival. Both blood acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities were significantly reduced at 15 minutes postchallenge but gradually returned to normal within 24 hours. Analysis of cerebral cortex showed that BChE, but not AChE, activity was reduced in animals that succumbed prior to 24 hours after challenge.

CONCLUSION

The results suggest that coadministration of atropine and 2-PAM Cl at the currently recommended human equivalent doses for use in the prehospital setting to treat organophosphorus nerve agent and pesticide poisoning would likely also be effective against aldicarb or methomyl poisoning.

Investigation of kinetic interactions between approved oximes and human acetylcholinesterase inhibited by pesticide carbamates

Carbamates are widely used for pest control and act primarily by inhibition of insect and mammalian acetylcholinesterase (AChE). Accidental or intentional uptake of carbamates may result in typical signs and symptoms of cholinergic overstimulation which cannot be discriminated from those of organophosphorus pesticide poisoning. There is an ongoing debate whether standard treatment with atropine and oximes should be recommended for human carbamate poisoning as well, since in vitro and in vivo animal data indicate a deleterious effect of oximes when used in combination with the N-methyl carbamate carbaryl. Therefore, we performed an in vitro kinetic study to investigate the effect of clinically used oximes on carbamoylation and decarbamoylation of human AChE. It became evident that pralidoxime and obidoxime in therapeutic concentrations aggravate the inhibition of AChE by carbaryl and propoxur, with obidoxime being substantially more potent compared to 2-PAM. However, obidoxime had no impact on the decarbamoylation kinetics. Hence, the administration of 2-PAM and especially of obidoxime to severely propoxur and carbaryl poisoned humans cannot be recommended.

Acute aldicarb toxicity in dogs: 15 cases (2001-2009)

OBJECTIVE

To describe the common clinical signs, laboratory abnormalities, treatment, and prognosis associated with acute aldicarb toxicosis in dogs.

DESIGN

Retrospective observational study from 2001 to 2009.

SETTING

Urban referral hospital.

ANIMALS

Fifteen client-owned dogs.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The most common clinical signs associated with acute aldicarb toxicosis were vomiting, ptyalism, diarrhea, and tremors. Of the 15 dogs, 11 were admitted to the hospital for treatment, 2 were euthanized at presentation and 2 were discharged against medical advice following minimal treatment and lost to follow-up. Laboratory abnormalities included lactic acidosis and hyperglycemia in 12 and 9 patients, respectively. Treatment of hospitalized dogs included induction of emesis with apomorphine (4 dogs), activated charcoal (5), IV fluids (11), atropine (7), methocarbamol (3), diazepam (1), pralidoxime (1) and diphenhydramine (1). Ten of 11 hospitalized dogs survived to discharge; 1 was euthanized following a respiratory arrest after 36 hours of hospitalization. One patient received mechanical ventilation and treatment for pneumonia before discharge from the hospital. The median duration of hospitalization was 22 hours (range 12-168 h).

CONCLUSIONS

Acute aldicarb toxicosis carries a good prognosis for survival and hospital discharge with treatment. Supportive care should be considered for at least 18-24 hours to monitor for response to therapy and development of respiratory failure.

Medical treatment of acute poisoning with organophosphorus and carbamate pesticides

Organophosphorus compounds (OPs) are used as pesticides and developed as warfare nerve agents such as tabun, soman, sarin, VX and others. Exposure to even small amounts of an OP can be fatal and death is usually caused by respiratory failure. The mechanism of OP poisoning involves inhibition of acetylcholinesterase (AChE) leading to inactivation of the enzyme which has an important role in neurotransmission. AChE inhibition results in the accumulation of acetylcholine at cholinergic receptor sites, producing continuous stimulation of cholinergic fibers throughout the nervous systems. During more than five decades, pyridinium oximes have been developed as therapeutic agents used in the medical treatment of poisoning with OP. They act by reactivation of AChE inhibited by OP. However, they differ in their activity in poisoning with pesticides and warfare nerve agents and there is still no universal broad-spectrum oxime capable of protecting against all known OP. In spite of enormous efforts devoted to development of new pyridinium oximes as potential antidotes against poisoning with OP only four compounds so far have found its application in human medicine. Presently, a combination of an antimuscarinic agent, e.g. atropine, AChE reactivator such as one of the recommended pyridinium oximes (pralidoxime, trimedoxime, obidoxime and HI-6) and diazepam are used for the treatment of OP poisoning in humans. In this article the available data related to medical treatment of poisoning with OP pesticides are reviewed and the current recommendations are presented.

The role of oximes in the management of organophosphorus pesticide poisoning

The number of intoxications with organophosphorus pesticides (OPs) is estimated at some 3,000,000 per year, and the number of deaths and casualties some 300,000 per year. OPs act primarily by inhibiting acetylcholinesterase (AChE), thereby allowing acetylcholine to accumulate at cholinergic synapses, disturbing transmission at parasympathetic nerve endings, sympathetic ganglia, neuromuscular endplates and certain CNS regions. Atropine is the mainstay of treatment of effects mediated by muscarine sensitive receptors; however, atropine is ineffective at the nicotine sensitive synapses. At both receptor types, reactivation of inhibited AChE may improve the clinical picture. The value of oximes, however, is still a matter of controversy. Enthusiastic reports of outstanding antidotal effectiveness, substantiated by laboratory findings of reactivated AChE and improved neuromuscular transmission, contrast with many reports of disappointing results. In vitro studies with human erythrocyte AChE, which is derived from the same single gene as synaptic AChE, revealed marked differences in the potency and efficacy of pralidoxime, obidoxime, HI 6 and HLö 7, the latter two oximes being considered particularly effective in nerve agent poisoning. Moreover, remarkable species differences in the susceptibility to oximes were revealed, requiring caution when animal data are extrapolated to humans. These studies impressively demonstrated that any generalisation regarding an effective oxime concentration is inappropriate. Hence, the 4 mg/L concept should be dismissed. To antagonise the toxic effects of the most frequently used OPs, pralidoxime plasma concentrations of around 80 mumol/L (13.8 mg/L pralidoxime chloride) should be attained while obidoxime plasma concentrations of 10 mumol/L (3.6 mg/L obidoxime chloride) may be sufficient. These concentrations should be maintained as long as circulating poison is expected to be present, which may require oxime therapy for up to 10 days. Various dosage regimens exist to reach this goal. The most appropriate consists of a bolus short infusion followed by a maintenance dosage. For pralidoxime chloride, a 1 g bolus over 30 minutes followed by an infusion of 0.5 g/h appears appropriate to maintain the target concentrtion of about 13 mg/L (70 kg person). For obidoxime chloride, the appropriate dosage is a 0.25 g bolus followed by an infusion of 0.75 g/24 h. These concentrations are well tolerated and keep a good portion of AChE in the active state, thereby retarding the AChE aging rate. AChE aging is particularly rapid with dimethyl phosphoryl compounds and may thwart the effective reactivation by oximes, particularly in suicidal poisoning with excessive doses. In contrast, patients with diethyl OP poisoning may particularly benefit from oxime therapy, even if no improvement is seen during the first days when the poison load is high. The low propensity to aging with diethyl OP poisoning may allow reactivation after several days, when the poison concentration drops. Rigorous testing of the benefits of oximes is only possible in randomised controlled trials with clear stratification according to the class of pesticides involved, time elapsed between exposure and treatment and severity of cholinergic symptoms on admission.

Butyrylcholinesterase in lumbar and ventricular cerebrospinal fluid

OBJECTIVES

This study establishes reference data for human lumbar CSF butyrylcholinesterase (E.C.3.1.1.8.) activity and investigates the enzyme activity in ventricular CSF. We comment on the relationship between CSF butyrylcholinesterase activity and other laboratory parameters.

SUBJECTS AND METHODS

We investigated 64 lumbar CSF samples obtained from a clinically healthy population and 169 ventricular CSF samples collected from 90 neurosurgical patients.

RESULTS

The reference range we recommend for lumbar CSF butyrylcholinesterase activity is 5.4 to 17.0 nmol/min x ml. The majority of ventricular butyrylcholinesterase activities in our patient subset ranged up to 5 nmol/min x ml.

CONCLUSIONS

We established the relative influence of serum and CNS components on total CSF butyrylcholinesterase activity. The CNS fraction predominates the total butyrylcholinesterase activity in normal lumbar CSF. In ventricular CSF enzyme influx from serum outweighs the CNS component.

Differentiation between organophosphate and carbamate poisoning

We propose a novel and simple assay for the real-time differentiation between carbamate and organophosphate inhibition of cholinesterase, based on our observations of the kinetic behavior of inhibited enzyme. The assay of carbamylated cholinesterase activity over time follows a non-linear kinetic pattern, whereas that of phosphorylated enzyme activity is linear. This feature can be exploited to differentiate between carbamate and organophosphate cholinesterase inhibition. The non-linear pattern characteristic of carbamates is easily discernible at degrees of inhibition of 40% or more. In this setting, cholinesterase activity ought to be measured continuously for about 1 h to obtain the kinetic pattern of enzyme activity. The initial activity, measured during the first 5 min of assay, represents the activity of enzyme in vivo. In vitro reactivation of inhibited cholinesterase allows the estimation of full potential activity of enzyme prior to poisoning, so that percentage of inhibition can be calculated. Reactivation of carbamylated cholinesterase is obtained by the incubation of diluted enzyme at 37 degrees C for 2.5 h prior to assay, whereas phosphorylated (non-aged) enzyme is reactivated by a 30 min incubation with oximes. In cases of mild exposure to cholinesterase inhibitors (< 40% inhibition), the response of enzyme to in vitro reactivation serves as a complementary test for exposure and for the nature of the inhibitor. All the results presented in this work refer to plasma cholinesterase. Erythrocyte cholinesterase was found to behave very similarly to plasma enzyme and its results have not been reported here.

Rate constants of carbamylation and decarbamylation of acetylcholinesterase for physostigmine and carbaryl in the presence of an oxime

Membrane-bound bovine erythrocyte acetylcholinesterase was inhibited with physostigmine or carbaryl, and the rate constants of carbamylation and decarbamylation were determined from the proportion of inhibited acetylcholinesterase in the steady state, and the rate of approach to the steady state. The oximes 2-PAM, HI-6, HS-6, TMB-4 and toxogonin, at 0.1 mM, all decreased the rate of carbamylation by physostigmine, but increased the rate of carbamylation by carbaryl. TMB-4 and toxogonin were the most effective oximes in potentiating carbamylation by carbaryl, with an enhancement of the second-order rate constant of 54- and 17-fold respectively. The greatest reduction in the rate constant for carbamylation by physostigmine (3.7-fold) was caused by HI-6. HS-6 and HI-6 increased the rate of decarbamylation, while 2-PAM reduced the rate of decarbamylation if physostigmine was the carbamate. 2-PAM and HI-6 were also studied with soluble bovine erythrocyte acetylcholinesterase, and similar results were obtained. The results extend those in a recent report by other authors who studied the half-life of carbamylation for acetylcholinesterase and butyrylcholinesterase in an attempt to understand the mechanism by which oximes increase the toxicity of carbaryl in vivo. These authors proposed binding of the oximes to an allosteric site on the enzyme. While not discounting this possibility, the present results, taken with other reports in the literature, suggest that binding of the oximes to the anionic subsite of the active site of the enzyme is also feasible. The present results also offer an explanation for another recent report, in which anomalous results were presented for decarbamylation of physostigmine-inhibited and carbaryl-inhibited erythrocyte acetylcholinesterase in the presence of 2-PAM or HI-6.