Synthesis and in vitro evaluation of novel non-oximes for the reactivation of nerve agent inhibited human acetylcholinesterase

Since several decades oximes have been used as part of treatment of nerve agent intoxication with the aim to restore the biological function of the enzyme acetylcholinesterase after its covalent inhibition by organophosphorus compounds such as pesticides and nerve agents. Recent findings have illustrated that, besides oximes, certain Mannich phenols can reactivate the inhibited enzyme very effectively, and may therefore represent an attractive complementary class of reactivators. In this paper we further probe the effect of structural variation on the in vitro efficacy of Mannich phenol based reactivators. Thus, we present the synthesis of 14 compounds that are close variants of the previously reported 4-amino-2-(1-pyrrolidinylmethyl)-phenol, a very effective non-oxime reactivator, and 3 dimeric Mannich phenols. All compounds were assessed for their ability to reactivate human acetylcholinesterase inhibited by the nerve agents VX, tabun, sarin, cyclosarin and paraoxon in vitro. It was confirmed that the potency of the compounds is highly sensitive to small structural changes, leading to diminished reactivation potency in many cases. However, the presence of 4-substituted alkylamine substituents (as exemplified with the 4-benzylamine-variant) was tolerated. More surprisingly, the dimeric compounds demonstrated non-typical behavior and displayed some reactivation potency as well. Both findings may open up new avenues for designing more effective non-oxime reactivators.

Resurrection and Reactivation of Acetylcholinesterase and Butyrylcholinesterase

Organophosphorus (OP) nerve agents and pesticides present significant threats to civilian and military populations. OP compounds include the nefarious G and V chemical nerve agents, but more commonly, civilians are exposed to less toxic OP pesticides, resulting in the same negative toxicological effects and thousands of deaths on an annual basis. After decades of research, no new therapeutics have been realized since the mid-1900s. Upon phosphylation of the catalytic serine residue, a process known as inhibition, there is an accumulation of acetylcholine (ACh) in the brain synapses and neuromuscular junctions, leading to a cholinergic crisis and eventually death. Oxime nucleophiles can reactivate select OP-inhibited acetylcholinesterase (AChE). Yet, the fields of reactivation of AChE and butyrylcholinesterase encounter additional challenges as broad-spectrum reactivation of either enzyme is difficult. Additional problems include the ability to cross the blood brain barrier (BBB) and to provide therapy in the central nervous system. Yet another complication arises in a competitive reaction, known as aging, whereby OP-inhibited AChE is converted to an inactive form, which until very recently, had been impossible to reverse to an active, functional form. Evaluations of uncharged oximes and other neutral nucleophiles have been made. Non-oxime reactivators, such as aromatic general bases and Mannich bases, have been developed. The issue of aging, which generates an anionic phosphylated serine residue, has been historically recalcitrant to recovery by any therapeutic approach-that is, until earlier this year. Mannich bases not only serve as reactivators of OP-inhibited AChE, but this class of compounds can also recover activity from the aged form of AChE, a process referred to as resurrection. This review covers the modern efforts to address all of these issues and notes the complexities of therapeutic development along these different lines of research.

A newly developed oxime K203 is the most effective reactivator of tabun-inhibited acetylcholinesterase

BACKGROUND

Based on in vitro and in vivo rat experiments, the newly developed acetylcholinesterase (AChE) reactivator, K203, appears to be much more effective in the treatment of tabun poisonings than currently fielded oximes.

METHODS

To determine if this reactivating efficacy would extend to humans, studies were conducted in vitro using human brain homogenate as the source of AChE. The efficacy of K203 was compared with commercially available oximes; pralidoxime, obidoxime and asoxime (HI-6).

RESULTS

Reactivation studies showed that K203 was the most effective reactivator with a second order kinetic constant (kr) of 2142 min- 1. M- 1, which was 51 times higher than that obtained for obidoxime (kr = 42 min- 1. M- 1). Both pralidoxime and asoxime (HI-6) failed to significantly reactivate tabun-inhibited human AChE.

DISCUSSION

According to these results and previous studies, using K203, it appears that oxime K203 is the most effective reactivator of tabun-inhibited cholinesterase in several species including humans and should be considered as a possible medical countermeasure to tabun exposure.

Kinetic analysis of oxime-assisted reactivation of human, Guinea pig, and rat acetylcholinesterase inhibited by the organophosphorus pesticide metabolite phorate oxon (PHO)

Phorate is a highly toxic agricultural pesticide currently in use throughout the world. Like many other organophosphorus (OP) pesticides, the primary mechanism of the acute toxicity of phorate is acetylcholinesterase (AChE) inhibition mediated by its bioactivated oxon metabolite. AChE reactivation is a critical aspect in the treatment of acute OP intoxication. Unfortunately, very little is currently known about the capacity of various oximes to rescue phorate oxon (PHO)-inhibited AChE. To help fill this knowledge gap, we evaluated the kinetics of inhibition, reactivation, and aging of PHO using recombinant AChE derived from three species (rat, guinea pig and human) commonly utilized to study the toxicity of OP compounds and five oximes that are currently fielded (or have been deemed extremely promising) as anti-OP therapies by various nations around the globe: 2-PAM Cl, HI-6 DMS, obidoxime Cl₂, MMB4-DMS, and HLö7 DMS. The inhibition rate constants (k_i) for PHO were calculated for AChE derived from each species and found to be low (i.e., 4.8×10³ to 1.4×10⁴M^-1min^-1) compared to many other OPs. Obidoxime Cl₂ was the most effective reactivator tested. The aging rate of PHO-inhibited AChE was very slow (limited aging was observed out to 48h) for all three species.

CONCLUSIONS

(1) Obidoxime Cl₂ was the most effective reactivator tested. (2) 2-PAM Cl, showed limited effectiveness in reactivating PHO-inhibited AChE, suggesting that it may have limited usefulness in the clinical management of acute PHO intoxication. (3) The therapeutic window for oxime administration following exposure to phorate (or PHO) is not limited by aging.

Interaction of the serine hydrolase KIAA1363 with organophosphorus agents: Evaluation of potency and kinetics

Oxons are bioactive metabolites of organophosphorus insecticides (OPs) that covalently inactivate serine hydrolases. KIAA1363 is one of the most abundant serine hydrolases in mouse brain. Although the physiological consequences related to the inhibition of KIAA1363 due to environmental exposures to OPs are poorly understood, the enzyme was previously shown to have a role in the detoxification of oxons. Here, we overexpressed human KIAA1363 and CES1 in COS7 cells and compared the potency of inhibition (IC50s, 15 min) of KIAA1363 and CES1 by chlorpyrifos oxon (CPO), paraoxon (PO), and methyl paraoxon (MPO). The order of potency was CPO > PO >> MPO for both enzymes. We also determined the bimolecular rate constants (kinact/Ki) for reactions of CPO and PO with KIAA1363 and CES1. KIAA1363 and CES1 were inactivated by CPO at comparable rates (4.4 × 10(6) s(-1) M(-1) and 6.7 × 10(6) s(-1) M(-1), respectively), whereas PO inactivated both enzymes at slower rates (0.4 × 10(6) s(-1) M(-1) and 1.5 × 10(6) s(-1) M(-1), respectively). Finally, the reactivation rate of KIAA1363 following inhibition by CPO was evaluated. Together, the results define the kinetics of inhibition of KIAA1363 by active metabolites of agrochemicals and indicate that KIAA1363 is highly sensitive to inhibition by these compounds.

Metabolism of agrochemicals and related environmental chemicals based on cytochrome P450s in mammals and plants

A yeast gene expression system originally established for mammalian cytochrome P450 monooxygenase cDNAs was applied to functional analysis of a number of mammalian and plant P450 species, including 11 human P450 species (CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1 and CYP3A4). The human P450 species CYP1A1, CYP1A2, CYP2B6, CYP2C18 and CYP2C19 were identified as P450 species metabolising various agrochemicals and environmental chemicals. CYP2C9 and CYP2E1 specifically metabolised sulfonylurea herbicides and halogenated hydrocarbons respectively. Plant P450 species metabolising phenylurea and sulfonylurea herbicides were also identified mainly as the CYP71 family, although CYP76B1, CYP81B1 and CYP81B2 metabolised phenylurea herbicides. The transgenic plants expressing these mammalian and plant P450 species were applied to herbicide tolerance as well as phytoremediation of agrochemical and environmental chemical residues. The combined use of CYP1A1, CYP2B6 and CYP2C19 belonging to two families and three subfamilies covered a wide variety of herbicide tolerance and phytoremediation of these residues. The use of 2,4-D-and bromoxynil-induced CYP71AH11 in tobacco seemed to enhance herbicide tolerance and selectivity.

[Specific features of the anticonvulsant effect of memantine in mice intoxicated with a model organic phosphate]

The effect of memantine administration has been studied on the model of mice poisoning with an anticholinesterase compound. It is established that the memantine action is due to its influence on the cholinesterase activity in the brain, blood plasma, and erythrocytes in addition to its NMDA-blocking action. Memantine promotes oxime-induced erythrocyte enzyme reactivation on the model of mice poisoning with anticholinesterase compound (0.8 LD50).

Nano-intercalated organophosphorus-hydrolyzing enzymes in organophosphorus antagonism

A dendritic poly(2-alkyloxazoline)-based polymer was studied as a new carrier system for the organophosphorus-hydrolyzing recombinant enzymes, organophosphorus acid anhydrolase and organophosphorus hydrolase. Paraoxon (PO) and diisopropylfluorophosphate (DFP) were used as model organophosphorus compounds. Changes in plasma cholinesterase activity were monitored. The cholinesterase activity was proportional to the concentrations of DFP or PO. Plasma cholinesterase activity was higher in animals receiving enzyme and oxime before the organophosphates than in the oxime-only pretreated groups. These studies suggest that cholinesterase activity can serve as an indicator for the in vivo protection by the nano-intercalated organophosphorus acid anhydrolase or organophosphorus hydrolase against organophosphorus intoxications. These studies represent a practical application of polymeric nano-delivery systems as enzyme carriers in drug antidotal therapy.

The effect of oxime reactivators on muscarinic receptors: functional and binding examinations

The antidotal treatment of organophosphorus poisoning is still a problematic issue since no versatile antidote has been developed yet. In our study, we focused on an interesting property, which does not relate to the reactivation of inhibited acetylcholinesterase (AChE) of some oximes, but refers to their anti-muscarinic effects which may contribute considerably to their treatment efficacy. One standard reactivator (HI-6) and two new compounds (K027 and K203) have been investigated for their antimuscarinic properties. Anti-muscarinic effects were studies by means of an in vitro stimulated atrium preparation (functional test), the [(3)H]-QNB binding assay and G-protein coupled receptor assay (GPCR, beta-Arrestin Assay). Based on the functional data HI-6 demonstrates the highest anti-muscarinic effect. However, only when comparing [(3)H]-QNB binding results and GPCR data, K203 shows a very promising compound with regard to anti-muscarinic potency. The therapeutic impact of these findings has been discussed.

Assessment of acetylcholinesterase activity using indoxylacetate and comparison with the standard Ellman's method

Assay of acetylcholinesterase (AChE) activity plays an important role in diagnostic, detection of pesticides and nerve agents, in vitro characterization of toxins and drugs including potential treatments for Alzheimer’s disease. These experiments were done in order to determine whether indoxylacetate could be an adequate chromogenic reactant for AChE assay evaluation. Moreover, the results were compared to the standard Ellman’s method. We calculated Michaelis constant Km (2.06 × 10⁻⁴ mol/L for acetylthiocholine and 3.21 × 10⁻³ mol/L for indoxylacetate) maximum reaction velocity V_max (4.97 × 10⁻⁷ kat for acetylcholine and 7.71 × 10⁻⁸ kat for indoxylacetate) for electric eel AChE. In a second part, inhibition values were plotted for paraoxon, and reactivation efficacy was measured for some standard oxime reactivators: obidoxime, pralidoxime (2-PAM) and HI-6. Though indoxylacetate is split with lower turnover rate, this compound appears as a very attractive reactant since it does not show any chemical reactivity with oxime antidots and thiol used for the Ellman’s method. Thus it can be advantageously used for accurate measurement of AChE activity. Suitability of assay for butyrylcholinesterase activity assessment is also discussed.

In vitro ability of currently available oximes to reactivate organophosphate pesticide-inhibited human acetylcholinesterase and butyrylcholinesterase

We have in vitro tested the ability of common, commercially available, cholinesterase reactivators (pralidoxime, obidoxime, methoxime, trimedoxime and HI-6) to reactivate human acetylcholinesterase (AChE), inhibited by five structurally different organophosphate pesticides and inhibitors (paraoxon, dichlorvos, DFP, leptophos-oxon and methamidophos). We also tested reactivation of human butyrylcholinesterase (BChE) with the aim of finding a potent oxime, suitable to serve as a “pseudocatalytic” bioscavenger in combination with this enzyme. Such a combination could allow an increase of prophylactic and therapeutic efficacy of the administered enzyme. According to our results, the best broad-spectrum AChE reactivators were trimedoxime and obidoxime in the case of paraoxon, leptophos-oxon, and methamidophos-inhibited AChE. Methamidophos and leptophos-oxon were quite easily reactivatable by all tested reactivators. In the case of methamidophos-inhibited AChE, the lower oxime concentration (10⁻⁵ M) had higher reactivation ability than the 10⁻⁴ M concentration. Therefore, we evaluated the reactivation ability of obidoxime in a concentration range of 10⁻³–10⁻⁷ M. The reactivation of methamidophos-inhibited AChE with different obidoxime concentrations resulted in a bell shaped curve with maximum reactivation at 10⁻⁵ M. In the case of BChE, no reactivator exceeded 15% reactivation ability and therefore none of the oximes can be recommended as a candidate for “pseudocatalytic” bioscavengers with BChE.

Tenocyclidine treatment in soman-poisoned rats--intriguing results on genotoxicity versus protection

This study aimed to evaluate the antidotal potency of tenocyclidine (TCP) that probably might protect acetylcholinesterase (AChE) in the case of organophosphate poisoning. TCP was tested alone as a pretreatment or in combination with atropine as a therapy in rats poisoned with (1/4) and (1/2) of LD(50) of soman. Possible genotoxic effects of TCP in white blood cells and brain tissue were also studied. Results were compared with previous findings on the adamantyl tenocyclidine derivative TAMORF. TCP given alone as pretreatment, 5 min before soman, seems to be superior in the protection of cholinesterase (ChE) catalytic activity in the plasma than in brain, especially after administration of the lower dose of soman. Plasma activities of the enzyme after a joint treatment with TCP and soman were significantly increased at 30 min (P<0.001) and 24 h (P=0.0043), as compared to soman alone. TCP and atropine, given as therapy, were more effective than TCP administered alone as a pretreatment. The above therapy significantly increased activities of the enzyme at 30 min (P=0.046) and 24 h (P<0.001), as compared to controls treated with (1/4) LD(50) of soman alone. Using the alkaline comet assay, acceptable genotoxicity of TCP was observed. However, the controversial role of TCP in brain protection of soman-poisoned rats should be studied further.

Alzheimer's disease: case-control association study of polymorphisms in ACHE, CHAT, and BCHE genes in a Sardinian sample

Alzheimer's disease (AD) is characterized by an extensive loss of cholinergic neurons, and their cortical projections, from the basal forebrain area. The resulting reduction in cholinergic activity is associated with decreased levels of the neurotransmitter acetylcholine (ACh), decreased activity of acetylcholinesterase (AChE), choline acetyltransferase (ChAT), and increased butyrylcholinesterase (BChE) activity. In the present study, we investigated whether the BCHE, ACHE, and CHAT genes were associated with AD and the possibility of a synergistic effect with APOE-epsilon4 in a Sardinian sample. AD patients (n = 158), exclusively of Sardinian ancestry, were recruited from the Division of Geriatrics Local Health Agency 8 and Unit of Clinical Pharmacology, Department of Neurosciences, University of Cagliari. Patients were diagnosed according to DSM-IV, and National Institute of Neurologic and Communicative Disorders and Stroke-AD and Related Disorders Association (NINCDS-ADRDA) criteria for possible or probable AD. Cognitive screening was performed by means of Mini-Mental State Examination (MMSE). Healthy controls (n = 118) of Sardinian ancestry were recruited from religious and sport associations. All patients and control subjects gave informed consent for participation in the study. Single nucleotide polymorphism (SNP) analysis was performed by PCR/RFLP or the TaqMan 5' exonuclease method. Our study confirms the association between APOE epsilon4 allele and AD (P < 0.000). No significant differences were observed in allele and genotype frequencies of BCHE, ACHE, and CHAT between AD and controls. Haplotype analysis of ACHE SNPs did not reveal a significant association between ACHE and AD. Our results suggest that the AChE, ChAT, and BChE polymorphisms do not constitute a major genetic risk factor for susceptibility to AD in a Sardinian population.

Structural Requirements of Acetylcholinesterase Reactivators

Nerve agents (sarin, soman, cyclosarin, tabun and VX agent) and pesticides (paraoxon, chlorpyrifos, TEPP) represent extremely toxic group of organophosphorus compounds (OPCs). These compounds inhibit enzyme acetylcholinesterase (AChE, EC 3.1.1.7) via its phosphorylation or phosphonylation at the serine hydroxy group in its active site. Afterwards, AChE is not able to serve its physiological function and intoxicated organism is died due to overstimulation of cholinergic nervous system. The current standard treatment of poisoning with highly toxic OPCs usually consists of the combined administration of anticholinergic drugs (preferably atropine) and AChE reactivators (called "oximes"). Anticholinergic drugs block effects of accumulated neurotransmitter acetylcholine at nicotinic and muscarinic receptor sites, while oximes reactivate AChE inhibited by OPCs. Unfortunately, none from the currently used oximes is sufficiently effective against all known nerve agents and pesticides. Therefore, to find new oximes able to sufficiently reactivate inhibited AChE (regardless of the type of OPCs) is still very important task for medicinal chemistry with the aim to improve the efficacy of antidotal treatment of the acute poisonings mentioned. In this paper, the relationship between chemical structure of AChE reactivators and their ability to reactivate AChE inhibited by several nerve agents and pesticides is summarized. It is shown that there are several structural fragments possibly involving in the structure of proposed AChE reactivators. Finally, an attempt of a future course of new AChE reactivators development is discussed.

Inhibition of plasma butyrylcholinesterase activity in the lizard Gallotia galloti palmae by pesticides: a field study

A field study was performed to evaluate the effect of exposure to organophosphorus (OP) and carbamate (CB) pesticides on the lizard Gallotia galloti palmae. Butyrylcholinesterase (BChE) activity was measured in the plasma of 420 lizards collected from agricultural and reference areas on the Island of La Palma (Canary Islands, Spain) in two sampling periods. Exposure to cholinesterase-inhibiting pesticides was evaluated by a statistical criterion based on a threshold value (two standard deviations below the mean enzyme activity) calculated for the reference group, and a chemical criterion based on the in vitro reactivation of BChE activity using pyridine-2-aldoxime methochloride (2-PAM) or after water dilution of the sample. Mean (+/- SD) BChE activity for lizards from agricultural areas was significantly lower (Fuencaliente site = 2.00 +/- 0.98 micromol min(-1) ml(-1), Tazacorte site = 2.88 +/- 1.08) than that for lizards from the reference areas (Los Llanos site = 3.06 +/- 1.17 micromol min(-1) ml(-1), Tigalate site = 3.96 +/- 1.62). According to the statistical criterion, the number of lizards with BChE depressed was higher at Fuencaliente (22% of males and 25.4% of females) than that sampled at Tazacorte (7.8% of males and 6.2% of females). According to the chemical criterion, Fuencaliente also yielded a higher number of individuals (112 males and 47 females) with BChE activity inhibited by both OP and CB pesticides. CBs appeared to be the pesticides most responsible for BChE inhibition because most of the samples showed reactivation of BChE activity after water treatment (63.3% from Fuencaliente and 29% from Tazacorte). We concluded that the use of reactivation techniques on plasma BChE activity is a better and more accurate method for assessing field exposure to OP/CB pesticides in this lizard species than making direct comparisons of enzyme activity levels between sampling areas.

Accidental injection of a U.S. Air Force aviator by a pralidoxime chloride auto-injector: a case report

To counter the threat of organophosphate nerve agents, military personnel may be issued auto-injectors containing pralidoxime chloride. This drug helps to dephosphorylate the nerve agent-acetylcholinesterase complex and, thus, regenerate the enzyme. In non-poisoned persons, pralidoxime chloride is rapidly excreted by the kidneys and is fairly well tolerated. We present the first reported case of an accidental injection of an Air Force aviator by an auto-injector. The patient recovered well with no specific treatment needed. The pharmacology and toxicology of pralidoxime chloride are discussed.

Autocatalytic degradation and stability of obidoxime

The degradation of obidoxime chloride (toxogonin), a reactivator of inhibited cholinesterase in organophosphorus poisoning, in concentrated (250 mg mL(-1)) acidic solutions was studied by HPLC at several temperatures to determine the degradation mechanism. The degradation had an autocatalytic profile, which was found to result from the formation of formaldehyde during the degradation process. The activation energy of the hydrolysis was 26.2 kcal mol(-1). The shelf-life (t90, the time by which 10% of the drug has degraded) at 25 degrees C was calculated by several methods and found to be more than 37 years. Autocatalysis, a mechanism found only rarely in the degradation of pharmaceuticals, has not been reported in previous studies of obidoxime hydrolysis.

Stability of trimedoxime in concentrated acidic injectable solutions

The degradation of trimedoxime in concentrated (114 mg/ml) acidic solutions was studied at several temperatures in the pH range 3.0 to 4.3. The degradation profile showed a relationship indicative of first-order kinetics. Trimedoxime was found to be stable in the pH range 3.0 to 3.8, with maximum stability at pH 3.0. The activation energy of the hydrolysis reaction at pH 3.0 was found to be 19.4 kcal/mol, and the half-life was 124 years. General equations were derived relating the half-life of trimedoxime solution to pH and temperature. The t90 value at 25 degrees C was calculated for each pH value studied and was found to be 11 to 18 years within the pH range 3.0 to 3.8.

Reactivating potency of obidoxime, pralidoxime, HI 6 and HLö 7 in human erythrocyte acetylcholinesterase inhibited by highly toxic organophosphorus compounds

The treatment of poisoning by highly toxic organophosphorus compounds (nerve agents) is unsatisfactory. Until now, the efficacy of new potential antidotes has primarily been evaluated in animals. However, the extrapolation of these results to humans is hampered by species differences. Since oximes are believed to act primarily through reactivation of inhibited acetylcholinesterase (AChE) and erythrocyte AChE is regarded to be a good marker for the synaptic enzyme, the reactivating potency can be investigated with human erythrocyte AChE in vitro. The present study was undertaken to evaluate the ability of various oximes at concentrations therapeutically relevant in humans to reactivate human erythrocyte AChE inhibited by different nerve agents. Isolated human erythrocyte AChE was inhibited with soman, sarin, cyclosarin, tabun or VX for 30 min and reactivated in the absence of inhibitory activity over 5-60 min by obidoxime, pralidoxime, HI 6 or HLö 7 (10 and 30 microM). The AChE activity was determined photometrically. The reactivation of human AChE by oximes was dependent on the organophosphate used. After soman, sarin, cyclosarin, or VX the reactivating potency decreased in the order HLö 7 > HI 6 > obidoxime > pralidoxime. Obidoxime and pralidoxime were weak reactivators of cyclosarin-inhibited AChE. Only obidoxime and HLö 7 reactivated tabun-inhibited AChE partially (20%), while pralidoxime and HI 6 were almost ineffective (5%). Therefore, HLö 7 may serve as a broad-spectrum reactivator in nerve agent poisoning at doses therapeutically relevant in humans.

[The presynaptic mechanism of interaction of cholinesterase reactivator and muscarinic antagonists in the animal's defence in organophosphate intoxication]

The presynaptic and antidote activity of various combinations of cholinesterase reactivators and that of muscarinic antagonist were compare it was show that the mechanism of the antidote effect of some cholinesterase reactivators in poisoning with organophosphorous compounds, in addition to restoring the activity of the enzyme, includes the effect of counteraction to the effect of muscarinic antagonists in increased acetylcholine secretion into the synaptic cleft. The expression of this effect depends on the presynaptic activity of the reactivator and on the selectivity of muscarinic antagonist. It is suggested that the type relation of the presynaptic muscarin receptors in the cerebral hemispheres and the brain stem is different and that the probability of the functional value of the ligand presynaptic interaction for the protection from organophosphorous compounds is higher in the brain higher in the brain stem than in the cerebral hemispheres of rats.

Interactions of oxime reactivators with diethylphosphoryl adducts of human acetylcholinesterase and its mutant derivatives

Diethylphosphoryl conjugates of human acetylcholinesterase (AChE) and selected mutants, carrying amino acid replacements at the active center and at the peripheral anionic site, were subjected to reactivation with the monopyridinium oxime 2-hydroxy-iminomethyl-1-methylpyridinium chloride and the bispyridinium oximes 1,3-bis(4'-hydroxyiminomethyl-1'-pyridinium),propane dibromide (TMB-4) and 1-(2'-hydroxyiminomethyl-1'-pyridinium)-3-(4"-carbamoyl-1"-pyridinium)-2 - oxapropane dichloride (HI-6). The kinetic profiles for all of the reactivation reactions indicate single populations of reactivatable species. Replacement of Trp86, the anionic subsite in the active center, lowered the affinity of the free enzyme toward all three reactivators, but in the corresponding diethylphosphoryl conjugate, only affinity toward TMB-4 was affected. Replacement of other constituents of the hydrophobic subsite (Tyr337, Phe338) had no major effect on either affinity to the free enzymes or rates of reactivation. Substitution of residues of the acyl pocket (Phe295, Phe297) lowered the affinities toward reactivators except for the 20-fold increase in affinity of F295A toward HI-6. Replacement of the acidic residues in the active center (Glu202, Glu450) affected mainly the rates of nucleophilic displacement of the phosphoryl moiety. The effect of substituting residues constituting the peripheral anionic site at the rim of the active site gorge (Tyr72, Asp74, Trp286) was particularly puzzling because for 2-hydroxy-iminomethyl-1-methylpyridinium chloride and HI-6, mainly the nucleophilic reaction rate constants were affected, whereas for TMB-4, the affinities of the phosphorylated enzymes were significantly reduced. The fact that perturbations of the functional architecture of HuAChE active center can account for only some of the observed effects on the reactivation rates suggests that the binding modes of oxime to the phosphorylated and nonphosphorylated enzymes are considerably different and/or that interactions of the reactivators with the phosphoryl moieties play a dominant role in the reactivation process.

Acetylcholinesterase inhibition in the crayfish Procambarus clarkii exposed to fenitrothion

Laboratory and field studies were conducted to evaluate the effects of fenitrothion (O,O-dimethyl O-4-nitro-m-tolyl phosphorothioate) on the crayfish Procambarus clarkii. Acetyl- and butyryl-cholinesterase activities were measured in the muscle of P. clarkii exposed to different doses of fenitrothion (4, 20, and 100 microg/liter) for different times (up to 48 hr). A positive correlation was found between both cholinesterases, and acetylcholinesterase (AChE) was selected as a biomarker of exposure to this compound. Significant AChE depression (20%) was detected 2 hr after exposure to 20 microg/liter of fenitrothion, reaching a maximum at 48 hr (47%), followed by a slow recovery. Reactivation techniques using the nucleophilic reagent pyridine 2-aldoxime methiodide were assayed in fenitrothion-poisoned specimens, and the results suggested the utility of this method to diagnose exposure, particularly when control animals are not available. Finally, AChE inhibition was used to test a field population of P. clarkii potentially exposed to high concentrations of the organophosphorus pesticide fenitrothion, and a 55% inhibition was detected.

In vitro binding of oxime acetylcholinesterase reactivators to proteoglycans synthesized by cultured chondrocytes and fibroblasts

The incorporation of the 14C-labelled acetylcholinesterase reactivators 1-(methyl-imidazolium)-3 (4-carbaldoxime-pyridinium) propane dibromide (pyrimidoxime) and N,N'-trimethylene bis(pyridinium-4-aldoxime) dibromide (TMB4) into cultured chondrocytes and fibroblasts was measured and their binding to macromolecules synthesized by these cells studied. The results showed that these drugs concentrated slowly and poorly into these cells, but bound firmly to high molecular mass materials in the culture supernatants. The chromatographic properties of these macromolecules on Sepharose CL-2B in non-dissociative or dissociative conditions were similar to those of the proteoglycans synthesized by these cells. Dialysis of the macromolecule-bound drugs against increasing pH buffers showed half-dissociation pH > 8, identical to those for chondroitin sulphate. These results suggest strongly that pyrimidoxime and TMB4 are bound to proteoglycans by ionic interactions, and this together with their poor lipophilicity can explain their high selectivity for the cartilaginous tissues as opposed to other proteoglycan-containing structures such as skin.

Therapy of organophosphate poisoning in the rat by direct effects of oximes unrelated to ChE reactivation

Isolated rat diaphragm preparations treated with soman or with the irreversible and oxime resistant cholinesterase (ChE) inhibitor S27 showed a considerable recovery of neuromuscular transmission (NMT) during incubation with the (bis)pyridinium oximes HI-6, HGG-12, P2S and obidoxime. In the soman-treated preparations this NMT recovery was predominantly caused by reactivation of acetylcholinesterase (AChE) but in the S27-treated preparations it was caused by a direct (pharmacological) effect unrelated to enzyme reactivation. Atropinized rats were artificially ventilated after injection with 3 x LD50 soman for 3 h and then treated with HI-6, i.e. at a time when oxime reactivation of soman inhibited ChE is no longer possible. Nevertheless, these rats started to breathe spontaneously and 50-60% survived more than 24 h, whereas all control animals (saline instead of HI-6) died within 10 min after artificial ventilation was terminated. In such animals no significant reactivation of ChE activity at various time intervals following HI-6 treatment was found, either in the diaphragms or in the brains. There was a significant amount of NMT (50%) in vitro in diaphragms obtained from these animals. This NMT did not improve in vitro in the presence of HI-6 and was not inhibited by soman administered to the medium. It is concluded that in this case the NMT found was based on synaptic adaptation to the continued inhibition of ChE and that the survival of the animals might be due to a combination of this synaptic adaptation and central direct effects of HI-6.

Disposition and metabolism of two acetylcholinesterase reactivators, pyrimidoxime and HI6, in rats submitted to organophosphate poisoning

1. The dispositions of two acetylcholinesterase reactivators, pyrimidoxime and HI6, labelled with 14C on the oxime group, have been studied in normal rats and rats poisoned by the organophosphates Soman and A4. 2. For both compounds, and for healthy and poisoned rats, radioactivity was eliminated essentially in the urine (85% dose in 24 h). Faecal elimination was low (4% in 72 h). 3. Both compounds were concentrated in kidney and mucopolysaccharide-containing tissues such as cartilage and intervertebral disc. Soman and A4 poisoning do not modify the kinetic parameters of pyrimidoxime, but A4 poisoning increases HI6 tissue concentration. 4. Chromatography of urine and plasma showed only unchanged pyrimidoxime in both healthy and poisoned animals. In contrast, HI6 in plasma and urine was strongly degraded by scission of the quaternary ammonium bond, and formation of 2-pyridine aldoxime.

Effects of dichlorvos on blood cholinesterase activities of cattle

Inhibitory effects of dichlorvos (2,2-dichlorovinyl dimethyl phosphate, DDVP) [corrected] on erythrocyte acetylcholinesterase (AChE) and plasma cholinesterase (ChE) activities of steers were characterized after treatments in vitro and in vivo (cutaneous application). The rates of in vitro inhibition were markedly influenced by DDVP concentration and incubation time. The activities of inhibited enzymes failed to reactivate spontaneously and had little response to treatment with 2-pyridine aldoxime methiodide (2-PAM). After gel-filtration chromatography, however, the inhibited enzymes had remarkable spontaneous reactivation and reactivation by 2-PAM treatment, indicating interference of excess unreacted DDVP in the reactivation process. Repeated cutaneous applications of a DDVP-containing livestock spray caused marked and characteristic decreases of AChE and ChE activities in blood of treated steers; however, the effects were transient because activities of both enzymes regenerated gradually. The nature of these in vivo trends suggests that spontaneous and de novo synthetic mechanisms could be responsible for complete recovery of both enzyme activities.

pH effects in the spontaneous reactivation of phosphinylated acetylcholinesterase

Previous studies on the spontaneous reactivation of phosphorylated and phosphonylated cholinesterases report bell-shaped curves with reaction rate maxima between pH values of 7 and 9. By way of contrast, we found reactivation rate minima in the same pH region for a phosphinylated bovine erythrocyte acetylcholinesterase and three phosphinylated eel acetylcholinesterases. To further elucidate these observations, eel acetylcholinesterase was inhibited with racemic 4-nitrophenyl ethyl(phenyl)phosphinate. The spontaneous reactivation of the inhibited enzyme over the pH range 6.00 to 9.00 was monitored following 1. both inhibition and spontaneous reactivation at the same pH, and 2. inhibition at pH 7.60 followed by spontaneous reactivation at the selected pH. The combined plots of both studies gave overlapping pH curves with minima around pH 7.60. The results indicate that the minima in the rates of the spontaneous reactivation of phosphinylated acetylcholinesterases are not the consequence of a pH-controlled change in the relative inhibition rates of the P(+)- and P(-)-enantiomers participating in the inhibition reaction. Our results suggest that the orientation of the phosphinyl group in the active site of phosphinylated acetylcholinesterase is quite different from that of the inhibitor groups in phosphonylated or phosphorylated enzyme.

[Pharmacokinetics of dipyroxime in the body of rats and dogs]

Pharmacokinetics of dipyroxime was studied following intravenous injection to noninbred albino male rats. Dipyroxime is a cholinesterase reactivating drug now widely used as an antidote in poisonings caused by organophosphorus pesticides. Dipyroxime pharmacokinetics analysis made it possible to propose a formula for approximation of the data on another animal species (dogs). The drug concentration in the dog blood plasma is in a good correlation with the predicted level.

Organophosphorus compounds. II. Metabolic considerations

Organophosphorus compounds are esters of alcohols with phosphoric acids or anhydrides of phosphoric acids with some other acids. The most important groups are the phosphates, phosphorothionates, phosphorothioates, phosphoroamidates, phosphorochloridate and phosphonates. The metabolism of phosphorothionates and phosphorothioates involves initial activation (oxidative desulphuration) followed by hydrolysis of the active metabolites. Activation is carried out by the action of microsomal oxidases, and degradation is performed by different types of hepatic and plasma esterases.

Pharmacokinetics and pharmacodynamics of the oxime HI6 in dogs

The pharmacokinetics and pharmacodynamics of the oxime HI6 were investigated in conscious and anesthetized beagle dogs following intramuscular injection. The absorption of HI6 (100 mumol/kg) was slower in conscious dogs as compared to the anesthetized dogs, and the maximum concentrations in plasma were lower (200 instead of 300 mumol/l). In comparison, the elimination of HI6 (100 mumol/kg) was twice as rapid in the conscious dogs (ke = 0.013 instead of 0.006 min-1) as in the anesthetized animals and was equal to the elimination after injection of 50 mumol/kg (likewise in anesthesia). The more rapid elimination was accompanied by a greater renal excretion of unchanged HI6 (60% instead of 40% in 3 h). HI6 penetrated the blood-brain barrier. The concentration of the oxime in CSF increased rapidly during the absorption phase (by 30 min after injection). The maximum concentrations (1-3 mumol/l) were reached between 60 and 120 min. The peak concentrations in plasma and CSF did not correlate with each other. In the anesthetized dogs the higher dose of HI6 (100 mumol/kg) caused a steady decrease in mean blood pressure (20 mm Hg) and blood flow (50%) in the femoral artery and a fall in left ventricular pressure (20 mm Hg), lasting for at least 60 min; the lower dose (50 mumol/kg) did not cause circulatory effects. EKG, respiration, hematocrit, arterial blood gases, and pH were not influenced.

Reactivation kinetics of diethylphosphoryl acetylcholine esterase

The kinetics of reactivation of diethylphosphorylated acetylcholine esterase by pyridine-2-aldoxime methochloride has been studied using the approach of following the course of the hydrolysis of acetylcholine during the reactivation of the phosphorylated enzyme by the reactivator [Tsou, C.-L. (1965) Acta Biochem. Biophys. Sin. 5, 398-417]. Equations are derived based on the scheme of the formation of a complex between the phosphorylated enzyme and the reactivator and the rate of dissociation of this complex is not necessarily faster than the dephosphorylation and regeneration of the active enzyme. The regenerated enzyme then reacts with the substrate through an acetyl-enzyme intermediate as generally depicted. The equation obtained for product formation during the course of reactivation contains two exponential terms and this is in accord with the experimentally observed biphasic reaction. By making the assumption that the dissociation of the phosphorylated enzyme-reactivator complex is much faster than the dephosphorylation reaction, the above equation can be simplified to a form containing only one exponential term. By following the course of the reactivation reaction with the conventional approach of taking aliquots and assaying for enzyme activity recovery, it would appear likely that one would miss the initial stage of this biphasic reaction.