Pharmacokinetics and pharmacodynamics of some oximes and associated therapeutic consequences: a critical review

Clinical manifestations, laboratory findings, treatment and outcome of acute organophosphate or carbamate intoxication in 39 cats

BACKGROUND

Organophosphates and carbamates are important sources of intoxication for humans and animals. However, large-scale studies of these intoxications in cats are unavailable.

METHODS

The medical records of 39 cats presented to a veterinary teaching hospital with acute organophosphate or carbamate intoxication were reviewed retrospectively.

RESULTS

Mortality in intoxicated cats was 15%. Low respiratory rate and low rectal temperature at presentation were associated with death. Other common clinical signs included weakness, ataxia, apathy, recumbency, anorexia and bradycardia, but these were unassociated with the outcome. The common biochemical abnormalities included decreased serum butyryl-choline esterase activity, acidaemia, hypercarbaemia and total hypocalcaemia, and increased creatine kinase activity and total plasma protein concentration. There were no significant differences in haematological, biochemical and blood gas analytes between survivors and non-survivors. Common medications and treatments included 2-pyridine aldoxime methyl-chloride-pralidoxime (2-PAM) (74%), metoclopramide (64%), antibiotics (64%), diphenhydramine (59%) and atropine sulphate (54%). There were no significant drug and treatment differences between survivors and non-survivors. The secondary complications of the intoxication included pneumonia (10%), acute kidney injury (10%) and pancreatitis (8%).

CONCLUSIONS

Acute cholinergic crisis due to organophosphate or carbamate intoxication has a fair prognosis in cats. Low respiratory rate and low rectal temperature at presentation were associated with death. The most commonly used specific medications in this study included 2-PAM, diphenhydramine and atropine sulphate.

Review about Structure and Evaluation of Reactivators of Acetylcholinesterase Inhibited with Neurotoxic Organophosphorus Compounds

BACKGROUND

Neurotoxic chemical warfare agents can be classified as some of the most dangerous chemicals for humanity. The most effective of those agents are the Organophosphates (OPs) capable of restricting the enzyme Acetylcholinesterase (AChE), which in turn, controls the nerve impulse transmission. When AChE is inhibited by OPs, its reactivation can be usually performed through cationic oximes. However, until today, it has not been developed one universal defense agent, with complete effective reactivation activity for AChE inhibited by any of the many types of existing neurotoxic OPs. For this reason, before treating people intoxicated by an OP, it is necessary to determine the neurotoxic compound that was used for contamination, in order to select the most effective oxime. Unfortunately, this task usually requires a relatively long time, raising the possibility of death. Cationic oximes also display a limited capacity of permeating the Blood-Brain Barrier (BBB). This fact compromises their capacity to reactivating AChE inside the nervous system.

METHODS

We performed a comprehensive search on the data about OPs available on the scientific literature today in order to cover all the main drawbacks still faced in the research for the development of effective antidotes against those compounds.

RESULTS

Therefore, this review about neurotoxic OPs and the reactivation of AChE, provides insights for the new agents' development. The most expected defense agent is a molecule without toxicity and effective to reactivate AChE inhibited by all neurotoxic OPs.

CONCLUSION

To develop these new agents, the application of diverse scientific areas of research, especially theoretical procedures as computational science (computer simulation, docking and dynamics), organic synthesis, spectroscopic methodologies, biology, biochemical and biophysical information, medicinal chemistry, pharmacology and toxicology, is necessary.

Synthesis, Molecular Docking, BSA, and in-vitro reactivation study of imidazopyridine oxime against paraoxon inhibited acetylcholinesterase

AIM

To synthesize and evaluate the fused heterocyclic imidazopyridine oxime as a reactivator against paraoxon inhibited acetylcholinesterase.

BACKGROUND

Organophosphorus compounds (OPs) include parathion, malathion, chlorpyrifos, monocrotophos, and diazinon which are commonly used in agriculture for enhancing agricultural productivity via killing crop-damaging pests. However, people may get exposed to OPs pesticides unintentionally/intentionally via ingestion, inhalation or dermal. The current treatment regimen includes reactivator such as mono or bis-pyridinium oximes along with anticholinergic and an anticonvulsant drugs are recommended for the treatment of OP poisoning. Unfortunately, the drawback of the existing reactivator is that owing to the permanent charge present on the pyridinium makes them inefficient to cross the blood-brain barrier (BBB) and reactivate OP-inhibited central nervous system (CNS) acetylcholinesterase. Therefore, there is a need of reactivator that could cross the BBB and reactivate the OP inhibited acetylcholinesterase.

OBJECTIVE

The objectives of the study were synthesis, molecular docking, BSA binding and in-vitro estimation of oximes of various substituted imidazo [1,2-a]pyridine against paraoxon inhibited acetylcholinesterase.

METHOD

The reactivators were synthesized in three steps and characterized using various spectroscopic techniques. Molecular docking study was performed on 2WHP and 3ZLV PDB using Autodock tool. The acid dissociation constant (pKa) of oximes was calculated experimentally and drug-likeness properties of the oximes were calculated In silico using mole inspiration and Swiss ADME software. The binding of oximes with bovine serum albumin (BSA) was also investigated by UV-Vis spectrophotometer. The reactivation potential of the oximes was determined by in vitro enzymatic assay.

RESULT

in-silico study inferred that synthesized molecules fulfilled the parameters that required for a successful CNS drug candidate. Further, in-vitro enzymatic assay indicated reasonable reactivation potential of the oximes against paraoxon-inhibited AChE. The binding of oximes with bovine serum albumin (BSA) revealed static quenching of intrinsic fluorescence of BSA by oxime. The binding constant value and number of binding sites were found 0.24 mol-1 and 1 respectively.

CONCLUSION

The results of study concluded that this scaffold could be used for further designing of more efficient uncharged reactivators.

Antiseizure and neuroprotective effects of delayed treatment with midazolam in a rodent model of organophosphate exposure

OBJECTIVE

Exposure to organophosphates (OPs) and OP nerve agents (NAs) causes status epilepticus (SE) and irreversible brain damage. Rapid control of seizure activity is important to minimize neuronal injury and the resulting neurological and behavioral disorders; however, early treatment will not be possible after mass release of OPs or NAs.

METHODS

We utilized a delayed-treatment model of OP exposure in adult rats by administration of diisopropyl fluorophosphate (DFP) to study the relationship between the antiseizure and neuroprotective effects of the "standard-of-care" benzodiazepine, midazolam (MDZ), when given at 30, 60, and 120 minutes after SE onset. After electroencephalography (EEG) recordings, neural damage in serial brain sections was studied with Fluoro-Jade B staining.

RESULTS

MDZ-induced seizure suppression was equivalent in magnitude regardless of treatment delay (ie, seizure duration). When assessed globally (ie, normalized across 10 different brain regions) for each treatment delay, MDZ administration resulted in only nonsignificant reductions in neuronal death. However, when data for MDZ treatment were combined from all three delay times, a small but significant reduction in global neuronal death was detected when compared to vehicle treatment, which indicated that the substantive MDZ-induced seizure suppression led to only a small reduction in neuronal death.

SIGNIFICANCE

In conclusion, MDZ significantly reduced DFP-induced SE intensity when treatment was delayed 30, 60, and even up to 120 minutes; however, this reduction in seizure intensity had no detectable effect on neuronal death at each individual delay time. These data show that although MDZ suppressed seizures, additional neuroprotective therapies are needed to mitigate the effects of OP exposure.

Oximes and Hydrazones in Bioconjugation: Mechanism and Catalysis

The formation of oximes and hydrazones is employed in numerous scientific fields as a simple and versatile conjugation strategy. This imine-forming reaction is applied in fields as diverse as polymer chemistry, biomaterials and hydrogels, dynamic combinatorial chemistry, organic synthesis, and chemical biology. Here we outline chemical developments in this field, with special focus on the past ∼10 years of developments. Recent strategies for installing reactive carbonyl groups and α-nucleophiles into biomolecules are described. The basic chemical properties of reactants and products in this reaction are then reviewed, with an eye to understanding the reaction's mechanism and how reactant structure controls rates and equilibria in the process. Recent work that has uncovered structural features and new mechanisms for speeding the reaction, sometimes by orders of magnitude, is discussed. We describe recent studies that have identified especially fast reacting aldehyde/ketone substrates and structural effects that lead to rapid-reacting α-nucleophiles as well. Among the most effective new strategies has been the development of substituents near the reactive aldehyde group that either transfer protons at the transition state or trap the initially formed tetrahedral intermediates. In addition, the recent development of efficient nucleophilic catalysts for the reaction is outlined, improving greatly upon aniline, the classical catalyst for imine formation. A number of uses of such second- and third-generation catalysts in bioconjugation and in cellular applications are highlighted. While formation of hydrazone and oxime has been traditionally regarded as being limited by slow rates, developments in the past 5 years have resulted in completely overturning this limitation; indeed, the reaction is now one of the fastest and most versatile reactions available for conjugations of biomolecules and biomaterials.

Cholinesterase reactivators and bioscavengers for pre- and post-exposure treatments of organophosphorus poisoning

Organophosphorus agents (OPs) irreversibly inhibit acetylcholinesterase (AChE) causing a major cholinergic syndrome. The medical counter-measures of OP poisoning have not evolved for the last 30 years with carbamates for pretreatment, pyridinium oximes-based AChE reactivators, antimuscarinic drugs and neuroprotective benzodiazepines for post-exposure treatment. These drugs ensure protection of peripheral nervous system and mitigate acute effects of OP lethal doses. However, they have significant limitations. Pyridostigmine and oximes do not protect/reactivate central AChE. Oximes poorly reactivate AChE inhibited by phosphoramidates. In addition, current neuroprotectants do not protect the central nervous system shortly after the onset of seizures when brain damage becomes irreversible. New therapeutic approaches for pre- and post-exposure treatments involve detoxification of OP molecules before they reach their molecular targets by administrating catalytic bioscavengers, among them phosphotriesterases are the most promising. Novel generation of broad spectrum reactivators are designed for crossing the blood-brain barrier and reactivate central AChE. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

A rodent model of human organophosphate exposure producing status epilepticus and neuropathology

Exposure to organophosphates (OPs) often results in seizures and/or status epilepticus (SE) that produce neural damage within the central nervous system (CNS). Early control of SE is imperative for minimizing seizure-related CNS neuropathology. Although standard therapies exist, more effective agents are needed to reduce OP-induced SE and neuronal loss, particularly therapies with efficacy when administered 10's of minutes after the onset of SE. To evaluate novel antiseizure compounds, animal models should simulate the CNS effects of OP exposure observed in humans. We characterized in rats the effects of the OP, diisopropyl flourophosphate (DFP) as a function of dose and route of administration of supporting agents (pyridostigmine, 2-PAM, atropine); outcome measures were mortality, electrographic seizure activity during SE, and subsequent CNS neuropathology. Doses of DFP between 3 and 7mg/kg consistently caused SE, and the latency to behavioral tremors and to subsequent initiation of SE were dose related. In distinction, all doses of DFP that resulted in electrographic SE (3-7mg/kg) produced seizures of similar intensity and duration, and similar CNS neuropathology (i.e., the effects were all-or-none). Although SE was similar across doses, mortality progressively increased with higher doses of DFP. Mortality was significantly lower when the route of administration of therapeutic agents was intramuscular compared to intraperitoneal. This rodent model of OP poisoning demonstrates pathological characteristics similar to those observed in humans, and thus begins to validate this model for investigating potential new therapeutic approaches.

Synthesis and in-vitro reactivation screening of imidazolium aldoximes as reactivators of sarin and VX-inhibited human acetylcholinesterase (hAChE)

Post-treatment of organophosphate (OP) poisoning involves the application of oxime reactivator as an antidote. Structurally different oximes are widely studied to examine their kinetic and mechanistic behavior against OP-inhibited cholinesterase enzyme. A series of structurally related 1,3-disubstituted-2-[(hydroxyiminomethyl)alkyl]imidazolium halides (5a-5e, 9a-9c) were synthesized and further evaluated for their in-vitro reactivation ability to reactivate sarin- and VX-inhibited human acetylcholinesterase (hAChE). The observed results were compared with the reactivation efficacy of standard reactivators; 2-PAM, obidoxime and HI-6. Amongst the synthesized oximes, 5a, 9a and 9b were found to be most potent reactivators against sarin-inhibited hAChE while in case of VX only 9a exhibited comparable reactivity with 2-PAM. Incorporation of pyridinium ring to the imidazole ring resulted in substantial increase in the reactivation strength of prepared reactivator. Physicochemical properties of synthesized reactivators have also been evaluated.

A rat model of nerve agent exposure applicable to the pediatric population: The anticonvulsant efficacies of atropine and GluK1 antagonists

Inhibition of acetylcholinesterase (AChE) after nerve agent exposure induces status epilepticus (SE), which causes brain damage or death. The development of countermeasures appropriate for the pediatric population requires testing of anticonvulsant treatments in immature animals. In the present study, exposure of 21-day-old (P21) rats to different doses of soman, followed by probit analysis, produced an LD50 of 62μg/kg. The onset of behaviorally-observed SE was accompanied by a dramatic decrease in brain AChE activity; rats who did not develop SE had significantly less reduction of AChE activity in the basolateral amygdala than rats who developed SE. Atropine sulfate (ATS) at 2mg/kg, administered 20 min after soman exposure (1.2×LD50), terminated seizures. ATS at 0.5mg/kg, given along with an oxime within 1 min after exposure, allowed testing of anticonvulsants at delayed time-points. The AMPA/GluK1 receptor antagonist LY293558, or the specific GluK1 antagonist UBP302, administered 1h post-exposure, terminated SE. There were no degenerating neurons in soman-exposed P21 rats, but both the amygdala and the hippocampus were smaller than in control rats at 30 and 90days post-exposure; this pathology was not present in rats treated with LY293558. Behavioral deficits present at 30 days post-exposure, were also prevented by LY293558 treatment. Thus, in immature animals, a single injection of atropine is sufficient to halt nerve agent-induced seizures, if administered timely. Testing anticonvulsants at delayed time-points requires early administration of ATS at a low dose, sufficient to counteract only peripheral toxicity. LY293558 administered 1h post-exposure, prevents brain pathology and behavioral deficits.

O-Benzoyl pyridine aldoxime and amidoxime derivatives: novel efficient DNA photo-cleavage agents

Pyridine oxime esters are effective DNA photocleavers, causing single-/double-stranded DNA cleavage at concentrations as low as 1 μM.

Assessment of antidotal efficacy of cholinesterase reactivators against paraoxon: In vitro reactivation kinetics and physicochemical properties

The search of proficient oximes as reactivators of irreversibly inhibited-AChE by organophosphate poisoning necessitates an appropriate assessment of their physicochemical properties and reactivation kinetics. Therefore, herein acid dissociation constant; pKa, lipophilicity; logP, polar surface area, hydrogen bond donor and acceptor counts of structurally different oximes (two tertiary oximes and thirteen pyridinium aldoxime derivatives) have been evaluated. The experimentally obtained data for pKa has been comparatively analyzed by using non-linear regression. Further the tested oximes were screened through in vitro reactivation kinetics against paraoxon-inhibited AChE. The pKa values of all the examined oximes were within the range of 7.50-9.53. pKa values of uncharged and mono-pyridinium oximes were in good correlation with their reactivation potency. The high negative logP values of pyridinium oxime reactivators indicate their high hydrophilic character; hence oximes with improved lipophilicity should be designed for the development of novel and more potent antidotes. Propane and butane linked oximes were superior reactivators than xylene linked bis-oxime reactivators. It is concluded from the present study that pKa value is not only ruled by the position of oximino functionality in the pyridinium ring, but also by the position of linker. Although, pyridinium oximes are proved to be better reactivators but their lipophilicity has to be improved.

Exploring the physicochemical properties of oxime-reactivation therapeutics for cyclosarin, sarin, tabun, and VX inactivated acetylcholinesterase

The inactivation of acetylcholinesterase (AChE) by organophosphorus agent (OP) compounds is a serious problem regardless of how the individual was exposed. The reactivation of OP-inactivated AChE is dependent on the OP conjugate, and commonly a specific oxime is better at reactivating a specific OP conjugate than several diverse OP conjugates. The presented research explores the physicochemical properties needed for the reactivation of OP-inactivated AChE. Four different OPs, cyclosarin, sarin, tabun, and VX, were analyzed using the same set of oxime reactivators. A trial descriptor pool of semiempirical, traditional, and molecular interaction field descriptors was used to construct an ensemble of QSAR models for each OP-conjugate pair. Based on the molecular information and the cross-validation ability, individual QSAR models were selected to be part of an OP-conjugate consensus model. The OP-conjugate specific models provide important insight into the physicochemical properties required to reactivate the OP conjugates of interest. The reactivation of AChE inactivated with either cyclosarin or tabun requires the oxime therapeutic to possess an overall polar-positive surface area. Oxime therapeutics for the reactivation of sarin-inactivated AChE are conformationally dependent while oxime reverse therapeutics for VX require a compact region with a highly hydrophilic region and two positively charged pyridine rings.

Conformational stability, vibrational spectra, molecular structure, NBO and HOMO-LUMO analysis of 5-nitro-2-furaldehyde oxime based on DFT calculations

The FTIR and FT-Raman spectra of 5-nitro-2-furaldehyde oxime (NFAO) have been recorded in the regions 4000-400 cm(-1) and 3500-50 cm(-1), respectively. The total energies of different conformations have been obtained from DFT (B3LYP) with 6-311++G(d,p) basis set calculations. The computational results identify the most stable conformer of NFAO as the C1 form. Utilizing the observed FTIR and FT-Raman data, a complete vibrational assignment and analysis of the fundamental modes of the compound were carried out. The optimum molecular geometry, harmonic vibrational frequencies, infrared intensities and Raman scattering activities, were calculated by density functional theory (DFT/B3LYP) method with 6-31+G(d,p) and 6-311++G(d,p) basis sets. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. A detailed interpretation of the infrared and Raman spectra of NFAO is also reported based on total energy distribution (TED). Stability of the molecule arising from hyperconjugative interactions, charge delocalization have been analyzed using natural bond orbital (NBO) analysis. Besides, molecular electrostatic potential (MEP), HOMO and LUMO analysis, and several thermodynamic properties were performed by the DFT method. Mulliken's net charges have been calculated and compared with the natural atomic charges. Ultraviolet-visible spectrum of the title molecule has also been calculated using TD-DFT method.

A physiologically based pharmacokinetic model for the oxime TMB-4: simulation of rodent and human data

Multiple oximes have been synthesized and evaluated for use as countermeasures against chemical warfare nerve agents. The current U.S. military and civilian oxime countermeasure, 2-[(hydroxyimino)methyl]-1-methylpyridin-1-ium chloride (2-PAM), is under consideration for replacement with a more effective acetylcholinesterase reactivator, 1,1'-methylenebis{4-hydroxyiminomethyl}pyridinium dimethanesulfonate (MMB-4). Kinetic data in the scientific literature for MMB-4 are limited; therefore, a physiologically based pharmacokinetic (PBPK) model was developed for a structurally related oxime, 1,1'-trimethylenebis{4-hydroximinomethyl}pyridinium dibromide. Based on a previous model structure for the organophosphate diisopropylfluorophosphate, the model includes key sites of acetylcholinesterase inhibition (brain and diaphragm), as well as fat, kidney, liver, rapidly perfused tissues and slowly perfused tissues. All tissue compartments are diffusion limited. Model parameters were collected from the literature, predicted using quantitative structure-property relationships or, when necessary, fit to available pharmacokinetic data from the literature. The model was parameterized using rat plasma, tissue and urine time course data from intramuscular administration, as well as human blood and urine data from intravenous and intramuscular administration; sensitivity analyses were performed. The PBPK model successfully simulates rat and human data sets and has been evaluated by predicting intravenous mouse and intramuscular human data not used in the development of the model. Monte Carlo analyses were performed to quantify human population kinetic variability in the human evaluation data set. The model identifies potential pharmacokinetic differences between rodents and humans, indicated by differences in model parameters between species. The PBPK model can be used to optimize the dosing regimen to improve oxime therapeutic efficacy in a human population.

Laboratory examination in nerve agent intoxication

Diagnosis of nerve agent intoxication is based on anamnestic data, clinical signs and laboratory examination. For acute poisoning, cholinesterase activity in the blood (erythrocyte AChE, plasma/serum BuChE) is sensitive, simple and most frequent laboratory examination performed in biochemical laboratories. Specialized examinations to precise treatment (reactivation test) or to make retrospective diagnosis (fluoride induced reactivation etc.) can be conducted. Other sophisticated methods are available, too.

Pharmacokinetic study of two acetylcholinesterase reactivators, trimedoxime and newly synthesized oxime K027, in rat plasma

K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-propane dibromide] is a promising new reactivator of organophosphate- or organophosphonate-inhibited acetylcholinesterase (AChE) with low acute toxicity and broad spectrum efficacy. The aim of the present study was to compare the pharmacokinetics of both compounds. Male Wistar rats (body weight = 320 ± 10 g) were administered a single intramuscular dose of K027 (22.07 mg kg(-1)) and an equimolar dose of trimedoxime. Blood was collected at various time intervals until 180 min. Plasma samples were analyzed by reversed-phase HPLC with ultraviolet (UV) detection. The recovery of both oximes from the plasma was approximately 90% and a linear relationship (R(2) > 0.998) was observed between the peak areas and concentrations of calibrated standards in the range 1-100 µg ml(-1). Near-identical plasma profiles were obtained for both compounds. No differences were found in the mean ± SD values of C(max) (18.6 ± 2.5 vs 20.0 ± 6.3 µg ml(-1), P = 0.72) and AUC(0-180min) (2290 ± 304 vs 2269 ± 197 min µg ml(-1), P = 0.84). However, the percentage coefficient of variation of the first-order rate constant of absorption (k(a)) was 3-fold higher (P < 0.01) providing evidence for more erratic absorption of intramuscular trimedoxime as compared with K027. In conclusion, oxime K027 might have superior pK properties that may be translated in its faster absorption and subsequent tissue distribution.