Oxime K027: novel low-toxic candidate for the universal reactivator of nerve agent- and pesticide-inhibited acetylcholinesterase

Experimental and Established Oximes as Pretreatment before Acute Exposure to Azinphos-Methyl

Poisoning with organophosphorus compounds (OPCs) represents an ongoing threat to civilians and rescue personal. We have previously shown that oximes, when administered prophylactically before exposure to the OPC paraoxon, are able to protect from its toxic effects. In the present study, we have assessed to what degree experimental (K-27; K-48; K-53; K-74; K-75) or established oximes (pralidoxime, obidoxime), when given as pretreatment at an equitoxic dosage of 25% of LD01, are able to reduce mortality induced by the OPC azinphos-methyl. Their efficacy was compared with that of pyridostigmine, the only FDA-approved substance for such prophylaxis. Efficacy was quantified in rats by Cox analysis, calculating the relative risk of death (RR), with RR=1 for the reference group given only azinphos-methyl, but no prophylaxis. All tested compounds significantly (p ≤ 0.05) reduced azinphos-methyl-induced mortality. In addition, the efficacy of all tested experimental and established oximes except K-53 was significantly superior to the FDA-approved compound pyridostigmine. Best protection was observed for the oximes K-48 (RR = 0.20), K-27 (RR = 0.23), and obidoxime (RR = 0.21), which were significantly more efficacious than pralidoxime and pyridostigmine. The second-best group of prophylactic compounds consisted of K-74 (RR = 0.26), K-75 (RR = 0.35) and pralidoxime (RR = 0.37), which were significantly more efficacious than pyridostigmine. Pretreatment with K-53 (RR = 0.37) and pyridostigmine (RR = 0.52) was the least efficacious. Our present data, together with previous results on other OPCs, indicate that the experimental oximes K-27 and K-48 are very promising pretreatment compounds. When penetration into the brain is undesirable, obidoxime is the most efficacious prophylactic agent already approved for clinical use.

Combined Pre- and Posttreatment of Paraoxon Exposure

AIMS

Organophosphates (OPCs), useful agents as pesticides, also represent a serious health hazard. Standard therapy with atropine and established oxime-type enzyme reactivators is unsatisfactory. Experimental data indicate that superior therapeutic results can be obtained when reversible cholinesterase inhibitors are administered before OPC exposure. Comparing the protective efficacy of five such cholinesterase inhibitors (physostigmine, pyridostigmine, ranitidine, tacrine, or K-27), we observed best protection for the experimental oxime K-27. The present study was undertaken in order to determine if additional administration of K-27 immediately after OPC (paraoxon) exposure can improve the outcome.

METHODS

Therapeutic efficacy was assessed in rats by determining the relative risk of death (RR) by Cox survival analysis over a period of 48 h. Animals that received only pretreatment and paraoxon were compared with those that had received pretreatment and paraoxon followed by K-27 immediately after paraoxon exposure.

RESULTS

Best protection from paraoxon-induced mortality was observed after pretreatment with physostigmine (RR = 0.30) and K-27 (RR = 0.34). Both substances were significantly more efficacious than tacrine (RR = 0.67), ranitidine (RR = 0.72), and pyridostigmine (RR = 0.76), which were less efficacious but still significantly reduced the RR compared to the no-treatment group (paraoxon only). Additional administration of K-27 immediately after paraoxon exposure (posttreatment) did not further reduce mortality. Statistical analysis between pretreatment before paraoxon exposure alone and pretreatment plus K-27 posttreatment did not show any significant difference for any of the pretreatment regimens.

CONCLUSIONS

Best outcome is achieved if physostigmine or K-27 are administered prophylactically before exposure to sublethal paraoxon dosages. Therapeutic outcome is not further improved by additional oxime therapy immediately thereafter.

Encapsulation of oxime K027 into cucurbit[7]uril: In vivo evaluation of safety, absorption, brain distribution and reactivation effectiveness

Oxime-based acetylcholinesterase reactivators (briefly oximes) regenerate organophosphate-inactivated acetylcholinesterase and restore its function. Poor blood-brain-barrier passage and fast elimination from blood limit their actual use in treatment of patients exposed to organophosphates. Previous in vitro results implicated further testing of cucurbit[7]uril as a delivery vehicle for bisquaternary oximes. The present paper focuses on cell toxicity, in vivo safety and influence of cucurbit[7]uril on oxime pharmacokinetics and pharmacodynamics. Neither the K027 nor the complex caused any cell toxicity, changes in blood biochemistry or hepato- or nephrotoxicity in tested concentrations. The encapsulation of K027 increased and accelerated the blood-brain-barrier penetration. The peripheral oxime exposure also increased, supporting the suggestion that cucurbit[7]uril protects the circulating oxime from rapid renal clearance. Contrary to the comparable in vitro reactivation power of K027 and the encapsulated K027, we failed to confirm this in vivo. In theory, this might result from the non-specific binding of molecules to the cucurbit[7]uril or the interaction of K027 with cucurbit[7]uril being too strong for acetylcholinesterase reactivation. Precise explanation requires additional in silico, in vitro and also in vivo experiments.

Oximes as pretreatment before acute exposure to paraoxon

Organophosphates, useful agents as pesticides, also represent a serious danger due to their high acute toxicity. There is indication that oximes, when administered before organophosphate exposure, can protect from these toxic effects. We have tested at equitoxic dosage (25% of LD₀₁ ) the prophylactic efficacy of five experimental (K-48, K-53, K-74, K-75, K-203) and two established oximes (pralidoxime and obidoxime) to protect from mortality induced by the organophosphate paraoxon. Mortalities were quantified by Cox analysis and compared with those observed after pretreatment with a strong acetylcholinesterase inhibitor (10-methylacridine) and after the FDA-approved pretreatment compound pyridostigmine. All nine tested substances statistically significantly reduced paraoxon-induced mortality. Best protection was conferred by the experimental oxime K-48, reducing the relative risk of death (RR) to 0.10, which was statistically significantly superior to pyridostigmine (RR = 0.31). The other oximes reduced the RR to 0.13 (obidoxime), 0.20 (K-203), 0.21 (K-74), 0.24 (K-75) and 0.26 (pralidoxime), which were significantly more efficacious than 10-methylacridine (RR = 0.65). These data support the hypothesis that protective efficacy is not primarily due to cholinesterase inhibition and indicate that the tested experimental oximes may be considered promising alternatives to the established pretreatment compound pyridostigmine.

The Experimental Oxime K027-A Promising Protector From Organophosphate Pesticide Poisoning. A Review Comparing K027, K048, Pralidoxime, and Obidoxime

Poisoning with organophosphorus compounds (OPCs) is a major problem worldwide. Standard therapy with atropine and established oxime-type enzyme reactivators (pralidoxime, obidoxime) is unsatisfactory. In search of more efficacious broad-spectrum oximes, new bispyridinium (K-) oximes have been synthesized, with K027 being among the most promising. This review summarizes pharmacokinetic characteristics of K027, its toxicity and in vivo efficacy to protect from OPC toxicity and compares this oxime with another experimental bisquaternary asymmetric pyridinium aldoxime (K048) and two established oximes (pralidoxime, obidoxime). After intramuscular (i.m.) injection, K027 reaches maximum plasma concentration within ∼30 min; only ∼2% enter the brain. Its intrinsic cholinesterase inhibitory activity is low, making it relatively non-toxic. In vitro reactivation potency is high for ethyl-paraoxon-, methyl-paraoxon-, dichlorvos-, diisopropylfluorophosphate (DFP)- and tabun-inhibited cholinesterase. When administered in vivo after exposure to the same OPCs, K027 is comparable or more efficacious than pralidoxime and obidoxime. When given as a pretreatment before exposure to ethyl-paraoxon, methyl-paraoxon, DFP, or azinphos-methyl, it is superior to the Food and Drug Administration-approved compound pyridostigmine and comparable to physostigmine, which because of its entry into the brain may cause unwanted behavioral effects. Because of its low toxicity, K027 can be given in high dosages, making it a very efficacious oxime not only for postexposure treatment but also for prophylactic administration, especially when brain penetration is undesirable.

Dose-response modeling of reactivating potency of oximes K027 and K203 against a direct acetylcholinesterase inhibitor in rat erythrocytes

Inhibition of acethylcholinesterase (AChE) as a key molecular event induced by organophosphate (OP) pesticides and nerve agents presents a human health concern. In efficacy testing of experimental oximes, potential antidotes in OP poisoning, reactivation of OP-inhibited AChE is used as specific endpoint. However, according to our best knowledge, so far oximes have not been quantitatively evaluated by comprehensive benchmark dose (BMD) approach, that would improve both identification and quantification of the effect and allow more rigorous comparison of efficacies. Thus, we have examined in vivo dose-response relationship for two promising experimental oximes, K203 and K027, concerning reactivation of erythrocyte AChE inhibited by dichlorvos (DDVP). Groups of Wistar rats were treated with six different doses of oximes (i.m) immediately after DDVP challenge (s.c) and AChE was measured 60 min later. Dose-response modeling was done by PROAST software 65.5 (RIVM, The Nederlands). BMD-covariate method resulted in four-parameter model from both exponential and Hill model families as the best estimate of relationship between AChE activity and oxime dose, with potency parameter being oxime-dependent. Oxime K027 was shown to be 1.929-fold more potent considering that 58% increase in AChE activity was achived with the dose BMD₅₈-K027 = 52 μmol/kg in contrast to BMD₅₈-K203 = 100 μmol/kg.

Pharmacokinetic profile of promising acetylcholinesterase reactivators K027 and K203 in experimental pigs

Standard treatment of organophosphorus compounds (OPs) poisoning includes administration of an anti-muscarinic (atropine), anticonvulsive (diazepam) and acetylcholinesterase reactivator (oxime). From a wide group of newly synthesized oximes, oxime K027 and oxime K203 seem to be perspective compounds in some specific OPs intoxication. The available in vitro and in vivo preclinical data indicate that both oximes may be considered for potential human use. The main aim of this study was to establish plasmatic concentration curves of both oximes after intramuscular (i.m.) and intragastric (i.g.) application with subsequent pharmacokinetic analysis and study distribution after (i.m.) application on a non-rodent animal model (experimental pigs; 1500mg/animal). According to the results, both oximes had similar C_max (K027: 106±19μg/mL and K203: 111±8μg/mL) in T_max 19±5min, respectively, in 22±3min. Bioavailability of oxime K027 calculated as AUC_total (8389±1024minμg/mL) was halved compared to oxime K203 (16938±795minμg/mL). The highest concentration from peripheral tissues was found in the kidney and lung, but the brain concentrations stay very low, the plasma/brain ratio being approximately 1%. The applied doses were derived from the recommendation where it is possible to use three autoinjectors to save human life. The results provide us with knowledge about the pharmacokinetics and distribution of these new oximes and may help us to better estimate the human pharmacokinetic profile.

Acetylcholinesterase Inhibitors and Drugs Acting on Muscarinic Receptors- Potential Crosstalk of Cholinergic Mechanisms During Pharmacological Treatment

BACKGROUND

Pharmaceuticals with targets in the cholinergic transmission have been used for decades and are still fundamental treatments in many diseases and conditions today. Both the transmission and the effects of the somatomotoric and the parasympathetic nervous systems may be targeted by such treatments. Irrespective of the knowledge that the effects of neuronal signalling in the nervous systems may include a number of different receptor subtypes of both the nicotinic and the muscarinic receptors, this complexity is generally overlooked when assessing the mechanisms of action of pharmaceuticals.

METHODS

We have search of bibliographic databases for peer-reviewed research literature focused on the cholinergic system. Also, we have taken advantage of our expertise in this field to deduce the conclusions of this study.

RESULTS

Presently, the life cycle of acetylcholine, muscarinic receptors and their effects are reviewed in the major organ systems of the body. Neuronal and non-neuronal sources of acetylcholine are elucidated. Examples of pharmaceuticals, in particular cholinesterase inhibitors, affecting these systems are discussed. The review focuses on salivary glands, the respiratory tract and the lower urinary tract, since the complexity of the interplay of different muscarinic receptor subtypes is of significance for physiological, pharmacological and toxicological effects in these organs.

CONCLUSION

Most pharmaceuticals targeting muscarinic receptors are employed at such large doses that no selectivity can be expected. However, some differences in the adverse effect profile of muscarinic antagonists may still be explained by the variation of expression of muscarinic receptor subtypes in different organs. However, a complex pattern of interactions between muscarinic receptor subtypes occurs and needs to be considered when searching for selective pharmaceuticals. In the development of new entities for the treatment of for instance pesticide intoxication, the muscarinic receptor selectivity needs to be considered. Reactivators generally have a muscarinic M2 receptor acting profile. Such a blockade may engrave the situation since it may enlarge the effect of the muscarinic M3 receptor effect. This may explain why respiratory arrest is the major cause for deaths by esterase blocking.

A 7-methoxytacrine-4-pyridinealdoxime hybrid as a novel prophylactic agent with reactivation properties in organophosphate intoxication

Chemical warfare agents constitute an increasing threat to both military and civilian populations. Therefore, effective prophylactic approaches are urgently needed. Herein, we present a novel hybrid compound which is able not only to keep acetylcholinesterase resistant to organophosphate (OP) inhibitors, but also to serve as an enzyme reactivator in the case of OP intoxication.

Therapeutic and reactivating efficacy of oximes K027 and K203 against a direct acetylcholinesterase inhibitor

As oxime-based structures are the only causal antidotes to organophosphate (OP)-inhibited acetylcholinesterase (AChE), the majority of studies on these have been directed towards their synthesis and testing. In this study, experimental bispyridinium oximes K027 and K203, which have shown promising results in the last decade of research, were examined in vivo for their therapeutic and reactivating ability in acute poisoning by the direct AChE-inhibitor dichlorvos (DDVP), used as a dimethyl OP structural model. Additionally, the efficacy of oximes K027 and K203 was compared with the efficacy of four oximes (pralidoxime, trimedoxime, obidoxime and HI-6), already used in efficacy experiments and human medicine. To evaluate therapeutic efficacy, groups of Wistar rats were treated with equitoxic doses of oximes (5% LD50, i.m.) and/or atropine (10mg/kg, i.m.) immediately after s.c. DDVP challenge (4-6 doses). Using the same antidotal protocol, AChE activity was measured in erythrocytes, diaphragm and brain 60min after s.c. DDVP exposure (75% LD50). The oxime K027 was the most efficacious in reducing the DDVP induced lethal effect in rats, while the oxime K203 was more efficacious than trimedoxime, pralidoxime and HI-6. Significant reactivation of DDVP inhibited AChE was achieved only with oxime K027 or its combination with atropine in erythocytes and the diaphragm. Moreover, the acute i.m. toxicity of oxime K027 in rats was lower than all other tested oximes. The results of this study support previous studies considering the oxime K027 as a promising experimental oxime structure for further testing against structurally-different OP compounds.

In vivo evaluation of cholinesterase activity, oxidative stress markers, cyto- and genotoxicity of K048 oxime–a promising antidote against organophosphate poisoning

K048 is a member of K-oximes, a new oxime class that has recently been confirmed effective against poisoning by the nerve agent tabun and several pesticides. The toxicity profile of the K048 oxime has not been fully characterized and its optimal therapeutic dose has not yet been established. Earlier studies report excellent results with K048 in reactivating tabun-phosphorylated AChE and in the therapy of tabun-poisoned mice. It possesses a low acute toxicity and exerts an acceptable toxicity profile on isolated human peripheral blood lymphocytes in vitro. Intraperitoneal administration of K048 in rats resulted in an LD50 of 238.3 mg/kg. In this in vivo study, we investigated cholinesterase (ChE) activity and oxidative stress marker levels (lipid peroxidation and superoxide dismutase activity) in the plasma of exposed rats after administering the compound at 25% of its LD50. Lymphocyte viability was evaluated using an acridine orange/ethidium bromide in situ fluorescent assay. The levels of primary DNA damage in rat white blood cells were measured using the alkaline comet assay. The compound applied at 25% of its LD50 did not significantly affect ChE activity and lipid peroxidation and did not cause significant changes in the SOD activity in plasma. The cytotoxicity profile of K048 in the tested dose was also acceptable, and it did not possess significant DNA-damaging potential. The obtained results are promising for further evaluations of the K048 oxime, which should include tests on a broader concentration range and longer incubation times.

Mini review on blood-brain barrier penetration of pyridinium aldoximes

This paper reviews the blood-brain barrier (BBB) penetration of newly developed pyridinium aldoximes. Pyridinium aldoximes are highly charged hydrophilic compounds used in the treatment of subjects exposed to organophosphonates because they are effective as acetylcholinesterase reactivators. Pyridinium aldoximes have antidotal effects against poisoning with cholinesterase inhibitors, a frequent problem affecting people working with organophosphate-based insecticides and pesticides. Toxic organophosphonate products such as sarin and tabun can be used by terrorists as chemical warfare agents. This poses a severe challenge to all innocent and peace-loving people worldwide. This review gives a brief summary of BBB transporters and description of the current in vitro and in vivo methods for the characterization of BBB penetration of established and novel pyridinium aldoximes. The authors provide a putative mechanism of penetration, outline some future ways of formulation and discuss the possible advantages and disadvantages of increasing BBB penetration.

Acetylcholinesterase reactivators (HI-6, obidoxime, trimedoxime, K027, K075, K127, K203, K282): structural evaluation of human serum albumin binding and absorption kinetics

Acetylcholinesterase (AChE) reactivators (oximes) are compounds predominantly targeting the active site of the enzyme. Toxic effects of organophosphates nerve agents (OPNAs) are primarily related to their covalent binding to AChE and butyrylcholinesterase (BChE), critical detoxification enzymes in the blood and in the central nervous system (CNS). After exposure to OPNAs, accumulation of acetylcholine (ACh) overstimulates receptors and blocks neuromuscular junction transmission resulting in CNS toxicity. Current efforts at treatments for OPNA exposure are focused on non-quaternary reactivators, monoisonitrosoacetone oximes (MINA), and diacylmonoxime reactivators (DAM). However, so far only quaternary oximes have been approved for use in cases of OPNA intoxication. Five acetylcholinesterase reactivator candidates (K027, K075, K127, K203, K282) are presented here, together with pharmacokinetic data (plasma concentration, human serum albumin binding potency). Pharmacokinetic curves based on intramuscular application of the tested compounds are given, with binding information and an evaluation of structural relationships. Human Serum Albumin (HSA) binding studies have not yet been performed on any acetylcholinesterase reactivators, and correlations between structure, concentration curves and binding are vital for further development. HSA bindings of the tested compounds were 1% (HI-6), 7% (obidoxime), 6% (trimedoxime), and 5%, 10%, 4%, 15%, and 12% for K027, K075, K127, K203, and K282, respectively.

Diet composition exacerbates or attenuates soman toxicity in rats: implied metabolic control of nerve agent toxicity

To evaluate the role of diet composition on nerve agent toxicity, rats were fed four distinct diets ad libitum for 28 d prior to challenge with 110 μg/kg (1.0 LD(50), sc) soman. The four diets used were a standard rodent diet, a choline-enriched diet, a glucose-enriched diet, and a ketogenic diet. Body weight was recorded throughout the study. Toxic signs and survival were evaluated at key times for up to 72 h following soman exposure. Additionally, acquisition of discriminated shuttlebox avoidance performance was characterized beginning 24h after soman challenge and across the next 8 d (six behavioral sessions). Prior to exposure, body weight was highest in the standard diet group and lowest in the ketogenic diet group. Upon exposure, differences in soman toxicity as a function of diet became apparent within the first hour, with mortality in the glucose-enriched diet group reaching 80% and exceeding all other groups (in which mortality ranged from 0 to 6%). At 72 h after exposure, mortality was 100% in the glucose-enriched diet group, and survival approximated 50% in the standard and choline-enriched diet groups, but equaled 87% in the ketogenic diet group. Body weight loss was significantly reduced in the ketogenic and choline-enriched diet groups, relative to the standard diet group. At 1 and 4h after exposure, rats in the ketogenic diet group had significantly lower toxic sign scores than all other groups. The ketogenic diet group performed significantly better than the standard diet group on two measures of active avoidance performance. The exacerbated soman toxicity observed in the glucose-enriched diet group coupled with the attenuated soman toxicity observed in the ketogenic diet group implicates glucose availability in the toxic effects of soman. This increased glucose availability may enhance acetylcholine synthesis and/or utilization, thereby exacerbating peripheral and central soman toxicity.