In vitro P-glycoprotein activity does not completely explain in vivo efficacy of novel centrally effective oxime acetylcholinesterase reactivators

Strategies for enhanced bioavailability of oxime reactivators in the central nervous system

Oxime reactivators of acetylcholinesterase are commonly used to treat highly toxic organophosphate poisoning. They are effective nucleophiles that can restore the catalytic activity of acetylcholinesterase; however, their main limitation is the difficulty in crossing the blood-brain barrier (BBB) because of their strongly hydrophilic nature. Various approaches to overcome this limitation and enhance the bioavailability of oxime reactivators in the CNS have been evaluated; these include structural modifications, conjugation with molecules that have transporters in the BBB, bypassing the BBB through intranasal delivery, and inhibition of BBB efflux transporters. A promising approach is the use of nanoparticles (NPs) as the delivery systems. Studies using mesoporous silica nanomaterials, poly (L-lysine)-graft-poly(ethylene oxide) NPs, metallic organic frameworks, poly(lactic-co-glycolic acid) NPs, human serum albumin NPs, liposomes, solid lipid NPs, and cucurbiturils, have shown promising results. Some NPs are considered as nanoreactors for organophosphate detoxification; these combine bioscavengers with encapsulated oximes. This study provides an overview and critical discussion of the strategies used to enhance the bioavailability of oxime reactivators in the central nervous system.

Pharmacokinetics of three novel pyridinium aldoxime acetylcholinesterase reactivators in female rats

A platform of novel lipophilic substituted phenoxyalkyl pyridinium oximes was invented to reactivate organophosphate-inhibited acetylcholinesterase. This platform has provided superior efficacy in rats to the current standard of care, 2-PAM, for survival of lethal doses of nerve agent surrogates as well as evidence of brain penetration and neuroprotection. The pharmacokinetics of three of these novel oximes in female rats was studied for comparison to previous data in male rats. Compared to the published half-life of 2-PAM (less than 2 h), the lead novel oxime, Oxime 20, displayed a plasma half-life of about 5 h in both sexes of rats following intramuscular administration. Very few sex differences in pharmacokinetic parameters were apparent. Oxime 20 displayed an increase in brain concentration to plasma concentration over the initial 2 h following intramuscular administration in male rats, with a plateau at 1 h; there were no differences in brain concentrations between the sexes at 2 h. Hepatic metabolism of Oxime 20 was higher in rat microsomes than in human microsomes. The relatively long plasma half-life is likely an important factor in both the enhanced survival and the neuroprotection previously observed for Oxime 20. The metabolism data suggest that the clearance of Oxime 20 could be slower in humans than was observed in rats, which might allow less frequent administration than 2-PAM for therapy of organophosphate acute toxicity. Therefore, the pharmacokinetic data combined with our earlier efficacy data suggest that Oxime 20 has potential as a superior therapeutic for nerve agent poisoning.

Effect of P-glycoprotein on the availability of oxime reactivators in the brain

The ability to overcome cellular barriers in the body is crucial for efficient delivery of drugs to the target where intervention is needed. For drugs acting in the brain it is essential to overcome the blood-brain barrier (BBB). Such drugs include antidotes for the treatment of organophosphate poisoning, a current warfare and terroristic threat. Being lipophilic compounds, organophosphates readily penetrate the brain and block the enzyme acetylcholinesterase (AChE). They cause severe symptoms which may have fatal consequences. A major drawback of currently available oxime reactivators is their inability to reactivate AChE in the central nervous system (CNS) as they are unable to cross the blood-brain barrier. An important obstacle preventing many drugs from reaching their therapeutic target in the brain is the efflux transporter P-glycoprotein (P-gp), whose function is to prevent the penetration of potentially harmful substances. The aim of this study was to evaluate the effect of P-gp on the permeation of oximes into the brain. The study of this interaction was carried out on the CACO-2 cell line, stably expressing P-gp. As it turned out, P-gp has no essential influence on the central availability of clinically used oxime reactivators within this study.

Oxime-mediated reactivation of organophosphate-inhibited acetylcholinesterase with emphasis on centrally-active oximes

This review provides an overview of the global research leading to the large number of compounds developed as reactivators of acetylcholinesterase inhibited by a variety of organophosphate compounds, most of which are nerve agents but also some insecticides. A number of these organophosphates are highly toxic and effective therapy by reactivators contributes to saving lives. Two major challenges for more effective therapy with reactivators are identification of a broad spectrum reactivator efficacious against a variety of organophosphate structures, and a reactivator that can cross the blood-brain barrier to protect the brain. The most effective of the reactivators developed are the nucleophilic pyridinium oximes, which bear a permanent positive charge from the quaternary nitrogen in the pyridinium ring. The permanent positive charge retards the oximes from crossing the blood-brain barrier and therefore restoration of normal cholinergic function in the brain is unlikely. A number of laboratories have developed nucleophiles, mostly oximes, that are theorized to cross the blood-brain barrier by several strategies. At the present time, no reactivator is optimally broad spectrum across the wide group of organophosphate chemistries. Some oximes, including the substituted phenoxyalkyl pyridinium oximes invented by our laboratories, have the potential to provide neuroprotection in the brain and show evidence of efficacy against both nerve agent and insecticidal chemistries, so these novel oximes have promise for future development. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Neuroprotection From Organophosphate-Induced Damage by Novel Phenoxyalkyl Pyridinium Oximes in Rat Brain

The nerve agents are extremely toxic organophosphates which lead to massive inhibition of acetylcholinesterase (AChE) in both the central and peripheral nervous systems. The currently approved pyridinium oxime reactivators of organophosphate-inhibited AChE (eg, 2-PAM in the United States) cannot penetrate the blood-brain barrier because of the permanent positive charge in the pyridinium ring. Therefore these current oximes cannot rescue inhibited AChE in the brain. Our laboratories have invented and patented a platform of substituted phenoxyalkyl pyridinium oximes that have been tested for efficacy as therapy within the brains of adult male rats which were challenged with a high sublethal dosage of highly relevant surrogates of sarin (nitrophenyl isopropyl methylphosphonate, NIMP) and VX (nitrophenyl ethyl methylphosphonate, NEMP). The histochemical astrocyte marker glial fibrillary acidic protein (GFAP) was investigated as an indication of neuropathology in two brain regions, the piriform cortex and the dentate gyrus of the hippocampus, which are regions known to be damaged by nerve agent toxicity. Rats treated with either NIMP or NEMP without therapy or with NIMP or NEMP plus 2-PAM therapy showed similar increases in GFAP compared with vehicle controls. However, the rats challenged with NIMP or NEMP plus therapy with our novel Oxime 20 (either a bromide or a mesylate salt) showed GFAP levels statistically undistinguishable from controls. These data provide highly supportive functional evidence of novel oxime entry into the brain. These novel oximes have the potential to provide central neuroprotection from organophosphate anticholinesterase-induced damage, which is a characteristic not displayed by most pyridinium oximes.

Novel centrally active oxime reactivators of acetylcholinesterase inhibited by surrogates of sarin and VX

A novel oxime platform, the substituted phenoxyalkyl pyridinium oximes (US patent 9,227,937), was invented at Mississippi State University with an objective of discovering a brain-penetrating antidote to highly potent organophosphate anticholinesterases, such as the nerve agents. The goal was reactivation of inhibited brain acetylcholinesterase to attenuate the organophosphate-induced hypercholinergic activity that results in glutamate-mediated excitotoxicity and neuropathology. The currently approved oxime antidote in the US, 2-PAM, cannot do this. Using highly relevant surrogates of sarin and VX that leave acetylcholinesterase phosphylated with the same chemical moiety as their respective nerve agents, in vitro screens and in vivo tests in rats were conducted to identify the most efficacious members of this platform. The most promising novel oximes provided 24-h survival of lethal level surrogate exposure better than 2-PAM in almost all cases, and two of the oximes shortened the time to cessation of seizure-like behavior while 2-PAM did not. The most promising novel oximes attenuated neuropathology as indicated by immunohistochemical stains for both glia and neurons, while 2-PAM did not protect either glia or neurons. These results strongly suggest that these novel oximes can function within the brain to protect it, and therefore show great promise as potential future nerve agent antidotes.