Sugar-oximes, new potential antidotes against organophosphorus poisoning

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.

Transitioning from Oxime to the Next Potential Organophosphorus Poisoning Therapy Using Enzymes

For years, organophosphorus poisoning has been a major concern of health problems throughout the world. An estimated 200,000 acute pesticide poisoning deaths occur each year, many in developing countries. Apart from the agricultural pesticide poisoning, terrorists have used these organophosphorus compounds to attack civilian populations in some countries. Recent misuses of sarin in the Syrian conflict had been reported in 2018. Since the 1950s, the therapy to overcome this health problem is to utilize a reactivator to reactivate the inhibited acetylcholinesterase by these organophosphorus compounds. However, many questions remain unanswered regarding the efficacy and toxicity of this reactivator. Pralidoxime, MMB-4, TMB-4, obidoxime, and HI-6 are the examples of the established oximes, yet they are of insufficient effectiveness in some poisonings and only a limited spectrum of the different nerve agents and pesticides are being covered. Alternatively, an option in the treatment of organophosphorus poisoning that has been explored is through the use of enzyme therapy. Organophosphorus hydrolases are a group of enzymes that look promising for detoxifying organophosphorus compounds and have recently gained much interest. These enzymes have demonstrated remarkable protective and antidotal value against some different organophosphorus compounds in vivo in animal models. Apart from that, enzyme treatments have also been applied for decontamination purposes. In this review, the restrictions and obstacles in the therapeutic development of oximes, along with the new strategies to overcome the problems, are discussed. The emerging interest in enzyme treatment with its advantages and disadvantages is described as well.

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'.