A COMPARISON OF THE POTENCY OF TRIMEDOXIME AND OTHER CURRENTLY AVAILABLE OXIMES TO REACTIVATE TABUN-INHIBITED ACETYLCHOLINESTERASE AND ELIMINATE ACUTE TOXIC EFFECTS OF TABUN

Recent developments on oximes to improve the blood brain barrier penetration for the treatment of organophosphorus poisoning: a review

Organophosphorus (OP) compounds are highly toxic synthetic compounds which have been used as pesticides and developed as warfare nerve agents. They represent a threat to both military and civilian populations. OP pesticides affect the nervous system and are thought to have caused at least 5 million deaths since their discovery in the 1930s. At present the treatment of OP nerve agent poisoning commonly involves the use of parenteral oximes. However, the blood brain barrier (BBB) remains a challenge in the delivery of oximes to the central nervous system (CNS). This is because almost all macromolecule drugs (including oximes) fail to pass through the BBB to reach the CNS structures. The presence of a permanent cationic charge in oximes has made these compounds inefficient in crossing the BBB. Thus, oximes are unable to reactivate acetylcholinesterase (AChE) in the CNS. Using current structural and mechanistic understanding of the BBB under both physiological and pathological conditions, it becomes possible to design delivery systems for oximes and other drugs that are able to cross the BBB effectively. This review summarises the recent strategies in the development of oximes which are capable of crossing the BBB to treat OP poisoning. Several new developments using oximes are reviewed along with their advantages and disadvantages. This review could be beneficial for future directions in the development of oxime and other drug delivery systems into the CNS.

Organophosphorus (OP) compounds are highly toxic synthetic compounds which have been used as pesticides and developed as warfare nerve agents.

In vivo protection studies of bis-quaternary 2-(hydroxyimino)- N-(pyridin-3-yl) acetamide derivatives (HNK oximes) against tabun and soman poisoning in Swiss albino mice

The study reports antidotal efficacy of three HNK [ bis quaternary 2-(hydroxyimino)-N-(pyridin-3yl) acetamide derivatives] and pralidoxime (2-PAM), against soman and tabun poisoning in Swiss albino mice. Protection index (PI) was determined (treatment doses: HNK oximes, ×0.20 of their median lethal dose (LD₅₀) and 2-PAM, 30 mg/kg, intramuscularly (im)) together with atropine (10 mg/kg, intraperitoneally). Probit log doses with difference of 0.301 log of LD₅₀ of the nerve agents administered and inhibition of acetylcholinesterase (AChE) activity by 50% (IC₅₀) was calculated at optimized time in brain and serum. Using various doses of tabun and soman (subcutaneously (sc)), in multiples of their IC₅₀, AChE reactivation ability of the oximes was studied. Besides, acute toxicity (0.8× LD₅₀, im, 24 h postexposure) of HNK-102 and 2-PAM was also compared by determining biochemical, hematological variables and making histopathological observations. Protection offered by HNK-102 against tabun poisoning was found to be four times higher compared to 2-PAM. However, nearly equal protection was noted with all the four oximes against soman poisoning. HNK-102 reactivated brain AChE activity by 1.5 times more than 2-PAM at IC₅₀ dose of soman and tabun. Acute toxicity studies of HNK-102 and 2-PAM showed sporadic changes in urea, uric acid, aspartate aminotransferase, and so on compared to control group, however, not supported by histopathological investigations. The present investigation showed superiority of newly synthesized HNK-102 oxime over standard 2-PAM, as a better antidote, against acute poisoning of tabun (4.00 times) and soman (1.04 times), in Swiss albino mice.

SAR study to find optimal cholinesterase reactivator against organophosphorous nerve agents and pesticides

Irreversible inhibition of acetylcholinesterase (AChE) by organophosphates leads to many failures in living organism and ultimately in death. Organophosphorus compounds developed as nerve agents such as tabun, sarin, soman, VX and others belong to the most toxic chemical warfare agents and are one of the biggest threats to the modern civilization. Moreover, misuse of nerve agents together with organophosphorus pesticides (e.g. malathion, paraoxon, chlorpyrifos, etc.) which are annually implicated in millions of intoxications and hundreds of thousand deaths reminds us of insufficient protection against these compounds. Basic treatments for these intoxications are based on immediate administration of atropine and acetylcholinesterase reactivators which are currently represented by mono- or bis-pyridinium aldoximes. However, these antidotes are not sufficient to ensure 100 % treatment efficacy even they are administered immediately after intoxication, and in general, they possess several drawbacks. Herein, we have reviewed new efforts leading to the development of novel reactivators and proposition of new promising strategies to design novel and effective antidotes. Structure-activity relationships and biological activities of recently proposed acetylcholinesterase reactivators are discussed and summarized. Among further modifications of known oximes, the main attention has been paid to dual binding site ligands of AChE as the current mainstream strategy. We have also discussed new chemical entities as potential replacement of oxime functional group.

In silico pharmacophore model for tabun-inhibited acetylcholinesterase reactivators: a study of their stereoelectronic properties

Organophosphorus (OP) nerve agents that inhibit acetylcholinesterase (AChE; EC 3.1.1.7) function in the nervous system, causing acute intoxication. If untreated, death can result. Inhibited AChE can be reactivated by oximes, antidotes for OP exposure. However, OP intoxication caused by the nerve agent tabun (GA) is particularly resistant to oximes, which poorly reactivate GA-inhibited AChE. In an attempt to develop a rational strategy for the discovery and design of novel reactivators with lower toxicity and increased efficacy in reactivating GA-inhibited AChE, we developed the first in silico pharmacophore model for binding affinity of GA-inhibited AChE from a set of 11 oximes. Oximes were analyzed for stereoelectronic profiles and three-dimensional quantitative structure-activity relationship pharmacophores using ab initio quantum chemical and pharmacophore generation methods. Quantum chemical methods were sequentially used from semiempirical AM1 to hierarchical ab initio calculations to determine the stereoelectronic properties of nine oximes exhibiting affinity for binding to GA-inhibited AChE in vivo. The calculated stereoelectronic properties led us to develop the in silico pharmacophore model using CATALYST methodology. Specific stereoelectronic profiles including the distance between bisquarternary nitrogen atoms of the pyridinium ring in the oximes, hydrophilicity, surface area, nucleophilicity of the oxime oxygen, and location of the molecular orbitals on the isosurfaces have important roles for potencies for reactivating GA-inhibited AChE. The in silico pharmacophore model of oxime affinity for binding to GA-inhibited AChE was found to require a hydrogen bond acceptor, a hydrogen bond donor at the two terminal regions, and an aromatic ring in the central region of the oximes. The model was found to be well-correlated (R = 0.9) with experimental oxime affinity for binding to GA-inhibited AChE. Additional stereoelectronic features relating activity with the location of molecular orbitals and weak electrostatic potential field over the aromatic rings were found to be consistent with the pharmacophore model. These results provided the first predictive pharmacophore model of oxime affinity for binding toward GA-inhibited AChE. The model may be useful for virtual screening of compound libraries to discover and/or custom synthesize more efficacious and less toxic reactivators that may be useful for GA intoxication.

Entry of two new asymmetric bispyridinium oximes (K-27 and K-48) into the rat brain: comparison with obidoxime

In the search for new oximes with higher reactivation potency and a broader spectrum, K-27 and K-48, have recently been synthesized. To test if their superior efficacy was related to better penetration across the blood-brain barrier, their brain entry was compared with that of obidoxime, when administered either alone or after the organophosphate paraoxon (POX). Rats received 50 micromol obidoxime, K-27 or K-48, either alone or in addition to 1 micromol POX. Oxime concentrations at various points in time in brain and plasma were measured using HPLC. The obidoxime C(max) in brain was 1.3% of the plasma C(max) when injected alone, and 1.5% when injected following POX. The ratio of the area under the curve (AUC) brain to plasma for obidoxime was around 6%, irrespective of whether it was administered alone or after POX. For K-27, C(max) (brain) was 0.6% of C(max) (plasma) when injected alone, and 0.7% when injected after POX (no significant difference). The AUC (brain) was 2% of AUC (plasma) for both K-27 groups. K-48, when injected alone reached 1.4% of C(max) (plasma) in the brain and 1.2% of C(max) (plasma), when injected following POX. The AUC (brain) was 5% of the AUC (plasma), both when K-48 was administered alone and in combination with POX. Entry of all three oximes into the brain is minimal and cannot explain the better therapeutic efficacy of K-27 and K-48. As already observed for pralidoxime, injection of POX before oxime administration had no influence upon penetration across the blood-brain barrier.