A Comparison of the Potency of Newly Developed Oximes (K074, K075) and Currently Available Oximes (Obidoxime, Trimedoxime, Hi-6) to Counteract Acute Toxic Effects of Tabun and Cyclosarin in Mice

Effects of memantine and its metabolite Mrz 2/373 on soman-induced inhibition of acetylcholinesterase in vitro

Memantine is the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, used in the treatment of Alzheimer's disease. It is also known that memantine pretreatment assured protection of skeletal muscles from poisoning with nerve agents and an interaction between memantine and AChE was proposed. In the study presented we examined interactions of memantine and its main metabolite (1-amino-3-hydroxymethyl-5-methyl adamantine, Mrz 2/373) with AChE in vitro as well as their effect on kinetics of the soman-induced AChE inhibition and aging. The results have shown that memantine and Mrz 2/373 exerted concentration-dependent inhibition of AChE, with Mrz 2/373 being a more potent inhibitor than the parent compound. Addition of soman 7.5 nmol/l induced gradual AChE inhibition that became almost complete after 20 min. Memantine (0.1, 0.5 and 1 mmol/l) and Mrz 2/373 (0.1, 0.5 and 1 mmol/l) concentration-dependently slowed down the AChE inhibition. After 30 min of incubation of AChE with soman, 5 min of aging and 20 min of reactivation by asoxime (HI-6 dichloride), AChE activity was 8.1% in control medium, 30.7% and 41.9% after addition of 1 and 10 mmol/l memantine, and 16.1% after addition of 1 mmol/l Mrz 2/373. It was concluded that it is possible that memantine and Mrz 2/373 can prevent AChE from inhibition by soman, which could, along with known memantine's neuroprotective activity, explain its potent antidotal effect in soman poisoning. The potential effect on aging of the soman-AChE complex warrants further studies.

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.

Advances in toxicology and medical treatment of chemical warfare nerve agents

Organophosphorous (OP) Nerve agents (NAs) are known as the deadliest chemical warfare agents. They are divided into two classes of G and V agents. Most of them are liquid at room temperature. NAs chemical structures and mechanisms of actions are similar to OP pesticides, but their toxicities are higher than these compounds. The main mechanism of action is irreversible inhibition of Acetyl Choline Esterase (AChE) resulting in accumulation of toxic levels of acetylcholine (ACh) at the synaptic junctions and thus induces muscarinic and nicotinic receptors stimulation. However, other mechanisms have recently been described. Central nervous system (CNS) depression particularly on respiratory and vasomotor centers may induce respiratory failure and cardiac arrest. Intermediate syndrome after NAs exposure is less common than OP pesticides poisoning. There are four approaches to detect exposure to NAs in biological samples: (I) AChE activity measurement, (II) Determination of hydrolysis products in plasma and urine, (III) Fluoride reactivation of phosphylated binding sites and (IV) Mass spectrometric determination of cholinesterase adducts. The clinical manifestations are similar to OP pesticides poisoning, but with more severity and fatalities. The management should be started as soon as possible. The victims should immediately be removed from the field and treatment is commenced with auto-injector antidotes (atropine and oximes) such as MARK I kit. A 0.5% hypochlorite solution as well as novel products like M291 Resin kit, G117H and Phosphotriesterase isolated from soil bacterias, are now available for decontamination of NAs. Atropine and oximes are the well known antidotes that should be infused as clinically indicated. However, some new adjuvant and additional treatment such as magnesium sulfate, sodium bicarbonate, gacyclidine, benactyzine, tezampanel, hemoperfusion, antioxidants and bioscavengers have recently been used for OP NAs poisoning.

Tabun-inhibited rat tissue and blood cholinesterases and their reactivation with the combination of trimedoxime and HI-6 in vivo

Up to now, intensive attempts to synthesize a universal reactivator able to reactivate cholinesterases inhibited by all types of nerve agents/organophosphates were not successful. Therefore, another approach using a combination of two reactivators differently reactivating enzyme was used: in rats poisoned with tabun and treated with combination of atropine (fixed dose) and different doses of trimedoxime and HI-6, changes of acetylcholinesterase activities (blood, diaphragm and different parts of the brain) were studied. An increase of AChE activity was observed following trimedoxime treatment depending on its dose; HI-6 had very low effect. Combination of both oximes showed potentiation of their reactivation efficacy; this potentiation was expressed for peripheral AChE (blood, diaphragm) and some parts of the brain (pontomedullar area, frontal cortex); AChE in the basal ganglia was relatively resistant. These observations suggest that the action of combination of oximes in vivo is different from that observed in vitro.

Efficacy of eight experimental bispyridinium oximes against paraoxon-induced mortality: comparison with the conventional oximes pralidoxime and obidoxime

Recently, several experimental K-oximes with two functional aldoxime groups have been synthesized that show excellent in vitro efficacy in protecting acetylcholinesterase (AChE) from inhibition by a broad variety of organophosphorus compounds (OPCs). However, oximes themselves are also AChE inhibitors, albeit at higher concentrations, which is a major cause of their toxicity and may be a dose-limiting factor in oxime therapy. To assess the efficacy of the experimental K-oximes in vivo, the extent of oxime-conferred protection from mortality induced by paraoxon was quantified. Rats received paraoxon in a dosage of 1, 5, or 10 mumol, and immediately thereafter intraperitoneal injections of the respective oxime at a dosage of half the LD(01). The relative risk of death (RR) over time was estimated by Cox survival analysis for treatment with experimental K-oximes (K-53, K-74, K-75, K-107, K-108, and K-113), with the clinically available oximes pralidoxime (2-PAM) and obidoxime, and with the well-characterized K-oximes K-27 and K-48, comparing results with the no-treatment group. Best protection was conferred by K-27, reducing the RR to 20% of controls (P </= 0.05), which was significantly (P <or= 0.05) better than all other tested oximes. Marked reduction in mortality was also achieved by K-48 and the three new bispyridinium oximes containing two aldoxime groups, but no xylene linker: K-48 (RR = 0.32), K-53 (RR = 0.36), K-74 (RR = 0.42), K-75 (RR = 0.35). This effect was significantly (P <or= 0.05) superior to that of all other oximes, except K-27. The remaining oximes, i.e., obidoxime (RR = 0.64), 2-PAM (RR = 0.78), K-107 (RR = 0.70), K-108 (RR = 0.77), and K-113 (RR = 0.87) reduced paraoxon-induced mortality only poorly, but significantly (P <or= 0.05). Our data show that K-27, K-48, K-53, K-74, and K-75, due to their far superior in vivo efficacy, are the most promising candidates to eventually replace the established oximes 2-PAM and obidoxime. Further studies in other species exposed to a broader spectrum of OPCs are, however, necessary before considering their use in humans.