Trimedoxime and HI-6: kinetic comparison after intravenous administration to mice

HI-6 assisted catalytic scavenging of VX by acetylcholinesterase choline binding site mutants

The high toxicity of organophosphorus compounds originates from covalent inhibition of acetylcholinesterase (AChE), an essential enzyme in cholinergic neurotransmission. Poisonings that lead to life-threatening toxic manifestations require immediate treatment that combines administration of anticholinergic drugs and an aldoxime as a reactivator of AChE. An alternative approach to reduce the in vivo toxicity of OPs focuses on the use of bioscavengers against the parent organophosphate. Our previous research showed that AChE mutagenesis can enable aldoximes to substantially accelerate the reactivation of OP-enzyme conjugates, while dramatically slowing down rates of OP-conjugate dealkylation (aging). Herein, we demonstrate an efficient HI-6-assisted VX detoxification, both ex vivo in human blood and in vivo in mice by hAChE mutants modified at the choline binding site (Y337A and Y337A/F338A). The catalytic scavenging of VX in mice improved therapeutic outcomes preventing lethality and resulted in a delayed onset of toxicity symptoms.

HI-6 modulates immunization efficacy in a BALB/c mouse model

HI-6 is used as an antidote to nerve agents. It can also act as an antagonist to acetylcholine receptors (AChRs) including the nicotinic receptor, α 7 nAChR which is involved in regulating the immune response through macrophages. This experiment investigated the efficacy of HI-6 to regulate the immune response. Laboratory BALB/c mice received HI-6 and/or keyhole limpet hemocyanin (KLH) as an antigen. Antibody production was investigated after either 21 or 65 days when either single or repeated dose of antigen was applied. We confirmed that HI-6 significantly improved vaccination efficacy when KLH was given in a dose of 1mg/kg. The effect was dose dependent. A combination of HI-6 and KLH produced a vaccination of almost the same efficacy as that for Freund's complete adjuvant. The findings point at the suitability of HI-6 for improving vaccination efficacy at the level of immunity regulation by the nervous system.

A physiologically based pharmacokinetic model for the oxime TMB-4: simulation of rodent and human data

Multiple oximes have been synthesized and evaluated for use as countermeasures against chemical warfare nerve agents. The current U.S. military and civilian oxime countermeasure, 2-[(hydroxyimino)methyl]-1-methylpyridin-1-ium chloride (2-PAM), is under consideration for replacement with a more effective acetylcholinesterase reactivator, 1,1'-methylenebis{4-hydroxyiminomethyl}pyridinium dimethanesulfonate (MMB-4). Kinetic data in the scientific literature for MMB-4 are limited; therefore, a physiologically based pharmacokinetic (PBPK) model was developed for a structurally related oxime, 1,1'-trimethylenebis{4-hydroximinomethyl}pyridinium dibromide. Based on a previous model structure for the organophosphate diisopropylfluorophosphate, the model includes key sites of acetylcholinesterase inhibition (brain and diaphragm), as well as fat, kidney, liver, rapidly perfused tissues and slowly perfused tissues. All tissue compartments are diffusion limited. Model parameters were collected from the literature, predicted using quantitative structure-property relationships or, when necessary, fit to available pharmacokinetic data from the literature. The model was parameterized using rat plasma, tissue and urine time course data from intramuscular administration, as well as human blood and urine data from intravenous and intramuscular administration; sensitivity analyses were performed. The PBPK model successfully simulates rat and human data sets and has been evaluated by predicting intravenous mouse and intramuscular human data not used in the development of the model. Monte Carlo analyses were performed to quantify human population kinetic variability in the human evaluation data set. The model identifies potential pharmacokinetic differences between rodents and humans, indicated by differences in model parameters between species. The PBPK model can be used to optimize the dosing regimen to improve oxime therapeutic efficacy in a human population.

Development and validation of a sensitive HPLC method for the quantification of HI-6 in guinea pig plasma and evaluated in domestic swine

A rapid and small volume assay to quantify HI-6 in plasma was developed to further the development and licensing of an intravenous formulation of HI-6. The objective of this method was to develop a sensitive and rapid assay that clearly resolved HI-6 and an internal standard in saline and plasma matrices. A fully validated method using ion-pair HPLC and 2-PAM as the internal standard fulfilled these requirements. Small plasma samples of 35 microL were extracted using acidification, filtration and neutralization. Linearity was shown for over 4 microg/mL to 1mg/mL with accuracy and precision within 6% relative error at the lower limit of detection. This method was utilized in the pharmacokinetic analysis HI-6 dichloride (2Cl) and HI-6 dimethane sulfonate (DMS) in anaesthetized guinea pigs and domestic swine following an intravenous bolus administration. From the resultant pharmacokinetic parameters a target plasma concentration of 100 microM was established and maintained in guinea pigs receiving an intravenous infusion. This validated method allows for the analysis of low volume samples, increased sample numbers and is applicable to the determination of pharmacokinetic profiles and parameters.

Unequal efficacy of pyridinium oximes in acute organophosphate poisoning

The use of organophosphorus pesticides results in toxicity risk to non-target organisms. Organophosphorus compounds share a common mode of action, exerting their toxic effects primarily via acetylcholinesterase (AChE) inhibition. Consequently, acetylcholine accumulates in the synaptic clefts of muscles and nerves, leading to overstimulation of cholinergic receptors. Acute cholinergic crisis immediately follows exposure to organophosphate and includes signs and symptoms resulting from hyperstimulation of central and peripheral muscarinic and nicotinic receptors. The current view of the treatment of organophosphate poisoning includes three strategies, i.e. the use of an anticholinergic drug (e.g., atropine), cholinesterase-reactivating agents (e.g., oximes) and anticonvulsant drugs (e.g., benzodiazepines). Oximes, as a part of antidotal therapy, ensure the recovery of phosphylated enzymes via a process denoted as reactivation of inhibited AChE. However, both experimental results and clinical findings have demonstrated that different oximes are not equally effective against poisonings caused by structurally different organophosphorus compounds. Therefore, antidotal characteristics of conventionally used oximes can be evaluated regarding how close the certain substance is to the theoretical concept of the universal oxime. Pralidoxime (PAM-2), trimedoxime (TMB-4), obidoxime (LüH-6), HI-6 and HLö-7 have all been demonstrated to be very effective in experimental poisonings with sarin and VX. TMB-4 and LüH-6 may reactivate tabun-inhibited AChE, whereas HI-6 possesses the ability to reactivate the soman-inhibited enzyme. An oxime HLö-7 seems to be an efficient reactivator of AChE inhibited by any of the four organophosphorus warfare agents. According to the available literature, the oximes LüH-6 and TMB-4, although relatively toxic, are the most potent to induce reactivation of AChE inhibited by the majority of organophosphorus pesticides. Since there are no reports of controlled clinical trials on the use of TMB-4 in human organophosphate pesticide poisoning, LüH-6 may be a better option.

Efficacy of Trimedoxime in Mice Poisoned with Dichlorvos, Heptenophos or Monocrotophos

The aim of the study was to examine antidotal potency of trimedoxime in mice poisoned with three direct dimethoxy-substituted organophosphorus inhibitors. In order to assess the protective efficacy of trimedoxime against dichlorvos, heptenophos or monocrotophos, median effective doses and efficacy half-times were calculated. Trimedoxime (24 mg/kg intravenously) was injected 5 min. before 1.3 LD50 intravenously of poisons. Activities of brain, diaphragmal and erythrocyte acetylcholinesterase, as well as of plasma carboxylesterases were determined at different time intervals (10, 40 and 60 min.) after administration of the antidotes. Protective effect of trimedoxime decreased according to the following order: monocrotophos > heptenophos > dichlorvos. Administration of the oxime produced a significant reactivation of central and peripheral acetylcholinesterase inhibited with dichlorvos and heptenophos, with the exception of erythrocyte acetylcholinesterase inhibited by heptenophos. Surprisingly, trimedoxime did not induce reactivation of monocrotophos-inhibited acetylcholinesterase in any of the tissues tested. These organophosphorus compounds produced a significant inhibition of plasma carboxylesterase activity, while administration of trimedoxime led to regeneration of the enzyme activity. The same dose of trimedoxime assured survival of experimental animals poisoned by all three organophosphorus compounds, although the biochemical findings were quite different.