Reactivator Report for: HS-6

Atropinized rats intoxicated with ethyl-S-2-diisopropyl aminoethyl methyl phosphonothioate (VX), 15 mg/kg iv, were divided into three groups and were treated with normal saline, iv, 30 mg/kg of 2-PAM C1, iv, and 30 mg/kg of HS-6, iv. One hr after administration of therapy they were decapitated and cholinesterase (ChE) activity was determined on blood, brain and diaphragm tissue. Both 2-PAM C1 and HS-6 markedly reactivated VX-inhibited blood and diaphragm ChE. Brain ChE activity was not significantly reactivated by either oxime. The effectiveness of these oximes in restoration of VX-inactivated ChE in vivo offers an explanation as to why conventional atropine/oxime therapy is so effective against VX intoxication.

Studied was the activity of low concentrations --- 1-10-12, and 1.10-18 -- of the specific reactivators of the enzyme cholinesterase (ChE): 2-PAM, TMB-4(Trimedoxime) Toxogonin, HS-3, and HS-6, in a chromodacryorrhea test (ChDT) with rats, daily (in the course of 30 days -- 1 ml/100 g), twice (at an interval of 10 days -- 1 ml/100 g) and once (5 mg/kg) at subcutaneous application. Described is an activating affect (observed for the first time in vivo) on ChE preduced by low and very low concentrations of reactivators of the group of oximes. HS-3 produced best activating effects. The low concentrations of the specific oxime ractivators of ChE, followed up by ChDT, can be used as sensitive quantitative indicators of the activity of ChE. They can also serve to specify the mode of action power and term of effectiveness of these reactivators.

Acetylcholinesterase (AChE; EC 3.1.1.7) reactivators are used as a part of the antidotal therapy of organophosphorus pesticide and nerve agent intoxications. Cyclosarin is one member of the nerve agent family. In this article, we compared the reactivation potency of five structurally different AChE reactivators (pralidoxime, trimedoxime, methoxime, HS-6, and BI-6) to reactivate cyclosarin-inhibited cholinesterases of human brain. The results demonstrate that the bisquaternary monooxime reactivator BI-6 seems to be the most potent reactivator of cyclosarin-inhibited cholinesterases. Moreover, according to the results, we can describe basic structural requirements, which are necessary for the efficacious reactivation process.

The effects of the oximes 2-pyridine aldoxime methiodide (PAM), HI-6, HS-6, toxogonin and TMB-4(Trimedoxime) on the rate of carbamylation of membrane-bound bovine erythrocyte acetylcholinesterase were studied. The second-order rate constant of carbamylation (ki) and the first-order rate constant of decarbamylation (k3) were calculated from the proportion of free acetylcholinesterase at equilibrium and the rate of approach to equilibrium. Twenty insecticidal carbamates plus physostigmine and pyridostigmine were studied. The oximes increased ki for several carbamates, with HI-6 causing an increase in the most number of cases (12) and PAM the least (3). HI-6 was also a potent accelerator of decarbamylation (increase in k3) in all cases, whereas PAM caused a significant decrease in k3 in 15 cases and a nonsignificant decrease in the other 7. Toxogonin and TMB-4(Trimedoxime) increased k3 or had no significant effect. The results were generally consistent with a proposal in the literature that there is a correlation between increased ki and increased toxicity of the carbamate in the presence of an oxime.

Membrane-bound bovine erythrocyte acetylcholinesterase was inhibited with physostigmine or carbaryl, and the rate constants of carbamylation and decarbamylation were determined from the proportion of inhibited acetylcholinesterase in the steady state, and the rate of approach to the steady state. The oximes 2-PAM, HI-6, HS-6, TMB-4(Trimedoxime) and toxogonin, at 0.1 mM, all decreased the rate of carbamylation by physostigmine, but increased the rate of carbamylation by carbaryl. TMB-4(Trimedoxime) and toxogonin were the most effective oximes in potentiating carbamylation by carbaryl, with an enhancement of the second-order rate constant of 54- and 17-fold respectively. The greatest reduction in the rate constant for carbamylation by physostigmine (3.7-fold) was caused by HI-6. HS-6 and HI-6 increased the rate of decarbamylation, while 2-PAM reduced the rate of decarbamylation if physostigmine was the carbamate. 2-PAM and HI-6 were also studied with soluble bovine erythrocyte acetylcholinesterase, and similar results were obtained. The results extend those in a recent report by other authors who studied the half-life of carbamylation for acetylcholinesterase and butyrylcholinesterase in an attempt to understand the mechanism by which oximes increase the toxicity of carbaryl in vivo. These authors proposed binding of the oximes to an allosteric site on the enzyme. While not discounting this possibility, the present results, taken with other reports in the literature, suggest that binding of the oximes to the anionic subsite of the active site of the enzyme is also feasible. The present results also offer an explanation for another recent report, in which anomalous results were presented for decarbamylation of physostigmine-inhibited and carbaryl-inhibited erythrocyte acetylcholinesterase in the presence of 2-PAM or HI-6.

Atropinized rats intoxicated with ethyl-S-2-diisopropyl aminoethyl methyl phosphonothioate (VX), 15 mg/kg iv, were divided into three groups and were treated with normal saline, iv, 30 mg/kg of 2-PAM C1, iv, and 30 mg/kg of HS-6, iv. One hr after administration of therapy they were decapitated and cholinesterase (ChE) activity was determined on blood, brain and diaphragm tissue. Both 2-PAM C1 and HS-6 markedly reactivated VX-inhibited blood and diaphragm ChE. Brain ChE activity was not significantly reactivated by either oxime. The effectiveness of these oximes in restoration of VX-inactivated ChE in vivo offers an explanation as to why conventional atropine/oxime therapy is so effective against VX intoxication.

Following intravenous administration of the cholinesterase reactivator HS-6 (30 mg/kg), blood pressure fell (up to 50 mmHg) and maximal blood levels of HS-6 reached 242 microgram/ml. HS-6 attenuated the pressor response resulting from carotid occlusion and the depressor effect of vagal stimulation. Doses of HS-6 below those used to protect against soman in different animal species (10--30 mumol/kg) progressively blocked the ganglion-stimulating effects of nicotine and dimethylphenylpiperazinium but not the pressor effect following adrenaline, a pattern similar to that produced by hexamethonium but only 1/84 as potent. HS-6, like hexamethonium and mecamylamine, progressively blocked the contraction of the nictitating membrane of the cat resulting from preganglionic stimulation. The results indicate that HS-6 possesses ganglion-blocking properties at doses likely to be used in the protection against soman poisoning. The ganglion-blocking properties of the drug may be a factor in the beneficial effects of HS-6.

Studied was the activity of low concentrations --- 1-10-12, and 1.10-18 -- of the specific reactivators of the enzyme cholinesterase (ChE): 2-PAM, TMB-4(Trimedoxime) Toxogonin, HS-3, and HS-6, in a chromodacryorrhea test (ChDT) with rats, daily (in the course of 30 days -- 1 ml/100 g), twice (at an interval of 10 days -- 1 ml/100 g) and once (5 mg/kg) at subcutaneous application. Described is an activating affect (observed for the first time in vivo) on ChE preduced by low and very low concentrations of reactivators of the group of oximes. HS-3 produced best activating effects. The low concentrations of the specific oxime ractivators of ChE, followed up by ChDT, can be used as sensitive quantitative indicators of the activity of ChE. They can also serve to specify the mode of action power and term of effectiveness of these reactivators.