Effect of HI-6, applied into the cerebral ventricles, on the inhibition of brain acetylcholinesterase by soman in rats

Development of the Bisquaternary Oxime HI-6 Toward Clinical Use in the Treatment of Organophosphate Nerve Agent Poisoning

The traditional therapeutic treatment of organophosphate cholinesterase inhibitor (nerve agents) poisoning consists of co-treatment with an antimuscarinic (atropine) and a reactivator of inhibited acetylcholinesterase (AChE), which contains a nucleophilic oxime function. Two oximes are presently widely available for clinical use, pralidoxime and obidoxime (toxogonin), but both offer little protection against important nerve agent threats. This has highlighted the real need for the development and availability of more effective oximes for human use, a search that has been going on for up to 30 years. However, despite the demonstration of more effective and safe oximes in animal experiments, no additional oximes have been licensed for human use. HI-6, (1-[[[4(aminocarbonyl)-pyridinio]methoxy]methyl]-2(hydroxyimino)pyridinium dichloride; CAS 34433-31-3) has been studied intensively and has been proved effective in a variety of species including non-human primates and appears from clinical experience to be safe in humans. These studies have led to the fielding of HI-6 for use against nerve agents by the militaries of the Czech republic, Sweden, Canada and under certain circumstances the Organisation for the Prohibition of Chemical Weapons. Nevertheless HI-6 has not been granted a license for clinical use, must be used only under restricted guidelines and is not available for civilian use as far as is known. This article will highlight those factors relating to HI-6 that pertain to the licensing of new compounds of this type, including the mechanism of action, the clinical and pre-clinical demonstration of safety and its efficacy against a variety of nerve agents particularly in non-human primates, since no relevant human population exists. This article also contains important data on the use of HI-6 in baboons, which has not been available previously. The article also discusses the possibility of successful therapy with HI-6 against poisoning in humans relative to doses used in non-human primates and relative to its ability to reactivate inhibited human AChE.

Pharmacological nature of soman-induced hypothermia in mice

The object of the study was to determine the pharmacological nature of pinacolyl methylphosphonofluoridate (soman)-induced hypothermia in mice. This was accomplished by examining the soman hypothermia dose response and the effect of various pharmacological antagonists in comparison to the hypothermia responses of muscarinic and nicotinic cholinergic agonists such as oxotremorine and nicotine and another anticholinesterase, physostigmine. Core temperature in mice was monitored by telemetry. In general, atropine antagonized oxotremorine, physostigmine, and soman hypothermia but not nicotine hypothermia whereas mecamylamine antagonized nicotine hypothermia but not that produced by the other agonists. Soman hypothermia was not affected significantly by various pharmacological antagonists, suggesting that other neurotransmitters were not involved in the expression of soman hypothermia. Soman hypothermia appears to be due to muscarinic receptor stimulation and can be effectively antagonized, but not completely, by the use of atropine. Acetylcholinesterase oxime reactivators, such as HI-6 and toxogonin, were ineffective in antagonizing soman-induced hypothermia and reactivating hypothalamic acetylcholinesterase, whereas HI-6 was effective in reactivating soman-inhibited diaphragm acetylcholinesterase when administered up to 10 min after soman, indicating that aging of the soman-inhibited acetylcholinesterase had not occurred. Soman hypothermia appears to be primarily a muscarinic receptor-related event.

Efficacy of antidotes against soman poisoning in female physostigmine-protected rats

Female rats have been found much more sensitive to lethal effects of soman than male rats. Therefore it was of interest to examine the efficacy of different antidotes against soman poisoning in females which are usually not being used in soman poisoning studies. The effects of acetylcholinesterase (AChE) non-reactivating antidotes atropine and diazepam were analyzed in combination with physostigmine prophylaxis against supralethal doses of soman. Physostigmine prophylaxis was much more effective when supplemented by atropine and diazepam therapy, applied at the onset of the first signs of poisoning. The interval between the injection of a supralethal dose of soman and the appearance of signs of poisoning was shorter in physostigmine pretreated animals than in non-pretreated controls poisoned with the same supralethal dose of soman. The prophylactic effect of physostigmine (used at maximal dose) disappeared in about 120 min. The addition of HI-6, an AChE-reactivating oxime, to atropine + diazepam therapy further increased the survival in soman-poisoned and physostigmine-pretreated rats, yielding the highest protective ratio of 6.4. Pretreatment with physostigmine offered marked protection against inhibition of AChE by soman, as shown by enzyme activity determination in different brain regions and in diaphragm muscle. Application of HI-6 in addition to the combination of the above mentioned antidotes even preserved more AChE activity in the skeletal muscle but did not influence inhibition of the enzyme in brain.

A comparison of cholinergic effects of HI-6 and pralidoxime-2-chloride (2-PAM) in soman poisoning

The effects of HI-6 and pralidoxime chloride (2-PAM) on soman-induced lethality, time to death and several cholinergic parameters in rats were compared to understand the beneficial action of HI-6. Treatment with atropine sulfate (ATS) or HI-6 alone protected against 1.2 and 2.5 LD50s of soman respectively, whereas 2-PAM or methylated atropine (AMN) alone afforded no protection. Addition of ATS, but not AMN, to HI-6-treated rats enhanced the protection from 2.5 to 5.5 LD50s. HI-6 increased the time-to-death, while 2-PAM had no effect; a combination of HI-6 and ATS provided the most significant increase in time-to-death. Cholinesterase (ChE) activity was not altered in any tissue by ATS, HI-6 or 2-PAM treatment individually, but was markedly inhibited in all tissues by 100 micrograms/kg of soman. In soman-poisoned rats, the HI-6, but not the 2-PAM, group had significantly higher levels of ChE in blood and other peripheral tissues than did the group given soman alone. Neither HI-6 nor 2-PAM affected soman-inhibited ChE in the brain. Additional ATS treatment had no effect on ChE activity. HI-6 and 2-PAM neither modified baseline brain acetylcholine (ACh) or choline (Ch) levels nor protected against soman-induced ACh or Ch elevation. 2-PAM exhibited a 4-fold more potent in vitro inhibition of 3H-quinuclidinyl benzilate (3H-QNB) binding and sodium-dependent high-affinity Ch uptake (HACU) than did HI-6 in brain tissues. The findings that 2-PAM is a more potent in vitro inhibitor of muscarinic receptor binding and HACU than HI-6, and yet neither elevates ChE activity in the periphery nor protects rats against soman poisoning, indicate the importance of higher ChE activity in the periphery of HI-6-treated rats. Maintenance by HI-6 of a certain amount of active ChE in the periphery appears to be important for survival after soman exposure.

Neurobehavioral effects of the pyridinium aldoxime cholinesterase reactivator HI-6

A series of neurobehavioral testing procedures was used to evaluate the behavioral effects of the pyridinium aldoxime cholinesterase reactivator HI-6 in male Sprague-Dawley rats. These procedures were fixed-ratio (FR) responding, shuttle-box conditioned avoidance response (CAR), conditioned taste aversion (CTA), drinking behavior, open-field exploratory behavior, negative geotaxis, and wire suspension time. Dose-response studies of HI-6 at dose-levels of 25, 50 and 100 mg/kg, or saline (IP) were evaluated. HI-6 disrupted FR responding in a dose-dependent fashion, with significant effects occurring at doses of 50 and 100 mg/kg. The pattern of disruption was characterized by extended periods of nonresponding having an abrupt onset and offset. HI-6 produced CTA in a dose-related manner, with significant effects at doses equal to those that disrupted FR performance. HI-6 did not alter CAR, drinking motivation, exploratory behavior, negative geotaxis, or wire suspension time. These data suggest that there may be a commonality in the underlying mechanism(s) for the disruption in FR performance and the induction of the CTA. This mechanism may relate to the presumed drug-induced adverse internal state inducing the CTA.

Attenuation of soman-induced lesions of skeletal muscle by acetylcholinesterase reactivating and non-reactivating antidotes

It has been reported recently that some oximes reactivating acetylcholinesterase (AChE) exhibit concomitant ganglion-blocking effects which presumably could contribute independently to their powerful antidotal action in organophosphate inhibitor (OPI) poisoning, thus mimicking some unrelated substances which are effective antidotes without reactivating AChE. This raises the question whether OPI-induced muscle lesions, like some other symptoms could also be attenuated by oximes and other antidotes in the absence of AChE reactivation. To test this possibility, the oxime HI-6 was applied at increasing time intervals after the injection of soman until and beyond the point when soman-AChE complex becomes completely "aged" and not capable of reactivation. As the examples of OPI antidotes which do not reactivate AChE, the muscarinic antagonist atropine and the ganglion-blocking agent hexamethonium were also tested on possible attenuation of muscle lesions. The proportions of fibers with lesions, AChE inhibition and muscle fasciculations in experimental groups relative to the controls treated with soman only were evaluated. The results show that HI-6 can attenuate lesions only if AChE is partially reactivated and muscle fasciculations are permanently eliminated. However, atropine does not affect either AChE inhibition or muscle fasciculations and is also ineffective in counteracting the lesions in spite of its potency as an effective general antidote. Hexamethonium also does not affect AChE inhibition, but abolishes fasciculations and effectively attenuates muscle lesions. The latter findings reveal the existence of lesion-protecting mechanisms unrelated to AChE reactivation, which if further elucidated might become potentially relevant for additional treatment in OPI poisoning.

Studies on the stability and decomposition of the Hagedorn-oxime HLö 7 in aqueous solution

HLö 7, (pyridinium, 1-[[[4-(aminoarbonyl)pyridinio]methoxy]methyl] -2,4-bis- [(hydroxyimino)methyl] diiodide) has been shown to be efficacious in soman poisoning of mice even in the absence of atropine. To assess possible risks involved in the administration of HLö 7 its degradation products were analyzed at pH 2.5 and pH 7.4, respectively. At pH 2.5, where HLö 7 in aqueous solution was assumed to possess maximal stability, the predicted shelf life (10% decomposition) was about 8 years for 10 mM solutions at 8 degrees C. The apparent energy of activation was 117 kJ/mol. At pH 2.5, attack on the aminal-acetal bond predominated with formation of pyridine-2,4-dialdoxime, 2-cyanopyridine-4-aldoxime, isonicotinamide, and formaldehyde. At pH 7.4, primary attack on the 2-aldoxime group resulted in formation of an intermediate 2-cyano-4-aldoxime derivative which mainly decomposed into cyanide and the corresponding 2-pyridinone, 1-[[[4-(aminocarbonyl)-pyridinio]methoxy]methyl]-4- [(hydroxyimino)methyl] diiodide. In addition, liberated cyanide reacted with the intermediate 2-cyano-4-aldoxime derivative with formation of 2-pyridinone, 1-[[[4-(aminocarbonyl)-pyridinio]-methoxy]methyl]-6-cyano-4- [(hydroxyimino)methyl] diiodide. This cyanide sequestering pathway became significant only at high concentrations (10 mM) of HLö 7, and was marginal at 1 mM HLö 7.

Studies on the decomposition of the oxime HI 6 in aqueous solution

HI 6 has been shown to be efficacious in soman intoxication of laboratory animals by reactivation of acetylcholinesterase. To assess possible risks involved in the administration of HI 6 its degradation products were analyzed at pH 2.0, 4.0, 7.4, and 9.0. At pH 2.0, where HI 6 in aqueous solution has its maximal stability, attack on the aminal-acetal bond of the "ether bridge" predominates, with formation of formaldehyde, isonicotinamide, and pyridine-2-aldoxime. Besides, HI 6 decomposes at the oxime group yielding 2-cyanopyridine. Liberation of hydrocyanic acid at pH 2.0 is below 5%. At pH 7.4, primary attack is on the oxime group, resulting in formation of the corresponding pyridone via an intermediate nitrile. The pyridone has been isolated and identified as 2-pyridinone, 1-[(4-carbamoylpyridinio)methoxy)methyl)formate. This major metabolite deaminates further to the 2-pyridinone, 1-[(4-carboxypyridinio)methoxy)methyl) derivative, which ultimately decomposes into formaldehyde, isonicotinic acid, and 2-pyridone. Hydrolysis of the acid amide group probably also occurs with HI 6 itself. Significant amounts of free hydrocyanic acid were only detected in the presence of an alkali trap; otherwise hydrocyanic acid reacts with formaldehyde to yield hydroxyacetonitrile from which hydrocyanic acid can be liberated again. Up to 0.6 equivalents of hydrocyanic acid were evolved at pH 7.4. After repetitive administration and impaired renal elimination of HI 6, e.g. during renal shock, there might be some risk of cyanide intoxication.

Pharmacokinetics and pharmacodynamics of the oxime HI6 in dogs

The pharmacokinetics and pharmacodynamics of the oxime HI6 were investigated in conscious and anesthetized beagle dogs following intramuscular injection. The absorption of HI6 (100 mumol/kg) was slower in conscious dogs as compared to the anesthetized dogs, and the maximum concentrations in plasma were lower (200 instead of 300 mumol/l). In comparison, the elimination of HI6 (100 mumol/kg) was twice as rapid in the conscious dogs (ke = 0.013 instead of 0.006 min-1) as in the anesthetized animals and was equal to the elimination after injection of 50 mumol/kg (likewise in anesthesia). The more rapid elimination was accompanied by a greater renal excretion of unchanged HI6 (60% instead of 40% in 3 h). HI6 penetrated the blood-brain barrier. The concentration of the oxime in CSF increased rapidly during the absorption phase (by 30 min after injection). The maximum concentrations (1-3 mumol/l) were reached between 60 and 120 min. The peak concentrations in plasma and CSF did not correlate with each other. In the anesthetized dogs the higher dose of HI6 (100 mumol/kg) caused a steady decrease in mean blood pressure (20 mm Hg) and blood flow (50%) in the femoral artery and a fall in left ventricular pressure (20 mm Hg), lasting for at least 60 min; the lower dose (50 mumol/kg) did not cause circulatory effects. EKG, respiration, hematocrit, arterial blood gases, and pH were not influenced.