Reactivator Report for: HGG-12

Acute sc toxicity of soman increased in the order, mice----rats----guinea pigs----dogs, being 12.6 times more toxic to dogs (LD50 = 0.05 mumol/kg) than to mice. It was 2.8 times more toxic than tabun to mice and 35 times more toxic to dogs. HI-6 was the least toxic and had similar toxicity values to the four animal species studied and HGG-12 the most toxic of the three oximes used. HGG-12 has shown the greatest interspecies variation (rats:dogs = 1:19.5). HI-6, HGG-12, and PAM-2 Cl (in conjunction with atropine and diazepam) revealed the best protective effect in soman-poisoned dogs, with the respective protective indices of 9, 6.3, and 3.5, followed by guinea pigs. In tabun poisoning the best, but relatively low, protective effect was found only in guinea pigs. The introduction of diazepam increased the protective effects of atropine-oxime combination in soman and tabun poisoning by 10 to 80%. We suggest that the high toxicity of soman and low toxicity of HI-6 may be anticipated in man. The inefficiency of HI-6, HGG-12, and PAM-2 Cl in tabun poisoning points either to the search of new compounds or to the use of the mixture of the oximes found to be effective against the known chemical warfare nerve agents.

Certain recently developed antidotes of the bispyridinium type, commonly called "H-oximes" (HGG 12, 21, 42, 52, 65, 70, 89, and HGG 90) have been investigated as to their effects on muscarinic and nicotinic acetylcholine receptors. These compounds clearly discriminate between these two types of receptors being more potent inhibitors of the muscarinic receptor with inhibitory constants in the micromole range. (The corresponding values for the nicotinic receptor are in the range of 0.1 mM.) However, the inhibitory potency in the binding assay does not correlate with the ED50 values obtained against soman in mice. The site of antidotal action therefore appears not to be the nicotinic acetylcholine receptor. Binding to the muscarinic receptors may partially contribute to the effects against soman in vivo.

The mode of interaction of bisquaternary pyridinium oximes with rat brain muscarinic receptors in cerebral cortex and brain stem preparations was studied by the use of the tritium-labeled antagonist N-methyl-4-piperidyl benzilate ( [3H] 4NMPB). Binding of the labeled muscarinic antagonist was inhibited by these drugs, the most potent inhibitors being 1-(2-hydroxyiminoethylpyridinium)-1-(3-cyclohexylcarboxypyridin ium)dimethyl-ether (HGG-42) and its 3-phenylcarboxypyridinium analog (HGG-12) (apparent KI = 1.3-1.7 and 1.8-2.2 microM, respectively). Analysis of the binding properties suggested that binding of the muscarinic antagonist and the bisquaternary pyridinium oximes was nonexclusive. Kinetic binding data provide evidence that the drugs inhibit binding of muscarinic antagonists in an allosteric manner, with a resulting decrease in the rates of both association of [3H]4NMPB to the receptor and its dissociation from it. These effects were observed both in brain stem and in cortical preparations even after pretreatment and washing out of the inhibitors. The selective natures of HGG-12 and HGG-42 were apparent from their irreversible effects on the number of muscarinic binding sites. In brain stem, the presence of these drugs resulted in a loss of about 30% of binding sites, which accounts in part for the apparent decrease in maximal binding capacity observed in the equilibrium binding of [3H]4NMPB. In the cortex, however, only approximately 10% of the muscarinic receptors were lost upon exposure to these drugs. The decrease in the muscarinic receptor population of the brain stem was dependent on both concentration and time and occurred both in vitro and in vivo following injection of HGG-12 into rats. Unlike the in vitro loss of receptor sites, which was irreversible, the in vivo effect was restored 2 hr after the injection. Taken together, the results suggest that the bisquaternary oximes are allosteric inhibitors of the muscarinic acetylcholine receptor and may be capable of distinguishing between receptor states and inducing specific irreversible effects. Because of these properties, the drugs may prove extremely useful as sensitive probes in studies on the nature of the agonist-receptor-effector relationship.

Acute sc toxicity of soman increased in the order, mice----rats----guinea pigs----dogs, being 12.6 times more toxic to dogs (LD50 = 0.05 mumol/kg) than to mice. It was 2.8 times more toxic than tabun to mice and 35 times more toxic to dogs. HI-6 was the least toxic and had similar toxicity values to the four animal species studied and HGG-12 the most toxic of the three oximes used. HGG-12 has shown the greatest interspecies variation (rats:dogs = 1:19.5). HI-6, HGG-12, and PAM-2 Cl (in conjunction with atropine and diazepam) revealed the best protective effect in soman-poisoned dogs, with the respective protective indices of 9, 6.3, and 3.5, followed by guinea pigs. In tabun poisoning the best, but relatively low, protective effect was found only in guinea pigs. The introduction of diazepam increased the protective effects of atropine-oxime combination in soman and tabun poisoning by 10 to 80%. We suggest that the high toxicity of soman and low toxicity of HI-6 may be anticipated in man. The inefficiency of HI-6, HGG-12, and PAM-2 Cl in tabun poisoning points either to the search of new compounds or to the use of the mixture of the oximes found to be effective against the known chemical warfare nerve agents.

Certain recently developed antidotes of the bispyridinium type, commonly called "H-oximes" (HGG 12, 21, 42, 52, 65, 70, 89, and HGG 90) have been investigated as to their effects on muscarinic and nicotinic acetylcholine receptors. These compounds clearly discriminate between these two types of receptors being more potent inhibitors of the muscarinic receptor with inhibitory constants in the micromole range. (The corresponding values for the nicotinic receptor are in the range of 0.1 mM.) However, the inhibitory potency in the binding assay does not correlate with the ED50 values obtained against soman in mice. The site of antidotal action therefore appears not to be the nicotinic acetylcholine receptor. Binding to the muscarinic receptors may partially contribute to the effects against soman in vivo.