Efficacy of HLö-7 and pyrimidoxime as antidotes of nerve agent poisoning in mice

The toxicity and efficacy of two oximes, HLö-7 and pyrimidoxime, were evaluated in mice and compared to those obtained with HI-6. HLö-7 and pyrimidoxime produced 24 h LD₅₀ values of 356 and 291 mg/kg (i.p.), respectively. In combination with atropine (17.4 mg/kg, i.p.), HLö-7 was a very efficient therapy against poisoning by 3×LD₅₀ dose of soman, sarin and GF and 2×LD₅₀ dose of tabun with ED₅₀ values of 12.4, 0.31, 0.32 and 25.2 mg/kg, respectively. In contrast, pyrimidoxime was a relatively poor therapy which resulted in ED₅₀ values of >150, 5.88, 100 and 71 mg/kg against poisoning by soman, sarin, GF and tabun, respectively. HLö-7 produced significant (p <0.05) reactivation of phosphorylated acetylcholinesterase, in vivo, resulting in 47, 38, 27 and 10% reactivation of sarin, GF, soman and tabun inhibited mouse diaphragm acetylcholinesterase, respectively. HLö-7 also antagonized sarin-induced hypothermia in mice suggesting that it reactivated central acetylcholinesterase. The potential of HLö-7 as a replacement oxime for the treatment of nerve agent poisoning is discussed.     

Therapy of organophosphate poisoning in the rat by direct effects of oximes unrelated to ChE reactivation

Isolated rat diaphragm preparations treated with soman or with the irreversible and oxime resistant cholinesterase (ChE) inhibitor S₂₇ (see Compounds) showed a considerable recovery of neuromuscular transmission (NMT) during incubation with the (bis)pyridinium oximes HI-6, HGG-12, P₂S and obidoxime. In the soman-treated preparations this NMT recovery was predominantly caused by reactivation of acetylcholinesterase (AChE) but in the S₂₇-treated preparations it was caused by a direct (pharmacological) effect unrelated to enzyme reactivation. Atropinized rats were artificially ventilated after injection with 3×LD₅₀ soman for 3 h and then treated with HI-6, i.e. at a time when oxime reactivation of soman inhibited ChE is no longer possible. Nevertheless, these rats started to breathe spontaneously and 50–60% survived more than 24 h, whereas all control animals (saline instead of HI-6) died within 10 min after artificial ventilation was terminated. In such animals no significant reactivation of ChE activity at various time intervals following HI-6 treatment was found, either in the diaphragms or in the brains. There was a significant amount of NMT (50%) in vitro in diaphragms obtained from these animals. This NMT did not improve in vitro in the presence of HI-6 and was not inhibited by soman administered to the medium. It is concluded that in this case the NMT found was based on synaptic adaptation to the continued inhibition of ChE and that the survival of the animals might be due to a combination of this synaptic adaptation and central direct effects of HI-6.     

Therapeutic efficacy of HI-6 in soman-poisoned marmoset monkeys.

The therapeutic efficacy of the oxime HI-6 against intoxication with the irreversible cholinesterase (ChE) inhibitor soman was tested in marmoset monkeys. Five out of six marmosets, intoxicated with 5 x LD50 soman and treated immediately with diazepam (0.2 mg.kg-1 iv) and 15 sec later with atropine (0.5 mg.kg-1 im) and HI-6 (50 mg.kg-1 im), survived for more than 24 hr. One of these animals died after 4 days. In the HI-6-treated marmosets blood ChE activity was inhibited at a rate slower than that in three animals treated similarly but with saline instead of HI-6. The latter marmosets died within 8 min after soman. HI-6 achieved its plasma peak 5 min after injection and was eliminated with a t1/2 of about 40 min. In a second experiment similarly treated marmosets were euthanized at 5 min (three saline-treated animals) or at 10 min (three HI-6-treated animals) after the soman intoxication to enable the determination of acetylcholinesterase (AChE) activities in diaphragm and brain tissue. In addition, in these animals blood AChE and butyrylcholine esterase (BuChE) activities were determined. Low AChE activities were encountered in diaphragms and brains. These levels were not significantly different between saline- and HI-6-treated marmosets. In vitro treatment with HI-6 at 40 min after soman still led to an increase of the AChE activity, which was significant in diaphragm, suggesting that postmortem AChE inhibition had occurred. The ratio of AChE to BuChE in blood was significantly enhanced in HI-6-treated animals, indicating that HI-6 preferentially reactivated AChE. It is concluded that (i) HI-6 is an effective treatment against soman poisoning in marmosets and (ii) AChE reactivation or protection by HI-6 contributed to the survival of the animals.     

Distribution of the bispyridinium oxime [14C] HI-6 in male and female rats

Female rats poisoned with multiple LD₅₀s of soman or tabun have been shown previously to respond to the protective effects of HI-6 more positively than male rats. This present study was designed first to determine the distribution pattern and concentration of [¹⁴C] HI-6 in rats, and secondly, to determine the possibility that HI-6 might be located in high concentrations in critical tissues in the female as opposed to the male. To these ends, [¹⁴C] HI-6 was administered to groups of male and female rats and its radiolabelled distribution determined by whole body autoradiography and/or by measurement of its actual concentration, by scintillation spectrometry. The experiments were repeated in the presence of 2 × LD₅₀ soman and supporting therapy with atropine. In both sexes, HI-6 levels were highest in the kidney, followed in order by cartilage >plasma >liver >heart >lung > diaphragm >brain and spinal cord. The relative distribution in the two sexes was confirmed by both methods and was not significantly altered in the presence of soman and atropine. The lack of a measurable difference in tissue distribution of [¹⁴C] HI-6 derived radioactivity between males and females suggested that the hormone-dependent difference in the protective effects previously observed was not due to selective accumulation of [¹⁴C] HI-6 in organs believed to be important in its therapeutic activity, such as brain or diaphragm.     

Delayed neuropathy by the organophosphorus nerve agents soman and tabun

The organophosphorus nerve agents soman and tabun were tested in the hen at doses 120–150 times higher than their acute LD₅₀, as it was assumed that these doses would produce delayed neuropathy. The animals were protected against the acute lethal effect of these agents by pretreatment with atropine, physostigmine, diazepam, and the oxime HI-6 or obidoxime.The surviving animals were followed for 30 days and the occurrence of delayed neuropathy was clinically diagnosed. Soman produced severe delayed neuropathy at a dose of 1.5 mg/kg, a dose which produced acute lethality in five animals out of six. Tabun elicited very mild neuropathic symptoms in one animal out of two at a dose of 6 mg/kg given on 2 consecutive days. Delayed neuropathy was not seen in the hens that survived the acute toxicity of a single dose of tabun, 12 mg/kg (three out of six) or 15 mg/kg (two out of six).     

Effect of PAM-2 Cl,HI-6, and HGG-12 in poisoning by tabun and its thiocholine-like analog in the rat

It has been shown that HI-6 was the most potent oxime so far known in poisoning by sarin, VX, and soman, but its protective effect in tabun poisoning, allegedly due to poor reactivation of inhibited ChE, was much less pronounced. We have found that the thiocholine-like analog of tabun, O-ethyl, N-N-dimethyamino-S-(2-diethylaminoethyl)-thiophosphatemethylsulfomethylate (Ta-S-N+), was very useful in resolving this problem and established the relationship between reactivating and protective effects of PAM-2 Cl, HI-6, and HGG-12 in rats. PAM-2 Cl (protective ratio (PR) = 22.1) and HI-6 (PR = 24.8), combined with atropine, were very effective against Ta-S-N+ poisoning and reactivating inhibited RBC AChE in vitro and rat blood ChE in vivo. The inefficiency of PAM-2 Cl (PR = 1.6) and HI-6 (PR = 2) in tabun poisoning was due to their inadequacy to reactivate tabun-inhibited ChEs. The protective effects of HGG-12 in tabun (PR = 2.8) and Ta-S-N+ poisoning (PR = 2.6) were low, and in the absence of any reactivation of inhibited ChEs, have been atributed to its direct pharmacological effects, which were much more potent in the comparison with PAM-2 Cl or HI-6. It is concluded that the reactivation of inhibited ChE is of decisive importance in the efficient protection in poisoning by tabun and other known chemical warfare nerve agents, whereas their direct pharmacological effects are of limited value, allowing survival of animals only against a few LD50s.     

The effect of pyridostigmine pretreatment on oxime efficacy against intoxication by soman or VX in rats.

This study was done to assess the effects of pyridostigmine (PYR) on a) the accumulation of labelled VX and soman within the brain, b) the therapeutic efficacy of atropine and oxime (2-PAM or HI-6) against intoxication by VX and soman and c) oxime-induced reactivation of inhibited acetylcholinesterase (AChE). In all experiments, rats were given PYR (131 micrograms/kg, im; I70 dose for whole blood AChE) or vehicle 30 min prior to nerve agent. In estimating 3H-agent the accumulation in the brain or estimating blood AChE activity, sufficient soman (47 micrograms/kg, iv) or VX (21.3 micrograms/kg, iv) was given to inhibit 50% of brain AChE activity. In assessing therapeutic efficacy and oxime-induced reactivation of blood AChE, rats were pretreated with PYR, challenged with agent and treated with atropine (16 mg/kg, im) and HI-6 or 2-PAM (100 umoles/kg, im) 30 sec post agent. Whole blood was collected by tail bleeding to monitor peripheral AChE activity at various time points before and after PYR and challenge. Pyridostigmine failed to alter covalent binding of labelled VX or soman in the brain. The 24-hr survival data showed that PYR reduced the therapeutic benefit of atropine and oxime against VX intoxication (but not soman). Protective ratios in VX-challenged rats given vehicle or PYR and treated with atropine + 2-PAM decreased slightly from 2.5 to 2.1 (p > .05), whereas with atropine + HI-6 they decreased significantly from 3.8 to 2.4. Also, AChE reactivation by HI-6 in VX-challenged rats was greater (p < .05) in vehicle- than in PYR-pretreated rats. HI-6 significantly reactivated AChE activity in both pretreatment groups (PYR or vehicle) given soman. The data suggest that PYR decreases the overall recovery of inhibited AChE in VX-challenged rats given HI-6; under the conditions used, this adverse effect decreases atropine+oxime efficacy against VX-induced lethality.     

In vitro and in vivo evaluation of pyridinium oximes: mode of interaction with acetylcholinesterase, effect on tabun- and soman-poisoned mice and their cytotoxicity

The increased concern about terrorist use of nerve agents prompted us to search for new more effective oximes against tabun and soman poisoning. We investigated the interactions of five bispyridinium oximes: K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide], K048 [1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium) butane dibromide], K033 [1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide], TMB-4 [1,3-bis(4-hydroxyiminomethylpyridinium) propane dibromide] and HI-6 [(1-(2-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-2-oxapropane dichloride)] with human erythrocyte acetylcholinesterase (AChE; E.C. 3.1.1.7) and their effects on tabun- and soman-poisoned mice. All the oximes reversibly inhibited AChE, and the enzyme-oxime dissociation constants were between 17 and 180 microM. Tabun-inhibited AChE was completely reactivated by TMB-4, K027 and K048, with the overall reactivation rate constants of 306, 376 and 673 min(-1)M(-1), respectively. The reactivation of tabun-inhibited AChE by K033 reached 50% after 24h, while HI-6 failed to reactivate any AChE at all. Soman-inhibited AChE was resistant to reactivation by 1mM oximes. All studied oximes protected AChE from phosphorylation with both soman and tabun. In vivo experiments showed that the studied oximes were relatively toxic to mice; K033 was the most toxic (LD50=33.4 mg/kg), while K027 was the least toxic (LD50=672.8 mg/kg). The best antidotal efficacy was obtained with K048, K027 and TMB-4 for tabun poisoning, and HI-6 for soman poisoning. Moreover, all tested oximes showed no cytotoxic effect on several cell lines in concentrations up to 0.8mM. The potency of the oximes K048 and K027 to protect mice from five-fold LD50 of tabun and their low toxicity make these compounds leading in the therapy of tabun poisoning. The combination of HI-6 and atropine is the therapy of choice for soman poisoning.     

Efficacy of a combination of acetylcholinesterase reactivators,HI-6 and obidoxime,against tabun and soman poisoning of mice

The bispyridinium oxime HI-6, 1-((((4-amino-carbonyl) pyridinio)methoxy) methyl)-2-(hydroxyimino)-methyl) pyridinium dichloride monohydrate, combined with atropine is an effective treatment for soman (pinacolyl methylphosphonofluoridate) poisoning but is relatively ineffective against tabun (ethyl N-dimethyl phosphoroamidocyanidate) poisoning in mice. This contrasts with those results obtained using the bispyridinium oxime obidoxime [1,1-(oxy bis(methylene)) bis(4-(hydroxyimino)-methyl) pyridinium dibromide]. The purpose of this study was to investigate the efficacy of the combination of HI-6 and obidoxime plus atropine against poisoning by tabun and soman in mice. The combination of ineffective single doses of obidoxime (5 or 10 mg/kg) and HI-6 (25 or 50 mg/kg) improved the treatment of tabun poisoning over either oxime alone. Combinations employing higher concentrations of obidoxime (25 or 50 mg/kg) and HI-6 (100 or 200 mg/kg) resulted in significant toxicity in the absence of organophosphate poisoning. Against soman poisoning the addition of obidoxime to HI-6 did not attenuate the efficacy of HI-6. The half-life of elimination and peak serum concentrations of HI-6 and obidoxime were not altered following administration of the combined injection. Reactivation of tabun-inhibited acetylcholinesterase was found consistently in the diaphragm but not in the brain. Using response surface methods it was possible to estimate the optimal therapy against soman and tabun poisoning (74.5 mg/kg HI-6+31.9 mg obidoxime against 1052 g/kg challenge of tabun and 129 mg/kg HI-6 +0 mg/kg obidoxime against 390 g/kg challenge of soman). It is proposed that reactivation of tabun inhibited acetylcholinesterase at the diaphragm may be responsible for the increased efficacy of the combination of HI-6 and obidoxime against tabun poisoning in mice.     

Efficacy of Mono- and Bis-Pyridinium Oximes Versus Soman, Sarin and Tabun Poisoning in Mice

Efficacy of Mono- and Bis-Pyridinium Oximes Versus Soman, Sarinand Tabun Poisoning in Mice. Clement, J.G. (1983). Fundam. Appl.Toxicol. 3:533–535. Various oximes (PAM, toxogonin, TMB-4,HS-6, HI-6, HGG-12, HGG-42) combined with atropine were comparedas antidotes of soman, sarin and tabun poisoning in non-fastedCD-1^® male mice. TMB-4 was the most toxic oxime with ani.p. LD₅₀ value of 80 mg/kg and HI-6 was the least toxic oximewith an i.p. LD₅₀ of 588 mg/kg. Upon comparing ED₅₀ values,HGG-42 was the most effective oxime versus soman and tabun poisoningwhereas, HI-6 was the most effective oxime versus sarin poisoning.Further research needs to be done to explain the distinct differencesin efficacy of the oximes versus poisoning by soman, sarin ortabun.     

Efficacy of biperiden and atropine as anticonvulsant treatment for organophosphorus nerve agent intoxication

The ability of the nerve agents tabun, sarin, soman, GF, VR, and VX to produce brain seizures and the effectiveness of the anticholinergics biperiden HCl or atropine SO₄ as an anticonvulsant treatment were studied in a guinea-pig model. All animals were implanted a week prior to the experiment with cortical electrodes for electroencephalogram (EEG) recordings. On the day of exposure, the animals were pretreated with pyridostigmine (0.026 mg/kg, i.m.) 30 min prior to challenge with a 2 × LD₅₀ dose (s.c.) of a given agent. In separate experiments, animals were challenged with 5 × LD₅₀ (sc) of soman. One minute after agent challenge, the animals were treated intramuscularly (i.m.) with 2 mg/kg atropine SO₄ admixed with 25 mg/kg 2-PAM Cl and then observed for the onset of seizure activity. Five minutes after the start of nerve agent-induced EEG seizures, animals were treated i.m. with different doses of biperiden HCl or atropine SO₄ and observed for seizure termination. The anticonvulsant ED₅₀ of biperiden HCl and atropine SO₄ for termination of seizures induced by each nerve agent was calculated and compared. With equally toxic doses (2 × LD₅₀) of these agents, continuous EEG seizures (status epilepticus) developed in all animals challenged with soman, tabun, or VR, and in more than 90% of the animals challenged with GF or sarin. In contrast, only 50% of the animals developed seizures when challenged with VX. The times to onset of seizures for soman, tabun, GF, and sarin were very similar (5–8 min) while for VR, it was about 10 min. In the case of VX, not only was the time to seizure development longer (20.7 min), but the seizure activity in 19% of the animals terminated spontaneously within 5 min after onset and did not return. Under these conditions, the anticonvulsant ED₅₀s of biperiden HCl for soman, GF, VR, tabun, sarin, and VX were 0.57, 0.51, 0.41, 0.2, 0.1, and 0.09 mg/kg, respectively, while those of atropine SO₄ for soman, VR, tabun, GF, sarin, and VX were 12.2, 11.9, 10.4, 10.3, 5.1, and 4.1 mg/kg, respectively. In separate experiments, the anticonvulsant ED₅₀ doses of biperiden for animals challenged with 2 or 5 × LD₅₀ of soman were 0.48 (95% confidence limits 0.25–0.73) or 0.57 (95% CI 0.38–0.84) mg/kg, respectively, while the anticonvulsant ED₅₀s for atropine (12.2 mg/kg, i.m.) were identical under these same two challenge conditions. The present study demonstrates that all nerve agents can produce status epilepticus and that the therapeutic effectiveness of atropine and biperiden roughly paralleled the seizurogenic potential of these agents. Received: 16 November 1999 / Accepted: 9 February 2000     

Protection of guinea pigs against soman poisoning with ferrocene carbamate

The protective effect of ferrocene carbamate pretreatment against soman poisoning was studied in guinea pigs. At doses corresponding to 1/20 x and 1/10 × LD₅₀ of this carbamate a 20% and 45% decrease of the acetylcholinesterase in blood and brain, respectively, was obtained. In combination with additional pretreatment, diazepam, and therapy, HI-6 and atropine, the protective ratios (LD₅₀ of soman in treated animals/LD₅₀ of soman in untreated animals) were around 20 and 40, respectively. Animals pretreated with the high dose of the ferrocene carbamate that survived 10 x and 15xLD_50s of soman showed no remaining signs of poisoning after 24 h. Thus, the ferrocene carbamate afforded a better protection against soman than physostigmine. The explanation for this could be due to the properties of the ferrocene carbamate, not correlated to its cholinesterase inhibiting activity. This hypothesis is discussed.     

Mechanism of action of HI-6 on soman inhibition of acetylcholinesterase in preparations of rat and human skeletal muscle; comparison to SAD-128 and PAM-2

HI-6 is presently considered the most potent oxime antidote against soman poisoning in mice, rats, dogs and monkeys. However, it is still an open question whether efficiency of HI-6, observed in experimental animals, can be extrapolated to soman intoxicated humans. In this paper efficiency of HI-6 and possible mechanisms of action were compared in rat and human fresh muscle preparations. In rat muscle, about 50% of control AChE activity could be recovered by both therapeutic (5 min after soman) and prophylactic (5 min before soman) application of HI-6. On the other hand, in human muscle therapeutic treatment restored only 5%, while prophylactic application of HI-6 again resulted in about 50% recovery of control AChE activity. As revealed by comparison of the prophylactic effects of HI-6 and the non-oxime bispyridinium compound SAD-128, competitive inhibition of AChE plays a minor role as a protective mechanism. Immediate reactivation of rapidly aging human AChE must therefore be instituted for successful protective treatment by HI-6. Retardation of aging, a direct effect of SAD-128, was roughly estimated to improve reactivation by HI-6 for about 10% of control AChE activity of the human muscle. PAM-2 proved completely inefficient as a therapeutic and as a prophylactic agent on both rat and human muscle preparations.     

Survivors of soman poisoning: recovery of the soman LD50 to control value in the presence of extensive acetylcholinesterase inhibition

Initially, mice were pretreated with atropine (17.4 mg/kg; IP) and the oxime reactivator HI-6 (50 mg/kg; IP) 5 min prior to an injection of soman (287 g/kg, SC); approximately 2.1 × LD₅₀ dose). More than 95% of the mice survived this dose of soman with atropine and HI-6 pretreatment. In these survivors of soman poisoning the return of the soman LD₅₀ value to control value (124 g/kg, SC) was determined at various times after the initial soman exposure. Mice which survived exposure to a lethal dose of soman by pretreatment with atropine and HI-6 were sensitized to the lethal effects of soman upon re-exposure. The SC soman LD₅₀ at 4 h, after surviving the initial soman exposure, was 20 g/kg. The normal soman LD₅₀ (as evidenced by a LD₅₀ value which was not significantly different from the control value) returned within 4 days, at which time there was still extensive acetylcholinesterase inhibition in all brain regions (striatum, pons-medulla, cerebellum, hypothalamus, hippocampus), diaphragm and erythrocytes. Serum carboxylesterase recovered to control levels within 48 h, whereas liver carboxylesterase activity was not inhibited following the initial soman exposure. The results demonstrate that there is an excess of acetylcholinesterase which is required for normal response in the toxicological sense.     

Dose-Response Effects of Atropine and Hi-6 Treatment of Organophosphorus Poisoning in Guinea Pigs

ABSTRACT

HI-6 (1-2-hydroxyiminomethyl-1-pyridino-3-(4-carbamoyl-1-pyridino)-2-oxapropane dichloride) has been evaluated as an oxime alternative to pralidoxime, and toxogonin in the treatment of organophosphorus (OP) poisoning. The dose response effects of atropine (ATR) and HI-6 were investigated to more fully explore the interaction of these compounds in the treatment of OP poisoning. ATR, HI-6 and various combinations of the two drugs were evaluated against lethal poisoning by soman (GD) and tabun (GA) in guinea pigs. The effect of adjunctive diazepam treattment on the efficacy of atropine and HI-6 against soman was also investigated. Animals of either sex were challenged s.c. with OP and treated i.m. 1 min later with ATR and/or HI-6. When used, diazepam was injected immediately after ATR+HI6. LD50s of each treatment were calculated from probit models based on 24-hour survival against 5 levels of nerve agent and 6 animals per challenge level. A protective index (PI) was calculated by dividing the nerve agent LD50 in the presence of treatment by the LD50 in the absence of treatment. Treatment with HI-6 alone had little effect on the toxicity of either OP Treatment with ATR alone was more effective than HI-6 alone and was significantly more effective against soman than against tabun. When used in combination atropine and HI-6 had a strong synergistic effect against both agents. The dose of atropine used with HI-6 was critical in determining the efficacy of HI-6 against either agent. The slopes of the dose-lethality curves were minimally affected by the dose of ATR or HI-6. Adjunctive treatment with diazepam enhanced the efficacy of HI-6 and atropine against soman. It is concluded that 1) ATR has a large effect on the efficacy of HI-6 against OP poisoning, 2) the dose of ATR must be carefully selected in studies investigating the efficacy of HI-6 against OP poisoning, 3) the effective dose of ATR in the guinea pig is approximately 16 mg/kg, and 4) diazepam is a useful adjunct to atropine and HI-6.     

Hormone- and Dose Schedule-Dependent Protection by HI-6 against Soman and Tabun Poisoning

Hormone- and Dose Schedule-Dependent Protection by HI-6 againstSoman and Tabun Poisoning. LUNDY, P. M., Gourzr, J. C., ANDHAND, B. T. (1989). Fundam. Appl. Toxicol. 12, 595–603.The protective ratio produced by HI-6 (with atropinc) againstsoman and tabun poisoning in rats and guinea pigs was determined.The amount of protection afforded by HI-6 decreased with timefollowing poisoning, prompting us to examine the effects ofrepeated doses of HI-6 (four additional) given over a 5-hr period.In addition, it was determined that HI-6 produced much betterprotection in female rats than male rats, which led to a studyof the hormone dependence of this activity. When in additionto the first dose of HI-6 four additional doses were given overa 5-hr period, the protective ratio, defined as LD50 in treatedanimals/LD50 in untreated animals, in males against soman, increasedfrom 4.2 to 7.8, and against tabun, from 2.5 to 6.6. A singledose of HI-6 produced a protective ratio in females of 10.5against soman and 4.3 against tabun, whereas multiple dosesincreased these values to greater than 27 and 22, respectively.A regimen consisting of gonad removal and long-term treatmentwith a sex hormone of the opposite gender reversed the sex-relateddifferences in response to the protective effects of HI-6. Inaddition a single injection of HI-6 (plus atropinc), 1 mm followingeither soman or tabun, produced good protective ratios in guineapigs against both soman (between 4 and 5) and tabun (5.1); however,there was no apparent hormone-dependent effect similar to thatobtained in rats. The results demonstrate unequivocal protectiveeffects of HI-6 against tabun and a very dramatic hormone-dependentfactor in its activity against either organophosphate. It canbe inferred from the results that for maximal effectivenessagainst soman or tabun, maintenance of adequate blood/tissuelevels of HI-6 is absolutely essential.     

The Influence of 2-/o-cresyl/-4 H-1 ∶ 3 ∶ 2-benzodioxa-phosphorin-2-oxide (CBDP) on organophosphate poisoning and its therapy

The aim of the experiments was to obtain more information on the toxicity of organophosphates and protection against them. Pretreatment of mice with CBDP increased the s.c. toxicity of soman 19.1-fold, and its i.p. toxicity 17.8-fold. The protective effect of atropine and the oximes HS-3, HS-6 and HI-6 in soman poisoning was much greater in CBDP pretreated than in control animals. Atropine + HI-6 raised the s.c. LD₅₀ of soman in the CBDP pretreated animals from 6.8 g/kg to 166 g/kg (PI = 24.3), but in control animals the i.p. LD₅₀ was only raised from 370 g/kg to 608 g/kg (PI = 0.6). CBDP inhibited blood and brain AChE activity, but had no effect on aliesterase (AE) activity in plasma, liver and brain of mice in vivo.CBDP increased s.c. toxicities of sarin 11-fold, of tabun 5-fold and of VX 0.24-fold. The protective index of atropine + HS-3 in sarin poisoning, as in the case of soman poisoning, was much higher in CBDP pretreated than in control animals (20.1 13.6), only slightly higher in tabun poisoning (4.3 3.4) and in the case of VX poisoning lower in CBDP pretreated than in control animals (32 47).The results indicate that CBDP potentiates soman, sarin and tabun toxicities mainly by blocking their binding to non-specific sites in the body.This paper was presented in part at the 7th International Congress of Pharmacology, Paris, July 16–21, 1978     

The influence of 2-/o-cresyl/-4 H-1 : 3 : 2-benzodioxa-phosphorin-2-oxide (CBDP) on organophosphate poisoning and its therapy.

The aim of the experiments was to obtain more information on the toxicity of organophosphates and protection against them. Pretreatment of mice with CBDP increased the s.c. toxicity of soman 19.1-fold, and its i.p. toxicity 17.8-fold. The protective effect of atropine and the oximes HS-3, HS-6 and HI-6 in soman poisoning was much greater in CBDP pretreated than in control animals. Atropine + HI-6 raised the s.c. LD50 of soman in the CBDP pretreated animals from 6.8 micrograms/kg to 166 micrograms/kg (PI = 24.3), but in control animals the i.p. LD50 was only raised from 370 micrograms/kg to 608 micrograms/kg (PI = 0.6). CBDP inhibited blood and brain AChE activity, but had no effect on aliesterase (AE) activity in plasma, liver and brain of mice in vivo. CBDP increased s.d. toxicities of sarin 11-fold, of tabun 5-fold and of VX 0.24-fold. The protective index of atropine + HS-3 in sarin poisoning, as in the case of soman poisoning, was much higher in CBDP pretreated than in control animals (20.1 : 13.6), only slightly higher in tabun poisoning (4.3 : 3.4) and in the case of VX poisoning lower in CBDP pretreated than in control animals (32 : 47). The results indicate that CBDP potentiates soman, sarin and tabun toxicities mainly by blocking their binding to non-specific sites in the body.     

HI-6, an oxime which is an effective antidote of soman poisoning: A structure-activity study

In contrast to other organophosphates, soman poisoning is resistant to atropine (AT) + oxime therapy. However, new bispyridinium-type oximes + AT are effective antidotes of soman poisoning. In structure-activity studies, T 4925 (1,1′-[oxybis(methylene)]bispyridinium dichloride) had an LD₅₀ of 178 mg/kg and HS-14 (1-[[[pyridinio]methoxy]methyl]-2[(hydroxyiminio)methyl]pyridinium dichloride) had an LD₅₀ of 130 mg/kg. DL-10 (1-[[[3-(aminocarbonyl)pyridinio]methoxy]methyl]pyridinium dichloride) and DL-11 (1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]pyridinium dichloride) had LD₅₀'s of 326 and 715 mg/kg, respectively, whereas HS-6 (Reg. No. 22625-23-6) and HI-6 (Reg. No. 34433-31-3) had LD₅₀s of 316 and 514 mg/kg (ip), respectively. T 4925, HS-14, DL-10, and DL-11 at an LD₁ dose + AT (17.4 mg/kg) were relatively ineffective against soman (540 μg/kg; sc) compared to HI-6 and HS-6. Prophylactic ED₅₀s for HI-6 and HS-6 vs soman were 17 and 110 mg/kg, producing safety ratios of 30 and 2.9, respectively. The therapeutic ED₅₀ for HI-6 was 20 mg/kg. Brain cholinesterase (ChE) in mice surviving soman (540 μg/kg; sc) + HI-6 (94 mg/kg) + AT was 0% of control activity at 1 and 2 hr, 5% at 24 hr, and 15% 48 hr later. Mice which died postsoman (540 μg/kg; sc) had 20% brain ChE activity. These results show that HI-6 was one of the least toxic and most efficacious compounds studied and suggest brain ChE inhibition was not the primary lesion in soman poisoning.     

Evaluation of the Influence of Three Newly Developed Bispyridinium Anti‐nicotinic Compounds (MB408, MB442, MB444) on the Efficacy of Antidotal Treatment of Nerve Agent Poisoning in Mice

The influence of three newly developed bispyridinium antinicotinic compounds (the non‐oximes MB408, MB442 and MB444) on the therapeutic efficacy of a standard antidotal treatment (atropine in combination with an oxime) of acute poisoning by the organophosphorus nerve agents tabun and soman was studied in mice. The therapeutic efficacy of atropine in combination with an oxime with or without one of the bispyridinium non‐oximes was evaluated by determination of the LD₅₀ values of the nerve agents and measurement of the survival time after supralethal poisoning. Addition of all the tested non‐oximes increased significantly the therapeutic efficacy of atropine in combination with an oxime against tabun poisoning. They also positively influenced the number of surviving mice 6 hr after supralethal poisoning with tabun. However, they were only slightly effective for the treatment of soman poisoning. The benefit of the tested bispyridinium non‐oximes was dose‐dependent. To conclude, the addition of bispyridinium non‐oximes to the standard antidotal treatment of acute poisoning with tabun was beneficial regardless of the chosen non‐oxime, but only slightly beneficial in the case of soman poisoning.