6-Methyluracil derivatives as peripheral site ligand-hydroxamic acid conjugates: Reactivation for paraoxon-inhibited acetylcholinesterase

New uncharged conjugates of 6-methyluracil derivatives with imidazole-2-aldoxime and 1,2,4-triazole-3-hydroxamic acid units were synthesized and studied as reactivators of organophosphate-inhibited cholinesterase. Using paraoxon (POX) as a model organophosphate, it was shown that 6-methyluracil derivatives linked with hydroxamic acid are able to reactivate POX-inhibited human acetylcholinesterase (AChE) in vitro. The reactivating efficacy of one compound (5b) is lower than that of pyridinium-2-aldoxime (2-PAM). Meanwhile, unlike 2-PAM, in vivo study showed that the lead compound 5b is able: (1) to reactivate POX-inhibited AChE in the brain; (2) to decrease death of neurons and, (3) to prevent memory impairment in rat model of POX-induced neurodegeneration.

Monooxime reactivators of acetylcholinesterase with (E)-but-2-ene linker—Preparation and reactivation of tabun- and paraoxon-inhibited acetylcholinesterase

Acetylcholinesterase reactivators are crucial antidotes for the treatment of organophosphate intoxication. Fifteen new monooxime reactivators of acetylcholinesterase with a (E)-but-2-ene linker were developed in an effort to extend the properties of K-oxime (E)-1-(4-carbamoylpyridinium)-4-(4-hydroxyiminomethylpyridinium)-but-2-ene dibromide (K203). The known reactivators (pralidoxime, HI-6, obidoxime, K075, K203) and the new compounds were tested in vitro on a model of tabun- and paraoxon-inhibited AChE. Monooxime reactivators were not able to exceed the best known compounds for tabun poisoning, but some of them did show reactivation comparable with known compounds for paraoxon poisoning. However, extensive differences were found by a SAR study for various substitutions on the non-oxime part of the reactivator molecule.

Novel series of bispyridinium compounds bearing a (Z)-but-2-ene linker—Synthesis and evaluation of their reactivation activity against tabun and paraoxon-inhibited acetylcholinesterase

Six novel AChE reactivators with a (Z)-but-2-ene linker were synthesized using the known synthetic pathways. Their ability to reactivate AChE, which had been previously inhibited by nerve agent tabun or pesticide paraoxon, was tested in vitro and compared to pralidoxime, HI-6, obidoxime, and K075. The novel synthesized compounds were found to be ineffective against GA-inhibited AChE but the ability of (Z)-1,4-bis(4-hydroxyiminomethylpyridinium)-but-2-ene dibromide to reactivate paraoxon-inhibited AChE was comparable with that of oxime K075. Notably, the oxime group in position four substantially increased the ability of the novel compounds to reactivate paraoxon-inhibited AChE.

Squaric monoamide monoester as a new class of reactive immunization hapten for catalytic antibodies

A squaric monoester monoamide motif was employed as an effective reactive immunogen for the discovery of monoclonal antibodies with reactive residue(s) in their combining sites. Two antibodies, 2D4 and 3C8, were uncovered that enhance paraoxon hydrolysis over background. Kinetic analysis of these antibodies was performed and interestingly both undergo a single turnover event due to covalent modification within the antibody combining site. Because antibodies 2D4 and 3C8 result in covalent attachment and thus inactivation of paraoxon, they could be useful probes for investigating paraoxon intoxication.

Interactive Effects of Paraoxon and Pyridostigmine on Blood-Brain Barrier Integrity and Cholinergic Toxicity

The effect of the organophosphorous insecticide paraoxon on the integrity of the blood-brain barrier (BBB) and permeability of pyridostigmine (PYR), a peripheral inhibitor of cholinesterase activity, was examined in Long Evans rats. The integrity of the BBB was examined by measuring the number of capillaries leaking horseradish peroxidase, which was injected into the heart. Treatment with paraoxon at 100 microg/kg, intramuscularly, resulted in a 3- to 4-fold increase in the number of leaky capillaries in young rats (25 to 30 days old) but not in older rats (90 days old). Interestingly, young rats treated with PYR (30 mg/kg, po) 50 min before treatment with paraoxon showed an inhibited effect of paraoxon on the BBB. Furthermore, no increase in the degree of inhibition of acetylcholinesterase activity was observed in young rats treated with PYR before paraoxon compared with young rats treated with paraoxon alone. Cholinergic toxicity, as assessed by changes in behavior, was not observed in young rats treated with paraoxon alone; but, slight signs of cholinergic toxicity were observed in rats treated with PYR. Young rats treated with both PYR and paraoxon did not exhibit more extensive signs of toxicity than rats treated with paraoxon alone or PYR alone. The results indicate that treatment with paraoxon can compromise BBB permeability at dosages that do not induce cholinergic toxicity, but only in young rats. Also, PYR pre-exposure appears to protect the BBB from the paraoxon-induced alterations.

Antagonism of paraoxon intoxication by recombinant phosphotriesterase encapsulated within sterically stabilized liposomes

This investigation effort is focused on increasing organophosphate (OP) degradation by phosphotriesterase to antagonize OP intoxication. For these studies, sterically stabilized liposomes encapsulating recombinant phosphotriesterase were employed. This enzyme was obtained from Flavobacterium sp. and was expressed in Escherichia coli. It has a broad substrate specificity, which includes parathion, paraoxon, soman, sarin, diisopropylfluorophosphate, and other organophosphorous compounds. Paraoxon is rapidly hydrolyzed by phosphotriesterase to the less toxic 4-nitrophenol and diethylphosphate. This enzyme was isolated and purified over 1600-fold and subsequently encapsulated within sterically stabilized liposomes (SL). The properties of this encapsulated phosphotriesterase were investigated. When these liposomes containing phosphotriesterase were incubated with paraoxon, it readily degraded the paraoxon. Hydrolysis of paraoxon did not occur when these sterically stabilized liposomes contained no phosphotriesterase. These sterically stabilized liposomes (SL) containing phosphotriesterases (SL)* were employed as a carrier model to antagonize the toxic effects of paraoxon by hydrolyzing it to the less toxic 4-nitrophenol and diethylphosphate. This enzyme-SL complex (SL)* was administered intravenously to mice either alone or in combination with pralidoxime (2-PAM) and/or atropine intraperitoneally. These results indicate that this carrier model system provides a striking enhanced protective effects against the lethal effects of paraoxon. Moreover when these carrier liposomes were administered with 2-PAM and/or atropine, a dramatic enhanced protection was observed.

Quinidine prevents paraoxon-induced necrotizing myopathy in rats

Acute organophosphorus anticholinesterase poisoning induces a necrotizing end-plate myopathy in rats and patients. Acetylcholine (ACh) excess leads to prolonged synaptic currents and increased influx of cations including calcium through the postsynaptic ACh receptor channels with prolonged muscle membrane depolarization, excess calcium influx into the sarcoplasm, and ultimately muscle fiber necrosis. Quinoline derivatives such as quinidine induce or worsen pre- and postsynaptic disorders of neuromuscular transmission in humans, and are beneficial in patients suffering from a rare congenital myasthenic syndrome called the slow channel congenital myasthenic syndrome. These drugs correct the prolonged opening times of the mutated acetylcholine receptor channels in this myasthenic syndrome. We treated paraoxon-poisoned rats with 4 x 10 or 4 x 50 mg/kg of quinidine and assessed the severity of the necrotizing myopathy in gastrocnemius and diaphragm muscle biopsies. Fasciculations were decreased and the necrotizing myopathy was prevented in most treated rats, with absence of necrotic muscle fibers in most animals in the high-dose group. Survival was not different from untreated poisoned animals. A number of physiological mechanisms, including blocking of presynaptic voltage-gated sodium or calcium channels or inhibition of the postsynaptic ACh receptors channels may have contributed to the attenuation of the myonecrosis. The optimal dose and the drug of choice amongst the clinically available quinoline derivatives remains to be determined.

[The antagonism of phosphorus-containing heterocyclics with the anticholinesterase agent Phosphacol (paraoxon)]

Effects of 15 derivatives of 2-R-oxo-(tioxo-3,5,5-trimethyl-1,2-oxaphospholan-3-ol (OPh) on lethality and protective action on cholinesterase (ChE) of mice brain and blood serum as a result of phosphacolum (paraoxonum) poisoning were investigated. Oxo and tioxo derivatives of OPh with O-methyl and O-ethyl, but not O-propyl and O-butyl radicals at phosphoric atom prevented the lethality of mice poisoned with phosphacolum and preserved brain and blood ChE of inhibition. Ethylic and phenylic OPh derivatives also prevented the lethality of poisoned animals, although without the protection of brain and blood plasma ChE. Combined application of OPh O-ethylic derivative with atropinum and dipiroxinum significantly potentiated antidotal effect m mice poisoned by phosphacolum.

Antagonism of the lethal effects of paraoxon by carrier erythrocytes containing phosphotriesterase

Annealed murine erythrocytes were employed as a carrier model to antagonize the toxic effects of organophosphorus agents. These resealed cells containing a recombinant phosphotriesterase provided striking protection against the lethal effect of paraoxon, an active metabolite of an agricultural pesticide, parathion. Phosphotriesterase hydrolyzes paraoxon to the less-toxic 4-nitrophenol and diethylphosphate. This enzyme was encapsulated into carrier erythrocytes by hypotonic dialysis with subsequent resealing and annealing. These carrier cells were administered to mice either alone or in combination with pralidoxime (2-PAM) and/or atropine. The recipient animals were subsequently challenged with paraoxon and a marked protection was noted. Protection of free enzyme and encapsulated enzyme was compared and the encapsulated enzyme was found to persist longer and possess much greater efficacy. Less serum cholinesterase inhibition also was observed with this enhanced protection. These results indicate that the erythrocyte carrier alone is quite effective in the antagonism of organophosphorus intoxication. Moreover, when these carrier cells were administered in combination with 2-PAM and/or atropine, a marked synergism was observed.

Prevention of organophosphate-induced toxicity in mice

bis-Quaternary amines, which are acetal analogues of hemicholinium-3, were synthesized and several compounds were potent chemicals to antagonize toxicity induced by the organophosphate, paraoxon. Structural requirements were specific and included two oxygen atoms (bis-acetal substitution) within 6 or 7 atom heterocyclic rings, oxygen atoms spaced 2-carbon atoms from the quaternary nitrogen, and carbonyl substitutions adjacent to the spacing moieties, either bicyclohexyl or biphenyl. Biological testing showed a positive potency correlation between the chemicals when data from the following tests were compared: antagonism in mice of paraoxon-induced motor impairment using the incline screen and toxicity, and ability to induce contractions of guinea-pig isolated ilea. The compounds were compared with the often used protective antagonist of organophosphate-induced toxicity, pyridostigmine. One compound, MFS-3, was seven times more efficacious and possessed a much higher therapeutic index. Possible mechanisms of action for these chemicals are discussed.

Encapsulation of phosphotriesterase within murine erythrocytes

A new conceptual approach was employed to antagonize organophosphorus intoxication by using resealed carrier erythrocytes containing a recombinant phosphotriesterase. This enzyme has been reported to hydrolyze many organophosphorus compounds, including paraoxon, a potent cholinesterase inhibitor. Paraoxon is rapidly hydrolyzed by this enzyme to p-nitrophenol and diethylphosphate. Incorporation of phosphotriesterase within resealed murine erythrocytes was accomplished by hypotonic dialysis. The properties of this enzyme within these resealed erythrocytes were investigated. Addition of paraoxon to reaction mixtures containing these resealed erythrocytes loaded with phosphotriesterase resulted in the rapid hydrolysis of paraoxon. Hydrolysis of paraoxon did not occur when these carrier erythrocytes contained no phosphotriesterase. These in vitro studies suggest that carrier erythrocytes may be developed as an approach for the prophylactic and therapeutic antagonism of organophosphorus intoxication.

Phosphotriesterase decreases paraoxon toxicity in mice

The effect of phosphotriesterase (PTE) on the ip toxicity of paraoxon was studied in mice. The PTE preparation (0.1 ml; paraoxon-hydrolyzing activity, 1.5 mumol/min) was given iv. Cholinesterase activities were measured 2 hr after paraoxon administration. The PTE treatment, given 10 min before paraoxon, did not protect serum cholinesterase (ChE) against the inhibiting effect of paraoxon, but it clearly prevented the decrease of the brain ChE activity. In PTE-nontreated animals ChE was reduced by 60% at the paraoxon dose of 0.5 mg/kg, whereas in PTE-treated mice a significant reduction was not seen until a paraoxon dose of 2.0 mg/kg. The iv injection of PTE did prevent the decrease in brain ChE activity by paraoxon, when it was administered before or immediately after the paraoxon. PTE, injected 15 min after paraoxon, resulted in a minor protection in the brain ChE activities. The iv injection of PTE increased the serum paraoxon-hydrolyzing activity up to 5.1-fold. When the same amounts of PTE were administered ip, im, or sc, the increases in the hydrolyzing activities were 4.7-, 2.5-, and 1.8-fold, respectively. The activities returned to the normal level within 24 hr after the PTE. The elimination half-life of the activity of PTE administered iv was approximately 5.5 hr. In conclusion, PTE substantially prevents the toxicity of paraoxon in mice by hydrolyzing paraoxon in circulation.

Structure-activity studies on a potent antagonist to organophosphate-induced toxicity

Molecular modifications have been made on a highly potent, active antagonist to organophosphate-induced toxicity, 4,4'-bis[1,3-dioxan-2-ylmethyl)methylamino]acetyl]biphenyl dimethobromide (1). Stepwise removal of the oxygen atoms from the dioxane rings, as well as changing the position of attachment of substituents on the 1,3-dioxane rings and decreasing the ring size from six-membered to five-membered caused drastic or complete loss of pharmacological effect. Partial structures of 1 were all inactive. Thus, the structure of 1 seems to be remarkably specific. Additional pharmacological data are reported for 1.

Mechanism for antagonism of paraoxon by hemicholinium-3 analogues

In a newly synthesized series of DMAE analogues (bis-diethyl analogue of hemicholinium-3), selected chemicals (TL-402 = NAM-242 greater than JGC-VII-110) showed significant protection of mouse lethality after acute toxic doses of paraoxon (in vivo). DMAE and NAM-250 (like hemicholinium-3) showed minimal or no antagonism against paraoxon-induced toxicity in mice. Studies with DMAE analogues demonstrate weak anticholinesterase activity. The pattern for the neuromuscular inhibition of TL-402, NAM-242 and JGC-VII-110 is different from that of hemicholinium-3. LD50 studies identified compounds with less inherent toxicity (TL-402, NAM-242 and JGC-VII-110) and showed significant antagonism in contrast to DMAE and NAM-250. These chemicals (DMAE and NAM-250) are as toxic as the parent compound hemicholinium-3. All compounds in this series showed potent antinicotinic activity in different nicotinic-receptor preparations. The antinicotinic activity correlates with their action on the acetylcholine receptor-ion channel complex at frog neuromuscular junctions (in vitro). Electrophysiological studies demonstrate that the antinicotinic agents significantly depressed both the end plate current (EPC) amplitude and the time constant of decay (tau EPC) at the end plate of frog. In presence of paraoxon, voltage- and concentration-dependent shortening of tau EPC is observed which is more prominent than the decrease of the amplitude of EPC. The antinicotinic agents which showed significant antagonism of paraoxon both in vivo and in vitro (TL-402, NAM-242 and JGC-VII-110) also produced profound tetanic rundown after neurally or ionophoretically evoked EPC. These effects are voltage-dependent. The marked shortening of tau EPC, linear relationship between 1/tau vs DMAE analogue concentrations and potential-dependent tetanic rundown suggest that these analogues produce antagonism of paraoxon primarily by reducing end plate permeability by blocking nicotinic ACh-R associated ion channels in their open form. The antinicotinic activity of these agents is related to acetal or corresponding ether substitution.

Hemicholinium-3 congeners as potential antagonists to organophosphate-induced toxicity

A series of congeners of hemicholinium-3, in which the 1,4-oxazinium rings of hemicholinium are replaced by pyrrolidine, piperidine, 1,3-dioxane, or 1,4-oxazine rings, is described. Several of the target compounds produced blockade of neuromuscular transmission in the rabbit, and three heterocyclic derivatives, 10, 11, and 13, significantly antagonized paraoxon-induced lethality in mice. 1,3-Dioxane derivative 11 was an extremely potent antagonist of paraoxon-induced toxicity in mice, compared with prototypical protective agents physostigmine and pyridostigmine. Compound 11 exhibited a much more favorable therapeutic ratio than the reference drugs. The mechanism of action of 11 has not been elucidated, although it is concluded that it differs from that of hemicholinium-3 (inhibition of high-affinity, sodium-dependent uptake of choline into nerve terminals).

Acetylcholinesterase reactivators antagonize epileptiform bursting induced by paraoxon in guinea pig hippocampal slices

The electrophysiological actions of paraoxon, an irreversible blocker of acetylcholinesterase, and their antagonism by a series of organophosphate cholinesterase reactivators, were studied in area CA1 of the guinea pig hippocampus in vitro. To avoid indirect effects elicited by excitation of CA3 neurons, the CA2/3 regions were removed routinely before the recording of extracellular field potentials in CA1. Under these conditions, paraoxon (1 microM) induced regular burst activity (rate, 2-10/min; amplitude, 0.2-1 mV; duration, 100-500 msec). The antagonism of this burst activity by atropine (0.3-1.0 microM) and pirenzepine (1.0 microM) suggested the involvement of muscarinic cholinoceptors in the mediation of this response. The reduction in frequency of paraoxon-induced bursting by the cholinesterase reactivators was taken as an index of their efficacies. The four oxime compounds tested were all active in the low micromolar range (rank order of potencies: obidoxime greater than HGG 12 = HLö 7 greater than Hl 6). In experiments without paraoxon, these oximes did not depress either evoked population spikes in normal artificial cerebrospinal fluid or bursts induced by superfusion with Mg++-free artificial cerebrospinal fluid. Thus, an unspecific inhibitory effect of oximes can be excluded. It is concluded that the in vitro hippocampus provides a suitable system for the quantitative electrophysiological evaluation of cholinesterase reactivators in the central nervous system.

Short-term effects of paraoxon and atropine on schedule-controlled behavior in rats

The effects of lethal (2.0 mg/kg) and high sublethal (1.3 mg/kg) dosages of the organophosphate acetylcholinesterase (AChE) inhibitor paraoxon on FR10 performance rate was determined 1 and 2 days after intoxication. The lethal doses were antidoted with either centrally acting atropine sulfate (AS), or atropine methyl bromide (AMB) or atropine methyl nitrate (AMN), both quaternary salts and not expected to act centrally. AChE inhibition in the brain was about 35-60% on the second day after treatment. AS yielded a small transient depression in performance, while AMB and AMN yielded severe deficits, with incomplete recovery. Performance was depressed by 1.3 mg/kg paraoxon by 52% and 34% on days 1 and 2, respectively, while performance was more greatly depressed by the lethal dose, especially with the noncentrally acting antidotes: AS, 67 and 48%; AMB, 81 and 55%; AMN, 91 and 78%. However, a low dose of AS with 2 mg/kg paraoxon resulted in very severe, nonrecovering deficits. A lethal dose of the nonpersistent anti-AChE eserine sulfate, antidoted with a low dose of AS, yielded no deficits. Thus, a high level, acute intoxication with paraoxon yields behavioral deficits which are attenuated by high levels of a centrally acting muscarinic receptor antagonist. The paraoxon-induced performance deficits or their recovery do not correlate directly with AChE inhibition.

Sugar conjugates of pyridinium aldoximes as antidotes against organophosphate poisoning

A series of pyridinium aldoximes having a sugar conjugated to the pyridine ring has been prepared as potential antidotes against organophosphate poisoning. The sugar residue was attached either directly through C-1 or C-6 of the pyranose ring or through a C3 bridge between the glycosyl group and the nitrogen atom of the pyridine moiety. Attachment of a sugar group to the oxime derivative seems to increase the bioavailability of the antidote. The clearance rate of the sugar conjugates was significantly lower than that of their non-sugar analogs and thus they were retained longer in the blood circulation. The sugar derivatives were more potent in decreasing paraoxon-induced hypothermia (which is regulated within the central nervous system) than N-methyl-2-pyridiniumaldoxime methanesulfonate, one of the most commonly used mono-oximes. The sugar analogs were also less toxic than the non-sugar analogs; some also displayed higher efficacy. The mechanism underlying the improved features of the sugar oximes, and the structural requirements in relation to the sugar attachment to the oxime function, are discussed.

[Electrocorticograms and the histochemical effects in the rat accompanying the reactivation of cholinesterases after the effects of an organophosphorus inhibitor]

The administration of paraoxon, an organophosphate inhibitor of cholinesterases, elicits theta rhythms, fast rhythms and paroxystical alterations on the electrocorticogram of the Rat. The disappearance of these abnormalities of the cortical activity after administration of aromatic oximes seems to be in good correlation with the restoration of cerebral cholinesterase activity revealed by histochemical method.

Penetration of oximes across the blood-brain barrier. A histochemical study of the cerebral cholinesterases reactivation

The comparison of the effects of 4 oximes upon the cerebral cholinesterases reactivation after intoxication with paraoxon shows that the best results are obtained with toxogonine and 1574 [(carbaldoxime-4 pyridinium)-1(methyl-1 imidazolium-3)-3 propane]. The reactivation power of this latter compound seems due to the ease with which it can pass through the blood-brain barrier.

Comparison of protective effects of ethylestrenol, norbolethone, and spironolactone against lethality from acute doses of parathion and paraoxon in female rats

Protection against the toxicity of parathion (increased LD50) was provided by preadministered ethylestrenol and, to a lesser extent, by norbolethone and spironlactone. Ethylestrenol and norbolethone also offered protection against paraoxon toxicity. With ethylestrenol and spironolactone, the protection against parathion lethality was greater than that against paraoxon lethality.