Dibromide – A New Nerve Agent Reactivator

Preparation of 1-(4-hydroxy-iminomethylpyridinium)-3-pyridiniumpropane dibromide is described. This compound represents a new acetylcholinesterase (AChE) reactivator, which has no substituents on the second pyridinium ring as found in other commonly used AChE reactivators. The reactivation ability of this reactivator was tested on tabun- and cyclosarin-inhibited AChE. According to the results obtained, the new compound (without substitution and with decreased molecule size) showed increased reactivation potency in case of cyclosarin inhibited AChE. A potent oxime for treatment of tabun and cyclosarin-caused intoxications was thus obtained via slight modification of the reactivator structure (compared to trimedoxime and K027).


Introduction
Organophosphorus inhibitors (OPI) are a large family of compounds with the common main target of action -the enzyme acetylcholinesterase (AChE; EC 3.1.1.7).These compounds irreversibly inhibit this enzyme by a covalent bond in its active site.After the inhibition, the enzyme is not able to fulfill its physiological role in an organism -splitting a neuromediator acetylcholine (ACh).Subsequently, ACh cumulates at the synaptic cleft and steadily over-stimulates nicotinic and muscarinic receptors.If no treatment is provided, a cholinergic crisis occurs and intoxicated organisms may die [1].
Nerve agents are very important group of OPI.Tabun (GA), sarin (GB), soman (GD), cyclosarin (GF) and agent VX (Figure 1) are probably the best known members of this family.Especially GB, well known after the Tokyo subway attack, has been discussed many times as a potential terrorist threat [2].GA and GF were also discussed frequently during last the few years in connection with Iraq's possible possession of this armament [3][4].However, currently commercially available compounds 1-4 are not sufficient [7][8][9][10][11] and many laboratories throughout the world are interested in the development of new AChE reactivators on account of the possible misuse of nerve agents.The promising new oxime HI-6 (1) seemed to be a solution, but its potency to reactivate tabun or pesticide poisoning is poor [12][13][14][15][16]. Due to this, new compounds are being developed and tested for their reactivation potency [17][18][19][20][21][22].
In 2003, a new promising AChE reactivator against tabun was prepared -K027 (5; Figure 3) [23].This reactivator, together with trimedoxime (4), is considered to be currently the most potent available for experimental treatment of GA poisoning [24][25][26][27].However, the potency of these reactivators in a case of GF poisoning was very poor [24,28].In this work, compound 6, a new potential cholinesterase reactivator derived from these compounds was prepared (Figure 3).Its structure was based on the removal of the functional group (trimedoxime -oxime group; K027 -carbamoyl group) on the second pyridinium ring to prevent interaction of this functional group with the enzyme cavity.A main aim was a design of a reactivator, which is able to access the enzyme active site inhibited by a bulky GF moiety.Thus, the new reactivator 1-(4-hydroxyiminomethylpyridinium)-3-pyridiniumpropane dibromide (6) was prepared.

Results and Discussion
In an in vitro assay the reactivator should exceed 10 % reactivation to be also able to reactivate OPI-inhibited AChE in vivo [31].Pralidoxime (1), the reactivator available worldwide, showed a very low reactivation of the tested OPI (Table 1).Focusing on GA-inhibited AChE, there were reactivators (3)(4)(5)(6), which fulfilled or exceed this requirement.The best tested compound for GA poisoning in vitro was trimedoxime (4) at 10 -3 M. The other compounds were less effective, whereas the new oxime 6 was comparable with the previously prepared K027 (5) at 10 -3 M.However, the maximal concentration attainable in vivo is 10 -4 M. Consequently, only the new reactivator 6 was able to exceed the required limit at a concentration attainable in vivo (10 -5 M) for GA intoxication.
For GF-inhibited AChE, only two oximes (2, 6) had any effective reactivation potency, while the best oximes for GA-inhibited AChE (3-4) were ineffective.Although the oxime HI-6 (2) showed massive reactivation of GF-inhibited AChE at both tested concentrations, it was almost ineffective against GA intoxication.In comparison with other tested reactivators, the new oxime 6 had promising potency both against GA and GF poisoning at concentrations attainable in vivo.
The SAR requirements of potent reactivators for GA-inhibited AChE were previously determined [32][33].Thus, the bisquaternary reactivators with the oxime group at the 4-position on heteroaromatic ring and a connecting linker ranging from 3 to 4 carbon-carbon bonds were recommended.The tested reactivators (3-6) fulfilled the mentioned recommendation completely.
Table 1.In vitro screening results (reactivation of tabun and cyclosarin inhibited acetylcholinesterase; source of the enzyme -rat brain acetylcholinesterase; nerve agents used -tabun and cyclosarin; time of inhibition by nerve agent -10 min; time of reactivation -30 min; pH 7.6; 25 °C).However, the GF inhibition demands other features in the reactivator molecule and SAR requirements are different from those of inhibition by GA.Bisquaternary reactivators with one oxime in the 2-position of the heteroaromatic ring and a connecting linker ranging from 3 to 4 carbon-carbon bonds were recommended.Oxime HI-6 (2), which was the best from all tested reactivators against GF, fulfilled the mentioned recommendation.Surprisingly, the new oxime 6 had modest potency against GF-inhibited AChE with an oxime in the 4-position of the pyridinium ring.This finding may be hypothetically explained by loss of a functional group on the second pyridinium ring, which is not included in the reactivation process and interacts with the enzyme.Consequently, a smaller reactivator molecule could better attack the bulky GF moiety in the enzyme active site.Nevertheless, the reactivation process in GF-inhibited AChE should be further described by molecular modeling or crystal structure methods [34][35].

Conclusions
In conclusion, a new cholinesterase reactivator without any substitution on the second pyridinium ring and thus different from other commonly used reactivators was prepared.Its reactivation of GAinhibited AChE was comparable to that achieved with K027, whereas it exceeded the reactivation potency of obidoxime and trimedoxime against GF-inhibited AChE in vitro.The results confirmed that only one oxime functional group is sufficient for reactivation.The other functional groups do not appear to be necessary in a reactivator molecule.

Synthesis
Novel reactivator 6 cannot be prepared in a one step synthesis due to the possible creation of a bisquaternary symmetrical byproduct (6A-6B -examples of side products; Scheme 1), which are very difficult to separate.Consequently, a two-step synthesis of the proposed compound was used (Scheme 2).In the first step, monoquaternary salt 6a was prepared using very mild conditions (acetone reflux; five molar excess of alkylating chain) to decrease the creation of bisquaternary salts 6A-6B [29].Owing to the mild conditions, the final yield (71 %) was not as high as was expected.In the second step, the monoquaternary salt 4 was dissolved in DMF (80 °C) with pyridine (monoquaternary salt to pyridine -

Biological activity
After preparation of the novel AChE reactivator, its reactivation potency was tested using a standard in vitro screening test.Its setup (including the right concentration choice) is described in detail in our previous article [30].GA and GF were used as nerve agent family members to see whether the new reactivator is able to reactivate both GA and GF-inhibited AChE.

Figure 1 .
Figure 1.Organophosphorus inhibitors of acetylcholinesterase known as nerve agents.

Figure 3
Figure 3 Novel reactivators designed for nerve agent poisonings.

Scheme 1 .
Proposed products via one step synthesis of bisquaternary non-symmetrical acetylcholinesterase reactivator.