Mil. Med. Sci. Lett. (Voj. Zdrav. Listy) 2013, vol. 82(1), p. 2-24 ISSN 0372-7025 DOI: 10.31482/mmsl.2013.001 REVIEW ARTICLE BINDING OF QUATERNARY AMMONIUM SALTS TO ACETYLCHOLINE RECEPTORS: POSSIBLE CHEMICAL WARFARE NERVE AGENTS James C. Ball 1083 Jewell Road, Milan, MI 48160, USA Received 22 th January 2013. Revised 12 th February 2013. Published 8 th March 2013. Summary Classical chemical nerve agents are organophosphate based compounds such as Sarin, Soman, Tabun, VX and others. These compounds inhibit acetylcholinesterases in the synapses of nerve junctions. Instead of inactivating acetylcholinesterases, compounds, quaternary amines, can block nerve transmission by binding primarily to the acetylcholine receptor sites. The U.S. Patent Office has published 23 unique patents or invention registrations on the synthesis of compounds that can bind to muscarinic and nicotinic acetylcholine receptors. It is likely that some of these compounds could serve as either polarizing or nonpolarizing nerve agents that bind to the acetylcholine receptor and block binding of acetylcholine. This paper has systematically reviewed a series of bisquaternary amines that could be potentially deadly nerve agents. Analogous tertiary amines, based on the structure of the parent bisquaternary amines, have been proposed as compounds that might be more soluble in membranes making them more bioavailable and toxic. Finally, methods have been discussed that could identify this class of compounds using a bioassay for binding to the acetylcholine receptor. A tool in the identification of these compounds, the Hofmann elimination reaction, has been proposed as a novel method for helping to establish that quaternary or tertiary amines are functional groups of the nerve agent. Key words: Quaternary ammonium salts; nerve agents; chemical warfare; acetylcholine receptor INTRODUCTION AND BACKGROUND the skin as well as through inhalation and oral expo - sure. These compounds act on acetylcholinesterases Classical chemical nerve agents are organophos - in the synapses of nerve junctions. The mode of ac - phate based compounds such as Sarin, Soman, tion of these agents involves phosphorylation Tabun, VX and others. These agents have properties of the active site of acetylcholinesterase followed by that make them uniquely suitable as chemical warfare “aging” in some compounds, a chemical reaction in - agents including high toxicity by all routes of expo - volving the spontaneous formation of a stable cova - sure. The hydrophobic nature of these chemicals lent bond in the active site of the enzyme. In a typical makes them susceptible to absorption through nerve conduction scenario, acetylcholine is released from the nerve terminus of the synaptic junction and rapidly diffuses across the synaptic cleft 1083 Jewell Road, Milan, MI 48160, USA to the postjunctional membrane where it binds [email protected] to acetylcholine receptors. When two acetylcholine (734) 429-1280 molecules bind to their respective sites within a re - Ball: Quaternary Ammonium Salts – Possible Chemical Agents ceptor, a conformational change occurs in the recep - receptors and opens the channel for the migration tor proteins allowing the opening of a channel with of cations. Thus, succinylcholine acts to depolarize the influx of Na +/Ca 2+ and efflux of K + [1]. the neuromuscular endplate. Succinylcholine is not The movement of cations through the channel causes hydrolyzed by the usual acetylcholinesterase but is the nerve to become depolarized; the nerve repolar - hydrolyzed by butyrylcholinesterase – a slower izes once the acetylcholine has diffused away from enzyme. The development of nondepolarizing the receptor causing the channel to close. Acetyl - compounds as an adjunct to anesthesia and used as choline is prevented from continually binding to a tool to relax muscles during surgery has resulted the receptor by rapid hydrolysis in a reaction catalyzed in the identification of several bisquaternary by acetylcholinesterase. If this enzyme is inactivated, ammonium compounds such as pancuronium as happens with the binding of the phosphorus-based (Figure 3), vercuronium (Figure 4) and tubocurarine nerve agents, the levels of acetylcholine do not dimin - (Figure 1) [3]. ish, the channel remains open and the nerve remains polarized; nerve conduction ceases to exist because the nerve cannot repolarize. Nicotinic acetylcholine O receptors are cholinergic receptors (responsive O H + to acetylcholine) found in the central nervous system, + N peripheral nervous systems and skeletal muscles. N H H O H OH O O + N O Figure 3. Pancuronium Cation [(2 S,3 S,5 S,8 R,9 S,10 S,13 S,14 S,16 S,17 R)- 17-acetyloxy- O 10,13-dimethyl-2,16-bis(1-methyl-3,4,5,6-tetrahydro-2 H- pyridin-1-yl)-2,3,4,5,6,7,8,9,11,12,14,15,16,17-tetra- + OH N decahydro-1 H-cyclopenta[ a]phenanthren-3-yl] acetate H O Figure 1. d-Tubocurarine 6,6’-dimethoxy-2,2,2’,2’-tetramethyltubocuraran-2,2’- O diium-7’,12’-diol O + H N N + OO+ H N N H O OO H Figure 2. Succinylcholine Cation O 2,2'-[(1,4-dioxobutane-1,4-diyl)bis(oxy)]bis( N,N,N - Figure 4. Vercuronium Cation trimethylethanaminium) [(2 S,3 S,5 S,8 R,9 S,10 S,13 S,14 S,16 S,17 S)-17-acetyloxy- 10,13-dimethyl-16-(1-methyl-3,4,5,6-tetrahydro-2 H-pyridin -1-yl)-2-(1-piperidyl)-2,3,4,5,6,7,8,9,11,12,14,15,16,17- Tubocurarine was the first compound identified tetradecahydro-1 H-cyclopenta[a]phenanthren-3-yl] acetate to cause muscle paralysis via binding to the nicotinic acetylcholine receptor. This compound was identified from curare, an extract of a South American plant A key feature of nondepolarizing compounds Pareira, Chondrodendron tomentosum [2]. is that they form a rigid molecular framework. Tubocurarine is a compound with two quaternary Decamethonium (Figure 5), with two quaternary ammonium functional groups (Figure 1) that binds to ammonium centers separated by 10 methylene (-CH 2-) the nicotinic acetylcholine receptor and blocks groups, is sufficiently flexible that both sites for the binding of acetylcholine. The binding of this acetylcholine binding are bound resulting in the compound does not open the membrane channel. nerve becoming depolarized [4, 5]. Two broad Succinylcholine (Figure 2, basically two classes of compounds that bind to acetylcholine acetylcholine molecules joined back to back) binds to receptors are the competitive inhibitors that block both acetylcholine binding sites in the acetylcholine binding of the acetylcholine neurotransmitters, but 3 Ball: Quaternary Ammonium Salts – Possible Chemical Agents do not activate the ion channel and compounds that The development of new agents, which do not bind tightly to both sites in the acetylcholine receptor depend on the inactivation of acetylcholinesterase, causing depolarization of the nerve. Depolarization would make the use of these three countermeasures of the nerve allows initial transmission of the nerve mute. New agents would also significantly delay impulse to the muscle resulting in twitching the forensic identification of the agent and its origin. of the muscles (fasciculations) followed by paralysis An example of potential new nerve agents comes of the affected muscles. from an examination of the U.S. Patent Office which has published 23 unique patents or invention Phosphorus-based nerve agents were first registrations on the synthesis of compounds that can synthesized by German chemists prior to and during block nerve transmission by binding to muscarinic World War II and were, fortunately, never used and nicotinic acetylcholine receptors [8-32]. It is in combat [6]. There has been little research likely that some of these compounds could serve as published on the development of new nerve agents either polarizing or nonpolarizing nerve agents that and it is difficult to determine the extent to which any bind to the acetylcholine receptor and block binding country has developed new and novel nerve agents. of acetylcholine. There are a number of motivations why countries would pursue such research. For example, some The primary purpose of this paper is to report on of the phosphorous-based nerve agents can be an investigation of the open literature for clues to counteracted three ways: possible new classes of chemical weapons, or in this case, a new class of nerve agents. This paper will 1) through the use of agents that reactivate endeavor to review some of the physical properties acetylcholinesterase (2-PAM, 2-pyridine aldoxime (e.g. solubility, hydrophobicity) and potential methyl chloride; Figure 6); toxicities of a new class of nerve agents and their potential for use in chemical warfare. There are 2) by protection of the enzyme from attack several objectives of this study: (pyridostigmine; Figure 7) [4]; 1) to understand the structure, properties and 3) or by replacement of inactivated possible modes of action of a “second class” of nerve acetylcholinesterase by the intravenous injection agents that does not involve the inhibition of (i.v.) of human butyrylcholinesterase [7]. acetylcholinesterase, a mechanism common to the phosphorous based nerve agents – the original class of nerve agents, + + N N 2) review structural characteristics of these compounds that appear to be important Figure 5. Decamethonium Cation in the structure-toxicity relationship, Trimethyl-(10-trimethylammoniodecyl)ammonium 3) review characteristics of quaternary ammonium compounds that optimize their binding + to the nicotinic or muscarinic acetylcholine receptors, N OH N 4) discuss the possibility that more potent nerve