Rational Design of Nicotinic Acetylcholine Receptor Antagonists for the Treatment of Nerve Agent Exposure

Current treatments for organophosphate poisoning (e.g. oximes or atropine) focus upon either the re-activation of acetylcholine esterase (AChE) or the inhibition of the muscarinic acetylcholine receptor. However, this means that the main target, the nicotinic acetylcholine receptor (located at both neuromuscular junctions and the central nervous system (CNS)), is currently not directly targeted. Medical treatment and recovery is only achieved through the reactivation of AChE and the subsequent breakdown of acetylcholine. This can be a problem, as for some of the organophosphorus nerve agents (e.g. soman); oximes may not always be efficient re-activators of AChE.
Objectives
The overall goal of this project is to improve our prospects for developing compounds that target the nicotinic acetylcholine receptor (nAChR) directly with negative allosteric modulators. Antagonists (or indeed negative allosteric modulators) that work at the orthosteric site of nAChRs are problematic because the binding site will share similar shape characteristics as the binding pocket within AChE. Thus, any compounds developed would likely inhibit both proteins, which is undesirable. The goal here is to develop compounds that bind to and negatively modulate the nAChR, but which don't bind to the AchE. The best route for achieving that is to develop negative allosteric modulators. Fortunately, the nAChRs are known to be highly allosteric proteins and indeed several compounds are known already which act in an allosteric fashion. This presents a viable long-term route for the development of new nerve-agent antidotes. To achieve this goal we have set the following aims:
1. To derive three dimensional structures of apo (i.e. unliganded) nAChR and explore the inherent conformational movements of this, and apo AChE, structures using MD simulations. We will explore apo models of the neuromuscular and CNS variants (alpha7 and alpha4beta2).
2. DSTL have already obtained some preliminary data on the effectiveness of a series of bispyridinium compounds, but it is not clear how they function at the molecular level. We will use a combination of docking and molecular dynamics to suggest working hypothesis, which can be readily tested with site-directed mutagenesis (SDM). The data suggests that at least some component of the effectiveness can be attributed to a negative allosteric modulator effect that is located at a site quite removed from the main orthosteric site. Previous work from the Cohen lab has identified TFD-etomidate as a negative allosteric modulator that likely acts at the gamma-alpha subunit interface and this will serve as useful control system.
3. We will also perform docking and simulation to help rationalise the current SAR profile of bispyridinium compounds against acetylcholine esterase. This should also provide testable (via SDM) hypothesis of the mode of action at this enzyme.
4. We will use the knowledge gained from the MD simulations to rationally design novel compounds predicted to have the appropriate activity on the proteins (i.e. antagonism towards nAChR but not AChE).
5. These compounds will be docked into the nAChR/AChE and simulations run to gain a better understanding of the mechanism.
6. The most promising new compounds will be selected and we will test their activity against the different proteins experimentally (placement at DSTL).
7. This knowledge can then be used in further in-silico refinement.
The project should improve our prospects for improved antidotes against nerve-agents.
EPSRC RESEARCH AREA: This project falls within the "Chemical Biology and Biological Chemistry" and "Computational and Theoretical Chemistry" research areas, both of which are under the Physical Sciences theme.