Synthesis of a Series of Non‐Symmetric Bispyridinium and Related Compounds and Their Affinity Characterization at the Nicotinic Acetylcholine Receptor

Abstract

The current standard therapy to counteract organophosphate intoxication is not effective in equal measure against all types of organophosphorus compounds (OPCs), as the outcome of oxime‐induced reactivation of inactivated acetylcholinesterase (AChE) strongly depends on the particular OPC. In case the reactivation is insufficient, acetylcholine concentrations that rise to pathophysiological levels force the nicotinic acetylcholine receptor (nAChR) into a desensitized state and hence a functionally inactive state. As a consequence, neurotransmission is irreversibly disrupted at the neuromuscular junction. Previous electrophysiological studies identified the symmetric bispyridinium compound 1,1′‐(propane‐1,3‐diyl)bis[4‐(tert‐butyl)pyridin‐1‐ium] diiodide (MB327) as a re‐sensitizer of the desensitized nAChR. MB327 is thereby capable of restoring the functional activity. Very recently, in silico modeling studies suggested non‐symmetric derivatives of MB327 as potential re‐sensitizers with enhanced binding affinity and thus possible enhanced efficacy. In this study, 26 novel non‐symmetric bispyridinium compounds and related derivatives were synthesized. For the synthesis of the highly polar target compounds in sufficient quantities, newly developed and highly efficient two‐step procedures were used. Compounds were characterized in terms of their binding affinity toward the MB327 binding site at the nAChR using recently developed mass spectrometry (MS) Binding Assays. Regarding structure–affinity relationships at the MB327 binding site, the presence of two quaternary aromatic nitrogen centers as well as pyridinium systems with a tert‐butyl group at the 4‐position or a NMe2 group at the 3‐ or 4‐positions appeared to be beneficial for high binding affinities.

Synthesis and Biological Evaluation of Novel MB327 Analogs as Resensitizers for Desensitized Nicotinic Acetylcholine Receptors after Intoxication with Nerve Agents

“…Non-symmetric MB327 analogs 6a - 6h bearing a 4-aminopyridinium ion moiety were readily accessible in one step by N -alkylation of 4-aminopyridines 5 with N -(3-iodopropyl)pyridinium building block 4 , analogous to the method described by Rappenglück et al . (Rappenglück et al, 2018) (Scheme 1). To cover a wider variety of 4-amino substituents in the target compounds, the set of commercially available 4-aminopyridines 5a , 5b and 5e - 5g was extended with some building blocks ( 5c , 5d and 5h ), synthesized according to literature (Hay et al, 2015; Price et al, 2006; Wang et al, 2019).…”

“…A solution of 4-( tert -butyl)-1-(3-iodopropyl)pyridin-1-ium iodide ( 4 ) (Rappenglück et al, 2018) (1.0 equiv) and the corresponding 4-amino-substituted pyridine 5 (1.05-1.1 equiv) or 7-hydroxychinoline ( 7 ) in acetonitrile (2.0-2.7 mL/mmol) was stirred at 90 °C under microwave irradiation (150 W) for 1 h unless otherwise stated. The reaction mixture was concentrated in vacuo, and the residue was purified by recrystallization from different solvent mixtures.…”

“…Thus, compound 6i was isolated in quantitative yield (99%) and with high purity (100%). PTMD90-0012 (8) was synthesized from building block 4 (Rappenglück et al, 2018) and 7-hydroxyquinoline (7) under the reaction conditions described for bispyridinium compounds 6a-6h. However, the reaction time had to be increased to 3 h to compensate for the lower reactivity of the sterically more demanding quinoline 7 as compared to the 4-aminopyridines 5.…”

“…This allowed us to predict structural modifications of MB327 that led to the more potent resensitizers PTM0062 (1) and PTM0063 (2), which have a more polar substituent, i.e., an amino and a methylamino group, respectively, instead of one of the two tert-butyl residues of MB327 (Figure 1) (Kaiser et al, 2023). Interestingly, the recently described dimethylamino analog PTM0056 (3) (Figure 1), which has a less polar substituent than PTM0062 (1) or PTM0063 (2), also shows a slightly higher affinity for the MB327-PAM-1 binding site than MB327 (Rappenglück et al, 2018) and a muscle force-restoring activity comparable to PTM0062 (1) and PTM0063 (2) in preliminary, unpublished ex vivo studies with soman-poisoned rat diaphragm hemispheres. Therefore, PTM0056 (3) appeared to be a promising starting point for the development of new, possibly even more potent, resensitizers for the desensitizied muscle-typ nAChR.…”

Diagnostics and treatment of nerve agent poisoning—current status and future developments

“…5). 134–136 The initial results showed that some of these compounds restore muscle function of soman‐poisoned rat diaphragm and restore the activity of desensitized Torpedo muscle–type nAChRs 118 . Patch clamp studies using human α7 nAChRs revealed that the pharmacological profile of these substances is similar to that of PNU120596, a well‐known type II PAM selective for the α7 nAChRs 126,137 .…”