Small Molecules Selectively Targeting Sigma-1 Receptor for the Treatment of Neurological Diseases

The sigma-1 (σ1) receptor, an enigmatic protein originally classified as an opioid receptor subtype, is now understood to possess unique structural and functional features of its own and play critical roles to widely impact signaling transduction by interacting with receptors, ion channels, lipids, and kinases. The σ1 receptor is implicated in modulating learning, memory, emotion, sensory systems, neuronal development, and cognition and accordingly is now an actively pursued drug target for various neurological and neuropsychiatric disorders. Evaluation of the five selective σ1 receptor drug candidates (pridopidine, ANAVEX2-73, SA4503, S1RA, and T-817MA) that have entered clinical trials has shown that reaching clinical approval remains an evasive and important goal. This review provides up-to-date information on the selective targeting of σ1 receptors, including their history, function, reported crystal structures, and roles in neurological diseases, as well as a useful collation of new chemical entities as σ1 selective orthosteric ligands or allosteric modulators.

In Pursuit of Fluorinated Sigma Receptor Ligand Candidates Related to [18F]-FPS

This paper describes the synthesis of N-arylmethyl(1-benzyl) and N-aroyl(1-benzoyl) 4-(4-fluoromethylphenoxymethyl)piperidines as potential sigma receptor ligands analogous to the potent and highly selective sigma-1 ligand [18F]-FPS, but with enhanced or alternative binding and transport profiles. The synthesis involves N-aroylation of 4-hydroxmethylpiperidine or ethyl nipecotate, functional group manipulation of the ester group or simple activation of the hydroxyl group to introduce the phenoxy component, and subsequent functional group manipulation to reduce the amide group and introduce the fluorine into the fluoromethyl substituent. In its development, the synthesis was found to require early N-aroylation of the piperidine precursor to avoid complications due to anchimeric assistance by its nitrogen in subsequent displacement reactions. New evidence is presented on the pathway followed in a literature report of direct displacement of a benzylic hydroxyl group by fluoride ion under Appel-like conditions. Relevant to the literature report, the halide ion in the fluoromethylphenoxy 1-benzylpiperidine derivatives was surprisingly labile to hydrolytic displacement on chromatography and this aspect is worthy of further study. Moreover, the NMR spectra of the amides were complicated by geometric isomerism about the amide C(O)–N bond, but detailed analysis of spectra from 2-anisoyl derivatives allowed the assignment of diastereomeric contributors to consistent, secondary atropisomerism about the aryl–C(O) bond.

Norbenzomorphan Scaffold: Chemical Tool for Modulating Sigma Receptor-Subtype Selectivity

Some norbenzomorphans exhibit high affinity for sigma 1 and sigma 2 receptors, and varying the position of substituents on the aromatic ring of this scaffold has a significant effect on subtype selectivity. In particular, compounds bearing several different substituents at C7 of the norbenzomorphan ring system exhibit a general preference for the sigma 1 receptor, whereas the corresponding C8-substituted analogues preferentially bind at the sigma 2 receptor. These findings suggest that the norbenzomorphan scaffold may be a unique chemical template that can be easily tuned to prepare small molecules for use as tool compounds to study the specific biological effects arising from preferential binding at either sigma receptor subtype. In the absence of structural characterization data for the sigma 2 receptor, such compounds will be useful toward refining the pharmacophore model of its binding site.

A systematic exploration of the effects of flexibility and basicity on sigma (σ) receptor binding in a series of substituted diamines

The sigma-1 receptor (S1R) has attracted a great deal of attention as a prospective drug target due to its involvement in numerous neurological disorders and, more recently, for its therapeutic potential in neuropathic pain. As there was no crystal structure of this membrane-bound protein reported until 2016, ligand generation was driven by pharmacophore refinements to the general model suggested by Glennon and co-workers. The generalised S1R pharmacophore comprises a central region where a basic amino group is preferred, flanked by two hydrophobic groups. Guided by this pharmacophore, S1R ligands containing piperazines, piperazinones, and ethylenediamines have been developed. In the current work, we systematically deconstructed the piperazine core of a prototypic piperazine S1R ligand (vide infra) developed in our laboratories. Although we did not improve the affinity at the S1R compared to the lead, we identified several features important for affinity and selectivity. These included at least one basic nitrogen atom, conformational flexibility and, for S1R, a secondary or tertiary amine group proximal to the anisole. Furthermore, S2R selectivity can be tailored with functional group modifications of the N-atom proximal to the anisole.