Conformationally Restricted σ₁ Receptor Antagonists from (−)-Isopulegol

Antagonists at σ1 receptors have great potential for the treatment of neuropathic pain. Starting from monoterpene (−)-isopulegol (1), aminodiols 8–11 were obtained and transformed into bicyclic 13–16 and tricyclic ligands 19–22. Aminodiols 8–11 showed higher σ1 affinity than the corresponding bicyclic 13–16 and tricyclic derivatives 19–22. (R)-configuration in the side chain of aminodiols (8 and 10) led to higher σ1 affinity than (S)-configuration (9 and 11). 4-Benzylpiperidines (b-series) revealed higher σ1 affinity than 4-phenylbutylamines (a-series). Aminodiol 8b showed very high σ1 affinity (Ki = 1.2 nM), excellent selectivity over σ2 receptors, and promising logD7.4 (3.05) and lipophilic ligand efficiency (5.87) values. Molecular dynamics simulations were conducted to analyze the σ1 affinity and selectivity on an atomistic level. In the capsaicin assay, 8b exhibited similar antiallodynic activity to the prototypical σ1 antagonist S1RA. The antiallodynic activity of 8b was removed by co-application of the σ1 agonist PRE-084, proving σ1 antagonism being involved in the antiallodynic effect.