Structural Determinants of Oxantel Analogs Reveal Modulatory Selectivity of α3β2 and α4β2 Neuronal Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs), ligand-gated ion channels involved in key physiological processes, show pharmacological diversity across receptor subtypes and species. The structurally similar anthelmintic compounds pyrantel, morantel, and oxantel differentially affect the α3β2 and α4β2 nAChR subtypes. Mutation analysis located the modulator binding sites to β(+)/α(−) interface pockets, homologous to the orthosteric agonist sites. We present here the synthesis and pharmacological characterization of 10 oxantel analogs with various phenyl substituents, planarity, and N-methylation, thereby elucidating the structural determinants of nAChR allosteric modulation by oxantel. Two-electrode voltage-clamp in Xenopus laevis oocytes expressing α3β2 and α4β2, respectively, revealed that selectivity and pharmacological profiles were most severely affected by the position of the hydroxy group (meta in oxantel) and the nature of the phenyl substituent. Oxantel is a PAM for α3β2 receptors, with EC₅₀ = 3.9 μM and E_max = 1.98 (relative to ACh alone, EC₅₀ = 3.4 μM), but a NAM for α4β2 receptors, with EC₅₀ = 200 μM and E_max = 0.75 (relative to ACh alone, EC₅₀ = 1.1 μM). Examples of large changes in modulatory activity of the analogs include the o-OH in 2a, resulting in a α3β2-selective PAM (EC₅₀ = 0.061 μM and E_max = 2.08), and the p-OH in 2c elucidated stricter requirement for activity at α3β2 (EC₅₀ = 5.8 μM and E_max = 1.01) compared to α4β2 (EC₅₀ = 96 μM and E_max = 0.88). These results, rationalized by in-silico docking studies, highlight distinct analog selectivity between the two subtypes and fine-tuning their pharmacological profiles.