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 Emax = 1.98 (relative to ACh alone, EC50 = 3.4
μM), but a NAM for α4β2 receptors, with EC50 = 200 μM and Emax = 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 Emax = 2.08), and the p–OH in 2c elucidated stricter requirement for activity at α3β2
(EC50 = 5.8 μM and Emax = 1.01) compared to α4β2 (EC50 = 96 μM
and Emax = 0.88). These results, rationalized
by in-silico docking studies, highlight distinct
analog selectivity between the two subtypes and fine-tuning their
pharmacological profiles.