Structure–activity relationship of ibogaine analogs interacting with nicotinic acetylcholine receptors in different conformational states

The interaction of ibogaine analogs with nicotinic acetylcholine receptors (AChRs) in different conformational states was studied by functional and structural approaches. The results established that ibogaine analogs: (a) inhibit (±)-epibatidine-induced Ca2+ influx in human embryonic muscle AChRs with the following potency sequence (IC50 in μM): (±)-18-methylaminocoronaridine (5.9 ± 0.3) ∼ (±)-18-methoxycoronaridine (18-MC) (6.8 ± 0.8) > (−)-ibogaine (17 ± 3) ∼ (+)-catharanthine (20 ± 1) > (±)-albifloranine (46 ± 13), (b) bind to the [3H]TCP binding site with higher affinity when the Torpedo AChR is in the desensitized state compared to that in the resting state. Similar results were obtained using [3H]18-MC. These and docking results suggest a steric interaction between TCP and ibogaine analogs for the same site, (c) enhance [3H]cytisine binding to resting but not to desensitized AChRs, with desensitizing potencies (apparent EC50) that correlate very well with the pKi values in the desensitized state, and (d) there are good bilinear correlations between the ligand molecular volumes and their affinities in the desensitized and resting states, with an optimal volume of ∼345 Å3 for the ibogaine site. These results indicate that the size of the binding sites for ibogaine analogs, located between the serine and nonpolar rings and shared with TCP, is an important structural feature for binding and for inducing desensitization.