Further Investigations on Natural and Synthetic Bis-(1,2,3,4-tetrahydroisoquinoline) Derivatives Interacting with the Apamin-Sensitive Site of Small-Conductance Calcium-Activated Potassium Channels

Our previous study demonstrated that bis-(1,2,3,4-tetrahydroisoquinoline) (bis-THIQ) compounds such as tetrandrine and AG525E1 exhibit significant affinity for SK channels. Interestingly, the stereoisomers of AG525 display markedly different affinities toward SK channels, with the (S,S) stereoisomer being the best ligand. Previous structure-based in silico investigations failed to provide a satisfactory explanation for these differences. Moreover, a series of simplified THIQ derivatives derived from AG525 showed little or no affinity for SK channels, raising questions about the structural features required for binding to SK channels. To better understand the structure–activity relationships within this chemical family with diverse three-dimensional features, a ligand-based approach is developed in this work. Using a dataset composed of AG525 stereoisomers and natural bis-THIQ derivatives, a pharmacophore model was generated to identify key molecular features required for binding to SK channels and to rationalize the observed differences in activity. Twenty pharmacophore hypotheses were generated, with the best-ranked models highlighting five major interaction features: an aromatic ring, one acceptor group associated with this ring, one lipophilic feature, one positive charge, and a second acceptor site located farther from the THIQ moiety. This model differs from previously proposed pharmacophores emphasizing the presence of two positive charges. Two alkaloids recently identified as SK channel ligands, (1) and (2), were found to match all five pharmacophoric features despite not being included in the training set. The absence of SK affinity of a series of simplified THIQ analogues derived from AG525 could be explained using the model. To go further, the predictive value of the model was evaluated by evaluating in silico the propensity of a series of cyclic and non-cyclic bis-THIQ alkaloids to bind to SK channels. High fitting scores were obtained for these compounds, and subsequent in vitro experiments, using radioligand competitive binding assays, were performed and confirmed that they belong as ligands.