Screening an In-House Isoquinoline Alkaloids Library for New Blockers of Voltage-Gated Na(+) Channels Using Voltage Sensor Fluorescent Probes: Hits and Biases

Voltage-gated Na(+) (Na(V)) channels are significant therapeutic targets for the treatment of cardiac and neurological disorders, thus promoting the search for novel Na(V) channel ligands. With the objective of discovering new blockers of Na(V) channel ligands, we screened an In-House vegetal alkaloid library using fluorescence cell-based assays. We screened 62 isoquinoline alkaloids (IA) for their ability to decrease the FRET signal of voltage sensor probes (VSP), which were induced by the activation of Na(V) channels with batrachotoxin (BTX) in GH3b6 cells. This led to the selection of five IA: liriodenine, oxostephanine, thalmiculine, protopine, and bebeerine, inhibiting the BTX-induced VSP signal with micromolar IC(50). These five alkaloids were then assayed using the Na(+) fluorescent probe ANG-2 and the patch-clamp technique. Only oxostephanine and liriodenine were able to inhibit the BTX-induced ANG-2 signal in HEK293-hNa(V)1.3 cells. Indeed, liriodenine and oxostephanine decreased the effects of BTX on Na(+) currents elicited by the hNa(V)1.3 channel, suggesting that conformation change induced by BTX binding could induce a bias in fluorescent assays. However, among the five IA selected in the VSP assay, only bebeerine exhibited strong inhibitory effects against Na(+) currents elicited by the hNav1.2 and hNav1.6 channels, with IC(50) values below 10 µM. So far, bebeerine is the first BBIQ to have been reported to block Na(V) channels, with promising therapeutical applications.