Discovery, Pharmacological Characterisation and NMR Structure of the Novel mu-Conotoxin SxIIIC, a Potent and Irreversible Na-V Channel Inhibitor

Voltage-gated sodium (Na-V) channel subtypes, including Na(V)1.7, are promising targets for the treatment of neurological diseases, such as chronic pain. Cone snail-derived mu-conotoxins are small, potent Na-V channel inhibitors which represent potential drug leads. Of the 22 mu-conotoxins characterised so far, only a small number, including KIIIA and CnIIIC, have shown inhibition against human Na(V)1.7. We have recently identified a novel mu-conotoxin, SxIIIC, from Conus striolatus. Here we present the isolation of native peptide, chemical synthesis, characterisation of human Na-V channel activity by whole-cell patch-clamp electrophysiology and analysis of the NMR solution structure. SxIIIC displays a unique Na-V channel selectivity profile (1.4 > 1.3 > 1.1 approximate to 1.6 approximate to 1.7 > 1.2 >> 1.5 approximate to 1.8) when compared to other mu-conotoxins and represents one of the most potent human Na(V)1.7 putative pore blockers (IC50 152.2 +/- 21.8 nM) to date. NMR analysis reveals the structure of SxIIIC includes the characteristic alpha-helix seen in other mu-conotoxins. Future investigations into structure-activity relationships of SxIIIC are expected to provide insights into residues important for Na-V channel pore blocker selectivity and subsequently important for chronic pain drug development.