Although the
sodium channel blocker,
mexiletine, is the first choice
drug in
myotonia, some myotonic patients remain unsatisfied due to
contraindications, lack of tolerability, or incomplete response. More therapeutic options are thus needed for myotonic patients, which require clinical trials based on solid preclinical data. In previous structure-activity relationship studies, we identified two newly-synthesized derivatives of
tocainide, To040 and To042, with greatly enhanced potency and use-dependent behavior in inhibiting
sodium currents in frog skeletal muscle fibers. The current study was performed to verify their potential as antimyotonic agents. Patch-clamp experiments show that both compounds, especially To042, are greatly more potent and use-dependent blockers of human skeletal muscle hNav1.4 channels compared to
tocainide and
mexiletine. Reduced effects on F1586C hNav1.4 mutant suggest that the compounds bind to the
local anesthetic receptor, but that the increased hindrance and lipophilia of the N-substituent may further strengthen
drug-receptor interaction and use-dependence. Compared to
mexiletine, To042 was 120 times more potent to block hNav1.4 channels in a
myotonia-like cellular condition and 100 times more potent to improve muscle stiffness in vivo in a previously-validated rat model of
myotonia. To explore toxicological profile, To042 was tested on hERG
potassium currents, motor coordination using rotarod, and C2C12 cell line for cytotoxicity. All these experiments suggest a satisfactory therapeutic index for To042. This study shows that, owing to a huge use-dependent block of
sodium channels, To042 is a promising candidate
drug for
myotonia and possibly other membrane excitability disorders, warranting further preclinical and human studies.