Hunting for lubeluzole analogues as antimyotonic agents with reduced cardiac liability

Lubeluzole is a neuroprotective agent displaying antimyotonic activity. Lubeluzole clinical development as an antiischemic drug was discontinued due to a lack of efficacy in human trials and possible cardiac toxicity. Since lubeluzole is a potent inhibitor of the hERG channel, involved in long QT syndromes and the potentially fatal cardiac arrhythmia Torsade de Pointes, a series of lubeluzole analogues were prepared to investigate the structural requirements to reduce the affinity for hERG channels to possibly obtain safe antimyotonic drugs. Compound 16o was identified as the less potent hERG blocker possibly endowed with lower cardiac liability in comparison with the parent compound. Antimyotonic activity of 16o was also investigated in vitro on hNav1.4 and higher use-dependence was observed in comparison to lubeluzole, thus suggesting greater selectivity toward highly excited tissues, such as the myotonic muscle. To further verify the cardiac safety of 16o, patch-clamp experiments on hNav1.5 were also carried out and a 3-fold reduction of potency in comparison with hNav1.4 in phasic block was observed. In vivo evaluation of the antimyotonic activity showed unintended effects on rat motor performance. Ex vivo studies suggested calcium channel blocking activity as a possible off-target source of the 16o unintended effects, also reinforced by possible interaction with β₂ receptors, as indicated by in vitro binding assays and in silico studies. In conclusion, we think our results may support the rational design of lubeluzole analogues endowed with both antimyotonic activity and lower hERG liability.