ATX-II

ATX-II is a Nav1.5 activator. ATX-II was originally discovered in 1976 by extraction from the tentacles of Anemonia sulcata (Bergman et al., 1976). At that time, it was already discovered that it has activity on voltage-dependent Na+ channels from the frog Rana esculenta by slowing the rate of inactivation. Later, it was found that the purified toxin has a positive inotropic effect on isolated guinea pig atria linked to delayed inactivation of the Na+ channel (Alsen et al., 1982). ATX-II acts as a late inward Na+ current inducer in the heart that produces atrial arrhythmias, partly because it also promotes Ca2+/calmodulin-dependent protein kinase activation and concomitant Nav1.5 channel phosphorylation and further activation (Liang et al., 2016). Because late inward Na+ current is difficult to witness, but is a risk factor for the induction of cardiac arrhythmias, it is now mandatory for the FDA that all drugs to be approved should lack effect on the ATX-II-induced late inward Nav1.5 Na+ current. ATX-II is a site 3 toxin and affects domain IV voltage-sensor movement. ATX-II is a carboxylated 47 amino acid peptides with 3 disulfide bridges and of 4934.7 Da molecular weight, recently produced by Smartox Biotechnology in its synthetic form. ATX-II has been tested on the human Nav1.5 channel at various concentrations at a holding potential of -90 mV and a test potential of 10 mV. Test depolarization lasted 50 ms. Appearance of a significant late Na current is visible at 1 nM and this effect is further enhanced at 10 nM ATX-II illustrating the potency of the synthetic compound of Smartox