Investigations of Thiosemicarbazides as Botulinum Toxin Active-Site Inhibitors: Enzyme, Cellular, and Rodent Intoxication Studies

Botulinum neurotoxin type A (BoNT/A) is an exceptionally potent neurotoxin of great therapeutic value; however, it is also considered a weapon of mass destruction, as it is one of the most poisonous biological substances known to man. The etiology behind BoNT/A is its action as a zinc-dependent protease, which can cause extended paralysis through the cleavage of SNARE proteins. Thiosemicarbazones, known zinc chelators, provide a privileged scaffold that can be leveraged for the development of BoNT/A LC inhibitors. Through a combination of biochemical and kinetic assays, it was demonstrated that the thiosemicarbazone ZMC1, an antitumor agent, is an effective competitive inhibitor of the BoNT/A LC. Based on these results, a series of thiosemicarbazones were designed/synthesized using structure-based analysis and examined in enzyme activity and cell-based assays. From this screen, two analogues presented noteworthy cellular activity. The most potent inhibitors were then tested in a BoNT/A mouse lethality assay, providing statistically significant prolonged survival.