Identification of Slow-Binding Inhibitors of the BoNT/A Protease

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.

Investigation of Salicylanilides as Botulinum Toxin Antagonists

Botulinum neurotoxin serotype A (BoNT/A) is recognized by the Centers for Disease Control and Prevention (CDC) as the most potent toxin and as a Tier 1 biowarfare agent. The severity and longevity of botulism stemming from BoNT/A is of significant therapeutic concern, and early administration of antitoxin-antibody therapy is the only approved pharmaceutical treatment for botulism. Small molecule therapeutic strategies have targeted both the heavy chain (HC) and the light chain (LC) catalytic active site and α-/β-exosites. The LC translocation mechanism has also been studied, but an effective, nontoxic inhibitor remains underexplored. In this work, we screened a library of salicylanilides as potential translocation inhibitors. Potential leads following a primary screen were further scrutinized to identify sal30, which has a cellular minimal concentration of a drug that is required for 50% inhibition (IC50) value of 141 nM. The inquiry of salicylanilide sal30's mechanism of action was explored through a self-quenched fluorogenic substrate conjugated to bovine serum albumin (DQ-BSA) fluorescence, confocal microscopy, and vacuolar H+-ATPase (V-ATPase) inhibition assays. The summation of these findings imply that endolysosomal proton translocation through the protonophore mechanism of sal30 causes endosome pH to increase, which in turn prevents LC translocation into cytosol, a process that requires an acidic pH. Thus, the inhibition of BoNT/A activity by salicylanilides likely occurs through disruption of pH-dependent endosomal LC translocation. We further probed BoNT inhibition by sal30 using additivity analysis studies with bafilomycin A1, a known BoNT/A LC translocation inhibitor, which indicated the absence of synergy between the two ionophores.