Searching for Therapeutics Against Botulinum Neurotoxins: A True Challenge for Drug Discovery

Botulinum neurotoxins (BoNTs), the most potent known toxins, cause severe muscle paralysis and death at nanogram exposures and are considered biothreat agents. BoNTs target the neuromuscular junction where they release smaller zinc metalloprotease light chains (LCs) into the neuron cytosol that selectively cleave SNARE proteins and thus block the exocytosis of acetylcholine neurotransmitters necessary for skeletal muscle contraction. The majority of efforts to develop post-symptomatic therapeutics for botulism poisoning have focused on inhibiting the LC and tremendous strides have been made in understanding how the LC binds to the SNARE proteins via X-ray crystallography. Subsequent homology modeling and structure based drug design have led to the discovery of multiple small molecule BoNT/A inhibitors in the 0.05 ~10 μ range, but to date none have shown significant post-symptomatic efficacy in an animal model of botulinum intoxication. With the lack of reported pharmacokinetic data, we have analyzed the BoNT/A inhibitor lead chemical matter from a physicochemical property point of view and have attempted to understand if bioavailability of drug at the neuromuscular junction is the root cause of this apparent in vitro/in vivo disconnect in the field.