Time-dependent botulinum neurotoxin serotype A metalloprotease inhibitors

Irreversible inhibition of BoNT/A protease: proximity-driven reactivity contingent upon a bifunctional approach

Botulinum neurotoxin A (BoNT/A) is categorized as a Tier 1 bioterrorism agent and persists within muscle neurons for months, causing paralysis. A readily available treatment that abrogates BoNT/A's toxicity and longevity is a necessity in the event of a widespread BoNT/A attack and for clinical treatment of botulism, yet remains an unmet need. Herein, we describe a comprehensive warhead screening campaign of bifunctional hydroxamate-based inhibitors for the irreversible inhibition of the BoNT/A light chain (LC). Using the 2,4-dichlorocinnamic hydroxamic acid (DCHA) metal-binding pharmacophore modified with a pendent warhead, a total of 37 compounds, possessing 13 distinct warhead types, were synthesized and evaluated for time-dependent inhibition against the BoNT/A LC. Iodoacetamides, maleimides, and an epoxide were found to exhibit time-dependent inhibition and their k GSH measured as a description of reactivity. The epoxide exhibited superior time-dependent inhibition over the iodoacetamides, despite reacting with glutathione (GSH) 51-fold slower. The proximity-driven covalent bond achieved with the epoxide inhibitor was contingent upon the vital hydroxamate-Zn2+ anchor in placing the warhead in an optimal position for reaction with Cys165. Monofunctional control compounds exemplified the necessity of the bifunctional approach, and Cys165 modification was confirmed through high-resolution mass spectrometry (HRMS) and ablation of time-dependent inhibitory activity against a C165A variant. Compounds were also evaluated against BoNT/A-intoxicated motor neuron cells, and their cell toxicity, serum stability, and selectivity against matrix metalloproteinases (MMPs) were characterized. The bifunctional approach allows the use of less intrinsically reactive electrophiles to intercept Cys165, thus expanding the toolbox of potential warheads for selective irreversible BoNT/A LC inhibition. We envision that this dual-targeted strategy is amenable to other metalloproteases that also possess non-catalytic cysteines proximal to the active-site metal center.

Catch and Anchor Approach To Combat Both Toxicity and Longevity of Botulinum Toxin A

Botulinum neurotoxins have remarkable persistence (∼weeks to months in cells), outlasting the small-molecule inhibitors designed to target them. To address this disconnect, inhibitors bearing two pharmacophores-a zinc binding group and a Cys-reactive warhead-were designed to leverage both affinity and reactivity. A series of first-generation bifunctional inhibitors was achieved through structure-based inhibitor design. Through X-ray crystallography, engagement of both the catalytic Zn2+ and Cys165 was confirmed. A second-generation series improved on affinity by incorporating known reversible inhibitor pharmacophores; the mechanism was confirmed by exhaustive dialysis, mass spectrometry, and in vitro evaluation against the C165S mutant. Finally, a third-generation inhibitor was shown to have good cellular activity and low toxicity. In addition to our findings, an alternative method of modeling time-dependent inhibition that simplifies assay setup and allows comparison of inhibition models is discussed.

Covalent Modifiers of Botulinum Neurotoxin Counteract Toxin Persistence

Botulinum neurotoxins (BoNTs) are the most potent toxins known to man and a significant threat as weapons of bioterrorism. BoNTs contain a metalloprotease domain that blocks neurotransmitter release in nerve terminals, resulting in a descending, flaccid paralysis with a 5-10% mortality rate. Existing treatment options cannot access or neutralize the toxin following its endocytosis, so there is a clear need to develop novel therapies. Numerous substrate-based and zinc-chelating small-molecule inhibitors have been reported; however, none have progressed to the clinic. This is likely due to the difficulty that reversible inhibitors have in achieving sustained inhibition of the toxin, which has a half-life of months in vivo. An alternative strategy for mitigating BoNT persistence is covalent, irreversible inhibition of toxin function. However, few examples of covalent BoNT inhibitors have been reported. Here, we describe a competition-based screen to identify covalent modifiers of the conserved active-site-adjacent cysteine C165 in the BoNT/A serotype. We found that compounds containing cysteine-reactive electrophiles designed to target cysteine proteases failed to bind C165 while selenide compounds were efficient covalent binders of this cysteine. Importantly, covalent modification at C165 resulted in sustained, irreversible inhibition of BoNT/A protease activity. Covalent selenide inhibitors were nontoxic and protective in a neuronal assay of intoxication, making them promising new scaffolds for the study of the BoNT/A toxin as well as for the design of novel therapy agents.

Phosphatase Inhibitors Function as Novel, Broad Spectrum Botulinum Neurotoxin Antagonists in Mouse and Human Embryonic Stem Cell-Derived Motor Neuron-Based Assays

There is an urgent need to develop novel treatments to counter Botulinum neurotoxin (BoNT) poisoning. Currently, the majority of BoNT drug development efforts focus on directly inhibiting the proteolytic components of BoNT, i.e. light chains (LC). Although this is a rational approach, previous research has shown that LCs are extremely difficult drug targets and that inhibiting multi-serotype BoNTs with a single LC inhibitor may not be feasible. An alternative approach would target neuronal pathways involved in intoxication/recovery, rather than the LC itself. Phosphorylation-related mechanisms have been implicated in the intoxication pathway(s) of BoNTs. However, the effects of phosphatase inhibitors upon BoNT activity in the physiological target of BoNTs, i.e. motor neurons, have not been investigated. In this study, a small library of phosphatase inhibitors was screened for BoNT antagonism in the context of mouse embryonic stem cell-derived motor neurons (ES-MNs). Four inhibitors were found to function as BoNT/A antagonists. Subsequently, we confirmed that these inhibitors protect against BoNT/A in a dose-dependent manner in human ES-MNs. Additionally, these compounds provide protection when administered in post-intoxication scenario. Importantly, the inhibitors were also effective against BoNT serotypes B and E. To the best of our knowledge, this is the first study showing phosphatase inhibitors as broad-spectrum BoNT antagonists.

SRC family kinase inhibitors antagonize the toxicity of multiple serotypes of botulinum neurotoxin in human embryonic stem cell-derived motor neurons

Botulinum neurotoxins (BoNTs), the causative agents of botulism, are potent inhibitors of neurotransmitter release from motor neurons. There are currently no drugs to treat BoNT intoxication after the onset of the disease symptoms. In this study, we explored how modulation of key host pathways affects the process of BoNT intoxication in human motor neurons, focusing on Src family kinase (SFK) signaling. Motor neurons derived from human embryonic stem (hES) cells were treated with a panel of SFK inhibitors and intoxicated with BoNT serotypes A, B, or E (which are responsible for >95 % of human botulism cases). Subsequently, it was found that bosutinib, dasatinib, KX2-391, PP1, PP2, Src inhibitor-1, and SU6656 significantly antagonized all three of the serotypes. Furthermore, the data indicated that the treatment of hES-derived motor neurons with multiple SFK inhibitors increased the antagonistic effect synergistically. Mechanistically, the small molecules appear to inhibit BoNTs by targeting host pathways necessary for intoxication and not by directly inhibiting the toxins' proteolytic activity. Importantly, the identified inhibitors are all well-studied with some in clinical trials while others are FDA-approved drugs. Overall, this study emphasizes the importance of targeting host neuronal pathways, rather than the toxin's enzymatic components, to antagonize multiple BoNT serotypes in motor neurons.

New targets in the search for preventive and therapeutic agents for botulism

Botulism is a severe neuroparalytic disease resulting from exposure to one of the most poisonous toxins to humans. Because of this high potency and the use of toxins as biological weapons, botulism is a public health concern and each case represents an emergency. Current therapy involves respiratory supportive care and anti-toxins administration. As a preventive measure, vaccination against toxins represents an effective strategy but is undesirable due the rarity of botulism and the effectiveness of toxins in treating several neuromuscular disorders. This paper summarizes the current issues in botulism treatment and prevention, highlighting the challenge for future researches.

Benzoquinones as inhibitors of botulinum neurotoxin serotype A

Although botulinum neurotoxin serotype A (BoNT/A) is known for its use in cosmetics, it causes a potentially fatal illness, botulism, and can be used as a bioterror weapon. Many compounds have been developed that inhibit the BoNTA zinc-metalloprotease light chain (LC), however, none of these inhibitors have advanced to clinical trials. In this study, a fragment-based approach was implemented to develop novel covalent inhibitors of BoNT/A LC. First, electrophilic fragments were screened against BoNT/A LC, and benzoquinone (BQ) derivatives were found to be active. In kinetic studies, BQ compounds acted as irreversible inhibitors that presumably covalently modify cysteine 165 of BoNT/A LC. Although most BQ derivatives were highly reactive toward glutathione in vitro, a few compounds such as natural product naphthazarin displayed low thiol reactivity and good BoNT/A inhibition. In order to increase the potency of the BQ fragment, computational docking studies were employed to elucidate a scaffold that could bind to sites adjacent to Cys165 while positioning a BQ fragment at Cys165 for covalent modification; 2-amino-N-arylacetamides met these criteria and when linked to BQ displayed at least a 20-fold increase in activity to low μM IC₅₀ values. Unlike BQ alone, the linked-BQ compounds demonstrated only weak irreversible inhibition and therefore acted mainly as non-covalent inhibitors. Further kinetic studies revealed a mutual exclusivity of BQ covalent inactivation and competitive inhibitor binding to sites adjacent to Cys165, refuting the viability of the current strategy for developing more potent irreversible BoNT/A inhibitors. The highlights of this study include the discovery of BQ compounds as irreversible BoNT/A inhibitors and the rational design of low μM IC50 competitive inhibitors that depend on the BQ moiety for activity.

Identification of clinically viable quinolinol inhibitors of botulinum neurotoxin A light chain

Botulinum neurotoxins (BoNT) are the most potent toxins known and a significant bioterrorist threat. Few small molecule compounds have been identified that are active in cell-based or animal models, potentially due to toxin enzyme plasticity. Here we screened commercially available quinolinols, as well as synthesized hydroxyquinolines. Seventy-two compounds had IC50 values below 10 μM, with the best compound exhibiting submicromolar inhibition (IC50 = 0.8 μM). Structure-activity relationship trends showed that the enzyme tolerates various substitutions at R1 but has a clear preference for bulky aryl amide groups at R2, while methylation at R3 increased inhibitor potency. Evaluation of the most potent compounds in an ADME panel showed that these compounds possess poor solubility at pH 6.8, but display excellent solubility at low pH, suggesting that oral dosing may be possible. Our data show the potential of quinolinol compounds as BoNT therapeutics due to their good in vitro potencies and favorable ADME properties.

Targeting botulinum A cellular toxicity: a prodrug approach

The botulinum neurotoxin light chain (LC) protease has become an important therapeutic target for postexposure treatment of botulism. Hydroxamic acid based small molecules have proven to be potent inhibitors of LC/A with nanomolar Ki values, yet they lack cellular activity conceivably due to low membrane permeability. To overcome this potential liability, we investigated two prodrug strategies, 1,4,2-dioxazole and carbamate, based on our 1-adamantylacetohydroxamic acid scaffold. The 1,4,2-dioxazole prodrug did not demonstrate cellular activity, however, carbamates exhibited cellular potency with the most active compound displaying an EC50 value of 20 μM. Cellular trafficking studies were conducted using a "fluorescently silent" prodrug that remained in this state until cellular uptake was complete, which allowed for visualization of the drug's release inside neuronal cells. In sum, this research sets the stage for future studies leveraging the specific targeting and delivery of these prodrugs, as well as other antibotulinum agents, into neuronal cells.

Structural framework for covalent inhibition of Clostridium botulinum neurotoxin A by targeting Cys165

Clostridium botulinum neurotoxin type A (BoNT/A) is one of the most potent toxins for humans and a major biothreat agent. Despite intense chemical efforts over the past 10 years to develop inhibitors of its catalytic domain (catBoNT/A), highly potent and selective inhibitors are still lacking. Recently, small inhibitors were reported to covalently modify catBoNT/A by targeting Cys(165), a residue located in the enzyme active site just above the catalytic zinc ion. However, no direct proof of Cys(165) modification was reported, and the poor accessibility of this residue in the x-ray structure of catBoNT/A raises concerns about this proposal. To clarify this issue, the functional role of Cys(165) was first assessed through a combination of site-directed mutagenesis and structural studies. These data suggested that Cys(165) is more involved in enzyme catalysis rather than in structural property. Then by peptide mass fingerprinting and x-ray crystallography, we demonstrated that a small compound containing a sulfonyl group acts as inhibitor of catBoNT/A through covalent modification of Cys(165). The crystal structure of this covalent complex offers a structural framework for developing more potent covalent inhibitors catBoNT/A. Other zinc metalloproteases can be founded in the protein database with a cysteine at a similar location, some expressed by major human pathogens; thus this work should find broader applications for developing covalent inhibitors.

Discovery of a fluorene class of compounds as inhibitors of botulinum neurotoxin serotype E by virtual screening

Botulinum neurotoxins are one of the most poisonous biological substances known to humans and present a potential bioterrorism threat. There are no therapeutic interventions developed so far. Here, we report the first small molecule non-peptide inhibitor for botulinum neurotoxin serotype E discovered by structure-based virtual screening and propose a mechanism for its inhibitory activity.