Synthesis/biological evaluation of hydroxamic acids and their prodrugs as inhibitors for Botulinum neurotoxin A light chain

Synthesis and activity evaluation of selenazole-coupled CPI-1 irreversible bifunctional inhibitors for botulinum toxin A light chain

A series of novel conjugates of benzoselenazole or selenazole and CPI-1 were designed, synthesized, and evaluated for inhibitory activities against the botulinum neurotoxin A (BoNT/A) light chain (LC) and BoNT/A in vivo. The results show that these compounds exhibit potent inhibitory activities to the LC with IC₅₀ of 0.5-4.1 µM. The reaction kinetics and the mass spectra of the reaction products of LC with benzoselenazole- or selenazole- coupled CPI-1 demonstrate that the benzoselenazole group of most inhibitors is coupled to the LC of BoNT/A. These data indicate that the CPI-1 conjugates can inhibit both the active center of BoNT/A LC as well as Cys165, therefore functioning as irreversible bifunctional inhibitors. The detoxification activities in vivo show that one of the benzoselenazole-CPI-1 compounds prolongs the survival time of mice challenged by 2 × LD₅₀ of BoNT/A. This work provides a new strategy to design potent antidotes of BoNT/A.

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

Botulinum neurotoxin A (BoNT/A) is a lethal toxin, which causes botulism, and is categorized as a bioterrorism threat, which causes flaccid paralysis and death. Botulinum A neurotoxicity is governed through its light chain (LC), a zinc metalloprotease. Pharmacological investigations aimed at negating BoNT/A's LC have typically looked to inhibitors that have been shown to inhibit the light chain's activity by reversible zinc chelation within its active site. This report outlines the first examples of nonpeptidic inhibitors of the BoNT/A LC that possess slow-binding kinetics, a needed logic to counteract the longevity of BoNT/A. Cyclopropane, alkyl, and alkenyl derivatives of 2,4-dichlorocinamic hydroxamic acid (DCHA) were shown to possess both one-step and two-step slow-binding kinetics. Structure-kinetic relationships (SKRs) were observed and were rationalized with the aid of docking models that predicted improved interactions with residues within a hydrophobic cleft adjacent to the active site.

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.

Synthesis and activity of isoleucine sulfonamide derivatives as novel botulinum neurotoxin serotype A light chain inhibitors

The botulinum neurotoxin (BoNT) is the most lethal protein known to man causing the deadly disease botulinum. The neurotoxin, composed of a heavy (HC) and light (LC) chain, work in concert to cause muscle paralysis. A therapeutic strategy to treat individuals infected with the neurotoxin is inhibiting the catalytic activity of the BoNT LC. We report the synthesis, inhibition study and computational docking analysis of novel small molecule BoNT/A LC inhibitors. A structure activity relationship study resulted in the discovery of d-isoleucine functionalized with a hydroxamic acid on the C-terminal and a biphenyl with chlorine at C- 2 connected by a sulfonamide linker at the N-terminus. This compound has a measured IC₅₀ of 0.587 µM for the BoNT/A LC. Computational docking analysis indicates the sulfonamide linker adopts a geometry that is advantageous for binding to the BoNT LC active site. In addition, Arg363 is predicted to be involved in key binding interactions with the scaffold in this study.

Design, synthesis, and biological evaluation of dual targeting inhibitors of histone deacetylase 6/8 and bromodomain BRPF1

Histone modifying proteins, specifically histone deacetylases (HDACs) and bromodomains, have emerged as novel promising targets for anticancer therapy. In the current work, based on available crystal structures and docking studies, we designed dual inhibitors of both HDAC6/8 and the bromodomain and PHD finger containing protein 1 (BRPF1). Biochemical and biophysical tests showed that compounds 23a,b and 37 are nanomolar inhibitors of both target proteins. Detailed structure-activity relationships were deduced for the synthesized inhibitors which were supported by extensive docking and molecular dynamics studies. Cellular testing in acute myeloid leukemia (AML) cells showed only a weak effect, most probably because of the poor permeability of the inhibitors. We also aimed to analyse the target engagement and the cellular activity of the novel inhibitors by determining the protein acetylation levels in cells by western blotting (tubulin vs histone acetylation), and by assessing their effects on various cancer cell lines.

Double prodrugs of a fosmidomycin surrogate as antimalarial and antitubercular agents

A series of eleven double prodrug derivatives of a fosmidomycin surrogate were synthesized and investigated for their ability to inhibit in vitro growth of P. falciparum and M. tuberculosis. A pivaloyloxymethyl (POM) phosphonate prodrug modification was combined with various prodrug derivatisations of the hydroxamate moiety. The majority of compounds showed activity comparable with or inferior to fosmidomycin against P. falciparum. N-benzyl substituted carbamate prodrug 6f was the most active antimalarial analog with an IC₅₀ value of 0.64 µM. Contrary to fosmidomycin and parent POM-prodrug 5, 2-nitrofuran and 2-nitrothiophene prodrugs 6i and 6j displayed promising antitubercular activities.

A Method for Bischler-Napieralski-Type Synthesis of 3,4-Dihydroisoquinolines

A new method for the Bischler-Napieralski-type synthesis of 3,4-dihydroisoquinolines was developed by a Tf₂O-promoted tandem annulation from phenylethanols and nitriles. Its success was mainly due to the fact that a phenonium ion was formed in the process and practically functioned as a stable and reactive primary phenylethyl carbocation.

Carbamates as Potential Prodrugs and a New Warhead for HDAC Inhibition

We designed and synthesized carbamates of the clinically-approved HDAC (histone deacetylase) inhibitor vorinostat (suberoylanilide hydroxamic acid, SAHA) in order to validate our previously-proposed hypothesis that these carbamates might serve as prodrugs for hydroxamic acid containing HDAC inhibitors. Biochemical assays proved our new compounds to be potent inhibitors of histone deacetylases in vitro, and they also showed antiproliferative effects in leukemic cells. These results, as well as stability analysis led to the suggestion that the intact carbamates are inhibitors of histone deacetylases themselves, representing a new zinc-binding warhead in HDAC inhibitor design. This suggestion was further supported by the synthesis and evaluation of a carbamate derivative of the HDAC6-selective inhibitor bufexamac.

Utilizing Ayurvedic literature for the identification of novel phytochemical inhibitors of botulinum neurotoxin A

ETHNOPHARMACOLOGICAL RELEVANCE

Ayurveda, an ancient holistic system of health care practiced on the Indian subcontinent, utilizes a number of multi-plant formulations and is considered by many as a potential source for novel treatments, as well as the identification of new drugs. Our aim is to identify novel phytochemicals for the inhibition of bacterial exotoxin, botulinum neurotoxin A (BoNT/A) based on Ayurvedic literature. BoNT/A is released by Clostridium species, which when ingested, inhibits the release of acetylcholine by concentrating at the neuromuscular junction and causes flaccid paralysis, resulting in a condition termed as botulism, and may also lead to death due to respiratory arrest.

METHODS

Fifteen plants were selected from the book 'Diagnosis and treatment of diseases in Ayurveda' by Vaidya Bhagwan Dash and Lalitesh Kashyap, based on their frequency of use in the formulations used for the treatment of six diseases with neuromuscular symptoms similar to botulism. Phytochemicals from these plants were screened using in silico, and in vitro methods. Structures of 570 reported phytochemicals from 14 plants were docked inside six reported BoNT/A light chain crystal structures using ensemble docking module in Maestro (Schrödinger, LLE).

RESULTS

From the docking scores and structural diversity, nine compounds including acoric acid 1, three flavonoids, three coumarins derivatives, one kava lactone were selected and screened using an in vitro HPLC-based protease assay. The bioassay results showed that several compounds possess BoNT/A LC inhibition of 50-60% when compared to positive controls NSC 84094 and CB7967495 (80-95%).

CONCLUSION

Further testing of the active compounds identified from Ayurvedic literature and structure-activity studies of acoric acid 1 using more sensitive bioassays is under way. The identification of acoric acid 1, a novel scaffold against BoNT/A, exemplifies the utility of Ayurvedic literature for the discovery of novel drug leads.

Cellular Protection of SNAP-25 against Botulinum Neurotoxin/A: Inhibition of Thioredoxin Reductase through a Suicide Substrate Mechanism

Botulium neurotoxins (BoNTs) are among the most lethal toxins known to man. They are comprised of seven serotypes with BoNT/A being the most deadly; yet, there is no approved therapeutic for their intoxication or one that has even advanced to clinical trials. Botulinum neurotoxicity is ultimately governed through light chain (LC) protease SNARE protein cleavage leading to a loss of neurotransmitter release. Pharmacological attempts to ablate BoNT/A intoxication have sought to either nullify cellular toxin entry or critical biochemical junctions found within its intricate mechanism of action. In these regards, reports have surfaced of nonpeptidic small molecule inhibitors, but few have demonstrated efficacy in neutralizing cellular toxicity, a key prerequisite before rodent lethality studies can be initiated. On the basis of a lead discovered in our BoNT/A cellular assay campaign, we investigated a family of N-hydroxysuccinimide inhibitors grounded upon structure activity relationship (SAR) fundamentals. Molecules stemming from this SAR exercise were theorized to be protease inhibitors. However, this proposition was overturned on the basis of extensive kinetic analysis. Unexpectedly, inhibitor data pointed to thioredoxin reductase (TrxR), an essential component required for BoNT protease translocation. Also unforeseen was the inhibitors' mechanism of action against TrxR, which was found to be brokered through a suicide-mechanism utilizing quinone methide as the inactivating element. This new series of TrxR inhibitors provides an alternative means to negate the etiological agent responsible for BoNT intoxication, the LC protease.

Toward the discovery of dual inhibitors for botulinum neurotoxin A: concomitant targeting of endocytosis and light chain protease activity

Dyngo-4a™ has been found to be an endocytic inhibitor of BoNT/A neurotoxicity through dynamin inhibition. Herein, we demonstrate this molecule to have a previously unrecognized dual activity against BoNT/A, dynamin-protease inhibition. To establish the importance of this dual activity, detailed kinetic analysis of Dyngo-4a's inhibition of BoNT/A metalloprotease as well as cellular and animal toxicity studies have been described. The research presented is the first polypharmacological approach to counteract BoNT/A intoxication.

Probing BoNT/A protease exosites: implications for inhibitor design and light chain longevity

Botulinum neurotoxin serotype A (BoNT/A) is one of the most lethal toxins known. Its extreme toxicity is due to its light chain (LC), a zinc protease that cleaves SNAP-25, a synaptosome-associated protein, leading to the inhibition of neuronal activity. Studies on BoNT/A LC have revealed that two regions, termed exosites, can play an important role in BoNT catalytic activity. A clear understanding of how these exosites influence neurotoxin catalytic activity would provide a critical framework for deciphering the mechanism of SNAP-25 cleavage and the design of inhibitors. Herein, based on the crystallographic structure of BoNT/A LC complexed with its substrate, we designed an α-exosite binding probe. Experiments with this unique probe demonstrated that α-exosite binding enhanced both catalytic activity and stability of the LC. These data help delineate why α-exosite binding is needed for SNAP-25 cleavage and also provide new insights into the extended lifetime observed for BoNT/A LC in vivo.

A platform stratifying a sequestering agent and a pharmacological antagonist as a means to negate botulinum neurotoxicity

Botulinum neurotoxicity is characterized by peripheral neuromuscular blockade/flaccid paralysis that can lead to respiratory failure and ultimately death. Current therapeutic options provide relief in a pre-exposure scenario, but there are no clinically approved postexposure medical countermeasures. Here, we introduce a platform that utilizes a combination of a toxin sequestering agent and a pharmacological antagonist to ablate botulinum neurotoxicity in a well-defined mouse lethality assay. The platform was constructed to allow for ready exchange of sequestering agent and/or pharmacological antagonist for therapeutic optimization. As such, we attempted to improve upon the pharmacological antagonist, a potassium channel blocker, 3,4-diaminopyridine, through a prodrug approach; thus, a complete kinetic decomposition pathway is described. These experiments provide the first proof-of-principle that a synergistic combination strategy can be used to reduce toxin burden in the peripheral using a sequestering antibody, while restoring muscle action via a pharmacological small molecule antagonist.

Discovery and SAR study of a sulfonamide hydroxamic acid inhibitor for the botulinum neurotoxin serotype A light chain

Through the use of chemical synthesis and high throughput screening, we discovered a sulfonamide hydroxamic acid inhibitor for the botulinum neurotoxin serotype A light chain. A structure activity relationship study of the parent inhibitor resulted in the synthesis of a new inhibitor with an IC₅₀of 0.95 ± 0.60 μM for the BoNT/A LC.