A matrix-focused structure-activity and binding site flexibility study of quinolinol inhibitors of botulinum neurotoxin serotype A.

Our initial discovery of 8-hydroxyquinoline inhibitors of BoNT/A and separation/testing of enantiomers of one of the more active leads indicated considerable flexibility in the binding site. We designed a limited study to investigate this flexibility and probe structure-activity relationships; utilizing the Betti reaction, a 36 compound matrix of quinolinol BoNT/A LC inhibitors was developed using three 8-hydroxyquinolines, three heteroaromatic amines, and four substituted benzaldehydes. This study has revealed some of the most effective quinolinol-based BoNT/A inhibitors to date, with 7 compounds displaying IC50 values ⩽1µM and 11 effective at ⩽2µM in an ex vivo assay.

Comparative genomic analyses reveal broad diversity in botulinum-toxin-producing Clostridia.

BACKGROUND: Clostridium botulinum is a diverse group of bacteria characterized by the production of botulinum neurotoxin. Botulinum neurotoxins are classified into serotypes (BoNT/A-G), which are produced by six species/Groups of Clostridia, but the genetic background of the bacteria remains poorly understood. The purpose of this study was to use comparative genomics to provide insights into the genetic diversity and evolutionary history of bacteria that produce the potent botulinum neurotoxin. RESULTS: Comparative genomic analyses of over 170 Clostridia genomes, including our draft genome assemblies for 59 newly sequenced Clostridia strains from six continents and publicly available genomic data, provided in-depth insights into the diversity and distribution of BoNT-producing bacteria. These newly sequenced strains included Group I and II strains that express BoNT/A,/B,/E, or/F as well as bivalent strains. BoNT-producing Clostridia and closely related Clostridia species were delineated with a variety of methods including 16S rRNA gene, concatenated marker genes, core genome and concatenated multi-locus sequencing typing (MLST) gene phylogenies that related whole genome sequenced strains to publicly available strains and sequence types. These analyses illustrated the phylogenetic diversity in each Group and the diversity of genomic backgrounds that express the same toxin type or subtype. Comparisons of the botulinum neurotoxin genes did not identify novel toxin types or variants. CONCLUSIONS: This study represents one of the most comprehensive analyses of whole genome sequence data for Group I and II BoNT-producing strains. Read data and draft genome assemblies generated for 59 isolates will be a resource to the research community. Core genome phylogenies proved to be a powerful tool for differentiating BoNT-producing strains and can provide a framework for the study of these bacteria. Comparative genomic analyses of Clostridia species illustrate the diversity of botulinum-neurotoxin-producing strains and the plasticity of the genomic backgrounds in which bont genes are found.

The C terminus of the catalytic domain of type A botulinum neurotoxin may facilitate product release from the active site.

Botulinum neurotoxins are the most toxic of all compounds. The toxicity is related to a poor zinc endopeptidase activity located in a 50-kDa domain known as light chain (Lc) of the toxin. The C-terminal tail of Lc is not visible in any of the currently available x-ray structures, and it has no known function but undergoes autocatalytic truncations during purification and storage. By synthesizing C-terminal peptides of various lengths, in this study, we have shown that these peptides competitively inhibit the normal catalytic activity of Lc of serotype A (LcA) and have defined the length of the mature LcA to consist of the first 444 residues. Two catalytically inactive mutants also inhibited LcA activity. Our results suggested that the C terminus of LcA might interact at or near its own active site. By using synthetic C-terminal peptides from LcB, LcC1, LcD, LcE, and LcF and their respective substrate peptides, we have shown that the inhibition of activity is specific only for LcA. Although a potent inhibitor with a Ki of 4.5 µm, the largest of our LcA C-terminal peptides stimulated LcA activity when added at near-stoichiometric concentration to three versions of LcA differing in their C-terminal lengths. The result suggested a product removal role of the LcA C terminus. This suggestion is supported by a weak but specific interaction determined by isothermal titration calorimetry between an LcA C-terminal peptide and N-terminal product from a peptide substrate of LcA. Our results also underscore the importance of using a mature LcA as an inhibitor screening target.

Three enzymatically active neurotoxins of Clostridium botulinum strain Af84: BoNT/A2, /F4, and /F5.

Botulinum neurotoxins (BoNTs) are produced by various species of clostridia and are potent neurotoxins which cause the disease botulism, by cleaving proteins needed for successful nerve transmission. There are currently seven confirmed serotypes of BoNTs, labeled A-G, and toxin-producing clostridia typically only produce one serotype of BoNT. There are a few strains (bivalent strains) which are known to produce more than one serotype of BoNT, producing either both BoNT/A and /B, BoNT/A and /F, or BoNT/B and /F, designated as Ab, Ba, Af, or Bf. Recently, it was reported that Clostridium botulinum strain Af84 has three neurotoxin gene clusters: bont/A2, bont/F4, and bont/F5. This was the first report of a clostridial organism containing more than two neurotoxin gene clusters. Using a mass spectrometry based proteomics approach, we report here that all three neurotoxins, BoNT/A2, /F4, and /F5, are produced by C. botulinum Af84. Label free MS(E) quantification of the three toxins indicated that toxin composition is 88% BoNT/A2, 1% BoNT/F4, and 11% BoNT/F5. The enzymatic activity of all three neurotoxins was assessed by examining the enzymatic activity of the neurotoxins upon peptide substrates, which mimic the toxins' natural targets, and monitoring cleavage of the substrates by mass spectrometry. We determined that all three neurotoxins are enzymatically active. This is the first report of three enzymatically active neurotoxins produced in a single strain of Clostridium botulinum.

EL4 cell-based colorimetric toxin neutralization activity assays for determination of neutralizing anti-ricin antibodies.

A recombinant ricin toxin A-chain 1-33/44-198 vaccine (RVEc), developed at the United States Army Medical Research Institute of Infectious Diseases as a vaccine candidate, is under investigation in a phase 1 clinical study. To effectively evaluate the immunogenicity of this ricin vaccine and to eliminate the use of radioactive material, an EL4 cell-based colorimetric toxin neutralization activity (TNA) assay using a CellTiter 96 AQueous One Solution Cell Proliferation Assay Reagent has been developed, optimized, and applied in the vaccine efficacy studies. The TNA assay measures the protective neutralizing anti-ricin antibodies in animal sera by determining the cell viability after ricin exposure in the assay system and comparing it to a purified mouse polyclonal antiricin IgG standard curve. The standard curve of the anti-ricin TNA assay closely fits a four-parameter logistic regression model. The unknown test sample concentration was expressed as microg/mL, but not the 50% effective concentration (EC50), which was determined by most TNA assays. The neutralizing endpoint titers, not the 50% effective dilution (ED50), of human specimens were measured with the TNA assay in support of the clinical study of the RVEc vaccine. The optimal amount of ricin toxin, EL4 cells, and concentration of standards used in the assay system was established to minimize false-negative and false-positive results of serum specimens from the nonclinical and clinical studies of RVEc. The testing conditions were adjusted to optimize assay performance. The colorimetric TNA assay replaced a radioactive TNA assay previously used in the ricin vaccine studies.

De novo subtype and strain identification of botulinum neurotoxin type B through toxin proteomics.

Botulinum neurotoxins (BoNTs) cause the disease botulism, which can be lethal if untreated. There are seven known serotypes of BoNT, A-G, defined by their response to antisera. Many serotypes are distinguished into differing subtypes based on amino acid sequence, and many subtypes are further differentiated into toxin variants. Previous work in our laboratory described the use of a proteomics approach to distinguish subtype BoNT/A1 from BoNT/A2 where BoNT identities were confirmed after searching data against a database containing protein sequences of all known BoNT/A subtypes. We now describe here a similar approach to differentiate subtypes BoNT/B1, /B2, /B3, /B4, and /B5. Additionally, to identify new subtypes or hitherto unpublished amino acid substitutions, we created an amino acid substitution database covering every possible amino acid change. We used this database to differentiate multiple toxin variants within subtypes of BoNT/B1 and B2. More importantly, with our amino acid substitution database, we were able to identify a novel BoNT/B subtype, designated here as BoNT/B7. These techniques allow for subtype and strain level identification of both known and unknown BoNT/B rapidly with no DNA required.

Different substrate recognition requirements for cleavage of synaptobrevin-2 by Clostridium baratii and Clostridium botulinum type F neurotoxins.

Botulinum neurotoxins (BoNTs) cause botulism, which can be fatal if it is untreated. BoNTs cleave proteins necessary for nerve transmission, resulting in paralysis. The in vivo protein target has been reported for all seven serotypes of BoNT, i.e., serotypes A to G. Knowledge of the cleavage sites has led to the development of several assays to detect BoNT based on its ability to cleave a peptide substrate derived from its in vivo protein target. Most serotypes of BoNT can be subdivided into subtypes, and previously, we demonstrated that three of the currently known subtypes of BoNT/F cleave a peptide substrate, a shortened version of synaptobrevin-2, between Q58 and K59. However, our research indicated that Clostridium baratii type F toxin did not cleave this peptide. In this study, we detail experiments demonstrating that Clostridium baratii type F toxin cleaves recombinant synaptobrevin-2 in the same location as that cleaved by proteolytic F toxin. In addition, we demonstrate that Clostridium baratii type F toxin can cleave a peptide substrate based on the sequence of synaptobrevin-2. This peptide substrate is an N-terminal extension of the original peptide substrate used for detection of other BoNT/F toxins and can be used to detect four of the currently known BoNT/F subtypes by mass spectrometry.

Analysis of Clostridium botulinum serotype E strains by using multilocus sequence typing, amplified fragment length polymorphism, variable-number tandem-repeat analysis, and botulinum neurotoxin gene sequencing.

A total of 41 Clostridium botulinum serotype E strains from different geographic regions, including Canada, Denmark, Finland, France, Greenland, Japan, and the United States, were compared by multilocus sequence typing (MLST), amplified fragment length polymorphism (AFLP) analysis, variable-number tandem-repeat (VNTR) analysis, and botulinum neurotoxin (bont) E gene sequencing. The strains, representing environmental, food-borne, and infant botulism samples collected from 1932 to 2007, were analyzed to compare serotype E strains from different geographic regions and types of botulism and to determine whether each of the strains contained the transposon-associated recombinase rarA, involved with bont/E insertion. MLST examination using 15 genes clustered the strains into several clades, with most members within a cluster sharing the same BoNT/E subtype (BoNT/E1, E2, E3, or E6). Sequencing of the bont/E gene identified two new variants (E7, E8) that showed regions of recombination with other E subtypes. The AFLP dendrogram clustered the 41 strains similarly to the MLST dendrogram. Strains that could not be differentiated by AFLP, MLST, or bont gene sequencing were further examined using three VNTR regions. Both intact and split rarA genes were amplified by PCR in each of the strains, and their identities were confirmed in 11 strains by amplicon sequencing. The findings suggest that (i) the C. botulinum serotype E strains result from the targeted insertion of the bont/E gene into genetically conserved bacteria and (ii) recombination events (not random mutations) within bont/E result in toxin variants or subtypes within strains.

Extraction and inhibition of enzymatic activity of botulinum neurotoxins /B1, /B2, /B3, /B4, and /B5 by a panel of monoclonal anti-BoNT/B antibodies.

BACKGROUND: Botulism is caused by botulinum neurotoxins (BoNTs), extremely toxic proteins which can induce respiratory failure leading to long-term intensive care or death. Treatment for botulism includes administration of antitoxins, which must be administered early in the course of the intoxication; therefore, rapid determination of human exposure to BoNT is an important public health goal. In previous work, our laboratory reported on Endopep-MS, a mass spectrometry-based activity method for detecting and differentiating BoNT/A, /B, /E, and /F in clinical samples. We also demonstrated that antibody-capture is effective for purification and concentration of BoNTs from complex matrices such as clinical samples. However, some antibodies inhibit or neutralize the enzymatic activity of BoNT, so the choice of antibody for toxin extraction is critical. RESULTS: In this work, we evaluated 24 anti-BoNT/B monoclonal antibodies (mAbs) for their ability to inhibit the in vitro activity of BoNT/B1, /B2, /B3, /B4, and /B5 and to extract those toxins. Among the mAbs, there were significant differences in ability to extract BoNT/B subtypes and inhibitory effect on BoNT catalytic activity. Some of the mAbs tested enhanced the in vitro light chain activity of BoNT/B, suggesting that BoNT/B may undergo conformational change upon binding some mAbs. CONCLUSIONS: In addition to determining in vitro inhibition abilities of a panel of mAbs against BoNT/B1-/B5, this work has determined B12.2 and 2B18.2 to be the best mAbs for sample preparation before Endopep-MS. These mAb characterizations also have the potential to assist with mechanistic studies of BoNT/B protection and treatment, which is important for studying alternative therapeutics for botulism.

Purification of a recombinant heavy chain fragment C vaccine candidate against botulinum serotype C neurotoxin [rBoNTC(H(c))] expressed in Pichia pastoris.

A purification process for the manufacture of a recombinant C-terminus heavy chain fragment from botulinum neurotoxin serotype C [rBoNTC(H(c))], a potential vaccine candidate, has been defined and successfully scaled-up. The rBoNTC(H(c)) was produced intracellularly in Pichia pastoris X-33 using a three step fermentation process, i.e., glycerol batch phase, a glycerol fed-batch phase to achieve high cell densities, followed by a methanol induction phase. The rBoNTC(H(c)) was captured from the soluble protein fraction of cell lysate using hydrophobic charge induction chromatography (HCIC; MEP HyperCel™), and then further purified using a CM 650M ion exchange chromatography step followed by a polishing step using HCIC once again. Method development at the bench scale was achieved using 5-100mL columns and the process was performed at the pilot scale using 0.6-1.6L columns in preparation for technology transfer to cGMP manufacturing. The process yielded approximately 2.5 g of rBoNTC(H(c))/kg wet cell weight (WCW) at the bench scale and 1.6 g rBoNTC(H(c))/kg WCW at the pilot scale. The purified rBoNTC(H(c)) was stable for at least 3 months at 5 and -80°C as determined by reverse phase-HPLC and SDS-PAGE and was stable for 24 months at -80 °C based on mouse potency bioassay. N-Terminal amino acid sequencing confirmed that the N-terminus of the purified rBoNTC(H(c)) was intact.

Mode of action of botulinum neurotoxins: current vaccination strategies and molecular immune recognition.

The action of a botulinum neurotoxin (BoNT) commences by binding at the nerve terminal via its H- (heavy) chain to a cell-surface receptor, which consists of a ganglioside and a cell-surface protein. Binding enables the L-chain, a Zn2+-dependent endopeptidase, to be internalized and act intracellularly, cleaving one or more SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins required for vesicle docking and fusion, which results in reduced neurotransmitter release. Sprouts emerge at motor-nerve terminals that reestablish synaptic contact and lead to restoration of exocytosis. As the terminals recover, sprouts retreat and synaptic function is fully re-established. Neutralizing antibodies (Abs) induced by vaccination can prevent the neuronal changes produced by BoNT. Until recently, vaccines against BoNT have been based on toxins inactivated by treatment with formaldehyde (toxoids) and contain either one (monovalent) or five (pentavalent) toxoids, but formalin-based toxoids have many undesirable side effects. Availability of the gene sequences of BoNT serotypes enabled design of recombinant subunit vaccines that have included the C-terminal domain of the H chain (HC, its subdomains (HC-N and HC-C), the L- (catalytic) chain, and the L-chain expressed with the translocation domain (LCHN). Of these, the HC displays the highest protective ability. Recent vaccines have used whole toxins inactivated by three key mutations at the enzyme active site, which have been found to be very effective in mice against the correlated toxin. Immune responses to BoNTs A and B epitopes are under the hosts MHC (major histocompatibility complex) control. Anti-BoNT/A blocking Abs bind at sites that coincide or overlap with those that bind synaptosomes and to BoNT/B at sites that overlap with synaptotagmin-II and ganglioside-binding sites. Therefore, locations occupied by blocking Abs preclude the respective toxin from binding to its receptor and thus from binding to cell surface. Information on BoNT epitopes for blocking Abs, sites for binding to cell surface receptors, and T-cell epitopes that provide help to B cells making blocking Abs afford a prospect for rational design of stable synthetic vaccines. These constructs should be clinically useful for epitope-selective modulation of Ab responses to restore effective BoNT treatment in immunoresistant patients.

Progress in biological threat agent vaccine development: a repeat-dose toxicity study of a recombinant ricin toxin A-chain (rRTA) 1-33/44-198 vaccine (RVEc) in male and female New Zealand white rabbits.

A recombinant ricin toxin A-chain 1-33/44-198 vaccine (RVEc) was administered to male and female New Zealand white (NZW) rabbits (10/sex/group) in a repeat-dose toxicity study. The RVEc vaccine was administered on study days 1, 29, 57, and 85 via intramuscular (IM) injection (0, 100, or 200 µg/dose). All study animals were observed throughout treatment until euthanized and submitted for necropsy on study day 88 or 99 (recovery period). There were no treatment-related or toxicologically significant effects observed. There were no statistically significant differences noted in the antibody titers and/or concentrations in 100 µg RVEc-treated animals when compared to 200 µg RVEc-treated animals, suggesting that both doses produced comparable antibody titers/concentrations during the study. The highest immune response was observed on study day 99 (ie, 2 weeks after the last dose). The immune response observed demonstrated that RVEc is biologically active in the rabbit model, with no apparent marked sex differences.

Development of an o-pthalaldehyde (OPA) assay to measure protein content in Ricin Vaccine E. coli (RVEc™).

A recombinant ricin vaccine from E. coli (RVEc™), was developed at the US Army Medical Research Institute of Infectious Diseases (USAMRIID) and assessed in an FDA sponsored Phase 1a clinical trial. At the maximum dosage, two of the study participants developed physiological responses that were elevated to the level of severe adverse reactions. To stay within safe dosing guidelines, the FDA recommended that an assay be developed to accurately quantify the recombinant protein content in the vaccine. The RVEc™ vaccine Final Drug Product (FDP) contains the adjuvant Alhydrogel®, which by its colloidal nature interferes with most conventional protein assay methods. We decided to develop an assay measuring RVEc™ FDP using o-pthalaldehyde (OPA) reagent. The OPA reagent reacts to the primary amines and lysine side chains of proteins in the presence of a thiol under alkaline conditions with a quantifiable fluorescent signature, but does not react with Alhydrogel®. Protein content in the RVEc™ FDP can be determined by comparing the fluorescence of the test sample to the fluorescence of a standard curve of defined concentration. Each phase of the assay was tested to optimize and simplify the assay procedure. The accuracy, specificity, reproducibility, and stability of the assay were evaluated. Results indicated that the optimized and modified OPA assay was simple and able to quantify antigen concentration from a standard curve in the 25 µg/mL-600 µg/mL range. The assay accuracy and coefficient of variation (CV) was 95% and less than 8%, respectively, when determining the ricin protein content in the 200 µg/mL vialed RVEc™ FDP. The assay was simple to perform and used conventional laboratory equipment. This assay could be adapted to measure the protein content in the FDP of other vaccines, but with the proviso that each step of the assay would need to be optimized for each antigen.

A Four-Monoclonal Antibody Combination Potently Neutralizes Multiple Botulinum Neurotoxin Serotypes C and D.

Human botulism can be caused by botulinum neurotoxin (BoNT) serotypes A to G. Here, we present an antibody-based antitoxin composed of four human monoclonal antibodies (mAbs) against BoNT/C, BoNT/D, and their mosaic toxins. This work built on our success in generating protective mAbs to BoNT /A, B and E serotypes. We generated mAbs from human immune single-chain Fv (scFv) yeast-display libraries and isolated scFvs with high affinity for BoNT/C, BoNT/CD, BoNT/DC and BoNT/D serotypes. We identified four mAbs that bound non-overlapping epitopes on multiple serotypes and mosaic BoNTs. Three of the mAbs underwent molecular evolution to increase affinity. A four-mAb combination provided high-affinity binding and BoNT neutralization of both serotypes and their mosaic toxins. The mAbs have potential utility as therapeutics and as diagnostics capable of recognizing and neutralizing BoNT/C and BoNT/D serotypes and their mosaic toxins. A derivative of the four-antibody combination (NTM-1634) completed a Phase 1 clinical trial (Snow et al., Antimicrobial Agents and Chemotherapy, 2019) with no drug-related serious adverse events.

Rapid product analysis and increased sensitivity for quantitative determinations of botulinum neurotoxin proteolytic activity.

The ultimate molecular action of botulinum neurotoxin (BoNT) is a Zn-dependent endoproteolytic activity on one of the three SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. There are seven serotypes (A-G) of BoNT having distinct cleavage sites on the SNARE substrates. The proteolytic activity is located on the N-terminal light chain (Lc) domain and is used extensively as the primary target toward therapeutic development against botulism. Here we describe an improved method using ultra-performance liquid chromatography (UPLC) whereby quantitative data were obtained in 1/10th the time using 1/20th the sample and solvent volumes compared with a widely used high-performance liquid chromatography (HPLC) method. We also synthesized a VAMP (vesicle-associated membrane protein)-based peptide containing an intact V1 motif that was efficiently used as a substrate by BoNT/D Lc. Although serotype C1 cleaves the serotype A substrate at a bond separated by only one residue, we were able to distinguish the two reactions by UPLC. The new method can accurately quantify as low as 7 pmol of the peptide substrates for BoNT serotypes A, B, C1, and D. We also report here that the catalytic efficiency of serotype A can be stimulated 35-fold by the addition of Triton X-100 to the reaction mixture. Combining the use of Triton X-100 with the newly introduced UPLC method, we were able to accurately detect very low levels of proteolytic activity in a very short time. Sensitivity of the assay and accuracy and rapidity of product analysis should greatly augment efforts in therapeutic development.

Recombination and insertion events involving the botulinum neurotoxin complex genes in Clostridium botulinum types A, B, E and F and Clostridium butyricum type E strains.

BACKGROUND: Clostridium botulinum is a taxonomic designation for at least four diverse species that are defined by the expression of one (monovalent) or two (bivalent) of seven different C. botulinum neurotoxins (BoNTs, A-G). The four species have been classified as C. botulinum Groups I-IV. The presence of bont genes in strains representing the different Groups is probably the result of horizontal transfer of the toxin operons between the species. RESULTS: Chromosome and plasmid sequences of several C. botulinum strains representing A, B, E and F serotypes and a C. butyricum type E strain were compared to examine their genomic organization, or synteny, and the location of the botulinum toxin complex genes. These comparisons identified synteny among proteolytic (Group I) strains or nonproteolytic (Group II) strains but not between the two Groups. The bont complex genes within the strains examined were not randomly located but found within three regions of the chromosome or in two specific sites within plasmids. A comparison of sequences from a Bf strain revealed homology to the plasmid pCLJ with similar locations for the bont/bv b genes but with the bont/a4 gene replaced by the bont/f gene. An analysis of the toxin cluster genes showed that many recombination events have occurred, including several events within the ntnh gene. One such recombination event resulted in the integration of the bont/a1 gene into the serotype toxin B ha cluster, resulting in a successful lineage commonly associated with food borne botulism outbreaks. In C. botulinum type E and C. butyricum type E strains the location of the bont/e gene cluster appears to be the result of insertion events that split a rarA, recombination-associated gene, independently at the same location in both species. CONCLUSION: The analysis of the genomic sequences representing different strains reveals the presence of insertion sequence (IS) elements and other transposon-associated proteins such as recombinases that could facilitate the horizontal transfer of the bonts; these events, in addition to recombination among the toxin complex genes, have led to the lineages observed today within the neurotoxin-producing clostridia.

Potency and stability of a trivalent, catalytically inactive vaccine against botulinum neurotoxin serotypes C, E and F (triCEF).

Botulism is an acute neuroparalytic affliction of the motor and autonomic neurons caused by the toxins produced from Clostridium botulinum and related bacterial strains. The botulinum neurotoxins, or BoNTs, consist of a phylogenetically diverse group of highly potent protein toxins. Current medical interventions for confirmed cases of botulism are limited to immediate administration of antitoxins and respiratory support. There is currently no licensed vaccine against botulism in the United States. The most widely distributed botulism vaccine was a pentavalent BoNT toxoid (PBT) against serotypes A-E administered until 2011 under an investigational new drug license. A binary vaccine composed of the recombinant, non-toxic, receptor binding domains (RBD) of serotypes/A1 and/B1 has completed a phase II clinical trial, but has yet to attain full licensure. We have previously published data demonstrating catalytically inactive, full length botulinum neurotoxin holoproteins (ciBoNT HPs) against serotypes/A1,/B1,/C1,/E1 and/F1 provide equivalent or superior potency against parental and dissimilar subtype toxins as compared the RBD vaccines. Here we describe the consistent potencies of the three independent lots each of ciBoNT/C1,/E1, and/F1 HPs against substantial monovalent challenges of the parental toxins. We also present data that a trivalent formulation of ciBoNT/C1,/E1 and/F1 (triCEF) maintains potency against both monovalent and polyvalent toxin challenges when stored as an adjuvanted vaccine at 4-8 °C for up to 2 years.

Identification and biochemical characterization of small-molecule inhibitors of Clostridium botulinum neurotoxin serotype A.

An integrated strategy that combined in silico screening and tiered biochemical assays (enzymatic, in vitro, and ex vivo) was used to identify and characterize effective small-molecule inhibitors of Clostridium botulinum neurotoxin serotype A (BoNT/A). Virtual screening was initially performed by computationally docking compounds of the National Cancer Institute (NCI) database into the active site of BoNT/A light chain (LC). A total of 100 high-scoring compounds were evaluated in a high-performance liquid chromatography (HPLC)-based protease assay using recombinant full-length BoNT/A LC. Seven compounds that significantly inhibited the BoNT/A protease activity were selected. Database search queries of the best candidate hit [7-((4-nitro-anilino)(phenyl)methyl)-8-quinolinol (NSC 1010)] were performed to mine its nontoxic analogs. Fifty-five analogs of NSC 1010 were synthesized and examined by the HPLC-based assay. Of these, five quinolinol derivatives that potently inhibited both full-length BoNT/A LC and truncated BoNT/A LC (residues 1 to 425) were selected for further inhibition studies in neuroblastoma (N2a) cell-based and tissue-based mouse phrenic nerve hemidiaphragm assays. Consistent with enzymatic assays, in vitro and ex vivo studies revealed that these five quinolinol-based analogs effectively neutralized BoNT/A toxicity, with CB 7969312 exhibiting ex vivo protection at 0.5 microM. To date, this is the most potent BoNT/A small-molecule inhibitor that showed activity in an ex vivo assay. The reduced toxicity and high potency demonstrated by these five compounds at the biochemical, cellular, and tissue levels are distinctive among the BoNT/A small-molecule inhibitors reported thus far. This study demonstrates the utility of a multidisciplinary approach (in silico screening coupled with biochemical testing) for identifying promising small-molecule BoNT/A inhibitors.

Quinolinol and peptide inhibitors of zinc protease in botulinum neurotoxin A: effects of zinc ion and peptides on inhibition.

Quinolinol derivatives were found to be effective inhibitors of botulinum neurotoxin serotype A (BoNT/A). Studies of the inhibition and binding of 7-(phenyl(8-quinolinylamino)methyl)-8-quinolinol (QAQ) to the light chain domain (BoNT/A LC) showed that QAQ is a non-competitive inhibitor for the zinc protease activity. Binding and molecular modeling studies reveal that QAQ binds to a hydrophobic pocket near the active site. Its inhibitor effect does not involve the removal of zinc ion from the light chain. A 24-mer SNAP-25 peptide containing E183 to G206 with Q197C mutation (Peptide C) binds to BoNT/A LC with an unusually slow second order binding rate constant of 76.7M(-1)s(-1). QAQ binds to Zn(2+)-free BoNT/A LC with a K(D) of 0.67microM and to Peptide C-BoNT/A LC complex with a K(D) of 2.33microM. The insights of the interactions of quinolinols and peptides with the zinc protease of BoNT/A should aid in the development of inhibitors of metalloproteases.