Strategies to design inhibitors of Clostridium botulinum neurotoxins

First-in-Human Clinical Trial to Assess the Safety, Tolerability and Pharmacokinetics of Single Doses of NTM-1633, a Novel Mixture of Monoclonal Antibodies against Botulinum Toxin E

Botulism is a rare, life-threatening paralytic disease caused by botulinum neurotoxin (BoNT). Available treatments including an equine antitoxin and human immune globulin are given postexposure and challenging to produce and administer. NTM-1633 is an equimolar mixture of 3 human IgG monoclonal antibodies, E1, E2, and E3, targeting BoNT serotype E (BoNT/E). This first-in-human study assessed the safety, tolerability, pharmacokinetics (PK), and immunogenicity of NTM-1633. This double-blind, single-center, placebo-controlled dose escalation study randomized 3 cohorts of healthy volunteers to receive a single intravenous dose of NTM-1633 (0.033, 0.165, or 0.330 mg/kg) or saline placebo. Safety monitoring included physical examinations, clinical laboratory studies, and vital signs. Blood sampling was performed at prespecified time points for PK and immunogenicity analyses. Twenty-four subjects received study product (18 NTM-1633; 6 placebo), and no deaths were reported. An unrelated serious adverse event was reported in a placebo subject. Adverse events in the NTM-1633 groups were generally mild and similar in frequency and severity to the placebo group, and no safety signal was identified. NTM-1633 has a favorable PK profile with a half-life >10 days for the 0.330 mg/kg dose and an approximately linear relationship with respect to maximum concentration and area under the concentration-time curve (AUC_0→t). NTM-1633 also demonstrated low immunogenicity. NTM-1633 is well tolerated at the administered doses. The favorable safety, PK, and immunogenicity profile supports further development as a treatment for BoNT/E intoxication and postexposure prophylaxis.

Clostridial Neurotoxins: Structure, Function and Implications to Other Bacterial Toxins

Gram-positive bacteria are ancient organisms. Many bacteria, including Gram-positive bacteria, produce toxins to manipulate the host, leading to various diseases. While the targets of Gram-positive bacterial toxins are diverse, many of those toxins use a similar mechanism to invade host cells and exert their functions. Clostridial neurotoxins produced by Clostridial tetani and Clostridial botulinum provide a classical example to illustrate the structure-function relationship of bacterial toxins. Here, we critically review the recent progress of the structure-function relationship of clostridial neurotoxins, including the diversity of the clostridial neurotoxins, the mode of actions, and the flexible structures required for the activation of toxins. The mechanism clostridial neurotoxins use for triggering their activity is shared with many other Gram-positive bacterial toxins, especially molten globule-type structures. This review also summarizes the implications of the molten globule-type flexible structures to other Gram-positive bacterial toxins. Understanding these highly dynamic flexible structures in solution and their role in the function of bacterial toxins not only fills in the missing link of the high-resolution structures from X-ray crystallography but also provides vital information for better designing antidotes against those toxins.

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

[Aptamers in the Treatment of Bacterial Infections: Problems and Prospects]

Aptamers are short single-stranded oligonucleotides which are selected via targeted chemical evolution in vitro to bind a molecular target of interest. The aptamer selection technology is designated as SELEX (Systematic evolution of ligands by exponential enrichment). SELEX enables isolation of oligonucleotide aptamers binding a wide range of targets of interest with little respect for their nature and molecular weight. A number of applications of aptamer selection were developed ranging from biosensor technologies to antitumor drug discovery. First aptamer-based pharmaceutical (Macugen) was approved by FDA for clinical use in 2004, and since then more than ten aptamer-based drugs undergo various phases of clinical trials. From the medicinal chemist’s point of view, aptamers represent a new class of molecules suitable for the development of new therapeutics. Due to the stability, relative synthesis simplicity, and development of advanced strategies of target specific molecular selection, aptamers attract increased attention of drug discovery community. Difficulties of the development of next-generation antibiotics basing on the conventional basis of combinatorial chemistry and high-throughput screening have also amplified the interest to aptamer-based therapeutic candidates. The present article reviews the investigations focused on the development of antibacterial aptamers and discusses the potential and current limitations of the use of this type of therapeutic molecules.

Small molecule metalloprotease inhibitor with in vitro, ex vivo and in vivo efficacy against botulinum neurotoxin serotype A

Botulinum neurotoxins (BoNTs) are the most toxic substances known to mankind and are the causative agents of the neuroparalytic disease botulism. Their ease of production and extreme toxicity have caused these neurotoxins to be classified as Tier 1 bioterrorist threat agents and have led to a sustained effort to develop countermeasures to treat intoxication in case of a bioterrorist attack. While timely administration of an approved antitoxin is effective in reducing the severity of botulism, reversing intoxication requires different strategies. In the present study, we evaluated ABS 252 and other mercaptoacetamide small molecule active-site inhibitors of BoNT/A light chain using an integrated multi-assay approach. ABS 252 showed inhibitory activity in enzymatic, cell-based and muscle activity assays, and importantly, produced a marked delay in time-to-death in mice. The results suggest that a multi-assay approach is an effective strategy for discovery of potential BoNT therapeutic candidates.

Antibody Mimetics, Peptides, and Peptidomimetics

In spite of their widespread applications as therapeutic, diagnostic, and detection agents, the limitations of polyclonal and monoclonal antibodies have enthused scientists to plan for next-generation biomedical agents, the so-called antibody mimetics, which offer many advantages compared to traditional antibodies. Antibody mimetics could be designed through protein-directed evolution or fusion of complementarity-determining regions with intervening framework regions. In the recent decade, extensive progress has been made in exploiting human, butterfly (Pieris brassicae), and bacterial systems to design and select mimetics using display technologies. Notably, some of the mimetics have made their way to market. Numerous limitations lie ahead in developing mimetics for different biomedical usage, particularly for which conventional antibodies are ineffective. This chapter presents a brief overview of the current characteristics, construction, and applications of antibody mimetics.

Small molecule non-peptide inhibitors of botulinum neurotoxin serotype E: Structure-activity relationship and a pharmacophore model

Botulinum neurotoxins (BoNTs) are the most poisonous biological substance known to humans. They cause flaccid paralysis by blocking the release of acetylcholine at the neuromuscular junction. Here, we report a number of small molecule non-peptide inhibitors of BoNT serotype E. The structure-activity relationship and a pharmacophore model are presented. Although non-peptidic in nature, these inhibitors mimic key features of the uncleavable substrate peptide Arg-Ile-Met-Glu (RIME) of the SNAP-25 protein. Among the compounds tested, most of the potent inhibitors bear a zinc-chelating moiety connected to a hydrophobic and aromatic moiety through a carboxyl or amide linker. All of them show low micromolar IC50 values.

Selection of RNA Aptamers Against Botulinum Neurotoxin Type A Light Chain Through a Non-Radioactive Approach

Botulinum neurotoxin (BoNT), a category A agent, is the most toxic molecule known to mankind. The endopeptidase activity of light chain domain of BoNT is the cause for the inhibition of the neurotransmitter release and the flaccid paralysis that leads to lethality in botulism. Currently, antidotes are not available to reverse the flaccid paralysis caused by BoNT. In the present study, a non-radioactive-based systematic evolution of ligands by exponential enrichment (SELEX) process is developed by utilizing surface plasmon resonance to monitor the binding enrichment. Two RNA aptamers have been identified as strong binders against light chain of botulinum neurotoxin type A. These two aptamers showed strong inhibition activity on LCA, with IC50 in nanomolar range. Inhibition kinetic studies reveal mid nanomolar KI and non-competitive nature of their inhibition, suggesting that they have strong potential as antidotes that can reverse the symptom caused by BoNT/A. More importantly, we observed that the 2'-fluorine-pyrimidine-modified RNA aptamers identified here do not change their binding and biological activities. This observation could lead to a cost-effective way for SELEX, by using regular nucleotide during SELEX, and 2'-fluorine-pyrimidine-modified nucleotide for final application to enhance their RNase-resistance.

Interactions of a potent cyclic peptide inhibitor with the light chain of botulinum neurotoxin A: Insights from X-ray crystallography

The seven antigenically distinct serotypes (A-G) of botulinum neurotoxin (BoNT) are responsible for the deadly disease botulism. BoNT serotype A (BoNT/A) exerts its lethal action by cleaving the SNARE protein SNAP-25, leading to inhibition of neurotransmitter release, flaccid paralysis and autonomic dysfunction. BoNTs are dichain proteins consisting of a ∼ 100 kDa heavy chain and a ∼ 50 kDa light chain; the former is responsible for neurospecific binding, internalization and translocation, and the latter for cleavage of neuronal SNARE proteins. Because of their extreme toxicity and history of weaponization, the BoNTs are regarded as potential biowarfare/bioterrorism agents. No post-symptomatic therapeutic interventions are available for BoNT intoxication other than intensive care; therefore it is imperative to develop specific antidotes against this neurotoxin. To this end, a cyclic peptide inhibitor (CPI-1) was evaluated in a FRET assay for its ability to inhibit BoNT/A light chain (Balc). CPI was found to be highly potent, exhibiting a Ki of 12.3 nM with full-length Balc448 and 39.2 nM using a truncated crystallizable form of the light chain (Balc424). Cocrystallization studies revealed that in the Balc424-CPI-1 complex, the inhibitor adopts a helical conformation, occupies a high percentage of the active site cavity and interacts in an amphipathic manner with critical active site residues. The data suggest that CPI-1 prevents SNAP-25 from accessing the Balc active site by blocking both the substrate binding path at the surface and the Zn(2+) binding region involved in catalysis. This differs from linear peptide inhibitors described to date which block only the latter.

Antibody mimetics: promising complementary agents to animal-sourced antibodies

Despite their wide use as therapeutic, diagnostic and detection agents, the limitations of polyclonal and monoclonal antibodies have inspired scientists to design the next generation biomedical agents, so-called antibody mimetics that offer many advantages over conventional antibodies. Antibody mimetics can be constructed by protein-directed evolution or fusion of complementarity-determining regions through intervening framework regions. Substantial progress in exploiting human, butterfly (Pieris brassicae) and bacterial systems to design and select mimetics using display technologies has been made in the past 10 years, and one of these mimetics [Kalbitor® (Dyax)] has made its way to market. Many challenges lie ahead to develop mimetics for various biomedical applications, especially those for which conventional antibodies are ineffective, and this review describes the current characteristics, construction and applications of antibody mimetics compared to animal-sourced antibodies. The possible limitations of mimetics and future perspectives are also discussed.

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.

Structural basis for recognition of synaptic vesicle protein 2C by botulinum neurotoxin A

Botulinum neurotoxin A (BoNT/A) belongs to the most dangerous class of bioweapons. Despite this, BoNT/A is used to treat a wide range of common medical conditions such as migraines and a variety of ocular motility and movement disorders. BoNT/A is probably best known for its use as an antiwrinkle agent in cosmetic applications (including Botox and Dysport). BoNT/A application causes long-lasting flaccid paralysis of muscles through inhibiting the release of the neurotransmitter acetylcholine by cleaving synaptosomal-associated protein 25 (SNAP-25) within presynaptic nerve terminals. Two types of BoNT/A receptor have been identified, both of which are required for BoNT/A toxicity and are therefore likely to cooperate with each other: gangliosides and members of the synaptic vesicle glycoprotein 2 (SV2) family, which are putative transporter proteins that are predicted to have 12 transmembrane domains, associate with the receptor-binding domain of the toxin. Recently, fibroblast growth factor receptor 3 (FGFR3) has also been reported to be a potential BoNT/A receptor. In SV2 proteins, the BoNT/A-binding site has been mapped to the luminal domain, but the molecular details of the interaction between BoNT/A and SV2 are unknown. Here we determined the high-resolution crystal structure of the BoNT/A receptor-binding domain (BoNT/A-RBD) in complex with the SV2C luminal domain (SV2C-LD). SV2C-LD consists of a right-handed, quadrilateral β-helix that associates with BoNT/A-RBD mainly through backbone-to-backbone interactions at open β-strand edges, in a manner that resembles the inter-strand interactions in amyloid structures. Competition experiments identified a peptide that inhibits the formation of the complex. Our findings provide a strong platform for the development of novel antitoxin agents and for the rational design of BoNT/A variants with improved therapeutic properties.

4-Amino-7-chloroquinolines: probing ligand efficiency provides botulinum neurotoxin serotype A light chain inhibitors with significant antiprotozoal activity

Structurally simplified analogues of dual antimalarial and botulinum neurotoxin serotype A light chain (BoNT/A LC) inhibitor bis-aminoquinoline (1) were prepared. New compounds were designed to improve ligand efficiency while maintaining or exceeding the inhibitory potency of 1. Three of the new compounds are more active than 1 against both indications. Metabolically, the new inhibitors are relatively stable and nontoxic. 12, 14, and 15 are more potent BoNT/A LC inhibitors than 1. Additionally, 15 has excellent in vitro antimalarial efficacy, with IC90 values ranging from 4.45 to 12.11 nM against five Plasmodium falciparum (P.f.) strains: W2, D6, C235, C2A, and C2B. The results indicate that the same level of inhibitory efficacy provided by 1 can be retained/exceeded with less structural complexity. 12, 14, and 15 provide new platforms for the development of more potent dual BoNT/A LC and P.f. inhibitors adhering to generally accepted chemical properties associated with the druggability of synthetic molecules.

Inhibition in vivo of the activity of botulinum neurotoxin A by small molecules selected by virtual screening

To search for small molecular size inhibitors of botulinum neurotoxin A (BoNT/A) endopeptidase activity, we have screened the NCI library containing about 1 million structures against the substrate binding pocket of BoNT/A. Virtual screening (VS) was performed with the software Glide (Grid-based ligand docking energetics) and the findings were confirmed by AutoDock. Ten compounds were found that had favorable energetic and glide criteria and 5 of these compounds were selected for their ability to protect mice in vivo against a lethal dose of BoNT/A. Each compound was incubated at different molar excesses with a lethal dose of the toxin and then the mixture injected intravenously into mice. At 4690 M excess, compounds NSC94520 and NSC99639 protected all (100%) the mice from lethal toxicity. Compounds NSC48461 and NSC627733 gave 75% protection. Compound NSC348884 showed the least inhibition of toxicity allowing only a fraction (25%) of the mice to survive challenge with a lethal dose; and in the case of the mice that did not survive there was a considerable delay of mortality. At 2400 M excess compounds NSC94520 remained fully protective while and NSC99639 afforded 75% protection and at 1200 M excess each of these two compounds gave 50% protection. The two compounds gave no protection at 600 or less molar excess. When each compound was administered intravenously at 4690 M excess at different times (from 1 h to 6 h) after the intravenous injection of the active toxin, none of the compounds was able to protect the animals from toxicity. The findings show the value of VS in identifying potential inhibitors of the toxin for further development and improvement.

Sublingual immunization with adenovirus F protein-based vaccines stimulates protective immunity against botulinum neurotoxin A intoxication

Sublingual (s.l.) vaccination is an efficient way to induce elevated levels of systemic and mucosal immune responses. To mediate mucosal uptake, ovalbumin (OVA) was genetically fused to adenovirus 2 fiber protein (OVA-Ad2F) to assess whether s.l. immunization was as effective as an alternative route of vaccination. Ad2F-delivered vaccines were efficiently taken up by dendritic cells and migrated mostly to submaxillary gland lymph nodes, which could readily stimulate OVA-specific CD4(+) T cells. OVA-Ad2F + cholera toxin (CT)-immunized mice elicited significantly higher OVA-specific serum IgG, IgA and mucosal IgA antibodies among the tested immunization groups. These were supported by elevated OVA-specific IgG and IgA antibody-forming cells. A mixed T(h)-cell response was induced as evident by the enhanced IL-4, IL-10, IFN-γ and TNF-α-specific cytokine-forming cells. To assess whether this approach can stimulate neutralizing antibodies, immunizations were performed with the protein encumbering the β-trefoil domain of C-terminus heavy chain (Hcβtre) from botulinum neurotoxin A (BoNT/A) as well as when fused to Ad2F. Hcβtre-Ad2F + CT-dosed mice showed the greatest serum IgG, IgA and mucosal IgA titers among the immunization groups. Hcβtre-Ad2F alone also induced elevated antibody production in contrast to Hcβtre alone. Plasma from Hcβtre + CT- and Hcβtre-Ad2F + CT-immunized groups neutralized BoNT/A and protected mice from BoNT/A intoxication. Most importantly, Hcβtre-Ad2F + CT-immunized mice were protected from BoNT/A intoxication relative to Hcβtre + CT-immunized mice, which only showed ∼60% protection. This study shows that s.l. immunization with Ad2F-based vaccines is effective in conferring protective immunity.

A chemotype that inhibits three unrelated pathogenic targets: the botulinum neurotoxin serotype A light chain, P. falciparum malaria, and the Ebola filovirus

A 1,7-bis(alkylamino)diazachrysene-based small molecule was previously identified as an inhibitor of the botulinum neurotoxin serotype A light chain metalloprotease. Subsequently, a variety of derivatives of this chemotype were synthesized to develop structure-activity relationships, and all are inhibitors of the BoNT/A LC. Three-dimensional analyses indicated that half of the originally discovered 1,7-DAAC structure superimposed well with 4-amino-7-chloroquinoline-based antimalarial agents. This observation led to the discovery that several of the 1,7-DAAC derivatives are potent in vitro inhibitors of Plasmodium falciparum and, in general, are more efficacious against CQ-resistant strains than against CQ-susceptible strains. In addition, by inhibiting β-hematin formation, the most efficacious 1,7-DAAC-based antimalarials employ a mechanism of action analogous to that of 4,7-ACQ-based antimalarials and are well tolerated by normal cells. One candidate was also effective when administered orally in a rodent-based malaria model. Finally, the 1,7-DAAC-based derivatives were examined for Ebola filovirus inhibition in an assay employing Vero76 cells, and three provided promising antiviral activities and acceptably low toxicities.

Pharmacophore Refinement Guides the Rational Design of Nanomolar-Range Inhibitors of the Botulinum Neurotoxin Serotype A Metalloprotease

Botulinum neurotoxins (BoNTs) are the deadliest of microbial toxins. The enzymes' Zinc(II) metalloprotease, referred to as the light chain (LC) component, inhibits acetylcholine release into neuromuscular junctions, resulting in the disease botulism. Currently, no therapies counter BoNT poisoning post-neuronal intoxication; however, it is hypothesized that small molecules may be used to inhibit BoNT LC activity in the neuronal cytosol. Herein, we describe the pharmacophore-based design and chemical synthesis of potent (non-Zinc(II) chelating) small molecule (non-peptidic) inhibitors (SMNPIs) of the BoNT serotype A LC (the most toxic of the BoNT serotype LCs). Specifically, the three-dimensional superimpositions of 2-[4-(4-amidinephenoxy)-phenyl]-indole-6-amidine-based SMNPI regioisomers (K(i) = 0.600 μM (± 0.100 μM)), with a novel lead bis-[3-amide-5-(imidazolino)-phenyl]-terephthalamide (BAIPT)-based SMNPI (K(i) = 8.52 μM (± 0.53 μM)), resulted in a refined 4-zone pharmacophore. The refined model guided the design of BAIPT-based SMNPIs possessing K(i) values = 0.572 μM (± 0.041 μM) and 0.900 μM (± 0.078 μM).

In vitro selection of RNA aptamers that inhibit the activity of type A botulinum neurotoxin

The category A agent, botulinum neurotoxin (BoNT), is the most toxic molecule known to mankind. The endopeptidase activity of light chain domain of BoNT is the cause for the inhibition of the neurotransmitter release and the flaccid paralysis that leads to lethality in botulism. Currently, antidotes are not available to reverse the flaccid paralysis caused by BoNT. In the present study, we have identified three RNA aptamers through SELEX-process, which bind strongly to the light chain of type A BoNT (BoNT/A) and inhibit the endopeptidase activity, with IC(50) in low nM range. Inhibition kinetic studies reveal low nM K(I) and non-competitive nature of their inhibition. Aptamers are unique group of molecules as therapeutics, and this is first report of their development as an antidote against botulism. These data on K(I) and IC(50) strongly suggest that the aptamers have strong potential as antidotes that can reverse the symptom caused by BoNT/A.

The identification and biochemical characterization of drug-like compounds that inhibit botulinum neurotoxin serotype A endopeptidase activity

A robust, high-throughput, two-tiered assay for screening small molecule inhibitors against botulinum neurotoxin serotype A was developed and employed to screen 16,544 compounds. Thirty-four compounds were identified as potent hits employing the first-tier assay. Subsequently, nine were confirmed as actives by our second-tier confirmatory assay. Of these, one displayed potent inhibitory efficacy, possessing an IC(50)=16 microM (+/-1.6 microM) in our in vitro assay. This inhibitor (0831-1035) is highly water-soluble, and possesses an IC(50)=47 microM (+/-7.0 microM) in our primary cell culture assay (with virtually no cytotoxicity up to 500 microM), suggesting that this inhibitor is a good candidate for further development as a therapeutic countermeasure to treat botulism resulting from botulinum neurotoxin serotype A intoxication. An enzyme kinetics study indicated that this inhibitor exhibits mixed non-competitive inhibition, with a K(I)=9 microM.

Antibody protection against botulinum neurotoxin intoxication in mice

Adulteration of food or feed with any of the seven serotypes of botulinum neurotoxin (BoNT) is a potential bioterrorism concern. Currently, there is strong interest in the development of detection reagents, vaccines, therapeutics, and other countermeasures. A sensitive immunoassay for detecting BoNT serotype A (BoNT/A), based on monoclonal antibodies (MAbs) F1-2 and F1-40, has been developed and used in complex matrices. The epitope for F1-2 has been mapped to the heavy chain of BoNT/A, and the epitope of F1-40 has been mapped to the light chain. The ability of these MAbs to provide therapeutic protection against BoNT/A intoxication in mouse intravenous and oral intoxication models was tested. High dosages of individual MAbs protected mice well both pre- and postexposure to BoNT/A holotoxin. A combination therapy consisting of antibodies against both the light and heavy chains of the toxin, however, significantly increased protection, even at a lower MAb dosage. An in vitro peptide assay for measuring toxin activity showed that pretreatment of toxin with these MAbs did not block catalytic activity but instead blocked toxin entry into primary and cultured neuronal cells. The timing of antibody rescue in the mouse intoxication models revealed windows of opportunity for antibody therapeutic treatment that correlated well with the biologic half-life of the toxin in the serum. Knowledge of BoNT intoxication and antibody clearance in these mouse models and understanding of the pharmacokinetics of BoNT are invaluable for future development of antibodies and therapeutics against intoxication by BoNT.

Foodborne illness

While few patients with foodborne illness present with life-threatening symptoms, there are a number of foodborne infectious diseases and toxins that the emergency physician or other health care provider must consider in the evaluation of these patients. Given the frequency of international travel, as well as the risk associated with recurrent outbreaks of foodborne illness from commercial food sources, it is important to recognize various syndromes of foodborne illness, including those which may require specific evaluation and management strategies. This article reviews a number of the most common causes of foodborne illness, as well as several less common pathogens with the potential for causing significant morbidity and mortality if not promptly identified and treated.