Pharmacokinetics of Human Recombinant Anti-Botulinum Toxin Antibodies in Rats

Neutralization mechanism of human monoclonal antibodies against type B botulinum neurotoxin

Botulism is a deadly neuroparalytic condition caused by the botulinum neurotoxin (BoNT) produced by Clostridium botulinum and related species. Toxin-neutralizing antibodies are the most effective treatments for BoNT intoxication. We generated human monoclonal antibodies neutralizing type B botulinum neurotoxin (BoNT/B), designated M2 and M4. The combination of these antibodies exhibited a strong neutralizing effect against BoNT/B toxicity. In this study, we analyzed the mechanisms of action of these antibodies in vitro. M4 binds to the C-terminus of the heavy chain (the receptor-binding domain) and inhibits BoNT/B binding to neuronal PC12 cells. Although M2 recognized the light (L) chain (the metalloprotease domain), it did not inhibit substrate (VAMP2) cleavage in the cleavage assay. M2 increased the surface localization of BoNT/B in PC12 cells at a later time point, suggesting that M2 inhibits the translocation of the L chain from synaptic vesicles to the cytosol. These results indicate that M2 and M4 inhibit the different processes of BoNT/B individually and that multistep inhibition is important for the synergistic effect of the combination of monoclonal antibodies. Our findings may facilitate the development of effective therapeutic antibodies against BoNTs.

New approach to generating of human monoclonal antibodies specific to the proteolytic domain of botulinum neurotoxin A

Introduction

Botulinum neurotoxins (BoNTs) cause botulism and are the most potent natural toxins known. Immunotherapy with neutralizing monoclonal antibodies (MAbs) is considered to be the most effective immediate response to BoNT exposure. Hybridoma technology remains the preferred method for producing MAbs with naturally paired immunoglobulin genes and with preserved innate functions of immune cells. The affinity-matured human antibody repertoire may be ideal as a source for antibody therapeutics against BoNTs. In an effort to develop novel BoNT type A (BoNT/A) immunotherapeutics, sorted by flow cytometry plasmablasts and activated memory B cells from a donor repeatedly injected with BoNT/A for aesthetic botulinum therapy could be used due to obtain hybridomas producing native antibodies.

Methods

Plasmablasts and activated memory B-cells were isolated from whole blood collected 7 days after BoNT/A injection and sorted by flow cytometry. The sorted cells were then electrofused with the K6H6/B5 cell line, resulting in a producer of native human monoclonal antibodies (huMAbs). The 3 antibodies obtained were then purified by affinity chromatography, analyzed for binding by Western blot assay and neutralization by FRET assay.

Results

We have succeeded in creating 3 hybridomas that secrete huMAbs specific to native BoNT/A and the proteolytic domain (LC) of BoNT/A. The 1B9 antibody also directly inhibited BoNT/A catalytic activity in vitro.

Conclusion

The use activated plasmablasts and memory B-cells isolated at the peak of the immune response (at day 7 of immunogenesis) that have not yet completed the terminal stage of differentiation but have undergone somatic hypermutation for hybridization allows us to obtain specific huMAbs even when the immune response of the donor is weak (with low levels of specific antibodies and specific B-cells in blood). A BoNT/A LC-specific antibody is capable of effectively inhibiting BoNT/A by mechanisms not previously associated with antibodies that neutralize BoNT. Antibodies specific to BoNT LC can be valuable components of a mixture of antibodies against BoNT exposure.

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 (AUC0→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.

Neutralizing Concentrations of Anti-Botulinum Toxin Antibodies Positively Correlate with Mouse Neutralization Assay Results in a Guinea Pig Model

Botulinum neurotoxins (BoNT) are some of the most toxic proteins known and can induce respiratory failure requiring long-term intensive care. Treatment of botulism includes the administration of antitoxins. Monoclonal antibodies (mAbs) hold considerable promise as BoNT therapeutics and prophylactics, due to their potency and safety. A three-mAb combination has been developed that specifically neutralizes BoNT serotype A (BoNT/A), and a separate three mAb combination has been developed that specifically neutralizes BoNT serotype B (BoNT/B). A six mAb cocktail, designated G03-52-01, has been developed that combines the anti-BoNT/A and anti-BoNT/B mAbs. The pharmacokinetics and neutralizing antibody concentration (NAC) of G03-52-01 has been determined in guinea pigs, and these parameters were correlated with protection against an inhalation challenge of BoNT/A1 or BoNT/B1. Previously, it was shown that each antibody demonstrated a dose-dependent mAb serum concentration and reached maximum circulating concentrations within 48 h after intramuscular (IM) or intraperitoneal (IP) injection and that a single IM injection of G03-52-01 administered 48 h pre-exposure protected guinea pigs against an inhalation challenge of up to 93 LD₅₀s of BoNT/A1 and 116 LD₅₀s of BoNT/B1. The data presented here advance our understanding of the relationship of the neutralizing NAC to the measured circulating antibody concentration and provide additional support that a single IM or intravenous (IV) administration of G03-52-01 will provide pre-exposure prophylaxis against botulism from an aerosol exposure of BoNT/A and BoNT/B.

Safety, Tolerability, and Pharmacokinetics of NTM-1632, a Novel Mixture of Three Monoclonal Antibodies against Botulinum Toxin B

Botulism is a rare, life-threatening paralytic disease caused by Clostridium botulinum neurotoxin (BoNT). Available treatments, including an equine antitoxin and human immune globulin, are given postexposure and challenging to produce and administer. NTM-1632 is an equimolar mixture of 3 human IgG monoclonal antibodies, B1, B2, and B3, targeting BoNT serotype B (BoNT/B). This first-in-human study assessed the safety, tolerability, pharmacokinetics (PK), and immunogenicity of NTM-1632. This double-blind, single-center, placebo-controlled dose escalation study randomized 3 cohorts of healthy volunteers to receive a single intravenous dose of NTM-1632 (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-1632; 6 placebo), and no deaths or serious adverse events were reported. Adverse events in the NTM-1632 groups were generally mild and similar in frequency and severity to the placebo group, and no safety signal was identified. NTM-1632 has a favorable PK profile with a half-life of >20 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-1632 demonstrated low immunogenicity with only a few treatment-emergent antidrug antibody responses in the low and middle dosing groups and none at the highest dose. NTM-1632 is well tolerated at the administered doses. The favorable safety, PK, and immunogenicity profile of NTM-1632 supports further clinical development as a treatment for BoNT/B intoxication and postexposure prophylaxis. (This study has been registered at ClinicalTrials.gov under identifier NCT02779140.).

Structural Insights into Rational Design of Single-Domain Antibody-Based Antitoxins against Botulinum Neurotoxins

Botulinum neurotoxin (BoNT) is one of the most acutely lethal toxins known to humans, and effective treatment for BoNT intoxication is urgently needed. Single-domain antibodies (VHH) have been examined as a countermeasure for BoNT because of their high stability and ease of production. Here, we investigate the structures and the neutralization mechanisms for six unique VHHs targeting BoNT/A1 or BoNT/B1. These studies reveal diverse neutralizing mechanisms by which VHHs prevent host receptor binding or block transmembrane delivery of the BoNT protease domain. Guided by this knowledge, we design heterodimeric VHHs by connecting two neutralizing VHHs via a flexible spacer so they can bind simultaneously to the toxin. These bifunctional VHHs display much greater potency in a mouse co-intoxication model than similar heterodimers unable to bind simultaneously. Taken together, our studies offer insight into antibody neutralization of BoNTs and advance our ability to design multivalent anti-pathogen VHHs with improved therapeutic properties.

Botulinum neurotoxins (BoNTs) are extremely toxic biothreats. Lam et al. report the crystal structures and neutralizing mechanisms of six unique antitoxin VHHs against BoNT/A1 and BoNT/B1, the two major human pathogenic BoNTs. They then develop a platform for structure-based rational design of bifunctional VHH heterodimers with superior antitoxin potencies.

Fully Human Monoclonal Antibodies Effectively Neutralizing Botulinum Neurotoxin Serotype B

Botulinum neurotoxin (BoNT) is the most potent natural toxin known. Of the seven BoNT serotypes (A to G), types A, B, E, and F cause human botulism. Treatment of human botulism requires the development of effective toxin-neutralizing antibodies without side effects such as serum sickness and anaphylaxis. In this study, we generated fully human monoclonal antibodies (HuMAbs) against serotype B BoNT (BoNT/B1) using a murine–human chimera fusion partner cell line named SPYMEG. Of these HuMAbs, M2, which specifically binds to the light chain of BoNT/B1, showed neutralization activity in a mouse bioassay (approximately 10 i.p. LD₅₀/100 µg of antibody), and M4, which binds to the C-terminal of heavy chain, showed partial protection. The combination of two HuMAbs, M2 (1.25 µg) and M4 (1.25 µg), was able to completely neutralize BoNT/B1 (80 i.p. LD₅₀) with a potency greater than 80 i.p. LD₅₀/2.5 µg of antibodies, and was effective both prophylactically and therapeutically in the mouse model of botulism. Moreover, this combination showed broad neutralization activity against three type B subtypes, namely BoNT/B1, BoNT/B2, and BoNT/B6. These data demonstrate that the combination of M2 and M4 is promising in terms of a foundation for new human therapeutics for BoNT/B intoxication.

Antibodies and Vaccines against Botulinum Toxins: Available Measures and Novel Approaches

Botulinum neurotoxin (BoNT) is produced by the anaerobic, Gram-positive bacterium Clostridium botulinum. As one of the most poisonous toxins known and a potential bioterrosism agent, BoNT is characterized by a complex mode of action comprising: internalization, translocation and proteolytic cleavage of a substrate, which inhibits synaptic exocytotic transmitter release at neuro-muscular nerve endings leading to peripheral neuroparalysis of the skeletal and autonomic nervous systems. There are seven major serologically distinct toxinotypes (A-G) of BoNT which act on different substrates. Human botulism is generally caused by BoNT/A, B and E. Due to its extreme lethality and potential use as biological weapon, botulism remains a global public health concern. Vaccination against BoNT, although an effective strategy, remains undesirable due to the growing expectation around therapeutic use of BoNTs in various pathological conditions. This review focuses on the current approaches for botulism control by immunotherapy, highlighting the future challenges while the molecular underpinnings among subtypes variants and BoNT sequences found in non-clostridial species remain to be elucidated.