A Quest for a Universal Plasma-Derived Antivenom Against All Elapid Neurotoxic Snake Venoms

Expression and characterization of scFv-6009FV in Pichia pastoris with improved ability to neutralize the neurotoxin Cn2 from Centruroides noxius

Small single-chain variable fragments (scFv) are promising biomolecules to inhibit and neutralize toxins and to act as antivenoms. In this work, we aimed to produce a functional scFv-6009FV in the yeast Pichia pastoris, which inhibits the pure Cn2 neurotoxin and the whole venom of Centruroides noxius. We were able to achieve yields of up to 31.6 ± 2 mg/L in flasks. Furthermore, the protein showed a structure of 6.1 % α-helix, 49.1 % β-sheet, and 44.8 % of random coil by CD. Mass spectrometry confirmed the amino acid sequence and showed no glycosylation profile for this molecule. Purified scFv-6009FV allowed us to develop anti-scFvs in rabbits, which were then used in affinity columns to purify other scFvs. Determination of its half-maximal inhibitory concentration value (IC50) was 40 % better than the scFvs produced by E. coli as a control. Finally, we found that scFv-6009FV was able to inhibit ex vivo the pure Cn2 toxin and the whole venom from C. noxius in murine rescue experiments. These results demonstrated that under the conditions assayed here, P. pastoris is suited to produce scFv-6009FV that, compared to scFvs produced by E. coli, maintains the characteristics of an antibody and neutralizes the Cn2 toxin more effectively.

Dual-Modal Cellular Nanoparticles for Continuous Neurotoxin Detoxification

Neurotoxins are known for their extreme lethality. However, due to their enormous diversity, effective and broad-spectrum countermeasures are lacking. This study presents a dual-modal cellular nanoparticle (CNP) formulation engineered for continuous neurotoxin neutralization. The formulation involves encapsulating the metabolic enzyme N-sulfotransferase (SxtN) into metal-organic framework (MOF) nanoparticle cores and coating them with a natural neuronal membrane, termed "Neuron-MOF/SxtN-NPs". The resulting nanoparticles combine membrane-enabled broad-spectrum neurotoxin neutralization with enzyme payload-enabled continuous neurotoxin neutralization. The studies confirm the protection of the enzyme payload by the MOF core and validate the continuous neutralization of saxitoxin (STX). In vivo studies conducted using a mouse model of STX intoxication reveal markedly improved survival rates compared with control groups. Furthermore, acute toxicity assessments show no adverse effects associated with the administration of Neuron-MOF/SxtN-NPs in healthy mice. Overall, Neuron-MOF/SxtN-NPs represent a unique biomimetic nanomedicine platform poised to effectively neutralize neurotoxins, marking an important advancement in the field of countermeasure nanomedicine.

Characterization of plant produced VHH antibodies against cobra venom toxins for antivenom therapy

Cobra (Naja kaouthia) venom contains many toxins including α-neurotoxin (αNTX) and phospholipase A2 (PLA2), which can cause neurodegeneration, respiratory failure, and even death. The traditional antivenom derived from animal serum faces many challenges and limitations. Heavy-chain-only antibodies (HCAb), fusing VHH with human IgG Fc region, offer advantages in tissue penetration, antigen binding, and extended half-life. This research involved the construction and transient expression of two types of VHH-FC which are specific to α-Neurotoxin (VHH-αNTX-FC) and Phospholipase A2 (VHH-PLA2-FC) in Nicotiana benthamiana leaves. The recombinant HCAbs were incubated for up to six days to optimize expression levels followed by purification by affinity chromatography and characterization using LC/Q-TOF mass spectrometry (MS). Purified proteins demonstrated over 92 % sequence coverage and an average mass of around 82 kDa with a high-mannose N-glycan profile. An antigen binding assay showed that the VHH-αNTX-Fc has a greater ability to bind to crude venom than VHH-PLA2-Fc.

Comparison of the intrageneric neutralization scope of monospecific, bispecific/monogeneric and polyspecific/monogeneric antisera raised in horses immunized with sub-Saharan African snake venoms

BACKGROUND

Snakebite envenomation inflicts a high burden of mortality and morbidity in sub-Saharan Africa. Antivenoms are the mainstay in the therapy of envenomation, and there is an urgent need to develop antivenoms of broad neutralizing efficacy for this region. The venoms used as immunogens to manufacture snake antivenoms are normally selected considering their medical importance and availability. Additionally, their ability to induce antibody responses with high neutralizing capability should be considered, an issue that involves the immunization scheme and the animal species being immunized.

METHODOLOGY/PRINCIPAL FINDINGS

Using the lethality neutralization assay in mice, we compared the intrageneric neutralization scope of antisera generated by immunization of horses with monospecific, bispecific/monogeneric, and polyspecific/monogeneric immunogens formulated with venoms of Bitis spp., Echis spp., Dendroaspis spp., spitting Naja spp. or non-spitting Naja spp. It was found that the antisera raised by all the immunogens were able to neutralize the homologous venoms and, with a single exception, the heterologous congeneric venoms (considering spitting and non-spitting Naja separately). In general, the polyspecific antisera of Bitis spp, Echis spp, and Dendroaspis spp gave the best neutralization profile against venoms of these genera. For spitting Naja venoms, there were no significant differences in the neutralizing ability between monospecific, bispecific and polyspecific antisera. A similar result was obtained in the case of non-spitting Naja venoms, except that polyspecific antiserum was more effective against the venoms of N. melanoleuca and N. nivea as compared to the monospecific antiserum.

CONCLUSIONS/SIGNIFICANCE

The use of polyspecific immunogens is the best alternative to produce monogeneric antivenoms with wide neutralizing coverage against venoms of sub-Saharan African snakes of the Bitis, Echis, Naja (non-spitting) and Dendroaspis genera. On the other hand, a monospecific immunogen composed of venom of Naja nigricollis is suitable to produce a monogeneric antivenom with wide neutralizing coverage against venoms of spitting Naja spp. These findings can be used in the design of antivenoms of wide neutralizing scope for sub-Saharan Africa.

Tissue damaging toxins in snake venoms: mechanisms of action, pathophysiology and treatment strategies

Snakebite envenoming is an important public health issue responsible for mortality and severe morbidity. Where mortality is mainly caused by venom toxins that induce cardiovascular disturbances, neurotoxicity, and acute kidney injury, morbidity is caused by toxins that directly or indirectly destroy cells and degrade the extracellular matrix. These are referred to as 'tissue-damaging toxins' and have previously been classified in various ways, most of which are based on the tissues being affected (e.g., cardiotoxins, myotoxins). This categorisation, however, is primarily phenomenological and not mechanistic. In this review, we propose an alternative way of classifying cytotoxins based on their mechanistic effects rather than using a description that is organ- or tissue-based. The mechanisms of toxin-induced tissue damage and their clinical implications are discussed. This review contributes to our understanding of fundamental biological processes associated with snakebite envenoming, which may pave the way for a knowledge-based search for novel therapeutic options.

Immune response to neurotoxic South American snake venoms

South American rattlesnakes (Crotalus durissus spp) and coral snakes (Micrurus sp) venoms are characterized by inducing a limited inflammatory innate immune response, in contrast to Bothrops sp snake venoms which exert a prominent inflammatory activity. Some Crotalus durissus spp venoms, in addition, exert immunosuppressive activities that hamper the development of neutralizing antibodies in animals immunized for antivenom production. Micrurus sp venoms are rich in low molecular mass neurotoxins that elicit a limited immune response. These characteristics make it difficult to generate antivenoms of high neutralizing activity. Therefore, the study of the mechanisms operating behind this limited immune response to venoms is relevant from both fundamental and practical perspectives. This review summarizes key aspects of the immune response to these venoms and discusses some pending challenges to further understand these phenomena and to improve antivenom production.

Polyvalent Snake Antivenoms: Production Strategy and Their Therapeutic Benefits

Snake envenomation remains an important yet neglected medical problem in many countries, with around five million people affected, and over a hundred thousand deaths annually. Plasma-derived antivenoms are the main therapeutic agent available. Monovalent antivenoms are produced via the immunization of large animals, e.g., horses, with one venom, after which the horse serum can neutralize the homologous venom, with minimal or no cross neutralization against other venoms. It is necessary, therefore, for the culprit snake to be identified, so that the appropriate specific antivenom can be selected. Polyvalent antivenoms (pAVs) are produced via immunization with a number of snake venoms, and the serum can neutralize all the venoms used in its production. Thus, pAVs can be used to treat several venoms from a country/region, and the identification of the culprit snake is not necessary. There are various parameters and processes involved in the production of pAVs, depending on the requirements and resources available. Most commercial pAVs use a mixture of both elapid and viperid venoms as immunogens, while some pAVs use either elapid or viperid venoms. Some pAVs are produced through the mixing of more than one monovalent or polyvalent antivenom. These various types of pAVs have their own characteristics, and have benefits and drawbacks. The major benefits of pAVs are the wide coverage of many medically important venoms, including many heterologous venoms. They also remove the need to identify the culprit snake, and they can be produced at a lower cost than several monovalent antivenoms. Interesting polyvalent antivenoms, termed 'syndromic pAVs' (s-pAVs), have recently gained attention. They are produced for use according to the syndromes manifested in snakebite patients. The venoms that produce these syndromes are used as immunogens in the production of 'syndromic antivenoms'. For example, 'neurotoxic polyvalent antivenom' and 'hematotoxic polyvalent antivenom' are produced using the neurotoxic elapid and hematotoxic viperid venoms as immunogens, respectively. They were first marketed by the Thai Red Cross in 2012, and have since gained attention as a possible therapeutic modality to help solve the problem of snakebite envenomation globally. The merits of these s-pAVs, including their efficacy and wide paraspecificities, are discussed.

Antibodies against a single fraction of Micrurus dumerilii venom neutralize the lethal effect of whole venom

The coralsnake Micrurus dumerilii (Elapidae) is reported to cause envenomings of medical importance. Previous studies characterized the protein composition of its venom, with phospholipase A₂ (PLA₂) proteins the most abundant. However, it is unknown which venom components are responsible for its lethal toxicity. Fractionation of M. dumerilii venom from Colombia was carried out using RP-HPLC and each fraction was screened for lethal effect in mice at a dose of 20 μg by intraperitoneal route. Results showed that only one fraction, F9, was lethal. This fraction displayed PLA₂ activity, induced indirect hemolysis in vitro, as well as edema and myotoxicity in vivo. SDS-PAGE of unreduced F9 evidenced two bands of 8 and 15 kDa, respectively, consistent with the detection of proteins with masses of 13,217.77 Da, 7144.06 Da, and 7665.55 Da. Tryptic digestion of F9 followed by nESI-MS/MS revealed peptide sequences matching proteins of the three-finger toxin (3FTx) and PLA₂ families. Immunization of a rabbit with F9 proteins elicited antibody titers up to 1:10,000 by ELISA. After serum fractionation with caprylic acid, the obtained IgG was able to neutralize the lethal effect of the complete venom of M. dumerilii using a challenge of 2 ×LD₅₀ at the IgG/venom ratio of 50:1 (w/w). In conclusion, present results show that the lethal effect of M. dumerilii venom in mice is mainly driven by one fraction which contains 3FTx and PLA₂ proteins. The antibodies produced against this fraction cross-recognized other PLA₂s and neutralized the lethal effect of whole M. dumerilii venom, pointing out to the potential usefulness of F9 as a relevant antigen for improving current coral snake antivenoms.

An immunoinformatic approach to assessing the immunogenic capacity of alpha-neurotoxins in elapid snake venoms

Introduction: Most elapid snakes produce venoms that contain alpha-neurotoxins (α-NTXs), which are proteins that cause post-synaptic blockade and paralysis in snakebite envenoming. However, existing elapid antivenoms are known for their low potency in neutralizing the neurotoxic activity of α-NTXs, while the immunological basis has not been elucidated. Methods: In this study, a structure-based major histocompatibility complex II (MHCII) epitope predictor of horse (Equus caballus), complemented with DM-editing determinant screening algorithm was adopted to assess the immunogenicity of α-NTXs in the venoms of major Asiatic elapids (Naja kaouthia, Ophiophagus hannah, Laticauda colubrina, Hydrophis schistosus, Hydrophis curtus). Results: The scoring metric M₂R, representing the relative immunogenic performance of respective α-NTXs, showed all α-NTXs have an overall low M₂R of <0.3, and most of the predicted binders feature non-optimal P1 anchor residues. The M₂R scores correlate strongly (R² = 0.82) with the potency scores (p-score) generated based on the relative abundances of α-NTXs and the neutralization potency of commercial antivenoms. Discussion: The immunoinformatic analysis indicates that the inferior antigenicity of α-NTXs is not only due to their small molecular size but also the subpar immunogenicity affected by their amino acid composition. Structural modification with conjugation and synthetic epitope as immunogen may potentially enhance the immunogenicity for improved antivenom potency against α-NTXs of elapid snakes.

Equatorial Spitting Cobra (Naja sumatrana) from Malaysia (Negeri Sembilan and Penang), Southern Thailand, and Sumatra: Comparative Venom Proteomics, Immunoreactivity and Cross-Neutralization by Antivenom

The Equatorial Spitting Cobra (Naja sumatrana) is a medically important venomous snake species in Southeast Asia. Its wide geographical distribution implies potential intra-specific venom variation, while there is no species-specific antivenom available to treat its envenoming. Applying a protein-decomplexing proteomic approach, the study showed that three-finger toxins (3FTX), followed by phospholipases A₂ (PLA₂), were the major proteins well-conserved across N. sumatrana venoms of different locales. Variations were noted in the subtypes and relative abundances of venom proteins. Of note, alpha-neurotoxins (belonging to 3FTX) are the least in the Penang specimen (Ns-PG, 5.41% of total venom proteins), compared with geographical specimens from Negeri Sembilan (Ns-NS, 14.84%), southern Thailand (Ns-TH, 16.05%) and Sumatra (Ns-SU, 10.81%). The alpha-neurotoxin abundance, in general, correlates with the venom’s lethal potency. The Thai Naja kaouthia Monovalent Antivenom (NkMAV) was found to be immunoreactive toward the N. sumatrana venoms and is capable of cross-neutralizing N. sumatrana venom lethality to varying degrees (potency = 0.49–0.92 mg/mL, interpreted as the amount of venom completely neutralized per milliliter of antivenom). The potency was lowest against NS-SU venom, implying variable antigenicity of its lethal alpha-neurotoxins. Together, the findings suggest the para-specific and geographical utility of NkMAV as treatment for N. sumatrana envenoming in Southeast Asia.

Cerebral Complications of Snakebite Envenoming: Case Studies

There are an estimated 5.4 million snakebite cases every year. People with snakebite envenoming suffer from severe complications, or even death. Although some review articles cover several topics of snakebite envenoming, a review of the cases regarding cerebral complications, especially rare syndromes, is lacking. Here, we overview 35 cases of snakebite by front-fanged snakes, including Bothrops, Daboia, Cerastes, Deinagkistrodon, Trimeresurus, and Crotalus in the Viperidae family; Bungarus and Naja in the Elapidae family, and Homoroselaps (rare cases) in the Lamprophiidae family. We also review three rare cases of snakebite by rear-fanged snakes, including Oxybelis and Leptodeira in the Colubridae family. In the cases of viper bites, most patients (17/24) were diagnosed with ischemic stroke and intracranial hemorrhage, leading to six deaths. We then discuss the potential underlying molecular mechanisms that cause these complications. In cases of elapid bites, neural, cardiac, and ophthalmic disorders are the main complications. Due to the small amount of venom injection and the inability to deep bite, all the rear-fanged snakebites did not develop any severe complications. To date, antivenom (AV) is the most effective therapy for snakebite envenoming. In the six cases of viper and elapid bites that did not receive AV, three cases (two by viper and one by elapid) resulted in death. This indicates that AV treatment is the key to survival after a venomous snakebite. Lastly, we also discuss several studies of therapeutic agents against snakebite-envenoming-induced complications, which could be potential adjuvants along with AV treatment. This article organizes the diagnosis of hemotoxic and neurotoxic envenoming, which may help ER doctors determine the treatment for unidentified snakebite.

Developing a computational pharmacokinetic model of systemic snakebite envenomation and antivenom treatment

Snakebite envenomation is responsible for over 100,000 deaths and 400,000 cases of disability annually, most of which are preventable through access to safe and effective antivenoms. Snake venom toxins span a wide molecular weight range, influencing their absorption, distribution, and elimination within the body. In recent years, a range of scaffolds have been applied to antivenom development. These scaffolds similarly span a wide molecular weight range and subsequently display diverse pharmacokinetic behaviours. Computational simulations represent a powerful tool to explore the interplay between these varied antivenom scaffolds and venoms, to assess whether a pharmacokinetically optimal antivenom exists. The purpose of this study was to establish a computational model of systemic snakebite envenomation and treatment, for the quantitative assessment and comparison of conventional and next-generation antivenoms. A two-compartment mathematical model of envenomation and treatment was defined and the system was parameterised using existing data from rabbits. Elimination and biodistribution parameters were regressed against molecular weight to predict the dynamics of IgG, F(ab')₂, Fab, scFv, and nanobody antivenoms, spanning a size range of 15-150 kDa. As a case study, intramuscular envenomation by Naja sumatrana (equatorial spitting cobra) and its treatment using Fab, F(ab')₂, and IgG antivenoms was simulated. Variable venom dose tests were applied to visualise effective antivenom dose levels. Comparisons to existing antivenoms and experimental rescue studies highlight the large dose reductions that could result from recombinant antivenom use. This study represents the first comparative in silico model of snakebite envenomation and treatment.

Snake Venom Proteomics of Samar Cobra (Naja samarensis) from the Southern Philippines: Short Alpha-Neurotoxins as the Dominant Lethal Component Weakly Cross-Neutralized by the Philippine Cobra Antivenom

The Samar Cobra, Naja samarensis, is endemic to the southern Philippines and is a WHO-listed Category 1 venomous snake species of medical importance. Envenomation caused by N. samarensis results in neurotoxicity, while there is no species-specific antivenom available for its treatment. The composition and neutralization of N. samarensis venom remain largely unknown to date. This study thus aimed to investigate the venom proteome of N. samarensis for a comprehensive profiling of the venom composition, and to examine the immunorecognition as well as neutralization of its toxins by a hetero-specific antivenom. Applying C₁₈ reverse-phase high-performance liquid chromatography (RP-HPLC) and tandem mass spectrometry (LC-MS/MS), three-finger toxins (3FTx) were shown to dominate the venom proteome by 90.48% of total venom proteins. Other proteins in the venom comprised snake venom metalloproteinases, phospholipases A_2, cysteine-rich secretory proteins, venom nerve growth factors, L-amino acid oxidases and vespryn, which were present at much lower abundances. Among all, short-chain alpha-neurotoxins (SαNTX) were the most highly expressed toxin within 3FTx family, constituting 65.87% of the total venom proteins. The SαNTX is the sole neurotoxic component of the venom and has an intravenous median lethal dose (LD₅₀) of 0.18 μg/g in mice. The high abundance and low LD₅₀ support the potent lethal activity of N. samarensis venom. The hetero-specific antivenom, Philippine Cobra Antivenom (PCAV, raised against Naja philippinensis) were immunoreactive toward the venom and its protein fractions, including the principal SαNTX. In efficacy study, PCAV was able to cross-neutralize the lethality of SαNTX albeit the effect was weak with a low potency of 0.20 mg/ml (defined as the amount of toxin completely neutralized per milliliter of the antivenom). With a volume of 5 ml, each vial of PCAV may cross-neutralize approximately 1 mg of the toxin in vivo. The findings support the potential para-specific use of PCAV in treating envenomation caused by N. samarensis while underscoring the need to improve the potency of its neutralization activity, especially against the highly lethal alpha-neurotoxins.

Elucidating the Venom Diversity in Sri Lankan Spectacled Cobra (Naja naja) through De Novo Venom Gland Transcriptomics, Venom Proteomics and Toxicity Neutralization

Inadequate effectiveness of Indian antivenoms in treating envenomation caused by the Spectacled Cobra/Indian Cobra (Naja naja) in Sri Lanka has been attributed to geographical variations in the venom composition. This study investigated the de novo venom-gland transcriptomics and venom proteomics of the Sri Lankan N. naja (NN-SL) to elucidate its toxin gene diversity and venom variability. The neutralization efficacy of a commonly used Indian antivenom product in Sri Lanka was examined against the lethality induced by NN-SL venom in mice. The transcriptomic study revealed high expression of 22 toxin genes families in NN-SL, constituting 46.55% of total transcript abundance. Three-finger toxins (3FTX) were the most diversely and abundantly expressed (87.54% of toxin gene expression), consistent with the dominance of 3FTX in the venom proteome (72.19% of total venom proteins). The 3FTX were predominantly S-type cytotoxins/cardiotoxins (CTX) and α-neurotoxins of long-chain or short-chain subtypes (α-NTX). CTX and α-NTX are implicated in local tissue necrosis and fatal neuromuscular paralysis, respectively, in envenomation caused by NN-SL. Intra-species variations in the toxin gene sequences and expression levels were apparent between NN-SL and other geographical specimens of N. naja, suggesting potential antigenic diversity that impacts antivenom effectiveness. This was demonstrated by limited potency (0.74 mg venom/ml antivenom) of the Indian polyvalent antivenom (VPAV) in neutralizing the NN-SL venom. A pan-regional antivenom with improved efficacy to treat N. naja envenomation is needed.