Key Approach In Increasing Antivenom Efficacy

Published Date: 02 Nov 2017

One of the first investigations into synthetic DNA-based toxin-specific antivenoms was produced by Harrison, et al (2000), within this experiment mice were immunised by a GeneGun loaded with DNA encoding for the carboxy-terminal JD9 domain of Jararhagin a metalloprotease found within Bothrops jararaca venom. The authors reported that the high-titre IgG antibody from these mice exclusively neutralised the lethal component of B. jararaca venom. It was noted that this new novel approach produced antivenoms with a greater immunological specificity compared within more traditional methods of antivenom production, leading to the belief that synthetic antivenoms could in fact be achieved with more than productive outcomes. In a follow up to the above paper Harrison, et al (2003) suggested through the use of bioinformatics that the JD9-specific antibody produced from B. jararaca venom possessed polyspecific immunological reactivity to venom components of a variety of snake species and genera. Outlining the potential of DNA immunisation to generate toxin-specific antibodies with polyspecific cover, however it was noted that a toxin-specific approach to future antivenom development would require an in depth knowledge of the target molecules compared to more traditional antivenom methods. The cross-reactivity between species witnessed within this investigation lead researchers to believe that the use of bioinformatic tools could be utilised to identify cross-specifically and cross-generically conserved regions of venom toxins that could be used to generate IgGs to neutralise the pathological effects associated with venoms regardless of phylogeny (Harrison, et al: 2011). While there is no doubt that the findings within Harrison, et al (2000) were more than promising, there were some issues associated with the fact that this technique relied heavily on PCR amplification of certain known toxins. The main issue being that this approach was restricted largely by primer design which in itself is decidedly subjective, it is for this reason that research has now progressed to integrate the use of venom gland transcriptomes (Harrison, et al: 2011).

An example of this technique can be seen within Wagstaff and Harrison (2006), within this paper the authors addressed issues surrounding the poor understanding of the contributing factors inducing the major haemorrhagic and coagulopathic pathology of Echis ocellatus venom. They created a toxin transcriptome based on 1000 expressed sequence tags (EST) from a cDNA library created from the pooled venom glands of 10 individual E. ocellatus snakes. This was then analysed using a variety of bioinformatic tools. The conclusions made within this report suggested that there was an abundant and diverse expression of snake venom metalloproteinases (SVMP) alongside a broad toxin-expression profile including important toxins such as phospholipase A2, C-type lectins and serine proteinases; making up the primary pathological toxins within E. ocellatus venom. This EST data provided an innovative data resource that allowed the future development of rational immunotherapies and a greater understanding of the composition of E. ocellatus venom. The authors also noted that since these venom toxins displayed a greater than 70% homology with orthologous there was the possibility to create toxin-specific antivenoms with a wide geographical cover, further backing up the findings made by Harrison, et al (2003). On an endnote the authors suggested that this paper created a greater knowledge of structure and function of venom toxins that could be used within the future in the rational design of toxin-specific antivenoms. Indeed it is this methodology that has been used within papers since to concentrate investigations into toxin-specific antivenoms, an example of this can be seen within Wagstaff, et al (2006). Through the isolation of EST data compiled from the venom gland of E. ocellatus, the authors developed a bioinformatic strategy that identified sequences encoding immunogenic and structurally important epitopes in efforts to design more rational toxin-specific antivenom. The focus of this investigation was SVMPs, identifying several parts of these genes that were considered clinically important and incorporated them into a single synthetic multiepitope DNA immunogen. Through the comparison of specificity and toxin-neutralizing efficacy of antiserum raised against this epitope string with antisera raised against a single SVMP toxin and traditional whole venom immunization protocols. The authors concluded that the SVMP string antiserum contained antibody specificities to several SVMPs within E. ocellatus venom and within the venoms of other African vipers, it was also noted that the antiserum cross-specifically reacted with venoms from the more distantly related Cerastes cerastes. The authors suggested that these findings outline the possibility of using cutting edge genetic techniques to design highly specific toxin-neutralising polyspecific antisera, which could be used regardless of phylogenic or geographical barriers.

All of these papers while only a small selection of the research under taken in efforts to create such toxin-specific antivenoms outline the substantial progress that has been made in regards to the creation of these novel therapies. Research has evolved to such a point where epitope-string therapies have been designed to produce IgGs specific to each of the major pathogenic toxins found within African Echis viper venom. These have been raised with the intent of creating polyspecific antivenom which will contain toxin-specific IgGs predetermined by their proportional representation within the proteome of the Echis species targeted (Harrison, et al: 2011). However while extremely encouraging much like other strategies surrounding the creation and implementation of antivenoms this technique does still have its challenges. Some of the challenges facing the application of this approach include obtaining venom gland transcriptomic resources from all medically important snakes, deciding which toxins provide the most attractive targets for IgG neutralization, the identification of phylogenetically conserved epitopes for all pathogenic toxins and the creation of systems to produce and deliver the identified epitopes as IgGs with the capability of neutralising the pathology of venom toxins. The ability to find solutions to these issues alongside the results for comparisons of pre-clinical and clinical efficacy with existing antivenoms and analysis of whether the cost and benefit of the toxin-specific, epitope-string technique will appeal to manufactures will essentially dictate the future success of this novel approach (Harrison, et al: 2011).

In conclusion the possibility of creating toxin-specific antivenoms is undoubtedly a primary aspect in efforts to improve antivenom productivity. This concept has developed rapidly within the last decade and has led to the realisation that this technique is more than viable as novel therapeutic treatment against the pathology of snake venoms. However with challenges like those highlighted above, this is a technique that is still not ready for practical use as a therapeutic treatment for snake bites. Nonetheless hopefully with further funding and developments in research epitope-string, toxin-specific antivenoms could be implemented as viable therapeutic strategies which are cheaper to produce and more effective relative to their more traditional counterparts.

This in turn could make antivenoms more readily available potently saving thousands of lives annually and possibly making the global problem of snakebite a more manageable burden within the not too distant future.