A team of experts spent years developing an antibody they believed would meet the standards for an effective, safe and efficacious pit viper anti-venom – which turned out to be an unsuccessful, yet invaluable experience.
Snakebites kill more than 100 000 people annually – and hundreds of thousands of survivors are left with long-term disabilities like amputations, write Christoffer Sørensen, Andreas Hougaard Laustsen, Bruno Lomonte and Julián Fernández in The Conversation.
Africa, Asia and Latin America are the regions most heavily affected. The most venomous snakes in Africa are the black mamba, cobras and saw-scaled and carpet vipers. In Asia, the Indian cobra, Russell’s viper, saw-scaled viper and common krait are the most venomous.
In the Central America and northern South America regions, the venomous pit viper Bothrops asper is responsible for most of the fatal and harmful bites.
As venom and anti-venom specialists who spent four years developing a therapeutic antibody to mitigate the effects of the pit viper’s bites, we were certain we’d met all the standards for an effective, safe and efficacious anti-venom.
But, at the last hurdle, we realised the antibody didn’t neutralise the snake’s toxins: in fact, it enhanced them, worsening the venom’s effects.
Initially this was, of course, very disappointing. But it was also a valuable lesson. By reporting this new way that future anti-venoms can fail, we have highlighted a problem with the current recommendations for testing anti-venoms that was – until now – hidden.
Our lesson is likely to have a much larger impact on the development of snakebite treatments than if the antibody had been a success, because the discovery will help anti-venom researchers focus their efforts so they don’t fail at the last hurdle as we did.
Developing our anti-venom
A large percentage of B. asper’s venom consists of potent muscle-damaging molecules called phospholipases A₂ (PLA₂s) and PLA₂-like toxins. These have severe effects, often leading to irreversible damage and disability.
Myotoxin II, a formidable PLA₂-like toxin within B. asper’s arsenal, is particularly significant. The precise mechanisms that underlie myotoxin II’s action aren’t fully understood. It is known to exert its effects locally, binding to muscle fibres and triggering muscle damage.
This localised action poses a challenge for traditional anti-venom treatments.
We have attempted to develop human monoclonal antibodies that target and neutralise this membrane-disrupting myotoxin II. For the first four years of our research project, the antibodies we discovered kept showing impressive effects in neutralising myotoxin II.
Even when tested in living mice, using the current gold standard for anti-venom testing, the antibodies continuously showed impressive neutralisation.
However, for our most promising antibody, we wanted to go a step further and carry out an experiment that more closely resembled a human envenoming, in which the antibody is injected after injection of the venom.
The results of this additional experiment were equal parts disappointing and surprising. Our most promising antibody in this last experiment changed its toxin-neutralising effect to toxin-enhancing instead, as we’ve documented in a research paper.
The results were so surprising that we decided to immediately repeat the experiment. We thought something must’ve gone wrong, like the antibody or other materials having gone bad. However, the results remained the same.
This curious phenomenon, which we termed “antibody-dependent enhancement of toxicity”, represents a novel discovery in toxin immunology. Similar phenomena have been observed in other contexts, such as with poisonous mushrooms and bacterial toxins, but never before with toxins from the animal kingdom.
Additional studies will be needed to fully understand what causes antibody-dependent enhancement of toxicity.
Reassessing preclinical models
There’s good news about this failure. It’s a chance for anti-venom researchers all over the world, no matter what snake species they’re working with, to reassess their preclinical models (like the current gold standard model).
We also think anti-venom researchers should consider incorporating more sophisticated experiments, like the ones used in our study, which more closely resemble a real-life envenoming case.
By doing so, the anti-venom research community can streamline the drug discovery process. This will expedite the identification and development of safer and more effective snakebite treatments.
Christoffer Vinther Sørensen – Postdoctoral researcher, Department of Biotechnology and Biomedicine, Center for Antibody Technologies, Technical University of Denmark; Andreas Hougaard Laustsen – Associate Professor at the Department of Biotechnology and Biomedicine, Technical University of Denmark; Bruno Lomonte – Emeritus Professor, Instituto Clodomiro Picado, Universidad de Costa Rica; Julián Fernández – Researcher at Instituto Clodomiro Picado, Universidad de Costa Rica.
Study details
Antibody-dependent enhancement of toxicity of myotoxin II from Bothrops asper
Christoffer V. Sørensen, Julián Fernández, Helen Wildenauer, Andreas Laustsen et al
Published in Nature Communications on 16 January 2024
Abstract
Improved therapies are needed against snakebite envenoming, which kills and permanently disables thousands of people each year. Recently developed neutralising monoclonal antibodies against several snake toxins have shown promise in preclinical rodent models. Here, we use phage display technology to discover a human monoclonal antibody and show that this antibody causes antibody-dependent enhancement of toxicity (ADET) of myotoxin II from the venomous pit viper, Bothrops asper, in a mouse model of envenoming that mimics a snakebite. While clinical ADET related to snake venom has not yet been reported in humans, this report of ADET of a toxin from the animal kingdom highlights the necessity of assessing even well-known antibody formats in representative preclinical models to evaluate their therapeutic utility against toxins or venoms. This is essential to avoid potential deleterious effects as exemplified in the present study.
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Snakebite: ‘The world’s biggest hidden health crisis’
Broad-spectrum snake venom antidote developed
SA feels the sting of snake anti-venom shortage