Snake bite antivenom assays
Key takeaway
Background
Antivenoms are unique in that they currently solely rely on animal models to predict their potential efficacy in humans. The long-established World Health Organisation endorsed ‘gold standard’ antivenom testing protocol is crude, involving the injection of premixed antivenom and an above-lethal dose of venom into a mouse. The efficacy of the antivenom is then determined based on mouse survival after 6 or 24 hours. The assay is listed as a requirement by many national regulatory agencies and the national pharmacopoeia as proof of antivenom efficacy before a product is approved for clinical use. Some countries have implemented humane endpoints and analgesia in these assays, but this is not common practice. Nor do these refinements ameliorate the severe suffering, distress, and pathological effects, e.g. breathing difficulties, bleeding, organ damage and paralysis. However, there are currently no alternative assays or non-animal alternatives due to the complex nature of venom, so this assay has been the mainstay for over 40 years, and has fulfilled a critical role in testing antivenoms that have saved many human lives.
Despite this, there are clearly ethical issues surrounding the use of this model, and also issues with the scientific validity. The assay does not reflect real-world envenoming, as venom would never be premixed or injected directly. The premixing also makes detailed assessments of the antivenom’s pharmacokinetic properties nearly impossible. This approach risks overestimating the effectiveness of an antivenom, which has unfortunately led to catastrophic product failures and loss of life.
Refinements
- Developing a new, non-lethal endpoint model which is sensitive enough to assess antivenom efficacy whilst minimising or negating severe suffering. This model uses the smallest possible dose of venom that can induce measurable changes in biomarkers. For example, measurements of blood fibrinogen, a protein which helps blood clots to form, and prothrombin time, a measurement of how long it takes for clots to form in a sample, both of which are greatly affected by particular venoms, can provide useful information on the effectiveness of the antivenom. These tests are also used to clinically assess human snakebite cases and their responses to antivenom. The use of these assays enables much more detailed and rigorous data versus the current binary, and not very informative, ‘alive or dead’ readout.
- Incorporating non-invasive monitoring of vital signs to give further data on subtle physiological changes.
Implications
Using this method, mice show no outward signs of envenoming despite quantifiable changes in blood chemistry, and the actual severity of mice undergoing this procedure is now mild or moderate.
Image courtesy of Simon Townsley