Measuring antiviral activity in a small animal model is an exciting confirmation of drug effectiveness, but it is not always possible, and regulators are aware of this. Often, there is no small animal model for the virus studied, and even when there is, it might not be representative of infection in human, making it difficult (if not impossible) to find correlatives of protection. In certain cases, the animal model can be too stringent and underestimate the effectiveness of a compound: it is important to be aware of this to avoid discarding a potentially useful drug too early. This is the case, for instance, when a virus that only causes non-lethal infection in human is instead lethal in mouse. An example is animal models for dengue or Zika, where high lethality even upon lowering of viral titres can mask the effect of the compound. It is, therefore, important to be aware of the limitations of these models and assess as many parameters as possible, including histological examination of multiple tissues, quantification of viral titers over time, isolation and characterization of resistant isolates in animals that experience viral rebound (more on this in the third part of this blog series), quantification of viral antigens and antibodies, and analysis of symptoms (e.g., behavioural, neurological, weight loss, and ruffling fur), as well as morbidity and mortality. Before clinical trials in human volunteers can begin, GLP-standard evaluation of toxicity is required in at least two animal species, generally a rodent and a non-rodent. The costs of testing in a non-rodent species are of course much higher, and they should only be undertaken when promising results from previous in vitro and in vivo studies have clearly highlighted the antiviral benefit.

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