Use of In Vivo Models in Preclinical Drug Discovery and Development

This chapter deals with the in vivo preclinical studies involving the COVID19 zebrafish model conducted on the Ayurvedic medicines mentioned in the last chapter to further validate their efficacy against COVID-19. Animal models are needed in order to understand the disease's progress and associated symptoms. While it is possible to understand the disease characteristics based on historical evidence and previous research on similar disease-causing organisms, newer species of diseasecausing agents have been recently discovered. These newer organisms without any previous history, pose the biggest challenge in drug discovery and development. In these cases, the use of relevant animal models of disease becomes important in order to understand the disease progression as well as the interaction of the body with the disease-causing agent. In the current SARS-CoV-2 infections, the virus is potentially lethal in humans. In such cases, the danger of using humans to test new drugs becomes ethically unacceptable unless the drug has been tested in animal models against the virus. The use of higher primates, like monkeys, or small animals like dogs and rodents, which are generally accepted pre-clinical models of drug discovery, has a myriad of ethical concerns. Despite this, several different models of SARS-CoV-2 infection are currently in use, ranging from non-human primates, such as rhesus macaques (Rhesus monkey), and rodent models, such as transgenic mice and hamsters. While it is difficult to incorporate all the different pathological features of the disease in a single model, it is important to choose the correct model animal in order to answer the primary question that the investigator seeks. For example, rodents lack the coagulopathy component, which is often seen in severe SARS-CoV-2 infections. By the same token, the narrow spectrum of viral infectivity and the inability to the crossspecies barrier by the virus is an important consideration while studying the disease pathology. This was seen in a rhesus monkey model where no overt clinical signs were detected even though prolonged viral shedding was detected in the upper respiratory tract of animals. With these issues in mind, we developed a humanized zebrafish model to test the efficacy of Coronil and Divya Swasari Vati in decreasing the pathogenic characteristics associated with SARS-CoV-2 spike protein expression. Zebrafish has proven to be a solid model system for investigating human viral pathophysiology, and various human viruses, including chikungunya and influenza, can colonize zebrafish, making it an appealing and alternative model system. Zebrafish have well-defined innate and adaptive immune systems that are strikingly comparable to those of humans. Unlike mouse models, zebrafish have swim bladders as buoyancy organs, and human cells could be transplanted into swim bladders to create xeno-transplanted humanized models for respiratory disorders, such as SARS-CoV-2 infection. The implantation of human lung cells into the zebrafish's air bladder increases the model's relevance and gives human-equivalent methods of inquiry. Different groups have successfully employed this strategy to replicate lung cancer in zebrafish as well as COPD and Pseudomonas aeruginosa pathogenesis. Results obtained from treating the humanized zebrafish model injected with the S protein of SARS-CoV-2 with either Coronil or Divya Swasari Vati are shared in this chapter, along with a proposed mode of action for both of these Ayurvedic formulations.

Keywords: Behavioural fever, Cytokine response, Humanized zebrafish, In vivo studies, Preclinical, SARS-CoV-2 S protein-induced inflammation.