**2. Identification of antiviral drug repurposing**

The drug repurposing approach is the process of finding new indications for existing FDA-approved drugs, is promising alternative to accelerate the process of drug development for infectious diseases and many other disorders and diseases, DR can be pursued by three possible strategies which are as follows.

**Pre-clinical studies:** This study was conducted in-vitro or in-vivo animal models before proceeding towards the clinical studies in the patient, whether healthy or sick. This also includes in silico in-vitro such as receptor-binding sassy to identify drug ligands and studies using cell line or tissue excised from animal or human representative of the specific disease, to examine how biological milieus of varying complexity response to the drug. In-vivo is also conduct in animals to test the biological parameters by sacrificing an animal for histological behaviors in rats or mice, here the available preclinical model can be used for repurposing to generate a hypothesis, this study is a preliminary study which is the base for setting up the clinical studies.

**Clinical studies:** Ranging from single-arm open-label **trial** to randomized controlled trial (RCTs) provides information on drug tolerance and efficacy, the notable interplay between regulatory and government bodies and research conducting clinical trial, as such entity have role in regulating clinical trial.

**Observational studies:** This study provides evidence to the findings it relays on data that is existing or data collected quickly in a prospective manner using previously established data system and conducted rapidly. The main aim of observational study evidence generation concerned drugs which use off-label for COVID-19 for example retrospective observational studies during the pandemic in Italy hospital. The limitation of observational studies is often based on secondary data. Refer **Figure 2**.


*Reprofiling of Octogenarian Antiviral Agent: A New Avenue Venture to Discover Viral Infection DOI: http://dx.doi.org/10.5772/intechopen.102825*

#### **Figure 2.**

*FDA-approved formulation proceeds through 3- strategy steps, identified molecule-based target similarity hypothesis on viral pathway, absence of activity shows poly-pharmacology interference with different function (cellular or viral).*


**Example-1:** Here approved drugs were exploited as an antiviral therapeutic agent (new indication).

**Figure 3.** *FDA-approved multiply-acting drugs.*

**Example-2:** Anticancer agents such as imatinib were found to be active against coronaviruses by inhibiting cellular Abelson (ABL) kinase

• **New target- new indication:** Here approved drugs were established with bioactivity in specific pathway/mechanism with new molecular target [Poly-Pharmacology (ability of approved drugs or pharmaceutical molecule to act on multiple targets might be toxicity or side effect) represent important opportunity to repurposing] molecule essential for virus replication.

**Example-1:** Antimicrobial agent which found to have targeted virus-infected cells as well as inhibition of viral replication (teicoplanin, ivermectin, itraconazole, and niazoxanide).

**Example-2:** Recently used SARS-CoV-2 and HIV drug to combat morbidity and mortality causes of viruses' outbreak examples of drugs are given (**Figure 3**).

Mentioned chemical structures are promising re-profiled candidates of FDAapproved drugs with its multiple actions, [3].
