**1. Introduction**

Repurposing of a drug is the process of reutilizing already utilized drugs for other treatment purposes. It is the use of a known drug for treating conditions other than their primary use [1]. Drug repurposing encourages disease-related drug development in a much cheaper, faster, and more accessible way for patients [2]. The drug studied for repurposing is the shelved drugs, drugs in use, discontinued drugs, and experimental drugs that either could not make it to the late phases of clinical trials or have failed in the market. Because the efficacy, safety, and toxicity of these drugs have already been established, the preliminary phases of the clinical trials can be omitted, minimizing the cost and length of the clinical trials. It takes about 15 years to deliver a new drug to the market [3]. The objective of drug repurposing is to identify new biological targets and different therapeutic uses of previously approved and/or investigational drugs, including drugs that did not meet primary therapeutic expectations. As such, a number of pre-clinical development and optimization issues, including negative toxicological profiles, can be avoided or at least minimized. Although most successful experiments in reallocating drugs are derived from coincidence, current research efforts focus on predicting opportunities for reallocating on rational grounds [4]. Interestingly, while most drug reallocation campaigns rely on chemical-based compounds, natural products can offer important opportunities. Natural products are characterized by unique and favorable properties, considerable structural diversity, and a large number of pharmacological activities [5]. Therefore, these are chemical entities preferred for the (re)discovery of medicines. Strategies that may bring to light new therapeutic uses that may not be related to their original biological space [6].

In the drug repurposing process, there are three important processes that are involved. They include (i) identification of the targets of interest for a new indication, (ii) assessment of mode of action intricate in drug or ailment of study, and (iii) establishment of the drug the usefulness in the second and third phases of a clinical trial. Of all the stages, finding a principal candidate is one of the most important. This is the stage where the most advanced and efficient techniques are required to be involved in generating new hypotheses in the reallocation of drugs. Drugs can be repurposed in multiple ways, which may be either experimentally, clinical-based, or computationally. The computational approach is an "*in-silico*" repurposing of drugs, which is divided into two sub-categories: centered drugs or diseases. Under the drug-based approach, we find new indications for existing drugs, while under the disease-based approach, we try new drugs for an existing disease (**Figure 1**) [7].

Recently, natural products have seen a revival of awareness in drug discovery, with a different approach. Newer and evolving technologies, such as computational screening, proteomics, metabolomics and big data analysis, have come to the fore to drive and speed up the "repurposing" of natural compounds and, more generally speaking, of nature-inspired compounds [8].

Even though a large number of natural product formulations are available as extracts, the phytocompounds components of these extracts can be utilized in drug repurposing, with the utilization of Computer-Aided Drug Design, after isolation, purification, and structural elucidation.

**Figure 1.** *Drug repurposing approaches.*

Approaches for Speeding Up Drug Repurposing


Therapeutic inhibitors are agents, compounds that could be of synthetic or natural source, with the ability to trigger the down-regulation or block the expression or overexpression of an enzyme or protein, or block protein-protein interactions or block the addition of phosphates to other molecules, thereby inducing therapeutic effect(s). Therapeutic inhibitors perform their functions either directly or indirectly by affecting the catalytic properties of the active site. Inhibitors can be extraneous to the cell or normal constituents of it. Inhibitors which are a normal component of a cell, can represent a significant component of the regulation of cell metabolism. Many toxins and also pharmacologically active agents (both illegal drugs and prescription and over-the-counter medicines) act by inhibiting specific enzymecatalyzed processes [12], which can be targeted *in-silico* using computer-aided drug design in the process of new therapeutic inhibitor development from natural products. There are thousands of natural products existing in natural product databases that can be utilized for this purpose. The protein and enzyme target are also readily available in protein databases in formats needed for computer simulation studies [13].
