**1. Introduction**

Drug discovery is a multifaceted process that aims at identifying a therapeutic agent that can be useful in treating and managing certain medical conditions. This process includes identification of candidates, characterization, validation, optimization, screening, and assays for therapeutic effectiveness. If a molecule achieves acceptable results in these studies, then the molecule has to go through drug development processes and be recruited to clinical trials [1]. Several drug candidates (about 90%) have collapsed in early clinical trials due to unexpected results such as adverse effects or inadequate effectiveness [2, 3]. Drug development is probably among the most complicated and challenging processes in biomedical research. Apart from the already enormous complexities underlying pharmacological drug designs, additional significant challenges arise from clinical, regulatory, intellectual property, and commercial constraints. Such as challenging atmosphere has made the drug development process very sluggish and unpredictable [4]. The process of discovering and developing a new drug is a lengthy and expensive process taking somewhere from 10 to 15 years and costs about US\$2–3 billion [1]. Despite massive sums of money being spent on drug development, no substantial rise in the new therapeutic drug agents in a clinical setting has been observed over several decades. Although overall global R&D spending for drug discovery has risen 10-fold from 1975 (the US \$4 billion) to 2009 (\$40 billion), the number of novel molecular entities (NMEs) approved has stayed essentially constant since 1975 (26 new drugs approved in 1976 and 27 new drugs approved in 2013) [5].

The essential step in discovering new drugs involves the evaluation of the safety and effectiveness of new drug candidates in human subjects, and it consists of four phases. In Phase I clinical trial, the candidate drug's safety is assessed in a small population (20–80 individuals) to establish safe dose range and uncover adverse effects. Phase II involves the examination of intervention for its effectiveness and safety in large populations (a few hundred people). Phase III further involves the assessment of drug efficacy in a large population (several thousand) and compares new drug candidates with standard or experimental treatments. Phase IV is conducted when the intervention is marketed. This study aims to track how well the approved treatment is performing in the general population and gather data on side effects that may arise from broad usage over time. Phase III studies determine whether or not a medication is effective, and if so, FDA clearance is granted. The FDA approves one anticancer treatment out of every 5000–10,000 applicants, and just 5% of oncology medicines entering Phase I clinical trials are approved in the end. Because of the increased cost and time frame for new medication development in recent years, patients with severe illness may die until alternative therapies are available if they develop resistance to current therapy [6]. In searching for an alternative treatment option for managing various diseases, including cancer, the researchers have shifted their focus to drug repurposing strategies.

The drug purposing or drug reprofiling or drug redesigning process explores the therapeutic use of existing clinically approved, off patent drugs with known targets for another indication to minimize the cost of therapy, time, and risk [7]. The huge benefit of drug repurposing is that the efficacy, pharmacokinetics, pharmacodynamics, and toxicity characteristics have previously been explored in preclinical and Phase I investigation. These drug moieties may thus be quickly made to proceed to Phase II and Phase III clinical trials, and hence related developmental costs might be substantially lowered [6, 8]. The failure risk in drug development is low because *in vitro* screening, *in vivo* screening, toxicity profile, chemical optimization, and formulation development have already been accomplished. Therefore, drug repurposing has made the pharmaceutical industry a desirable choice for investors. So the pharmaceutical companies and researchers have begun to make significant investments in drug repurposing, which offers a tremendous benefit over *de novo* drug design and development [9]. Therefore this new approach of drug purposing has reduced the timeline and cost of the drug development, notably in the case of FDA-approved repurposed pharmaceuticals, which will undergo faster clinical

**Figure 1.** *Outline of developing new drug versus repurposing.*

#### **Figure 2.**

*The time taken by the conventional of process of drug development with respect to drug repurposing. Conventional drug development process takes around 5 years and the same can be minimized to 5 years.*

trials because of already well-known safety and toxicological profile [10]. Outline of developing new drug verses repurposing is represented in **Figure 1**.

The development of new drugs for breast cancer like any other cancer is a multistep process that includes drug designing, synthesis, characterization, safety and efficacy assessment, and finally, regulatory approval (**Figure 2**). The overall process is very lengthy and involves significant financial expenditure [11]. Further, the sky-high cost of the therapies and associated side effects make it desirable to look for other approaches to manage cancer effectively. Therefore, concurrently with the synthesis and design of new therapeutic modalities, various strategies should be considered for repurposing various already approved drugs that may target this deadly disease.
