**1. Introduction**

Antibiotic resistance is a major threat that may lead to approximately 10 million deaths per year by 2050 [1]. Nevertheless, pharmaceutical companies' entire economic model for antibiotic drug discovery has clashed with the low profitability index. An estimated cost of developing an antibiotic in 2017 was nearly US \$1.5 billion, whereas the average revenue generated per year is nearly US \$46 million, which cannot be justified in any way [2]. Therefore, in an attempt to accelerate the identification of potential and safe anti-infective drugs, reduce discovery research expenses, and minimize drug development timeline, "drug repurposing" and/or "drug repositioning" has arisen as an excellent alternative approach because the developer already has the complete pharmacological and toxicological data of the drug candidate from preclinical and clinical trials. Drug repurposing and/or repositioning simply mean new treatment indication or pharmacological use of an old drug [3]. For example, Aspirin, the first-ever drug repurposed, was originally indicated as an analgesic but later repurposed for various pharmacological effects such as anti-platelet in cardiovascular events [4].

"Drug repurposing," "drug repositioning," and "drug rescuing" are the terms generally used interchangeably; however, these terms may slightly differ from each other. Drug repurposing means, "approved drug for one disease is identified potentially useful and repurposed in another disease" such as aspirin, whereas drug repositioning explains a situation when "an approved drug for one disease is used as a template and derivatized to a different form for use in another disease" [5, 6]. Nevertheless, drug rescuing is the term given to the concept where "the clinically failed or market abandoned drugs for one clinical indication is rescued or used for another clinical indication" such as thalidomide which was banned initially but later rescued to multiple myeloma [5]. However, the ultimate goal remains the same and that is "repurposing of old drugs for new diseases."
