**2. Need for drug repurposing**

Nobel Laureate Sir James Whyte Black (1924–2010) had once said that "the most fruitful basis for the discovery of a new drug is to start with an old drug" [7]. However, the systematic screening approach introduced by Paul Ehrlich became the cornerstone of antibiotic search strategies for pharmaceutical industries and along with further advancement in *de novo* drug design methods, various potential novel classes of antibiotics were discovered. Nevertheless, the rate of discovery of a novel

*Traditional* de novo *drug discovery process versus drug repurposing process.*

*Trends in Molecular Aspects and Therapeutic Applications of Drug Repurposing for Infectious… DOI: http://dx.doi.org/10.5772/intechopen.100858*

class of drugs suddenly dropped by 1970 with the increasing rate of resistance [8]. Even with all the scientific tools of traditional methods of drug discovery such as 'omics (genomics, proteomics, and metabolomics), virtual high throughput screening (vHTS), phenotypic, and whole cell-based high throughput screening (HTS), no new class of antibiotics are getting discovered [8]. On the contrary, there is an overall increase in the expenses leading to a collapse in the economic model of antibiotic drug discovery research [2]. Therefore, a change in the financial models is required to translate scientific advances into clinically approved antibiotics [9]. Drug repurposing is the best possible way to escape from this dilemma and reposition the drug candidates from the approved pharmacopeia. Drug repurposing offers great advantages over traditional drug discovery methods such as no chemical optimization and reduced developmental risk because the drug candidates have often been through several stages of preclinical and clinical trials and therefore have well-known toxicological safety and pharmacokinetics profile. Even formulation stages and bulk manufacturing are also bypassed, enabling a shorter route to the market [3]. A comparison of traditional *de novo* drug discovery versus drug repurposing is summarized in **Figure 1**.
