Abstract
Drug repurposing refers to finding a new indication for a pre-existing drug. It is a novel way of drug discovery that greatly reduces the time and money spent in the drug discovery process. This approach is associated with a better chance of successful drug approval. Both previously approved drugs as well as drugs that have failed in the trials conducted for their original indication can be repurposed. Even drugs withdrawn from market for their original indication can be repurposed for a new indication. Starting with Sildenafil which is the oldest example of repurposed drug to the recently repurposed drug tocilizumab for COVID-19, the list of repurposed drugs is a big one. The regulatory pathway to be followed for a repurposed drug is different from that for a new chemical entity. Furthermore, the period of marketing exclusivity for repurposed drug is only 3 years as against the 20 years of patent protection period for new drug. The strategy for drug repurposing may be a serendipitous one or hypothesis driven one. The hypothesis driven strategy includes the experimental and computational approaches. Computational approaches for drug discovery, especially the Connectivity map approach, offer a lot of scope to understand the drug-disease-gene link, thereby acting as a kick-starter for drug repurposing. Drug repurposing has real potential to offer a cure for rare genetic conditions and cancers. This review covers the various drug repurposing approaches in detail, the regulatory pathway for repurposed drugs, salient examples of repurposed drugs and also the challenges associated with drug repurposing.