**Notes/thanks/other declarations**

None.

*Drug Repurposing and Orphan Disease Therapeutics DOI: http://dx.doi.org/10.5772/intechopen.91941*

*Drug Repurposing - Hypothesis, Molecular Aspects and Therapeutic Applications*

representing large commercial possibility [35].

**6. Conclusion**

for repurposing.

None.

**Conflict of interest**

There are no conflicts of interests.

**Notes/thanks/other declarations**

Cytomegalovirus in 1998 but it has been withdrawn from the market due to the development of highly active antiretroviral therapy (HAART) but this was the "first antisense oligonucleotide therapy" [35]. Imatinib was approved by FDA in 2001 for Philadelphia chromosome-positive chronic myelogenous leukemia (CML) as "first targeted cancer therapy" [35]. Alipogene tiparvovec was the "first targeted gene therapy" to be approved in Europe in 2012 for reversing lipoprotein lipase deficiency (LPLD) in patients with pancreatitis [35]. Strimvelis was the "First ex-vivo gene therapy" approved for patients with Severe Combined Immunodeficiency due to Adenosine Deaminase deficiency (ADA-SCID) [35]. USFDA has approved Holoclar as orphan drug for the treatment of limbal stem cell deficiency (LSCD). Currently, this drug is in Phase IV trial and completing in mid of the year 2020 [36]. To overcome R&D-associated financial challenges and clinical trials-related failures of novel drugs, at present scenario, pharmaceutical companies are much more interested in switching to "drug repurposing" or "drug repositioning" drug development by adopting various computational approaches rather than going for de novo drug discovery, which is relatively expensive, risky and time consuming [37]. According to Southall et al. [35], the global market for drug repositioning will possibly be hiked to over \$31 billion by 2020, up from about \$24 billion in 2015, thus

*De novo* drug development is being time consuming, costly and risk-prone venture 'drug repurposing' has drawn attention of all pharma companies and R and D sectors which offers faster and cheaper ways for bringing new drugs into the market especially for targeting ODs. Based upon the benefits provided by drug repurposing, the development of more sophisticated and systematic approach is required to find more drug candidates that can be fitted into the picture of promising targets

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