Repurposing or repositioning drugs is the process of using a drug that is already in use for a disease it was not initially indicated for1. There are both benefits and challenges to repurposing drugs, but in general this strategy has proved useful and has led to life-saving treatments. It has been estimated that almost 30% of drugs approved by the US Food and Drug Administration are repurposed2.
Bringing a new drug to market costs pharmaceutical companies a considerable amount of money and time, which is usually spent developing the drug preclinically and then carrying out numerous clinical trials. Repurposing a drug has a few advantages over developing a drug de novo. One of the main advantages is that the risk of failure is much lower if the drug has already been through clinical trials that have demonstrated that it is safe for human use3. In addition, repurposing drugs can save a huge amount of time and money. Thus, getting a novel drug to market currently takes on average around 15 years and costs between $2 billion and $3 billion, whereas it is estimated that repositioning a drug costs on average $300 million and takes around 6.5 years, as preclinical phase 1 and occasionally phase 2 trials can be omitted4. Finally, repurposing drugs can reveal new targets and pathways that can be targeted when developing drugs in the future3. Nonetheless, even if a drug has been previously shown to be safe for treating a particular disease it may not be safe when indicated for another, since it may need to be administered at a different concentration or by a different route.
Now for some famous examples of repurposed drugs that you are likely to have heard of:
Probably the most successful of all repurposed medicines is Sildenafil, which is more commonly known by its brand name Viagra. It was initially researched as a potential antihypertensive drug, but after showing effective inhibition of platelet aggregation by inhibiting phosphodiesterase type 5 (PDE5) it was developed to treat angina. Finally, it was established as an effective treatment for erectile dysfunction in 1998 and has since generated sales of over $2 billion5. More recently, additional therapeutic targets for Sildenafil and other PDE inhibitors have been identified and there is good evidence to suggest that they can be repurposed to treat a range of cancers6.
Another example of a well-known repurposed drug is thalidomide. This notorious drug was initially used to treat anxiety and morning sickness but was banned in 1961 due to its teratogenicity. It has since been approved for the treatment of the rare disease leprosy as well as multiple cancers. This shows that even drugs that failed in their initial usage can be repurposed effectively5.
One of the most famous drugs that has been repurposed is aspirin. It was originally marketed in 1899 as an analgesic, which is still its main use, but in the 1980s it was successfully used as a blood thinning medicine5. Now there is research being undertaken to see if aspirin could be used in the treatment of triple-negative breast cancer, in combination with the immunotherapy drug Avelumab7. According to the Repurposing Drugs in Oncology (ReDO) project, there are currently more than 300 non-cancer drugs that have shown some published evidence of anticancer effects and at least some of these could be developed to treat a range of cancers8.
A recently repurposed drug is rapamycin, which was initially approved as an immunosuppressant for kidney transplant patients. Since 2015, rapamycin has been used to treat lymphangioleiomyomatosis (LAM), a rare progressive lung disease that predominantly affects young women. This is a major breakthrough because before rapamycin was approved to treat LAM, the only treatment was lung transplantation. Rapamycin has now mostly replaced the need for lung transplantation, improving and saving lives of many women living with the condition9.
The COVID-19 pandemic has also shown the importance of repurposing drugs. Dexamethasone, a corticosteroid used to treat inflammatory diseases, made headlines in summer 2020 as it was shown to reduce fatality in hospitalised COVID-19 patients10. It is often used in combination with Tocilizumab, another repurposed immunosuppressive drug that is usually indicated for arthritis. Tocilizumab was shown to be effective at treating COVID-19 pneumonia in hospitalised patients and reduced the risk of death by 14%11.
In 2017 the NHS released a report called ‘Facilitating adoption of off-patent, repurposed medicines into NHS clinical practice’, which identified the benefits of repurposing drugs11. This resulted in the Repurposing Medicines Programme being set up with the aim of clinical use by the NHS of cost-effective drugs that are currently unlicensed or off-label. By doing this there will hopefully be more opportunities to repurpose drugs successfully and create more effective treatments that are widely available within the NHS budget, thus saving lives.
Despite all these positives of repurposing drugs, there is evidence that success rates have been overestimated, and in 2018 it was reported that it is unlikely that a drug will be successfully repurposed, especially if it is intended to be indicated for a different therapeutic area than was initially intended12. It may therefore be a risk for pharmaceutical companies to put too much effort into repurposing drugs, especially since many of the discoveries of the most successful repurposed drugs were serendipitous. However, this may be because programmes focusing on repurposing drugs have only come into effect relatively recently and there is a need to follow up analysis in the next few years.
Overall, drug repurposing is an important effort that has many benefits, and with more research and delivery of programmes promoting new therapeutic use for existing drugs, there is potential for many drugs to be used to treat diseases they were not initially indicated for. This has incredible potential to improve lives globally.
 Osakwe, O. and Rizvi, S., 2016. Introduction. Social Aspects of Drug Discovery, Development and Commercialization, [online] p.xvii-xxx. Available at: <https://www.sciencedirect.com/science/article/pii/B978012802220700017X> [Accessed 16 September 2021].
 Rudrapal, M., J. Khairnar, S. and G. Jadhav, A., 2020. Drug Repurposing (DR): An Emerging Approach in Drug Discovery. Drug Repurposing – Hypothesis, Molecular Aspects and Therapeutic Applications, [online] Available at: <https://www.intechopen.com/chapters/72744> [Accessed 16 September 2021].
 Pushpakom, S., Iorio, F., Eyers, P., Escott, K., Hopper, S., Wells, A., Doig, A., Guilliams, T., Latimer, J., McNamee, C., Norris, A., Sanseau, P., Cavalla, D. and Pirmohamed, M., 2018. Drug repurposing: progress, challenges and recommendations. Nature Reviews Drug Discovery, [online] 18(1), pp.41-58. Available at: <https://www.nature.com/articles/nrd.2018.168> [Accessed 16 September 2021].
 Nosengo, N., 2016. Can you teach old drugs new tricks?. Nature, [online] 534(7607), pp.314-316. Available at: <https://www.nature.com/articles/534314a> [Accessed 16 September 2021].
 Jourdan, J., Bureau, R., Rochais, C. and Dallemagne, P., 2020. Drug repositioning: a brief overview. Journal of Pharmacy and Pharmacology, [online] 72(9), pp.1145-1151. Available at: <https://onlinelibrary.wiley.com/doi/full/10.1111/jphp.13273> [Accessed 16 September 2021].
 Cruz-Burgos, M., Losada-Garcia, A., Cruz-Hernández, C., Cortés-Ramírez, S., Camacho-Arroyo, I., Gonzalez-Covarrubias, V., Morales-Pacheco, M., Trujillo-Bornios, S. and Rodríguez-Dorantes, M., 2021. New Approaches in Oncology for Repositioning Drugs: The Case of PDE5 Inhibitor Sildenafil. Frontiers in Oncology, [online] 11. Available at: <https://www.frontiersin.org/articles/10.3389/fonc.2021.627229/full> [Accessed 16 September 2021].
 the Guardian. 2021. Aspirin trialled as potential treatment for aggressive breast cancer. [online] Available at: <https://www.theguardian.com/science/2021/aug/18/aspirin-trialled-as-potential-treatment-for-aggressive-breast-cancer> [Accessed 16 September 2021].
 Anticancerfund. 2021. Drug repurposing. [online] Available at: <https://www.anticancerfund.org/en/redo-db> [Accessed 16 September 2021].
 Crombie, C., 2021. Repurposing drugs: the opportunity and the challenges. [online] Association of Medical Research Charities. Available at: <https://www.amrc.org.uk/blog/repurposing-drugs-the-opportunity-and-the-challenges> [Accessed 16 September 2021].
 New England Journal of Medicine, 2021. Dexamethasone in Hospitalized Patients with Covid-19. [online] 384(8), pp.693-704. Available at: <https://www.nejm.org/doi/full/10.1056/NEJMoa2021436> [Accessed 16 September 2021].
 England, N., n.d. NHS England » Repurposing medicines in the NHS in England. [online] England.nhs.uk. Available at: <https://www.england.nhs.uk/medicines-2/medicines-repurposing-programme/> [Accessed 16 September 2021].
 Oraiopoulos, N., 2018. Limits of drug repurposing – News & insight – Cambridge Judge Business School. [online] Cambridge Judge Business School. Available at: <https://www.jbs.cam.ac.uk/insight/2018/limits-of-drug-repurposing/> [Accessed 16 September 2021].