Drug repurposing is the use of a drug in an indication other than the one that it was initially developed for. It seeks to discover new applications for an existing drug that were not previously referenced, are not currently prescribed, and have not been investigated.
This article looks at the advantages of drug repurposing with some familiar examples, and how AI identifies existing drugs that may be repurposed for new therapeutic uses.
Drug repurposing offers many advantages over conventional drug discovery, where it takes at least ten years and an average of $2.6 billion to get a new drug to the market. It builds upon previous research and development efforts, so R&D costs are considerably lower and, while clinical trials are still required to demonstrate the drug’s effectiveness in the new area, the notoriously expensive and lengthy phase 1 trials can be avoided as safety in humans has already been established.
There will also be plenty of previously generated data available, including comprehensive information on the drug’s pharmacology, dose, possible toxicity, and formulation. So therapies can be ready for phase 2 clinical trials quickly.
Numerous examples exist for the identification of new indications for existing drugs. Ideas for potential drug repurposing can come via unexpected routes, historically serendipitous side effects often provided the springboard for breakthrough therapies. For example, the first FDA-approved chemotherapy to treat leukemia was developed by repurposing mustard gas after researchers noticed that World War I soldiers who had been gassed had lower white blood cell counts.
Here are some other familiar examples of successful drug repurposing:
Probably the oldest and best known example of drug repositioning is aspirin or acetylsalicylic acid. Initially marketed by Bayer in 1899 as an analgesic, aspirin was repositioned in the 1980s as an antiplatelet aggregation drug to help prevent cardiovascular events, after it had been found to increase bleeding in tonsillectomy patients given it for pain relief.
Aspirin has been repositioned again since, this time in oncology, after taking aspirin daily for at least five years was shown to prevent the development of many cancers, particularly colorectal, due to COX‐2 inhibition, which prevents apoptosis in malignant cells.
Originally prescribed as an antiemetic for pregnant women, the World Health Organization (WHO) banned thalidomide in 1962 on discovery of the severe physical defects it caused to unborn fetuses. More than 10,000 children were affected in 46 countries, as well as thousands of miscarriages.
Then a 1964 study demonstrated thalidomide’s dramatic efficacy against an autoimmune complication of leprosy called erythema nodosum leprosum. It took until 1998 for thalidomide to be officially repositioned, with its use accompanied by strict contraceptive measures to ensure there was no exposure to the drug during pregnancy.Like aspirin, thalidomide was repositioned for a second time in the field of oncology. In 2006, studies showed the antiangiogenic activity that caused the severe limb development issues in utero was found to block or destroy blood vessels supplying malignant tumors. It’s now a first‐line treatment for multiple myeloma, with the same contraception precautions applied.
Sildenafil, best known as Viagra, was not originally intended to help with erectile dysfunction. This was an unexpected side effect of the drug that was developed by Pfizer in 1985 to treat high blood pressure and angina. This physiological effect led Pfizer to market sildenafil in 1998 under the brand name Viagra. Sildenafil was repositioned again in 2005, further exploiting the vasodilation it produces to treat a rare form and potentially fatal of pulmonary arterial hypertension. This time, it was rebranded as Revatio.
Botox is a neurotoxin that was originally approved to treat abnormal contractions in the eyelid muscles. It corrected the position of the eyes by blocking nerve signals and temporarily inhibiting contraction of targeted eye muscles. But then doctors administering botox injections observed how it reduced the appearance of wrinkles. The use of Botox as a muscle paralytic has been further extended to other muscle groups to relieve migraines and treat spasticity in cerebral palsy.
AI can help make drug repurposing a speedier and more streamlined process, with the capacity to quickly and efficiently sweep a search space of drug candidates far beyond the capability of individual researchers or clinicians. AI is able to give insight into the biomolecules involved in disease causation and treatment, generate better matches between repurposed drugs and target proteins, and identify any potential adverse side effects.
In 2021, researchers in Ohio built a customizable AI framework that emulates randomized clinical trials, the gold standard for drug discovery, using data from electronic health records and medical claims for a given disease to extract a list of drugs or active ingredients with potential for repurposing.
Using the framework, the team successfully identified six potential coronary artery disease (CAD) drug repurposing candidates, among them metformin, a diabetes treatment, and escitalopram, prescribed for depression and anxiety. They were also able to assess drug combinations for CAD using this framework, and found that using lisinopril and atorvastatin together substantially improved CAD outcomes, even though individually they were not found to be statistically significant.
The AI framework outperformed three existing preclinical drug repurposing methods, which was very exciting when also factoring in the increased speed and significantly reduced costs. And this same approach can be applied to drug repurposing for most diseases. It has already been used to expedite the development of drugs related to COVID-19, and there have been a number of studies looking at its potential in the fight against Alzheimer’s disease.