There are a number of reasons that drugs become available for repurposing. It could be that the drug’s original patents expired. Maybe a better drug came to market for that indication and the company decided to shelf the program. Sometimes the company itself develops a next-generation drug and retires the older version. Or it could be that the discovery of the drug was for an indication that at the time didn’t provide enough financial incentive to be developed. Those are just a few of the reasons drugs become available for repurposing.
Sometimes a promising drug fails, only for investigators to discover its effectiveness in a different condition.
For example, sildenafil, developed for cardiovascular disease, failed in a clinical trial, but a side effect appeared that indicated the drug might treat erectile dysfunction, and thus the $2 billion-a-year blockbuster drug Viagra was born.
In the 1960s, AZT was originally developed to fight cancer, but it didn’t work. In the 1980s, after AIDS emerged as a new infectious disease, a pharmaceutical company began a massive study of potential agents. Among those tested was AZT, which blocked the virus’ activity. This “break-through” became the first live-saving FDA approved AIDS medication.
One of the most astonishing cases of a repurposed drug involves thalidomide. Originally approved in Europe in the 1950s, doctors were infamously prescribing it to prevent nausea in pregnant women. It was discovered to cause severe birth defects in new-borns and was withdrawn from the market. In 1998, thalidomide found a new use as a treatment for leprosy and in 2006 it was approved for multiple myeloma, a bone marrow cancer.
With 30,000+ approved drugs and more either discontinued or abandoned for one reason or another, there’s a rich pool of potential compounds for repurposing.