The class of drugs currently prescribed to treat male erectile dysfunction has the potential to be repurposed for use in cancer treatment. Cancer specialists would like to prescribe existing medications like Viagra and Cialis since they are affordable and approved for use, thus save significant research money otherwise spent on expensive drug discovery.

Cancer drugs cost a median $750 million to develop. Therefore researchers came together in the Repurposing Drugs in Oncology (ReDO) project to try and find new purposes for treatment in commercially-available drugs. Previous papers from the ReDO project explored how inexpensive drugs such as beta-blockers and anti-fungal remedies can be used as part of cancer treatment.

In their new study published in the journal, eCancer Medical Science, researchers identified that selective phosphodiesterase 5 (PDE5) inhibitors have the potential to be used in new drug trials. These PDE5 inhibitors include sildenafil, tadalafil and vardenafil, commonly known by their market names Viagra, Cialis and Levitra.

“In many respects, sildenafil is the ultimate repurposing success story,” Dr. Pan Pantziarka of the Anticancer Fund said for eCancer. “It was originally developed for angina, repurposed for erectile dysfunction and then again for pulmonary arterial hypertension, and now it has the potential to be repurposed again as an anti-cancer drug.”

The study summarizes 25 studies that suggest Viagra may help fight different types of cancer, such as breast, colorectal, lung, pancreatic, and others. Past experiments show that PDE5 delays and stops the growth of tumors in mice and some human patients. Also,  inhibitors show a wide range of mechanisms of action in different cancer types. The paper brings attention to 11 ongoing clinical trials that are evaluating Viagra.

Checkpoint inhibitors have radically altered the landscape in oncology, but there remain significant challenges in terms of increasing the number and duration of responses,” Pantziarka explains. “It would be ironic if the key to improving outcomes from some of the most expensive drugs in oncology comes from repurposing some of the cheapest non-oncology drugs.”

Researchers also explored the possibilities of finding new agents that are able to cross the blood-brain-barrier (BBB). It represents a challenge which severely limits the range of drugs available to treat brain tumors. Some researches indicate PDE5 inhibitors are able to increase the permeability of the BBB, hence improving the drug delivery to the brain tumors. In case PDE5 proves to be efficient, patients could have a brand new therapeutic option.

For now, PDE5 is not an approved cancer medication but a number of small, early phase trials are already in motion. These are fairly preliminary, in the first or second of the three required testing phases in the life cycle of every medication before it can be approved for a specific use. There is no guarantee yet that these drugs will make it to your local pharmacy. The ReDO group believes, however, that it is time for much larger efficacy trials to begin.

After summarizing all data, researchers believe the PDE5 inhibitors, as low-toxicity and low-cost drugs, are very strong candidates for being repurposed as anti-cancer agents. ReDO scientists are optimistic and hopeful that the paper will bring much-needed attention to this class of drugs and drug repurposing in general.

Learn more about drug repurposing in the video below:

If you are interested in less known facts about Viagra watch the video below:

By Andreja Gregoric, MSc