Viagra, also known as sildenafil, is used by many men to treat erectile dysfunction and has been on the market since the 1990s. But researchers have found a signaling pathway in melanoma cells that is affected by the drug, indicating that sildenafil actually stimulates skin tumors to grow.

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Sildenafil, also known as the popular ED drug Viagra, encourages skin tumors to grow, researchers say.

The researchers, led by Prof. Robert Feil from the University of Tübingen in Germany, publish their results in the journal Cell Reports.

A class of medicines called phosphodiesterase (PDE) inhibitors, sildenafil treats erectile dysfunction (ED) by increasing blood flow to the penis, facilitating an erection.

The team explains that normally, cells contain PDE type 5 (PDE5), which is an enzyme that ensures signaling molecules are constantly broken down. The signaling molecule is called cyclic guanosine monophosphate (cGMP), and it plays an important part in complex metabolic pathways, including blood vessel cells, the heart, neurons and sensory cells.

The effects of cGMP on various growth processes in the body are not well understood, but Prof. Feil explains that he and his team "have discovered that the cells of malignant melanoma also use the cGMP signaling pathway for their growth," after conducting experiments in animals and human cell cultures.

He adds that sildenafil appears to inhibit PDE5, which acts "like a brake on cGMP." As such, he equates taking sildenafil as disabling that brake, allowing melanoma to grow more robustly.

The researchers say this mechanism could explain the increased risk of melanoma in men who take sildenafil.

Melanoma patients should consult doctor before use

Previous studies have pointed to a possible link between sildenafil and cancer. In 2014, for example, a long-term study involving 15,000 men in the US suggested a link between higher risk of malignant melanoma and sildenafil use.

And in 2015, another study in 24,000 men in Sweden found this same correlation.

However, neither of these studies were able to pin the increased melanoma risk to a biological effect of sildenafil on tumor cells, because they could not discount confounding lifestyle factors from the men, including sunbathing holidays and tanning bed use.

Although their latest findings are revelatory, Prof. Feil still says there is no reason that men should refrain from taking PDE5 inhibitors to treat ED from time to time, as it is unlikely that the drug makes new cancers form.

He says:

"We are assuming that sildenafil and possibly other PDE5 inhibitors could first and foremost reinforce the growth of existing melanomas - particularly if these medications are taken frequently and in high dosages."

Interestingly, other studies have suggested that sildenafil may have a positive effect on inhibiting other types of tumors, stopping the growth of certain intestinal tumors, for example.

While further studies are needed to assess these effects in humans, Prof. Feil warns that melanoma patients should consult their doctors before using sildenafil. "Ultimately," he says, "we should all be thinking about reducing our risk of skin cancer and cutting our exposure time to the sun, as well as using effective UV protection."

Medical News Today recently reported on a study that suggested sildenafil relieves pulmonary edema for swimmers and divers.