Viagra may cut colorectal cancer risk by half

New research, now published in the journal Cancer Prevention Research, suggests that a small dose of the popular male impotence drug Viagra, when administered daily, may significantly reduce the risk of colorectal cancer.

The American Cancer Society (ACS) write that colorectal cancer is the third leading cause of cancer death among men and women in the United States.

It is the third most commonly diagnosed form of cancer overall; around 1 in 22 men and 1 in 24 women are likely to develop it at some point.

A significant risk factor for developing the illness is a mutation in a gene called the adenomatous polyposis coli (APC), a tumor suppressor. People with an APC genetic mutation might develop hundreds of colorectal polyps, which may eventually result in cancer.

New research uses a mouse model of this genetic mutation to test the effect of sildenafil — which is marketed as the popular erectile dysfunction drug Viagra — on colorectal cancer risk.

The study — led by Dr. Darren D. Browning, a cancer researcher at the Georgia Cancer Center and the Department of Biochemistry and Molecular Biology at Augusta University in Georgia — claims that a small daily intake of the drug could cut the number of colorectal tumors by half.

The researchers added sildenafil to the drinking water of mice that had been genetically modified to develop hundreds of polyps — which, in humans, almost always lead to colorectal cancer.

The beneficial mechanism of Viagra

The study found that sildenafil raises levels of a substance called cyclic guanosine monophosphate (cGMP), which is an intracellular calcium regulator.

cGMP contributes to the good physiological functioning of smooth muscle cells, pituitary cells, and retinal cells, among others.

As Dr. Browning and colleagues explain in their study, cGMP has also been shown to regulate the homeostasis of the intestinal epithelium, or the layer of cells inside the intestine that forms a physical barrier against foreign substances and bacteria.

The epithelium plays a key role in how our immune system responds to such foreign agents, and in conditions such as inflammatory bowel disease, the epithelium is inflamed.

In their study, Dr. Browning and team investigated the impact of sildenafil on cGMP because they knew that sildenafil inhibits another substance that has the potential to increase cGMP.

This substance is called phosphodiesterase-5, an enzyme that occurs naturally in colon cells, and in some others. The enzyme can break down cGMP, making more of it available for creating cells that form the protective layer that is the epithelium.

Viagra cuts cancerous polyps by 50 percent

The study revealed that Viagra increased cGMP, which, in turn, suppressed some of the cells that were proliferating in excess in the gut.

A second beneficial effect of the Viagra-boosted cGMP was that it aided the natural process of abnormal cell death and elimination.

"When we give Viagra," explains Dr. Browning, "we shrink the whole proliferating compartment in an area of our body that directly deals with whatever we put in our mouths and normally experiences high cell turnover."

"Proliferating cells are more subject to mutations that cause cancer," he explains.

In the mouse model, small doses of Viagra reduced the formation of polyps by 50 percent. As Dr. Browning says, "Giving a [tiny] dose of Viagra can reduce the amount of tumors in these animals by half."

He adds that the next steps should comprise human clinical trials of the already approved drug, with a focus on people who are already at high risk of colorectal cancer.

Dr. Browning also notes that, given in such small doses, Viagra is unlikely to cause side effects.