When combined with the erectile-dysfunction drug Viagra, a long-used chemotherapy drug called doxorubicin may be even more effective as a treatment for prostate cancer, according to a new study.

Viagra improves doxorubicin's tumor-fighting power and also makes doxorubicin a safer drug, the researchers said.

However, the studies were conducted in mice and more research will be needed to find out if the results hold true for humans.

For more than four decades, doxorubicin has been used to treat a number of human cancers , including prostate cancer. However, its use is associated with irreversible heart damage , which often shows up several years after treatment stops, according to researchers from the Virginia Commonwealth University School of Medicine.

Scientists have been working for 15 years to find a treatment to protect the heart against doxorubicin's harmful side effects, they said.

Now, it seems combining Viagra with doxorubcin increases the production of molecules called reactive oxygen species that trigger death in prostate cancer cells. The combination did not harm the normal, healthy prostate cells.

In the mice used in the study, Viagra, generically known as sildenafil, also increased levels of nitric oxide in the heart. Nitric oxide relaxes blood vessels and allows blood to flow to the heart.

"We believe sildenafil could be an excellent candidate for incorporation into cancer treatment protocols with the potential of enhancing the anti-tumor efficacy, while protecting the heart against both short-term and long-term damage from doxorubicin," said study researcher Rakesh C. Kukreja, a professor of cardiology at the university.

"My team and I are hoping to move the research forward to a clinical trial and plans are under way to do so," he said. The clinical trial would evaluate the effectiveness of the drug combination in cancer patients.

The study was funded by the National, Heart, Lung and Blood Institute of the National Institutes of Health, and will be published online this week in the journal Proceedings of the National Academy of Sciences.