A pair of cancer drugs used in combination has shown an ability to shrink tumors in nearly 60 percent of people suffering from advanced melanoma, or skin cancer, doctors say.

The two drugs, nivolumab and ipilimumab, stopped such cancer from showing any signs of advancing for almost a year in an international trial with 945 patients, researchers say.

When the two drugs were combined, the delay in progression was nearly four times that offered by either drug alone, suggesting the remaining life span of people with advanced melanoma could be doubled or even quadrupled, the researchers report in their study published in the New England Journal of Medicine.

The researchers did acknowledge that using the two together increased the severity of side effects, meaning the treatment may not be appropriate for patients in otherwise frail health.

Still, the researchers note there were no deaths reported during the global, large-scale clinical trial.

"This trial was conducted at 137 sites globally, and the safety guidelines that were in place clearly were able to handle these side effects," says study author Dr. Jedd Wolchok, chief of the Melanoma and Immunotherapeutics Service at Memorial Sloan Kettering Cancer Center in New York City. "This treatment can be safely applied in a global setting."

Melanoma is the most serious form of skin cancer and is one of the most aggressive of all cancers.

The drugs have a particular effect on the human immune system, and using that system in the fight against all cancers is a rapidly growing area of cancer research.

Although a powerful defense against diseases that would assault the body, the immune system includes a number of internal "brakes" that hold it back from attacking healthy tissues in the body, brakes that cancer often takes advantage of to escape detection by the immune system as it spreads.

Both ipilimumab and nivolumab take some of those brakes off, the researchers say.

"By giving these drugs together you are effectively taking two brakes off the immune system rather than one so the immune system is able to recognize tumors it wasn't previously recognizing and react to that and destroy them," says study investigator James Larkin of the Royal Marsden Hospital in Great Britain.

"For immunotherapies, we've never seen tumor shrinkage rates over 50 percent so that's very significant to see," he added.

Much more research is needed to answer some questions, including why some people in the trial responded exceptionally well to the combination therapy while other displayed no improvement at all, the researcher note.

Bristol-Myers Squibb, the manufacturer of both drugs, funded the study.