Human melanoma cells growing in tissue culture. (City of Hope National Medical Center)

In a head-to-head comparison of two immunotherapy drugs used to prevent relapse in certain patients with advanced melanoma, one treatment was the clear winner — and it's not the one that most people get.

The international study, released Sunday, involved 900 patients whose tumors were removed by surgery but who remained at high risk of recurrence of melanoma, an often aggressive form of skin cancer.

The drugs were used as “adjuvant” therapy, meaning they were used after surgery, which was the primary therapy, to lower the risk of the cancer coming back. Both drugs work with the patient's immune system to fight cancer cells.

The trial compared Yervoy, the current standard postoperative treatment, with Opdivo, a newer drug. After a year of therapy, 71 percent of patients on Opdivo hadn't had a recurrence, compared with 61 percent of those on Yervoy.

“Results like this will change how we practice medicine,” said Jeffrey Weber, a professor of medicine at NYU Langone School of Medicine, who presented the findings at the annual meeting of the European Society for Medical Oncology in Madrid. The results were published simultaneously online in the New England Journal of Medicine.

Weber said Opdivo was also much better tolerated by patients. Fourteen percent of patients treated with Opdivo experienced severe side effects, compared with 45 percent of patients on Yervoy. Just 5 percent of Opdivo patients had to discontinue treatment because of severe side effects, compared with 31 percent on Yervoy. The most serious problems for both treatments were diarrhea and fatigue.

Given Opdivo's superior safety and effectiveness profile, Weber said, the drug “could realistically become the new standard of care” for melanoma treatment after surgery.

Asked why Opdivo might be safer and more effective than Yervoy, Weber described Opdivo as “a sniper rifle, much more directed,” and Yervoy as “more of a shotgun.”

Both drugs are made by Bristol-Myers Squibb, which funded the research.

Read more:

'This is not the end': Using immunotherapy and a genetic glitch to give cancer patients hope 

These experimental treatments target brain cancer like John McCain's

HPV researchers, Planned Parenthood win prestigious Lasker medical awards

How immunotherapy helps cancer patient's own body attack tumors