NEW YORK - Two drugs from Bristol-Myers Squibb shrank tumors in as many as half of patients with advanced melanoma, according to early research that may pave the way for cocktails that trigger the immune system to destroy cancer.

In the study, 52 melanoma patients were simultaneously treated with Bristol-Myers's melanoma drug Yervoy and nivolumab, its experimental therapy that targets the immune system in a different way. Tumors shrank in 40 percent of patients, and in 53 percent of those who got the most effective dose combination, according to data released Wednesday in advance of the American Society of Clinical Oncology meeting scheduled to begin May 31.

"The responses are occurring faster and the absolute amount of shrinkage is more than we have seen with either drug alone with this frequency," said Jedd Wolchok, an oncologist at Memorial Sloan-Kettering Cancer Center in New York, and lead author on the study. "What we are seeing that is so impressive is the number of patients that have had very deep responses, their tumor burden decreasing by 80 percent or more." Tumors disappeared entirely in 10 percent of patients, he said.

Bristol-Myers, Merck & Co. and Roche Holding are competing to devise new cancer therapies and drug cocktails that use the human immune system to seek and destroy tumor cells. The treatments have the potential to reap billions of dollars in sales while lengthening patient remissions, analysts and oncologists say.

The new drugs are designed to prevent flicking what is essentially a molecular off switch, called PD-1, for immune system T-cells, the body's key defenders against attacks from dangerous germs and infections. Many tumors may hide from the immune system by triggering this switch.

While none of the drugs has been shown in large trials to extend life, doctors are awaiting early results to gain clues about which drugs may work best in which patients.

Data was released Wednesday from a preliminary study of Whitehouse Station, N.J.-based Merck's lambrolizumab that also targets the PD-1 system. It shrank tumors in at least 35 percent of patients with advanced melanoma, according to an abstract posted on the American Society of Clinical Oncology website from the first 85 patients in a 294-patient study.

In a third study, Basel, Switzerland-based Roche's experimental drug MPDL3280A reduced tumor size in 29 of 140 patients with a variety of late-stage cancers. The drug helped those with lung cancer, melanoma, kidney cancer, colorectal cancer and stomach cancer. All but three of the 29 patients who responded to the drug were still being helped by it, according to a statement from the cancer doctors' group.

Doctors are enthusiastic about immunotherapy because "this type of approach may lead to much longer lasting benefit for patients," than many existing therapies, said Sandra Horning, global head of clinical development for cancer drugs at Roche.