“It was very temporary; it was not enough to call it a response,” Dr. Topalian said. “But it was a signal; it was there.”

That was surprising because researchers had assumed the cancers most vulnerable to an immune system attack were melanoma and kidney cancer. Lung cancer was supposed to be out of the question.

“Julie and I got on the phone with Medarex and said, ‘You have to include lung cancer in your next clinical trial,’ ” Dr. Topalian said, referring to her colleague Dr. Julie Brahmer.

That led to studies of two Bristol-Myers drugs: one that blocks PD-1 and another that blocks PD-L1. The studies included a 503 patients with a variety of advanced cancers who had exhausted other options.

The findings, presented in October last year at a meeting of the American Society of Clinical Oncology, were striking. A significant proportion of patients responded, including 18 percent of 76 lung cancer patients who got the PD-1 drug and 10 percent of 49 who got PD-L1 drug. Dr. Pardoll, who is married to Dr. Topalian, said that when she and her colleagues presented the data, “it was almost like a hush fell over the room: ‘Can this really be?’ ”

Emblems of Hope

As researchers continue to study the new drugs and ask if they can improve their results by combining them with other therapies, they are heartened by some of the rare patients whose cancers were halted by the drugs. They caution that these patients are unusual; critical studies to reveal the drugs’ effects on populations of cancer patients are still under way.

“What you really want to know,” said Dr. Roger M. Perlmutter, the president of Merck Research Laboratories, “is, are people living longer?” For that, “you just have to wait,” he continued, adding, “What I don’t want to do is give people false hope.”