CHICAGO – It’s way too soon to declare success, but an experimental drug for lung cancer patients with a certain gene showed extraordinary promise in early testing, doctors reported at a cancer conference on Saturday.

More than 90 percent of the 82 patients in a study saw their tumors shrink after two months on the drug, Pfizer Inc.’s crizotinib, researchers reported.

Doctors had expected only about 10 percent of these very sick patients to respond to the drug, according to one of the study’s leaders, Dr. Yung-Jue Bang of the Seoul National University College of Medicine in South Korea.

These were people with advanced disease, including some whose cancers had spread to the brain. They had already tried an average of three other drugs. Responses to crizotinib have lasted up to 15 months so far, and the drug has been rushed into late-stage testing, Bang said.

Many leading cancer specialists, who normally don’t get excited until a drug proves effective in large studies against existing treatments, said the research so far on crizotinib was promising.

“It’s early, but I’m impressed by it. It looks extremely effective,” said Dr. Roy Herbst, lung cancer chief at the University of Texas M.D. Anderson Cancer Center in Houston. He has consulted for makers of other lung cancer drugs but not this one.

Dr. Alice Shaw, the Massachusetts General Hospital doctor who is leading a larger study of crizotinib, agreed.

“I don’t think there is false hope. The data are so strong,” she said.

The drug targets a gene that promotes tumor growth and is found in about 4 percent of lung cancers, especially among younger, non-smokers. This small percentage is still a lot of people: nearly 220,000 new cases of lung cancer are diagnosed each year in the United States alone, and it is the world’s top cancer killer.

That means that up to 10,000 people in the U.S. annually could benefit, said Dr. Mark Kris, a lung cancer specialist at Memorial Sloan-Kettering Cancer Center in New York, who has consulted for Pfizer.

Two other gene-targeted treatments, Tarceva and Iressa, help about 20,000 lung cancer patients annually in the U.S.

“We’re chipping away at large numbers of patients,” and future gene discoveries should add to the number helped, Herbst said.

The gene targeted by crizotinib was discovered in 2007, Kris noted.

“Once we understand a cancer cell, we can come up with a treatment very quickly,” he said.

No show-stopping side effects were seen in the first study — half of patients on crizotinib had diarrhea, nausea or vomiting — but much larger tests are needed to establish safety.

Even more testing is needed to see if the drug is more effective than existing treatments, how long any benefits last, and whether it improves survival — not just shrinks tumors.