WASHINGTON, United States—An experimental drug has shown encouraging results in treating advanced breast cancer in an early clinical trial, pharmaceutical giant Pfizer reported Sunday.

Pfizer, the world’s second largest drugmaker, said the drug prevented breast cancer from worsening for 20.2 months in a trial involving 165 patients. Current medications do so for 10.2 months. The drug, known as palbociclib, is among a new class of cancer drugs that target specific proteins to block tumors.

The outcome was not as positive as some initial results reported earlier in the tests, said University of Michigan business professor Erik Gordon, who studies the biomedical industry but is not affiliated with the trial.

But, he added, “there’s been a lot of hope surrounding this class of cancer drugs, and this keeps that hope alive.”

Wall Street analysts have been closely watching the tests given the potential market for palbociclib. Breast cancer is the most common cancer to strike women.

“It’s good news, but some investors are disappointed, because they expected more,” Gordon said. In earlier results, the drug had kept the cancer from worsening for 18 additional months, rather than 10.

The results were presented by the lead researcher, Dr. Richard S. Finn, associate professor of medicine at UCLA, during the annual meeting of the American Association of Cancer Research in San Diego.

The drug will likely need to undergo broader testing with more patients before the Food and Drug Administration will approve it. The FDA sometimes accelerates approval for drugs that show promise.

Other companies, including Eli Lilly and Novartis, have similar medications that are also in clinical trials.

Pfizer’s fledgling cancer drug business has been a recent bright spot for the company, which has many older drugs that will soon lose patent protection.