SAN DIEGO — A new type of drug can help prevent advanced breast cancer from worsening, potentially providing an important new treatment option for women, researchers say.
In a clinical trial, the drug cut in half the risk that cancer would worsen, or progress, researchers said here Sunday. The median time before the disease progressed or the woman died was 20.2 months for those who received the drug, compared with 10.2 months for the control group.
“The magnitude of benefit we are seeing is not something commonly seen in cancer medicine studies,” said Dr. Richard S. Finn, a principal investigator in the study. Finn, an oncologist at the University of California Los Angeles, called the results “quite groundbreaking.”
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The drug, made by Pfizer Inc. and known as palbociclib, also appeared to prolong survival but not by a statistically significant amount. Those who received the drug lived a median of 37.5 months, compared with 33.3 months for those in the control group.
The results from the mid-stage study were presented here at the annual meeting of the American Association for Cancer Research. They are being closely watched on Wall Street, because palbociclib is considered a jewel in Pfizer’s product pipeline, with analysts predicting annual sales of billions of dollars.
Amgen Inc. is entitled to an 8 percent royalty on sales of the drug.
Strong as the results were, it is possible they will be a slight letdown to some investors. That is partly because they were not quite as good as interim results presented about halfway through the trial. At that point, the difference in median progression-free survival was 26.1 months for palbociclib versus 7.5 months for the control group.
The lack of a statistically significant survival benefit could also give investors pause.
Finn said, however, that a statistically significant survival benefit should not have been expected at this point because only 61 of the 165 patients in the trial had died. Also, patients can use other drugs after leaving the trial, which can dilute any effect of palbociclib.
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Palbociclib slows the runaway proliferation of cancer cells by inhibiting the activity of two related enzymes involved in cell division: cyclin-dependent kinases 4 and 6.
While Pfizer is in the lead to bring this class of drugs to market, Novartis has begun late-stage testing of its own CDK 4/6 inhibitor. Eli Lilly and Co. is at an earlier stage, with some results for its drug scheduled to be presented here. While breast cancer is the initial focus, the drugs are being tested for other cancers.
Breast cancer specialists not involved in the study were encouraged but somewhat cautious. “These results are strikingly positive and with a large potential impact to patients,” Dr. Jose Baselga said in a speech at the conference discussing the results.
But Baselga, who is the physician-in-chief at Memorial Sloan-Kettering Cancer Center, said the results might have been biased because the study’s investigators, who determined whether tumors had progressed, knew which patients were getting palbociclib.
Dr. Eric P. Winer, chief of women’s cancers at the Dana-Farber Cancer Institute in Boston, said larger studies are needed.
“This is a small Phase 2 trial — not tiny, but not the kind of study that would typically lead to a change in practice,” he said.
The study, sponsored by Pfizer, involved 165 post-menopausal women who were receiving their initial treatment for recurring or metastatic breast cancer.
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The cancers were estrogen receptor-positive, meaning their growth was fueled by that hormone, but negative for Her2, a different protein.
A big question is whether Pfizer will be able to win approval of the drug based on this study. The Food and Drug Administration normally requires larger Phase 3 studies, but sometimes makes exceptions for drugs for cancers and other life-threatening illnesses.
If Pfizer can get early approval, the drug could probably reach the market next year. If the company must complete a Phase 3 study, which is underway, approval might be delayed a couple of years, according to the ISI Group.
Dr. Mace L. Rothenberg, chief medical officer for Pfizer’s oncology division, said Thursday that the company was in discussions with the FDA and had not yet decided whether to seek approval now.
In 2008, the FDA granted accelerated approval to Genentech Inc.’s Avastin as a treatment for breast cancer based on a single trial in which the drug delayed disease progression by about 5½ months.
But the women who got Avastin did not live significantly longer, and subsequent studies showed a smaller effect in delaying progression. In 2011, the FDA revoked approval of Avastin for breast cancer treatment.