Researchers say that a new type of drug can help prevent advanced breast cancer from worsening, potentially providing an important new treatment option for women and a blockbuster product for Pfizer.
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,” Dr. Richard S. Finn, a principal investigator in the study, said in an interview. Dr. Finn, an oncologist at the University of California, Los Angeles, called the results “quite groundbreaking.”
The drug, 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.
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The results from the Phase 2, or mid-stage, study were presented here on Sunday 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 is entitled to an 8 percent royalty on sales of the drug.
Strong as the results were, it is possible they will be a bit of a 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.
Dr. 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.
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 new class of drugs to market, Novartis has begun late-stage testing of its own CDK 4/6 inhibitor. Eli Lilly 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. “This is a whole new avenue that we are going to exploit now,” said Dr. Adam M. Brufsky, professor of medicine at the University of Pittsburgh. “Most of us who do breast cancer are very excited by this.”
But Dr. Eric P. Winer, chief of women’s cancers at the Dana-Farber Cancer Institute in Boston, said larger studies were still 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. The cancers were estrogen receptor-positive, meaning their growth was fueled by that hormone, but negative for Her2, a different protein.
About 60 to 65 percent of breast cancers fit that description, according to Dr. Dennis J. Slamon of U.C.L.A., another investigator in the study. Analysts at ISI Group, an investment research firm, estimate that about 50,000 American women a year would be eligible for palbociclib.
All the women in the trial took letrozole, a drug that blocks the synthesis of estrogen. Such drugs are standard initial therapy for this type of breast cancer. About half the women also received palbociclib, which was taken orally once a day for three out of every four weeks.
The biggest side effect, experienced by about three quarters of patients, was a decreased white blood cell count. But that did not lead to infections as it usually does, according to Dr. Finn, who said that the drug was generally well tolerated.
Still, many patients had their doses reduced because of side effects, and 13 percent of patients who received palbociclib dropped out of the study because of side effects, compared to 2 percent in the control group.
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 already underway, approval might be delayed a couple of years, according to ISI Group.
Dr. Mace L. Rothenberg, chief medical officer for Pfizer’s oncology division, said Thursday that the company was in discussions with the F.D.A. and had not yet decided whether to seek approval now.
In 2008, the F.D.A. granted accelerated approval to Genentech’s Avastin as a treatment for breast cancer based on a single trial in which the drug delayed disease progression by about five and a half 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 F.D.A. revoked approval of Avastin for breast cancer treatment.