US Food and Drug Administration has labeled the medicine as a “breakthrough”

Researchers from Pfizer say that they have developed a new drug that slowed down the pace of advanced breast cancer, doubled the amount of time patients live, and prevented additional tumor growth. The drug, known as palbociclib, is among a new class of cancer drugs that target specific proteins to block tumors.

A trial of the drug involved165 postmenopausal women with locally advanced or newly diagnosed breast cancer that had metastasized. All women had cancer that was estrogen receptor positive, meaning their tumors grew in response to estrogen. Such patients make up about 60% of all breast cancers cases. Researchers found that palbociclib prevented breast cancer from worsening for 20.2 months when taken with letrozole, an estrogen blocker, as compared to 10.2 months for those in the control group, who only took letrozole.

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The US Food and Drug Administration has labeled the medicine as a “breakthrough,” and Pfizer is discussing a regulatory pathway for the drug, but has not decided whether to seek accelerated approval based on the results of the trial. “The magnitude of benefit we are seeing is not something commonly seen in cancer medicine studies,” said Dr. Richard S. Finn, the lead author of the study, who called the results “quite groundbreaking.”

However, overall survival in patients taking the drug was not statistically significant. The average survival for women on palbociclib was 37.5 months, compared to 33.5 months in the control group. Because only about 30 patients on each side of the trial had died, it’s still too early to tell how the drug affects survival. “The curves are starting to separate,” said Dr. Finn. “It hasn’t reached statistical significance, but patients are still being followed.”

A drug developed by Eli Lilly & Co. also targets hormone receptor–positive cancer and has shown positive results. The drug, called bemaciclib, was given to 47 patients who had been previously treated, and 33 of those patients showed antitumor activity. Tumors decreased in size in 9 of the patients participating in phase 1 of the study, while tumors neither grew nor shrunk in another 24 patients. Novartis AG is also testing a similar drug, but the data released to date were only from testing animals.


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  3. Rockoff JD, Winslow R. Two drugs show promise in slowing breast cancer. Wall Street Journal website. April 6, 2014.