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* Tested against locally advanced, or newly diagnosed metastatic cancer
* Pfizer to discuss favorable data with FDA
* Pfizer shares rise nearly 4 pct (Adds analyst and company comments, details on drug, trials, stock price)
By Ransdell Pierson
Feb 3 (Reuters) - Pfizer Inc's experimental breast cancer drug significantly delayed progression of symptoms in a mid-stage trial, meeting the study's primary goal and keeping Pfizer in the forefront of a race for a new standard of care for the disease.
Pfizer shares were up nearly 4 percent on positive findings for the oral treatment, called palbociclib, one of the company's most important experimental drugs that some analysts believe could eventually claim annual sales of more than $5 billion, if approved.
The trial tested the pill in post-menopausal patients with locally advanced or newly diagnosed breast cancer that had spread to other parts of the body.
"We are delighted with the final data," Pfizer said in a release, adding the company will discuss the successful data on progression-free survival with the U.S. Food and Drug Administration "to determine next steps" for palbociclib.
A secondary goal of the study is to determine whether palbociclib can prolong overall survival. But Pfizer spokeswoman Sally Beatty said the company does not yet have such data, because it takes longer periods to follow patients and gauge survival trends.
Although Pfizer is conducting two larger late-stage trials of palbociclib, ISI Group analyst Mark Schoenebaum noted the company has previously said it might discuss with the FDA the potential to seek approval of the medicine based upon mid-stage trial data, if the data warranted.
Palbociclib, which blocks enzymes known as cyclin-dependent kinases (CDKs) 4 and 6, was taken in combination with a standard treatment called letrozole among women who were estrogen receptor positive - meaning tumors grow in response to estrogen - and HER2-negative, meaning that the HER2 protein is not causing the cancer. Such patients make up about 60 percent of advanced breast cancer cases.
Letrozole is the chemical name of Femara, a Novartis AG drug that belongs to a class of treatments called aromatase inhibitors that block production of estrogen.
Pfizer on Monday did not present detailed data from the trial results, saying they would be presented at a scientific meeting in San Diego in April.
The success of the trial, called Paloma-1, follows highly favorable interim data that Pfizer unveiled in December 2012. At that point in the study, those taking both palbociclib and letrozole went an average of 26.1 months before tumors worsened. That compared with 7.5 months for those taking letrozole, but not palbociclib. The 18.6-month difference was considered statistically significant.
Side effects from palbociclib were similar to those previously reported, Pfizer said on Monday. The earlier interim results had shown that patients taking the drug combination experienced anemia, fatigue and neutropenia - a decline in white blood cells called neutrophils that can put patients at higher risk of infection.
Palbociclib was designated a "breakthrough" drug by the FDA last April based on initial data from the trial.
The FDA grants "breakthrough" status to drugs that may offer substantial improvement over existing treatments for serious or life-threatening diseases. The status helps to speed up the approval process.
Pfizer has started two late-stage trials of the drug - Paloma-2 and Paloma-3 - in patients with advanced breast cancer.
Paloma-2 will test the same drug combination, palbociclib and letrozole, as an initial treatment for breast cancer.
Paloma-3 will evaluate palbociclib in combination with another hormone therapy, fulvestrant, in women whose cancer has progressed after hormonal therapy. The drug, sold under the brand name Faslodex by AstraZeneca Plc, is called an estrogen receptor antagonist.
Companies trying to develop treatments similar to palbociclib include Novartis and Eli Lilly & Co.
Breast cancer killed more than 508,000 women worldwide in 2011, according to the World Health Organization.
Pfizer shares were up 3.7 percent at $31.53 on Monday morning on the New York Stock Exchange. (Reporting by Ransdell Pierson in New York; additional reporting by Esha Dey in Bangalore; editing by Ted Kerr and Matthew Lewis)