A Phase III trial of Pfizer Inc's Ibrance showed that, in combination with hormone therapy, the drug more than doubled the duration of disease control for women with the most common type of breast cancer.

At the time of an interim analysis, patients given Ibrance and AstraZeneca Plc's Faslodex (fulvestrant), a widely used treatment to block estrogen, lived an average of 9.2 months before their cancer worsened. This compared with 3.8 months for patients treated with Faslodex and a placebo.

The trial, presented in Chicago at a meeting of the American Society of Clinical Oncology, enrolled 521 patients whose breast cancer was classified as estrogen-receptor positive, human epidermal growth factor receptor 2-negative. This category accounts for about 75 percent of all breast cancers.

Ibrance, or palbociclib, was given conditional approval by the U.S. Food and Drug Administration in February for such patients, but only those who had not previously been treated for advanced breast cancer.

"This represents a new standard of care option ... we await follow-up for overall survival," said Dr. Don Dizon, a gynecologic oncologist at the Massachusetts General Hospital Cancer Center in Boston and an ASCO spokesperson.

Ibrance works by blocking two enzymes, cyclin-dependent kinase 4 and 6, that are involved in cell growth. Some Wall Street analysts have predicted the drug could eventually generate annual sales of more than $5 billion.

Pfizer announced in April that the pivotal trial was stopped early after meeting its goal of demonstrating that Ibrance delayed disease progression. Overall survival results are not yet available, said Dr. Nicholas Turner, a consultant medical oncologist at The Royal Marsden and a team leader at The Institute of Cancer Research in London, and the study's lead author.

The most common adverse side effects seen in the trial involved blood count irregularities, but the number of infections caused by the irregularities was low and similar for both arms of the trial, Dr Turner said. The trial showed that 2.6 percent of Ibrance patients stopped treatment due to side effects.

Pfizer is currently conducting a study of Ibrance in women with advanced breast cancer not previously treated with hormone therapy.

The company said it is in discussions with global regulatory authorities about potentially making Ibranceb available for women with hormone positive, HER2 negative metastatic breast cancer whose disease has progressed following endocrine therapy.