SEATTLE (The Seattle Times) -- Women with metastatic breast cancer treated with a combination of two estrogen-blocking drugs survived more than six months longer than those treated with just one of the drugs or one followed by the other, according to a study involving nearly 700 women.

It's the first time such improvement in overall survival has been seen in trials of first-line hormonal therapy for hormone-receptive metastatic breast cancer, the study authors said.

"We're finding a lot of other ways of treating these estrogen-receptor-positive breast cancers that don't include chemotherapy," said co-author Dr. Julie Gralow, director of breast medical oncology at Seattle Cancer Care Alliance.

Such hormone-targeting therapy is a key focus in breast-cancer research, in part because it avoids the toxic effects of chemotherapy, Gralow said. However, she cautioned, these first results need to be repeated and verified by other researchers and may only apply to a subgroup of women with this type of breast cancer.

The report, from the University of Michigan-based research group SWOG, one of five cooperative groups that comprise the National Cancer Institute's National Clinical Trials Network, was published Wednesday in the New England Journal of Medicine.

The SWOG Statistical Center is based at Fred Hutchinson Cancer Research Center; and co-authors also include Dr. William Barlow, professor of biostatistics at the University of Washington.

The two drugs, anastrozole (brand name Arimidex) and fulvestrant (brand name Faslodex), are both called endocrine therapies because they work through action on a hormone -- in this case, estrogen. About three-quarters of women with breast cancer and metastatic breast cancer have tumors that are responsive to estrogen, Gralow said. Anastrozole inhibits estrogen synthesis, while fulvestrant works on estrogen receptors.

The combination therapy increased the median survival of the women in the trial by 6.4 months compared with those who took only anastrozole. The combination also lengthened by 1.5 months the time before patients' disease progressed. More than 40 percent of the women in anastrozole-only group switched to fulvestrant when their disease progressed.

Although at this point metastatic breast cancer isn't cured with such endocrine therapies, Gralow said researchers are making a lot of progress in this area.

"We're trying to get away from toxic, nonspecific therapies, like chemotherapy, and better understand how we can affect the estrogen receptors and its pathways and get even better results than with chemo, with fewer side effects. That's where we're going in treating breast cancer."

About 700 women with metastatic breast cancer were enrolled in the study. Gralow said she was surprised to find that nearly 40 percent of them had received no treatment for breast cancer before coming in for evaluation of a breast lump and being informed their cancer had already spread.

"When I first saw that I didn't believe it," Gralow said. While such late diagnosis is common in the rest of the world, she said, it's troubling to see so many women from this country whose breast cancer was not caught in the early stages.

"That means there are a lot of those patients out there," she said.

For the study, having such a large percentage of previously untreated patients may mean the results would not hold for women who had received estrogen-blocking drugs in the past, Gralow said. "What the results mostly relate to is a patient who is just starting endocrine therapy for the first time," she said.

(c)2012 The Seattle Times Distributed by McClatchy-Tribune News Service.