By Amy Norton
The study found that for women with "triple-negative" breast cancer, adding Keytruda to standard chemotherapy improved their odds of responding.
And in the months afterward, women treated with the drug were less likely to see their cancer come back.
The findings are encouraging in a disease that is challenging to treat, said Dr. Skip Burris, president of the American Society of Clinical Oncology.
"I think these results will be greeted enthusiastically by doctors and patients," said Burris, who was not involved in the trial.
Triple-negative breast cancers account for about 10% to 15% of all breast cancers, according to the American Cancer Society. They are so called because the cancer's growth is not fueled by estrogen, progesterone or a protein called HER-2.
Unfortunately, Burris said, that means women with the disease are unlikely to benefit from the hormone therapies and "targeted" drugs that have greatly improved breast cancer survival for U.S. women.
Instead, the mainstays of treatment are surgery and chemotherapy.
On top of the lack of treatment options, triple-negative breast cancer is typically more aggressive than other forms of the disease, Burris said.
Most breast cancers related to the BRCA1 gene mutation are triple-negative. The tumors are also more common among younger women and African Americans.
Keytruda (pembrolizumab) belongs to a newer class of cancer drugs called immune checkpoint inhibitors. They work by releasing a particular "brake" on the immune system, which allows it to find and attack cancer cells.
The drugs are given intravenously, and approved for several cancers, including advanced cases of melanoma, lung, liver, bladder and stomach cancers.
And last year, the U.S. Food and Drug Administration approved a different immune checkpoint inhibitor -- Tecentriq (atezolizumab) -- to treat some women with advanced triple-negative breast cancer that has spread beyond the breast.
The new trial, published Feb. 27 in the New England Journal of Medicine, was funded by drugmaker Merck. It focused on women newly diagnosed with earlier-stage triple-negative cancer. The goal was to see whether Keytruda might help prevent recurrences and spread of the disease in the first place.
An international research team led by Dr. Peter Schmid of Barts Cancer Institute in London randomly assigned close to 1,200 women with stage 2 or 3 cancer to one of two groups. One received standard chemotherapy along with Keytruda before undergoing surgery to remove the tumor; the other group received chemo and a placebo.
After surgery, the patients received either Keytruda or the placebo every three weeks, for up to nine treatment cycles.
At the time of surgery, just under 65% of Keytruda patients showed a "pathological complete response." That means there were no signs of cancer in the breast tissue or lymph nodes removed during surgery.
That was true of only 51% of the placebo group.
The study patients were followed for anywhere from three months to two years, depending on when they enrolled. During that time, Keytruda patients were 37% less likely to die or suffer a cancer progression or recurrence: Just over 7% did, versus almost 12% of placebo patients.
The difference started to become clear around the 18-month mark, Burris noted.
Susan Brown is senior director of education and patient support at the nonprofit Susan G. Komen. She reviewed the results, describing them as "promising," but said more research is needed to know whether Keytruda can ultimately extend the lives of women with this disease.
The long-term safety is also unknown, Brown said.
During treatment, nearly all study patients had side effects like nausea, fatigue and hair loss. Of women given Keytruda, one-third had side effects considered "serious" -- including anemia and fever and other signs of infection related to a drop in white blood cells.
Those problems are expected with the drug. "There weren't any surprises," Burris said. "The side-effect profile looks favorable."
But while Keytruda is used for other cancers, it is not yet approved for triple-negative breast cancer. That means the drug -- whose list price is over $9,700 per infusion -- would not be "broadly covered" by insurers, Burris said.
He and Brown said that for now, women with the disease can ask their doctors about the possibility of getting into an ongoing clinical trial.