Fourteen women with breast cancer that had spread were injected with a vaccine that targets a specific protein, known as mammaglobin-A, that is found in high amounts in breast tumors.
Although the study was small, the findings suggest that the vaccine may also boost a patient's immune response and help slow disease progression.
"I don't want to oversell this," cautioned study co-author Dr. William Gillanders, vice chairman for research in the department of surgery at the Washington University School of Medicine in St. Louis. "This is a small clinical trial. But we can say confidently that the vaccine was safe," he said.
"We can also say with confidence that we were able to generate an immune response in almost all the patients who were vaccinated," he added. "And there is preliminary evidence that the vaccine may have an impact on breast cancer progression. But that needs to be studied further to be confirmed."
Gillanders and his colleagues reported their findings in the Dec. 1 issue of Clinical Cancer Research.
The study authors noted that overexpression of mammaglobin-A is found in up to 80 percent of breast cancer patients.
The vaccine prompts a specific kind of white blood cell in the immune system to track this protein down and eliminate it.
The women in the study were classified as having "advanced" disease. This meant they had prior exposure to chemotherapy, a process known to undermine a patient's immune function.
That said, none of the patients had undergone chemotherapy in the month leading up to vaccination.
The team found that side effects from the vaccine after one year were minimal, and included rashes, tenderness and mild flu-like symptoms.
What's more, by the one-year mark roughly 50 percent of the patients showed no sign of disease progression, the investigators found.
By comparison, only 20 percent of a similar group of 12 patients showed no signs of disease progression one year out, according to the study authors.
Even though the researchers stressed the need for a larger, longer study, they theorized that if the vaccine were given to newly diagnosed breast cancer patients who had not yet been exposed to chemotherapy, the vaccine might prove even more effective at halting disease.
"This trial wasn't really designed to look at this question, which makes it difficult to interpret the results so far," Gillanders noted. "But there's been a lot of interest in the development of a prevention vaccine for breast cancer and other cancers. And this work confirms the promise of such a strategy."
Dr. Courtney Vito, a breast surgeon and assistant clinical professor of surgical oncology with the City of Hope Comprehensive Cancer Center in Duarte, Calif., suggested the vaccine approach "makes a lot of sense and is very promising."
Vito explained that "our bodies all, at one time or another, have probably had a cancerous cell somewhere. It's just an error in cell replication when the DNA doesn't get copied correctly. Our cells have their own machinery, or 'kill switches,' to self-destruct if there's a problem in replication. However, sometimes this mechanism fails. The next line of defense is for the immune system to recognize a sick or cancerous cell and destroy it," she said.
"Tumors occur when the single cancerous cell escapes destruction by self-death and then also the immune system," Vito said. "Once it evades those systems, it can replicate quickly, producing a tumor, and then, with time, it can spread to other parts of the body," she noted.
"Ramping up the immune system specifically against breast cancer cells is really just augmenting nature's own lines of defense, possibly without the side effects of drugs like chemotherapy, which is what this trial showed," Vito added.
The findings are "exciting," said Sarah Hawley, an associate professor of internal medicine at the University of Michigan Medical School in Ann Arbor.
"The preliminary finding that the vaccine improved progression-free survival in patients with metastatic cancer [when the disease has spread to other parts of the body] is especially exciting because of the lack of good treatments for metastatic breast cancer," Hawley said.
"If the authors can replicate this result in future trials in metastatic and non-metastatic or newly diagnosed patients, this will represent an important direction for the field of breast cancer treatment research," Hawley concluded.
Visit the U.S. National Cancer Institute for more on cancer vaccines.