WASHINGTON - A therapeutic vaccine designed for breast cancer appears to be safe in women with advanced disease and shows signs of actually slowing down tumor growth, U.S. researchers reported on Friday.

Dendreon Corporation, maker of the Provenge prostate cancer vaccine, calls the new vaccine Neuvenge. It targets a type of breast cancer called her2/neu-positive breast cancer, which affects between 20 percent and 30 percent of breast cancer patients.

Like Provenge, Neuvenge is made using immune cells from the cancer patient, so it is a tailor-made vaccine.

Dr. John Park of the University of California, San Francisco and colleagues tested it in a phase I trial with 18 women with advanced her2/neu-positive breast cancer, whose cancer had spread despite treatment.

In the Journal of Clinical Oncology, the researchers report that the vaccine did not cause any serious side effects and appeared to help at least one patient.

"We saw a partial response, meaning a reduction in the size of tumor area in one patient that was certainly attributable to the treatment," Park said in a telephone interview.

In three other women, their cancer appeared to stabilize for as long as a year, something that could have been due to treatment, Park said.

Park said the effects justify moving from the phase I safety trial to a phase II trial, which would be designed to show the treatment actually helps patients. But that may not happen for a while, he said.

"The sponsor of this vaccine technology, Dendreon, is also associated with Provenge," Park said. "We on the breast cancer side are trying to encourage them to develop Neuvenge with the same vigor that they have tried to apply to Provenge, but it is not a company that has unlimited resources."

In January, the U.S. Food and Drug Administration accepted Dendreon's marketing application for Provenge on a "fast-track" basis, and in March an advisory panel said Provenge appeared reasonably safe. But the FDA has asked Dendreon to show the vaccine helps prostate cancer patients actually live longer.

Dr. Dave Urdal, chief scientific officer at Seattle-based Dendreon, said that data would not be available any sooner than next year, and perhaps not until 2010.

"The company currently is focused on its lead product, Provenge, so moving forward with the Neuvenge product is something that we haven't given guidance on," Urdal said in a telephone interview.

The delay in approval of Provenge has angered some patient advocacy groups.

Urdal said both Provenge and Neuvenge make use of immune system cells called CD4 helper T-cells. "This validates the approach. So there is more than one target and more than one cancer that potentially will benefit from this form of therapy," he said. Other companies are also working on cancer vaccines.

The her2/neu protein is over-expressed, meaning it is over-active, in several tumors including breast, colorectal and ovarian cancer.