"From a clinical standpoint, the assertion that the women had a 19-month progression-free survival is astounding, because these are all recurrent disease patients," said Dr. Robert Morgan Jr., section head of medical gynecologic oncology at City of Hope Cancer Center in Duarte, Calif.

"In general, response rates and progression-free survival you see from someone is usually around six to eight months before the tumors relapse. This is really encouraging," added Morgan, who was not involved in the study.

He did sound one note of caution, however. "Eighteen patients is obviously not going to be enough to draw any firm conclusions," Morgan said. "But I think that this data is certainly data which should be encouraging to continue in larger trials."

Another expert echoed those sentiments.

"This is a very well-done study showing what we all know: that the immune system can play a very important role in our fight against cancers," said Dr. David A. Fishman, director of gynecologic oncology, cancer prevention and early detection at New York University Cancer Institute in New York City. "From a scientific standpoint, it's a very interesting study that shows a lot of promise, but it does not show that we're ready for prime-time yet."

The study, which was led by Dr. Kunle Odunsi of the Roswell Park Cancer Institute, in Buffalo, N.Y., was published Monday in the online edition of the Proceedings of the National Academy of Sciences.

Ovarian cancer is the fifth leading cancer killer of women. Some 20,000 women in the United States are diagnosed with the disease every year, and about 15,000 women will die from it during the same time frame.

As with most cancers, a woman's chances of survival are better if the disease is found early, but ovarian tumors are a "stealth killer," because they are notoriously difficult to detect in their early stages. Only about 20 percent of ovarian cancers are found before the malignancy has spread to other tissues, according to information from the Mayo Clinic.

Although most women with advanced-stage ovarian cancer do respond to chemotherapy, 70 percent still die of the disease within five years of diagnosis.

Researchers are exploring vaccines as one way to extend remission. Vaccines essentially harness the body's own immune system to recognize some component of the cancer, then turn the body's forces against it.

Immune agents called cancer-testis antigens are found in abundance in a variety of cancer types and so are good candidates for vaccination, the researchers explained. An antigen known as NY-ESO-1 has shown particular promise.

"NY-ESO-1 peptide has been recognized and known as a potentially tumor-specific antigen, and that's the one single peptide that they're using for this particular vaccine," Morgan explained.

The Roswell Park team tested the vaccine in 18 women with epithelial ovarian cancer, which originates in the covering of the ovaries. All participants had undergone surgery and had completed chemotherapy. Most of the patients had advanced disease.

The vaccine turned out to be safe and also induced patients' antibody and T-cell responses, both measures of immune responses. The T-cells were able to recognize tumor cells containing NY-ESO-1.

Vaccine-induced immune cells were also found to be present in patients up to a year after immunization.

One woman experienced a complete regression of her disease after 10 vaccinations, although the disease did recur again eight months after the vaccinations were discontinued.

While phase I trials like this one are intended mainly to look at safety issues, they can often yield more information, Morgan said.

"While the classical goal of phase I trials is toxicity evaluation, these kinds of agents are different, because they usually don't have much in the area of toxicity," he explained. "What you're really looking for is a response to the proteins."

"I'm very encouraged," he continued.

Fishman said the findings are promising, but even better vaccine candidates may lie ahead.

NY-ESO-1 "is probably not the best peptide to be used as a vaccine because all vaccines are based on targeting something unique to the disease, and this is not unique to ovarian cancer," Fishman said.

However, the study "does show the promise of immunotherapy," he added, "and as we become more sophisticated, hopefully someday we will be able to have vaccines that will treat or ultimately prevent one from developing cancer. That is the goal -- to vaccinate so you never get the disease."

More information

There's more on ovarian cancer at the U.S. National Cancer Institute.