By Martha Kerr

NEW YORK (Reuters Health) - A new leukemia vaccine under investigation extends patients' event-free survival by more than three-fold, from 2.4 month with convention treatment to 8.7 months, investigators at the University of Texas M. D. Anderson Cancer Center in Houston reported this week at the 49th annual meeting of the American Society of Hematology underway in Atlanta.

The phase I/II trial involved 66 patients with acute myeloid leukemia, chronic myeloid leukemia, or myelodysplastic syndrome. Among them, 53 had active disease and 13 were in remission.

The vaccine was designed to stimulate the body's own immune system to recognize and mount a defense against tumor cells, resulting in their destruction, according to the researchers at National Cancer Institute.

The first 54 patients received three vaccine injections and the last 12 received six vaccine injections, all given three weeks apart. Three different dosages were tested.

An immune response was observed in 25 of the 53 patients with active disease (47 percent). There were clinical responses in 9 of those 25 patients (36 percent), compared with 3 of 28 patients (10 percent) who did not have an immune response.

As noted, the vaccine-induced immune response was associated with a longer event-free survival of 8.7 months compared with 2.4 months, and was also associated with a trend toward longer overall survival.

Among the 13 patients who were in complete remission at the beginning of the study, 4 have remained in remission for an average of 30.5 months, with a range of 11 to 59 months among all 13 patients. The vaccine-induced immune response has lasted for up to 4 years in some patients.

A low level of "blast" cells in the bone marrow "is a significant predictor of an immune response and a longer event-free survival," lead investigator Dr. Muzaffar Qazilbash told Reuters Health. "Patients with an immune response also showed a clinical response and longer overall survival."

This vaccine supplements the body's immune system, "rather than targeting the leukemia cells specifically," he said. "We have seen some relapses in these patients, but fewer than expected and fewer than in those who did not have a clinical response after immunization.

"The residual leukemia cells disappear over time in the responders and we cannot detect them after a time," Qazilbash said. "The molecular markers of leukemia disappear."

The best time to use the vaccine is after first-line treatment fails, he suggests. "We don't see this used as first-line treatment."

Phase III trials, larger more advanced studies that are usually the last stage before submission for regulatory approval, with this peptide vaccine are already ongoing.