Adding an experimental Celldex Therapeutics Inc vaccine that enlists the immune system to fight cancer to standard therapy helped patients with the deadliest type of brain cancer live longer, according to data from a midstage trial presented on Sunday.

In the trial of 73 patients whose glioblastoma multiforme (GBM) had recurred after prior therapy, 30 percent of those given Celldex's Rintega along with Roche Holding's Avastin were alive after 18 months versus 15 percent of those who received only Avastin.

"We are observing an extremely rare overall survival advantage that is now translating into long-term survival for a number of patients," said Dr. David Reardon, the trial's lead investigator, who presented the results at the American Society of Clinical Oncology meeting in Chicago.

Celldex said it is in discussions with regulators on finding a pathway to approval. "We certainly think the data is good enough to justify accelerated approval," said Celldex Chief Medical Officer Thomas Davis.

Rintega received breakthrough designation from the U.S. Food and Drug Administration, which is given to therapies seen as a potential advance in serious or life-threatening diseases.

There is no cure for GBM, in which highly malignant brain tumors overpower healthy cells by consuming space, blood and nutrients in the brain. About 9,000 new cases are diagnosed every year in the United States.

GBM tumors tend to become resistant to initial treatment relatively quickly. For newly diagnosed patients, the median survival rate is about 14 or 15 months if given standard chemotherapy treatment, such as Merck & Co's Temodar, explained Reardon, a neuro-oncology specialist expert from Dana-Farber Cancer Institute in Boston.

Rintega belongs to an emerging class of drugs that spur the immune system to recognize and attack cancer. It targets tumors that carry a genetic mutation found in about a third of all glioblastoma cases. That amounts to roughly 4,000 patients in the United States.

The vaccine was associated with a notable decrease in the need for steroids, and the numerous side effects associated with their use, researchers said.

Data from a late-stage trial in newly diagnosed patients is expected in the next couple of months, Celldex Chief Executive Anthony Marucci said.

Oppenheimer & Co analyst Christopher Marai forecast annual global Rintega sales reaching about $400 million for recurrent patients. If newly diagnosed patients were included, the figure would likely climb to $1 billion, he said.