By Deena Beasley

CHICAGO, June 2 (Reuters) - Adding cancer drug Avastin to standard chemotherapy doubled the length of time a certain group of advanced ovarian cancer patients lived without their disease getting worse, according to results of a clinical trial.

The study involved 361 women whose cancer had stopped responding to traditional platinum-based chemotherapy.

After a median follow-up of 13.5 months, 75 percent of Avastin patients had a recurrence of cancer, compared to 91 percent of those who received chemotherapy alone.

The median time to disease progression or death was 6.7 months in the combination group and 3.4 months in the chemotherapy group.

"A lot of drugs have been tested in this situation," said Dr Eric Pujade-Lauraine, head ofG roup d'Investigateurs Nationaux pour l'Etude des Cancers Ovariens, an ovarian cancer clinical trials cooperative group based in Paris.

"It is the first time that a Phase 3 trial has shown a significant difference," said Pujade-Lauraine, the study's lead investigator.

The study, to be presented on Saturday at a meeting here of the American Society of Clinical Oncology, involved 361 women whose cancer had stopped responding to traditional platinum-based chemotherapy.

Avastin, developed by Roche's Genentech unit, is an antibody that blocks the growth of blood vessels tumors need to survive and grow. It is approved in the U.S. for treating glioblastoma, colorectal, lung and kidney cancers.

The drug, also known as bevacizumab, is also approved in Europe for previously untreated ovarian cancer and is under review for use in previously treated ovarian cancer.

The U.S. Food and Drug Administration last year revoked its conditional approval of Avastin as a treatment for breast cancer because, although there was evidence that it slowed progression of the disease, there was no conclusive data showing that it extended the lives of breast cancer patients.

A Genentech spokeswoman said the company is waiting for final overall survival results from all of its ovarian cancer studies, expected by next year, and th en plans to discuss its next steps with the FDA.

An estimated 230,000 women worldwide are diagnosed with ovarian cancer each year. Most are not diagnosed before the cancer has spread, and up to 70 percent of them die within five years.

"Ultimately, almost all the patients with ovarian cancer will develop chemotherapy resistance," Pujade-Lauraine said.