Shares of Genentech fell nearly 10 percent to a 2.5-year low before being halted after the panel's recommendation. Sanford Bernstein analyst Geoffrey Porges said he believed so-called "off-label" use of Avastin for breast cancer accounts for $200 million to $250 million in sales.

Avastin currently is approved by the U.S. Food and Drug Administration (FDA) for treating lung and colon cancer, but many doctors use it to treat breast cancer.

"I think there are too many uncertainties in the way the data was collected," said Dr. Maha Hussain, an oncologist at the University of Michigan and chairwoman of the panel.

FDA officials will consider the panel's recommendation in deciding whether to approve Avastin for breast cancer. The FDA does not have to follow the panel's advice but usually does. A final decision is expected by Feb. 23.

Avastin already has blockbuster sales, with $2.65 billion in the first nine months of this year. FDA approval would allow the drug to be promoted specifically for breast cancer and could help to secure reimbursement by insurers.

Breast cancer is one of the top 10 killers of women in the United States, claiming about 41,000 in 2004, the most recent available from the Centers for Disease Control and Prevention.

Globally, breast cancer affects 1.2 million people a year and kills 500,000, according to the World Health Organization.

STUDY WEIGHS ON RECOMMENDATION

In a major study, Avastin did not extend overall survival, although it did meet its main goal of extending progression-free survival -- a patient remaining alive without the disease getting worse.

Patients in the study were given a commonly used treatment called paclitaxel, or paclitaxel combined with Avastin, known generically as bevacizumab.

Median progression-free survival was 5.5 months longer in patients who got Avastin. FDA reviewers asked the panel to weigh that against serious side effects.

Among 722 total patients, those in the Avastin group had a 20-percent increase in serious side effects including hypertension, blood clots and heart attacks, FDA staff said.

Death attributed to the drug by the FDA was about 1.7 percent of the Avastin group versus zero in the control group.

"This is what we would call a split vote," said Dr. Richard Pazdur, director of the FDA's Office of Oncology Drug Products. "We will review the discussion and discuss it internally."

Genentech said it was disappointed with the vote and would discuss the committee's comments with the FDA.

"We remain convinced Avastin is helpful for patients with metastatic breast cancer," Genentech's president for product development, Dr. Susan Desmond-Hellmann, told reporters.

Avastin, which works by choking off the blood supply that tumors need to grow, has been cleared in Europe as a breast cancer treatment.

Shares of Genentech fell 9.1 percent to $66.13 before being halted on the New York Stock Exchange on Wednesday.

Sanford Bernstein's Porges said the sell-off was too steep. "The stock has more than fully priced the downside of Avastin not being approved in breast cancer," he said. "To completely write off the breast cancer indication is a mistake."

Porges said he did not expect an approval in February but, he said, he thinks Avastin ultimately will be cleared for breast cancer treatment after future studies.

Regarding off-label use of the drug for breast cancer, Porges said: "I don't think that goes away. The people that believe in this will continue to use it."

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