The Food and Drug Administration said it's "unclear" whether positive responses seen in clinical trials involving Genentech Inc.'s (DNA) Avastin are sufficient to support accelerated approval of the drug to treat an aggressive type of brain cancer.

Genentech is seeking FDA approval to market Avastin, already approved to treat advanced breast, lung and colorectal cancer, for patients who suffer from an aggressive form of brain cancer in which the survival rate is 9 months to 12 months.

A panel of outside medical experts will discuss Avastin at an FDA sponsored meeting Tuesday. The panel will decide whether to recommend approval to expand the drug to treat brain cancer. The FDA generally follows the group's advice.

The FDA said it's "unclear weather the response rate and duration of response seen" in Genentech's application are sufficient, according to briefing documents posted on the agency's Web site.

Glioblastoma multiforme, a form of brain cancer, accounts for about 20% of all brain cancer. In 2008, for instance, there were nearly 22,000 new cases of brain cancer and cancers of the nervous system. There are few FDA-approved treatments for this type of drug, and getting approval would boost Avastin's sales.

The drug has already been a top-seller for Genentech, bringing in 2008 sales of $2.7 billion.

To get approval, Avastin must be safe for patients and show it can shrink tumors. The FDA said it's also judging the drug on what the chances are that someone taking the drug for six months will end up disease free.

The FDA data showed 36% of patients taking Avastin had at least a 6-month period in which their condition didn't worsen. One of the main studies involved 85 patients who received Avastin, and 82 who received Avastin and the chemotherapy agent irinotecan.

The FDA said only the Avastin-only data can be used to support the drug's approval because the agency couldn't separate the contribution of irinotecan from the results of the other trial.

-By Jared A. Favole, Dow Jones Newswires; 202.862.9207; jared.favole@dowjones.com