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.

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