SOUTH SAN FRANCISCO (AP) — Biotechnology company Genentech Inc. said Monday it asked the Food and Drug Administration to approve its drug Avastin as a secondary treatment for glioblastoma, an aggressive brain cancer.Avastin is Genentech’s top-selling drug, and is on the market as a treatment for lung, breast and colorectal cancer. Genentech reported $2.3 billion in Avastin sales last year. The company said it filed a supplemental Biologics License Application, requesting that the FDA approve Avastin for use in glioblastoma patients who have already received previous treatment. The company said glioblastoma is the most common type of brain tumor in the U.S. In more than 90 percent of cases, the cancer returns after initial treatment, and patients typically survive between three and six months after that time. In a recent clinical trial, Genentech said 43 percent of patients treated with Avastin survived for at least six months without the disease progressing. Median overall survival was 9.3 months.