MATTHEW PERRONE AP Business Writer WASHINGTON (AP) — Federal health regulators said Friday it's unclear whether Genentech's blockbuster cancer drug Avastin significantly shrinks the deadliest type of brain tumor. The Food and Drug Administration is reviewing the company's drug for patients with recurring glioblastoma multiforme, a form of brain cancer that is generally fatal within six months. South San Francisco, Calif.-based Genentech has asked the FDA to give its drug accelerated approval. That designation gives market access based on promising early results. However regulators said in documents posted online that they are unsure that the company's results are strong enough to rush out the new indication. The company's application relies on imaging scans that claim to show a reduction in tumor size. Patients were considered responsive to the drug if their tumor shrunk at least 50 percent over the course of two consecutive visits to their physician. In two separate studies, Genentech reported that roughly 25 percent and 20 percent of brain cancer patients responded successfully to the drug. However, FDA reviewers said they have never used "response rate" as a measurement to grant accelerated approval for a brain cancer drug. They noted the difficulty of measuring tumor size via medical imaging. Avastin is known to decrease swelling caused by excess fluid in the brain, which could give the false impression of tumor reduction, reviewers noted. In its own briefing documents, Genentech noted that 38 percent of patients treated with its drug were alive after one year. The company said there was a statistically significant connection between response to its drug and survival time. FDA's panel of outside cancer experts previously stated that a drug would have to have a response rate above 30 percent — higher than either of Genentech's studies — to outweigh the uncertainties connected with brain tumor imaging. Next Tuesday the FDA will ask its panel to weigh in on whether Avastin should be granted accelerated approval for patients whose brain cancer has returned. The agency is not required to follow the panel's advice, though it usually does. About 10,000 U.S. patients are affected each year by glioblastoma multiforme, according to the National Cancer Institute. The cancer quickly returns in nearly all patients treated for the disease. Avastin is already approved to treat colon and breast cancer, along with the most common form of lung cancer. The drug was Genentech's top-selling product last year with revenue of $2.69 billion, however sales growth has been slowing. Avastin is marketed in Europe by Swiss drug giant Roche, which completed its $48.6 billion takeover of Genentech on Thursday. Roche shares rose 2.3 percent to 150 Swiss francs on the Zurich exchange at midday Friday. In the last quarter, Genentech reported Avastin sales of $731 million, falling short of Wall Street forecasts for $740 million. Analysts said the drug is likely reaching a saturation point in the market and will need additional FDA approvals to continue growing. Initially approved in 2004, Avastin was the first drug to fight cancer by chocking off blood flow to tumors. Such "targeted therapies" were considered a significant advance beyond older chemotherapy drugs.