UPDATE 1-Exelixis brain cancer drug response rates lowered

* Independent review shows 15 pct response rate

* Trial will enroll 100 more patients

* Lower dose to be studied (Recasts; adds analyst comment)

By Deena Beasley

ORLANDO, Fla., May 31 (Reuters) - An independent review of mid-stage trial results for an experimental brain cancer drug being developed by Exelixis Inc (EXEL.O) and Bristol-Myers Squibb Co (BMY.N) shows that fewer patients than previously reported benefited from the drug.

An outside radiology review of trial results shows that 15 percent of 46 evaluated patients with advanced glioblastoma multiforme, the most common and deadly form of brain cancer, had tumor shrinkage, the companies said on Sunday.

Looking only at patients not previously treated with anti-angiogenic drugs, 20 percent responded to the drug.

Trial investigators had previously reported that out of 26 assessed patients, 38 percent had tumor shrinkage of at least 50 percent.

The median duration of response was 2.9 months, according to the latest data, reported at a meeting of the American Society of Clinical Oncology.

The independent review "is a very conservative picture," said Renzo Canetta, head of global oncology development at Bristol-Myers.

The revised results still "more or less compare favorably to Avastin," Cory Kasimov, an analyst with J.P. Morgan, said in a research note.

Roche Holding AG's (ROG.VX) Avastin was approved recently by U.S. regulators to treat glioblastoma multiforme once the disease has recurred.

Exelixis and Bristol-Myers said they plan to enroll 100 additional patients in the mid-stage trial of their drug, known as XL184, and will also test it at a lower dose.

XL184 is an oral drug designed to block the vascular endothelial growth factor, the same target as Avastin, as well as MET and RET, two other drivers of tumor formation,

"We can only hope that broadening the targets can broaden the clinical benefits," Canetta said.

The companies are also planning a Phase III trial program, but said the current trial of patients with previously treated brain cancer may be sufficient for a regulatory filing.