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: Quote, Profile, Research) and Bristol-Myers Squibb Co (BMY.N: Quote, Profile, Research) 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.