* Roche says encouraged by recent findings with Biogen drug

* Looking again at prospects for gantenerumab and crenezumab

* Swiss drugmaker expects to give update later in year

By Ben Hirschler

LONDON, April 22 (Reuters) - Swiss drugmaker Roche has "renewed confidence" that drugs targeting potentially brain-destroying plaque can fight Alzheimer's disease, following promising results with a product from a rival.

Pharmaceuticals head Daniel O'Day said on Wednesday Roche would look again at prospects for two of its experimental Alzheimer's drugs, both of which suffered setbacks in 2014, in the wake of recent data on Biogen's drug.

The U.S. biotech group made headlines last month with better-than-expected clinical trial results for its experimental Alzheimer's medicine, aducanumab.

The small trial showed the treatment significantly slowed cognitive impairment in patients with mild symptoms, a rare bit of good news in a field littered with high-profile failures from the likes of Pfizer and Eli Lilly.

Aducanumab is similar to Roche's experimental product gantenerumab in the way it blocks beta amyloid, a protein that forms toxic brain plaques that are theoretically an underlying cause of the memory-robbing disease.

Yet Biogen's early success contrasts with disappointing results from a late-stage Phase III study with gantenerumab and a separate setback with Roche's Phase II drug crenezumab.

The Basel-based company has insisted all along it is not giving up on Alzheimer's and O'Day told reporters, after presenting quarterly sales figures, that Roche was evaluating in detail earlier research on gantenerumab and crenezumab in light of the Biogen data.

"We're not at a stage where we've made a final decision on those two programmes, but we're encouraged because the data that was presented from Biogen showed a concordance between dose level, between plaque removal and between clinical effect," he said.

Roche, which stated in a results presentation it had "renewed confidence" in the beta amyloid hypothesis, expects to be in a position to give an update on its Alzheimer's work later this year.

Analyst Tim Anderson at brokerage Bernstein said the latest comments suggested crenezumab would be advanced into late-stage Phase III studies.

Dementia, of which Alzheimer's is the most common form, affects close to 50 million people worldwide, a total set to reach 135 million by 2050, according to non-profit campaign group Alzheimer's Disease International.

Unlike heart disease and cancer, which have seen major strides in drug development, there is still no treatment that can slow the progression of Alzheimer's. Current drugs can do no more than ease some of the symptoms.

(Additional reporting by Katherina Bart in Zurich; Editing by David Holmes)