Swiss drugmaker Roche said its experimental Alzheimer's drug failed to meet its main goals in a mid-stage study, a result likely to bolster the belief that drugs need to be given in earlier stages of the disease to slow patients' decline.

In a Phase II study involving 431 patients, it was found that the drug, crenezumab, failed to significantly slow cognitive and functional decline compared to placebo, missing two main goals.

However an exploratory analysis of patients with a milder form of the disease who received a higher dose of crenezumab via an intravenous infusion showed a statistically significant reduction in cognitive decline, Roche said.

Carole Ho, director of early clinical development at Roche's biotech unit Genentech told the news agency Reuters she was encouraged by the data, even though it missed its main goals, since it demonstrated that treating the disease earlier could increase the benefit.

Ho said Roche would decide on any future plans for additional clinical studies following an analysis of the data in conjunction with health authorities.

Analysts had expected crenezumab to fail its main goals, after a similar treatment from Pfizer and Johnson & Johnson called bapineuzumab and solanezumab, a drug from Eli Lilly failed in late-stage trials.

Lilly has since started a new clinical trial focusing only on patients with mild signs of the disease.

Crenezumab, which was licensed from Swiss biotech company AC Immune in 2006, works by blocking the toxic protein beta-amyloid that forms plaques in the brain believed to signal the onset of the disease.

Roche said a smaller Phase II biomarker study also showed an effect of slowing cognitive decline in milder patients. Details of this study will be presented at the Clinical Trials in Alzheimer's Disease meeting in November.

A startling 99.6% of clinical trials in Alzheimer's failed between 2002 and 2012, the Cleveland Clinic study found.

Crenezumab has been picked for a US government-backed trial in a group of Colombians with a genetic mutation that causes them to develop Alzheimer's early. Results of that trial are due in 2020.

A second Alzheimer's drug from Roche, known as gantenerumab, is also being investigated in a late-stage trial with patients who are yet to develop any signs of the disease. Another Swiss drugmaker, Novartis, has also begun studies with this type of patient.