INDIANAPOLIS (Reuters) - The failure of Eli Lilly and Co's Alzheimer's treatment to slow declines in mental capacity of patients with even mild symptoms cast serious doubt over whether a long-held approach to the memory-robbing disease could provide meaningful benefit to millions of people at risk.

The drug, solanezumab, failed to slow loss of cognitive ability in patients of mild Alzheimer's disease, a major setback for patients, investors and the drug maker.