Researchers say an experimental drug from the biotech company Genentech failed to slow mental decline in tests on more than 500 people with mild to moderate Alzheimer's disease.
The drug did show some promise in the least-impaired participants who received the higher of two doses in the mid-stage studies.
The drug, crenezumab, aims to clear away the sticky amyloid plaques clogging Alzheimer's patients' brains.
The company says it is encouraged by the possible benefit for mildly impaired patients and will talk with regulators about the next steps to test the drug.
Results were discussed Wednesday at the Alzheimer's Association International Conference in Copenhagen.