Preventing memory loss and restoring it once it’s gone is the holy grail of brain science. In the United States alone, an estimated 5 million people are living with Alzheimer’s disease. But recently, unexpected and disappointing findings have underlined just how difficult it is to develop therapies for cognitive decline.

In November, Eli Lilly announced that its Alzheimer’s drug solanezumab failed to show a significant benefit for people with mild forms of Alzheimer’s. “The outcome was not what we hoped for, and that is disappointing for the millions of people waiting for a potential disease-modifying treatment for Alzheimer’s disease,” Dr. Eric Siemers, medical director of the Alzheimer’s Disease Team at Lilly said during a teleconference.