Eli Lilly said Wednesday its experimental Alzheimer’s drug appeared to slow worsening of the mind-robbing disease in a large study, sending its stock up by 5% on investor optimism about the results.

In the 18-month trial, people in the early stages of Alzheimer’s who received infusions of donanemab showed 35% less decline in thinking skills compared to those given a dummy drug, Lilly announced in a press release. About 6 million people in the U.S. have Alzheimer’s, which gradually attacks areas of the brain needed for memory, reasoning, communication and daily tasks.

Shares of Lilly jumped nearly $19, or 4.7%, to $423 in morning trading.

The Eli Lilly drug is designed to target and clear away a sticky protein called beta-amyloid that builds up into brain-clogging plaques that are one hallmark of Alzheimer’s.

A similar amyloid-targeting drug, Eisai and Biogen’s Leqembi, recently hit the market with similar evidence that it could modestly slow Alzheimer’s — and also some safety concerns, brain swelling or small brain bleeds.

Donanemab also comes with that risk. Lilly said in its study, the brain side effects caused the deaths of two participants and a third also died after a serious case.

The preliminary study results haven’t been vetted by outside experts. Indianapolis-based Lilly plans to release more details at an international Alzheimer’s meeting this summer and is seeking Food and Drug Administration approval of the drug.