In this study, people will be placed into one of two groups by chance. One group will receive the study medicine and the other group will receive a placebo, which has no active medicine. The study medicine is given one or two times through a small needle in the lower back into the area around the spinal cord. The medicine is designed to lower a protein that builds up in the brains of people with Alzheimer disease. Participants will stay in the research center for a few days during treatment so doctors can watch them closely. They will also need to have a study partner who can go with them to many of the study visits.