I believe a prevention trial would require a very large n and very lengthy not to mention very expensive. A large, deep pocket BP partner would probably be needed.
I am puzzled at how (without very accurate diagnoses and testing) they would prove that an Alzheimer’s patient has been prevented from getting Alzheimer’s?
Very good question, which a few years ago would have been more difficult to answer. But in the last year, several new diagnostic tests have been formulated to accurately detect solid vulnerabilities for the eventual onset of Alzheimer's. All of these will need their own clinical validation, but that shouldn't be difficult. Merely take a blood, genomic or other sample and see if the new Alzheimer's vulnerability test is successful.
When a test is confirmed, a new blarcamesine trial will be in order. Get a thousand older people enrolled in the trial, all of whom have been diagnosed with a valid Alzheimer's vulnerability. Give half a placebo starch pill (with the name "Anavex" imprinted on it) and the other half get a visually identical pill containing blarcamesine. In two or three years, compare the frequencies of Alzheimer's in the two cohorts. The people in the placebo arm will have a certain frequency. Those in the blarcamesine arm will have fewer or no cases appearing. Matter settled. Health insurance companies eager to get all of their paying clients older than 50 on blarcamesine. Everybody (well, except for nursing homes and Alzheimer's care facilities and doctors) wins.