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Secondary prevention trials in Alzheimer disease: the challenge of identifying a meaningful end point.

The chief difference between a primary prevention trial (PPT) and secondary prevention trial (SPT) in Alzheimer's disease (AD) is related to the targeted participant group. As currently conceptualized, a PPT recruits asymptomatic individuals from the general population while an SPT recruits only asymptomatic (or preclinical) individuals who are biomarker-positive for AD1. Recent examples of the latter include the AD kindred trial in Antioquia, Colombia, which recruits only subjects who are E280A mutation carriers with a familial history of early onset disease, and the Anti-Amyloid Treatment in Asymptomatic Alzheimer's Disease (A4) trial, which targets adults age 70 years and older with high brain amyloid levels as determined by a PET scan. The latter is the subject of the Donohue et al. manuscript2, which seeks to identify a primary endpoint for that SPT.