Evolving early (pre-dementia) Alzheimer's disease trials: suit the outcomes to the population and study design.

Assuming that some cases of Alzheimer's disease (AD) could be prevented or delayed, prevention trials will be developed for this neurodegenerative condition. Initially, stakeholders will have to agree about the definition of prevention-true primary prevention, meaning the prevention of AD neuropathological changes; the prevention of clinical signs and symptoms that often augur AD; or preventing the progression of signs and symptoms to full-blown dementia. True primary prevention trials will have to rely completely upon neuroimaging or biomarker outcomes that reflect AD pathology. On the other hand, trials designed to prevent signs and symptoms of dementia will require researchers to agree on the phenomenology that would constitute an unequivocal endpoint: cognitive worsening on one or more measure compared to a normative group; development of Mild cognitive impairment (MCI); or development of Alzheimer's dementia. Prevention trials utilizing any of these outcomes in the general public will be large, will have to utilize low risk public health interventions, and might therefore have only a small impact (treatment effect size), especially if the studies are too short or the study populations are too diverse. An alternative to interventions aimed at the general public would be any attempt to prevent signs and symptoms of dementia in individuals thought to be at an increased risk for clinical dementia. These trials could try to reduce the development of signs and symptoms of dementia in cognitively normal subjects, or they could try to prevent progression from some form of Mild Cognitive Impairment to AD, or they could have the more subtle goal of reducing the accumulation of subclinical deficits in MCI subjects. If the populations for these trials are limited to individuals who have abnormal laboratory and neuroimaging studies associated with AD neuropathology, the results will not generalize to biomarker-negative, at risk individuals, who are likely to constitute the majority of any clinically relevant study population. Outcome measures for each study design will depend upon the characteristics of the study.