Evaluation of a calibrated (18)F-FDG PET score as a biomarker for progression in Alzheimer disease and mild cognitive impairment.

Increasingly, clinical trials are being planned in patients with mild cognitive impairment (MCI) to prevent or delay the onset of dementia in Alzheimer disease (AD) by disease-modifying intervention. Inclusion of imaging techniques as biomarkers for patient selection and assessment of outcome is expected to increase trial efficacy. PET using (18)F-FDG provides objective information about the impairment of synaptic function and could, with appropriate standardization, qualify as a biomarker. We evaluated a predefined quantitative measure (PET score) that is extracted automatically from (18)F-FDG PET scans using a sample of controls (n = 44), patients with MCI (n = 94), and patients with mild AD (n = 40) from the Alzheimer Disease Neuroimaging Initiative (ADNI). Subjects received 4 scans and clinical assessments over 2 y. PET scores provide much higher test-retest reliability than standard neuropsychologic test scores (Alzheimer's Disease Assessment Scale-Cognitive [ADAS-cog] and Mini-Mental State Examination) and superior signal strength for measuring progression. At the same time, they are related linearly to ADAS-cog scores, thus providing a valid measure of cognitive impairment. In addition, PET scores at study entry in MCI patients significantly predict clinical progression to dementia with a higher accuracy than Mini-Mental State Examination and ADAS-cog. (18)F-FDG PET scores are a valid imaging biomarker to monitor the progression of MCI to AD. Their superior test-retest reliability and signal strength will allow the reduction in the number of subjects needed or shortening of study duration substantially.