The study will evaluate the safety & efficacy of AC-1204, a ketogenic compound, administered orally on a daily basis for 6 months. Following the 6 month double-blind phase of the study, subjects may enroll in an optional 6 month open-label extension phase. Efficacy will be evaluated by standard tests of memory and cognition, along with other measurements of activities of daily living and quality of life. Safety will be assessed by frequency of adverse events and changes in laboratory test results. Subjects will be stratified and outcomes will be separately analyzed based on apolipoprotein E4 genotype (APOE4).