<qa>  What is the primary endpoint of the study?Cognitive testing will be carried out at baseline and every three months over a period of 18 months. Results of cognitive testing will constitute the primary endpoint of this study. Positron Emission Tomography PET imaging substudies will be performed at two time points during the study. Safety laboratories and assessments will be carried out at regular intervals. Subjects will not be responsible for any research study-related costs but will be responsible for the costs of evaluations required to establish diagnosis. 