 This study will assess the effects of administration of VX-745 for 12 weeks on amyloid plaque burden in Alzheimers disease AD. Subjects who meet entry criteria will undergo 11C-PiB Carbon-11-labeled Pittsburgh Compound B positron emission tomography PET at baseline and after 45 days of dosing with VX-745. Cognitive testing will also be conducted at baseline and day 45. 