It is a single-centre, randomised, parallel-group, double-blind controlled Phase 3 trial to compare the efficacy of standard cholinergic therapy (Donepezil, Rivastigmine or Galantamine) plus extended release guanfacine (2mg GXR) versus standard cholinergic therapy plus placebo on Cognition (as measured by the ADAS-Cog) in patients with mild to moderate Alzheimer's Disease.