Rivastigmine Monotherapy and Combination Therapy With Memantine in Patients With Moderately Severe Alzheimer's Disease Who Failed to Benefit From Previous Cholinesterase Inhibitor Treatment

Inclusion Criteria: * Outpatients who have probable Alzheimer's disease according to the DSMIV criteria * Patients treated with donepezil (5-10 mg ) or galantamine (16- 24 mg) for at least 6 months * Patients, in the investigator's clinical judgment, not stabilized on treatment with donepezil or galantamine Exclusion Criteria: * Patients with evidence of severe or unstable physical illness, i.e., acute and severe asthmatic conditions, severe or unstable cardiovascular disorders, active peptic ulcer disease, hypersensitivity to cholinesterase inhibitors or memantine, clinically significant laboratory abnormalities or any patient with a medical condition which would prohibit them from completing the clinical trial