The purpose of this study is to find out if SB-742457 is a safe treatment and what effects it has on the symptoms of mild to moderate Alzheimer's disease. SB-742457 is a new treatment which is thought to increase the levels of certain chemicals in the brain that are often decreased in patients with Alzheimer's disease.

Inclusion criteria:

• Clinical diagnosis of probable mild-to-moderate Alzheimer's disease as determined by the NINCDS-ADRDA and DSM-IV criteria with an MMSE score of 12-24.
• Subjects and their caregivers must provide informed consent prior to study entry.
• Adequate blood pressure and laboratory values.

Exclusion criteria:

• Females of child-bearing potential.
• Have other causes of dementia such as vascular damage, depression, bipolar affective disorder, schizophrenia, syphilis, vitamin B12 deficiency or thyroid deficiency.
• Subjects taking medication for Alzheimer's disease or centrally acting agents which might impact study outcomes.
• Subjects taking agents for which there is a theoretical risk of interaction with SB-742457.
• Subjects with known hypersensitivity to sunlight or seizures.