Inclusion Criteria:

• Male or female subjects with a clinical diagnosis of probable Alzheimer's disease
• The subject has an MMSE score at screening of 12 to 26 for Part A and 16-26 for Part B.
• Age ≥ 50 and above.
• If female, the subject must be post-menopausal (i.e. 12 months without menstrual period) or surgically sterile.
• Male subjects must be willing to abstain from sexual intercourse with pregnant or lactating women; or be willing to use a condom/spermicide in addition to having their female partner use another form of contraception if the woman could become pregnant, from the time of the first dose of GSK239512 until 84 days following completion of the study.
• The subject has the ability to comply with the study procedures.
• The subject has a permanent caregiver and is willing to attend all study visits for Parts A and B.
• The subject has provided full written informed consent prior to the performance of any protocol specific procedure, or if unable to provide informed consent due to cognitive status, full written informed consent on behalf of the subject has been provided by a legally acceptable representative.
• The caregiver has provided his / her written consent prior to the performance of any protocol specific procedure.

Exclusion Criteria:

• In the opinion of the investigator, following review of CT/MRI scans in the past 12 months and completion of neurological review there could be other probable causes of dementia
• History of significant psychiatric illness such as schizophrenia or bipolar affective disorder that in the opinion of the Investigator would interfere with participation in the study, or current depression (a score of ≥8 on the Cornell Scale for Depression in Dementia), or subjects with other psychiatric features in their AD which would in the opinion of the investigator, would increase risk to safety.
• History of significant sleep disturbance, for example, when it is associated with nocturnal wandering, nocturnal confusion / disorientation / agitation, which in the opinion of the investigator, may increase safety risk.
• History or presence of known or suspected seizures, unexplained significant loss of consciousness within last 6 months. Subjects who had febrile seizures in childhood may be included if these ceased by age 10 and they have had no other type of seizure in their medical history and have not been on anti-epileptic medications.
• History or presence of significant cardiovascular, gastro-intestinal, hepatic, or renal disease or other condition known to interfere with the absorption, distribution, metabolism, or excretion of drugs, or any other clinically relevant abnormality, medical or psychiatric condition, which, in the opinion of the Investigator, makes the subject unsuitable for inclusion in the study.
• History of alcohol or other substance abuse, according to the Diagnostic and Statistical Manual of Mental Disorders - Substance related disorders (DSM-IV) criteria.
• Clinically significant abnormalities in laboratory tests, including subjects with active liver disease or uncontrolled thyroid disease.
• Uncontrolled hypertension with systolic BP ≥160 and/or diastolic ≥95 mmHg. Subjects with controlled hypertension with systolic BP < 160 mmHg and diastolic <95 mmHg for at least 4 weeks are acceptable.
• Systolic BP <100 mmHg and/or diastolic <60 mmHg.
• Subjects with ECG criteria outside ranges specified in the protocol
• History of hypersensitivity to GSK239512 or its excipients.
• Treatment with cholinesterase inhibitors, (including Tacrine), memantine or selegiline within the previous month. No patients with AD who are already on these medications at the time of screening will be recruited, as it would be unethical to withdraw these medications for study participation. Only AD subjects who are not yet on these medications, or who have withdrawn from these medications for other reasons previously, may be enrolled into this study.
• Subjects who are currently taking or who have taken in the last month anti-psychotic drugs (typical or atypical dopaminergic antagonists or modulators) or mood stabilization drugs (including SSRI, DNRI, SNRI, MAO inhibitors, tricyclic antidepressants, lithium, valproate, carbamazepine).
• Subjects who are currently taking Pgp inhibitors or any CYP3A4 inhibitors.
• Subjects on chronic sedative medications (≥ 4 days per week for the past 4 weeks).
• Subject or caregiver is an immediate family member or employee of the participating Investigator, any of the participating site staff or GSK staff.
• Has received any other investigational treatment in the previous 3 months.