Inclusion Criteria:

• a diagnosis of Parkinson's disease (Larsen and Dupont, 1994) and dementia (DSM IV(1987; 1994), or Dementia with Lewy bodies (McKeith et al. Neurology 2005)
• mild-to-moderate or moderate dementia (i.e. MMSE 12-26, inclusive)
• the subject has given a written informed consent
• the subject is able and willing to comply with the study procedures and has a reliable caregiver (i.e. relative or nurse/nurse assistant who sees the patient at least weekly)

Exclusion Criteria:

• other brain disease of sufficient severity to cause dementia
• mental retardation
• terminal illness with life expectancy shorter than 6 months
• recent major changes in health status
• known epilepsy or previous convulsive seizure
• major depression
• severe dementia as defined by a Mini-mental State Examination score of 12 or lower
• moderate to severe renal impairment (i.e. serum creatinine > 1,5 upper limit normal (ULN) or creatinin clearance < 40ml/minute/1,73 m2
• moderate or severe heart disease (NYHA III-IV)
• moderate or severe pulmonal disease
• moderate to severe hepatic impairment (bilirubin or transaminases > 2 times ULN
• women of childbearing potential (i.e. not post-menopausal and not taking contraceptive
• the subjects is lactating
• any laboratory value(s) exceeding the limits of normality if deemed to be clinically relevant by the study physician
• known allergies to the investigational product