The main purpose of this study is to evaluate the cognitive changes after administration of tideglusib versus placebo at two oral doses and two treatment regimes for 26 weeks in patients with mild to moderate Alzheimer's disease.

After the 26 week core treatment period, the patients may continue in the study under blinded conditions for an optional extension period up to a maximum of 39 additional weeks (total study duration up to 65 weeks), until the last patient in the study has completed the 26 week of treatment.

This double-blind, placebo-controlled, randomized, parallel group study will be conducted at multiple centers in the European Union. Patients with mild to moderate Alzheimer's disease will undergo a screening period, and then they will be randomized to one of these four groups: tideglusib 1000 mg once daily (Q.D.), tideglusib 1000 mg every other day (Q.O.D.), tideglusib 500 mg Q.D., or matching placebo, for a 26-week, double-blind, placebo-controlled treatment period.

Main Inclusion Criteria:

1. Men and women (of non-childbearing potential) with a diagnosis of probable Alzheimer's disease.
2. Age of 50 to 85 years.
3. MMSE score 14 to 26.
4. Well-tolerated treatment with one of the approved Acetylcholinesterase-Inhibitors and/or Memantine in a stable dose

Main Exclusion Criteria:

1. Significant psychiatric on medical disease.
2. Any chronic liver disease as indicated by out of range values of ALAT, ASAT or direct bilirubin, clinically relevant hepatic steatosis or other clinical manifestations of liver disease
3. Chronic daily drug intake of excluded concomitant medications.
4. Enrollment in another investigational drug study within 3 months before the baseline visit.