The purposes of this study are to determine:

• The safety of LY450139 dihydrate and any side effects that might be associated with it.
• How much LY450139 dihydrate should be given and how long it may be detected in blood.
• To determine if LY450139 dihydrate may have an effect on a protein found in blood, called A beta. This protein is studied in subjects with Alzheimer's disease.
• To collect and store samples from blood and spinal fluid for research related to Alzheimer's disease and similar (neurodegenerative) diseases or inflammation (irritation) that may provide information on how subjects respond to LY450139 or other medications.

Length of study: Approximately 29 weeks.

Number of office visits: 11 for most subjects: initial visit, every other week during 14 weeks of study drug treatment, and 2 follow-up visits.

At no cost, approximately 45 eligible participants will receive:

• Study medication
• Study-related diagnostic and laboratory evaluations

Inclusion Criteria:

• At least 50 years of age and diagnosed with mild to moderate probable Alzheimer's disease
• Fluent in reading and speaking English and have a reliable study partner (caregiver) (someone who is in frequent contact and can make sure study medication is being taken correctly).
• If currently treated with certain drugs for Alzheimer's disease, doses must be the same for at least the last 2 months prior to starting study drug.

Exclusion Criteria:

• Cannot be currently using or require MAOI antidepressants (such as selegiline or Eldepryl(R)), antipsychotics, benzodiazepines, calcium channel blockers for heart disease, and propanolol.
• Cannot have a clinically significant and/or uncontrolled condition or other significant disease, including a condition requiring treatment with warfarin or heparin; or have had or currently have a peptic ulcer, reflux disease, or gastrointestinal (GI) bleeding.