The purpose of this study is to determine whether Zydena (Udenafil) has positive effect on cognitive function in patients with Alzheimer's disease.

This study is a randomized, double blind, placebo-controlled multicenter study.

Inclusion Criteria:

• Signed written informed consent;
• Male or female subjects 50 to 90 years of age;
• Diagnosis of probable Alzheimer's disease according to National Institute of Neurological Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association (NINCDS/ADRDA) criteria;
• A Mini-Mental State Examination (MMSE) score of ≥10 and ≤26;
• Global Clinical Dementia Rating ≥ 0.5;
• Mild to moderate (not severe) white matter hyperintensities on brain MRI performed within three years from screening;
• Good enough hearing and visual function to complete neuropsychological tests
• Caregivers living with patients or spending 10 or more hours a week with patients;
• Stable dose of donepezil (5mg to 10mg) for at least 60 days;
• If patients have been on memantine, it should be washed out for at least 60 days;
• Medications including anxiolytics, antipsychotics, and hypnotics may be taken if the dose has been stable for at least two weeks

Exclusion Criteria:

• History of stroke within 6 months;
• Previous diagnosis of severe (more than 80%) intracranial artery stenosis;
• History of heart failure, ischemic heart disease (myocardial infarction, unstable angina, and stable angina), hypertrophic cardiomyopathy, and life-threatening arrhythmia;
• Previous history of coronary artery bypass graft surgery;
• Severe symptom of orthostatic hypotension (orthostatic syncope or presyncope), especially when patients take alpha-adrenergic blocker (Alfuzosin, Doxazosin, Naftopidil, Tamsulosin, Terazosin, Arotinolol, Carvedilol, Labetalol, Trazodone, typical and atypical antipsychotics);
• Uncontrolled diabetes mellitus;
• Proliferative diabetic retinopathy;
• Severe hypotension (blood pressure less than 90/50mmHg) or severe hypertension (blood pressure more than 170/100mmHg);
• Hepatic dysfunction (AST or ALT more than three times of upper normal limit) or renal dysfunction (serum creatinine more than 2.5mg/dL);
• Retinitis pigmentosa;
• Previous history of active peptic ulceration within one year before screening;
• Hematodyscrasia susceptible to priapism including sickle cell anemia, multiple myeloma, leukemia, and various bleeding disorders;
• History of drug abuse;
• Medication including nitrates/nitric oxide donor (ex: nitroglycerin, isosorbide mononitrate, isosorbide dinitrate, amyl nitrate/nitrite, and Sodium nitroprusside), androgen (ex: testosterone), anti-androgen, and anticoagulants;
• Current cancer chemotherapy;
• Usage of PDE5i (Zydena, Viagra®, Levitra®, or Cialis®) within two weeks before study start;
• History of hypersensitive reaction to PDE5i (Zydena, Viagra®, Levitra®, or Cialis®)