FDA Phase IV - Commitment - Retinal Function Study

Inclusion Criteria:- Healthy adult male subjects between 18 and 55 years of age, inclusive, at screening- Body mass index (BMI) 19 to 34 kg/m2- Negative for HIV, Hepatitis B and Hepatitis C at screening Exclusion Criteria:- Any unstable medical, psychiatric, or substance abuse disorder- History of moderate or severe hepatic impairment- Creatinine clearance- A resting systolic blood pressure of \< 100 mm Hg or \> 170 mm Hg or a resting diastolic blood pressure \> 110 mm Hg- Blindness, color blindness or vision in either eye judged as abnormal by the investigator- History of trauma or surgery to either eye, of retinitis pigmentosa, of diabetes or current diabetes, of epilepsy or of seizure disorders- Symptomatic hypotension within 6 months of start of study- History of sickle cell anemia or sickle cell trait or bleeding disorder- Significant active peptic ulceration or other gastrointestinal disorder such as colitis or Crohn's Disease- Use of alpha blockers, nitrates, ritonavir, indinavir, ketoconazole, itraconazole, or erythromycin, ciprofloxacin, tadalafil, isotretinoin, ibuprofen, unoprostone isopropyl, Lily-of-the-Valley, Strophanthus, ranitidine and sildenafil