Study Comparing the Safety of ASP2151 to Valacyclovir and Placebo in Healthy Volunteers

Inclusion Criteria: * Healthy subject weighing at least 45 kg with a Body Mass Index (BMI) of 18-35 kg/m2, inclusive * If female, subject is using a medically acceptable contraceptive method along with a double-barrier method to prevent pregnancy and agrees to continue using this method throughout study; and is not lactating or pregnant as documented by a negative serum pregnancy test * The subject is medically healthy, with a normal 12-lead electrocardiogram (ECG) Exclusion Criteria: * History of any clinically significant disease or malignancy excluding non-melanoma skin cancer * History of liver disease, liver dysfunction, liver enzyme elevations, or Gilbert's disease * History of acute or chronic pancreatitis or pancreatic insufficiency * History of gout, hyperuricemia, or crystalluria * History of chronic pain requiring medical therapy * Positive test for hepatitis C virus (HCV) antibody or hepatitis B surface antigen (HBsAg) * Positive test for human immunodeficiency virus (HIV) antibody * Clinical laboratory tests outside the normal limits and considered by the investigator to be clinically significant * The subject has difficulty swallowing tablets * Clinically significant illness within one month prior to study drug administration * History of drug or alcohol abuse within 2 years prior to study drug administration * Treatment with prescription medication (with the exception of contraceptives and hormone replacement therapy (HRT)) or complementary and alternative medicines (CAM) within 14 days; over-the-counter products with exception of ibuprofen within 14 days; or alcohol, grapefruit or grapefruit juice containing beverage within 72 hours * Donated blood or has had significant blood loss within 56 days prior to study drug administration or has donated plasma within 7 days prior to study drug administration