A Study of Switching From Entecavir to Tenofovir Disoproxil Fumarate in Subjects With Chronic Hepatitis B

Inclusion Criteria

• Subjects must be 20 to 69 years of age inclusive, at the time of signing the informed consent
• Male and female subjects. A female subject is eligible to participate if she is not pregnant and not breastfeeding, and at least one of the following conditions applies:
• Not a woman of childbearing potential (WOCBP), OR
• A WOCBP who agrees to follow the contraceptive guidance during the treatment period and for at least 4 days after the last dose of study treatment
• Capable of giving signed informed consent form (ICF)
• Subjects with chronic hepatitis B (CHB) (excluding hospitalized subjects)
• Subjects treated with ETV for at least 2 years prior to initiation of study treatment.
• The serum HBV-DNA level at screening is below the limit of quantitation ( =800 IU/mL
• Meet all of the following criteria at screening:
• Creatinine clearance (CLcr) >=70 mL/minute. CLcr is calculated using the following Cockcroft-Gault formula.

Male: CLcr = (body weight in kilogram [kg] multiplied by [140 minus age in years]) divided by (72 multiplied by serum creatinine [milligram {mg}/deciliter {dL}]) Female: CLcr = CLcr (male) multiplied by 0.85

• Hemoglobin >= 8 gram/dL
• WBC >=1000 per cubic millimeter (mm^3)

Exclusion Criteria

• QTc > 450 millisecond (msec) or > 480 msec for subjects with bundle branch block
• Received any interferon or Hepatitis B vaccine therapy within 24 weeks prior to initiation of the study treatment.
• Received overdose of nonsteroidal anti-inflammatory drugs (NSAIDs) (excluding temporary or topical use) within 7 days prior to initiation of the study treatment.
• Received any of the following drugs within 8 weeks prior to initiation of the study treatment (excluding topical products such as ointment and/or cream etc).
• Drugs causing renal impairment (examples: aminoglycosides, amphotericin B, vancomycin, foscarnet, cisplatin, pentamidine, tacrolimus, cyclosporine, some contrast mediums [ionic high-osmolar contrast media, ionic low-osmolar contrast media]
• Competitors of renal excretion (except temporary use, example: probenecid)
• Immunosuppressants (examples: azathioprine, cycolphosphamide) or chemotherapeutics (example: etoposide)
• Glucocorticoid preparation
• Received TDF, Adefovir pivoxil (ADV) or Tenofovir Alafenamide Fumarate (TAF) within 2 years prior to initiation of the study treatment
• Participation in another clinical study within 6 months prior to screening, or planned participation in another clinical study simultaneously with this study.
• Co-infection with human immunodeficiency virus (HIV) or hepatitis C virus (HCV)
• Subjects with serious complication other than compensated CHB (cancer, significant renal, cardiovascular, pulmonary, or neurological disease, uncontrollable diabetes, etc.)
• Received or have a plan for solid organ or bone marrow transplantation
• Has proximal tubulopathy.
• Subjects with decompensated CHB who meet the following: direct bilirubin > 1.5 times upper limit of normal (ULN), Prothrombin Time (PT) 1:160), excluding CHB
• Subjects with or suspected of having hepatocellular carcinoma (HCC) (including both primary and metastatic) from diagnostic imaging at screening, or with serum alpha-fetoprotein (AFP) > 50 nanogram (ng)/mL at screening
• History of HCC (except subjects who underwent resection or received curative treatment by radiofrequency, and with AFP <=10 ng/mL at screening)
• Woman who is pregnant, possibly pregnant, lactating or planning a pregnancy during the study period.
• Psychiatry disorder or cognitive disorder that may affect the subject's ability to give informed consent or to follow specified study procedures.
• Subjects with a history of alcohol or drug abuse
• Subjects whom the investigator (or sub-investigator) considers ineligible for the study.
• Subjects with hypersensitivity to study treatments or their components, nucleoside and/or nucleotide analogues. Subjects with drug allergy that, in the investigator's (sub-investigator