This study explores the efficacy, safety and tolerability of tenofovir DF (TDF) 300 mg once daily monotherapy versus the combination of emtricitabine 200 mg plus tenofovir DF 300 mg (FTC/TDF) once daily in subjects currently being treated with adefovir dipivoxil (Hepsera) for chronic hepatitis B who have persistent viral replication (detectable hepatitis B virus deoxyribonucleic acid [HBV DNA]).

Subjects with confirmed (within 4 weeks) plasma HBV DNA ≥ 400 copies/mL during double blind treatment at Week 24 or any time thereafter have the option of receiving 12 weeks of open-label FTC/TDF which may be continued through the end of the 168-week treatment period if there is a virologic response (HBV DNA < 400 copies/mL). Alternatively, subjects with confirmed HBV DNA < 400 copies/mL at or any time after Week 24 of double-blind treatment may continue blinded therapy up to Week 168 at the discretion of the investigator. If, in the investigator's opinion, it is felt that continued blinded treatment beyond 24 weeks in subjects with confirmed HBV DNA ≥ 400 copies/mL is not beneficial, the subject may discontinue the study and begin commercially available HBV therapy rather than initiate open-label FTC/TDF.

Inclusion Criteria:

• 18 through 69 years of age, inclusive

• Chronic HBV infection, defined as positive serum HBsAg for at least 6 months

• Active chronic HBV infection with all the following:

  1. Currently treated with adefovir dipivoxil 10 mg QD (for at least 24 weeks but not more than 96 weeks)
  2. HBeAg positive or negative at screening
  3. Plasma HBV DNA >/= 1000 copies/mL at screening (irrespective of HBeAg status)
  4. Serum ALT less than 10 times the upper limit of normal (ULN)
  5. Calculated creatinine clearance of at least 70 mL/min using the Cockcroft-Gault formula
  6. Hemoglobin at least 8 g/dL
  7. Neutrophils at least 1,000 /mm3

• Nucleoside naive except for lamivudine (>/= 12 weeks of therapy)

• Negative serum beta human chorionic gonadotropin

• Compliant with adefovir dipivoxil

• Willing and able to provide written informed consent

Exclusion Criteria:

• Pregnant women, women who are breastfeeding or who believe they may wish to become pregnant during the course of the study
• Male or females of reproductive potential who are unwilling to use an effective method of contraceptive while enrolled in the study. For males, condoms should be used and for females, a barrier contraception method should be used
• Decompensated liver disease defined as conjugated bilirubin greater than 1.5 times ULN, prothrombin time (PT) greater than 1.5 times ULN, platelets less than 75,000/mm3, serum albumin less than 3.0 g/dL, or prior history of clinical hepatic decompensation (eg, ascites, jaundice, encephalopathy, variceal hemorrhage)
• Prior use of tenofovir DF or entecavir
• Received treatment with interferon or pegylated interferon within 6 months of the screening visit
• Evidence of hepatocellular carcinoma (HCC); for example, alpha-fetoprotein greater than 50 ng/mL or by any other standard of care measure.
• Co-infection with HCV (based on serology), human immunodeficiency virus (HIV), or hepatitis delta virus (HDV)
• Significant renal, cardiovascular, pulmonary, or neurological disease.
• Received solid organ or bone marrow transplantation.
• Is currently receiving therapy with immunomodulators (eg, corticosteroids, etc.), investigational agents, nephrotoxic agents, or agents capable of modifying renal excretion
• Has proximal tubulopathy
• Known hypersensitivity to the study drugs (tenofovir DF or emtricitabine/tenofovir DF), the metabolites (tenofovir or emtricitabine) or formulation excipients