Evolution of hepatitis B virus mutation during entecavir rescue therapy in patients with antiviral resistance to lamivudine and adefovir

Background: The efficacy of entecavir (ETV) monotherapy in treatment-experienced patients with chronic hepatitis B (CHB) is debatable. Methods: A total of 22 hepatitis B e antigen (HBeAg)positive CHB patients who had shown viral breakthrough or suboptimal response with lamivudine (3TC) and adefovir disoproxil (ADV) therapy were treated with 1.0 mg of ETV. Clinical and virological parameters were monitored every 3 months. Restriction fragment mass polymorphism assays were used to detect antiviral resistance. Results: During 3TC and ADV therapy, 11 patients had rtM204V/I mutations, 2 had rtA181V/T or rtN236T. 7 had both and 2 had no 3TC- or ADV-related mutations. After switching to ETV monotherapy, the median change in serum hepatitis B virus (HBV) DNA level was -2.1 log(10) copies/ml. Virological response (HBV DNA<300 copies/ml) was achieved in 1 of 18 patients with pre-existing rt204 mutations, whereas it was achieved in all 4 patients without pre-existing rt204 mutations regardless of the presence of rt181 or rt236 mutations. Changes in mutational patterns during ETV therapy showed that rt204 mutations persisted or re-emerged. Relative abundances of rtM204V/I mutations in total viral populations gradually increased under ETV rescue, whereas those with rtA181V/T and rtN236T mutations decreased. ETV resistance mutations (rtL180M+rtT1841/L[rtS2026]+rtM204V) were detected in five patients with pre-existing rt204 mutations. Conclusions: ETV monotherapy resulted in a limited virological response in patients who had previously failed 3TC and ADV rescue therapy. The limited efficacy might be associated with residual or reselected rtM204V/I mutations leading to ETV resistance. Combination treatment including potent antiviral agents should be recommended for patients with pre-existing rtM204V/I mutations.