Evolution of adefovir-resistant HBV polymerase gene variants after switching to tenofovir disoproxil fumarate monotherapy

Background: Tenofovir disoproxil fumarate (TDF), an acyclic nucleotide analogue was shown to be effective in many HBV-infected patients with resistance to adefovir dipivoxil (ADV). This observation is intriguing because in vitro studies show that HBV mutations selected by ADV confer cross-resistance to TDF. To assess the clinical relevance of this cross-resistance, we studied the evolution of HBV polymerase gene variants in patients with genotypic resistance against ADV (rtN236T and/or rtA181V/T) during TDF treatment. Methods: In 10 HBV-monoinfected patients (9 male, mean age 47 11 [range 27-67] years, 6 hepatitis B e antigen--positive) with virological breakthrough during ADV treatment associated with the mutations rtN236T and/or rtA181T/V, HBV polymerase gene variants were studied during up to 24 months of consecutive monotherapy with TDF by population sequencing, line probe assay and clonal analysis. Results: In all patients, switching to TDF resulted in a continuous reduction of HBV DNA from a median of 7.6 (4.6-9.4) log 10 copies/ml to 3.3 (2-5) log 10 copies/ml, remaining in 7 patients > 400 copies/ml at 12 months. ADV-resistance mutations remained detectable throughout the whole observation period in most patients. Apart from an M204Q mutation in one sample, no new HBV polymerase gene mutations were found. In two patients with low level viraemia after 72 weeks of TDF, adding lamivudine led to a complete response within a few weeks. Conclusions: ADV-resistant HBV variants may further become selected during TDF treatment, however they cause only a mild decrease in TDF susceptibility.