title: In vitro susceptibility of adefovir-associated hepatitis B virus polymerase mutations to other antiviral agents.
abstract: BACKGROUND
  Adefovir dipivoxil is a nucleotide prodrug approved for the treatment of chronic hepatitis B. During clinical trials, ADV-associated mutations were observed in 0, 3, 11, 18 and 29% of patients after 48, 96, 144, 192 and 240 weeks of therapy, respectively. Hepatitis B virus (HBV) polymerase mutations associated with virological breakthrough to ADV include rtA181V and rtN236T, which occur alone or in combination. The rtA181T mutation has also been observed at low frequency, alone or in combination with rtN236T.
  
  
  METHODS
  To investigate the in vitro activity of adefovir and other anti-HBV agents against these mutants, we generated five stable cell lines that each expressed one of the following HBV mutants: rtN236T, rtA181V, rtA181V + rtN236T, rtA181T + rtN236T and rtA181T. Using these cell lines, we quantified in vitro changes in drug susceptibility for eight nucleotide/nucleoside analogues.
  
  
  RESULTS
  The rtN236T mutant had 7-fold resistance to adefovir but remained sensitive to entecavir, telbivudine and torcitabine (53.2-fold reduced susceptibility). The A181V mutant had 4.3-fold resistance to adefovir and had reduced susceptibility to multiple other agents ranging from 3.2-fold (tenofovir) to >191-fold (clevudine). The A181V + rtN236T double mutant was the most highly resistant showing 18-fold resistance to adefovir and higher levels of resistance to other tested drugs with the exception of tenofovir (10-fold reduced susceptibility). Our results and preliminary clinical data suggest that patients with rtN236T or rtA181V remain susceptible to tenofovir, entecavir and lamivudine. Further clinical data are necessary to precisely define in vitro cutoffs indicative of clinically-relevant resistance, particularly for drugs in development such as emtricitabine, telbivudine, torcitabine and clevudine.
authors.affiliation: Department of Clinical Virology, Gilead Sciences, Foster City, CA, USA.
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Qi
authors.forename: Shelly
authors.initials: S
authors.lastname: Xiong
authors.forename: Huiling
authors.initials: H
authors.lastname: Yang
authors.forename: Michael
authors.initials: M
authors.lastname: Miller
authors.forename: William E
authors.initials: WE
authors.lastname: Delaney
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology / Q000627:therapeutic use; D000998:Antiviral Agents / Q000737:chemistry / Q000494:pharmacology* / Q000627:therapeutic use; D045744:Cell Line, Tumor; D004279:DNA, Viral / Q000235:genetics; D004259:DNA-Directed DNA Polymerase / Q000235:genetics; D024921:Drug Resistance, Multiple, Viral; D006509:Hepatitis B / Q000188:drug therapy / Q000821:virology; D006515:Hepatitis B virus / Q000187:drug effects* / Q000235:genetics; D006801:Humans; D008826:Microbial Sensitivity Tests; D009154:Mutation; D063065:Organophosphonates / Q000737:chemistry / Q000494:pharmacology* / Q000627:therapeutic use; D014162:Transfection; D014764:Viral Proteins / Q000235:genetics
chemical_list: D000998:Antiviral Agents; D004279:DNA, Viral; D063065:Organophosphonates; D014764:Viral Proteins; D004259:DNA-Directed DNA Polymerase; D000225:Adenine; C106812:adefovir dipivoxil