icon-folder.gif   Conference Reports for NATAP  
  HEP DART 2003: Frontiers in drug development for Viral Hepatitis
December 14-18, 2003
Kauai, Hawaii
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Adefovir Dipivoxil Therapy for Chronic Hepatitis B: Durable Viral Suppression and Consistent Efficacy Across All Patient Populations
  L Brosgart
Gilead Sciences, Foster City, CA
Adefover dipivoxil (ADV), a prodrug of adefovir, is an acyclic nucleotide analog of adenosine monophosphate with potent in vitro and in vivo activity against both wild-type and lamivudine-resistant HBV. ADV 10 mg has shown durable viral suppression, consistent efficacy and long-term safety in a broad range of patient populations with chronic hepatitis B (CHB). ADV is licensed for the treatment of CHB in adults. This indication is based on efficacy (histological, virological, biochemical, and serological responses) and safety similar to placebo in adult patients with HBeAg+ and HBeAg- chronic hepatitis B with compensated liver function, and in adult patients with clinical evidence of lamivudine-resistant HBV with either compensated or decompensated liver function.
ADV conferred benefit relative to placebo by all baseline demographic (gender, ethnicity, age) and disease (prior IFN use, Knodell score, serum HBV DNA, ALT levels) characteristics. Efficacy is consistent in all patient populations: HBeAg-positive and HBeAg-nagative; treatment-naïve and treatment-experienced; patients with lamivudine-resistant HBV; including patients pre-and post-liver transplantation and with HIV/HBV co-infection; patients with compensated and decompensated liver function; and in Asians and Caucasians. Efficacy is similar regardless of fibrosis score at baseline and against all patterns of lamivudine-resistant HBV.
Recent data from a study of ADV 10 mg therapy for 3 years (144 weeks) show ADV mg provides significant, potent and durable viral suppression (4.0, 4.8, 5.5 log10 copies/ml reduction in serum HBV DNA at weeks 48, 96, and 144 respectively) that is associated with sustained ALT reduction.
No adefovir-associated resistance was identified through 48 weeks of therapy; through 96 weeks, the incidence of resistance is delayed and infrequent (<2%). Three year resistance analyses are ongoing. The safety profile with extended therapy is similar to that observed over the first 48 week placebo-controlled trials.
ADV is a significant advance in the treatment of chronic hepatitis B. The magnitude of the reductions in HBV DNA and ALT increases with greater duration of treatment, conferring significant clinical benefit over time due to ADV’s favorable resistance profile.
H Yang and C Brosgart (Gilead Sciences) presented a second paper at HEP DART on adefovir resistance.
In Vitro Characterization and Molecular Modeling Analysis of Adefovir Resistance Mutation rtN236T in the H Polymerase
Lamivudine resistance occurs in 24% and 70% of chronic hepatitis B patients after 1 and 4 years, respectively. Lamivudine resistance is frequently associated with YMDD (rtM204V/I) mutations in HBV reverse transcriotase (RT). A novel mutation rtN236T in the D domain of HBV RT was identified to confer resistance to adefovir in vitro and in vivo. The rtN236T mutation emerged slowly and infrequently in adefovir dipivoxi-treated patients (0% after 1 year and 1.7% after two years). In this study, we have evaluated the cross-resistance profile and viral replication fitness of the rtN236T mutant in vitro. A hypothesis on the mechanism of rtN236T induced resistance to adefovir is also proposed.
A patient-derived full-length HBV genome (Genotype D) with the rtN236T mutation was cloned into a plasmid, pHY106, to generate a replication competent clone. Using this as atemplate, a paired wild-type HBV clone was generated by reversion of the rtT236 mutation to rtN236 via site-directed mutagenesis. In vitro drug susceptibility and viral replication fitness were determined by transient transfection of HepG2 cells. Intracellular HBV replication signals were determined by Southern blotting. A model HBV polymerase structure based on HIV-1 and murine leukemia virus (muLV) RT crystal structures was used to understand the possible mechanism of rtN236T induced resistance to adefovir.
In vitro phenotypic analysis demonstrated that the rtN236T clone conferred 9.6 fold decrease in adefovir susceptibility. The rtN236T mutant also caused 4.2 and 8.6 fold reduced susceptibility to the acyclic nucleotides tenofovir and MCC-478, respectively. However, the rtN236T mutant remained sensitive to the nucleotides lamivudine, L-dT, and entecavir with IC50 changes <2.4 fold. Similar to lamivudine-resistant HBV (rtL180M+rtM204V), the rtN236T HBV mutants resulted in over 60% reduced replication capacity compared to wild-type HBV.
A modeled structure of HBV RT indicated that the side-chain of T236 in the rt236T mutant HBV ploymerase may have a favorable interaction with the g-phosphate of the natural substrate dATP (or other nucleosides). In case of adefovir diphosphate (or other acyclic nucleotides), the g-phosphate is positioned further away from T236. This difference in the relative positions of the g-phosphates with respect to the mutated T236 may allow a discrimination of the acyclic nucleotides from the cyclic nucleotides.
The authors concluded that the rtN236T mutation in HBV RT, infrequently selected during adefovir therapy, conferred reduced susceptibility to adefovir and decreased replication capacity. RtN236T remained sensitive to lamivudine, eny=tecavir, and L-dT in vitro, suggesting that patients who breakthrough adefovir therapy may be successfully treated with lamivudine or investigational agents entecavir and L-dT.