Murat Sayan 43 Entecavir (Baraclude) ETV, a novel carbocyclic analogue of 2’ - deoxyguanosine, inhibits HBV replication at three different stages: (1) the priming of HBV DNA polymerase, (2) the reverse transcription of the negative-strand HBV DNA from the pregenomic RNA, and (3) the synthesis of the positive-strand HBV DNA. It was approved in 2005 in USA and in 2006 in Europe for naïve and LAM-resistant chronic hepatitis B treatment (9). ETV has been shown to have more potent antiviral activity than LAM or ADV in head-to-head comparison studies (10, 11). Entecavir Resistance ETV resistance was first identified in two patients with LAM-resistant strains, who experienced virologic breakthrough after more than 1 year of ETV therapy (12). Preliminary data indicate that ETV resistance increased to 51% of LAM-refractory patients after 5 years of ETV treatment (13, 14). More importantly, ETV resistance is rare (~1%) in nucleoside- naive patients after up to 5 years of treatment (Table 1) (15). Resistance to ETV appears to occur through a two – hit mechanism with initial selection of rt M204V/I mutation followed by amino acid subtitutions at rtT184, rtS202, or rtM250 (Table 1) (12). Patients who had previous resistance to LAM have lower treatment response and higher resistance rates, because only one or two additional mutations in the HBV polymerase (pol) gene are required for the development of ETV resistance, in contrast to three mutations required in treatment-naive patients. Moreover, LAM-resistance substitutions are necessary for the development of an ETV- resistant mutant (16). However, some compensatory mutations, such as rtV173L, rtL180M occurring in ETV therapy, help to restore the replication efficiency of the mutant virus (17). Some specific mutations in the HBV pol region are associated with multidrug failure. A recent example includes rtA181T + rtI233V + rtN236T + rtM250L. It is important to note that rtI233V and rtM250L (in domain E, related to ETV) substitutions do not confer significant drug resistance, nor do they significantly reduce replication capacity in the absence of selection pressure, but appear to act to compensate for the replication defects associated with multidrug resistance (18). Genotypic Resistance Genotypic resistance is generally defined by revelation of viral populations characterized by amino acid substitutions in the HBV pol gene that have been shown to confer resistance to antiviral drugs by in-vitro phenotypic assays (19). These Table 1. Cumulative incidence of drug resistant HBV with lamivudine and entecavir (1, 7, 12-15, 18, 39). Rate of genotypic resistance (year) 1 2 3 4 5 Major mutations conferring resistance Cross resistance Diagnosis of drug resistant HBV Lamivudine Treatment naive 24 % 38 % 49 % 67 % 70 % rtV173L, rtL180M, rtA181V/T, rtM204V/I/S Entecavir Treatment naive 0,2 % 0,5 % 1,2 % 1,2 % 1,2 % rtI169T, rtT184S/A/I/L/G/C/M, rtS202C/I/G, rtM250I/V (with rtL180M + rtM204V/I) (((TEEArrretttnmdLAMltebte1f1ir2cov8i80cauv01vi4idMtVri/arinbI/(+)eiTinn)ertteMrm2e0d4iVat/eI)) ((rLTErrteattLmMMlmb1t8iri22vvi000cuuMi44ddt/Vaii)nbnI/)eieIn/eS ± rtV173L ± (rtV173L, rtL180M, rtM204V/I) (rtV173L, rtL180M, rtM204V/I) Direct sequencing Line probe assay a Clonal analysis Direct sequencing Clonal analysis RFLP analysis Entecavir Lamivudine resistance 6 % 15 % 36 % 46 % 51 % rtL180M + rtM204V/I ± rtI169T ± rtV173L ± rtM250I/V rtL180M + rtM204V/I ± rtT184S/A/I/L/G/C/M, ± rtS202C/I/G Lamivudine (rtM204V/I/S ± rtV173L ± rtL180M) Telbivudine (rtM204/I) Emtricitabine (rtV173L, rtL180M, rtM204V/I) Direct sequencing Clonal analysis RFLP analysis RFLP analysis b a INNO- LiPA DR v2 (Innogenetics, Ghent, Belgium) b RFLP: restriction fragment length polymorphism Hepatitis Monthly, Winter 2010; 10(1): 42-47