Primary resistance, multidrug resistance, and cross-resistance pathways in HBV as a consequence of treatment failure.

Antiviral resistance is now the single most important factor in treatment failure using nucleos(t)ide analogues (NA). Primary drug resistance mutations refer to amino acid change(s) that result in reduced susceptibility to an antiviral agent. Secondary compensatory mutations restore replication defects associated with primary drug resistance and may be associated with low level reduced susceptibility. Several evolutionary pathways of drug resistant HBV have been observed in patients treated with NAs. It is possible that the drug resistance mutations selected with one agent may affect the efficacy of other NAs. Several major HBV-evolutionary NA-resistance pathways (rtM204I/V, rtN236T and rtA181T/V) have now been characterised. The rtM204V/I pathway is responsible for resistance to the L: -nucleosides, such as lamivudine (LMV), telbivudine (LdT) and clevudine (CLD), and also entecavir (ETV), whilst the rtN236T pathway is responsible for adefovir (ADV) and tenofovir (TFV) resistance. Both pathways are associated with clusters of secondary mutations that can affect subsequent treatment with NAs (rtT184G, rtS202I) such as ETV. The third pathway, rtA181T/V, is associated with resistance to LMV and ADV and is a potential multi-drug resistance pathway and will probably have an impact on TFV sensitivity, either alone or with the rtN236T. In naïve patients treated with ETV, atleast three mutations arising at the same time are required: rtL180M + rtM204V plus either one of rtT184, rtS202 or rtM250 codon changes. Finally, in highly drug-experienced patients, clusters of mutations such as rtA181T/I233V/N236T/M250L, all on the one dominant HBV genome, are being detected which are associated with multi-drug resistance. Sequential treatment with nucleos(t)ide analogue reverse transcriptase inhibitors (NRTI) promotes multidrug resistance. It is likely, therefore, that development of multi-drug resistance could be reduced by combination therapy optimised to individual viral phenotypes.