| Summary: | Introduction A viral breakthrough occurs when a patient achieves a response while on interferon (IFN) therapy and then loses the response despite continued IFN therapy. The cause of viral breakthroughs is not well understood. We encountered three cases with viral breakthrough during treatment with pegylated interferon (PEG-IFN) and ribavirin (RBV). Case presentation The three cases were all late virological responders. They did not express anti-IFN alpha-2b antibodies after PEG-IFN and RBV therapy. We analyzed amino acid substitutions of core 70, core 91, and interferon sensitivity-determining region (ISDR), which significantly influence sustained virological response (SVR). Their amino acid substitutions of core 91 were mutant in two cases. Amino acid substitutions of ISDR were wild pattern in two cases. PEG-IFN adherence was above 80% in three cases, and RBV adherence was below 80% in two cases. Conclusion During PEG-IFN and RBV therapy, we should watch for viral breakthrough in late virological responders with mutant type of amino acid substitutions of core 91, wild pattern of amino acid substitution of ISDR, and decrease of RBV adherence. Viral breakthrough is an important problem in PEG-IFN and RBV therapy for chronic hepatitis C. Therefore, it should be investigated more thoroughly in more cases.
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