Tenofovir disoproxil fumarate (TDF) is widely used to treat hepatitis B virus (HBV) patients worldwide. We previously reported a patient with CHB and cirrhosis in whom viral breakthrough occurred during combination therapy with TDF and entecavir (ETV) against ETV-resistant virus. A recent Korean report showed that two patients with viral breakthrough during treatment with TDF-containing regimens were found to carry five reverse transcriptase (rt) mutations ([rt]S106C[C], rtH126Y[Y], rtD134E[E], rtM204I/V, and rtL269I [I]), with the C, Y, E, and I mutations being associated with tenofovir resistance. We report the clinical course up to September 2019 in our patient, and compare the HBV mutations to those of the two Korean patients. Four mutations (rtS106C, rtD134N/S[N/S], rtM204V, and rtL269I) plus ETV resistance (rtL180M and rtS202G) existed when she developed viral breakthrough during ETV and TDF combination therapy in April 2013. Moreover, three mutations (rtS106C, rtD134N, and rtL269I) existed at baseline. Our patient\'s father is Korean. Considering these factors, patients with these three or four mutations (CYEI or CN/SI) at baseline could experience tenofovir resistance in addition to lamivudine (LAM) or ETV resistance. In addition, HBV DNA levels fluctuated during tenofovir alafenamide (TAF) and LAM therapy in our patient, although treatment was switched from LAM, TDF, and ETV to LAM and TAF combination therapy in April 2018. In conclusion, three mutations (CN/SI) plus ETV resistance (rtL180M, rtM204V, and rtS202G) can cause tenofovir resistance. Long-term therapy with tenofovir against ETV-resistant virus has the potential to induce viral breakthrough and resistance, necessitating careful follow-up.

BACKGROUND: We determined the antiviral potency and viral breakthrough rate after 10 years of continuous entecavir treatment in patients with chronic hepatitis B (CHB) infection.
METHODS: The cumulative rates of undetectable hepatitis B virus DNA (HBV-DNA, < 2.1 log copies/mL), alanine aminotransferase (ALT) normalization, hepatitis B e antigen (HBeAg) seroclearance, hepatitis B surface antigen (HBsAg) seroclearance, and viral breakthrough of 1094 nucleos(t)ide analogue-naïve CHB patients (HBeAg-positive: 47%) who were on continuous entecavir treatment for 10 years were calculated.
RESULTS: The median age was 50 years and follow-up period was 5.5 years, with 999, 804, 591, 390, 182 and 87 patients followed up for at least 1, 3, 5, 7, 9 and 10 years, respectively. Incremental increases were noted in the rates of undetectable HBV-DNA, ALT normalization, HBeAg seroclearance, and HBsAg seroclearance, reaching 96, 79, 38 and 3.7%, respectively, by the tenth year. The mean decline in HBsAg level from baseline was - 0.08 log IU/mL/year. Multivariate analysis identified HBsAg level and genotype (A) as independent predictors of HBsAg seroclearance. Sixteen patients experienced viral breakthrough. The cumulative percentages of patients with viral breakthrough analyzed by the Kaplan-Meier test were 1.5 and 2.5% at years 5 and 10, respectively. There were no serious adverse events during treatment.
CONCLUSIONS: Long-term entecavir treatment of nucleos(t)ide analogue-naïve CHB patients was associated with an excellent viral response and a low rate of entecavir-resistant mutations at 10 years. Baseline HBsAg levels and genotype were predictors of HBsAg seroclearance during entecavir treatment.

OBJECTIVE: Entecavir (ETV) is approved for the treatment of chronic hepatitis B virus (HBV) infections, but the virus can acquire resistance to the drug. This requires lamivudine resistance mutations (LAMr) and at least one additional mutation. Here, we characterized two novel mutations, rtI163V and rtA186T, associated with viral breakthrough (VBT) in an ETV-refractory patient.
METHODS: HBV from an ETV-refractory patient was sequenced, and newly identified mutations were inserted into a replication-competent clone by mutagenesis. Clones were analyzed for replication efficacy and susceptibility to ETV in vitro. Chimeric mice with human hepatocytes were inoculated with the patient\'s serum at VBT, and monitored for viral mutation pattern using a next-generation sequencing approach.
RESULTS: RtI163V and rtA186T mutations were detected together with LAMr (rtL180M and rtM204V) at VBT. RtA186T plus LAMr reduced susceptibility to ETV more than 111.1-fold compared with the wild-type clone, while rtI163V plus LAMr resulted in a 20.4-fold reduction. RtA186T significantly reduced viral replication efficacy, while the rtI163V mutation rescued it. Interestingly, the viral mutation pattern in the chimeric mice indicated dominant (or selective) proliferation of a clone containing rtI163V and rtA186T mutations plus LAMr under ETV treatment. Three-dimensional docking simulation indicated that rtA186T reduced the binding affinity of the HBV polymerase to ETV.
CONCLUSIONS: VBT in this ETV-refractory patient is attributable to the novel ETV resistance mutations rtI163V and rtA186T. RtA186T was apparently responsible for ETV resistance but the selection of a clone with the double mutation plus LAMr suggests that rtI163V is required to sustain viral fitness.

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