Viral evolutionary changes during tenofovir treatment in a chronic hepatitis B patient with sequential nucleos(t)ide therapy

•Tenofovir (TDF) showed a favorable resistance profile.•The resistance mutations of TDF have not been identified through several years.•This patient experienced virological breakthrough under TDF without rtA194T.•TDF resistance may emerge due to multi-site polymerase mutations.•Patients with sustained detectable HBV DNA should be monitored for TDF resistance.

A 54-year-old man diagnosed with HBeAg-positive chronic hepatitis B (CHB) was treated with entecavir (ETV) 1 mg/day following an initial unsuccessful lamivudine (LAM) treatment (rtL180M, rtM204V/I). Subsequently, virological breakthrough with ETV mutation (rtT184A/L) developed. The LAM and adefovir combination therapy was followed by virological breakthrough. The therapy had been switched to TDF monotherapy. However, this patient experienced virological breakthrough under TDF with a HBV strain bearing rtL80M, rtL180M, rtM204V/I, rtA200V, rtF221Y, rtS223A, rtT184A/L, rtR153Q, and rtV191I combined mutations without rtA194T mutation. TDF resistance may emerge due to multi-site polymerase mutations rather than single-site polymerase mutation.