ADV is a nucleotide analogue of adenosine monophosphate. ADV has recently been approved for treatment, and it is a potent antiviral drug against both the wild‐type and LMV resistant HBV.3,4,5,6,7,16,17,18,19
In contrast with LMV therapy, ADV therapy is associated with delayed and infrequent selection of drug resistant viruses in the nucleos(‐t)ide naïve patient.7,8,20
Angus and colleagues10
reported the first case of a novel HBV polymerase N236T mutation against ADV in a nucleotide naïve liver cirrhosis patient at treatment week 80. Villeneuve and colleagues11
also reported a case in which there was selection of the N236T mutant in a post‐liver transplantation patient with LMV resistant HBV at ADV treatment month 20. However, selection of the ADV mutation in a large number of LMV resistant CHB patients exposed to ADV has not yet been reported.
A total of 67 LMV resistant CHB patients treated with ADV for a minimum of three months were included in our study. ADV resistance was confirmed by RFMP genotypic analysis and real time PCR quantification of HBV DNA.
A total of 11 ADV mutations occurred in nine of 67 LMV resistant CHB patients during the course of their ADV treatment, and the cumulative incidence of genotypic mutation in our cases at treatment weeks 48 and 96 was 6.4% and 25.4%, respectively, and this was higher than that of a previous report in nucleoside/‐tide naïve patients.8,20
In our study, the first cases of the rtN236T, rtA181V, and rtA181T mutations were detected at treatment months 3, 12, and 7. As previously described,14
RFMP analysis has the advantage of detecting a “minor” population comprising up to 1% of the total viral population.14,21
In contrast with RFMP, detection of mutation using the PCR sequencing method almost certainly could not have detected the minor species comprising less than 30% of the entire HBV population. In our study, a sensitive and specific RFMP method was used for genotypic mutation analysis. These factors might explain the earlier and higher detection rate of the ADV mutations in our cases.
Eight of nine patients with the ADV mutation had cirrhosis (compensated (n
5) or decompensated (n
3)). Differences in baseline characteristics, including histological severity, that might have contributed to the development of the ADV mutation needs to be studied further.
We selected a definition of HBV DNA rebound as a 1.0 log10 or greater increase in serum HBV DNA level from the treatment nadir. In our study, the cumulative incidence of HBV DNA rebound associated with the ADV mutation was 0 of 67 patients (0%) and three of 67 patients (4.4%) at treatment months 12 and 24, respectively.
In 58 patients with no detectible ADV mutation, serial quantification of HBV DNA revealed HBV DNA rebound in three patients. Westland and colleagues8
reported that 24 of 294 patients on ADV 10 mg displayed an unconfirmed 1.0 log10
or greater increase in serum HBV DNA levels at week 48 or at the last visit. They suggested that this reflects the fluctuating nature of HBV replication in CHB disease. Although patients showed good compliance to the drug and coinfection with other viruses was not detected, serial quantification and mutation analysis will be able to provide more information, including the interaction between the previous LMV resistant mutation and ADV.
The clinical course of patients after emergence of the ADV mutation was variable. Although exacerbation of serum bilirubin and serum ALT was observed in two patients, most of the nine patients with the ADV mutation maintained their clinical stability. In our study, two rtA181V mutant patients with or without the rtN236T mutation developed HBV rebound three months after the genotypic mutation, but one patient with the rtA181T mutation developed HBV DNA rebound at the time that the mutant was detected.
In our cases, fractionation of the mutant population by RFMP analysis revealed that the mutant comprised 40–100% of the total population in three patients with HBV DNA rebound. The other cases with no HBV DNA rebound showed the wild‐type dominant or transient mutation. As this was observed in a rather small study population, it will be important to confirm these results in a larger cohort of patients.
Angus and colleagues10
reported that ADV mutation patients did not show any further HBV DNA decrement after an initial log reduction to 2.4, even after six months of therapy. They suggested that antiviral resistance may have begun to emerge towards the end of the first year of therapy. In our study, patients with the ADV mutation showed a significantly lower magnitude of HBV DNA reduction at treatment months 9 and 12 compared with the non‐ADV mutation group. Further studies are needed to analyse the importance of the magnitude of HBV DNA reduction for inducing the ADV mutation.
We verified the results of the RFMP analysis using sequence analysis of the HBV polymerase domain. Bartholomeusz and colleagues22
reported additional possible mutations in the rt domains D, A, and B. According to our sequence analysis data, we could not identify any additional mutations.
The ultimate goal of HBV therapy is to heal hepatic inflammation and necrosis, and thereby halt the progression to cirrhosis and hepatocellular carcinoma. The key to successful therapy for chronic viral hepatitis is to induce durable suppression of viral replication without selection of drug resistant mutations.23,24,25
In our LMV resistant CHB patients treated with ADV, emergence of the ADV mutation appeared to present earlier and more frequently than was reported in previous studies in nucleoside/‐tide treatment naïve patients. Further studies are needed to define the potential role of the ADV resistant mutations and clinical consequences of those mutations for LMV resistant CHB patients who are treated with long term ADV.