Efficacy of different nucleoside analog rescue therapies for entecavir-resistant chronic hepatitis B patients

The World Health Organization estimates that 257 million people around the world are currently infected with hepatitis B virus (HBV) and approximately 63 million new cases will occur between 2015 and 2030 [1]. Management of HBV infection remains a global public health challenge. At present, curing HBV is challenging in most patients and they need long-term antiviral treatment. Entecavir (ETV) is recommended as the first-line antiviral treatment in the APASL, AASLD, and EASL guidelines, and the drug resistance of ETV is only 1% over 5 years in treatment-naive patients [2‐4].

It is reported that approximately 81% of chronic hepatitis B (CHB) patients receive antiviral drugs with low barrier to resistance such as lamivudine (LMV) and telbivudine (LDT) or adefovir (ADV) before ETV treatment in China [5]. The rate of ETV resistance could increases to 51% in LMV-resistant patients, because if primary LMV resistance mutations occur, compensatory resistance mutations to ETV may arise even if primary LMV treatment is stopped [6]. Drug-resistant patients have higher rates of hepatitis flares and disease progression [7]. Therefore, management of ETV resistance has become an essential clinical issue in China.

Among rescue therapies for patients with ETV resistance, according to the APASL, AASLD, and EASL guidelines [2‐4], for patients with only ETV resistance, switching to TDF is recommended, while in patients with multi-drug resistance, TDF or a combination of ETV and TDF are recommended. Because TDF has a high barrier to drug resistance [8], both TDF monotherapy and TDF combined with ETV showed high virologic responses in patients with ETV resistance [9]. Moreover, HBV variants of ADV resistance are also not cross-resistant to ETV, so ETV and ADV combination therapy could be considered in theory, and data in a recent report supported this view [10]. Combining ETV and ADV is also recommended in the APASL guidelines. However, the AASLD and EASL guidelines do not recommended combining ETV and ADV as ETV-resistant rescue therapy. Many patients receive ADV treatment in China because of its relatively low cost. ETV and ADV combination therapy is an alternative rescue therapy for ETV-resistant patients according to the Chinese CHB guidelines by the Chinese Society of Infectious Diseases and Chinese Society of Hepatology. Due to the lack of data on comparative research into ETV–ADV combination therapies to TDF monotherapy or TDF–ETV combination therapy, whether combination ETV and ADV therapy has a comparable efficacy with TDF or TDF plus ETV is worth evaluating. Therefore, this study compared the efficacy of ETV plus ADV combination therapy, TDF monotherapy, and ETV and TDF combination therapy in ETV-resistant patients.

The study protocol was approved by the ethics committee of West China Hospital at Sichuan University. Patients were included from the Center of Infectious Diseases, West China Hospital, from 2011 to 2017. Written informed consent was obtained from each patient or his/her legal guardian. Patients with persistent HBV viremia (persistent HBV DNA > 100 IU/ml after 48 weeks of antiviral treatment in plasma) or virologic breakthrough (an increase in HBV DNA levels ≧ 1 log IU/mL in patients who initially responded to antiviral therapy and are compliant with therapy) were eligible for enrollment at screening. Resistance mutations were determined by direct sequencing of the reverse transcriptase region of the HBV polymerase gene (pol/RT). Patients with confirmed ETV genotypic resistance mutations (the presence of rtT184A/C/F/G/I/L/S, rtS202G, or rtM250L/V, in addition to L180M + M204V/I mutation) were included.

Patients with underlying liver diseases such as non-HBV viral hepatitis, nonalcoholic fatty liver diseases, and autoimmune hepatitis were excluded. Patients with underlying severe chronic respiratory diseases, cardiovascular disease, and chronic kidney injury were not enrolled. Patients lost to complete follow-up were also excluded. Ultimately, 72 patients with ETV resistance (rtL180M, rtT184A/C/F/G/I/L/S, rtS202G, rtM250L/V, and rtM204V/I) combined with ADV resistance (rtA181V/T and/or rtN236T) were included. The patients switched to three rescue therapies, including ETV/ADV combination therapy, TDF monotherapy, and ETV/TDF combination therapy. After 48 weeks of rescue treatment in patients with ETV resistance, parameters including HBV DNA levels (ranging from 100 IU/mL to 5 * 107 IU/mL), HBV serological markers (HBsAg, antibody to HBsAg, HBeAg, antibody to HBeAg, antibody to HBcAg), liver function (TB: total bilirubin, DB: direct bilirubin, IB: indirect bilirubin, ALT: alanine aminotransferase, AST: aspartate aminotransferase, TP: total protein, ALB: albumin, GLB: globin, GGT: gamma-glutamyl transpeptidase, TBA: total bile acid), renal function (urea, creatine, uric acid, Cys-C, eGFR) were analyzed. In addition, the virologic response, rate of normal alanine aminotransferase (ALT), and incidence of HBeAg loss/seroconversion were compared in the three groups. Adverse events were also assessed throughout 48 weeks. The glomerular filtration rate was estimated using the modification of diet in renal disease equation as follows: estimated glomerular filtration rate (eGFR; milliliters per minute per 1.73 m²) = 186 * serum creatinine^−1.154 * age^−0.203 * (0.742 if female) * 1.233 (Chinese) (Additional file 2: Figure S1).

From 2011 to 2017, the rate of ETV resistance among all HBV-resistant variants increased from 6.04 to 15.02%. ETV resistance is becoming severe in China since antiviral drugs such as LMV, LDT, and ADV with low barriers to resistance are commonly used, and the number of CHB patients in China constitutes approximately one-half of the global CHB population [11]. As for the mechanism of ETV resistance, ETV resistance barrier is lower by initial selection of LMV-resistant HBV mutation. The primary mutations of LMV resistance are the methionine residues at amino acid 204 conferred to isoleucine or valine M204 I/V, while methionine or serine changes at 180 (rtL180M/S) usually accompany this [12]. Once such primary resistance mutation occurred, the ETV resistance, which needs a mutation at B domain (rtI169T or rtS184G), C domain (rtS202G/I), or E domain (rtM250V) on basis of rtM204V/I ± rtL180M/S mutation, is much more likely to achieve [13]. Therefore, because of a large accumulation number of LMV resistance, the rate of ETV resistance increased during the long-term course in NA-experienced patients, which indicated that monitoring the resistance of ETV requires more attention.

Regarding the efficacy of different rescue therapies for patients with ETV resistance, the combination of ETV and TDF has potential benefit on minimizing the risk of potential mutations and improving the antiviral efficacy during the TDF rescue therapy. In our study, TDF and TDF plus ETV combination therapy showed comparable virologic response at 24 or 48 weeks, which was similar to previous studies on patients with partial virologic response to ETV [14, 15]. In multi-drug-resistant CHB patients, TDF also demonstrated comparable efficacy and safety to TDF plus ETV combination therapy [16]. Theoretically, mutations such as N236T and A194T are potential variants with resistance to TDF, which has no cross-resistance to ETV. TDF monotherapy is likely to have comparable antiviral effects compared with TDF plus ETV combination therapy. However, one major concern is that whether TDF genetic resistance occurred in LMV-experienced patients [17]. It is reported that rtL180M/T184L/A200V/M204V mutation with resistance to TDF was found in ETV-resistant patients receiving TDF monotherapy [18]. Whether primary resistant mutations to ETV resistance could increase the rate of mutations resistant to TDF in long-term TDF monotherapy is unknown. Long-term clinical trials on TDF and TDF plus ETV combination therapy demonstrated that no persistent HBV viremia or virologic breakthroughs occurred in TDF monotherapy at 144 weeks or 240 weeks [16, 19]. In an in vitro study, TDF was also susceptible in both LMV-resistant clones and ETV-resistant clones [20]. Therefore, we hypothesis the combination of ETV and TDF didn’t have better antiviral efficacy and lower risk of potential mutations resistance than TDF monotherapy.

Regarding the safety profile of the TDF monotherapy group and TDF plus ETV groups, the baseline eGFR and eGFR at 48 weeks were comparable, no significance exists in renal safety between these two groups. As for the potential mechanism of renal toxicity of TDF, TDF is excreted via glomerular filtration, and active tubular transport and may cause proximal tubular dysfunction [21]. Which was not found in ETV treatment. However, there was no evidence that ETV has renal protective effect, also a combination of ETV and TDF didn’t show better renal safety than TDF in previous researches [22, 23]. Therefore, we posit that no difference in renal safety exists between TDF monotherapy and TDF plus ETV combination therapy. Consider that long-term adherence and the cost-effectiveness of monotherapy is better than combination therapy, TDF monotherapy could be optimal treatment strategy for patients with underlying renal or bone metabolism diseases.

The virologic response of ETV plus ADV combination therapy is the worst among the three therapies regardless of multi-drug resistance. Only 28% of patients achieved virologic response, and 72% of patients had persistent HBV viremia at 48 weeks. Totally, three patients in ETV + ADV group discontinued the rescue therapy due to poor antiviral efficacy. They all switched to TDF monotherapy. One patient achieved virologic response 12 weeks after switching to TDF and another patient achieved virologic response 24 weeks after switching to TDF. The 3rd patient’s HBV DNA load reduced to 6.76E + 03 IU/mL 48 weeks after switching to TDF, who suffered decompensation of liver cirrhosis for three times during this period. Therefore, more effective rescue therapy should be selected to avoid persistent viremia, which is associated with a disease progression in long-term treatment. Although major ADV-resistant mutations were rtA181V/T mutations and rtN235T without cross-resistance to ETV, the relatively weak antiviral efficacy of ADV limits its use in ETV-resistant patients. In previous research, ADV had an estimated 30% resistance rate after 5 years of treatment in LMV-resistant patients [24]. Moreover, ADV and LMV dual resistant mutations may occur in LMV-resistant patients receiving ADV rescue therapy [25]. Additionally, TDF is easy to access and extremely inexpensive because of the new government procurement policy in China. ETV and ADV combination therapy has lower antiviral efficacy without the advantage of safety or cost-effectiveness. Therefore, TDF monotherapy may be the preferred rescue therapy rather than ETV and ADV combination therapy.

Since the rate of ETV resistance is relatively low, although we have screened a large cohort including 1837 HBV patients with genomic resistance to antiviral treatment, the number of patients with ETV resistance were limiting.

ETV resistance has gradually become a severe clinical problem in China because of the large number of antiviral treatment-experienced patients. TDF showed comparable virologic response and tolerance to TDF plus ETV combination therapy and better virologic response than ETV plus ADV combination therapy. TDF monotherapy may be the optimal strategy for CHB patients with ETV resistance.