Treatment for hepatitis B in patients with drug resistance

Exchanges in the 'a' determinant of the hepatitis B virus surface antigen revisited

The hepatitis B virus surface antigen's (HBsAg) 'a' determinant comprises a sequence of amino acid residues located in the major hydrophilic region of the S protein, whose exchanges are closely associated with compromising the antigenicity and immunogenicity of that antigen. The HBsAg is generally present in the bloodstream of individuals with acute or chronic hepatitis B virus (HBV) infection. It is classically known as the HBV infection marker, and is therefore the first marker to be investigated in the laboratory in the clinical hypothesis of infection by this agent. One of the factors that compromises the HBsAg detection in the bloodstream by the assays adopted in serological screening in both clinical contexts is the loss of S protein antigenicity. This can occur due to mutations that emerge in the HBV genome regions that encode the S protein, especially for its immunodominant region - the 'a' determinant. These mutations can induce exchanges of amino acid residues in the S protein's primary structure, altering its tertiary structure and the antigenic conformation, which may not be recognized by anti-HBs antibodies, compromising the infection diagnosis. In addition, these exchanges can render ineffective the anti-HBs antibodies action acquired by vaccination, compromise the effectiveness of the chronically HBV infected patient's treatment, and also the HBsAg immunogenicity, by promoting its retention within the cell. In this review, the residues exchange that alter the S protein's structure is revisited, as well as the mechanisms that lead to the HBsAg antigenicity loss, and the clinical, laboratory and epidemiological consequences of this phenomenon.

Elimination of the hepatitis B virus: A goal, a challenge

The hepatitis B elimination is a goal proposed by the WHO to be achieved by 2030 through the adoption of synergistic measures for the prevention and chronic HBV infection treatment. Complete cure is characterized by the HBV elimination from the body and is the goal of the chronic hepatitis B treatment, which once achieved, will enable the hepatitis B elimination. This, today, has been a scientific challenge. The difficulty in achieving a complete cure is due to the indefinite maintenance of a covalently closed episomal circular DNA (cccDNA) reservoir and the maintenance and persistence of an insufficient and dysfunctional immune response in chronically infected patients. Among the measures adopted to eliminate hepatitis B, two have the potential to directly interfere with the virus cycle, but with limited effect on HBV control. These are conventional vaccines-blocking transmission and antiviral therapy-inhibiting replication. Vaccines, despite their effectiveness in protecting against horizontal transmission and preventing mother-to-child vertical transmission, have no effect on chronic infection or potential to eliminate the virus. Treatment with antivirals suppresses viral replication, but has no curative effect, as it has no action against cccDNA. Therapeutic vaccines comprise an additional approach in the chronic infection treatment, however, they have only a modest effect on the immune system, enhancing it temporarily. This manuscript aims to address (1) the cccDNA persistence in the hepatocyte nucleus and the immune response dysfunction in chronically infected individuals as two primary factors that have hampered the treatment and HBV elimination from the human body; (2) the limitations of antiviral therapy and therapeutic vaccines, as strategies to control hepatitis B; and (3) the possibly promising therapeutic approaches for the complete cure and elimination of hepatitis B.

FIB‑4 index and serum α‑fetoprotein are useful predictors of hepatocellular carcinoma occurrence in hepatitis B patients with nucleos(t)ide analogs therapy

Current antiviral therapies cannot achieve eradication of hepatitis B virus (HBV) and can reduce but not eliminate the risk of hepatocellular carcinoma (HCC) in patients with chronic HBV infection. The present study aimed to identify the risk factors for HCC development by analyzing nucleoside analogue (NA)-treated patients as a retrospective cohort using fibrosis-4 index (FIB-4 index) as a non-invasive fibrosis marker. A total of 260 patients with HBV receiving NAs without a history of HCC between January 2001 and January 2021 were included in the present study. The incidence of HCC in patients with HBV during NA therapy and the factors contributing to HCC occurrence were identified using clinical characteristics and blood test results. Among the 260 patients, 40 patients (15.4%) developed HCC. Univariate and multivariate analysis showed that age [hazard ratio (HR), 1.03; P=0.045], male sex (HR, 3.14; P<0.01) and FIB-4 index at 6 months after NA treatment <1.95 (HR, 4.35; P<0.01) correlated with the incidence of HCC. The cumulative incidence of HCC in patients with FIB-4 index at 6 months after NA treatment >1.95 was significantly higher compared with that in patients with FIB-4 index ≤1.95 (P<0.01). Multivariate analysis in patients in which serum α-fetoprotein (AFP) level at 6 months after NA treatment was measured showed that FIB-4 index >1.95 (HR, 8.27; P=0.014) and serum AFP level >4 ng/ml (HR, 4.26; P=0.033) contributed to HCC occurrence. FIB-4 index at 6 months after NA treatment and serum AFP levels at 6 months after NA treatment were predictors for the development of HCC in patients with HBV during NA treatment. Further study of hepatocarcinogenesis during NA with a longer follow-up period and larger numbers of participants is required.

The premise of capsid assembly modulators towards eliminating HBV persistence

INTRODUCTION

The burden of chronic hepatitis B virus (HBV) results in almost a million deaths per year. The most common treatment for chronic hepatitis B infection is long-term nucleoside analogs (NUC) or one-year interferon-alpha (pegylated or non-pegylated) therapy before or after NUC therapy. Unfortunately, these therapies rarely result in HBV functional cure because they do not eradicate HBV from the nucleus of the hepatocytes, where the covalently closed circular DNA (cccDNA) is formed and/or where the integrated HBV DNA persists in the host genome. Hence, the search continues for novel antiviral therapies that target different steps of the HBV replication cycle to cure chronically infected HBV individuals and eliminate HBV from the liver reservoirs.

AREAS COVERED

The authors focus on capsid assembly modulators (CAMs). These molecules are unique because they impact not only one but several steps of HBV viral replication, including capsid assembly, capsid trafficking into the nucleus, reverse transcription, pre-genomic RNA (pgRNA), and polymerase protein co-packaging.

EXPERT OPINION

Mono- or combination therapy, including CAMs with other HBV drugs, may potentially eliminate hepatitis B infections. Nevertheless, more data on their potential effect on HBV elimination is needed, especially when used daily for 6-12 months.

Resistant mutations within the hepatitis B virus reverse transcriptase sequence in treatment failure patients with chronic HBV infection in Vietnam

OBJECTIVES

We conducted this study to describe whether mutations in the gene coding for the enzyme reverse transcriptase (RT) were related to drugs used in the treatment of hepatitis B in Vietnam.

METHODS

Patients receiving antiretroviral therapy with evidence of treatment failure were included in the study. The RT fragment was cloned using the polymerase chain reaction technique after being extracted from patients' blood samples. The nucleotide sequences were analysed using Sanger method. The HBV drug resistance database contains mutations associated to resistance to existing HBV therapies. Medical records were accessed to collect information on patient parameters, such as treatment, viral load, biochemistry, and blood count.

RESULTS

Resistance mutations to lamivudine, telbivudine, and entecavir were found in the highest proportion (75-91.7%) of HBV samples from patients who had failed antiretroviral therapy. Only 20.8% of HBV strains had mutations exhibiting adefovir resistance, while none had mutations conferring tenofovir resistance. M204I/V, L180M, and L80I are frequent variants linked with resistance to lamivudine, telbivudine, and entecavir. In contrast, the A181L/T/V mutation was detected predominantly in tenofovir-resistant HBV strains. Following the drug resistance mutation test, patients achieved the greatest virological response after 24 weeks of therapy with tenofovir and entecavir at a daily dose of one tablet.

CONCLUSION

Lamivudine, telbivudine, and entecavir were all highly resistant to the RT enzyme modifications in 24 treatment failure patients, with M204I/V, L180M, and L80I being the most prevalent mutations. Tenofovir resistance mutations have not been found in Vietnam.

Long-term follow-up of treatment-naïve HBeAg-negative patients with chronic hepatitis B

BACKGROUND/AIMS

We aimed to explore long-term results of oral antiviral agents in treatment-naïve "HBeAg negative chronic hepatitis B (CHB)" and determine the factors affecting the complete virological response.

METHOD

Patients with HBeAg-negative CHB who used oral antiviral agents for at least 3 years were evaluated retrospectively.

RESULTS

A total of 173 patients were recorded. The mean duration of treatment was 62.2 ± 28.9 months. Complete virological responses (CVR) were 82.8% (n = 53/64) in tenofovir disoproxil fumarate (TDF), 84.4% (n = 49/58) in lamivudine (LAM), 83.9% (n = 26/31) in entecavir (ETV), 95% in telbivudine (LdT) (n = 19/20) (p = 0.290). Multivariate analysis revealed age ≤ 40 (p = 0.012, 95%CI = 1.38-13.76, OR = 4.36) and baseline HBV DNA value (p = 0.003, 95%CI = 1.23-2.63, OR = 1.78) as independent factors for CVR. Virological breakthrough was detected in 29 (50%) patients on LAM therapy, two (6.4%) patients on ETV therapy, and two (10%) patients on LdT therapy. Treatment was switched to another antiviral agent due to osteoporosis in four patients in the TDF group, muscle pain in nine patients in the LDT group, and headache in one patient in the ETV group. Hepatocelluler cancer was detected in five patients. HBsAg seroclearance developed in two patients. Anti-HBs seroconversion was not detected.

CONCLUSION

CVR was achieved at similar rates with all four antiviral agents, while younger age (≤ 40) and low baseline viral load were the main factors for virological response. However, drug resistance and virological breakthrough in the LAM group and side effects in the LdT group were detected during the long-term follow-up. Moreover, HBsAg seroclearance was achieved at very low rates with oral antiviral agents.

Asian Pacific association for the study of liver (APASL) guidelines: hepatitis B virus in pregnancy

Hepatitis B virus (HBV) infection still remains a major public health issue in the Asia-Pacific region. Most of the burden of HBV-related disease results from infections acquired in infancy through perinatal or early childhood exposure to HBV in Asia-Pacific. Hepatitis B during pregnancy presents unique management issues for both the mother and fetus. These APASL guidelines provide a comprehensive review and recommendations based on available evidence in the literature, for the management of females with HBV infection through every stage of pregnancy and postpartum. These also address the concerns, management challenges, and required follow-up of children born to hepatitis B-positive mothers.

Applicability of oral fluid samples for tracking hepatitis B virus mutations, genotyping, and phylogenetic analysis

Little is known about the usefulness of saliva samples for hepatitis B virus (HBV) genotyping and mutation analysis. The aim of this study was to evaluate the usefulness of oral fluid samples to determine HBV genotype distribution, S/polymerase mutations, and HBV subpopulation diversity among chronically HBV-infected individuals. Serum and oral fluid samples were obtained from 18 individuals for PCR and nucleotide sequencing of the HBV surface antigen gene. Biochemical analysis of liver enzymes (ALT, AST, GGT) and HBV, HCV, and HIV serological tests were also performed. All serum samples were HBsAg (+), anti-HBc (+), and anti-HBs (-); 55.6% were HBeAg (+)/anti-HBe (-), and 11.1% were anti-HIV (+). The mean HBV DNA viral load was 6.1 ± 2.3 log IU/mL. The HBV genotype distribution was as follows: A, 72.2%; D, 11.1%; E, 5.6%; F, 11.1%. A concordance of 100% in genotype classification and 99.8% in sequence similarity between paired oral fluid and serum samples was observed. HBsAg mutations were detected in all samples, but no resistance mutations were found in the polymerase gene. This study demonstrates that oral fluid samples can be used reliably for tracking HBV mutations, genotyping, and phylogenetic analysis. This could be important for molecular epidemiology studies with hard-to-reach populations.

Detection of Hepatitis B Virus M204V Mutation Quantitatively via Real-time PCR

BACKGROUND AND AIMS

Drug-resistant DNA mutations of the hepatitis B virus (HBV) affect treatment response in chronic hepatitis B patients. We have established a new, sensitive, specific, accurate and convenient real-time PCR method to detect HBV mutations quantitatively.

METHODS

Blood samples were collected from patients showing viral breakthrough, primary nonresponse, or poor response during treatment, and mutations were detected via direct sequencing to assess our method. A plasmid containing the M204V mutation was synthesized and standard curves plotted.

RESULTS

The determination coefficient for linear correlation between Ct and log plasmid copy numbers was 0.996, where Ct value was -3.723log (DNA concentration) +48.647. Coefficients of variation indicated good reproducibility. Correctness was within tolerable bias. Limit of detection was 10³ copies/mL. Specificity, accuracy, positive predictive value and negative predictive value were 92.86%, 100%, 96.88%, 100% and 94.74%, respectively.

CONCLUSIONS

These results show that our method can be used to detect HBV M204V mutations with the advantages of sensitivity, specificity and efficiency, providing a new choice for monitoring drug resistance.

Scalable Synthesis, In Vitro cccDNA Reduction, and In Vivo Antihepatitis B Virus Activity of a Phosphonomethoxydeoxythreosyl Adenine Prodrug

Standard literature procedures for the chemical synthesis of l-threose nucleosides generally employ l-ascorbic acid as starting material. Herein, we have explored two alternative routes that start from either l-arabitol or l-diethyl tartrate, both affording 2-O-methyl-l-threofuranose as a key building block for nucleobase incorporation. The access to multigram quantities of this glycosyl donor in a reproducible fashion allows for the preparation of 2'-deoxy-α-l-threofuranosyl phosphonate nucleosides on a large scale. This methodology was applied to the gram scale synthesis of an aryloxy amidate prodrug of phosphonomethoxydeoxythreosyl adenine. This prodrug exerted potent activity against an entecavir-resistant hepatitis B virus (HBV) strain, while leading to a significant reduction in the levels of HBV covalently closed circular DNA in a cellular assay. Furthermore, its remarkable anti-HBV efficacy was also confirmed in vivo using a hydrodynamic injection-based HBV mouse model, without relevant toxicity and systemic exposure occurring.

COLD-PCR Method for Early Detection of Antiviral Drug-Resistance Mutations in Treatment-Naive Children with Chronic Hepatitis B

We investigated Nucleos(t)ide-analogue (NA)-resistance mutations (mt) in 142 treatment-naive children with Chronic Hepatitis B (CHB), using a sensitive co-amplification at lower denaturation temperature (COLD)-PCR with Sanger DNA sequencing. An NA resistance-associated mt in the hepatitis B virus (HBV) reverse transcriptase (RT) was found in 66.2% of the patients, with nonclassical mt contributing the most (64.8%). Significantly higher frequencies of Lamivudine (LMV) and Adefovir dipivoxil (ADF) resistance-associated mt were found in genotypes B and C, respectively (OR_LMV/ADF: 1495.000; 95% CI: 89.800–24,889.032; p < 0.001). Single-point mt associated to LMV and ADF resistance were detected in 59.9% of the tested children with rtV207M (38.0%) and rtN238T (9.9%) being the most frequent. Multiple-point mt were found only in 8 cases (5.6%): 6 children carried double mt (rtV207M + rtL229V; rtV207M + rtI233V; rtV207I + rtV207M × 2 cases; rtV207M + rtS213T; rtN238A + rtS256G) relating to LMV or/and ADF resistance and 3 children carried triple mt (rtL180M + rtM204I + rtN238T; rtV207M + rtS213T + rtS256G) or quadruple mt (rtL180M + rtM204V + rtV207I/M) for LMV-ADF resistance and Entecavir-reduced susceptibility. Our data indicate that significantly higher frequencies of LMV and ADF-associated mutations were found in treatment-naïve children infected with HBV genotypes B and C, respectively. The developed COLD-PCR method and obtained data may contribute to the development of suitable treatments for children with CHB.

Brca1 is involved in tolerance to adefovir dipivoxil‑induced DNA damage

Nucleos(t)ide analogues (NAs) are currently the most important anti‑viral treatment option for patients with chronic hepatitis B (CHB). Adefovir dipivoxil (ADV), a diester pro‑drug of adefovir, has been widely used for the clinical therapy of hepatitis B virus infection. It has been previously reported that adefovir induced chromosomal aberrations (CAs) in the in vitro human peripheral blood lymphocyte assay, while the genotoxic mechanism remains elusive. To evaluate the possible mechanisms, the genotoxic effects of ADV on the TK6 and DT40 cell lines, as well as DNA repair‑deficient variants of DT40 cells, were assessed in the present study. A karyotype assay revealed ADV‑induced CAs, particularly chromosomal breaks, in wild‑type DT40 and TK6 cells. A γ‑H2AX foci formation assay confirmed the presence of DNA damage following treatment with ADV. Furthermore, Brca1‑/‑ DT40 cells exhibited an increased sensitivity to ADV, while the knockdown of various other DNA damage‑associated genes did not markedly affect the sensitivity. These comprehensive genetic studies identified the genotoxic capacity of ADV and suggested that Brca1 may be involved in the tolerance of ADV‑induced DNA damage. These results may contribute to the development of novel drugs against CHB with higher therapeutic efficacy and less genotoxicity.

Evaluation of drug resistance mutations in patients with chronic hepatitis B

Mutations occurring in viral polymerase gene of hepatitis B virus (HBV) due to the use of nucleos(t)id analogs reduce the activity of the drugs by causing antiviral resistance. In this study, it was aimed to evaluate mutations responsible for drug resistance and drug resistance mutation rates in patients followed up by the diagnosis of chronic hepatitis B (CHB). A total of 318 CHB patients were included in the study. HBV mutations were detected using the INNO-LiPA commercial kit based on the reverse hybridization principle. Drug resistance mutation was detected in 46.86% (149/318) of the patients. The rates of drug resistance were found 36.79% (117/318) for lamivudine resistance, 12.58% (40/318) for entecavir (ETV), and 7.86% (25/318) for adefovir. In 10 patients, the possible tenofovir (TDF) resistance (3.14%) was found. Single-drug and double-drug resistances were detected in 34.59% and in 11.01% of the patients, respectively. Triple drug resistance was detected in only 1.26% of the patients. Unlike various studies in Turkey and in other countries, remarkable resistance to ETV and TDF were found in this study. The high rate of the probable TDF resistance was striking, with 3.14%.

Pharmacokinetics studies of 4'-cyano-2'-deoxyguanosine, a potent inhibitor of the hepatitis B virus, in rats

OBJECTIVES

4'-cyano-2'-deoxyguanosine (CdG), a novel nucleoside analogue, has a high degree of antiviral activity against the chronic hepatitis B virus (HBV). The objective of this study was to develop an analytical method for quantitatively determining CdG levels in biological samples by liquid chromatography-mass spectrometry (LC/MS) and to investigate the pharmacokinetic properties of CdG in rats after intravenous and oral administration.

METHODS

An analytical method using a UPLC system interfaced with a TOF-MS system was developed and validated. The pharmacokinetic properties after the intravenous and oral administration of CdG to rats were evaluated. In vivo pharmacokinetic interactions between CdG and entecavir were also investigated.

KEY FINDINGS

A rapid, simple and selective method for the quantification of CdG in biological samples was established using LC/MS with solid-phase extraction. In vivo pharmacokinetic studies of CdG in rats demonstrated that CdG is highly bioavailable, is rapidly absorbed from the intestinal tract, is then distributed to the liver rather than kidney and is ultimately excreted via the urine in an unchanged form. The co-administration of CdG and entecavir led to pharmacokinetic interactions with each other.

CONCLUSIONS

The data generated in this study provide support for the clinical development of CdG for use in the treatment of HBV.

Kushenin combined with adefovir dipivoxil affects the HBV-DNA load in serum, immune functions and liver functions of patients with chronic hepatitis B

This study aimed to explore the effect of kushenin combined with adefovir dipivoxil on the load of hepatitis B virus DNA (HBV-DNA) in serum, in immune functions and in liver functions of patients with chronic hepatitis B. A sample of 80 patients with chronic hepatitis B was selected who were admitted to Weifang People's Hospital for treatment between January, 2013 and December, 2015. They were divided into the observation group (n=40) and the control group (n=40). The patients in both groups received adefovir dipivoxil, while those in the observation group additionally received the kushenin. Variations in HBV-DNA load and transforming growth factor-β1 (TGF-β1) in the two groups were detected before intervention, at 1 month, 3 months and 6 months after intervention. In addition, after intervention, we also observed the changes in CD4+, CD8+ and CD4+/CD8+, as well as the levels of immune globulin. Furthermore, in these two groups, we detected the changes in endotoxin in serum before and after intervention, the liver function after intervention, and the variations of hyaluronic acid (HA) and type III procollagen (PCIII) before and after intervention which were used to serve as the indicators for hepatic fibrosis. Results showed that at one month, 3 months and 6 months after intervention, HBV-DNA load and the level of TGF-β1 in the observation group were lower than those in the control group (P<0.05). In the observation group, the HBV-DNA load at 6 months after intervention was the lowest, sequentially followed by the levels at 3 months, at 1 month and before intervention (P<0.05). After intervention, the levels of CD4+ and CD8+ and CD4+/CD8+ in the observation group were higher than those in the control group (P<0.05). Moreover, the levels of immunoglobulin M (IgM), immunoglobulin G (IgG) and immunoglobulin A (IgA) were elevated in the observation group compared to the levels in the control group (P<0.05). Additionally, the level of endotoxin in serum, the levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and total bilirubin (TBil). Besides, after intervention, the levels of HA and PCIII in the observation group were found to be lower than those in the control group before and after intervention (P<0.05). This study concludes that, for patients with chronic hepatitis B, kushenin combined with adefovir dipivoxil can remarkably decrease the HBV-DNA load, improve their immunity, ameliorate the liver function and delay the onset of liver cirrhosis.

Comparing Efficacy of Lamivudine, Adefovir Dipivoxil, Telbivudine, and Entecavir in Treating Nucleoside Analogues Naïve for HBeAg-Negative Hepatitis B with Medium Hepatitis B Virus (HBV) DNA Levels

Background

The antiviral effect of HBV in different nucleos (t) ide analogues is still not well known. This study was conducted to compare the effectiveness of lamivudine (LMV), adefovir dipivoxil (ADV), telbivudine (LdT), and entecavir (ETV) monotherapy in chronic HBeAg-negative hepatitis B patients with medium load of HBV DNA.

Material/Methods

The effective data of 207 patients treated by LMV (n=43), ADV (n=57), LdT (n=54) or ETV (n=53) were collected and analyzed during 144-week follow-up by retrospective analysis.

Results

Serum HBV DNA levels were significantly lower in the ETV group 1.91±0.45 log₁₀ IU/ml) than in the LdT group (2.09±0.62 log₁₀ IU/ml), ADV group (2.26±0.73 log₁₀ IU/ml), and LMV group (2.08±0.75 log₁₀ IU/ml) at 12 weeks (P=0.0464). HBV DNA levels were maintained at lower levels in the ETV group compared to other 3 groups during follow-up (48 weeks, P<0.001; 96 weeks, P<0.001). Multivariate Cox regression analysis showed that LMV (P=0.001), ADV, (P<0.001), and LdT (P<0.001) were all negative predictors of HBV DNA-negative time, but ETV was not. Viral breakthrough occurred in 34.8% (15/43) of patients in the LMV group; 5.26% (3/57) in the ADV group, 7.4.0% (4/54) in the LdT group, and 0% (0/53) in the ETV group at the end of follow-up. No significant differences were found in mean ALT levels (all P>0.05) or in cumulative normalization rates (P=0.473).

Conclusions

ETV was more potent and faster for viral response and lower viral breakthrough in medium load of HBV DNA when compared to LMV, ADV, or LdT monotherapy in HBeAg-negative CHB.

TDF Monotherapy Is Effective Regardless of Prior Nucleos(t)ide Analogue Treatment in Chronic Hepatitis B Patients in China

Background/Aims

Many patients had to transfer to tenofovir disoproxil fumarate (TDF) if there is other nucleos(t)ide analogue (NA) resistance. We aimed to investigate antiviral effects of TDF monotherapy between NA-naive and NA-experienced chronic hepatitis B (CHB) patients in China.

Methods

A total of 102 NA-naive and NA-experienced CHB patients with TDF monotherapy (300 mg/day) were retrospectively analyzed for useful parameters up to 72 weeks.

Results

There were 36 and 66 patients with matched HBV DNA baseline level in NA-naïve and NA-experienced group, respectively. There were no significant differences between NA-naïve and NA-experienced groups in HBV DNA levels (all P > 0.05) and HBV DNA undetectable rates (all P > 0.05) at all time points. At the end of follow-up, HBV DNA undetectable rates in NA-naïve and NA-experienced group were 96.2% (25/26) and 91.8% (45/49), respectively (P = 0.476). Baseline HBV DNA level was the only independent predictor for HBV DNA negative time (P = 0.018). In addition, 27.8% (5/18) and 11.4% (4/35) achieved HBeAg seroconversion at the end of the follow-up, respectively (P = 0.133).

Conclusions

TDF monotherapy was effective regardless of prior NA experienced. Baseline HBV DNA was a key predictive factor for HBV DNA negative time in TDF monotherapy.