Abstract: Chronic hepatitis B virus (HBV) infection remains a major global health burden. Millions of people are at risk for complications of chronic HBV infection, despite the widespread availability of an effective prophylactic vaccine. The current available treatments for HBV infection—interferon and nucleos(t)ide analogues—are effective at suppressing viral replication and decreasing the risk of cirrhosis. However, these treatments have a number of limitations, creating the need for alternative therapeutic agents. Recent advances in drug therapy have heralded a new horizon of novel therapeutic approaches for chronic HBV infection, with several promising antiviral and immunomodulatory agents currently in preclinical or clinical testing. This article reviews the current landscape of HBV treatments and highlights the most recent therapeutic strategies designed to directly target HBV or to improve immune response during chronic infection.
Hepatitis B virus (HBV) infection is a leading cause of chronic liver disease, affecting more than 290 million people worldwide.1 Annually, 750,000 people die from complications of chronic HBV infection.2 Elimination or suppression of HBV reduces the risk of developing chronic liver disease, cirrhosis, and hepatocellular carcinoma (HCC). Current treatments are associated with improved liver-related outcomes but relatively low rates of sustained hepatitis B surface antigen (HBsAg) seroconversion. Novel and potentially promising therapeutic strategies are in development and may result in more durable and complete responses. This article reviews the current landscape of HBV treatments and highlights the most recent therapeutic strategies designed to directly target HBV or to improve immune response during chronic infection.
Phases of Chronic Hepatitis B Virus Infection
The natural history of chronic HBV infection is a dynamic interaction between viral replication and the host’s immune response. Patients with chronic HBV infection can transition through 4 clinical phases, defined by the following clinical parameters: HBV DNA level, alanine aminotransferase (ALT) level, hepatitis B e antigen (HBeAg) status, and liver histology (Table 1).3 Active HBV replication, defined as the presence of HBeAg or HBV DNA, is associated with disease progression and increased risk of HCC.4
The first phase of chronic HBV infection is the immune-tolerant phase, which is associated with high levels of viremia and HBeAg but normal ALT levels. During the second phase, the HBeAg-positive chronic HBV immune-active phase, ALT elevates. After HBeAg loss, most patients enter the third phase, inactive chronic HBV, in which they are HBeAg-negative, have low or undetectable HBV DNA, and have normal ALT. In the fourth phase, patients are HBeAg-negative and have moderate to high HBV DNA levels and elevated ALT. The duration of each phase varies from months to decades. Transitions to a later phase and regression back to an earlier phase can occur.5 Complete suppression of HBV with antiviral therapy greatly reduces the risk of adverse clinical outcomes.6
Life Cycle and Replication of Hepatitis B Virus Infection
Advances in understanding the molecular biology and replication cycle of HBV have allowed for the development of new therapeutic drug targets. A member of the Hepadnaviridae family, HBV is a small, enveloped, hepatotropic DNA virus that replicates and persists in the nucleus of infected hepatocytes (Figure).7 The HBV virion is composed of an envelope and a nucleocapsid that contains a partially double-stranded, relaxed circular DNA (rcDNA). The virion enters the hepatocyte via the sodium taurocholate cotransporting polypeptide (NTCP) receptor, the viral envelope is shed, and the nucleocapsid is transported through the nuclear pore complex.8 Capsid dissociation in the nucleus leads to the release of rcDNA, which is then converted to covalently closed circular DNA (cccDNA).9 The cccDNA integrates with the host DNA and is transcribed into pregenomic RNA (pgRNA), which is then transcribed into viral messenger RNAs and reverse-transcribed into HBV DNA. The viral messenger RNAs are translated into 4 major proteins: HBsAg, HBeAg/core protein (Cp), polymerase, and x protein, whose function remains unclear.7,9 The nucleocapsid with the partially double-stranded HBV DNA is re-enveloped, and the virion is secreted back into the cytoplasm; however, the virion also can be recycled from the cytoplasm back into the nucleus. In this way, cccDNA can replenish and persist without the need for entry of new virions. Each step of the HBV life cycle is a potential therapeutic target (Figure).
Current Treatments
To date, there are 2 classes of drugs approved by the US Food and Drug Administration (FDA) for the treatment of HBV: interferon (IFN) and nucleos(t)ide analogues (NAs) (Table 2).
Interferon
Standard IFN a was approved as the first agent for the treatment of HBV infection 40 years ago.10 IFNs are cytokines with potent antiviral, antiproliferative, and immunomodulatory properties. Although the exact mechanism(s) remain elusive, IFNs ultimately induce IFN-stimulated genes, resulting in the degradation of viral mRNA, inhibition of viral protein synthesis and HBV replication, and prevention of viral infection of cells.11 IFNs also may augment cell-mediated immunity, thus promoting clearance of HBV-infected cells.12
In 2005, pegylated (PEG)-IFN a largely replaced standard IFN a as first-line treatment for chronic HBV infection owing to its improved pharmacokinetics and prolonged half-life.13 Treatment of HBeAg-positive chronic HBV infection with PEG-IFN a2a for 1 year was associated with HBeAg seroconversion in 32% of patients vs 19% with lamivudine monotherapy (P<.001) at 6 months following therapy.14 Sustained HBeAg seroconversion was confirmed in a small cohort of patients with HBeAg-positive chronic HBV infection, with the rate of seroconversion rising progressively from 37% at the end of treatment to 60% at 5 years.15 Safety of PEG-IFN α2b was evaluated in patients with advanced fibrosis and compensated cirrhosis. Although use of PEG-IFN α2b did not precipitate immunologic flares, fatigue, anorexia, and thrombocytopenia occurred more often in patients with advanced fibrosis when compared with those without (P<.01).16
Preferred Nucleos(t)ide Analogues
Developed in the 1980s, NAs have similar structures to natural nucleos(t)ides and competitively inhibit HBV polymerase activity, thereby preventing synthesis of viral DNA. Long-term treatment with NAs has demonstrated improved survival rates and decreased rates of HCC.17 NAs have an overall favorable safety profile but are associated with adverse events (AEs) such as abdominal pain/discomfort, upper respiratory tract infections, fatigue, and headache.18 There are currently 5 FDA-approved NAs for the treatment of chronic HBV infection, although major professional societies researching liver disease recommend only entecavir (ETV) and tenofovir disoproxil fumarate (TDF) as monotherapies.3,19
The FDA approved ETV, a cyclopentyl guanosine analogue and potent selective inhibitor of HBV replication, in 2005.20 In a phase 3, double-blind trial, ETV administered for 48 weeks in patients with HBeAg-positive chronic HBV yielded significantly greater histologic improvements and undetectable HBV DNA levels when compared with lamivudine (72% vs 62%; P=.009 and 67% vs 36%; P<.001, respectively).21 Extended treatment through 96 weeks showed continued benefits with suppression of HBV DNA to less than 300 copies/mL in ETV-treated vs lamivudine-treated patients (80% vs 39%; P<.0001).22 Treatment with ETV up to 5 years was associated with viral suppression in 94% of patients with chronic HBV infection.23 In a study of 1315 patients who received ETV for 4 years, results demonstrated a 60% risk reduction of HCC and significant risk reduction of cirrhosis-related events, including variceal bleeding (hazard ratio [HR], 0.38; 95% CI, 0.20-0.74), spontaneous bacterial peritonitis (HR, 0.06; 95% CI, 0.01-0.32), and hepatic encephalopathy (HR, 0.78; 95% CI, 0.28-2.14), and liver-related mortality (HR, 0.14; 95% CI, 0.07-0.13) in patients with chronic HBV–related cirrhosis.24 ETV has a similar safety profile to lamivudine, but without the drug resistance that limits lamivudine’s efficacy.21 Unlike older NAs, ETV is effective against lamivudine-resistant HBV.25
TDF is recommended as a first-line agent for chronic HBV infection. The FDA approved TDF, a prodrug of tenofovir, for chronic HBV infection in 2008. The safety and efficacy of TDF for the treatment of chronic HBV infection were evaluated in patients with HBeAg-positive and -negative chronic HBV infection, with results demonstrating higher rates of viral suppression after 48 weeks of treatment with TDF when compared with adefovir (ADV; 93% vs 63%; P<.001).26 Virologic suppression occurred in patients with HBeAg-positive and -negative chronic HBV infection who received extended treatment with TDF for 5 years (84.5% and 87.9%, respectively) and for 7 years (99.3% and 80%, respectively).27,28 Moreover, in a 10-year study of 641 HBeAg-positive and -negative patients, more than 98% of patients achieved HBV viral suppression with no evidence of drug resistance.29 In a real-world study of 92 patients with chronic HBV infection treated with TDF for 3 years, there was no difference in achieving complete virologic response between treatment-naive and -experienced patients (P=.6207).30 TDF monotherapy for the treatment of patients with chronic HBV infection and resistance to ETV or ADV was still associated with significant virologic suppression and was not significantly different between groups (84.4% vs 73.5%, respectively; P=.07).31 In a 5-year study of patients with HBeAg-positive and -negative chronic HBV infection treated with TDF, 51% of patients had regression of fibrosis (P<.0001) and 71% of patients had regression of cirrhosis (P<.0001).32 Genotypic resistance to TDF has not been detected.27,33,34 Reductions in bone mineral density and increases in renal toxicity from long-term use of TDF have been reported in patients with HIV infection; however, similar AEs have not been reported in patients with chronic HBV infection.29
Tenofovir alafenamide fumarate (TAF) is a tenofovir prodrug with more efficient delivery of active metabolite to hepatocytes than TDF, leading to higher intrahepatic concentrations of active drug.35 Owing to lower systemic exposure, TAF offers the potential for an improved safety profile. Administration of TAF at a dose of 25 mg daily was associated with a decrease in serum HBV DNA comparable to that of the standard TDF dose of 300 mg daily.36 In a large randomized trial of TAF vs TDF in 873 patients with HBeAg-positive chronic HBV infection, both treatment groups had similar results in achieving HBV DNA of less than 29 IU/mL at 48 weeks (64% vs 67%; P=.25).37 TAF also was compared with TDF in 426 patients with HBeAg-negative chronic HBV, and, again, both treatment groups had similar results in achieving HBV DNA of less than 29 IU/mL at 48 weeks (94% vs 93%; P=.47).38 In HBeAg-positive patients, the viral suppression rate in TAF-treated vs TDF-treated patients was 73% vs 75% (P=.47) and in HBeAg-negative patients, the viral suppression rate in TAF-treated vs TDF-treated patients was 90% vs 91% (P=.83), respectively.39 Treatment with TAF had significantly smaller decreases in bone mineral density of the hip (mean % change −0.33% vs −2.51%; P<.001) and lumbar spine (mean % change −0.75% vs −2.57%; P<.001), as well as a significantly smaller median change in estimated glomerular filtration rate (eGFR) by Cockcroft-Gault method (−1.2 vs −4.8 mg/dL; P<.001) when compared with patients who received TDF.39 In 2017, TAF was added to the list of first-line treatments for chronic HBV infection. TAF and TDF are both well tolerated, but changes in renal parameters, specifically eGFR, favor use of TAF over TDF.40
The current recommended first-line agents for immune-active chronic HBV infection are PEG-IFN a, ETV, or tenofovir (TDF or TAF). ETV should not be used in patients with lamivudine or telbivudine resistance because of the high risk of ETV resistance. TDF has been shown to be effective in patients with lamivudine-, ADV-, or ETV-resistant HBV and is preferred to ETV in these patients.41,42
Nonpreferred Nucleos(t)ide Analogues
Lamivudine, the oldest oral NA approved for the treatment of chronic HBV infection, is a cytidine analogue that competes with cytosine in viral DNA synthesis.43 In a 1-year double-blind trial of 358 patients with chronic HBV infection, lamivudine 100 mg daily significantly improved hepatic necroinflammation when compared with placebo (56% vs 25%; P=.001).44 Lamivudine is a relatively inexpensive drug with favorable efficacy and safety profiles; thus, it was previously widely used as the first-line treatment for chronic HBV infection. However, prolonged lamivudine monotherapy is associated with a high rate of resistance owing to frequent mutations in the viral polymerase gene. Therefore, lamivudine is no longer used as a first-line agent for chronic HBV infection.45
ADV, an adenosine NA, was the second NA approved for the treatment of chronic HBV infection. ADV 10 mg daily administered for 48 weeks in patients with HBeAg-positive and -negative chronic HBV infection effectively reduced serum HBV DNA levels (P<.001 vs
placebo for both groups) and improved liver histology by 53% and 64% (P<.001 vs placebo for both groups).46,47 Treatment at 30 mg daily is more effective at reducing serum HBV DNA levels; however, this approach has an increased risk of nephrotoxicity.48 Drug resistance mutations have been identified with a reported cumulative probability of mutations with virologic resistance of 20% in patients with chronic HBV infection within 240 weeks of treatment.49,50 Higher rates of nephrotoxicity and drug resistance limit the utility of ADV as a monotherapy for patients with chronic HBV infection.
Telbivudine, a thymidine NA, was compared with lamivudine for efficacy and safety in the GLOBE trial and demonstrated significantly greater therapeutic response in HBeAg-positive (63% vs 48%; P<.001) and HBeAg-negative (78% vs 66%; P=.007) patients treated for 2 years with significantly less resistance (10.8% vs 25.9%; P<.001).51 However, in a comparative study of 179 patients with chronic HBV infection receiving telbivudine vs ETV, results demonstrated high rates of viral mutations (9.1% vs 1.1%) and elevations in creatinine kinase levels (8.0% vs 0%).52 In 2016, production of telbivudine was discontinued in the United States.53
Other Nucleos(t)ide Analogues
Emtricitabine is a cytidine analogue that inhibits HBV polymerase. It is currently not approved for the treatment of chronic HBV infection. In a randomized controlled trial comparing emtricitabine 200 mg daily with placebo once daily for 48 weeks, results demonstrated a reduction of HBV DNA to less than 400 copies/mL in 54% and 2% of patients, respectively (P<.001).54 However, 13% to 18% of patients on emtricitabine developed resistance mutations, thus limiting its use as a monotherapy.54,55
Clevudine, a pyrimidine NA, inhibits the synthesis of positive-strand DNA, which may have additional effects on cccDNA.56 In a randomized trial comparing clevudine 30 mg daily with placebo for 24 weeks in patients with HBeAg-positive and -negative chronic HBV infection, results demonstrated sustained viral suppression (92.1% vs 0%; P<.0001) and normalization of ALT (74.6% vs 33.3%; P=.0006).57 Although there have been no safety or drug resistance issues detected with short-term treatment, extended durations of therapy are associated with increased rates of myopathy and drug resistance.57 Currently, clevudine is approved for the treatment of chronic HBV infection only in South Korea.
Combination Therapy
Theoretically, combination therapy with PEG-IFN α and NAs is an attractive approach that may potentially increase efficacy owing to differing mechanisms of action. However, strategies to combine these therapies have been met with varying levels of success.
In a randomized trial of HBeAg-positive and
-negative patients, 48-week treatment with simultaneous (de novo) combination therapy of TDF and PEG-IFN α resulted in significantly more HBsAg loss after 0.5 years of treatment (9.1%, 2.8%, and 0%; P<.05 in combination, PEG-IFN α monotherapy, and TDF monotherapy, respectively).58 In a large randomized trial of HBeAg-
negative patients with HBV DNA loads of less than 20,000 IU/mL, simultaneous combination therapy of 48 weeks of TDF and PEG-IFN α did not improve the rate of HBsAg loss compared with the no-treatment group after 0.5 years of therapy (4% vs 0%; P=.38).59
Sequential (switch to) combination therapy, in which treatment with NAs (ETV or TDF) is followed by switching to PEG-IFN α, has been evaluated.60 In the New Switch trial, HBeAg-positive patients achieved HBeAg loss and HBV DNA loads to less than 200 IU/mL 1 year after treatment with sequential NA therapy for 1 to 3 years, followed by 48-week or 96-week treatment with PEG-IFN α (9.8% vs 15.3%; P=.17).61
Add-on combination therapy for chronic HBV infection, in which PEG-IFN α is added to ongoing NA therapy (ETV or TDF), has shown declines in HBsAg levels but low rates of HBsAg seroclearance in the short term. PEGON, a randomized trial of patients with HBeAg-positive chronic HBV infection on ETV or TDF for greater than 1 year along with an additional 48 weeks of PEG-IFN α, demonstrated a decline in HBsAg levels but not HBsAg loss after 0.5 years posttreatment, compared with NA monotherapy (0.4 vs 0.2 log10 IU/mL; P=.01).62
Currently, practice guidelines do not support the use of combination therapy for the treatment of chronic HBV infection owing to the lack of robust evidence demonstrating superiority over monotherapy. However, these conflicting results demonstrate the need for further studies to assess the safety and efficacy of combination therapy to increase rates of HBsAg seroclearance.
Advantages and Limitations of Current Treatments
Advantages of chronic HBV treatment with PEG-IFN α include finite treatment duration and the absence of drug resistance.63 Patients who respond to PEG-IFN α have a greater chance of HBeAg and HBsAg seroconversion and immune-mediated clearance after treatment.64 However, treatment with PEG-IFN α has severe limitations, including frequent AEs such as thrombocytopenia and leukopenia, which require dose adjustment or even medication discontinuation.65 PEG-IFN α is contraindicated in pregnancy and in patients with decompensated cirrhosis.66 Additional contraindications include patients with a history of suicidal tendency or active psychiatric illness.
Patients generally have better adherence to oral NAs compared with subcutaneously injected PEG-IFN α. Newer NAs are preferred over PEG-IFN α because of their potent antiviral activity, high barrier to antiviral resistance, and more favorable safety profile.67 However, the main disadvantages of NAs are lower rates of HBeAg and HBsAg seroconversion and the need for long-term therapy in the vast majority of patients.68 In addition, NAs do not provide a direct effect on the level and activity of cccDNA, which can persist in the infected liver despite successful antiviral treatment. Therefore, the durability of response to NAs is generally low, usually requiring indefinite therapy.
Potential Treatment Options
The availability of a simple, safe, and highly effective cure for hepatitis C virus infection has reignited the search for a cure for HBV infection. The limited efficacy of currently approved treatments for chronic HBV infection underscores the urgent need for more effective agents that not only suppress viral replication, but also completely eradicate HBV infection. Several novel approaches are currently in preclinical or early clinical development.
Nucleos(t)ide Analogues
Besifovir dipivoxil maleate (Besivo, Ildong Pharmaceutical) is a guanine monophosphate that has shown potent suppression of HBV DNA. Treatment with besifovir dipivoxil maleate for 48 weeks was noninferior to TDF in achieving virologic response (80.9% vs 84.9%; P=.46).69 In a 144-week, open-label, phase 3 study evaluating long-term use of besifovir dipivoxil maleate, results demonstrated good virologic response when compared with TDF (87.7% vs 92.1%; P=.36).70 No drug-resistant mutations were found, and besifovir dipivoxil maleate had a better safety profile than TDF.69,70
Pradefovir mesylate (Metabasis Therapeutics), a prodrug of ADV, has been evaluated for its efficacy and safety for chronic HBV infection. In a phase 2 study of 51 patients with chronic HBV infection, short-term treatment with pradefovir mesylate was associated with a decline in serum HBV DNA levels with a similar safety profile to TDF.71 A phase 3 clinical trial comparing pradefovir mesylate to TDF is currently in the enrollment stage (NCT04543565).
Entry Inhibitors
Bulevirtide (formerly Myrcludex-B; Hepcludex, Gilead) is a synthetic N-acetylated lipopeptide that binds to NTCP surface receptors and interferes with de novo HBV infection by decreasing viremia, HBsAg, and cccDNA levels.72 Most studies of bulevirtide have focused on chronic hepatitis δ virus infection or its coinfection with HBV. Bulevirtide 2 mg once daily subcutaneous injection was compared with bulevirtide combined with IFN a2a and IFN a2a alone in a phase 2 study. Results showed an HBV DNA decline greater than or equal to 1 log in the bulevirtide cohort, which was not significant (P=.2); however, there was a significant HBV DNA decline greater than or equal to 1 log in the bulevirtide–IFN a2a cohort (P=.04).73 Bulevirtide is currently approved in the European Union for chronic hepatitis δ virus infection.
RNA Interference
RNA interference is a potent and specific posttranscriptional gene silencing mechanism that can inhibit translation of viral proteins needed for cccDNA formation, thus effectively reducing cccDNA.74 Small interfering RNAs are large double-stranded RNAs processed to shorter nucleotide duplexes that enable gene silencing.75 In an ongoing phase 2 study, JNJ-3989 (Janssen and Arrowhead) administered subcutaneously at doses up to 400 mg concomitantly with ETV or TDF for 24 weeks reduced HBsAg to less than 100 IU/mL in 88% of patients.76 ARC-520 (Arrowhead) 2 mg/kg administered monthly along with a NA to HBeAg-positive and -negative patients with chronic HBV demonstrated a HBsAg mean reduction of 0.38 and 0.54 IU/mL, respectively. Treatment was generally well tolerated, with pyrexia as the only observed AE.77 VIR-2218 (Vir Biotechnology) in patients with chronic HBV infection demonstrated marked reductions in HBsAg.78 In HBeAg-negative patients, HBsAg load through 48 weeks dropped average maximums of 1.03 log10 IU/mL with 20 mg of VIR-2218 and 1.65 log10 with 200 mg, whereas in HBeAg-positive patients, average maximum drops in HBsAg measured 1.16 log10 IU/mL with 20 mg of VIR-2218 and 1.57 log10 with 200 mg.79
Antisense oligonucleotides are small, single-stranded nucleic acid sequences that selectively bind to their target RNAs to trigger degradation. Phase 2 data suggest that treatment with GSK3228836 (formerly IONIS-HBVRX; Ionis and GlaxoSmithKline) for 4 weeks is associated with a reduction in HBsAg when compared with placebo in patients with chronic HBV infection on NA therapy (mean HBsAg log10 IU/mL ± standard error change from baseline, –2.514 ± 0.783 vs –0.008, respectively).80 In NA-naive patients, HBsAg mean change was –1.556 ± 0.398 (P=.001 vs placebo) and HBV DNA mean change was –1.655 ± 0.427 (P<.001 vs placebo).80
Capsid Assembly Modulators
The HBV Cp is a multifunctional protein critical for the HBV life cycle. Capsid assembly modulators are small molecules that bind HBV Cps and interfere with the encapsidation of pgRNA and formation of viral nucleocapsids.81 ABI-H0731 (Assembly Biosciences) is a potent and selective first-generation capsid assembly modulator that inhibits HBV replication and cccDNA formation in in-vitro research.82 Treatment with ABI-H0731 for 28 days in patients with HBsAg-positive and -negative chronic HBV was safe and demonstrated potent antiviral activity with mean maximum HBV DNA reductions from baseline of 1.7, 2.1, and 2.8 log10 IU/mL in the 100-, 200-, and 300-mg dose cohorts, respectively.83 Reductions in pgRNA correlated with reductions in HBV DNA. A number of additional capsid assembly modulators are under clinical development.
Hepatitis B Surface Antigen Release Inhibitors
HBsAg is the most abundant circulating viral antigen and has direct immunoinhibitory activity against both innate and adaptive immune responses.84 Nucleic acid polymers are emerging antiviral therapies that block assembly of subviral particles, which effectively reduces both circulating and intracellular HBsAg.85 In phase 2 research, the addition of nucleic acid polymers to TDF and PEG-IFN α2α did not alter tolerability and significantly increased rates of HBsAg loss (P<.001 vs control) and HBsAg seroconversion (P=.046 vs control).86
Immunotherapy
Toll-like receptors are the initial sensors of viral infection because they initiate intracellular signaling pathways that induce antiviral mediators such as cytokines.87 Vesatolimod (Gilead) and selgantolimod (Gilead and Vir Biotechnology) are toll-like receptor agonists under clinical study for chronic HBV infection. Although vesatolimod was well tolerated, an initial study did not demonstrate a significant decline in HBsAg. At week 48, 6%, 16%, and 15% of patients on vesatolimod 1, 2, and 4 mg, respectively, achieved median HBsAg declines of at least 0.5 log10 IU/mL compared with placebo (18%).88 Early phase 2 data on selgantolimod were promising and demonstrated a small number of patients achieving HBsAg loss.89
In chronic HBV infection, antiviral B- and T-cell responses are defective. The so-called exhaustion state is characterized by poor cytotoxic activity, impaired cytokine production, and sustained expression of multiple inhibitory receptors and pathways.90 These receptors and pathways, which include programmed death 1 and cytotoxic T-lymphocyte–associated antigen 4, are thought to play a role in immune dysfunction, making them attractive therapeutic targets.91 However, a major concern of immunotherapy is uncontrolled immune activation, which can lead to fatal hepatitis flares or extrahepatic organ damage.
Covalently Closed Circular DNA Inhibitors
One of the key mechanisms for the persistence of the HBV genome is the presence of cccDNA in the nucleus of infected hepatocytes, allowing for continued viral transcription. Specifically targeting cccDNA may be an effective curative option for chronic HBV. A cleaving sequence-specific DNA using transcription activator-like effector nucleases or genome editing using clustered regularly interspaced short palindromic repeats could potentially block the formation of cccDNA and silence its transcription. These direct effects on cccDNA may offer a cure not feasible with IFN α or a NA. Potential cccDNA inhibitors are currently in very early development.92,93
Vaccines
The aim of therapeutic vaccines for chronic HBV infection is to achieve a functional cure. The first HBV vaccine therapy was studied in 1995 and was only able to achieve undetectable HBV DNA levels in 37.5% of study participants.94 Since then, therapeutic vaccines have been studied widely but have been largely unsuccessful owing to challenges including HBV genetic variability, persistence, and immune evasion.95 In a phase 2 study of patients with chronic HBV infection, the combination of GS-4774 (GlobeImmune and Gilead) with TDF demonstrated increased immune response but no significant reduction in HBsAg levels at week 24 when compared with the TDF-only group. The mean HBsAg declines for the 2-, 10-, and 40-YU GS-4774 arms were –0.096, –0.016, and –0.135 log10 IU/mL, respectively, which did not statistically differ from the placebo arm (–0.079 log10 IU/mL).96 Several DNA vaccines are under development, including heterologous prime-boost approaches, vaccines against multiple HBV proteins, and combination vaccine and/or antiviral drugs and immune modulators. However, these studies are in preclinical or phase 1 stages and therefore are not reliable treatment options for the immediate future.
Conclusion
A strong need to develop new and effective treatments for chronic HBV infection persists. The mechanisms and novel drug targets under current exploration are multifaceted. Attaining the goal of a clinical cure for chronic HBV infection likely will require combinations of immunomodulatory and antiviral therapies that target multiple steps in the HBV life cycle, including the elimination of cccDNA. Recent advances in technology provide a reasonable hope of a cure in our lifetime.
Disclosures
The author has no relevant conflicts of interest to disclose.
References
1. Polaris Observatory Collaborators. Global prevalence, treatment, and prevention of hepatitis B virus infection in 2016: a modelling study. Lancet Gastroenterol Hepatol. 2018;3(6):383-403.
2. Lozano R, Naghavi M, Foreman K, et al. Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010. Lancet. 2012;380(9859):2095-2128.
3. Terrault NA, Lok ASF, McMahon BJ, et al. Update on prevention, diagnosis, and treatment of chronic hepatitis B: AASLD 2018 hepatitis B guidance. Hepatology. 2018;67(4):1560-1599.
4. Lin C-L, Kao J-H. Hepatitis B viral factors and clinical outcomes of chronic hepatitis B. J Biomed Sci. 2008;15(2):137-145.
5. Lok ASF, McMahon BJ. Chronic hepatitis B. Hepatology. 2007;45(2):507-539.
6. Nam JY, Chang Y, Cho H, et al. Delayed viral suppression during antiviral therapy is associated with increased hepatocellular carcinoma rates in HBeAg-positive high viral load chronic hepatitis B. J Viral Hepat. 2018;25(5):552-560.
7. Beck J, Nassal M. Hepatitis B virus replication. World J Gastroenterol. 2007;13(1):48-64.
8. Yan H, Zhong G, Xu G, et al. Sodium taurocholate cotransporting polypeptide is a functional receptor for human hepatitis B and D virus. Elife. 2012;1:e00049.
9. Seeger C, Mason WS. Molecular biology of hepatitis B virus infection. Virology. 2015;479-480:672-686.
10. Greenberg HB, Pollard RB, Lutwick LI, Gregory PB, Robinson WS, Merigan TC. Effect of human leukocyte interferon on hepatitis B virus infection in patients with chronic active hepatitis. N Engl J Med. 1976;295(10):517-522.
11. Xu C, Guo H, Pan X-B, et al. Interferons accelerate decay of replication-competent nucleocapsids of hepatitis B virus. J Virol. 2010;84(18):9332-9340.
12. Isaacs A, Lindenmann J, Valentine RC. Pillars article: virus interference. II. Some properties of interferon. Proc R Soc Lond B Biol Sci. 1957. 147: 268-273. J Immunol. 2015;195(5):1921-1926.
13. Woo ASJ, Kwok R, Ahmed T. Alpha-interferon treatment in hepatitis B. Ann Transl Med. 2017;5(7):159.
14. Lau GKK, Piratvisuth T, Luo KX, et al; Peginterferon Alfa-2a HBeAg-Positive Chronic Hepatitis B Study Group. Peginterferon alfa-2a, lamivudine, and the combination for HBeAg-positive chronic hepatitis B. N Engl J Med. 2005;352(26):2682-2695.
15. Wong VW-S, Wong GL-H, Yan KK-L, et al. Durability of peginterferon alfa-2b treatment at 5 years in patients with hepatitis B e antigen-positive chronic hepatitis B. Hepatology. 2010;51(6):1945-1953.
16. Buster EHCJ, Hansen BE, Buti M, et al; HBV 99-01 Study Group. Peginterferon alpha-2b is safe and effective in HBeAg-positive chronic hepatitis B patients with advanced fibrosis. Hepatology. 2007;46(2):388-394.
17. Papatheodoridis GV, Sypsa V, Dalekos G, et al. Eight-year survival in chronic hepatitis B patients under long-term entecavir or tenofovir therapy is similar to the general population. J Hepatol. 2018;68(6):1129-1136.
18. de Fraga RS, Van Vaisberg V, Mendes LCA, Carrilho FJ, Ono SK. Adverse events of nucleos(t)ide analogues for chronic hepatitis B: a systematic review. J Gastroenterol. 2020;55(5):496-514.
19. Lampertico P, Agarwal K, Berg T, et al; European Association for the Study of the Liver. EASL 2017 Clinical Practice Guidelines on the management of hepatitis B virus infection. J Hepatol. 2017;67(2):370-398.
20. Innaimo SF, Seifer M, Bisacchi GS, Standring DN, Zahler R, Colonno RJ. Identification of BMS-200475 as a potent and selective inhibitor of hepatitis B virus. Antimicrob Agents Chemother. 1997;41(7):1444-1448.
21. Chang T-T, Gish RG, de Man R, et al; BEHoLD AI463022 Study Group. A comparison of entecavir and lamivudine for HBeAg-positive chronic hepatitis B. N Engl J Med. 2006;354(10):1001-1010.
22. Gish RG, Lok AS, Chang T-T, et al. Entecavir therapy for up to 96 weeks in patients with HBeAg-positive chronic hepatitis B. Gastroenterology. 2007;133(5):1437-1444.
23. Chang T-T, Lai C-L, Kew Yoon S, et al. Entecavir treatment for up to 5 years in patients with hepatitis B e antigen-positive chronic hepatitis B. Hepatology. 2010;51(2):422-430.
24. Su T-H, Hu T-H, Chen C-Y, et al; C-TEAM Study Group and the Taiwan Liver Diseases Consortium. Four-year entecavir therapy reduces hepatocellular carcinoma, cirrhotic events and mortality in chronic hepatitis B patients. Liver Int. 2016;36(12):1755-1764.
25. Sherman M, Yurdaydin C, Simsek H, et al; AI463026 Benefits of Entecavir for Hepatitis B Liver Disease (BEHoLD) Study Group. Entecavir therapy for lamivudine-refractory chronic hepatitis B: improved virologic, biochemical, and serology outcomes through 96 weeks. Hepatology. 2008;48(1):99-108.
26. Marcellin P, Heathcote EJ, Buti M, et al. Tenofovir disoproxil fumarate versus adefovir dipivoxil for chronic hepatitis B. N Engl J Med. 2008;359(23):2442-2455.
27. Liang X, Gao Z, Xie Q, et al. Long-term efficacy and safety of tenofovir disoproxil fumarate in Chinese patients with chronic hepatitis B: 5-year results. Hepatol Int. 2019;13(3):260-269.
28. Buti M, Tsai N, Petersen J, et al. Seven-year efficacy and safety of treatment with tenofovir disoproxil fumarate for chronic hepatitis B virus infection. Dig Dis Sci. 2015;60(5):1457-1464.
29. Marcellin P, Wong DK, Sievert W, et al. Ten-year efficacy and safety of tenofovir disoproxil fumarate treatment for chronic hepatitis B virus infection. Liver Int. 2019;39(10):1868-1875.
30. Lovett GC, Nguyen T, Iser DM, et al. Efficacy and safety of tenofovir in chronic hepatitis B: Australian real world experience. World J Hepatol. 2017;9(1):48-56.
31. Lim Y-S, Gwak G-Y, Choi J, et al. Monotherapy with tenofovir disoproxil fumarate for adefovir-resistant vs. entecavir-resistant chronic hepatitis B: a 5-year clinical trial. J Hepatol. 2019;71(1):35-44.
32. Marcellin P, Gane E, Buti M, et al. Regression of cirrhosis during treatment with tenofovir disoproxil fumarate for chronic hepatitis B: a 5-year open-label follow-up study. Lancet. 2013;381(9865):468-475.
33. Pan CQ, Trinh H, Yao A, Bae H, Lou L, Chan S; Study 123 Group. Efficacy and safety of tenofovir disoproxil fumarate in Asian-Americans with chronic hepatitis B in community settings. PLoS One. 2014;9(3):e89789.
34. Cathcart AL, Chan HL-Y, Bhardwaj N, et al. No resistance to tenofovir alafenamide detected through 96 weeks of treatment in patients with chronic hepatitis B infection. Antimicrob Agents Chemother. 2018;62(10):e01064-18.
35. Babusis D, Phan TK, Lee WA, Watkins WJ, Ray AS. Mechanism for effective lymphoid cell and tissue loading following oral administration of nucleotide prodrug GS-7340. Mol Pharm. 2013;10(2):459-466.
36. Agarwal K, Fung SK, Nguyen TT, et al. Twenty-eight day safety, antiviral activity, and pharmacokinetics of tenofovir alafenamide for treatment of chronic hepatitis B infection. J Hepatol. 2015;62(3):533-540.
37. Chan HLY, Fung S, Seto WK, et al; GS-US-320-0110 Investigators. Tenofovir alafenamide versus tenofovir disoproxil fumarate for the treatment of HBeAg-positive chronic hepatitis B virus infection: a randomised, double-blind, phase 3, non-inferiority trial. Lancet Gastroenterol Hepatol. 2016;1(3):185-195.
38. Buti M, Gane E, Seto WK, et al; GS-US-320-0108 Investigators. Tenofovir alafenamide versus tenofovir disoproxil fumarate for the treatment of patients with HBeAg-negative chronic hepatitis B virus infection: a randomised, double-blind, phase 3, non-inferiority trial. Lancet Gastroenterol Hepatol. 2016;1(3):196-206.
39. Agarwal K, Brunetto M, Seto WK, et al; GS-US-320-0110; GS-US-320-0108 Investigators. 96 weeks treatment of tenofovir alafenamide vs. tenofovir disoproxil fumarate for hepatitis B virus infection. J Hepatol. 2018;68(4):672-681.
40. Byrne R, Carey I, Agarwal K. Tenofovir alafenamide in the treatment of chronic hepatitis B virus infection: rationale and clinical trial evidence. Therap Adv Gastroenterol. 2018;11:1756284818786108.
41. Fung S, Kwan P, Fabri M, et al. Tenofovir disoproxil fumarate (TDF) vs. emtricitabine (FTC)/TDF in lamivudine resistant hepatitis B: a 5-year randomised study. J Hepatol. 2017;66(1):11-18.
42. Berg T, Zoulim F, Moeller B, et al. Long-term efficacy and safety of emtricitabine plus tenofovir DF vs. tenofovir DF monotherapy in adefovir-experienced chronic hepatitis B patients. J Hepatol. 2014;60(4):715-722.
43. Chang CN, Doong SL, Zhou JH, et al. Deoxycytidine deaminase-resistant stereoisomer is the active form of (+/-)-2′,3′-dideoxy-3′-thiacytidine in the inhibition of hepatitis B virus replication. J Biol Chem. 1992;267(20):13938-13942.
44. Lai CL, Chien RN, Leung NW, et al; Asia Hepatitis Lamivudine Study Group. A one-year trial of lamivudine for chronic hepatitis B. N Engl J Med. 1998;339(2):61-68.
45. Fischer KP, Gutfreund KS, Tyrrell DL. Lamivudine resistance in hepatitis B: mechanisms and clinical implications. Drug Resist Updat. 2001;4(2):118-128.
46. Marcellin P, Chang T-T, Lim SG, et al; Adefovir Dipivoxil 437 Study Group. Adefovir dipivoxil for the treatment of hepatitis B e antigen-positive chronic hepatitis B. N Engl J Med. 2003;348(9):808-816.
47. Hadziyannis SJ, Tassopoulos NC, Heathcote EJ, et al; Adefovir Dipivoxil 438 Study Group. Adefovir dipivoxil for the treatment of hepatitis B e antigen-negative chronic hepatitis B. N Engl J Med. 2003;348(9):800-807.
48. Simsek H, Shorbagi A, Balaban Y, Tatar G. What is the optimum dose of adefovir in the treatment of chronic hepatitis B infection? J Hepatol. 2008;49(3):464-465.
49. Marcellin P, Chang T-T, Lim SGL, et al. Long-term efficacy and safety of adefovir dipivoxil for the treatment of hepatitis B e antigen-positive chronic hepatitis B. Hepatology. 2008;48(3):750-758.
50. Hadziyannis SJ, Tassopoulos NC, Heathcote EJ, et al; Adefovir Dipivoxil 438 Study Group. Long-term therapy with adefovir dipivoxil for HBeAg-negative chronic hepatitis B for up to 5 years. Gastroenterology. 2006;131(6):1743-1751.
51. Liaw Y-F, Gane E, Leung N, et al; GLOBE Study Group. 2-year GLOBE trial results: telbivudine is superior to lamivudine in patients with chronic hepatitis B. Gastroenterology. 2009;136(2):486-495.
52. Pan H-Y, Pan H-Y, Song W-Y, et al. Long-term outcome of telbivudine versus entecavir in treating higher viral load chronic hepatitis B patients without cirrhosis. J Viral Hepat. 2017;24(suppl 1):29-35.
53. Han DH. FDA: hepatitis B drug discontinued. MPR. Published October 5, 2016. Accessed January 31, 2021. https://www.empr.com/home/news/fda-hepatitis-b-drug-discontinued/.
54. Lim SG, Ng TM, Kung N, et al; Emtricitabine FTCB-301 Study Group. A double-blind placebo-controlled study of emtricitabine in chronic hepatitis B. Arch Intern Med. 2006;166(1):49-56.
55. Gish RG, Trinh H, Leung N, et al. Safety and antiviral activity of emtricitabine (FTC) for the treatment of chronic hepatitis B infection: a two-year study. J Hepatol. 2005;43(1):60-66.
56. Jones SA, Murakami E, Delaney W, Furman P, Hu J. Noncompetitive inhibition of hepatitis B virus reverse transcriptase protein priming and DNA synthesis by the nucleoside analog clevudine. Antimicrob Agents Chemother. 2013;57(9):4181-4189.
57. Yoo BC, Kim JH, Kim T-H, et al. Clevudine is highly efficacious in hepatitis B e antigen-negative chronic hepatitis B with durable off-therapy viral suppression. Hepatology. 2007;46(4):1041-1048.
58. Marcellin P, Ahn SH, Ma X, et al; Study 149 Investigators. Combination of tenofovir disoproxil fumarate and peginterferon α-2a increases loss of hepatitis B surface antigen in patients with chronic hepatitis B. Gastroenterology. 2016;150(1):134-144.e10.
59. de Niet A, Jansen L, Stelma F, et al. Peg-interferon plus nucleotide analogue treatment versus no treatment in patients with chronic hepatitis B with a low viral load: a randomised controlled, open-label trial. Lancet Gastroenterol Hepatol. 2017;2(8):576-584.
60. Yoshida K, Enomoto M, Tamori A, Nishiguchi S, Kawada N. Combination of entecavir or tenofovir with pegylated interferon-α for long-term reduction in hepatitis B surface antigen levels: simultaneous, sequential, or add-on combination therapy. Int J Mol Sci. 2021;22(3):1456.
61. Hu P, Shang J, Zhang W, et al. HBsAg loss with peg-interferon alfa-2a in hepatitis B patients with partial response to nucleos(t)ide analog: New Switch study. J Clin Transl Hepatol. 2018;6(1):25-34.
62. Chi H, Hansen BE, Guo S, et al. Pegylated interferon alfa-2b add-on treatment in hepatitis B virus envelope antigen-positive chronic hepatitis B patients treated with nucleos(t)ide analogue: a randomized, controlled trial (PEGON). J Infect Dis. 2017;215(7):1085-1093.
63. Rijckborst V, Sonneveld MJ, Janssen HLA. Review article: chronic hepatitis B—anti-viral or immunomodulatory therapy? Aliment Pharmacol Ther. 2011;33(5):501-513.
64. Sonneveld MJ, Janssen HLA. Chronic hepatitis B: peginterferon or nucleos(t)ide analogues? Liver Int. 2011;31(suppl 1):78-84.
65. Perrillo R. Benefits and risks of interferon therapy for hepatitis B. Hepatology. 2009;49(5)(suppl):S103-S111.
66. Hoofnagle JH, Di Bisceglie AM, Waggoner JG, Park Y. Interferon alfa for patients with clinically apparent cirrhosis due to chronic hepatitis B. Gastroenterology. 1993;104(4):1116-1121.
67. Ono A, Suzuki F, Kawamura Y, et al. Long-term continuous entecavir therapy in nucleos(t)ide-naïve chronic hepatitis B patients. J Hepatol. 2012;57(3):508-514.
68. Seto W-K, Wong DK-H, Fung J, Huang F-Y, Lai C-L, Yuen M-F. Reduction of hepatitis B surface antigen levels and hepatitis B surface antigen seroclearance in chronic hepatitis B patients receiving 10 years of nucleoside analogue therapy. Hepatology. 2013;58(3):923-931.
69. Ahn SH, Kim W, Jung YK, et al. Efficacy and safety of besifovir dipivoxil maleate compared with tenofovir disoproxil fumarate in treatment of chronic hepatitis B virus infection. Clin Gastroenterol Hepatol. 2019;17(9):1850-1859.e4.
70. Yim HJ, Kim W, Ahn SH, et al. Besifovir dipivoxil maleate 144-week treatment of chronic hepatitis B: an open-label extensional study of a phase 3 trial. Am J Gastroenterol. 2020;115(8):1217-1225.
71. Zhang H, Wu M, Zhu X, et al. Safety, efficacy, and pharmacokinetics of pradefovir for the treatment of chronic hepatitis B infection. Antiviral Res. 2020;174:104693.
72. Volz T, Allweiss L, Ben MBarek M, et al. The entry inhibitor Myrcludex-B efficiently blocks intrahepatic virus spreading in humanized mice previously infected with hepatitis B virus. J Hepatol. 2013;58(5):861-867.
73. Bogomolov P, Alexandrov A, Voronkova N, et al. Treatment of chronic hepatitis D with the entry inhibitor Myrcludex B: first results of a phase Ib/IIa study. J Hepatol. 2016;65(3):490-498.
74. van den Berg F, Limani SW, Mnyandu N, Maepa MB, Ely A, Arbuthnot P. Advances with RNAi-based therapy for hepatitis B virus infection. Viruses. 2020;12(8):E851.
75. Nayagam JS, Cargill ZC, Agarwal K. The role of RNA interference in functional cure strategies for chronic hepatitis B. Curr Hepatol Rep. 2020;19(4):362-369.
76. Yuen M-F, Locarnini SA, Lim T-H, et al. Short term RNA interference (RNAi) therapy in chronic hepatitis B (CHB) using JNJ-3989 brings majority of patients to HBsAg <100 IU/ml threshold. Presented at: The International Liver Congress; April 10-14, 2019; Vienna, Austria.
77. Yuen M-F, Schiefke I, Yoon J-H, et al. RNA interference therapy with ARC-520 results in prolonged hepatitis B surface antigen response in patients with chronic hepatitis B infection. Hepatology. 2020;72(1):19-31.
78. Gane E, Lim YS, Tangkijvanich P, et al. Preliminary safety and antiviral activity of VIR-2218, an X-targeting HBV RNAi therapeutic, in chronic hepatitis B patients. J Hepatol. 2020;73:S50. Abstract AS068.
79. Gane E, Lim Y-S, Cloutier D, et al. Safety and antiviral activity of VIR-2218, an X-targeting RNAi therapeutic, in participants with chronic hepatitis B infection: week 48 follow-up results. Presented at: The International Liver Congress; June 23-26, 2021.
80. Yuen M-F, Heo J, Jang JW, et al. Hepatitis B virus (HBV) surface antigen (HBsAg) inhibition with ISIS 505358 in chronic hepatitis B (CHB) patients on stable nucleos(t)ide analogue (NA) regimen and in NA-naive CHB patients: phase 2a, randomized, double-blind, placebo-controlled study. Presented at: The Digital International Liver Congress; August 27-29, 2020. Abstract AS067.
81. Ko C, Bester R, Zhou X, et al. A new role for capsid assembly modulators to target mature hepatitis B virus capsids and prevent virus infection. Antimicrob Agents Chemother. 2019;64(1):e01440-19.
82. Huang Q, Cai D, Yan R, et al. Preclinical profile and characterization of the hepatitis B virus core protein inhibitor ABI-H0731. Antimicrob Agents Chemother. 2020;64(11):e01463-20.
83. Yuen M-F, Agarwal K, Gane EJ, et al. Safety, pharmacokinetics, and antiviral effects of ABI-H0731, a hepatitis B virus core inhibitor: a randomised, placebo-controlled phase 1 trial. Lancet Gastroenterol Hepatol. 2020;5(2):152-166.
84. Kondo Y, Ninomiya M, Kakazu E, Kimura O, Shimosegawa T. Hepatitis B surface antigen could contribute to the immunopathogenesis of hepatitis B virus infection. ISRN Gastroenterol. 2013;2013:935295.
85. Lee HW, Lee JS, Ahn SH. Hepatitis B virus cure: targets and future therapies. Int J Mol Sci. 2020;22(1):213.
86. Bazinet M, Pântea V, Placinta G, et al. Safety and efficacy of 48 weeks REP 2139 or REP 2165, tenofovir disoproxil, and pegylated interferon alfa-2a in patients with chronic HBV infection naïve to nucleos(t)ide therapy. Gastroenterology. 2020;158(8):2180-2194.
87. Ma Z, Cao Q, Xiong Y, Zhang E, Lu M. Interaction between hepatitis B virus and toll-like receptors: current status and potential therapeutic use for chronic hepatitis B. Vaccines (Basel). 2018;6(1):6.
88. Agarwal K, Ahn SH, Elkhashab M, et al. Safety and efficacy of vesatolimod (GS-9620) in patients with chronic hepatitis B who are not currently on antiviral treatment. J Viral Hepat. 2018;25(11):1331-1340.
89. Gane E, Zhao Y, Tan SK, et al. Efficacy and safety of oral TLR8 agonist selgantolimod in virally suppressed adult patients with chronic hepatitis B: a phase 2, randomized, double-blind, placebo-controlled, multicenter study. Presented at: The Liver Meeting; November 8-12, 2019; Boston, Massachusetts.
90. Ye B, Liu X, Li X, Kong H, Tian L, Chen Y. T-cell exhaustion in chronic hepatitis B infection: current knowledge and clinical significance. Cell Death Dis. 2015;6:e1694.
91. Maini MK, Schurich A. The molecular basis of the failed immune response in chronic HBV: therapeutic implications. J Hepatol. 2010;52(4):616-619.
92. Hong X, Kim ES, Guo H. Epigenetic regulation of hepatitis B virus covalently closed circular DNA: implications for epigenetic therapy against chronic hepatitis B. Hepatology. 2017;66(6):2066-2077.
93. Liu X, Hao R, Chen S, Guo D, Chen Y. Inhibition of hepatitis B virus by the CRISPR/Cas9 system via targeting the conserved regions of the viral genome. J Gen Virol. 2015;96(8):2252-2261.
94. Pol S. Immunotherapy of chronic hepatitis B by anti HBV vaccine. Biomed Pharmacother. 1995;49(3):105-109.
95. Lim SG, Agcaoili J, De Souza NNA, Chan E. Therapeutic vaccination for chronic hepatitis B: a systematic review and meta-analysis. J Viral Hepat. 2019;26(7):803-817.
96. Boni C, Janssen HLA, Rossi M, et al. Combined GS-4774 and tenofovir therapy can improve HBV-specific T-cell responses in patients with chronic hepatitis. Gastroenterology. 2019;157(1):227-241.e7.