In Search of a Cure: First Steps Toward the Conquest of Hepatitis B
Hepatitis B is a quiet virus that can take decades before its damaging effects on the liver are realized. However, with approximately 257 million people globally living with hepatitis B and an accounted 887,000 hepatitis B-related deaths in 2015 from complications such as cirrhosis and hepatocellular carcinoma, it requires as much attention as has been recently given to the elimination of hepatitis C.
The current state for HBV care includes vaccination for uninfected individuals or long-term treatment. While treatment is effective at suppressing the virus, the potential for a cure is years away and will most likely require a combination therapy approach, as the structure of the virus is even more complicated than that of HCV. This leaves those who are infected at a continued risk for liver-related complications.
“For hepatitis C, everything is now about access, elimination strategies, and risk reduction, as the therapies work exceptionally well,” Paul Y. Kwo, MD, director of hepatology at Stanford University, told Healio Gastroenterology and Liver Disease. “In hepatitis B, which is a far more complicated virus to treat than hepatitis C, we are just beginning to see the earliest efforts to probe the various strategies that are going to lead to a therapy with a finite duration that allows clearance of hepatitis B surface antigen.”
According to Kwo, the focus for HBV will be to find and develop new mechanisms of action for a “functional cure,” which would mean clearance of HBV surface antigen, also known as seroclearance. This would be different from the concept of a “full cure” in which the virus would be eliminated — a far more daunting task given the nature of the integrated viral DNA.
“It’s a new era for hepatitis B,” Kwo said, explaining that a multitude of investigators and companies are looking to develop or combine sets of therapies to interfere with the replication or life cycle of the virus as well as immunomodulation strategies, which play an important role in the clearance of hepatitis B surface antigen.
The Character of HBV
The structure of the virus is composed of the HBV surface antigen (HBsAg), a protein that makes up the outer envelope of the virus with other proteins, and an inner core protein shell known as the HBV core antigen that contains the replicating DNA (HBcAg). The presence of HBsAg is used to establish the diagnosis of HBV infection. Chronic HBV is defined by the presence of HBsAg for at least 6 months duration.
The complex life cycle of HBV begins when the virus enters the host liver cell and is transported into the nucleus, at which point the viral DNA is transformed into a covalently closed circular DNA (cccDNA). The cccDNA serves as a template for viral replication, remains in the nucleus, and integrates into the DNA of the host liver cell.
“Like a ‘bear hiding in a cave,’ the cccDNA reflects the challenge of how to cure a virus that has this dormant form available to it that is impervious to the agents we currently give patients to repress replication in hepatitis C and hepatitis B,” Ira M. Jacobson, MD, director of hepatology at NYU Langone Health, told Healio Gastroenterology and Liver Disease. “What’s interesting is that when you suppress replication in hepatitis C, it appears to decay in such a way that it disappears, whereas in hepatitis B, this staple form of the archived DNA remains stable in the nucleus and persists despite whatever you try to give it in the form of currently available agents.”
The AASLD released an updated guidance document in 2018 for the prevention, diagnosis and treatment of chronic HBV. Along with revised recommendations for screening, testing, and counseling, the document included information on the newest recombinant, adjuvanted vaccine, Heplisav-B (Dynavax), and the newest antiviral treatment, tenofovir alafenamide (tenofovir alafenamide fumarate [TAF], Gilead Sciences).
Heplisav-B received FDA approval for adult patients in November 2017 and was the first new HBV vaccine in the U.S. in more than 25 years. While previous HBV vaccines have an excellent safety record, they require a three-dose series over 6 months or a four-dose series for the combination HBV and hepatitis A vaccine over 1 year. In contrast, Heplisav-B requires only two doses over 1 month. In February 2018, the CDC’s Advisory Committee on Immunization Practices voted unanimously to recommend Heplisav-B for HBV based on studies that showed 90% to 100% of patients achieved seroprotection.
For patients with HBV, preferred treatment options include Baraclude (entecavir, Bristol-Myers Squibb), Viread (tenofovir disoproxil fumarate [TDF], Gilead Sciences), pegylated interferon (peg-IFN), along with TAF.
Like TDF, TAF is a nucleos(t)ide analogue (NA) approved to treat chronic HBV in adults. TAF is more stable and efficient than TDF and can be administered at a lower dose, according to the guidance authors.
Results from a study published in Journal of Hepatology showed that TAF remained as effective as TDF in suppressing HBV replication after 2 years of treatment with no virologic resistance. Additionally, patients who received TAF had significantly less decreases in bone mineral density in the hip (0.33% vs. 2.51%; P< .001) and lumbar spine (0.75% vs. 2.57%; P< .001), and superior rates of alanine aminotransferase normalization compared with those who received TDF.
“TAF was specifically designed to substantially reduce systemic exposure to tenofovir, which was thought to be causing these known abnormalities associated with long-term TDF use,” Kosh Agarwal, MD, from Kings College Hospital, United Kingdom, and colleagues wrote in the study, referring to renal and bone safety. “While longer term follow-up will be needed to fully characterize the renal safety profile of TAF, these findings of lower renal impact compared to TDF are important given the reports that link TDF use to kidney injury.”
According to the AASLD guidance, adults with HBV without cirrhosis who seroconvert from positive to negative for HBV e-antigen (HBeAg) can potentially discontinue their NA therapy after a period of treatment consolidation. The recommendation for cessation of therapy, however, is conditional and supported by limited data.
The guidance states that the period of consolidation therapy generally involves treatment of persistently normal ALT levels and undetectable serum HBV DNA levels for at least 12 months. Patients who stop antiviral therapy should also be monitored every 3 months for at least 1 year for recurrent viremia, ALT flares, seroconversion, and clinical decompensation.
“What we’ve learned is that if you take a large population of who have been on antiviral therapy for years, a prohibitively small percentage of those patients will achieve functional cure of HBsAg clearance,” Jacobson said. “This is not an exercise in which we would engage people with cirrhosis, for example, because we can’t afford to have a flare. But if we have solid evidence that the patient never had significant scar tissue, it might become something worth doing.”
Thomas Berg, MD, from the University Clinic Leipzig in Germany, and colleagues conducted a study that included 21 patients with chronic HBV who had received TDF for 4 years or more with suppressed HBV DNA for 3.5 years or more. They were randomly assigned to either stop or continue TDF therapy. During the study, eight patients qualified to restart TDF due to persistently high levels of viremia or early flares of HBV DNA and alanine aminotransferase levels.
At week 144, four patients who stopped therapy achieved a significant loss of HBsAg and three patients achieved a functional cure of seroconversion, where no patients who continued therapy achieved HBsAg loss or reached seroconversion.
“Although there were clear fluctuations in HBV DNA levels and ALT levels in the patients who discontinued TDF therapy, there were no unexpected safety issues in this patient group,” Berg and colleagues wrote. “The results of this study support the concept of stopping antiviral therapy in long-term HBV DNA-suppressed patients without cirrhosis, under strict observation.”
The best predictor of functional cure in an individual patient after stopping therapy is a quantitative level of HBsAg test that shows low levels of circulating surface antigen, typically between 100 IU/mL and 200 IU/mL or less, according to Jacobson.
“It’s only really fruitful to do this if you can get a quantitative HBsAg titer at the time you’re thinking of stopping and use that to guide you,” he said. “We theorize that what we’re doing by stopping therapy and allowing the virus to wake up again is triggering heightened immunologic response that is now available because of the long years of viral suppression. It’s as if you’re releasing the breaks on a hitherto suppressed immune system that has tightened its activity during the long years of viral replication.”
He noted, however, that this novel therapeutic approach has not been employed by most clinicians as it has “just appeared on the horizon” and most clinicians adhere to the tradition of treating patients until they achieve seroclearance on therapy.
Full vs. Functional Cure
In 2016, the AASLD organized the Hepatitis B Treatment Endpoints Workshop to establish a definition of cure for HBV and develop a consensus on treatment endpoints. Most participants (87.9%) agreed that “functional cure” (sustained HBsAg loss) should be the goal for new HBV therapies. However, a few participants advocated that elimination of cccDNA was a mandatory criterion for functional cure, and less than half required that cccDNA be rendered transcriptionally inactive.
“There are two sources of cccDNA: incoming virions and recycling of encapsidated DNA from the hepatocyte cytoplasm,” Anna S. Lok, MD, FAASLD, from University of Michigan, and colleagues wrote in the report published in Hepatology. “The half-life of cccDNA is long, thus explaining why it is difficult to cure HBV infection and why HBV can reactivate either spontaneously or following immune suppression, many years after clearance of HBsAg. Unfortunately, current assays for circulating HBsAg cannot distinguish the transcription of HBsAg from cccDNA vs. integrated HBV DNA.”
The report concluded that a functional cure characterized by sustained loss of HBsAg with or without HBV antibody seroconversion after a finite course of novel antiviral and immune modulatory therapies was a currently achievable goal, whereas the elimination or permanent silencing of viral DNA remains unlikely feasible.
Healio Gastroenterology and Liver Disease spoke with Harry L.A. Janssen, MD, PhD, from the University Health Network in Toronto, who agreed that functional cure is the most important target right now, but also returned the spotlight to viral DNA.
“If you look at the life cycle of the virus, there are several steps where you can try to intervene,” Janssen said. “Hepatitis B is treatable currently, but not a curable disease. The next step is trying to create a cure for hepatitis B, which will be a challenge. The holy grail is the cccDNA, which is truly the template of viral replication in the nucleus of the hepatocyte. It will be difficult — we’re trying to find clever ways to do it so that you don’t have a lot of off-target effects.”
Janssen likened the discussion of HBV treatment and potential cure to a room filled with virologists and immunologists each highlighting a direction of intervention with clinicians standing in the middle. On one side, antiviral therapy has a “larger therapeutic window and tends to work homogenously,” compared with immunomodulatory treatment, which as yet appears to have a more heterogenous response.
“One immunomodulatory treatment might work for one patient but not for another,” he said. “To find out why, you have to dive into the complex interaction between the virus and the immune system, which is not easy.”
Whether the focus is more near at hand with a functional cure or looking far into the future at the concept of a full cure, medical research and development experts are expanding not only the health care community’s knowledge of how best to tackle the virus but also the clinical pipeline.
Among the different types of compounds under investigation for HBV, Hepatera and MYR Pharma have reported positive results from early studies of their entry inhibitor myrcludex B in patients with HBV and hepatitis D coinfection. Future studies will evaluate how effectively the drug blocks viral DNA from entering liver cells.
Small interfering RNAs (siRNAs), have demonstrated the ability to inactivate messenger RNA and thereby block production of essential viral proteins such as HBsAg. Several companies have recently initiated phase 1 and phase 2 trials for proprietary candidates including one collaboration between Vir Biotechnology and Alnylam Pharmaceuticals and another between Janssen Pharmaceuticals and Arrowhead Pharmaceuticals.
Jacobson referred to siRNA technology as an “elegant line of attack, because it turns out you can engineer a small interfering RNA particle to target any messenger RNA that you want ... just by knowing the genetic sequence. There are companies now making siRNA products that can be delivered as a package to interfere with more than one messenger RNA at a time.”
Capsid assembly modulators, or capsid inhibitors, are another DAA option for HBV. These inhibit the formation and assembly of core particles that make up the viral DNA protein shield and block cccDNA replenishment. Currently, Assembly Biosciences and Janssen Pharmaceuticals each have a capsid assembly modulator candidate in phase 2 trials.
Immunomodulators are used to help activate, regulate, or normalize the immune system and include therapeutic vaccines, toll-like receptor agonists (TLRs), and checkpoint inhibition, among others. Immunomodulatory therapies for HBV are in relative infancy with most trials in preclinical or phase 1 stages. Experts agree that these options will play a role in combatting HBV in the future and likely in combination therapies for a potential cure, but immune system therapies also raise safety concerns.
“We try to modulate the immune system with what we call pattern recognition receptors (TLRs) or RIG-I agonists which both interfere rather early in the cascade of immune reactions,” Janssen explained. “One other approach is to use therapeutic vaccination, where you design a vaccine in such a way that it doesn’t prevent disease but cures a disease like hepatitis B, which is a true challenge as these patients have been exposed to the virus for decades and are pretty tolerant to it, so to have a successful response from a few injections of such a vaccine is unlikely to happen. We may be more successful if we prime the immune system beforehand with other treatment options.”
“The third point on the immunology side is checkpoint inhibition or PD1-PDL1 blocking agents,” he continued. “It is the therapy that moved the needle quite a bit in field of anti-cancer therapy. Those drugs essentially revitalize the immune cells, the T-cells, to attack hepatitis B. The T-cells that are supposed to attack the hepatitis B virus become very lazy after 20 to 30 years of being exposed to the virus and these checkpoint inhibitors have the ability to revitalize these T-cells.”
Yuchen Xia, MD, and T. Jake Liang, MD, from the National Institutes of Health, detailed the opportunity of using the gene editing tool, clustered regularly interspaced short palindromic repeats associated nuclease 9 (CRISPR/Cas9), as an innovative application for targeting HBV.
“The CRISPR/Cas9 system originates from the immune system of bacteria and archaea, which detects and degrades foreign DNA from bacteriophages and plasmids,” they wrote. “The promise of CRISPR/Cas9 as a tool for the cleavage and elimination, or at least inactivation, of HBV cccDNA and HBV genome integration, has prompted many studies. These have provided a clear proof of concept that this approach has the potential to treat or even cure patients with chronic HBV infection.”
Excision BioTherapeutics and Intellia Therapeutics recently announced the first steps in developing CRISPR/Cas9 candidates for HBV infection.
“One of the lines of attack that we’re thinking about is to target the cccDNA directly with agents that can directly destroy it, hopefully without destroying human chromosomal DNA, at the same time,” Jacobson said. “With cccDNA in mind as a potential target, we can develop technologies like CRISPR that can be engineered to target any DNA you want, whether it’s viral or human. If we could eliminate all of the cccDNA, at that point you would have no template of DNA from which any new viral proteins or viral particles can be formed and would get us very close to cure. Theoretically, it’s a very appealing approach, but it’s probably one of the furthest from realization because of the safety issues.”
As excited as many physicians and researchers are regarding the prospects of HBV treatment and potential cure, it is also agreed that there is a desperate need to increase awareness about the long-term harm of living with HBV, not only for outreach to undiagnosed or untreated patients but also for financing much-needed research and care programs.
“In hepatitis B, just like in hepatitis C, the problem is that the majority of patients are not diagnosed,” Janssen said. “These diseases are silent killers; for many decades, you have no complaints, but the hepatitis moves along and very slowly healthy liver cells are replaced with fibrous tissue and you develop fibrosis, and cirrhosis, and liver cancer, and unfortunately we still see patients coming in with advanced liver cancer that have had hepatitis B for decades without treatment.”
Janssen went on to note that because HBV and HCV are “quiet diseases” with an extensive interval between time of infection and the development of infection-related end-stage liver disease, they do not generally receive the same level of attention as acute infectious diseases such as HIV.
He also advocated for more studies focused on intrahepatic analyses alongside current ongoing drug trials to discover why some patients do not respond to certain agents. “If we investigate the immune system and HBV just in the blood we may well look into the wrong compartment,” he said.
“The awareness and underdiagnosis and undertreatment of hepatitis B is our main point of concern nowadays,” Janssen concluded. “If we can reach a cure, that will make life much easier for patients who otherwise face lifelong treatment that comes with complications such as adherence, costs, and possible long-term side effects.” - by Talitha Bennett
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Disclosures: Jacobson reports consulting for AbbVie, Bristol-Myers Squibb, Gilead, Intercept, Janssen, Merck, Novo Nordisk and Trek; and receiving grant or research support from Assembly, Bristol-Myers Squibb, Enanta, Gilead, Janssen and Merck. Janssen reports he has received grants AbbVie, Arbutus, Bristol Myers-Squibb, Gilead Sciences, Janssen, Medimmune, Merck and Roche; and is a consultant for Arbutus, Enyo, Gilead Sciences, Janssen, Medimmune, Merck, Roche, Vir Biotechnology and Viroclinics. Kwo reports no relevant financial disclosures.