Lonafarnib

NEW TREATMENT OPTION FOR DELTA VIRUS: IS A CURE IN SIGHT?

CIHAN YURDAYDIN
Department of Gastroenterology Ankara University Medical School Cebeci Tıp Fakültesi Hastanesi Dikimevi, 06100 Ankara

Abstract:
Current treatment of chronic hepatitis D viral infection with interferons is poorly tolerated and effective only in a minority of patients. Despite Delta virus causing the most severe form of chronic viral hepatitis no other treatments are available. After many years of inactivity there is now hope for new treatment approaches for Delta virus and some are likely to enter clinical practice in the near future. Four new treatment approaches are currently being evaluated in phase 2 studies. These involve the hepatocyte entry inhibitor myrcludexB, the farnesyl transferase inhibitor lonafarnib, the nucleic acid inhibitor REP 2139 Ca and pegylated interferon lambda. Results obtained so far are promising and Phase 3 studies are expected shortly. This review summarises the available data on the efficacy and safety of these new drugs.
Some 40 years ago, Rizzetto and co-workers noticed that while studying, by immune fluorescence, liver biopsies from patients with hepatitis B an antiserum against the hepatitis B core antigen (HBcAg), also reacted with biopsies not containing core particles at electron microscopy. This led to the discovery of a novel antigen-antibody system which they called the delta antigen and delta antibody (1). A follow-up study using chimpanzees at the National Institutes of Health in the US provided showed that the delta antigen was an internal component of HBsAg particles and it was soon recognized that the delta agent was a new virus and not a HBV mutant (2). It was considered a defective virus requiring helper function from the hepatitis B virus (HBV) and to quote the wording used in the paper, “encapsidation of delta agent with HBsAg could represent one such helper function of HBV to provide this agent a mode of transmission and access to susceptible hepatocytes.” (2). Thirty eight years later this prediction based on data still holds true. It was also reported that the RNA of the virus represented the smallest genome of known RNA viruses, information which has also endured till today.
Clinical studies have revealed that both acute and chronic (CHD) infection with HDV lead to a more severe form of viral hepatitis compared to infection with HBV alone. In acute HBV/HDV co-infection this was very evident in early studies (3, 4) but a more recent study suggests that disease severity may have waned (5), probably due to the slower turnover of HDV in the community which may account for emergence of less pathogenic strains of HDV as a consequence of the slow passage of HDV form person to person, similar to data obtained in studies using chimpanzees (6). For chronic hepatitis delta (CHD) the more severe form of CHD has been observed both in early studies and also in recent studies (7-11), suggesting that individuals with severe disease persist. The course of disease may be influenced by HDV and HBV genotypes and some HDV genotypes (such as genotype 2 seen in the Far East, 5 and, probably, the African strains 6 to 8) may have a milder clinical course than the other, more widely, distributed genotype 1 (12, 13). However, as with acute HDV population based studies suggest that in CHD, the disease may be not as severe as in the past and may in particular slow down at advanced stages (14, 15). Still, HDV despite its relative rarity is currently a major health problem in many parts of the world and current treatment is, to a large extent unsatisfactory, and HDV is independently associated with mortality as has been elegantly shown in long-term follow-up studies in HIV-HDV co- infected patients (16, 17). The impact of HDV infection is enhanced by the striking advances achieved in the management of chronic hepatitis B and C, witnessed in the last decades.
Since its discovery, many drugs have been tested for the treatment of CHD (Table 1). Among them, prednisolone, azathioprine, interferon alpha 2a and 2b, pegylated interferon alpha 2a and 2b, thymic humoral factor-gamma 2 and levamisole have been used based on their general immunomodulatory properties and other agents, chiefly nucleos(t)ide analogs, have been tested in CHD patients due to their antiviral properties (18).

Current treatment of CHD:
Treatment with interferons:
Interferons are currently the mainstay of treatment in CHD. In addition, of the NAs used for the treatment of CHD, tenofovir will be mentioned here as it may be beneficial, at least in some patients with CHD. Interferons have been used for the treatment of CHD since the 1980s. They represent the only treatment of proven efficacy (18) and as has been shown in the two largest randomized controlled clinical trials, interferons appear to lead to a post- treatment week 24 viral response (undetectable HDV RNA by a sensitive PCR assay) in 25 to 30% of patients (19, 20). It is important to note that post-treatment week 24 viral response borrowed from the hepatitis C literature is apparently not equal to a sustained viral response in CHD since more than 50% of patients with negative HDV RNA at 6 months post- treatment developed detectable HDV RNA after 5 years of follow-up (21). Optimal treatment duration is not established. In most studies interferons have been used for a duration of one year. In some studies longer treatment duration has been tested. In particular, two years of treatment did not appear to be superior to one year of treatment (20, 22-25). A direct comparison is available however in only one study (25) which is heavily underpowered. In contrast, a study from Turkey (26) and the famous case report from the National Institutes of Health in the US (27) suggest that some patients may benefit from prolonged treatment in line with in vitro data (28, 29) and viral kinetic studies which suggest that HDV may respond very slowly interferon compared to hepatitis C (30). An analysis of the Delta Hepatitis database of the Ankara University Medical School has convincingly shown that some patients may respond to prolonged courses of treatment and as such interferon treatment may need to be individualized according to patient’s response to treatment (31). Predictors of response to pegylated interferon therapy have been investigated and based on an analysis of the HIDIT-1 Study the only independent factor predicting viral response at post-treatment week 24 was HDV RNA negativity at Week 24 (32). This had also been observed in patients treated with conventional interferon (33). Earlier on-treatment time points, e.g. HDV RNA kinetics at treatment weeks 4, 8 or 12, were less predictive, as shown by a sub-analysis of HIDIT-2 Study data (34), leading to recommendations that a decision on continuing therapy should be made after 24 weeks. However, it remains important to find early predictors of response. Detectable HDV RNA at the end of treatment cannot be targeted and used to determine treatment outcome as such patients can end up with post-treatment week 24 HDV RNA negativity (19) and a favorable clinical outcome (21). In the sub-analysis of the HIDIT-1 Study we had arbitrarily chosen a less than 1 log decline of HDV RNA at end of treatment compared to baseline as null response to treatment (32). On-treatment week 24 HDV RNA decrease of less than 1 log combined with no decrease of quantitative HBsAg levels at the same time point compared to baseline had a 83% PPV for predicting “null responders”. Therefore it remains impossible to define a robust stopping rule as 17% of patients identified in trials as ‘treatment failures’ may respond. On the other hand, on treatment 2 log decline of serum HDV RNA compared to baseline had a 95% NPV for “null response” prediction (32).
Interferon treatment was equally effective in patients with compensated advanced disease vs early disease in the 2 largest randomized clinical trials (19, 20) whereas in other studies patients with advanced disease were less responsive (23, 31). Although effective in 25 to 30% of patients, successful treatment with interferons decreases liver related complications and mortality as has been shown in long-term follow-up studies from Greece, Turkey and Germany (11, 31, 35). Complications of liver disease are better prevented with early interferon treatment (31) and interferon treatment should not be deferred.

Effect of treating HBV infection with nucleos(t)ide analogs (NAs) in CHD:
Famciclovir, lamivudine, clevudine, entecavir, adefovir and tenofovir was used for treating CHD. Except for the use of tenofovir in a Spanish cohort (36), nucleotides in HCV have proved to be ineffective. (19, 35, 37-41). In the Spanish studies which reported a beneficial effect of tenofovir, it is important to note that the reported beneficial effect was observed in HIV-HDV co-infected patients and in this retrospective analysis patients had received tenofovir for a median duration of 6 years, whilst in the studies with other NAs duration of treatment was much less -6 to 12 months. Lack of efficacy of short-term NA treatment is not unexpected since NAs do not affect HBsAg synthesis which is the only HBV function needed by HDV. Longer duration of NA treatment on the other hand may have had an indirect effect on HBsAg synthesis and may lead to decreased re-cycling of HBV DNA to the nucleus thus decreasing the hepatitis B virus covalently closed circular DNA (HBV ccc DNA) pool with consequent decreases in HBsAg production and hence serum HBsAg levels (42, 43). One other theoretical mechanism that might explain the long-term efficacy of tenofovir may be immune re-constitution secondary to effective antiretroviral therapy (44). The Spanish group published a second study, again in HIV-HDV co-infected patients, with similar results (45) although studies from France in HIV-HDV co-infected patients challenge the conclusions of the Spanish studies (46, 47). Of importance in the initial study showing efficacy of tenofovir the beneficial effect was not associated with an increase in CD 4 counts or a decrease in HBsAg levels contrary to theoretical considerations (36). In the most recent study from France, out of 21 patients 3 were HDV RNA negative after approximatly 3 years of tenofovir treatment. Baseline HDV RNA levels were significantly lower in these 3 patients compared to the rest of the population (47). Similar data has been reported in entecavir- treated CHD patients without HIV (41) suggesting that CHD disease with less dominant HDV may be more responsive to NA treatment than previously believed. Further data on larger cohorts with appropriate durations of treatment is awaited with interest.
Finally, several combinations of interferons with NAs have been tested in CHD (Table 1). These include combinations of conventional or pegylated interferon with ribavirin (23, 48) lamivudine (33, 49) adefovir (19) and tenofovir (20). Although the combination of pegylated interferon with adefovr led to a significant decrease in HBsAg levels at end of treatment and at post-treatment week 24 compared to baseline which was not observed with pegyated interferon monotherapy, similar data could not be repeated with the tenofovir combination. It remains poorly understood why the adefovir combination had an effect on HBsAg whereas tenofovir did not augment interferon. However, there was a trend for a better viral response to the pegylated interferon tenofovir combination compared to pegylated interferon monotherapy which was more evident in female patients (20). Combinations of interferons with lamivudine and ribavirin did not provide better antiviral efficacy results and it is fair to say that convincing evidence for a better response of combination treatments of interferons with any nucleos(t)ide analogs does not exist.

New treatments for CHD
New treatments have been tested in phase 2 studies in the last few years. An important distinguishing factor with these new treatment modalities compared to drugs used in the past is that they are antiviral agents targeting various steps of the HDV life cycle (Fig. 1), and represent good examples of translational science. Some of these agents are directed against HDV only such as the farnesyl transferase inhibitors, others may be considered for the treatment for CHB as well. Examples of the latter are hepatocyte entry inhibitors and nucleic acid polymers (13). It is worth mentioning that these new treatments in CHD do not target HDV RNA polymerase directly as in CHB or CHC where targeting HBV DNA or HCV RNA polymerase, respectively, led to breakthrough new treatments. The main reason for this is that HDV does not possess an HDV RNA polymerase of its own but depends on the polymerase of the host for its replication. It also suggests that an expectation to develop an antiviral agent against HDV with immediate strong potency similar to the situation in CHB and CHC may not be realistic. Given that CHD represents the most aggressive form of chronic viral hepatitis and where no drug of proven efficacy other than interferons exist, availability of new drugs for its treatment is a matter of urgency. In contrast to CHB and CHC, surrogate markers of treatment efficacy have not been well defined in CHD. Therefore a group of experts in the field have proposed a realistic surrogate of treatment efficacy for CHD to be considered in phase 3 studies which are expected to start very soon. They proposed a 2 log decline of HDV RNA at end of treatment compared to baseline as a surrogate marker for initial treatment efficacy (50).
While there is a great deal of interest in developing new treatments for CHD and since HDV is in need of a helper function from HBV in the form of production of HBsAg it is clear that any treatment leading to a functional cure for HBV would also be beneficial for HDV patients (51) and these have recently been reviewed (INSERT REFERENCE FROM ZOULIM JVH 73- 2019 HERE). Taken aside approaches targeting functional cure for HBV, a list of new treatments for CHD is shown in table 2.

Main new treatments in CHD
Hepatocyte entry inhibitors: The pre-S1 domain of the L protein of HBsAg is critical for the attachment of both HBV and HDV to the hepatocyte (52). Attachment of HBV/ HDV virions starts with the virus approaching heparan sulphate proteoglycans on the hepatocyte membrane (53, 54). Then the HBV or HDV virion attaches to the entry receptor sodium taurocholate co-transpoting polypeptide (NTCP) (55). At this stage the integrity of the amino acids in the pre-S1 region is crucial. Myristoylation of glycine in the preS1 domain and 77 amino acids in the preS1 domain of the HBV L-surface protein appears to be important for cellular attachment (56, 57). Myrcludex B is is the first hepatocyte entry inhibitor and consist of a myristoylated lipopeptide comprising 47 amino acids of the preS1 domain of the HBV L-surface protein. Human myrcudex B application started with a phase I study in healthy volunteers where the drug was given as both an intravenous and subcutaneous formulations in ascending daily doses from 0.3μg to 20mg in the intravenous group and from 800μg to 10mg in the subcutaneous group. The subcutaneous route displayed 85% bioavailability. A transient, asymptomatic, dose-independent serum lipase elevation was observed in five of 36 healthy volunteers (58). NTCP is also a bile salt transporter exclusively expressed on hepatocytes. It is reported that IC50 of Myrcludex B for HBV and HDV entry inhibition differs > 500-fold from its inhibitory effect on bile salt transport and it was therefore suggested that HBV/HDV receptor blockade can be achieved without blocking the bile acid transporter function of NTCP (52). In human studies with myrcludex B elevation of glycine and taurine-conjugated bile satls was observed without clinical consequences. Further, mild and transient neutropenia, thrombocytopenia and eosinophilia was observed (59). Overall administration of Myrcludex B was reported to be well tolerated in phase 1 and 2 clinical studies (Table 2). In the proof-of-concept phase 2 study, 24 patients received either 48 weeks of peg-IFN monotherapy or 24 weeks of myrcludex B, 2mg/kg, sc, daily for 6 months followed by 48 weeks of peg IFN monotherapy or 6 months of combination of myrcludex B+ peg-IFN, followed by 6 months of peg-IFN. Myrcludex B, peg-IFN alpha and their combination was associated with a mean 1.67, 2.17 and 2.59 log10 reduction in HDV RNA, respectively at the end of 24 weeks treatment (59). HDV RNA became negative in 2 patients during myrcludex B monotherapy and in 5 patients in combination with pegIFN-α (58).
However, the primary endpoint, of the study, a 0.5 log decrease in HBsAg was not observed with myrcludex B as monotherapy or in combination with peg IFN alpha.
In a follow-up study, 120 patients pretreated with tenofovir (TDF) monotherapy for at least 3 months were randomly assigned into 4 groups which consisted of daily subcutaneous administration of myrcludex 2, 5 and 10 mg, respectively, in combination with TDF or tenofovir TDF alone (60). Duration of treatment was 6 months with 6 months of follow-up where patients continued to receive TDF monotherapy. The primary endpoint, a 2 log decrease or undetectable HDV RNA at end of treatment was reached by 46, 47 and 77% with escalating doses of myrcludex B whereas in the TDF monotherapy this endpoint was reached by a mere 3% of patients. HDV RNA at end of treatment displayed a delta decline compared to baseline of −1.75 log, −1.60 log, −2.70 log in the 2, 5 and 10mg myrcludex B arms, respectively, and a -0.18 log decline with TDF. ALT normalization was observed in 43, 50, 40 and 7% of patients, respectively. At follow-up week 12, HDV RNA relapse occurred in 60, 80 and 83% of treatment responder patients in the myrcludex B arms, respectively. End of treatment results of a new phase 2 study were presented at the 2018 AASLD Meeting.
The study was a 4 arm randomized multicenter study conducted in Russia where 48 weeks of myrcludex B, 2 mg and peg IFN monotherapies were compared with peg-IFN in combination with either 2 or 5mg myrcludex B in compensated CHD patients (61). Each arm contained 15 patients. Median HDV RNA log10 change at week 48 compared to baseline was higher with combination treatment (-3.62 and -4. 48 logs for combination with 2 and 5 mg myrcludex B) than with peg IFN and 2 mg myrcludex B monotherapies (-1.14 and -2.84 log, respectively). While 2 of 15 patients (13%) in each monotherapy group patients became HDV RNA negative at end of treatment, 15 of 30 patients (50%) did so with combination treatment. Interestingly, a > 1log decline in quantitative HBsAg levels was observed in 9 of 30 patients (30%) with the combination treatments but in none of the patients in the monotherapy groups. The data were suggestive of an additive if not synergistic effect of combination treatment. In these studies myrcludex B was in general well tolerated and aymptomatic rises in bile acids returned to baseline levels one week after treatment discontinuation.
Farnesyl transferase inhibitors: After replication in the nucleus the newly formed ribonucleoportein complex is exported to the cytoplasm where it is covered by HBsAg and new virion synthesis completed. During this process the lipophobic nucleoprotein complex is converted to a lypophilic complex through attachment of a polycarbon prenyl group to the carboxyterminal CXXX box (C for cysteine) of the large hepatitis D antigen (L-HDAg). Through this posttranslational modifications the isoprenylated L-HDAg-modified ribonucleoprotein enters the endoplasmic reticulum through which the newly formed virion will leave the hepatocyte through most likely by budding of HBsAg-membrane derived complexes (62).
Thus, prenylation of large hepatitis delta antigen is required for HDV virion assembly. Of the two available cellular prenyl tansferases, farnesyl- and geranylgeranyl transferases, farnesyl transferase is specific for HDV and catalyzes the attachment of the 15 carbon farnesyl group to L-HDAg (63). Farnesyl-transferase inhibitors abolished HDV-like particle production in vitro (64) and in vivo (65). In the proof-of-concept study, the farnesyl transferase inhibitor lonafarnib (LNF), given for 28 days in 14 CHD patients, dose-dependently reduced serum HDV RNA levels (66). Several phase 2 Studies were then conducted at the University of Ankara Medical School in Turkey, at Hannover Medical School in Germany and at the National Institutes of Health (NIH) in the US (67-69). At the NIH, efficacy of once daily dosing of LNF with ritonavir (RTN) was explored. In Hannover, a strategy to increase LNF dosing with intervals of 2 weeks was explored- here too, LNF was combined with RTN. In Ankara, on the other hand, a dose finding study was performed where various doses of LNF were combined with mainly RTN , 100 mg, bid, dosing with and without the addition of pegylated interferon (pegIFN). In the LOWR HDV-1 (LNF with and without RTV in HDV-1) study, LNF was tested at the higher doses of 200 and 300 mg, bid, as monotherapy and at lower doses in combination with peg-IFN or RTN (67) for up to 3 months. Two findings of this study are worth mentioning. First, combination treatment with RTN or pegIFN appear to best combine efficacy with tolerability. Second, short-term treatment of LNF for 3 to 6 months was associated in some patients with post-treatment viral and biochemical flares. This led to undetectable HDV RNA levels along with ALT normalization and also to suppression of HBV DNA. The mechanism of these favorable post-lonafarnib responses is considered immune-mediated but is not entirely understood. In the dose escalation study in Hannover, patients started with a dose of LNF/RTN (50mg/100mg, bid) and the LNF dose was increased, provided patients had tolerated the dose, first to 75mg and then to 100mg, bid, at 4 week intervals. In 10 out of 15 patients dose escalation achieved the aimed dose. One patient had undetectable HDV RNA and one patient had HDV RNA levels below level of quantification at end of treatment (68). LNF RTN once daily dosing combination regimen for 24 weeks was tested in a double-blind approach at NIH (69). LNF at doses of 50/75/100mg, qd with RTN 100mg, qd was tested. After 24 weeks of treatment, 6 out of 21 patients had HDV RNA levels below 250 IU/mL. Studies on optimal combination treatment regimens were explored in the LOWR HDV-2 Study (70). The study results revealed that 50mg LNF bid had better antiviral efficacy than 25 mg LNF, bid, both combined with RTN 100 mg, bid.
However, the best results combining efficacy with tolerability was obtained with the triple combination treatment consisting of 50mg LNF, bid with RTN, 100 mg, bid and peg-IFN (70). Thus, all oral combination with 24 weeks of LNF 50mg, bid, led to a > 2log decrease of HDV RNA at end of treatment in 6 of 12 (50%) patients. ALT normalization occurred in 7 out of 10 patients with baseline elevated ALT. Triple therapy with 24 weeks of bid dosing of 25 or 50mg LNF and 100mg RTV bid in combination with weekly peg-IFNα was associated with a >2 log HDV RNA decrease in 8 of 9 patients and ALT normalization in all 8 patients with high baseline ALT. Combination of LNF at doses of 75mg bid and higher as dual or triple therapy were not well tolerated and did also not lead to better antiviral efficacy (70). LNF is associated with dose limiting side effects, mainly related to gastrointestinal toxicity. These side effects comprise anorexia, nausea, diarrhea and weight loss. A concern could have been accumulation of the ribanucleoprotein complex inside hepatocytes. However, in vitro studies have not revealed cellular toxicity (71).
Nucleic acid polymers: Nucleic acid polymers (NAPs) are sequence-independent phosphorothioated oligonucleotides which exert their pharmacological effect in a sequence independent manner. They bind with high affinity to amphipathic protein structures, a consequence of a hydrophobic-based interaction (72). Their mechanism of action is not entirely clear but it is suggested that NAPs inhibit assembly and/or secretion of subviral particles. Further a large hydrophobic domain of large HDAg and a nucleic acid binding domain in small HDAg may also be targeted by NAPs (72). In preclinical studies, the NAP NAP REP9-AC was effective both in vitro (73) and in vivo (74). Phase 2 Studies have been conducted both in chronic hepatitis B and in CHD patients (75, 76). The phase 2 study in CHD was a pilot study encompassing 12 patients with compensated liver disease. Patients received the NAP REP 2139-Ca once weekly as an intravenous infusion for 15 weeks as monotherapy followed by add-on Peg-IFN for another 15 weeks (76). Finally, patients received Peg-IFN alone for another 33 weeks. Similar to the phase 2 study in chronic hepatitis B (75), striking declines in HBsAg levels was observed and 5 out of 12 patients displayed negative HBsAg with rising HBs antibody titres at end of treatment. Similarly, serum HDV RNA was undetectable in 9 patients at end of treatment. Seven patients continued to have undetectable HDV RNA and 5 patients were HBsAg negative eighteen months off treatment (77). One concern with NAP treatment are ALT flares observed on- treatment. These flares did not lead to hepatic decompensation in any of the patients but their mechanism is still poorly understood. Anorexia, hair loss, dysphagia and dysgeusia has been reported in the chronic hepatitis B trial and attributed by the investigators to heavy metal exposure at the trial site (75). Leucopenia, thrombocytopenia have been reported in 7 out of 12 patients. Further, administration route related side effects such as fever, chills, peripheral hyperemia have been reported. There are plans to develop a subcutaneous formula (CY, personal communication with Michel Bazinet).
Pegylated Interferon lambda: Type III interferons, discovered in 2002/ 2003 and designated as interferon lambda or interleukin 28/29, share with type I interferons similar expression patterns. They activate through similar intracellular signaling pathways (Jak-STAT pathway) interferon stimulated genes, despite utilizing distinct receptor complexes for signaling (78). An important difference between type 1 and III interferons is that the type I interferon/receptor family is expressed more widespread whereas the IL-29 cytokine/receptor family is expressed in hepatocytes but beyond that displays a more restricted expression pattern (79, 80). These observations were predictive of less systemic effects with interferon lambda (80) which was later confirmed in human studies in patients with chronic hepatitis C and chronic hepatitis B (81, 82). IFN lambda has been tested in vivo in mice with humanized HBV/HDV infected livers where antiviral efficacy against HDV was observed (83). This cytokine is currently tested in phase 2 studies in patients with CHD. In a randomized, open-label multi-centre trial involving 33 patients with CHD, patients received either 120 or 180 μg PegIFN lambda weekly for 48 weeks. An interim analysis was recently reported on a limited number of patients reaching week 24 of treatment; the overall tolerability was reported to be better than PegIFN alpha (84), although on-treatment ALT flares and hyperbilirubinemia was observed which were reversible with dose reductions and without any clinical consequences. All patients have recently reached week 48 end of dosing and end of treatment results display the superiority of the 180ug dosing over the 120 ug dose. A > 2log decline of HDV RNA was observed in 90% and 54% of patients of the high dose vs low dose groups, respectively (J Glenn, personal communication).
Beyond these studies which were performed in patients with CHD, studies with new treatment approaches aiming for functional cure in chronic hepatitis B, i.e. clearance of HBsAg with or without anti-HBs seroconversion, are investigated in large. A breakthrough result in these efforts is expected to be beneficial for CHD as well. Some of these approaches have gone beyond preclinical stages of development and are currently being tested in phase 1b or 2 studies in CHB patients. Prominent examples of such approaches include capsid assembly inhibitors and gene silencing through small interfering RNAs (85). These 2 approaches are attractive since they potentially target more than one site of the HBV life cycle; capsid inhibitors’ main function is the inhibition of encapsidation of pregenomic RNA in the cytoplasm blocking HBV replication but core/capsid proteins are multifunctional proteins with other less understood functions such as transcriptional regulation of cccDNA, microtubule-dependent nucleocapsid trafficking to nucleus and others with the potential to affect the cccDNA pool (85). The potential of capsid inhibitors to lead to functional cure rely on the efficiency of these secondary mechanisms and more than a dozen of capsid inhibitors have been and continue to be investigated (86, 87). Small interfering RNAs can also target multiple steps. Current siRNA approaches are designed to reduce all transcripts from cccDNA and also integrated HBV DNA; hence they affect both serum HBVDNA and HBsAg levels. On-treatment mean quantitative HBsAg levels decreased by more than 1 log after 3 sc injections of a siRNA molecule (88). Immune stimulators may need mentioning since there is scientific reasoning that immunological approaches are required for achieving a functional cure and also as they also are being tested in phase 1b or 2 trials. Among them Toll-like receptors as inducers of type 1 interferon responses raised some interest but so far despite induction of the immune system virologic efficacy is lacking (89, 90). Check point inhibitors gained attention in recent years in cancer immunotherapy. Chronic viral hepatitis is a condition characterized by T-cell exhaustion. Blocking programmed cell death protein 1 (PD1) may activate HBV-specific T cell response. In a phase 1b clinical study the immune check point inhibitor nivolumab led to significant declines in HBsAg levels after a single dose of nivolumab (91). Finally, a drug with both antiviral and immune modulatory effects will be mentioned: iragivir. This drug is an oral RIG-1 (retinoic acid-inducible gene 1) receptor agonist and has both antiviral efficacy against HBV and also leads to selective activation of the interferon signaling pathway. Twelve weeks of oral iragivir was associated with declines in serum HBV DNA, HBV RNA and quantitative HBsAg levels and was well tolerated (92).
In conclusion, it appears that treatment of Lonafarnib will soon enter a new phase most likely very soon, where we will witness the licensing of drug or drugs for the treatment of CHD. This will be good news for patients and their treating physicians. At least for the time being, this new drugs will be aiming at an immune control of HDV rather than a cure of this disease. It is hoped that at least in some patients this immune control will eventually lead to a functional cure, i.e. the clearance of HBsAg with the development of HBs antibodies.