Table 1 shows the data from the clinical trials. They suggested that the addition of a nucleos(t)ide analogue to PegIFN-α added no antiviral effect. In fact, HDV RNA undetectability at the end of follow up (EOFU), 24 weeks after stopping PegIFN-alpha of each study ranged from 16% to 52.6%, with ALT normalization ranging from 19% to 46% with PegIFNα monotherapy, and from 18% to 75% and 19%-36%, respectively, with combination therapy ( Table 1 ). Only two studies evaluated HBsAg loss at EOFU, an event not frequent but possible ( Table 1 ).

Patients treated with IFN therapy typically have multiple adverse events. Commonly, the patients reported a flu-like syndrome, myalgia, fatigue, weight loss, depression and a local reaction at the site of injection (European Association for the Study of the Liver, 2017). If the reaction is defined severe (Grade III) this could lead to a drug dose reduction or discontinuation. In clinical studies ( Table 1 ) the adverse drug reaction leading to discontinuation ranged from 5% to 24%.

Table 2 shows the results of real-life studies. They were heterogeneous in type of treatment and sample size, but confirmed the data from clinical trials. In fact, in observational studies HDV RNA undetectability ranged from 7.1% to 47% (Karaca et al., 2013; Abbas et al., 2014; Keshvari et al., 2014; Bahcecioglu et al., 2015; Niro et al., 2016; Soyer et al., 2016; Wranke et al., 2017; Boglione et al., 2019) and ALT normalization at EOFU ranged from 27% to 50% (Karaca et al., 2013; Abbas et al., 2014; Keshvari et al., 2014; Bahcecioglu et al., 2015; Soyer et al., 2016). In only four studies, HBsAg loss at EOFU ranging from 0% to 22% (Karaca et al., 2013; Niro et al., 2016; Wranke et al., 2017; Boglione et al., 2019) was included. In these studies, the prevalence, type and severity of the adverse events were similar to those observed in clinical trials ( Table 2 ).

The use of pegIFNλ is under clinical investigation for the treatment of CHD. PegIFNλ differs from PegIFN-α because it recognizes a different heterodimeric receptor complex that is largely restricted to cells of epithelial origin (liver, lung and gut) (Donnelly and Kotenko, 2010).

In a phase II Pegylated Interferon Lambda Monotherapy in Patients with Chronic Hepatitis Delta Virus Infection (LIMT-1) clinical trial (Etzion et al., 2023), 33 patients were treated with pegIFN λ (120 or 180 µg subcutaneously once weekly for 48 weeks): 7/14 (50%) of the patients treated with 180 µg had a >2 log HDV RNA decline or negative HDV RNA compared with 4/19 (21%) patients receiving 120 µg. However, only 5 of the 14 patients (36%) and three of 19 (16%) in the two groups reached the end-point (undetectable HDV RNA 24 weeks after the end of therapy).

A phase III LIMT-2 trial of pegIFNλ 180 µg for 48 weeks with 24 weeks of post-treatment follow-up is ongoing.

According to the data shown, PegIFN therapy is unsatisfactory in terms of tolerability and virological efficacy, especially in long-term follow-up. In fact, sustained virological response was achieved in only a minority of cases, and the addition of nucleoside/nucleotide analogues has not been shown to significantly improve the outcome. However, at the moment it is the only therapy that can guarantee the loss of HBsAg, a fundamental endpoint for therapeutic success. Another point of interest is that it is not clear whether the HDV genotype can influence the outcome of therapy; in this regard, interesting are the results of a retrospective study showing that in 24 patients treated with peg-IFN, the post-treatment response was significantly better in patients infected with HDV genotype 5 (10% GT1 vs. 64% GT5, p = 0.013) (Spaan et al., 2020).

Moreover, it is important topoint out that the management of Peg-IFN-based therapy is very complex. In fact, the frequent clinical and biochemical side effects of PegIFN therapy are well known and affect different sites of the organism: dermatological manifestations, neuropsychiatric disorders, alteration of thyroid function and thrombocytopenia and a multidisciplinary approach is often necessary to manage these complications, the severity of which may be such as to lead to premature discontinuation of PegIFN treatment. Moreover, PegIFN is contraindicated not only in subjects with advanced liver disease but also in subjects with neurological, dermatological and psychiatric comorbidities as well as in the case of autoimmune diseases. In rare cases, PegIFN can be a trigger for autoimmune hepatitis.

Table 3 summarizes the results of BLV therapy both in monotherapy and in combination therapy with PegIFN derived from RTCs.

In this section we will review real life studies published across Europe (Degasperi et al., 2023b; Dietz-Fricke et al., 2023a; Loglio et al., 2019; Degasperi et al., 2022; Herta et al., 2022; Jachs et al., 2022; Loglio et al., 2022; Zöllner et al., 2022; Anolli et al., 2023b; Comandini et al., 2023; Dietz-Fricke et al., 2023b; Jachs et al., 2023; Sandmann et al., 2023; https://www.natap.org/2022/EASL/EASL_44.htm; https://www.natap.org/2021/AASLD/AASLD_18.htm), in the majority of cases on BLV in mono-therapy. Table 4 summarizes these studies.

One of the first real-life studies was done in France: thanks to the availability of BLV within an early access program in September 2019 and to a conditional marketing authorization since September 2020, 138 patients were included in the BuleDelta cohort, also known as ANRS HD EP01 (Agence Nationale de Recherches sur le Sida et les hépatites virales, for HDV code EP01). The preliminary analysis included 98 of 138 patients with available data at week 24: 54 (55%) were treated with BLV in mono-therapy, and 44 (45%) in association with Peg-IFNα; overall 74 (76%) were concomitantly treated with nucleos(t)ide analogues (NUC). The mean decrease in HDV RNA at week 24 was 1.9 ± 1.4 log IU/ml (2.6 and 1.6 log IU/ml with and without Peg-IFNα, respectively). The virologic response, i.e. a decrease in HDV RNA of at least 2 log and/or HDV RNA undetectability, at week 24 was observed in 55 (56%) patients (80% and 37%, with and without Peg-IFNα, respectively, among which undetectability of HDV RNA was reached in 44% ad 8%, respectively). The biochemical response, i.e. normalization of ALT, was observed in 36 of 98 (37%) patients at week 24 (34% and 40%, with and without Peg-IFNα, respectively) and the combined response in 25 (26%) patients at week 24 (36% and 17%, with and without Peg-IFNα, respectively) (https://www.natap.org/2022/EASL/EASL_44.htm).

Dietz-Fricke et al (Dietz-Fricke et al., 2023b). evaluated 114 patients from various German centers receiving a 2mg/day BLV therapy were followed-up for 38 ± 17.6 weeks: 59 patients had cirrhosis, of which 4 were Child-Pugh B and 1 was Child-Pugh C; 55 patients had previously received Peg-IFNα therapy. A virologic response defined as an HDV RNA decline of at least 2 log or undetectable HDV RNA was observed in 87/114 (76%) cases over the mean observation time of 38 ± 17.6 weeks, 21.9% reached undetectability of HDV RNA; however, in 11 cases a virologic breakthrough (>1 log-increase in HDV RNA after virologic response) was observed. Cross sectionally, at 24 weeks of treatment, 19/33 patients (58%) had a virologic response, while three patients (9%) did not achieve a 1 log HDV RNA decline. No patient lost hepatitis B surface antigen. Alanine aminotransferase levels improved even in patients not achieving a virologic response, including five patients who had decompensated cirrhosis at the start of treatment. In 6 patients of the 114 analyzed, BLV was discontinued, in one of these because of a lack of response; no serious adverse events were described.

In the Sandmann study (Sandmann et al., 2023), a German study conducted in a Hannover center, 16 patients undergoing 6 months of BLV therapy were included, 7 with compensated cirrhosis, all receiving concomitant NUC therapy. HDV RNA declined in all patients during treatment; at 6 months, 38% (6/16) showed ≥ 2 log HDV RNA decline from baseline levels and 11 patients (69%) normalized ALT levels, while 4 patients (25%) achieved both outcomes.

The data from Austrian patients are described in two studies by Jachs et al, in 2022 and 2023 (Jachs et al., 2022; Jachs et al., 2023). In the first study (Jachs et al., 2022), 23 patients were treated for various lengths of time, with the peculiarity of adjunction of Peg-IFNα treatment in patients fit for treatment and who did not reach virological outcome within 24 weeks. Interesting are the data on the efficacy and tolerability of BLV in cirrhotic patients. According to literature data, BLV monotherapy seems to be safe and useful in patients with severe CHD, as in patients with cirrhosis and significant portal hypertension for which treatment with Peg-IFNα is contraindicated and BLV, therefore, represents the only therapeutic option. Certainly, a good safety and tolerability profile is even more important if antiviral therapy is to be administered long-term in patients with advanced liver disease.

Loglio et al (Loglio et al., 2019; Loglio et al., 2022). described the first 3 European patients with HDV-related compensated cirrhosis who were treated with BLV 10 mg/day for 48 weeks as a compassionate therapy. Liver function tests, bile acids, and virological markers were monitored every 4 weeks. During treatment, HDV RNA levels progressively declined from 4.4 and 5.6 logs IU/ml to undetectability in 2 cases, and from 6.8 log copies/ml to 500 copies/ml for the other patient. Alanine aminotransferases normalized after 20, 12 and 28 weeks, respectively, and there was a significant improvement in features of portal hypertension, in liver function tests and in alpha-fetoprotein levels with a good safety profile.

Anolli et al (Anolli et al., 2023b). evaluated in a retrospective multicenter Italian real-life study the safety and efficacy of BLV 2 mg monotherapy up to 72 weeks in HDV patients with compensated cirrhosis; 93 patients were included: median age was 52 years, 52% males, median liver stiffness measurement (LSM) was 17.4 kPa, 55% had varices, 22% had previous ascites, 53% were IFN-experienced, 97% under NUC treatment. Median ALT levels were 79 U/L, albumin 3.9 g/dL, platelets 70 x 103/mm3, HDV RNA 5.2 log IU/mL, Child Turcotte Pugh (CTP) score A in all patients. A virological response (undetectable HDV RNA or ≥2 log decline vs. baseline) was achieved by 67%, 77% and 75% of patients at weeks 24, 48 and 72, respectively, HDV RNA becoming undetectable in 10%, 16% and 38%. At the same timepoints, ALT normalization was observed in 67%, 67% and 81% of the patients while a combined response (undetectable HDV RNA or ≥2 Log decline vs. baseline ALT normalization) was achieved by 45%, 56% and 63% of patients. Besides ALT, significant on-treatment declines were also observed for AST, GGT, IgG (p<0.001 versus. baseline), while albumin values increased (p=0.02). BLV was well tolerated and no patient discontinued treatment for adverse events, an asymptomatic increase in bile acids occurred in all patients. During BLV treatment, liver decompensation occurred in one patient, de-novo HCC in two, three underwent liver transplantation and one died because of BLV-unrelated causes.

Interesting also are the results of Save-D study (Degasperi et al., 2023a; https://www.who.int/news-room/fact-sheets/detail/hepatitis-d), a retrospective multicenter European study, showing the virological and clinical outcomes of patients with HDV-related compensated cirrhosis treated with BLV monotherapy. In this study, 176 patients receiving BLV monotherapy up to 96 weeks (median follow-up: 48 (8–96) weeks) were included. At enrollment the median age was 50 (range 19-82) years, 59% were men, median ALT 77 (23–1, 074) U/L, liver stiffness measurement (LSM) 18.3 (6.4-75.0) kPa (30% with LSM >25kPa), platelets 89 (17-330) G/L (80% with PLT<150 G/L), 100% CTP score A, 9% HIV-positive, 46% with esophageal varices, 12% with a history of previous ascites, 6% with active HCC, 91% on NUC. Rates of virological responses i.e. HDV RNA undetectable or ≥2-log decline versus baseline) at week 24, 48 and 96 were 48%, 66% and 77%, respectively; HDV RNA was undetectable in 16%, 33%, and 43% patients, respectively. Biochemical response, i.e. ALT <40 U/L, was achieved by 60%, 69% and 73%, respectively, while rates of combined response were 28%, 48% and 58%, respectively. The cumulative risk of de-novo HCC at week 96 and decompensation was 5.9% (95% CI 2-12%) and 2.4% (95% CI 1-5%) (n=2 ascites, n=1 variceal bleeding), respectively. Six (3%) patients underwent liver transplantation (n=4 for HCC, n=2 for end-stage liver disease) and 2 patients died of BLV-unrelated causes (pneumonia and intestinal infarction). The overall cumulative survival rate was 93% (95% CI 88-98%) at week 96. In this study, according to the safety profile, the authors reported that bile acids significantly increased, 10% patients reported mild and transient pruritus. Injection site reactions occurred in 3% of cases; 1 patient discontinued BLV due to a grade 3 maculopapular rash with mild eosinophilia.

Degasperi et al. (Degasperi et al., 2022) evaluated 18 patients treated for 48 weeks with BLV 2mg/d, all undergoing nucleos(t)ide treatment, for HDV-related severe cirrhosis; in fact all BLV patients had significant portal hypertension. During 48 weeks of BLV monotherapy, HDV RNA declined by 3.1 (0.2-4.3) log IU/ml (p <0.001 vs. baseline), becoming undetectable in 5 patients (23%). Overall, a virological response, defined as undetectable HDV RNA or a ≥2 log decline versus baseline, was observed in 14 (78%) patients and a virological non response, defined as <1 log decline of viremia at week 24, was observed in 2 (11%) patients. ALT decreased to 35 (15–86) U/L (p <0.001 versus baseline), normalizing in 83% of patients. At week 48, 12 patients (67%) achieved a combined virological and biochemical response. Regarding markers of liver function, albumin and cholinesterase levels significantly improved, so 4 out of 5 Child-Pugh A6 patients at baseline improved to Child-Pugh A5 at week 48. Regarding the 14 patients achieving a virological response, the liver stiffness measurement decline was significant (16.4 kPa at baseline vs. 14.1 kPa at week 48, p = 0.03). None of the patients developed decompensating events (including ascites, encephalopathy, bleeding) or de novo or recurrent HCC during the study period. None of the patients discontinued BLV and no symptomatic adverse effects were reported, and bile acid increase was not symptomatic. The results of this study showed a good safety profile of BLV in patients with severe cirrhosis.

Dietz-Friecke et al (Dietz-Fricke et al., 2023a). showed data on off-label BLV monotherapy in 15 Austrian, Italian and German patients with decompensated liver disease. CTP stage B and C or with clinical signs of decompensated cirrhosis were included. Virological response (decline in HDV-RNA levels by ≥2 log) was observed in 66% of patients, in 47% of patients ALT normalization was observed, in 4 patients improvement of liver function was observed from CTP B to C, in 4 patients an improvement in ascites was observed. According to the safety profile the results showed that in one patient a worsening in liver function to CTP C was observed, in one patient there was further decompensation, in 3 subjects BLV was terminated at liver transplantation.

All these data, although on a small number of patients, support the antiviral efficacy of BLV in decompensated cirrhosis and encourage to investigate in clinical trials the long term efficacy and safety profile of BLV in this highly vulnerable population.

The innovative therapy with BLV, both in monotherapy and combination therapy, shows interesting results, although it is necessary to wait for a longer follow-up.

As regards the safety profile, the data from both clinical and real life trials are encouraging: BLV treatment was well tolerated without any serious drug-related adverse events or treatment discontinuations (https://www.natap.org/2023/EASL/EASL_31.htm; https://www.natap.org/2021/AASLD/AASLD_56.htm; https://www.natap.org/2022/HDV/070122_07.htm; https://www.natap.org/2021/AASLD/AASLD_17.htm; Wedemeyer et al., 2019a; Buti et al., 2023; Wedemeyer et al., 2023b; Wedemeyer et al., 2023a; Asselah et al., 2024). A minority of patients complained of mild symptoms like fatigue, nausea, headache, dizziness or showed a reduction in platelets or white blood cells; adverse reactions at the injection site were mild, transient and only occasionally required specific treatment (Behrendt et al., 2022; Schwarz et al., 2022). Since BLV inhibits the bile acid transporter function of NTCP as expected, a transient increase in total bile acids was reported in all studies without clinical significance (Hagenbuch and Meier, 1994; Stieger, 2011). Also encouraging were the data from an Italian study (Degasperi et al., 2022) on patients with HDV-related cirrhosis and significant portal hypertension showing that 48 weeks of BLV 2 mg/day monotherapy was safe in patients with compensated cirrhosis and clinically significant portal hypertension. Moreover, in a clinical case described by Anolli et al (Anolli et al., 2023a), a patient with compensated cirrhosis and esophageal varices treated for 76 weeks with 10mg/d, and with 5mg/d for the following 68 weeks, a combined response was observed, platelet count and liver stiffness improved (liver stiffness also improved at the end of follow up even when compared with end of therapy evaluation, in which ALT was already negative) and small esophageal varices without red color sign, at the start, were not detected at the end. In conclusion, BLV is generally well-tolerated with fewer side effects compared to Peg-IFN, since it has been shown to be effective in long-term use, even in patients with advanced compensated cirrhosis.

As regards the virological and clinical efficacy, data from clinical trials and real word studies demonstrated significant positive effects of BLV-based therapy, although the heterogenicity of the treatment, virological endpoints and the length of therapy, make comparison of results difficult. In this innovative therapeutic approach, it is necessary to clearly identify the outcomes of virological response. It is well known that the efficacy of an antiviral is assessed by the undetectability of viral genome. Although the mechanism of action of BLV is peculiar in that it acts by saturating NTCP, an entry receptor of HDV, therefore without a role on HDV replication, the virological efficacy should be assessed by the undetectability of the viral genome. However, many studies, both RTCs and real-life studies, considered a virological response that also includes a two-log reduction in HDV viremia and it is not always possible to extrapolate the pure data of viral suppression. Among RTCs undetectability is part of the primary outcome in MYR 204 and MYR 203 with better results when BLV is associated with pegIFNα compared to PEG IFN monotherapy and BLV monotherapy, while in in MYR 202 and MYR 301 the data is extractable from secondary outcomes ( Table 3 ). Most of the real-life studies ( Table 4 ) include BLV monotherapy and data on viral suppression are present in Fontaine et al., 2022 (https://www.natap.org/2022/EASL/EASL_44.htm), in Visco Comandini et al., 2023 (Comandini et al., 2023), in de Lédinghen et al., 2021 (https://www.natap.org/2021/AASLD/AASLD_18.htm), Dietz-Fricke et al., 2023 (Dietz-Fricke et al., 2023b), Degasperi et al., 2022 (Degasperi et al., 2022), Zollner et al., 2022 (Zöllner et al., 2022), Dietz-Fricke et al., 2023 (Dietz-Fricke et al., 2023a) in Degasperi et al., 2023 (Degasperi et al., 2023b).

However, several questions are again open: for example the role of Peg-IFN, the possible identification of stopping rules during treatment, the duration of BLV monotherapy and the causes of non-response to BLV therapy.

The role of the addition of Peg-IFNα to BLV remains unclear since the data are few and especially from RTCs with short follow-up after stopping treatment; thus, Peg-IFNα therapy adjunction, given the high burden of its side effects but its potential benefit, was one of the most difficult clinical decisions. MYR 204, recently published in the New England Journal of Medicine (Asselah et al., 2024), the most ambitious RCT in terms of primary endpoint, demonstrated how a sustained viral response could more easily be achieved when Peg-IFNα therapy was added to BLV alone. The few real world studies seem to be coherent with RCTs: in de Lédinghen et al (https://www.natap.org/2021/AASLD/AASLD_18.htm). a virological response after 12 months BLV therapy seems to be more likely when therapy with Peg-IFNα is added. In Jachs, et al, 2022 (Jachs et al., 2022), Peg-IFNα therapy was added when no significant benefit was observed after the first 24 weeks of therapy, with good results in terms of virological outcome.

However, real-world studies, such as those of de Lédinghen and Jachs, while providing useful insights into real-world clinical settings, cannot influence guidelines. These data should therefore be interpreted as complementary, while the main decisions remain guided by the results of the RCTs.

It is not easy to compare the different strategies used in RTCs and Real life studies, as they are heterogeneous in terms of therapeutic protocol and virological outcomes. The data show that the association of BLV and peg-IFN improves the virological outcome, but the optimal length of these treatments has not yet been established furthermore and unfortunately, not all patients tolerate peg-IFN. Therefore, it will be necessary to test head to head, or combine together, in specific studies, the drugs currently available for HDV, peg-IFN and BLV, with experimental drugs characterized by other mechanisms of action, hoping to soon have a poly-pharmacological treatment available that is able to suppress HDV replication.

Another open question is to identify in the first months of treatment the patients that will not respond. However, today we have no stopping rules. To this regard Degasperi et al. offer interesting attempts to identify baseline and on-therapy predictors of response evaluating 49 HDV patients with compensated cirrhosis treated with BLV monotherapy 2 mg/day up to 96 weeks, enrolled in a single-center study (Degasperi et al., 2023b). The results of this study showed that different baseline and on-treatment HDV RNA cut-offs may predict virological, biochemical and combined response rates. In particular, for these patients clinical and virological variables were assessed at baseline and every 8 weeks. Virological response rates (undetectable or HDV RNA ≥2 log decline versus baseline) showed 71% at week 24, 84% at week 48, 75% at week 72 and 80% at week 96, respectively; biochemical response rates were 57%, 68%, 81% and 67%, while combined response was 46%, 63%, 63% and 66%, respectively. The Authors showed that baseline HDV RNA <1000 IU/ml was associated also with virological response at week 48 (p = 0.007), 72 (p = 0.04) and 96 (p = 0.04), while it did not predict combined response at any timepoint. HDV RNA <1000 IU/ml at week 24 predicted virological response (p = 0.03), combined response (p =0.02) and ALT<1.5 ULN (p = 0.02) at week 96, but not biochemical response.

Also very interesting are the results of a study that evaluated whether continued therapy could give benefits to patients without virological response after 24 weeks of treatment with BLV. In particular Lampertico et al (https://www.natap.org/2023/AASLD/AASLD_59.htm). evaluated this considering 114 CHD patients who completed BLV monotherapy for 96 weeks in the MYR301 and MYR204: after 24 weeks of therapy, 34/141 (24%) had a partial response (PR), defined as HDV RNA decline ≥1 but <2 log10 IU/mL, and 15/141 (11%) had a non-response (NR), defined as HDV RNA declines of <1 log10. Of the 34 PR patients at week 24, 25 (74%) had a virological response (HDV RNA negativity decline of ≥2log) by week 96. Of the 15 NR patients at week 24, 7 (47%) had a virological response and 3 (20%) had PR by week 96. Moreover, a higher proportion of NR at week 24 achieved a virological response at week 96 among those receiving BLV 10mg (4/5, 80%) versus BLV 2mg (3/10, 30%) and among week 24 NR or PR, the mean baseline HDV RNA did not predict viral response at week 96. These results provide evidence for continuing BLV therapy despite early (24week) suboptimal virologic responses.

Another open question remains the duration of treatment. On this point the paper by Jachs, Panzer et al. evaluating patients who discontinued BLV treatment is interesting (Jachs et al., 2023). The seven patients enrolled (age, 31-68 years, four with cirrhosis) discontinued BLV treatment after 46-141 weeks of treatment after long-term HDV suppression (time of HDV-RNA negativity: 12-69 weeks) and were followed-up for 14 to 112 weeks. One patient was lost to follow-up before the 24th week. HDV-RNA became detectable again in three patients within 24 weeks, while another patient subsequently showed a recurrence of HDV-RNA after almost 1 year; HDV-RNA remained undetectable in two patients treated with BLV + peg-IFN- α2a. BLV was reintroduced in three patients after 13-62 BLV-free weeks and was well tolerated and all patients achieved a virological response again (Jachs et al., 2023)

Finally, it would be important to identify the causes of non-response to BLV. On this point the paper by Hollnberger et al. is interesting (Hollnberger et al., 2023). The authors, in patients who were non-responders (20 patients) or who experienced virologic breakthrough (1 patient) to BLV in MYR202 and MYR301 study, performed deep-sequencing of the BLV-corresponding region in HBV PreS1 and of the HDV HDAg gene, and in vitro phenotypic testing at baseline and week 24. No amino acid exchanges associated with reduced susceptibility to BLV within the BLV-corresponding region or within HDAg were identified at baseline or at week 24. These results suggest that BLV has a high barrier to resistance, and this would make it a suitable drug for long-term treatment, but long-term surveillance for resistance needs to be performed with other studies to confirm these results.