Correspondence Edward Gane, New Zealand Liver Transplant Unit, 15th Floor Support Building, Auckland City Hospital, Park Road, Grafton, Auckland 92024, New Zealand Tel: 642 154 8371 Fax: 649 529 4061 e-mail: firstname.lastname@example.org
An estimated million people have chronic hepatitis C virus (HCV) infection. With current treatment success rates, by 2030, more than 40% will be cirrhotic and the number of cases with end-stage liver disease is projected to treble. Current standard-of-care is the combination of pegylated interferon plus ribavirin for 24–48 weeks. Unfortunately this is associated with poor efficacy (45% in HCV GT1; 75% in GT2 and 65% in GT 3) and tolerability. Many patients are either unsuitable for or decline current treatment infection because of the significant side-effects associated with this treatment, including those with decompensated cirrhosis or sever psychiatric illness. It is hoped that the development of direct acting antiviral agents (DAAs) will address this huge unmet medical need. The addition of a protease inhibitor to pegylated interferon plus ribavirin is associated with increase in efficacy and shortened duration of therapy in patients with HCV GT1 and is likely to become the new standard-of-care. However, triple therapy will not be suitable for patients with non-1 HCV infection, or contraindications to interferon. It is hoped that the combination of multiple DAAs which target different steps of HCV replication should provide interferon-free treatment regimen. Current and planned studies will determine which combination (protease, nonnucleoside polymerase, nucleoside polymerase, NS5A, cyclophyllin B inhibitors), how many DAAs and duration of therapy will be required to optimise cure. It will also be important to minimise the emergence of multi-resistance, which would jeopardise future retreatment options
Chronic hepatitis C is the global ‘epidemic’ of the new millennium, with an estimated 200 million people currently infected with the hepatitis C virus. The incidence of hepatitis C virus (HCV) infection has decreased by more than 50% over the last decade, reflecting reduced exposure risk (1, 2). Although the size of the population with chronic HCV infection has been stable since 2000 (3), this is an ageing cohort and the proportion of this cohort with cirrhosis will double over the next decade from 16 to over 35% (4). As a result, the annual rate of both HCV-related hepatocellular carcinoma and HCV-related-mortality is projected to triple again by 2030 (5–7). The only way to prevent this projected health burden is to reduce the pool of infected patients through successful antiviral therapy. However, <10% of the infected population has been treated, and less than half of these have been cured. It is estimated that these numbers would need to increase almost 10-fold to prevent the projected increase in HCV-related complications (1, 8).
Unfortunately, current treatment options are limited by both efficacy and tolerability. The current standard of care (SOC) treatment for chronic HCV infection is 48 weeks of combination therapy with subcutaneous injections of pegylated interferon (PEG-IFN) plus orally administered ribavirin. SOC achieves a sustained virological response (SVR) in only 45% of patients infected with HCV genotype (GT)1 and 65% of those infected with GT2 or 3 (9). HCV GT1 is the predominant genotype globally, accounting for between 55 and 95% of infections.
Baseline patient predictors of non-response to SOC other than HCV genotype include older age, advanced fibrosis, high body mass index, insulin resistance and African ethnicity. A specific inherited polymorphism on chromosome 19 at rs12979860, close to the interleukin (IL)28B gene, is strongly associated with SVR across all patient groups (T/T vs. T/C or C/C), independent of all other predictors including ethnic origin (10).
Subsequently, several other independent genome-wide association studies have also identified additional SNPs, in the IL28B region, associated with response to treatment (11–13).
In those patients with favourable baseline predictors of response, adherence to therapy is an important determinant of outcome. Both PEG-IFN and ribavirin are associated with significant adverse effects, from flu-like symptoms, fever, rash, anorexia, thyroid dysfunction, to dose-related life-threatening cytopaenias and mood disorders. Side effects result in a dose reduction in 60–80% of patients and treatment withdrawal in 5–10%. Another inherited polymorphism on Chromosome 20 at rs1127354, which determines the activity of inosine triphosphatase, reliably predicts protection from ribavirin-induced haemolysis (A/A and C/A vs. C/C) (14). In addition, many patients never start SOC because of real or perceived medical or psychosocial contraindications to either IFN or ribavirin. Many more defer therapy because of anecdotal stories about severe adverse effects.
Finally, there is a large and growing pool of largely GT1 patients who have not previously responded to PEG-IFN-a and ribavirin treatment, in whom retreatment no alternative treatment options are currently available.
New therapeutic approaches offering improvements in efficacy, safety and tolerability are urgently needed to address these unmet medical needs.
Direct-acting antivirals and triple therapy
The five steps in HCV replication that are potential targets for direct-acting antivirals (DAAs) include initial binding of HCV to hepatocyte surface receptors (via LDLR and CD81), translation and polyprotein processing (via the HCV protease complex), RNA replication (via the HCV RNA-dependent polymerase complex), virion assembly and maturation, followed by release from the hepatocyte (Fig. 1). To date, the most successful approaches have been targeting the HCV protease (via inhibition of NS3A4 protease) and the HCV polymerase complex (via inhibition of NS5A, NSAb and indirectly through NS3A4). The in vitro replicon and transgenic models for HCV replication and the application of rapid screening techniques for small molecules have triggered an explosion in drug development. Over the last 5 years, more than 90 protease and polymerase inhibitors have entered clinical development, of which several have halted because of toxicity (BILN2061, NM283, HCV796, R1626) and many more have been abandoned because of preclinical toxicity signals or lack of clinical efficacy.
The DAAs closest to being marketed are the protease inhibitors, boceprevir and telaprevir. Phase 3 global registration studies of both will be completed this year and these protease inhibitors are expected to be the first DAAs to gain regulatory approval as add-on therapy to current SOC PEG-IFN plus ribavirin. The benefits in terms of efficacy will be significant – 48 weeks of boceprevir plus SOC increased the SVR rates in treatment-naïve GT1 patients from 38 to 66%, while 12 weeks of telaprevir plus 24 weeks SOC increased the SVR rates from 43 to 75% (15, 16). Triple therapy may also offer hope in treatment-experienced patients, especially previous responder relapsers and partial responders (17– 19). However, both have specific toxicities (notably anaemia and dysgeusia with boceprevir and anaemia and rash with telaprevir), which increased the rate of treatment withdrawal in the DAA combination arms.
Although triple therapy (addition of either telaprevir or boceprevir to PEG-IFN plus ribavirin) is likely to become the new SOC in late 2011, this will not be suitable for patients either intolerant of or with contraindications to IFN or ribavirin, including patients with decompensated cirrhosis or following solid organ transplantation. Also, the efficacy of this triple therapy will probably be reduced in treatment-experienced patients, especially those who were non-responders to a previous treatment with SOC. Moreover, although telaprevir has similar antiviral activity against HCV GT2, this agent has no effect in patients with HCV GT3 infection (20, 21). All current protease inhibitors and most non-nucleoside polymerase inhibitors in development are active primarily against HCV GT1. PEG-IFN plus ribavirin will remain the SOC for non-GT1 HCV until nucleoside polymerase inhibitors and cyclophyllin inhibitors enter clinical practice.
Finally, the 30–40% of GT1 patients who do not respond to this new triple therapy will have developed resistance to protease inhibitors, which will limit future treatment options.
Therefore, although the addition of a single DAA to PEG-IFN and ribavirin may improve cure and shorten the treatment duration of SOC, this approach will not meet the needs of many difficult-to-treat patient groups.
Towards an all oral regimen
The successful development of an all-oral, IFN-free regimen of multiple DAAs should address this current extensive unmet medical need and could potentially become the standard of care for all patients with chronic HCV infection. The rationale for this approach is based on the current human immunodeficiency virus (HIV) treatment paradigm, in which a combination of different DAA agents, which target different steps of viral replication, has been shown to increase viral suppression as well as delay or prevent the emergence of antiviral resistance. There is a rapidly increasing list of potential candidates for such a combination including NS3 helicase inhibitors, NS3/4A protease inhibitors, cyclophyllin B inhibitors, NS5A inhibitors, NS5B nucleoside inhibitors and NS5B non-nucleoside inhibitors (see Table 1). The primary criteria for a DAA combination should be to increase antiviral efficacy without increasing toxicity. The combination of DAAs should exhibit in vivo at least additive and preferably synergistic antiviral efficacy [i.e. rather than interference as observed with telbivudine plus lamivudine in patients with hepatitis B virus (HBV) infection]. The combination should lack cross resistance and should prevent virological breakthrough from the emergence of resistance mutants to either or both DAAs. Finally, the combination should lack direct drug interactions and overlapping toxicities.
Table 1. Direct-acting antivirals in clinical development in 2010
Phase of development
HCV, hepatitis C virus.
NS3/4a serine protease inhibitor
NS3 Helicase inhibitor
HCV replication complex
NS5B nucleoside polymerase inhibitors
NS5B non-nucleoside polymerase inhibitors
The first in vitro study of combination DAAs was performed in the replicon model (22). The addition of RG7128, a nucleoside polymerase inhibitor, to RG7227, a protease inhibitor, provided additive viral suppression and completely prevented the development of phenotypic resistance to the protease inhibitor. Similar in vitro effects have been demonstrated for other DAA combinations, including a protease inhibitor plus a non-nucleoside polymerase inhibitor and also the combination of two nucleoside polymerase inhibitors (23, 24). The in vivo efficacy of combining DAAs was first reported in chimpanzees treated with a combination of Merck NS3/4A protease inhibitor MK-7009 and non-nucleoside inhibitor MK-608 for 7 days, whereby both had rapid and sustained viral suppression and one animal eradicated the HCV infection (25). The first study of combination DAAs in patients was the proof-of-concept INFORM-1 study completed last year (26). In this randomised, placebo-controlled double-blind trial, 87 patients with HCV GT1 infection were randomised to receive up to 13 days of either oral combination therapy with RG7128, a nucleoside polymerase inhibitor, and RG7227/danaprevir, an NS3/4A protease inhibitor or with matched placebos. Both agents had already been administered to patients for 12 weeks in combination with SOC. Direct drug interactions between RG7128 and danaprevir were considered very unlikely, because of the different mechanisms of action and routes of elimination and the lack of overlapping toxicities identified in any of the preclinical or human clinical studies. This combination achieved profound antiviral suppression, greater than the additive effects of either treatment alone. The median reduction in HCV RNA from baseline was 5 logs, falling below the level of detection in 88% in the cohort who received the highest dose of both RG7128 (1000 mg b.i.d.) and danaprevir (900 mmg b.i.d.). No evidence of the emergence of resistance to either compound was observed during this study. This combination was well tolerated, with no serious adverse events, treatment-related dose modifications, discontinuations or study withdrawals. An important observation was that antiviral efficacy was similar in treatment-naïve and treatment-experienced patients including non-responders. Because the total duration of therapy was only 13 days, all patients rolled over into SOC. The rates of RVR, EVR and ETR were markedly increased by the 2 weeks of pretreatment. In the final cohort of patients who received the highest dose of RG7227 and RG7128, 100% achieved ETR after 24 weeks SOC. Although SVR results are still pending, the benefit of pretreatment with combination DAA on subsequent responses to SOC suggests that the strategy of combination of DAA lead-in before starting SOC could be an alternative strategy to IFN-free DAA therapy.
However, the primary goal of combination DAA therapy in HCV infection will be to provide a safe and effective substitute for IFN regimens in all treatment-naïve and -experienced patients.
Although the new treatment paradigm for HCV is based on HIV, the goals are very different. In HIV infection, a cure is not achievable because it is impossible to eradicate infection from lymphocytes and macrophage reservoirs and from nuclear integration. Therefore, lifelong combination DAA therapy is needed to maintain viral suppression and prevent disease progression. In HCV infection, however, replication is entirely cytoplasmic and limited to hepatocytes. Therefore, viral eradication should be possible with short-course combination DAA.
The duration of combination DAA necessary to eradicate HCV infection is unknown. The early viral kinetic profile of single DAA therapy demonstrates a rapid Phase 1 decline in serum HCV RNA levels of 3–4 log in the initial 36 h, attributed to the clearance of free virions from the circulation. The addition of a second DAA appears to increase this initial slope, suggesting at least additive effects of both agents. This is followed by Second Phase decline in serum HCV RNA of 1-1.2 log/week, attributed to the loss of infected hepatocytes (Fig. 2). This rate of viral decline continues until the infection is eradicated, unless DAA-resistant variants emerge. Therefore, based on the estimated total body viral burden of 1011 virions, between 8 and 12 weeks of DAA therapy should be sufficient to eradicate HCV infection in most patients. The addition of a second DAA targeting a different step of HCV replication and lacking cross resistance should both increase the slope of the Phase 1 decline as well as prevent virological breakthrough during Phase 2.
A finite duration of combination DAA therapy without IFN assumes that viral suppression alone will eradicate HCV, which seems reasonable in the absence of evidence of either viral latency or extrahepatic reservoirs of replication (as seen in HBV). Another important factor for the maintenance of end-of-treatment response may be the indirect effect of combination DAA therapy on host immune responses. In chronic HCV infection, the HCV NS3 protease may directly impair host IFN responses through the inhibition of phosphorylation of IFN regulatory factor-3 (27). Administration of the NS3/4A protease inhibitor should restore this immune responsiveness. Chronic HCV infection is also associated with high levels of IP-1(CXCL-10), a chemokine involved in lymphocyte chemotaxis, reflecting endogenous IFN activation levels. High levels of IP-10 during SOC are correlated with the risk of post-treatment relapse (28). In the INFORM study, the rapid and profound viral suppression achieved with 13 days of combination RG7128 and danaprevir (5 logs within 14 days) correlated with normalisation of IP-10 levels (29). Furthermore, it has been shown that in non-responders, some IFN-stimulated genes were highly expressed; thus, preactivation of the IFN system in patients appears to limit the effect of IFN antiviral therapy (30). Current and future studies will determine whether the IL28 genotype influences sustained response rates in an IFN-free DAA regimen.
Future studies of combination DAA therapy in chronic HCV should include the evaluation of HCV-specific and non-specific immune responses to determine whether immune reconstitution does occur and whether this is a prerequisite for the prevention of relapse after treatment withdrawal.
First studies with all-oral treatments
The shift from triple therapy (single DAA plus SOC) to IFN-free combination DAA studies has been impeded by the reluctance of regulatory authorities to approve the combination of two experimental compounds still in early-phase clinical development. However, such studies should be reasonable as long as safety data are available for each candidate DAA for the duration of the proposed treatment. The rapid emergence of resistant variants during monotherapy with either NS3/4a or NS5b non-nucleoside inhibitors has restricted the duration of DAA monotherapy studies to 3–5 days; thus, longer duration safety data must be obtained from studies of DAA in combination with SOC. An additional requirement before embarking on combination DAA studies in patients should be data from preclinical and human clinical studies for each candidate DAA, confirming lack of cross-resistance, lack of overlapping toxicities and a low likelihood of any drug–drug interactions, which could affect antiviral activity, bioavailability or clearance. INFORM-1 fulfilled all of these requirements but was performed in Australia and New Zealand, because the conservative regulatory environment of the FDA and EMEA did not allow this study to be performed in either Europe or the US at that time. The success of this proof-of-concept and widespread enthusiasm in the HCV field over these results will push the regulatory authorities to modify their previously conservative position.
Following the INFORM proof-of-concept study, seven Phase 2 studies of combination DAA studies are already entering Phase 2 clinical trials in patients with treatment-naïve HCV infection (see Table 2), with many more planned. All studies include an NS3/4a protease inhibitor with or without ritonavir boosting, combined with an agent targeting the HCV polymerase complex – either a non-nucleoside NS5b (n=4), nucleoside NS5b (n=1) or an NS5a inhibitor (n=2). The potential advantage of the nucleoside polymerase inhibitor is the relatively high genetic barrier to resistance and the low prevalence of pre-existing nucleoside polymerase resistance. In a recent survey, no treatment-naïve GT 1 patients had detectable signature S282T mutations, while the prevalence of mutations conferring resistance to either NS3/4a inhibitors or non-nucleoside polymerase inhibitors could be detected in more than 8% of treatment-naïve patients (31). However, it is not clear whether these pre-existing mutations will affect the response to combination DAA therapy. Most of these mutations are associated with decreased replicative fitness and none confer cross-resistance to either NS3/4a inhibitors or non-nucleoside polymerase inhibitors. Unlike INFORM-1, the aim of these studies is curative, with combination DAAs administered for 12–24 weeks. Most study designs incorporate response-guided therapy, with the 2-week rather than the 4-week RVR adopted as a predictor of efficacy (and shortened treatment duration). Although IFN-free, most studies have retained ribavirin as a third oral agent (based on observations of higher relapse rates in the ribavirin-sparing treatment arms in the telaprevir plus SOC studies) (32). However, the impact of ribavirin on the efficacy and tolerability of combination DAA therapy needs to be established.
Table 2. Combination direct-acting antivirals' clinical studies in 2010/2011
The addition of a protease inhibitor to PEG-IFN plus ribavirin will increase the cure rate in both treatment-naïve and treatment-experienced patients and is likely to become the new SOC. However, there will still be a large ‘unmet need’, including patients unable to or unwilling to receive IFN or ribavirin therapy and previous non-responders to SOC. The rationale for combining different DAAs is to increase viral suppression and prevent or delay the emergence of antiviral resistance. The ultimate goal is to develop a short-duration, IFN-free oral combination, with excellent tolerability and efficacy in both treatment-naïve and treatment-experienced patients.
Conflicts of interest
Edward Gane is a member of local or international Advisory Boards and Invited Speaker for GSK, Roche, Pharmasset, Abbott, Novartis and Merck.