HCV New Drug Research

David R. Nelson

Article first published online: 4 JAN 2011
DOI: 10.1111/j.1478-3231.2010.02391.x
© 2011 John Wiley & Sons A/S

With the introduction of direct-acting oral antiviral agents we are on the verge of a new era that will transform the treatment landscape. This review discusses recent developments in drug discovery for hepatitis C protease inhibitors. First generation protease inhibitors will offer higher sustained viral response rates in naïve populations when combined with standard pegylated interferon and ribavirin. However, these dramatic gains will be partially offset by new challenges in viral resistance and increased adverse events.

Abbreviations
BOC,
boceprevir;
DAA,
direct acting antiviral agents;
PEG-IFN,
pegylated interferon;
SOC,
standard of care;
SVR,
sustained virological response;
TPR,
telaprevir

Hepatitis C virus (HCV) infection is a global problem with an estimated prevalence of 170 million worldwide and 4 million (1.6%) in the US (1–3). Most patients with acute HCV infection become chronically infected, which increases the risk of developing further complications associated with advanced liver disease (4). Given the projected increase in HCV-related cirrhosis (the proportion of chronic HCV infection with cirrhosis is now 25% and projected to reach 45% in 2030) and hepatocellular carcinoma (HCC), optimal treatment of chronic HCV is a high priority (5). Current standard therapy for HCV includes pegylated interferon (PEG-IFN) and ribavirin (RBV), a combination which is effective in approximately 40–50% of genotype 1-infected patients and 80% of genotype 2 and 3-infected patients (6–8). Unfortunately, most patients in the US and Western Europe are infected with HCV genotype 1. The relatively low response rates in treating genotype 1-infected patients, as well as the long treatment durations and adverse side effects has meant that a small minority of patients opt for treatment. Less than 20% of the HCV-infected population in the US is estimated to have been treated. However, the introduction of oral, direct-acting antiviral agents (DAA) is now on the horizon with anticipated higher cure rates, and potentially shorter treatment durations. Approval of the first oral inhibitors is expected by mid-2011 and many patients are awaiting these new therapies.

The class of drugs in the latest stages of development is HCV serine protease NS3–NS4A inhibitors. The NS3/4A protease is essential to viral replication and is responsible for cleaving the HCV polyprotein and releasing most of the nonstructural proteins. The design of NS3/4A inhibitors is complicated because the active site of the NS3/4A protease is located in a very shallow groove composed of three highly conserved amino acid residues. This key concept explains why most NS3/4A protease inhibitors under development display high antiviral efficacy but a low genetic barrier to resistance and will frequently cause the selection of resistant mutants which can lead to viral breakthrough. The most mature protease inhibitors are telaprevir (TVR) and boceprevir (BOC), which have now completed phase II and III trials and will probably be approved in mid 2011. These programmes have yielded some consistent early lessons for the protease inhibitor class. For naïve, genotype 1 patients, higher cure rates and a shorter duration of therapy can be expected, which will be partially offset by new issues of resistance and increased adverse events.

The recently published Protease Inhibition for Viral Evaluation (PROVE 1 and 2, evaluating TVR) and Serine Protease Inhibitor Therapy (SPRINT-1, evaluating BOC) studies evaluated protease inhibitors in combination with PEG-IFN/RBV in genotype 1, naïve patients. In PROVE 1, TVR was dosed at 750 mg every 8 h for 12 weeks in combination with PEG-IFN and RBV followed by an additional 12 weeks or 36 weeks of standard of care (SOC). The results were compared with 48 weeks of SOC (see Table 1). The sustained virological response (SVR) rate in SOC was 41%, compared with 61% (P=0.02) in the 24-week treatment group and 67% (P=0.002) in the 48–week treatment group (9). Relapse rates were highest in the control group (23%) compared with the 24- (2%) and 48-week TVR treatment group (6%). However, more patients discontinued therapy in the TVR treatment groups secondary to adverse side effects, with a rash being the most common reason for discontinuation. In the PROVE 2 trial, shorter treatment duration was investigated with treatment groups receiving triple therapy (TVR+PEG-IFN/RBV) for only 12 weeks (with and without RBV) compared with an additional 12 weeks of SOC (See Table 1). SVR was 46% in the control group, compared with 36% in the non-RBV group (P=0.20), 60% in the 12 weeks triple therapy TVR group (P=0.12) and 69% in the 24 weeks triple therapy TVR group (P=0.004) (10). Relapse rates were highest in the non-RBV treated group (48%) compared with the control group (22%), 12 weeks triple therapy group (30%) and 24 weeks triple therapy group (14%). The most important side effects with TVR were rash, gastrointestinal disorders and anaemia. Although severe rash may require treatment discontinuation, moderate forms can be successfully treated with topical steroids. The median decline in blood haemoglobin concentrations with TVR was approximately 1 g/dl.

Table 1. Lessons learned from Phase II data
PEG-IFN, pegylated interferon; RBV, ribavirin; SVR, sustained virological response.
Higher SVR (60–70%) in genotype 1
Response-guided therapy
Resistance emergence
Differences in genetic barrier to resistance for subtypes (1a vs. 1b)
PEG-IFN and RBV necessary to maximize efficacy

The PROVE 1 and 2 seem to indicate that TVR can help overcome negative host and viral factors. A recent pooled analysis looked at a subgroup of patients with characteristics associated with low virological response (11). The overall SVR for the pooled TVR treatment groups was 65 vs. 44% in the control group (P less then 0.001). SVR rates were significantly higher with TVR-based vs. SOC among patients with baseline HCV RNA more then 800 000 IU/ml (P less then 0.05), patients with genotype 1a HCV infection (P less then 0.05), patients with genotype 1b HCV infection (P less then 0.05), men (P less then 0.05), patients more then 50 years of age (P less then 0.05) and those with bridging fibrosis (P less then 0.05). The conclusion of this analysis is that TVR is effective in all subgroups of patients who have traditionally been considered difficult to treat. Another phase II trial with TVR was recently released (Study C208) that suggests that SVR rates in naïve patients may be higher than previously reported, especially when a response-guided duration is followed. In this study, treatment-naïve, genotype 1 patients (n=161) were administered triple therapy for 12 weeks with the subsequent PEG-IFN/RBV treatment duration determined according to a response-guided strategy (12). Patients who achieved a rapid virological response (RVR) received a total of 24 weeks of therapy and those who did not have RVR continued PEG-IFN/RBV to weeks 48. The SVR rates in this study ranged from 81 to 85%, higher than those observed in the phase II PROVE trials. These high overall SVR rates emphasize the potential of the triple therapy approach. Results may be explained in part by experienced study centres with very low discontinuation rates (5%) compared with the PROVE studies. In addition, treatment duration was shortened to 24 weeks in patients who achieved RVR, while the remaining patients received 48 weeks of therapy. Between 80 and 83% of patients treated with PEG-IFN-2a, and 67–69% of patients treated with PEG-IFN-2b achieved RVR and could therefore be treated for 24 weeks. This study clearly suggests that response-guided therapy based on RVR at week 4 may optimize SVR and provides a useful guide for determining which patients should be treated for 24 vs. 48 weeks.
,

In the phase II SPRINT-1 trial, triple combination therapy with BOC and the current SOC, PEG-IFN and RBV, was found to induce high rates of SVR (54–75%) in genotype 1 treatment-naive patients, depending on the duration of therapy (13). Unlike TVR, BOC was administered for the duration of treatment. The treatment regimens included a control group treated with 48 weeks of SOC compared with five BOC treatment regimens (4 weeks of PEG-IFN/RBV lead-in followed by triple therapy for 24 or 44 weeks; triple therapy for 28 or 48 weeks; triple therapy, but with low-dose RBV for 48 weeks). The ideal duration of therapy appears to depend upon early viral kinetics. Patients who cleared the virus by week 4 of triple therapy had 82 and 94% chances of achieving SVR after 28 and 48 weeks of treatment respectively. If HCV RNA is detectable after week 4, but becomes undetectable by week 12, 48 weeks of treatment resulted in a 79% SVR rate; shortened treatment was significantly inferior, with only 21% of patients achieving SVR after 28 weeks. Clearance after week 12 was associated with a negligible chance of SVR and appears to indicate an early stopping rule at week 12. In addition to the expected side effects associated with the SOC, treatment with a BOC-containing regimen was associated with increased dysgeusia and anaemia. Anaemia (defined as a decline in haemoglobin level less then 10 g/dl) occurred in 52–56% of patients in the triple-therapy groups despite administration of epoetin-at the investigator's discretion, compared with 34% in controls. Higher rates of discontinuation secondary to adverse side effects and viral breakthrough occurred in the BOC treatment groups compared with the control group and anaemia appeared to be a significant problem, with up to 50% of patients receiving erythropoietin. Of note, the highest reported viral breakthrough was seen in the low-dose RBV group

Figure 2. ADVANCE: Phase 3: 12-week duration telaprevir optimal. PEG-IFN, pegylated interferon; RBV, ribavirin; RVR, rapid virological response; SVR, sustained virological response; TVR, telaprevir.