Therapy of hepatitis C: From empiricism to eradication

Authors


  • Potential conflict of interest: Nothing to report.

Abstract

The complications of chronic hepatitis C virus infection can be prevented by antiviral therapy. The initial choice of interferon alfa and, subsequently, ribavirin as potential treatments for chronic hepatitis C was empirical. Nevertheless, the combination of pegylated interferon alfa and ribavirin has become the standard treatment of chronic hepatitis C. Since the advent of interferon-based therapy, enormous progress has been made in understanding the mechanisms of treatment efficacy and failure, and in everyday patient management. The principal advances are: a better understanding of hepatitis C virus steady-state kinetics and the antiviral mechanisms of interferon and ribavirin; easier treatment decisions thanks to novel assays to assess liver disease severity and the virological characteristics of infection; a better use of virological tests to tailor therapy; a better management of adverse effects; a better understanding of virological treatment failure; and a better management of “special” populations, including patients with decompensated cirrhosis and end-stage liver disease, liver transplant recipients, hemodialysis patients and renal transplant recipients, human immunodeficiency virus-coinfected patients, intravenous drug users and patients on opiate replacement therapy, or virological non responders to previous therapies. Steady-state HCV kinetics offers several potential targets for new drugs. These targets should ideally be hit simultaneously in order to achieve viral eradication within a reasonable time frame. Future drugs for HCV infection will belong to four main categories, including new interferons, alternatives to ribavirin, specific HCV inhibitors, and immune modulators. New treatments and vaccines might make it possible to eradicate HCV in the future. (Hepatology 2006;43:S207–S220.)

Chronic hepatitis C virus (HCV) infection affects more than 170 million people worldwide. Its prevalence is 1% to 2% in industrialized countries. An estimated 20% of HCV-infected patients have or will develop cirrhosis, with an annual risk of developing hepatocellular carcinoma of 1% to 4% among patients with cirrhosis. Chronic HCV infection has become the most common indication for liver transplantation. HCV infection is curable. Thus, its complications can be prevented by antiviral therapy. Treatment of chronic hepatitis C is currently based on a combination of pegylated interferon (IFN) alfa and ribavirin, and a number of new anti-HCV therapies are in development.

Empiricism

Interferon alfa

The initial choice of interferon (IFN) alfa as a potential treatment for chronic hepatitis C was an empirical one. The causative agent of chronic “non-A, non-B” hepatitis had not yet been identified, nothing was known of its virological characteristics that could have helped to design specific drugs, and there was no way of evaluating antiviral activity. In their seminal work on the treatment of chronic non-A, non-B hepatitis with recombinant human IFN alfa, published in 1986, Hoofnagle et al.1 wrote: ”Alfa interferon was a natural choice as a possible therapeutic agent for chronic non-A, non B hepatitis. This agent has a wide spectrum of antiviral activity and has been used to treat many acute and chronic viral illnesses. Alfa interferon has already been shown to inhibit replication of several human hepatitis viruses, including hepatitis A virus (in cell culture), hepatitis B virus and the hepatitis delta agent.” Retrospectively, the rationale for using IFN to combat infection by a member of the Flaviviridae family was weak. Yet a significant decline in ALT was observed in 8 of 10 patients receiving various doses and schedules of IFN alfa for up to 12 months, and liver histology had improved by the end of therapy in the three patients who underwent biopsy.1 A 10-year follow-up study of the same cohort of patients2 revealed that 5 of the 10 patients had cleared the infection on IFN alfa monotherapy (a higher rate than obtained in later studies). According to the authors, “The reasons for these differences are probably in the selection of patients in this study, who were generally young, without evidence of other disease. Furthermore, most of the responders in this study happened to have only mild or moderate degrees of fibrosis on liver biopsy and hepatitis C virus (HCV) genotypes 2 or 3, factors that are known to be associated with high rates of response to IFN.” Patients and their physicians should salute Hoofnagle's intuition: nobody can say what the treatment of chronic hepatitis C would be now if these investigators had decided to include in their initial study only elderly patients with advanced fibrosis or cirrhosis, HCV genotype 1 infection, and a high baseline level of replication.

Several randomized controlled trials were subsequently performed, and the first NIH Consensus Development Conference on Management of Hepatitis C, held in 1997,3 recommended standard IFN alfa at a dose of 3 million units three times a week for 48 weeks as the standard treatment for chronic hepatitis C. However, the rates of sustained virological response, defined as undetectable HCV RNA 6 months after the end of therapy in an assay with a detection limit of 50 international units/mL, were only 12% to16% with this schedule.4

Ribavirin

Reichard et al. were the first to report the use of ribavirin in patients with chronic hepatitis C, in 1991.5 The authors justified their choice of ribavirin as follows: “Ribavirin is a non-interferon-inducing nucleotide analogue with a broad spectrum of activity against RNA and DNA viruses, including those from the flavivirus family”. Luckily, this trial was performed before accurate molecular assays were widely available. The significant reduction in ALT levels observed during therapy was encouraging, and the authors concluded that “ribavirin is the first drug to offer a potentially effective oral treatment for chronic hepatitis C”.5 Subsequent studies showed that ribavirin improved ALT levels but did not affect viral replication,6–8 somewhat tempering the initial enthusiasm. How could a drug with no apparent antiviral effect in vivo be beneficial in chronic hepatitis C? What would be the rationale for combining such a drug with standard IFN alfa to improve the rate of sustained viral clearance ? Luckily again, Brillanti et al.9 started their trial of standard IFN alfa and ribavirin combination therapy after the encouraging ALT data had been reported but before ribavirin's lack of antiviral efficacy could be shown in sensitive quantitative HCV RNA assays. Their results for ever changed the landscape of hepatitis C therapy, and the lives of many patients: 40% of patients who received a combination of standard IFN alfa three times per week plus ribavirin eradicated HCV, whereas none of those on IFN alfa monotherapy did so. This was enough to initiate randomized controlled trials and to seek marketing authorization for IFN-ribavirin combination therapy.

Further randomized controlled trials10, 11 led to approval of the IFN alfa-ribavirin combination as the standard treatment for chronic hepatitis C, as recommended by the European Association for the Study of the Liver (EASL) Consensus Conference on Hepatitis C in 1999.12 The results of combination therapy were subsequently improved by pegylation of IFN alfa molecules in order to improve their pharmacokinetic and pharmacodynamic properties (allowing weekly injections) and to enhance their efficacy.13–17 Now, the standard treatment of chronic hepatitis C is pegylated IFN alfa combined with ribavirin, as established in 2002 by the NIH Consensus Development Conference on Management of Hepatitis C.18

Abbreviations

IFN, interferon; HCV, hepatitis C virus; EASL, European Association for the Study of the Liver; IMPDH, inosine monophosphate dehydrogenase; Th1, T-helper 1; HRQOL, health-related quality of life; G-CSF, granulocyte-colony stimulating factor; IL, interleukin; MELD, Model for End-Stage Liver Disease; ELTR, European Liver Transplant Registry; UNOS, the United Network for Organ Sharing; HAART, highly active antiretroviral therapy; IVDU, Intravenous drug users; IRES, internal ribosome entry site; siRNA, small interfering RNA.

Reason

Two molecular variants of pegylated IFN alfa can be used to treat chronic hepatitis C, namely pegylated IFN alfa-2a, administered at a fixed dose of 180 μg/week, and pegylated IFN alfa-2b, administered at a weight-adjusted dose of 1.5 μg/kg/week. Ribavirin is administered at a dose of 0.8 to 1.2 g/day according to body weight and the HCV genotype. In the three main registration trials (randomized controlled studies involving patients without cirrhosis),15–17 the rate of sustained virological responses ranged from 76% to 84% in patients with HCV genotype 2 or 3 infection, and from 42% to 52% in patients with HCV genotype 1 infection. Pretreatment variables that correlated with sustained viral eradication included HCV genotypes 2 and 3, lower baseline viral load, lower body weight, younger age, and milder hepatic fibrosis.15–17

Since the advent of IFN-based therapy, enormous progress has been made in understanding the mechanisms of treatment efficacy and failure, and in everyday patient management. The principal advances are described below.

Better Understanding of HCV Steady-State Kinetics in Chronically Infected Patients.

Recent mathematical models of viral decay during IFN alfa therapy19 have helped to understand the steady-state kinetics of HCV infection, which represents the treatment target (Fig 1). The liver is composed of infected and uninfected hepatocytes. Collectively, infected hepatocytes continuously produce large amounts of HCV virions, the vast majority of which are released into the peripheral circulation. Meanwhile, a small proportion of newly produced virions infect naïve liver cells, that join the pool of infected hepatocytes. Both infected and uninfected hepatocytes die, essentially by apoptosis. Circulating virions are continuously degraded in a virtual degradation compartment, by unknown mechanisms. Smaller compartments with similar properties may also exist outside the liver. The steady-state viral kinetics observed during chronic hepatitis C is characterized by: (1) equilibrium between infection of new cells and death of infected cells, ensuring that the size of the pool of infected hepatocytes remains stable; and (2) equilibrium between the release of newly produced HCV virions into the peripheral circulation and their degradation, ensuring stable viral load. The estimated half-life of free HCV virions in peripheral blood is about 2.7 hours, and an estimated 1012 virions are produced and cleared each day in a given patient.19 The goal of anti-HCV therapy is to disrupt this equilibrium by reducing virion release into peripheral blood and by promoting the clearance of infected cells.20 Cure is achieved when all infected cells have been cleared from the body.

Figure 1.

Schematic representation of steady-state HCV kinetics during chronic infection, based on mathematical modeling of viral decay during IFN-α therapy.19 The liver of the chronically infected patient is composed of infected and uninfected hepatocytes. The infected hepatocytes continuously produce large amounts of HCV virions, the vast majority of which are released into the peripheral circulation. A small proportion of newly produced virions infect new liver cells that join the pool of infected hepatocytes. Both infected and uninfected hepatocytes die, essentially by apoptosis. Circulating virions are continuously degraded in a virtual degradation compartment by unknown mechanisms. Smaller compartments with similar properties may exist outside the liver, but they probably contribute only a small fraction of daily virus production. Adapted from Pawlotsky JM. Current and future concepts in hepatitis C therapy. Semin Liver Dis 2005;25:72–83. Reprinted with permission.

Better Understanding of the Antiviral Mechanisms of IFN and Ribavirin.

Mathematical modeling of the first phase of viral decay during therapy suggests that IFN alfa directly inhibits HCV replication.19 The antiviral effect of IFN alfa was subsequently demonstrated in the “subgenomic replicon”, a synthetic in vitro replication system based on non structural HCV proteins and cultured HuH7 cells.21–23 IFN alfa was also shown to inhibit HCV replication of the FL-J6/JFH infectious clone in a virus productive cell culture.24 Finally, we showed this direct effect in primary cultures of normal human hepatocytes, the model closest to the naturally infected human liver.25 The IFN-induced proteins and enzymatic pathways that establish an antiviral state in infected and uninfected cells are not yet fully identified. IFN alfa binding to its receptors at the surface of immune cells triggers complex and intricate effects, such as class I major histocompatibility complex antigen expression, activation of effector cells, and complex interactions with the cytokine. cascade.26, 27 Mathematical modeling of the second slope of viral decay during therapy points to gradual clearance of infected cells. It is unclear whether IFN alfa, through its immunomodulatory properties, accelerates the clearance of infected cells at the same time as it inhibits viral replication. Thus, IFN alfa could act mainly, or solely, as an antiviral agent.

Ribavirin is a synthetic guanosine analog that is transformed into ribavirin triphosphate, the active form of the drug, by intracellular phosphorylation. We recently showed that ribavirin has only a moderate and transient dose-dependent inhibitory effect on HCV replication in vivo, the mechanisms of which remain unclear.28 The principal clinical effect of ribavirin is to prevent relapses in patients who respond to the antiviral effect of IFN alfa,29 probably by shortening the half-life of infected cells in the presence of IFN alfa. However, the precise mechanisms underlying the action of ribavirin in chronic hepatitis C are unknown. Ribavirin weakly inhibits HCV RNA-dependent RNA polymerase in vitro.30 Its main properties may be related to inhibition of inosine monophosphate dehydrogenase (IMPDH), an enzyme that transiently depletes intracellular GTP pools.30 However, other IMPDH inhibitors have not proven effective on HCV infection (Hézode et al., unpublished data). Ribavirin has also been suggested to tilt the T-helper 1 (Th1)/Th2 balance toward Th1,31 but these findings have not yet been confirmed. Finally, a recent mathematical model of HCV kinetics during IFN-alfa-ribavirin therapy suggests that, during combination therapy, ribavirin acts mainly by making HCV virions less infectious.32 This implies that virus production is inhibited by IFN while de novo hepatocyte infection is diminished by ribavirin. It is important to note, however, that this hypothesis has not been confirmed experimentally. Elsewhere, it was recently shown in vitro and with other viruses that ribavirin, an RNA mutagen, can drive viral quasispecies to “error catastrophe”, i.e., loss of fitness by lethal accumulation of nucleotide mutations during replication.23, 33, 34 Nevertheless, studies of human HCV infection have shown no acceleration of mutagenesis during ribavirin therapy.35, 36 Thus, the mechanism by which ribavirin promotes sustained viral eradication when combined with IFN alfa remains unknown, and the use of ribavirin in hepatitis C is basically empirical.

Easier Treatment Decisions.

The decision to treat chronic hepatitis C was long based on the severity of the accompanying liver disease, as assessed by biopsy. Although biopsy provides a wealth of information, it has a number of adverse effects, some of which can be life-threatening. Thus, liver biopsy has represented a barrier to treatment for a substantial number of patients. Better knowledge of the results of therapy according to the HCV genotype, and the development of alternatives to liver biopsy, have considerably increased access to therapy. Approximately 75% to 90% of patients infected by HCV genotypes 2 and 3 will have a sustained virological response to pegylated IFN alfa and ribavirin,15–17 and treatment will thus be beneficial in the vast majority of these patients, whatever the initial severity of their liver disease. This is why the 2002 NIH Consensus Development Conference on Management of Hepatitis C recommended that, in the absence of contraindications, patients with HCV genotype 2 or 3 infection should be treated, without undergoing liver biopsy.18 In contrast, disease assessment is still recommended for patients infected by HCV genotype 1 or by genotypes 4, 5 or 6.18 The recent development of noninvasive markers of liver fibrosis is currently changing the landscape of HCV therapy. Both serological markers (interpreted using complex algorithms) and ultrasound-based technologies (e.g., transient elastography) are currently being evaluated.37–40 Their use, alone or in combination, has already been shown to avoid liver biopsy in a large proportion of patients.37–40 New algorithms will have to be developed and tested in prospective clinical trials. If their specificity can be sufficiently improved, these techniques might entirely replace liver biopsy in the hepatitis C treatment decision-making process.

Better Use of Virological Tests to Tailor Therapy.

Virological tests have become key tools in the management of HCV infection, not only to diagnose the infection, but especially to guide treatment decisions and to assess the virological response.41 The main determinant of the response to therapy is the HCV genotype,15–17 which must be systematically determined before treatment, as it determines the indication, the duration of treatment, the dose of ribavirin (the approved dose of pegylated IFN is identical for all HCV genotypes) and the virological monitoring procedure.18

Patients infected by HCV genotype 1 (Fig. 2A) have a 40% to 50% likelihood of eradicating the infection during therapy. They should receive a high dose of ribavirin, i.e., 1000 to 1200 mg daily (for patients with a body weight of less or more than 75 kg, respectively), and require 48 weeks of treatment.18 Heavier patients may benefit more from 1400 mg of ribavirin daily.42 HCV RNA load monitoring during therapy is recommended in order to avoid the dangers and cost associated with the full treatment course in patients who have little or no chance of having a sustained virological response.15, 43, 44 For this purpose, HCV RNA is quantified at baseline and after 12 weeks of treatment. Treatment is continued when, at week 12, there is at least a 2-log drop in HCV RNA (i.e., when the baseline HCV RNA level is divided by a factor of at least 100) or when HCV RNA is undetectable (<50 IU/mL). If HCV RNA is undetectable (<50 IU/mL) at week 24 in these patients, there is a high likelihood of achieving a sustained virological response, and treatment is thus continued until week 48.18 It was recently suggested that 24 weeks of therapy might be sufficient for patients with a baseline viral load below 600,000 IU/mL in whom pegylated IFN alfa-2b-based treatment yields a 2-log decline at week 12 and undetectable HCV RNA at week 24.45 In contrast, if HCV RNA is still detectable at week 24 the likelihood of sustained viral eradication is virtually zero and treatment can be stopped (or continued with the sole aim of slowing the progression of liver disease in patients who have a poor prognosis).44 The lack of a virological response at 12 weeks (HCV RNA decline of less than 2 log, or no change) is associated with virtually no chance of achieving a sustained virological response.15, 43, 44 Treatment can thus be stopped at week 12 in these patients or, once again, can be continued with the sole aim of slowing the progression of liver disease.18 New algorithms are currently being derived from clinical trial datasets for early prediction and decision-making at week 4 of therapy.

Figure 2.

Current algorithms for the use of HCV virological tools in the treatment of chronic hepatitis C: HCV genotype 1 (A), HCV genotypes 2 and 3 (B) and HCV genotypes 4, 5 and 6 (C). In figure 2A, HCV RNA (−) means below the lower limit of detection of qualitative polymerase chain reaction assays, i.e., below 50 international units per ml. The dashed arrow relates to a recent recommendation from the European Agency for the Evaluation of Medicinal Products (EMEA) based on a single study.45

Patients infected by HCV genotype 2 or 3 (Fig. 2B) have a 70% to 80% likelihood of viral eradication with a low dose of ribavirin (800 mg/day) and only 24 weeks of treatment.17 Preliminary data suggest that even lower doses of ribavirin and/or shorter treatment could be sufficient to achieve a sustained virological response in certain subgroups of patients with genotype 2 or 3 infection, such as those with a low baseline viral load and no extensive fibrosis or cirrhosis. It should be noted that patients who have several baseline characteristics associated with poor treatment response, such as extensive fibrosis, old age, male gender or comorbidity, may need 48 weeks of therapy to clear the infection. Monitoring of the HCV RNA level during therapy is not recommended for the patients with genotype 2 or 3 infection, as the vast majority of them become HCV RNA-negative early during treatment.18

There are too few clinical trials, with too few patients, to determine the optimal treatment schedule and the likelihood of an SVR in patients infected by HCV genotype 4, 5 or 6 (Fig. 2C). It is thus recommended to use the same treatment as for HCV genotype 1, i.e., pegylated IFN alfa at the usual dose, combined with a high dose of ribavirin (1000-1200 mg per day, according to body weight less or greater than 75 kg).18 In the absence of published data, no stopping rules have been defined and it is recommended to treat these patients for a total of 48 weeks.

Better Management of Adverse Effects.

The adverse effects of pegylated IFN-ribavirin therapy limit patients' ability to remain on treatment, lower their chances of achieving viral eradication, and impair their health-related quality of life (HRQOL). The use of paracetamol and nonsteroidal antiinflammatory drugs after the first IFN injections has been shown to prevent “flu-like” symptoms.18 Growth factors can be used to maintain the dose of IFN alfa or ribavirin if severe cytopenias occur. IFN-related leukopenia and thrombocytopenia, and ribavirin-related hemolytic anemia, are among the principal reasons for dose reduction or treatment discontinuation. Reducing the ribavirin dose, or withdrawing ribavirin altogether, is associated with frequent relapses after an initial response to combination therapy, and can ultimately lead to treatment failure.15, 16 Erythropoietin alfa and its genetically engineered version epoietin alfa have been shown to increase hemoglobin levels during ribavirin administration, to maintain the ribavirin dose, and to improve HRQOL during therapy, principally by reducing fatigue.46–50 Erythropoietin alfa is increasingly used in clinical practice to maintain the ribavirin dose, although its beneficial effect on treatment adherence and final outcome has not been firmly demonstrated so far. One should however be cautious when using erythropoietin alfa because the occurrence of pure red cell aplasia secondary to the development of anti-erythropoietin antibodies has been reported.51, 52 Ongoing studies will establish the optimal timing and duration of erythropoietin administration.

Severe neutropenia can occur during IFN administration. Although the risk of infections in severely neutropenic patients has recently been shown to be low,53, 54 neutropenia is a frequent cause of IFN dosage reduction or withdrawal, and this has been shown to impact on treatment efficacy. Recombinant human granulocyte-colony stimulating factor (G-CSF), a molecule that enhances granulopoiesis, has been shown to increase mean and peak white blood cell counts. A pegylated form of G-CSF is currently under study. The utility of G-CSF to maintain the IFN dosage and to prevent severe complications of neutropenia is uncertain, however. Recombinant human interleukin (IL)-11 is currently being studied for its capacity to increase platelet counts during therapy, although thrombocytopenia rarely necessitates a dose reduction (except in patients with hypersplenism or portal hypertension).

A better understanding of the psychological and psychiatric complications of therapy has also resulted in better treatment adherence and outcome.55, 56 Full information on the disease and its treatment must be given before starting therapy, together with psychological support when necessary during treatment, as this can greatly enhance patient motivation. Early detection of psychiatric disorders and preventive use of antidepressants can also improve adherence to therapy.

Better Understanding of Virological Treatment Failure.

Several factors are known to play a role in treatment outcome, such as the treatment regimen, host factors, disease-related factors and viral factors that account for so-called “HCV resistance”.20, 57 The sustained high concentrations obtained after a single weekly injection of pegylated IFN alfa, together with body weight adjustment of the ribavirin dose and, in some instances, of the pegylated IFN dose, have considerably improved treatment efficacy compared to thrice-weekly standard IFN alfa monotherapy.13–17, 42, 50 Patient characteristics such as older age, male gender, and race (African American) are associated with higher treatment failure rates.13–17, 58 Overweight can also negatively influence the chances of successful IFN alfa-ribavirin treatment, as can active alcohol or intravenous drug use. Adherence to therapy remains a major determinant of outcome. Finally, advanced fibrosis and cirrhosis are associated with lower response rates.13–17

Viral factors also play an important role. HCV genotypes 1 and 4 are intrinsically more resistant to the antiviral action of IFN alfa than are genotypes 2 and 3.59 In addition, within each genotype, different strains may display very different sensitivities to IFN. The molecular mechanisms underlying these differences remain elusive, however.57 In addition, the clearance of infected cells in patients who respond to IFN alfa often occurs later and more slowly in patients infected by HCV genotypes 1 or 4 than in those infected by genotypes 2 or 3.59 Possible explanations are direct interactions with cellular mechanisms and/or genotype-specific immune modulation.

Better Management of “Special” Populations.

The management of HCV infection in several “special” populations has improved significantly over the past few years, through a better knowledge of the disease in these patients, publication of several clinical trials, and the development of new tools to manage therapy and its adverse effects.

Decompensated Cirrhosis and End-Stage Liver Disease.

Experience in the treatment of patients who have decompensated cirrhosis or who are on the waiting list for liver transplantation is relatively limited. Administration of pegylated IFN alfa and ribavirin is recommended whenever possible in these patients, as it leads to sustained viral eradication in 13% to 41% of cases of HCV genotype 1 infection and in 50% to 73% of cases of genotypes 2 and 3 infection.60–64 However, the indications for therapy should be carefully weighed up. Patients with Child-Turcotte-Pugh score higher than 11 or Model for End-Stage Liver Disease (MELD) score >25 should be excluded from treatment.60 In addition, treatment success is frequently impaired by dose reductions and discontinuations due to poor tolerance (these patients frequently have cytopenias prior to treatment initiation). There is no consensus on the optimal treatment regimen for this population.65 A low-accelerating-dose regimen was recently advocated.61 Growth factors could be particularly useful in this population.

Liver Transplant Recipients.

Currently, HCV infection recurs in nearly all patients who are transplanted for end-stage HCV-related liver disease. HCV replication starts to be detected within hours or days after transplantation.66 HCV recurrence is associated with acute hepatitis on the graft, and the infection subsequently becomes chronic in the vast majority of cases. Chronic graft infection is associated with more severe outcome, including more rapid progression of liver fibrosis, than in non transplanted patients.67 Various parameters appear to affect the outcome of chronic hepatitis C in liver transplant recipients, such as viral load before and after transplantation, renal function, the HCV genotype, the year of transplantation, donor and recipient age, and the nature of immunosuppressive therapy. Overall, it appears that severe hepatitis occurs in approximately 50% of transplanted patients, while 15% of them develop cirrhosis and less than 5% require retransplantation.67 The latter number could increase with longer follow-up, and more severe outcomes have been described in recent years, potentially owing to more potent post-transplant immunosuppressive therapies. Severe fibrosing cholestatic hepatitis has been reported in 2% to 8% of cases. The European Liver Transplant Registry (ELTR) and the United Network for Organ Sharing (UNOS) both report poorer survival among HCV-infected liver transplant recipients than among patients transplanted for causes unrelated to HCV infection.67 Together, these findings emphasize the need for effective therapies for HCV recurrence after liver transplantation. Preliminary results indicate that pegylated IFN alfa monotherapy, or a combination of pegylated IFN alfa and ribavirin, may achieve a sustained virological response in a substantial number of patients (25% to 35%).68–72 However, tolerance is poor in these patients and the rate of treatment discontinuation is higher than in non transplanted patients. In addition, IFN alfa could increase the risk of liver graft rejection. The timing of therapy relative to transplantation may also be important, because recent studies suggest better outcome when treatment is started early after transplantation or at the time of acute recurrent hepatitis. New treatments are eagerly awaited, but the best way of preventing HCV infection of the graft is to eradicate the infection before transplantation.

Hemodialysis and Renal Transplantation.

HCV infection is frequent in hemodialysis patients and renal transplant recipients. Whether or not HCV infection is associated with increased morbidity and mortality in these populations is controversial, as available results are probably subject to cohort effects.73 Antiviral therapy is indicated for hemodialysis patients with acute hepatitis, extra-hepatic complications of HCV infection (e.g., cryoglobulinemia-associated disease), histological activity and/or fibrosis on liver biopsy (Metavir score ≥A2F2), and for all candidates for renal transplantation. Renal impairment contraindicates the use of ribavirin when the serum creatinine level is higher than 200 μmol/L, because of the risk of severe anemia. Treatment of such patients is generally based on IFN alfa monotherapy. Standard IFN alfa therapy has given sustained virological responses in a significant number of cases, but the incidence of severe adverse effects is higher than in the non-hemodialysis population.74, 75 Ongoing clinical trials are currently assessing pegylated forms of IFN alfa, the potential use of ribavirin combined with erythropoietin alfa, and the value of ribavirin-like molecules that are less prone to inducing hemolytic anemia. IFN alfa-based therapy is contraindicated in renal transplant recipients because of a high risk of kidney graft rejection.

Human Imunodeficiency Virus-Coinfected Patients.

With the advent of highly active antiretroviral therapy (HAART), liver disease-related morbidity and mortality have increased considerably in patients coinfected by HCV and human immunodeficiency virus (HIV). In these patients, recent large-scale clinical trials have shown: (1) that current treatment with pegylated IFN alfa and ribavirin can achieve sustained virological responses in a substantial number of cases, but (2) that the results of therapy are at least 10% worse than in the HIV-seronegative population.76–78 The reason for this difference is unclear, but treatment of HIV-coinfected patients with pegylated IFN alfa plus ribavirin is nonetheless warranted. The last European Consensus Conference on the Treatment of Chronic Hepatitis B and C in the Coinfected Patient made the following recommendations.79 When chronic hepatitis C is detected before HAART initiation is necessary, it should be treated. In contrast, in a patient with severe immune deficiency (CD4 count >200 cells/mm3), HAART should be initiated and efficient on the CD4 cell count before commencing anti-HCV therapy. The same treatment schedules as in non-HIV-infected patients should be used. Especially, although clinical trials in coinfected patients used a fixed dose of 800 mg ribavirin daily for all genotypes, 1000-1200 mg ribavirin daily should be used for treatment of infections with genotypes 1 and 4, and 800 mg ribavirin daily for genotypes 2 and 3.79 In contrast with monoinfected patients, the recommendation is that duration of treatment should be 48 weeks regardless of the HCV genotype, pending new data eventually showing that shorter duration is sufficient for genotypes 2 and 3. The stopping rules based on a less than 2 log reduction in viral load compared to baseline at week 12 and a positive HCV RNA reduction at week 24 should also be applied in HIV-coinfected patients.78 In the patients who did not respond appropriately based on virological assessment, continuation with pegylated IFN monotherapy can be considered if they have biopsy-proven advanced fibrosis or cirrhosis in order to delay or prevent disease progression. Importantly, didanosine is contraindicated in patients with cirrhosis and should be avoided in patients with less severe liver disease who receive pegylated IFN alfa-ribavirin therapy. Stavudine should be avoided, especially in combination with didanosine, because it is associated with an excess risk of lactic acidosis. The use of zidovudine should also be avoided due to an excess risk of anemia and neutropenia. Finally, nevirapine should not be used or used with caution because of potential hepatic toxicity.79 It is recommended to use paracetamol, possibly combined with nonsteroidal anti-inflammatory drugs, for flu-like syndrome, erythropoietin in case of severe anemia, growth factors to correct severe neutropenia, antidepressants in case of clinically-relevant depression, thyroid hormone substitution in hypothyroidism, and beta-blockers to relieve symptoms of hyperthyroidism.

Intravenous Drug Users and Patients on Opiate Replacement Therapy.

Intravenous drug users (IVDUs) and patients on opiate replacement therapy have long been excluded from clinical trials and everyday therapy. Although initiation of anti-HCV therapy in active drug users must be considered on a case-by-case basis, it is now accepted that patients on opiate replacement therapy should not be refused treatment, provided psychological and social support by a multidisciplinary team is available.

Virological Non Responders to Previous Therapies.

There is no consensus on the treatment and follow-up of patients in whom the pegylated IFN alfa-ribavirin combination fails to eradicate the infection. The French Consensus Conference on Hepatitis C in 2002 recommended that these patients be kept on maintenance therapy with a lower dose of pegylated IFN alfa monotherapy when they have severe or progressive liver disease.80 The results of ongoing clinical trials are needed to confirm the validity of this widely used approach. In most cases, and in the absence of alternatives to the pegylated IFN alfa-ribavirin combination, non responder patients should be included in clinical trials of new IFN-based approaches or novel anti-HCV drugs. They must be followed on a regular basis until new drugs become commercially available.

Technology

The goal of hepatitis C therapy is to disrupt steady-state HCV kinetics (Fig. 1) by significantly reducing virion production and allowing all infected cells to be gradually cleared.20 Steady-state HCV kinetics offers several potential targets for new drugs, including inhibition of virion production by antiviral drugs, inhibition of de novo HCV infection, and accelerated clearance of infected cells. Ideally, these targets should be hit simultaneously in order to achieve viral eradication within a reasonable time frame. Recent technological advances have helped to identify new candidate drug targets.81 Future drugs for HCV infection will belong to four main categories, namely new IFNs, alternatives to ribavirin, specific HCV inhibitors, and immune modulators.

New IFN Molecules.

New forms of IFN alfa are currently being developed. They are expected to have more potent antiviral effects, and possibly more potent immunomodulatory effects, together with improved pharmacokinetic and pharmacodynamic properties and, hopefully, fewer adverse effects. Albumin-linked IFN alfa (Albuferon, Human Genome Sciences) has been shown to induce a significant biphasic decline in HCV replication after monthly or bi-monthly injections, and is currently in phase III trials.82, 83 Other IFNs could have better intrinsic antiviral efficacy than IFN alfa-2a and -2b. These include consensus IFN (IFN alfacon-1, Infergen, Amgen and InterMune/Yamanouchi), which does not appear to be much superior to other IFNs at equivalent doses, and gene-shuffled IFN, whose specific activity in vitro is 135,000 to 270,000 times higher than that of human IFN alfa-2a.81 The future place of these molecules, and IFN beta, in the treatment of HCV infection will depend on the development of pegylated or other pharmacologically modified forms that can compete with current pegylated IFNs.

It is unclear whether or not IFN gamma or omega might be beneficial in chronic hepatitis C, either alone or in combination with IFN alfa. The nucleoside analogue isatoribine (Anadys Pharmaceuticals) is a Toll-like receptor 7 agonist that is believed to induce the IFN system. It was recently shown to induce a modest reduction in viral replication in some patients. An oral prodrug is currently being investigated.84 CPG 10101 is a Toll-like receptor 9 agonist that has been shown to activate and mature several immune cell types and to induce moderate HCV RNA load reductions in infected patients.85 The future of such so-called “oral IFN inducers” in the treatment of HCV infection is uncertain, however.

Alternatives to Ribavirin.

Alternatives to ribavirin should have the same beneficial effects as ribavirin but not its hemolytic properties. IMPDH inhibitors have proven disappointing, both alone and in combination with IFN alfa. The prodrug viramidine (Valeant Pharmaceuticals) is converted into ribavirin by adenosine deaminase, an enzyme primarily present in hepatocytes, leading to preferential accumulation of ribavirin in the liver rather than in erythrocytes. In a phase II trial, in combination with pegylated IFN alfa, viramidine gave a slightly (but not significantly) lower rate of sustained virological responses.86 The position of viramidine in hepatitis C treatment will depend on the results of ongoing phase III trials. Higher doses than those currently tested could eventually yield improved efficacy with an acceptable tolerance.

Specific HCV Inhibitors.

The recent resolution of the 3D structure of various functional viral elements (including HCV proteins and genome structures) and the development of in vitro assays to assess the antiviral potency of molecules targeting these elements has made it possible to screen and develop specific small inhibitory molecules.81 Virtually, all steps of the HCV life cycle can be targeted by specific inhibitors, as shown in Fig. 3 (adapted from87). Currently, the principal targets of new antivirals are the HCV internal ribosome entry site (IRES, the RNA structure that drives HCV polyprotein translation), HCV NS3 serine proteinase (the enzyme that ensures polyprotein processing downstream of the NS3-NS4A junction), and HCV RNA–dependent RNA polymerase (the enzyme that catalyzes HCV replication).

Figure 3.

The HCV lifecycle. Virtually all steps of the HCV lifecycle can be targeted by specific inhibitors (indicated by parenthesis when compounds already exist at the preclinical or clinical developmental stage): (1) virus binding to cellular receptor(s) (small molecule inhibitors of cell attachment, monoclonal antibodies, hyperimmune anti-HCV immunoglobulins); (2) receptor-mediated endocytosis; (3) membrane fusion and nucleocapsid release; (4) nucleocapsid uncoating; (5) translation and polyprotein processing (IRES inhibitors, NS3 serine protease inhibitors, NS2 zinc-dependent auto-protease inhibitors); (6) HCV RNA replication (NS5B RNA-dependent RNA polymerase inhibitors, NS5A inhibitors, inhibitors of replication complex formation); (7) virion formation and budding in intracellular vesicles; (8) virion transport and maturation; (9) virion release. ER, endoplasmic reticulum; (+) positive-strand RNA; (−) negative-strand RNA. Adapted from F. Penin.87

IRES Inhibitors.

Nucleic-acid-based strategies have been developed to inhibit IRES function. IRES inhibitors include antisense oligodeoxynucleotides, ribozymes, and small interfering RNAs (siRNAs). All of these molecules potently inhibit the function of IRES models in vitro. However, clinical use of ribozymes and antisense oligonucleotides has been disappointing,88, 89 probably because these molecules are designed to target the two-dimensional IRES structure, whereas the IRES is present in infected cells as a complex functional three-dimensional structure involving various canonical and noncanonical factors and ribosome subunits. Novel approaches targeting the functional three-dimensional structure of the IRES are currently at the preclinical developmental stage.

NS3 Serine Protease Inhibitors.

The three-dimensional structures of NS3 combined with its cofactor NS4A have been resolved. Various classes of NS3 protease inhibitors have been developed that potently inhibit the protease function in cell-free systems, and also HCV replication in various models. Three drugs belonging to different classes have already been administered to HCV genotype 1-infected patients in clinical trials. All three induced major reductions in HCV RNA (2 to 4 log). BILN 2061 (Boehringer-Ingelheim)90 will not be developed for clinical use, because of its myocardial toxicity in animals. VX-950 (Vertex Pharmaceuticals) and SCH 503034 (Schering-Plough Pharmaceuticals) appear to be safe and are currently undergoing clinical evaluation.91, 92 Partial relapse occurred in a substantial number of patients after only one week of VX-950 administration,91 and this was shown to be due to selection of resistant variants,93 raising serious doubts as to the efficacy of single-agent therapy with such drugs. No resistance data are yet available for SCH 503034. Several other companies have developed NS3 proteinase inhibitors targeting various sites of the enzyme, and some are ready to enter clinical development.

NS5B RNA-Dependent RNA Polymerase Inhibitors.

The HCV RNA–dependent RNA polymerase harbors several potential target sites for nucleoside/nucleotide and non nucleoside inhibitors. Valopicitabine (NM283, Idenix Pharmaceuticals) is a nucleoside analogue that targets the HCV polymerase catalytic site. Valopicitabine administration has been reported to induce a moderate (up to about one log) and sustained dose-related viral load reduction. This antiviral effect appears to be additive with that of pegylated IFN, including in previous non sustained virological responders to IFN-based therapy.94 A large number of other companies are developing similar drugs, some of which could soon enter clinical evaluation.

Other HCV Inhibitors.

Virtually all steps of the HCV lifecycle can be targeted by specific inhibitors. Inhibitors of p7, a protein creating a putative ion channel in infected cells, and inhibitors of the NS3 HCV helicase, an enzyme that unwinds the HCV RNA molecule and is necessary for viral replication, have already entered preclinical evaluation. The early steps of the HCV lifecycle, such as viral attachment to host cells, internalization of the virus-receptor complex, and the fusion that releases the nucleocapsid into the cytoplasm, are attractive targets for novel therapies, but development has been slowed by the lack of appropriate models of the early steps of viral entry. The recent development of a productive cell culture system for HCV, although only based so far on a genotype 2a clone, should improve our understanding of these crucial steps of the HCV lifecyle and help to develop potent specific inhibitors.24, 95, 96

Resistance.

Experience in the treatment of human immunodeficiency virus and hepatitis B virus infection with specific inhibitors of viral enzymes and cell entry shows that, apart from non adherence to therapy, the principal cause of treatment failure is viral resistance. Viral resistance is characterized by the selection of minor viral populations bearing mutations that confer resistance to a specific drug (Fig. 4).97 Treatment withdrawal is usually followed by recovery of the “sensitive” wild-type genotype and phenotype. Viral resistance is a major clinical issue. In the case of HCV, there are already strong arguments that specific HCV inhibitors will select resistant variants. Indeed, HCV is a highly variable virus with rapid viral kinetics, large viral population sizes and a quasispecies distribution.98 Some minor variants have been already shown to bear mutations within or close to functionally important drug-binding sites. Several drugs currently in preclinical development were recently reported to select variants bearing mutations that confer resistance in the Huh7 cell replicon system.99–101 Finally, rapid selection of resistance has been described with the NS3 protease inhibitor VX-950.93 Rapid onset of viral resistance to specific HCV inhibitors is therefore foreseeable if these drugs are used alone rather than in combination. However, resistance can be prevented by the following measures: (1) maximally reducing virus production by using highly potent antiviral molecules; (2) raising the “pharmacologic barrier” to viral escape by reaching high trough levels, having a tissue distribution of the drug that permits no sanctuaries, and optimizing patient adherence to therapy; (3) raising the “genetic barrier” to viral escape by using combinations of drugs with different targets and no cross resistance.102

Figure 4.

Simplified model of HCV resistance to specific inhibitors. The baseline quasispecies harbors a major population, sensitive to the drug and highly fit in the absence of drug administration, and a minor drug-resistant population bearing genome mutations conferring resistance, poorly fit in the absence of drug administration. In the presence of the drug, the sensitive viral population becomes unfit but does not disappear, whereas the resistant population has been selected because of improved fitness and fills in the replication space. Acquisition of additional “secondary” mutations may further improve its fitness and replication capacities during therapy. Drug withdrawal restores pretreatment fitness and the baseline viral population distribution.

Immune Therapies.

Various non specific immunomodulatory agents such as thymosin alfa-1 (Thymalfasin, SciClone), IL-10 and histamine have been administered to patients with chronic hepatitis C, but the results have been disappointing. Pilot studies have evaluated passive immunization with hyperimmune hepatitis C immunoglobulin (HCIg, Civicir, Nabi Pharmaceuticals) in order to prevent HCV recurrence in patients undergoing liver transplantation, but so far with no success. Monoclonal antibodies to HCV (HepeX-C, XTL Biopharmaceuticals) are in early-phase clinical trials.

Various therapeutic vaccine strategies are currently under study. Preclinical and early human studies indicate that therapeutic vaccines based on different forms of recombinant HCV proteins and containing various adjuvants could upregulate both cellular and humoral immune responses in patients with chronic hepatitis C.103 However, there is currently no evidence that therapeutic vaccines alone can affect HCV RNA levels. It remains to be determined whether therapeutic vaccines might be useful if combined with potent antiviral molecules.

Cure, Finally?

The treatment goal in HCV infection is permanent viral eradication, i.e., cure. This is currently achieved in a substantial proportion of patients by standard therapy with pegylated IFN alfa and ribavirin. The recent report that HCV RNA sequences persist in peripheral blood mononuclear cells of a small number of patients who have had a sustained virological response to IFN alfa-based therapy104, 105 should be seen in the light of clinical evidence that late recurrence is extremely rare, even in patients receiving immunosuppressive therapy. Some such “recurrences” may in fact correspond to re-infection.

HCV infection is currently the only curable chronic human viral infection and the goal is now to increase the cure rate. New treatments and vaccines might even make it possible to eradicate HCV, at least in rich parts of the world. The confrontation between human ingenuity and hepatitis C virus is far from over, but there are promising signs that we may one day (soon?) prevail.

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