Over the past three decades Hepatitis C has become one the most prevalent causes of liver disease in the world. It now affects over 180 million people worldwide (3% of the population), with over 30,000 new cases being diagnosed in the US alone every year. Though treatments for hepatitis C exist, only 50% of those treated show any response resulting in an urgent need for new therapies. This review will aim to cover the current treatment options available and the most relevant clinical and pre-clinical trials for the treatment of hepatitis C.
More than 180 million individuals across the globe are infected with the hepatitis C virus (HCV) (Waters et al., 2006). Infection can lead to fatal liver diseases; cirrhosis and primary liver cancers known as hepatocarcinomas (Waters et al., 2006). Indeed, HCV is considered by the World Health Organisation to be a silent but nevertheless deadly epidemic. The Centre for Disease Control (CDC) has declared that 20-30% of individuals with chronic hepatitis C will eventually develop potentially fatal symptoms (Webster et al., 2009). When compared to Human Immunodeficiency Virus (HIV), the most well known and well publicised viral infection, HCV is found to be more prevalent. For each individual infected with HIV, there are more than four infected with HCV (Webster et al., 2009). In addition, the number of HCV cases in the developing world is rapidly increasing with 6% of the population of central Africa infected (Madhava et al., 2002). The scale of the level of hepatitis infection worldwide has lead to many avenues of research being pursued. However, though therapies currently exist for the treatment of HCV, their efficacy is low; effective in only 50% of subjects (Waters et al., 2006). This has driven the development of more efficacious and less toxic treatments.
This review will aim to cover the cause, symptoms, current and potential future therapies for HCV. The review will be divided into several sections including;
- The Hepatitis C Virus
- Infection with Hepatitis C
- Causes and Symptoms of Hepatitis C
- Mechanisms of Current Theraphy
- Interferon Treatment
- Interferon and Ribavirin Treatment
- New Theraphy Strategies
- Inhibitors of Viral Replication
- Prevention of Viral Binding
- Inhibitors of Viral Release
- New Interferons
- Hepatitis C Vaccines
2. The Hepatitis C Virus:
The term “hepatitis” is of Latin origin and describes inflammation of the liver. The HCV is an ribonucleic acid (RNA) virus of the Flaviviridae family (Tan et al., 2004). The HCV was first identified in 1989 and specifically targets the liver causing inflammation and preventing it from effectively removing toxins from the blood (Stremmel et al., 1989). During chronic HCV infection the liver initially becomes fibrotic due to overproduction of extracellular matrix (ECM) components such as collagen which are deposited between liver cells. If inflammation continues, the liver becomes progressively hardened, resulting in cirrhosis.
Though there are vaccines available to counter some viruses of the hepatitis family including hepatitis A and B, there is currently no vaccine available for HCV (2008). Based on sequence homology there are six HCV genotypes currently described (Qureshi, 2007). These can vary up to 40% at the nucleotide level and have differing geographical distributions; genotypes 1-3 are found worldwide, whilst genotypes 4-6 are found primarily in Egypt, Africa and South East Asia (Qureshi, 2007). The genotypes also vary in disease progression and responsiveness to treatment.
3. Infection with Hepatitis C:
The HCV is a blood borne virus and primarily transmitted by direct blood-blood contact such as the sharing of contaminated needles by drug users, and recipients of blood transfusions (Madhava et al., 2002). Transmission can also occur via sexual intercourse with a partner who has hepatitis C, or to children borne to mothers with hepatitis C (though these are relatively inefficient modes of transmission). Unsterile medical or dental procedures, body piercing, acupuncture and tattooing are also potential infection routes (Madhava et al., 2002).
4. Causes and Symptoms of Hepatitis C:
Many individuals are infected with HCV for several decades before symptoms begin to appear making the infection difficult to diagnose. One common indication of HCV infection is an abnormal level of the enzyme Alanine aminotransferase (ALT) in a patient’s serum. This enzyme interconverts L-alanine and D-alanine and is normally contained within liver cells. However when the liver becomes inflamed ALT is released into the bloodstream, indicating hepatocellular damage. A diagnosis of HCV infection is obtained initially by determining the antibody titre to see if the individual has been infected with HCV in the past. If antibodies to HCV are present, a polymerase chain reaction is carried out to determine the current level of HCV RNA, indicative of the level of infection (Di Bisceglie et al., 2002; Madhava et al., 2002; Tan et al., 2004; Waters et al., 2006; Webster et al., 2009).
Approximately 25% of those infected with HCV clear the virus at the acute stage. Of the 75% of individuals who do not clear the HCV infection some will never develop symptoms, some will develop moderate liver damage, while 5-20% will develop cirrhosis of the liver over a 20 year period. Of those with liver cirrhosis, some will then develop hepatocarcinomas and liver failure which generally prove fatal (Madhava et al., 2002). Indeed, in the western world HCV infection accounts for 40% of all end-stage liver cirrhosis, 60% of all primary hepatocarcinomas and 30-40% of all liver transplantation (Tan et al., 2004).
5. Mechanisms of Current Theraphy
All treatments for HCV aim to reduce inflammation of the liver by eliminating the virus. The effectiveness of therapy is assessed by monitoring the level HCV RNA in a patient’s serum. If HCV RNA remains undetectable six months after completing the course of therapy, this is designated as a sustained virological response (SVR) and associated with a favorable prognosis (Tan et al., 2004).
5.1 Interferon treatment:
All current treatment protocols for HCV are based on the use of interferon alpha (IFN-α), first developed in 1986 and administered either by intramuscular or subcutaneous injection (Feld et al., 2005; Stremmel et al., 1989). IFN-α is an endogenous glycoprotein normally secreted in response to viral infections. When administered therapeutically IFN-α is thought to function by binding to IFN cell-surface-receptors resulting in their dimerisation and activation of Janus-activated kinase 1 (Jak1) and tyrosine kinase 2 (Tyk2) (Gale, 2003). Jak1 and Tyk2 then phosphorylate and activate signal transducer and activator of transcription proteins 1 and 2 (STAT1 and STAT2). The STAT1/2 complex then translocates to the nucleus to combine with IFN-regulatory factor 9 (IRF-9) and initiate transcription of genes with antiviral activity, as well as those involved in apoptosis, protein degradation and lipid metabolism. These include the gene for protein kinase R which may suppress viral protein synthesis by inhibiting eukaryotic initiation factor 2 (Feld et al., 2005). This results in increased target cell killing by lymphocytes and inhibition of virus replication in infected cells, reducing HCV RNA levels in serum and liver to resolve the infection (Feld et al., 2005).
Several interferons are currently approved for the treatment of chronic HCV infections including; IFN-α 2a (produced by Hoffmann-La Roche), IFN α -2b (produced by Schering-Plough) and interferon alfacon-1 (produced by Intermune). These are usually administered for between six months to two years. After its initial trials, IFN-α monotherapy was adopted as the standard treatment recommendation for HCV, but is relatively ineffective, indeed treatment with IFN-α for 12months only resulted in a successful response in 16-20% of cases (Feld et al., 2005).
5.2 Interferon and Ribavirin treatment:
The efficacy of IFN-α treatment was increased by the coadministration of the broad-spectrum antiviral synthetic nucleoside ribavirin (1-b-D-ribofuranosyl-1,2,4-triazole-3-carboxyamide) (McHutchison et al., 1998). Ribavirin has been shown to upregulate IFN stimulated genes and increase the interaction of STAT1 with DNA, possibly potentiating the actions of IFN (Zhang et al., 2003). This combination treatment increased the sustained response rate to 35-40% and was approved for use in December 1998 (Feld et al., 2005). Results from three double-blind, placebo-controlled trials supporting this were published in 1998 (McHutchison et al., 1998; Poynard et al., 1998; Reichard et al., 1998). However, 10-25% of patients relapse even when treated with this combination, and though retreatment in these cases can result in a SVR, this usually requires a longer course of drugs and a high drug dose. One third of patients treated with IFN-α and ribavirin show no response to treatment and never become HCV RNA negative (Feld et al., 2005).
HCV treatment was further improved by the use of pegylated IFN-α, where a molecule of poly(ethylene glycol), known as PEG is attached to IFN-α. This increases the active half-life of the IFN-α, increasing its virological activity so that more constant blood levels are achieved with less frequent administration (Feld et al., 2005; Lindsay et al., 2001; Manns et al., 2001). There are two preparations of peginterferon alpha now available: peginterferon alpha-2b (known as Peg-Intron; produced by Schering-Plough) and peginterferon alpha-2a (known as Pegasys; produced by Hoffmann-La Roche). Based on these studies the currently recommended therapy for chronic hepatitis C is a 24-48 week course of pegylated interferon and ribavirin which achieves a SVR in 50% of patients. Figure 1 illustrates the increasing sustained virological response that has been achieved by developing HCV treatments, first by the combination of ribavirin with IFN-α (Study A, black bar), the addition of a PEG moiety onto IFN-α (studies B and C, black bars) and finally the current most potent combinations of pegylated IFN-α with ribavirin (studies D and E, black bars):
Study A (McHutchison et al., 1998): Comparison of interferon α -2b with ribavirin (black) versus interferon α -2b alone (grey), both for 48 weeks .Study B (Lindsay et al., 2001)): pegylated interferon α -2b (black) versus interferon α -2b (grey). Study C (Zeuzem et al., 2000) : pegylated interferon α -2a (black) versus interferon α -2a (grey). Study D (Manns et al., 2001): pegylated interferon α -2b with ribavirin (black) versus interferon α -2b with ribavirin (grey). Study E (Fried et al., 2002): pegylated interferon α -2a with ribavirin (black) versus interferon α -2b with ribavirin (grey).
Adapted from Di Bisceglie et al 2002
Responsiveness to treatment is dependent upon the age, weight and gender of the patient as well as the genotype of the virus (Kemmer et al., 2007; Montalbano et al., 2008). Evidence suggests that a sustained response is only achieved in approximately 45% of genotype 1 cases, which represent 70% of all cases in the US, whilst 76-80% of genotype 2 and 3 cases show a sustained response to combination therapy (2002b; Di Bisceglie et al., 2002). In addition, IFN-α can produce side effects such as fatigue, flu-like symptoms, nausea and depression (Laguno et al., 2004), while ribavirin can affect fetal development, and exacerbate anaemia, heart and kidney disease (Ferm et al., 1978; Kilham et al., 1977). Furthermore, those who do not respond to the current treatments can further damage their liver as the current regimes available for HCV eradication can accelerate hepatic dysfunction (Di Bisceglie et al., 2002).
6. New Theraphy Strategies:
New treatment strategies for HCV generally focus on targeting specific stages of the viral life cycle; inhibiting key enzymes, targeting viral RNA and attempting to prevent viral replication.
6.1 Inhibitors of viral replication
In an attempt to increase the synergistic effect of pegylated interferon plus ribavirin, researchers have recently been testing the nucleoside polymerase inhibitor R1626, which has previously been shown to inhibit replication of the HCV in vitro. In two recent phase two short trials up to 74% of individuals additionally taking R1626 had undetectable levels of HCV RNA after 2-4 weeks, compared to 5% of individuals taking the standard IFN-ribavirin combination therapy (Pockros et al., 2008; Roberts et al., 2008). However both these trials used low numbers of HCV infected individuals, and follow up studies determining whether the levels of HCV RNA remain undetectable in the blood are ongoing. Therefore the long-term efficacy of these recent trials remains to be determined.
A viral protein called p7 is another potential novel therapeutic target (Griffin et al., 2008). This protein is a virus-coded ion channel and has been shown to function in HCV replication in chimpanzees. p7 had been thought to be a potential therapeutic target, however there was significant variability between p7 inhibition and efficacy against infection. Research revealed that different HCV genotypes show genetic variability in p7 and that this correlates with the sensitivity of HCV to p7 inhibitors (Griffin et al., 2008).
Therefore genotype specific p7 inhibitors may be suitable therapies for chronic HCV infection (Griffin et al., 2008). A further potential therapeutic target in HCV is a cellular peptidyl-prolyl cis-trans isomerase (PPIase), known as cyclophilin B (CyPB) (Rice et al., 2005). This protein is critical for the efficient replication of the hepatitis C virus genome as evidenced by the reduction in HCV replication mediated by RNA mediated interference against it (Rice et al., 2005).
6.2 Prevention of viral binding
Another method of eliminating HCV infection is by preventing or reducing the ability of the virus to bind to and thereby infect host cells. This can be achieved by antibodies to prevent binding or inhibition of the host receptor which mediated the interaction with HCV. On human cells the CD81 surface protein has been identified as a potential receptor for HCV, acting by binding to a glycoprotein known as E2 on the exterior of HCV (Cao et al., 2007). Antibodies have been developed via phage display which bind to CD81, thereby competing with the viral E8 glycoprotein and may act as an antiviral by preventing HCV cell attachment (Cao et al., 2007).
6.3 Inhibitors of viral release
In order to reduce HCV infectivity, the endoplasmic reticulum glucosidase I inhibitor Celgosivir is currently being characterised. This is a potent inhibitor of a host enzyme that is hijacked by HCV to aid viral assembly and release. Initial results from a recent clinical trial suggest that Celgosivir can reduce HCV RNA, suggesting with further characterisation it may prove a novel therapy (Durantel et al., 2007).
6.4 New interferons:
Several research groups are attempting to increase the efficacy of the current treatment by the use of novel interferons. One such interferon is consensus interferon (cIFN), which contains the most common amino acid sequences from a variety of natural interferons. Initial trials suggest that cIFN in combination with ribavirin can treat patients who showed no response to the standard pegylated IFN and ribavirin treatment (Leevy, 2008). Indeed, in a recent study in 2008, of 137 patients who had not responded to standard treatments, 37% showed undetectable HCV RNA when treated with cIFN and ribavirin (Leevy, 2008). A second novel interferon under investigation is Albuferon, a protein fusion of IFNα and human serum albumin, which has a longer active half life (Chemmanur- et al., 2006). A comparison of albuferon and ribavirin with pegIFN and ribavirin found similar responses between the two treatments, but those using the albuferon treatment were scored with a higher quality of life (2003; Masci et al., 2003; Sung et al., 2003).
6.5 Hepatitis C virus vaccines
Though vaccines exist for hepatitis A and B, there are as yet no prophylactic vaccines available for HCV. A number of potential vaccines are under development including a synthetic E1 vaccine, which is directed against an HCV surface glycoprotein (Lorent et al., 2008). Initial clinical trials have suggested that this vaccine did not reduce the level of HCV RNA in individuals but did slow the liver damage observed in non-responders to standard therapy, suggesting that this warrants further characterisation (Leroux-Roels et al., 2005; Nevens et al., 2003). Another vaccine currently under investigation is the synthetic peptide vaccine IC41 (Firbas et al., 2006). In a clinical trial IC41 could increase IFN producing T-cells, particularly in non-responders to standard therapy, but could not significantly reduce the level of HCV RNA (Klade et al., 2008). Current research in this area aims to optimize the dose of these vaccines and assess whether they can potentiate the benefits observed with combination therapies (Klade et al., 2008).
Though new strategies for the treatment of HCV show significant promise for the future, it should also be noted that in those responsive to combination therapies, 99% of patients had no detectable virus up to a decade later (Adamek et al., 2007; Lau et al., 1998). Thus the current combination therapy represents a cure for the 50% of HCV infected individuals who respond.
The wealth of research continuing into the treatment of HCV brings significant hope that an effective treatment for this virulent and widespread virus may be found in the near future. Inhibitors of viral replication, virus attachment and virus release are all currently undergoing clinical trials and are likely to provide the next round of HCV therapeutics. The geographical diversity of the different genotypes and the significant influence of genotype on therapy responsiveness mean that trials conducted on one continent or even in different countries of regions within a continent are not comparable. Therefore it is likely that a worldwide single dosing strategy will not be appropriate for all patients. Differences in genotype and individual responsiveness may warrant local modifications in interferon dosing. Improvements in the dosing, dose regime and support infrastructure of patients receiving pegylated interferon and ribavirin could also significantly improve response rates. However, further studies are required to gain a better understanding of the mechanisms of IFN and ribavirin activity in order to substantially improve their actions. Recent innovation in the development of a cell-culture system that allows complete HCV replication may provide further information on the antiviral mechanisms of IFN activity and increase the ease with which new antiviral agents can be assessed. Crucially, novel antivirals may act synergistically with current therapies by reducing the ability of HCV to replicate and by increasing its susceptibility to IFN, reducing the level of non-responders. In summation, these rapidly increasing combinations of therapeutics hold the promise of greatly improved rates of response in the treatment of hepatitis C.