Market Insight - Hepatitis C: in search of a breakthrough

Infection with the hepatitis C virus (HCV) represents an important healthcare problem worldwide. Current estimates suggest that globally about 170 million people, representing approximately 3% of the world's population, are chronically infected, and are at risk of developing liver cirrhosis and/or hepatocellular carcinoma. Estimates indicate that in the US, three to four million people are infected annually. The prevalence of HCV-related disease is increasing, but no vaccine is yet available.

Since HCV was identified as being the causative agent of Hepatitis C (non-A, non-B hepatitis), treatment has progressed rapidly, but morbidity and mortality rates are still predicted to rise. Novel, more efficacious and tolerable therapies are urgently needed, and a greater understanding of the viral life cycle has led to an increase in the number of possible targets for antiviral intervention, including polymerase and protease inhibitors, immune modulators, and biologics.

HCV (originally referred to as "non-A non-B hepatitis") is caused by a virus with an RNA genome that is a member of the Flaviviridae family. HCV may lead to a chronic form of hepatitis, culminating in cirrhosis. It can remain asymptomatic for 10–20 years. Patients with HCV are susceptible to severe hepatitis if they contract either hepatitis A or B. Projections in the US suggest that if half of all patients infected with HCV are identified, even with the most aggressive treatment at optimal doses and durations, the best possible outcome is a 24% reduction in the incidence of decompensated cirrhosis after 20 years.¹

The antiviral sector continues to be a major focus for biotech companies – in terms of the number of biopharmaceuticals in development, it is second only to cancer. The number of approved antiviral drugs has increased substantially during the past decade. Accelerated by the constant threat of drug resistance, lower-than-desired cure rates and the need for drugs that offer improved quality of life during treatment, the worldwide antiviral market is expected to grow from $18 billion to as much as $25 billion by 2011.²

HCV remains one of the antiviral market's largest unmet medical needs. Since the discovery of the virus in 1989 by Chiron researchers,³ the development of HCV therapy has progressed significantly with the introduction of IFN monotherapy, and the current recommendation of peg IFN-alpha (Schering–Plough) and ribavirin (Sandoz,Teva), the proportion of patients achieving sustained antiviral response (SVR) has increased significantly.⁴

Importantly, ribavirin prevents relapse after the end of antiviral treatment. Despite this, the morbidity and mortality rates associated with HCV are predicted to rise, and more efficacious and tolerable therapies are urgently required, particularly for the increasing proportion of patients who are refractory to treatment with IFN-alpha and ribavirin.

Recent developments in the design of vaccines and novel antiviral agents targeting new mechanisms, second-generation molecules, combination treatments and novel administration routes, along with some of the newer drug candidates are attracting investor attention. Furthermore, it is hoped that a combination of complementary approaches and individualisation based on genotype, viral load and early virological response will improve outcomes. Driven by the potential for substantial profits to be gained from successful antiviral drugs, investors will doubtless continue to view drugs combating HCV as offering significant lucrative potential over the coming years.

emerging drug targets and therapies

Efforts in the search for new treatments for HCV-infection are either focused on direct antiviral drugs, targeting the structural components of enzymes encoded by the virus, or indirect antiviral drugs, targeting host-cell components (immunomodulators, etc). Among the direct acting antiviral agents, inhibitors of the NS3 protease, the NS5B polymerase and the viral RNA are the most intensively explored. However, there is also ongoing and promising preclinical research on such other potential targets as the structural protein E2 (for cell-entry inhibitors), the NS3 helicase, the p7 ion-channel, and the multifunctional NS5A protein.

new drugs in development

For the development of new, specific anti-HCV drugs, an understanding of the HCV life cycle (in particular the genomic organisation and polyprotein processing) is essential. This has resulted in the development of several agents that target specific stages of the life cycle; the so-called specifically targeted antiviral therapy for HCV (STAT-C) drugs.

Potential processes for viral inhibition include virus entry into the host-cell, proteolytic processing, RNA replication, and the assembly and release of the new virions. Among the most promising new agents in development are the protease and polymerase inhibitors.

RNA-targeted therapies, such as antisense oligonucleotides, ribozymes and small interfering RNA (siRNA)-targeting structures, have shown substantial success at inhibiting the HCV life cycle in vitro, but not in vivo. As yet, no prophylactic vaccine is available for HCV, but extensive studies of a recombinant vaccine in chimpanzees showed encouraging results.

Among the specific antivirals, inhibitors of the nonstructural protein 3 (NS3) protease are the furthest along in development. HCV is a single-strand, positive sense RNA virus whose tiny genome encodes a single polyprotein. This is cleaved by proteases, including NS3, to produce 10 viral proteins. NS3 activity is absolutely necessary for viral replication. It is almost certain that a protease inhibitor will be the first targeted antiviral on the market.

Vertex's NS3 inhibitor, telaprevir, is generating significant interest, not just because of its excellent potency (a median 3.4 to 4.8 log drop in viral RNA after a few days of treatment in Phase I trials), but because the company is using the drug to cut current treatment times in half. The drug, like all targeted therapies to date, is under development as combination therapy with interferon and ribavirin. Radically shortening therapy times could be the key to eventual approval by the US FDA.

protease inhibitors

Proof-of principle for the protease inhibitor class of compounds was provided by BILN 2061 (Boehringer Ingelheim), an NS3 protease inhibitor that provides at least a 2−3 log10 decrease in HCV load within 48 hours. However, the clinical development of BILN 2061 was stopped owing to significant side-effects. Protease inhibitors have been associated with substantial reductions in serum HCV RNA in clinical studies when given alone or in combination with peg IFN-alpha and are in preclinical and clinical development as shown in Table 1.

Table 1: Protease inhibitors in clinical trials

Compound	Company	Clinical stage
ITMN-191	InterMune & Roche	Phase I
MK-7009	Merck & Co	Phase I
B112202	Boehringer Ingelheim	Phase I
telaprevir	Vertex Pharmaceuticals	Phase II
boceprevir	Schering-Plough	Phase II
TMC 435350	Tibotec & Medivir	Phase II

polymerase inhibitors

Several inhibitors of the NS5B polymerase are, or have been, studied in the clinic (see Table 2).

Table 2: Nucleoside polymerase inhibitors in early-stage clinical trials

Compound	Company	Clinical stage
MK-0608	Merck & Co	Preclinical
R7128	Pharmasset & Roche	Phase I
PSI-6130	Pharmasset	Phase I
R1626	Roche	Phase I

Two separate classes of compounds have shown inhibitory effects on the NS5B polymerase through two distinct mechanisms: first, nucleoside polymerase inhibitors, which directly inhibit the active site causing chain termination for example, MK-0608 (Merck & Co), R1626 (Roche),5 PSI-6130 (Pharmasset) and its prodrug R7128.

The two agents furthest in clinical development (late Phase II) are the protease inhibitors telaprevir (Vertex Pharmaceuticals) and boceprevir (Schering-Plough). Valopicitabine (Idenix) was the first polymerase inhibitor to reach Phase IIb clinical testing,⁶ but was recently placed on clinical hold in the US following a review by the FDA due to safety concerns. These agents have been shown to have significant antiviral activity in patients with HCV genotype 1, including treatment-naive patients and those not responding to other therapies.

Preclinical studies have also shown that agents targeting the HCV RNA polymerase are associated with significant reductions in serum HCV RNA51 and clinical studies have demonstrated the promising antiviral effects of NS5B inhibitors when used either as monotherapy or in combination with peg IFN-alpha. However, due to safety concerns and unfavourable risk-benefit profiles, the development of several polymerase inhibitors, including HCV-796 (ViroPharma & Wyeth), BILB 1941 (Boehringer Ingelheim) and valopicitabine (Idenix),⁷ are on hold.

immune modulators

Immune modulators targeting the cellular immune response, which plays a major role in HCV infection are also being investigated. Examples include agents that generate and/or promote an effective immune response by inducing or modulating cytokine responses, such as the toll-like receptor (TLR) agonists, which have shown antiviral efficacy in initial clinical studies.

In a Phase 1b clinical trial of Coley Pharmaceutical's CPG 10101, patients with HCV genotype 1 who received at least 1mg CPG 10101 twice a week for four weeks experienced increases in IFN-alpha and other markers of immune response along with a mean 1 log10 decline in HCV RNA levels. However, improved SVR results have not been reported so far. The clinical development of the TLR7 and TLR9 agonists is currently on hold – Coley Pharmaceuticals has stopped further development of CPG 10101 for viral hepatitis and is concentrating its efforts on the more promising use of the compound as an anticancer drug. The development of Anadys Pharmaceuticals' TLR7 agonist was stopped owing to preclinical safety issues, as it was found to induce a general inflammatory response in animals.

novel investigational agents

The effectiveness of inhibitors of cyclophilin B, a host factor involved in viral replication, is being evaluated in patients with HCV (see Table 3). NIm-811 (Novartis),8 a cyclosporin A analogue, suppresses HCV genome replication in a cell culture system and may provide a novel strategy for anti-HCV treatment. DeBIo-025 Debiopharma) has demonstrated strong antiviral activity in vitro against HCV genotype 1 and HIV-1.9.

Table 3: Cyclophilin inhibitors and glucosidase inhibitors

Compound	Company	Clinical stage
NIM-811	Novartis	Phase I
DEBIO-025	Debiopharm	Phase II
Celgosivir (Glucosidase inhibitor)	Boehringer Ingelheim	Phase I

Furthermore, glucosidase inhibitors such as Celgosivir (Boehringer Ingelheim) have been in development for many years, albeit with slow progress.

combination therapies

With the introduction of IFN monotherapy, and the current recommendation of peg IFN-gamma and ribavirin as the standard of care, the proportion of patients achieving sustained antiviral response (SVR) has increased significantly.

The mechanism of action of IFN-gamma and ribavirin is still not completely understood. IFN has a direct antiviral effect and acts on the immune system of the host; ribavirin alone does not inhibit HCV replication significantly, but augments the antiviral action of IFN. Importantly, ribavirin prevents relapse after the end of antiviral treatment. However, the morbidity and mortality rates associated with HCV are predicted to rise in the coming years, and more efficacious and tolerable therapies are urgently required, particularly for the increasing proportion of patients who are refractory to treatment with IFN-gamma and ribavirin.

There are no approved treatment options available for patients who have failed to respond to previous treatments. Studies suggest that in non-responders to IFN monotherapy, re-treatment with pegIFN and ribavirin can achieve SVR rates of 25−40%. In non-responders to IFN and ribavirin, re-treatment can achieve SVR rates of up to 10%.

Longer-acting IFNs and IFN-inducing molecules are in development. One example is albinterferon-alpha2b (albIFN-alpha2b),¹⁰ a fusion protein comprising albumin and IFN-alpha2b, which has been shown to have antiviral activity in a clinical trial setting, with a less frequent dosing regimen than current peg IFNs IFN variants112 and the development of long-lasting IFNs, such as albIFN-alpha2b (Human Genome Sciences and Novartis), locteron (OctoPlus) and omega IFN with a subcutaneous delivery device (Intarcia Therapeutics) lasting 12 weeks. Furthermore, ribavirin derivatives have been developed to improve efficacy and tolerability – these include levovirin and viramidine (taribavirin). However, the combination of levovirin and peg IFN-alpha 2a fails to generate virological responses comparable with ribavirin–pegIFN-alpha2a combination therapy in patients with chronic HCV.

second-generation molecules

The need to improve quality of life for patients on existing antiviral regimens has driven industry efforts to develop second-generation molecules with improved pharmacokinetics and pharmacodynamics. Biotech companies, such as Human Genome Sciences and Biolex Therapeutics, are evaluating several enhanced IFN-alpha candidates that offer reduced side-effect profiles and the need for fewer injections.

Investors are closely watching Albuferon, a long-acting form of IFN-α2b genetically fused to human albumin in Phase III clinical trials by Human Genome Sciences. Albuferon has reportedly shown an acceptable safety profile and produced a 17% sustained virologic response rate, in combination with ribavirin, in individuals with HCV who failed to respond to previous pegylated IFN and ribavirin therapy. Another candidate, Locteron (OctoPlus) − a controlled-release formulation in which IFN-α2b is encapsulated in biodegradeable poly(ether ester) multiblock copolymers based on poly(ethylene glycol) and poly (butylene terephthalate) − is in Phase IIa evaluation by Biolex Therapeutics. Locteron produced antiviral activity and a favourable side-effect profile when administered every two weeks to treatment-naive individuals with chronic type-1 HCV.

HCV vaccine development

Although there is currently no prophylactic vaccine available for HCV, extensive clinical studies by InnoVac-C of a recombinant vaccine in chimpanzees have shown encouraging results. Based on the viral envelope proteins E1/E2, the vaccine protected more than 80% of the animals from developing chronic infection following experimental challenge with either homologous or heterologous HCV-1a viral strains. A T-cell vaccine eliciting broad cellular responses to HCV- 1b non-structural proteins 3, 4 and 5 was also shown to exhibit prophylactic activity in chimpanzees after heterologous HCV-1a challenge. The structural viral envelope proteins E1 and E2, as well as their assembly, represent other potential antiviral targets.

Other approaches being taken to develop therapeutic vaccines include a clinical-grade HCV E1 protein produced and purified from mammalian cells (InnoVac-C). In a Phase IIa study involving 35 patients with chronic HCV infection, cellular immune responses were boosted with a recombinant e1 vaccine, including a significant T-cell response. However, these cellular immune responses were not accompanied by any significant reductions in serum HCV RNA.¹¹

Another peptide-based therapeutic HCV vaccine, IC-41, also induced significant T-cell responses, but HCV decay was not more than 1 log10 in individual patients. The only parameter that was shown to correlate with RNA response to IC-41 was an increase in HCV-specific IFN-secreting CD8+ cytotoxic T-cells above a critical threshold.¹²

Such immune boosting in HCV carriers is likely to be most effective when used as an adjunct therapy with standard-of-care antiviral drugs. Other approaches to therapeutic HCV vaccines include the use of the recombinant core protein adjuvanted with Iscomatrix. This combination elicited an unusually strong T-helper and cytotoxic T-cell response to HCV in rhesus macaques109, and a clinical trial in HCV patients who previously failed IFN therapy is under way by InnoVac C.

antiviral monoclonal and polyclonal antibodies

An area that is increasing in prominence is the use of biologics as antivirals, specifically monoclonal antibodies (mAbs) and polyclonal antibodies. Although biologics are driving growth in some disease markets, antiviral biologic development has been slow because of the lack of success of high-throughput screens in identifying potential small-molecule regulators of HCV replication and the minimal viral reduction observed with early-stage biologic candidates. Even so, Peregrine Pharmaceuticals is developing bavituximab, which targets aminophospholipids on the surface of HCV-infected cells. The company announced Phase Ib results, at the November 2007 meeting of the American Association for the Study of Liver Diseases (AASLD), demonstrating that the chimeric mAb is well tolerated in patients and produces antiviral activity at all doses.

Pursuing a niche market, Nabi Biopharmaceuticals is developing Civacir, a preparation of polyclonal antibodies, for prevention of HCV recurrence in liver transplant recipients. It is currently in Phase II and results of a previous Phase I/II trial showed Civacir to be well-tolerated and able to reduce a key liver enzyme, although HCV RNA levels in serum were not suppressed.

In addition, XTL Biopharmaceuticals is developing XTL-6865, a combination of two fully human mAbs (Ab68 and Ab65) that target the HCV E2 envelope protein. Phase I results showed XTL-6865 to be safe but not to have an affect on HCV RNA levels during the trial's short administration period. XTL is expected to evaluate XTL-6865 in individuals with HCV undergoing liver transplantation and to seek partnerships for future clinical development.

Table 4: Selected HCV therapies

Compound	Company	Target/ mechanism	Clinical stage
albuferori	Human Genome Sciences	interferon-albumin fusion protein	Phase III
telaprevir	Vertex Pharmaceuticals	NS3 protease inhibitor	Phase II
boceprevir	Schering-Plough	NS3 protease inhibitor	Phase II
R1626	Roche	NS5B polymerase inhibitor	Phase II
1C41	Intercell	therapeutic vaccine	Phase II
VGX-410C	VGX Pharmaceuticals	oral IRES inhibitor	Phase II
Debio-025	Debiopharm	clophilin inhibitor	Phase II
celgosivir	Migenix	a-glucosidase 1 inhibitor	Phase II
SCV-07	SciClone Pharmaceuticals	immune stimulant peptide	Phase II
Nitazoxanide	Romark Laboratories	undisclosed	Phase II
R7128	Pharmasset/Roche	NS5B polymerase inhibitor	Phase I/II
ITMN-191	InterMune	NS3 protease inhibitor	Phase Ib
Gl-5005	Globeimmune	targeted molecular immunogen	Phase I
GS-919O	Gilead Sciences	NS5B polymerase inhibitor	Phase I
AZD-28.36	AstraZeneca	NS5A protein inhibitor	Phase I
lMO-2 125	Idera Pharmaceuticals	toll-like receptor 9 agonist	Phase I

outlook

Because of extensive HCV sequence diversity, it has been difficult to develop an effective prophylactic vaccine against the virus. The current standard of care treatment for the disease is considered suboptimal and it is not well tolerated by many individuals, often causing side-effects. Depending on the specific HCV genotype, as little as half of those treated can expect a cure. This has added urgency to the search for new drugs with alternative mechanisms that might have greater efficacy and fewer side-effects.

As the ultimate goal of HCV therapy is the complete elimination of the virus in all patients, new strategies for treatment are needed. Prophylactic and therapeutic vaccines are in development, and new approaches include the development of innovative new agents targeting different stages of the viral life cycle, as well as improvements to current strategies. Furthermore, a combination of complementary approaches and individualisation based on genotype, viral load and early virological response will improve outcomes and are being actively pursued.

References

1.GL Davis et al. ‘Projecting future complications of chronic hepatitis C in the United States', Liver Transpl 9, 331–338 (2003).

2.K Gaber. ‘Hepatitis C: Staying the course', Nature Vol 25, 1379-1381, 12 Dec 2007.

3.QL Choo et al. ‘Isolation of a cDNA clone derived from a blood-borne non-A, non-B viral hepatitis genome',.Science 244, 359-362, 1989.

4.GL Davis et al. ‘Treatment of chronic hepatitis C with recombinant interferon a. A multicenter randomized, controlled trial', Hepatitis Interventional Therapy Group, NEJM, 321, 1501-1506, 1989.

5.S Roberts et al. ‘Interim results of a multiple ascending dose study of R1626, a novel nucleoside analog targeting HCV polymerase in chronic HCV patients', J Hepatol, 44 (Suppl. 2), S269, 2006.

6.DB Olsen et al. ‘HCV antiviral activity and resistance analysis in chronically infected chimpanzees treated with NS3/4A protease and NS5B polymerase inhibitors', J Hepatol, 46 (Suppl. 1), S298, 2007.

7.D Dieterich et al. ‚Early clearance of HCV RNA with valopicitabine (NM283) plus peg-interferon in treatment-naive patients with HCV-1 infection: first results from a Phase IIb trial', J Hepatol, 44 (Suppl. 2), S271, 2006.

8.S Ma et al. ‘NIM811, a cyclophilin inhibitor, exhibits potent in vitro activity against hepatitis C virus alone or in combination with a interferon', Antimicrob Agents Chemother, 50, 2976–2982, 2006.

9.R Flisiak et al. ‘The cyclophilin inhibitor DEBIO-025 has a potent dual anti-HIV and anti-HCV activity in treatment-naive HIV/HCV co-infected subjects', National AIDS Treatment Advocacy Project, www.natap.org/2006/AASLD/ AASLD_30.htm

10.S Zeuzem et al. ‘Sustained virologic response rates with albinterferon a-2b plus ribavirin treatment in IFN-naive, chronic hepatitis C genotype I patients', Hepatology 46 (Suppl. 1), 317A, 2007.

11.F Nevens et al. ‘A pilot study of therapeutic vaccination with envelope protein E1 in 35 patients with chronic hepatitis C', Hepatology 38, 1289-1296, 2003.

12.G Leroux-Roels et al. ‘Immunogenicity and tolerability of intradermal administration of an HCV E1-based vaccine candidate in healthy volunteers and patients with resolved or ongoing chronic HCV infection', Hum Vaccin 1, 61–65, 2005.

Gautam Thor is a freelance writer. Email: [email protected].