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Beyond interferon and ribavirin: Antiviral therapies for hepatitis C virus
Drug Discovery Today: Therapeutic Strategies
Vol. 3, No. 2 2006
Editors-in-Chief Raymond Baker – formerly University of Southampton, UK and Merck Sharp & Dohme, UK Eliot Ohlstein – GlaxoSmithKline, USA DRUG DISCOVERY
TODAY THERAPEUTIC
STRATEGIES
Infectious diseases
Beyond interferon and ribavirin: Antiviral therapies for hepatitis C virus Ann D. Kwong, Sarah Cowherd, Peter Mueller* Vertex Pharmaceuticals, Inc., 130 Waverly St, Cambridge, MA 02138, USA
Hepatitis C virus (HCV) infection in humans is associated with the progressive liver failure. A new stage in HCV therapy has emerged with the development of Specifically Targeted Antiviral Therapies for HCV
Section Editors: Gary Woodnutt – CovX, San Diego, USA. Paul-Henri Lambert – Centre of Vaccinology, University of Geneva, Switzerland.
(STAT-C) directly inhibiting viral replication. In combination with pegylated interferon, HCV protease inhibitors have the potential to transform patient care by increasing the cure rate, decreasing side effects and shortening the duration of treatment. In the future, we foresee a shift in the current treatment regiment towards all oral STAT-C inhibitors. Introduction Chronic viral infection with hepatitis C virus (HCV) is an epidemic with approximately 170 million infected individuals worldwide [1,2]. Persistent infection with HCV in patients is associated with serious liver diseases such as chronic hepatitis, fibrosis and hepatocellular carcinoma [3]. Indeed, the burden of liver disease associated with HCV infection is increasing and a major cause for liver transplants in the United States. Currently, no specifically targeted antiviral therapies for HCV are available. The current therapies consists of standard of care (SOC) consists of weekly injections of pegylated interferon plus daily oral dosing with ribavirin [2,4,5], neither of which do not specifically target HCV. The failure rate for achieving a sustained viral response (SVR) using SOC for patients infected with HCV genotype 1 in the United States, Europe, and Japan is 50%. The long duration of treatment (48 weeks) is particularly difficult for patients to tolerate given significant side *Corresponding author: P. Mueller ([email protected]) 1740-6773/$ ß 2006 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddstr.2006.06.008
effects such as fatigue, flu-like symptoms, depression and suicide associated with interferon treatment, and side affects such as hemolytic anemia associated with ribavirin treatment. In addition, there are several contraindications for taking SOC including pregnancy, depression, anemia and decompensated liver disease that severely limit access to treatment for many HCV patients. As a result, many patients are ineligible or reluctant to be treated. For example, upwards of 50% of HCV genotype 1 infected patients elect to delay or refuse treatment due to the unfavorable benefit/risk trade-off associated with currently available options.
New HCV therapies The goal of any new HCV antiviral therapy is to improve the efficacy and tolerability of the current therapies and reduce the number of contraindications for starting therapy to allow more infected individuals to be treated. Because HCV replication is dependent on both viral and cellular factors, anti-HCV agents could be designed to either be Specifically Targeted Antiviral Therapies for HCV (STAT-C) directly inhibiting viral replication, or Non-Specific Targeted Antiviral Therapies for HCV (non-STAT-Cs) indirectly affecting viral replication through a cellular target [6].
STAT-C targets for therapeutic intervention in the HCV life cycle As shown in Fig. 1, there are five major steps in the HCV life cycle [7] in which an essential activity for HCV replication 211
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Figure 1. Multiple points in the HCV life cycle where STAT-C inhibitors can intervene and break the chain of infection.Schematic depiction of five steps in HCV viral replication where inhibitors targeting a viral-specific function can act. Briefly, these steps include: (1). Entry and uncoating: Attachment of the infecting virus to a hepatocyte, penetration into the host cell, and release of the (+)-strand HCV RNA genome into the cytoplasm. (2). IRES-dependent polyprotein synthesis: Binding of host ribosomes to the internal ribosome entry site (IRES) in the HCV viral genome and translation of the HCV polyprotein. (3). NS34A protease polyprotein cleavage: The HCV polyprotein is proteolytically processed by host and viral proteases. The NS34A protease is responsible for processing all of the non-structural proteins, which are part of the HCV replication machinery of replicase. (4). NS5B RNA polymerase viral RNA synthesis: The HCV replicase synthesizes new infectious viral RNA through two steps: (i) the ( )-strand or intermediate copy of the viral genome is synthesized using the incoming RNA as a template and (ii) more (+)-strand progeny HCV RNA is synthesized using the ( )-strand RNA as a template. (5). Packaging and egress: Capsids are assembled into which progeny RNA is packaged, during egress from the cell through the Golgi apparatus; a membrane is acquired into which viral glycoproteins are inserted. When the progeny virus exits the cell, it can initiate a new round of infection.
can be inhibited. After HCV infects the host hepatocyte (step 1), the viral RNA must be unpackaged in the cytoplasm of the cell so that the operating instructions encoded in its genome can be activated. HCV is a positive (+)-stranded RNA virus, which means that the genome can be directly translated (step 2) using the HCV internal ribosomal entry site (IRES) to produce a single polyprotein chain containing all the structural and non-structural viral proteins required for replication. This viral polyprotein is then proteolytically cleaved (step 3) by host and viral proteases forming structural proteins for packaging progeny virus and the multi-protein replicase complex required for viral RNA replication [8–10]. In step 4, the replicase complex synthesizes negative ( )-stranded replicative HCV 212
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RNA intermediates from the genomic (+)-strand RNA template, which in turn are used as a template to synthesize more progeny HCV RNA. In step 5, progeny HCV RNA is packaged and matured into infectious virus as it egresses through the endoplasmic reticulum out of the cell. Several companies have preclinical research programs to identify molecules that will attack and disable the virus at each of the potential points for STAT-C and non-STAT-C intervention. Several excellent, comprehensive reviews of HCV inhibitors in research, preclinical and clinical development are available [4,11–14]. For brevity, as shown in Table 1, only the most advanced STAT and non-STAT programs in Phase 2 or 3 clinical trials will be discussed.
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Table 1. Small molecule HCV inhibitors in Phase 2/3 clinical development Non-STAT HCV inhibitors Compound description
STAT HCV inhibitors Structure
Company
Status
Compound description
Viramidine Ribavirin pro drug
Valeant
Phase 3
Isotoribine (ANA975) oligonucleotide Toll-like receptor-7 agonist
Anadys/ Novartis
ActilonTM (CPG10101) Toll-like receptor-9 agonist
Structure
Status
VX-950 Peptidomimetic active site protease inhibitor
Vertex
Phase 2
Phase 2
SCH 503034 Peptidomimetic active site protease inhibitor
ScheringPlough
Phase 2
Coley
Phase 2
Valopicitabine (NM-283) Ribonucleoside polymerase inhibitor
Idenix/ Novartis
Phase 2
Celgosivir (MX-3253) 6 O-butanoyl castanospermine alpha-glucosidase inhibitor
Migenix/ ScheringPlough
Phase 2
ISIS-14803 Anti-IRES antisense oligodeoxynucleotide
Isis/Elan
Phase 2
Ceplene Histamine dichloride
Maxim/ ScheringPlough
Phase 2
The table is divided into Specifically Targeted Antiviral Therapies for HCV (STAT-C) or Non-Specific Targeted Antiviral Therapies for HCV (non-STAT-Cs).
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Company
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Non-STAT-C inhibitors Ribavirin-like molecules The most advanced non-STAT inhibitor in clinical development is Viramidine, which is currently in Phase III trials. Viramidine is a prodrug of ribavirin, which was designed to target the ribavirin component of SOC to the liver through adenosine deaminase conversion to ribavirin [15,16]. Ribavirin is a guanosine analogue, whose mechanisms of action include inhibition of inosine monophosphate dehydrogenase (IMPDH), a key step in de novo guanine synthesis, which is required for viral replication (see recent review [12]). Although ribavirin significantly increases the SVR for the SOC regimen, it is also associated with hemolytic anemia. At the recent 41st Annual Meeting of the European Association for the Study of the Liver held from 26 to 30 April 2006 in Vienna, Austria (the 2006 EASL mtg), it was reported that Viramidine, in combination with PEG-IFN, met its safety endpoint and demonstrated improved erythrocyte-sparing properties compared with ribavirin. However, Viramidine failed to meet its efficacy endpoint of noninferiority to ribarivirin in SOC [17].
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administered twice a week by intravenous infusion for 2 months, a HCV viral load drop of 1.0–3.8 log10 was observed in some patients [22].
HCV polymerase inhibitors (step 4) The HCV NS5B RNA-dependent RNA polymerase is the last viral protein to be translated from the HCV genome. It must be proteolytically released from the viral polyprotein through the action of the NS34A protease to form an active replicase complex. Although the structure of NS5B has all of the canonical features of other polymerases, it is unique in the manner in which it encircles the active site via extensive interactions between the fingers and thumb polymerase subdomains [23–25]. In enzymatic assays, NS5B lacks template specificity for HCV RNA and has poor catalytic activity, which might reflect a requirement for additional viral and host proteins in the HCV replicase complex to ensure efficient replication of progeny viral RNA molecules. As shown in Fig. 2A, the availability of the NS5B crystal structure has facilitated the development of four classes of inhibitors targeted to four distinct binding sites – one nucleoside active site and three non-nucleoside sites [26].
Immune modulators Compounds targeting host-immunomodulation include Ceplene, a histamine dichloride in Phase 2 clinical trials [18] and Isotoribine and Actilon (CPG 10101), which are oligonucleotide agonists of TLR-7 and TLR-9, respectively. By virtue of its effect on TLR-9, Actilon leads to activation of dendritic cells and B cells to induce production of antiviral interferons and drive virus-specific memory immune responses to help clear HCV infection. In a 4-week monotherapy trial involving HCV genotype 1 infected patients injected with Actilon once a week; a mean 1.63 log10 reduction in HCV viral load was observed [13]. In a 12 week Phase 1b study in treatment-refractory HCV patients, the triple combination of Actilon plus SOC achieved a mean 3.3 log10 decrease in HCV RNA levels from baseline compared with a 2.3 log10 decrease in patients receiving SOC alone (P < 0.05) [19,20].
HCV maturation inhibitors (step 5) Celgosivir is an alpha-glucosidase 1 inhibitor in Phase 2 clinical trials; in a monotherapy study, a HCV viral load decrease of <1.0–2.6 log10 was observed. Celgosivir acts via inhibition of host-directed glycosylation of viral glycoproteins, which are attached to the viral envelope during the maturation phase of the virus life cycle [21].
STAT-C inhibitors IRES inhibitors (step 2) ISIS-14803 is a 20-base phosphorothioate antisense oligodeoxynucleotide STAT-C inhibitor that targets the IRESdependent translation of the HCV genome into the viral polyprotein. In a Phase 2 study in which ISIS-14803 was 214
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Valopicitabine (NM283)
NM283 is an orally bioavailable pro-drug of a novel ribonucleoside analog and is the most advanced HCV polymerase inhibitor in development. NM283 has provided proof of concept for active site polymerase inhibitors with 0.7 log10 reduction in viral titer in 22 days [27,28] NM283 appears to act in two ways: (i) it inhibits the viral polymerase directly and (ii) it is incorporated into growing strands of viral RNA, which terminates RNA chain extension. In a 12 week Phase IIb study, patients treated with NM283 plus PEG-IFN decreased their HCV viral load by 2.5–2.8 log 10 compared with patients on SOC who achieved a mean HCV viral load reduction of 1.9 log10 [29,30]. At the 2006 EASL mtg, 12-week treatment data were presented from a Phase IIb study in which naı¨ve genotype 1 patients who were treated with various regimens of 200 or 800 mg of NM383 in combination with PEG-IFN [31–33]. The 200 mg and 800 mg/day combination (combo) arms demonstrated comparable antiviral potency with a decrease in HCV RNA of 3.93 log10 and 4.26 log10 for the 200 mg/day and pooled 800 mg/day arm, respectively. The 200 mg/day combo arm was better tolerated and the amount of NM293 in the 800 mg combo had to be dose-reduced through a protocol amendment. Severe adverse events associated with dehydration were attributed to NM283 and 2/12 treatment discontinuations due to the adverse events occurred in the 200 mg combination arm.
NS34A protease inhibitors (step 3) HCV NS34A protease is essential for the enzymatic cleavage of the HCV polyprotein, an early event in viral replication.
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Figure 2. Binding sites of HCV polymerase and protease inhibiitorsStructure-based drug design-driven targets for novel HCV inhibitors. (A). Space-filling model of the crystal of NS5B RNA polymerase showing four classes of inhibitor binding sites. NAS = Nucleoside Active Site inhibitor binding site. NN-A,B,C = Non-nucleoside inhibitor class A, B and C. (B). Space-filling model of the crystal structure of NS3 protease catalytic domain (silver), complexed with a NS4A peptide co-factor (not visible) and a HCV polypeptide substrate derived for the NS5A-5B cleavage site (green).
In vitro studies have suggested a potential second mechanism of action wherein two members of the host innate response pathway (TRIF [34] and CARDIF [35]), can be cleaved by HCV NS34A protease, resulting in the loss of innate immunity. CARDIF is targeted to the mitochrondrial membrane through a signal in the C-terminal portion of the molecule. When CARDIF is cleaved by HCV protease, it is separated from the C-terminal mitochondrial anchoring domain and moves to the cytoplasm. Recent studies have revealed that CARDIF is cytoplasmically localized in the liver of patients chronically infected with HCV, which is the first evidence that inhibition of innate immunity by HCV protease might occur in patients [35]. To date, significantly more potent antiviral activity has been observed clinically with HCV protease inhibitor monotherapy than with HCV polymerase inhibitors. Whether this discrepancy is due to the combination of directly inhibiting viral replication and restoring innate immunity with a single protease inhibitor remains to be determined. The success of HIV protease inhibitors gave hope that structure-based drug design of HCV NS34A protease inhibitors could be quickly developed for oral, small molecule-based antiviral therapy [36,37]. However, as shown in Fig. 2B, the crystal structure of the HCV protease domain [38] contains a shallow hydrophobic substrate binding site (substrate is shown in green) which makes it difficult to design high affinity inhibitors. Thus, it is remarkable that development of HCV protease inhibitors has advanced such that they are now among the most promising of the new STAT-C inhibitors, with three compounds (BILN 2061, SCH 503034 and VX-950) showing substantial antiviral activity in clinical trials.
BILN 2061
Although BILN 2061 is currently not in active clinical development, it is worth mentioning because it was the first compound to demonstrate proof of concept for a HCV protease inhibitor in a clinical trial that expanded the boundary of what could be accomplished in treating HCV-infected patients. In this paradigm-shifting study, BILN 2061 achieved a mean viral load drop of 2.5 log10 in 2 days (Fig. 3A), which is similar to the drop in viral load of patients infected with HCV genotype 1 of 2.8 log10 with 28 days of SOC therapy [39]. SCH 503034 and VX-950
Both SCH 503034 and VX-950 are oral peptidomimetic inhibitors that were designed by Schering-Plough and Vertex Pharmaceuticals Incorporated, respectively. In Phase Ib monotherapy dose-ranging studies with patients infected with HCV genotype 1, a maximum median decrease in plasma HCV RNA of 2.05 log10 was obtained after 14 days of dosing with SCH 503034 [40,41]. A similar antiviral effect with VX-950 was observed during dosing with VX-950 in a Phase 1b monotherapy study; the maximum median decrease in viral load was 4.4 log10 IU/mL in a monotherapy dose-ranging study [42–44], or was 4.0 log10 IU/mL in the monotherapy arm of a VX-950/PEG-IFN combination study [45,46] as illustrated in Fig. 3C. The kinetics of HCV viral load reduction in dual combination clinical trials with PEG-IFN and SCH 503034 [40,42] or VX-950 [45,47] is shown in Fig. 3B and C, respectively. Because the pattern of suppression of HCV RNA with PEG-IFN alone differed between in the two studies, it should be noted that the form of PEG-IFN used in each trial was different; SCH 503034 was combined with PEG-IFN-a-2b (PEG-Intron) and VX-950 www.drugdiscoverytoday.com
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was combined with PEG-IFN-a-2a (Pegasys). PEG-IFN-a-2b induced a transient decline of 1 log10 which rebounded close to baseline before each subsequent dose of interferon (Fig. 3B). By contrast, PEG-IFN-a-2b treatment alone resulted in a similar magnitude of decline in viral load of 1.1 log10 IU/mL with no rebound (Fig. 3C). In combination with their respective forms of PEG-IFN, both SCH 503034 and VX-950 induced a steep first
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phase drop in HCV viral load during the first 2 days of dosing of 2.5 log10 and 3.5 log10 IU/mL, respectively. However, the HCV RNA level transiently rebounded with SCH 505034 plus PEG-IFN-a-2b in a manner similar to that observed in the PEGIFN-a-2b arm alone, and the median maximum decrease in viral load observed in this study was 2.8 log10 (Fig. 3B). By contrast, HCV RNA levels in the PEG-IFN plus VX-950 arm did
Figure 3. Comparison of patient viral load reduction curves for HCV protease inhibitors (A). BILN 2061 viral response curve from a 2-day monotherapy Phase 1 study. (B). SCH 505034 viral response curve from a 14-day Phase Ib combination study with PEG-IFN-a-2b. (C). VX-950 viral response curve from a 14-day Phase Ib combination study with PEG-IFN-a-2b.
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not rebound and continued to decline, resulting in a median maximum decrease of 5.5 log10 IU/mL (Fig. 3C). SCH 503034
In summary, SCH 503034 monotherapy in HCV genotype 1 patients induced a mean maximal decrease in plasma HCV RNA levels of 2.06 log10 in a 14-day Phase I dose-ranging study. In a second Phase 1 crossover study, the double combination of SCH 503034 plus PEG-IFN-a-2b produced a mean maximum viral load drop of 2.9 log10, compared with a drop of 1.1 log10 with PEG-IFN-a-2b alone (Fig. 3B). VX-950
VX-950 monotherapy induced a maximum median reduction in plasma HCV RNA of 4.0 and 4.4 log10 in two separate clinical trials with HCV genotype 1 patients; 1/8 patient’s HCV RNA level became undetectable (<10 IU/mL) after 14 days (2005 AASLD ref). In a 14-day Phase Ib double combination study, VX-950 combined with PEG-IFN a-2a produced an rapid median HCV viral load drop of 3 log10 on Day 2 and a maximum median decrease in plasma HCV RNA of 5.5 log10 on Day 14; 4/ 8 patient’s HCV RNA levels became undetectable (<10 IU/mL). By contrast, patients who received PEG-IFN a-2a alone for 14 days had a median plasma HCV RNA decrease of 1.0 log10 and no patient’s viral RNA levels became undetectable in 14 days. In a 28-day Phase II triple combination study in HCV genotype 1 patients with VX-950 plus PEG-IFN a-2a and ribavirin, all patients (n = 12) achieved undetectable plasma HCV RNA levels of <10 IU/mL. No patients showed evidence of viral breakthrough on the triple therapy arm [47]. One interesting observation from both protease inhibitor clinical trials was the normalization of ALT levels among patients receiving the active drug who had entered the trial with elevated ALT levels [40,43]. Because HCV is a hepatotrophic virus, HCV protease inhibitor drug discovery has intentionally focused on targeting inhibitors to the liver and achieving high liver: plasma ratios. Thus, it is reassuring that high liver concentrations of SCH 503034 and VX-950 did not result in overt liver toxicity, which is often associated with elevated ALT levels. It is intriguing and encouraging that there was a rapid normalization of ALT levels, given that they are thought to be owing to chronic infection with HCV.
Conclusions: toward a new treatment landscape Chronic HCV infection remains a pressing medical issue. Improvement of the tolerability and efficacy of the current standard of care is an important goal and several exciting compounds with varied mechanisms are in clinical and preclinical development. Numerous points in the viral replication cycle have been targeted, and can be broadly classed as molecules targeting viral proteins essential to HCV replication (STAT-Cs) or host cell factors which aid in the replication of the virus or support or supplement the body’s innate
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immunity defense mechanism to viral pathogens (nonSTATs). To date some of the most promising results, both in terms of tolerability and efficacy, have been obtained with STAT-Cs targeting the HCV protease and polymerase and these might provide the vehicle to move toward a new era in HCV therapy. The entry of these new therapies to the market is expected to have a dramatic impact on the HCV treatment landscape. A model for this process is shown in Fig. 4 and can be subdivided into three stages starting with the current era and moving forward in time: Stage 1 is the present era in which a combination of pegylated interferon or consensus interferon and ribavirin are the only approved treatment regimens. Attempts to improve current SOC and clinical practice are primarily focused on two approaches to increase tolerability and compliance. The first approach is aimed at improving the pharmacologic properties of the current SOC. Examples of this approach include (i) improving the potency of interferon, (ii) improving the pharmacokinetic properties of interferon (e.g. Albuferon or albumin-interferon-a-2b [48,49] and (iii) decreasing the hemolytic anemia associated with ribavirin (e.g. Viramidine). The second approach is focused on optimizing the use of the individual components of SOC. Examples include (i) weight-based dosing of ribavirin, (ii) prophylactic dosing with antidepressants before starting IFN-based therapy, (iii) dosing with erythropoietin to treat ribavirin-induced hemolytic anemia and (iv) individualizing treatment by decreasing the length of dosing for patients who achieve a rapid viral response (RVR). Stage 2 is the era we will enter in approximately 3 years (2009–2010) and it will probably be marked by a major transformation in HCV treatment paradigms. On the basis of the currently available early clinical data, the new therapies in Stage 2, which will have the biggest impact are HCV protease inhibitors in combination with PEG-IFN and ribavirin or with PEG-IFN alone. The rapid and substantially increased potency of the HCV protease/PEG-IFN combination is expected to have a higher cure rate than the current SOC and shorter treatment duration. The addition of potent STAT-C drugs to HCV therapy is probably going to significantly increase the number of patients who enter therapy, who are infected with HCV genotype 1 (the hardest group to treat with current SOC), those who are currently in the ‘watch and wait’ mode, who have mild liver disease and who are waiting for more efficacious and better tolerated therapy. A second significant transformation of the HCV treatment landscape will occur in Stage 3 when all oral STAT-C combination treatment regimens are approved which will probably eliminate interferon from HCV treatment regimens. Timelines are hard to predict, but Stage 3 has the potential to launch 6–8 years from now (2012–2014) and a plethora of new oral treatment options might become available. www.drugdiscoverytoday.com
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Figure 4. Impact of STAT-Cs on the treatment landscapeSchematic depiction of stages in HCV therapy with increasing numbers of patients treated over time. 1980–1990s IFN or PEG-IFN monotherapy 1998 Stage 1: Non-STAT-C, IFN-based combination therapies with ribavirin with low efficacy and low tolerability 2009 Stage 2: STAT-C combination therapy, especially protease and polymerase inhibitors in combination with PEG-IFN 2012 Stage 3: All oral STAT-C therapy Abbreviations: SVR, sustained viral response; Rx, treatment; RBV, ribavirin; wks, weeks.
For example, new second generation HCV protease and polymerase inhibitors are probably be launched with one or more of the following properties: improved potency against wildtype and resistant virus, lower pill burden and twice a day (BID) or once a day (QD) dosing. Combination treatment with two STAT-C inhibitors such as a protease and polymerase inhibitor is also probably to be approved. In addition to HCV protease and polymerase inhibitors, new STAT-Cs with different mechanisms of action might be approved in Stage 3. Examples include, but are not limited to (i) virus entry inhibitors such as receptor binding and fusion inhibitors, (ii) inhibitors of the HCV IRES function, (iii) HCV core protein capsid assembly inhibitors and (iv) HCV helicase inhibitors. Similar to changes, which took place in the historical development of new HIV treatment regiments, the availability of STAT-Cs with different mechanisms of actions might result in the evolution of HCV patient care towards alloral combinations of targeted therapies. The current move to improve and individualize treatment is based on monitoring patient response to therapy by measuring the patients HCV viral load in their plasma. Because the field is trending towards adjusting duration of treatment based on the time it takes a patient’s plasma viral load to become ‘undetectable’, a standard assay for ‘undetectability’ needs to be implemented. Thus, it is probable that new diagnostics will be developed with one or more of the following properties: increased sensitivity to detect low HCV RNA 218
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plasma levels, (ii) increased accuracy in HCV RNA measurements, (iii) faster turn around times for HCV viral load measurements which will allow monitoring of the patient’s viral response to treatment to occur in real time, making it possible to individualize treatment options and (iii) more accurate genotype assays.
Acknowledgements We would like to thank Govinda Rao for providing chemical structures and the HCV polymerase model, James Griffith for providing the HCV protease model, and Mark Namchuk, John Randle, Jennifer Jackson and Lisa Dixon and Dasˇa Lipovsˇek for editorial assistance.
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CCO (2005) CCO Independent Conference Coverage of the 2005 Annual Meeting of the American Association for the Study of Liver Disease: Valopicitabine Plus Peginterferon Alfa-2a Highly Effective at Suppressing HCV in Peginterferon Plus Ribavirin Nonresponders (http:// www.clinicaloptions.com/Hepatitis/Conference%20Coverage/ San%20Francisco%202005/Capsules/95.aspx) Adfal, N. et al. (2006) Valopicitabine (NM283), alone or with peginterferon, compared to peg-interferon/ribavirin (pegIFN/RBV) retreatment in hepatitis C patients with prior non-response to pegIFN/RBV: week 24 results. 41st Annual Meeting of the European Association for the Study of the Liver, Vienna, Austria, 26–30 April 2006 (Abstract #39) CCO (2005) CCO Independent Conference Coverage of the 2006 Annual Meeting of the European Association for the Study of the Liver: Valopicitabine/Peginterferon alfa-2a More Effective Than Peginterferon alfa-2a/Ribavirin at Producing Virologic Response in HCV Genotype 1Infected Prior Nonresponders (http://www.clinicaloptions.com/ Hepatitis/Conference%20Coverage/Vienna%202006.aspx) Anon (2006) Anti-infective drug news: preliminary data show comparable activity with 200 and 800 mg/day doses of valopicitabin. Espicom Business Intelligence. West Sussex, UK, 9 May 2006, 18 Li, K. et al. (2005) Immune evasion by hepatitis C virus NS3/4A proteasemediated cleavage of the Toll-like receptor 3 adaptor protein TRIF. Proc. Natl. Acad. Sci. U S A 102, 2992–2997 Meylan, E. et al. (2005) Cardif is an adaptor protein in the RIG-I antiviral pathway and is targeted by hepatitis C virus. Nature 437, 1167–1172 Perni, R.B. and Kwong, A.D. (2002) Inhibitors of hepatitis C virus NS3.4A protease: an overdue line of therapy. Prog. Med. Chem. 39, 215–255 Goudreau, N. and Llinas-Brunet, M. (2005) The therapeutic potential of NS3 protease inhibitors in HCV infection. Expert Opin. Investig. Drugs 14, 1129–1144 Kim, J.L. et al. (1996) Crystal structure of the hepatitis C virus NS3 protease domain complexed with a synthetic NS4A cofactor peptide. Cell 87, 343– 355 Lamarre, D. et al. (2003) An NS3 protease inhibitor with antiviral effects in humans infected with hepatitis C virus. Nature 426, 186–189 Zeuzem, S. et al. Anti-viral activity of SCH 503034, a HCV protease inhibitor, administered as monotherapy in hepatitis C genotype-1 (HCV-1) patients refractory to pegylated interferon (Peg-IFN-a). 6th annual meeting of the American Association for the Study of Liver Diseases (56th AASLD) CCO (2005) CCO Independent Conference Coverage of the 2005 Annual Meeting of the American Association for the Study of Liver Diseases: SCH 503034 Demonstrated Potent Activity in Genotype 1 HCV Patients Previously Failing Peginterferon. CCO Independent Conference Coverage of the 2005 Annual Meeting of the American Association for the Study of Liver Diseases: SCH 503034 Demonstrated Potent Activity in Genotype 1 HCV Patients Previously Failing Peginterferon (http:// www.clinicaloptions.com/Hepatitis/Conference%20Coverage/ San%20Francisco%202005.aspx) Reesink, H.W. et al. (2005) Initial results of a phase 1b multiple dose study of VX-950, a hepatitis C virus protease inhibitor. Gastroenterology 128 (Suppl. 2), A-697 Reesink, H.W. et al. (2005) Final results of a phase 1b, multiple-dose study of VX-950, a hepatitis C virus protease inhibitor. Program and abstracts of the 56th Annual Meeting of the American Association for the Study of Liver Diseases, 11–15 November 2005, San Francisco, CA (Abstract 96) CCO (2005) Independent Conference Coverage of the 2005 Annual Meeting of the American Association for the Study of Liver Diseases: HCV NS34A Protease Inhibitor VX-950 Safe, Highly Effective in HCV-Infected Patients (http://www.clinicaloptions.com/Hepatitis/ Conference%20Coverage/San%20Francisco%202005.aspx) Reesink, H.W. et al. Initial results of a 14-day study of the hepatitis C virus inhibitor VX-950, in combination with peginterferon-alfa-2a. Program and abstracts of the 41st Annual Meeting of the European Association for the Study of the Liver, 6–30 April 2006, Vienna, Austria (Abstract 737) CCO (2006) CCO Independent Conference Coverage of the 2006 Annual Meeting of the European Association for the Study of the Liver: Combination Treatment With VX-950 Plus Peginterferon alfa-2a Highly Potent, Well Tolerated in Treatment-Naive Patients With HCV Genotype 1
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(http://www.clinicaloptions.com/Hepatitis/Conference%20Coverage/ Vienna%202006.aspx) Vertex Pharmceuticals Incorporated (2006) VX-950, investigational oral hepatitis C protease inhibitor, demonstrates rapid and dramatic reduction in viral levels in combination with pegylated interferon – Median 5.5 log10 reduction in HCV RNA achieved in patients receiving VX-950 and peg-IFN in 14-day clinical study (http://www.vrtx.com/Pressreleases2006/ pr010906.html)
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Zeuzem, S. (2006) 2006 #1503; The American Association For the Study of Liver Diseases: new perspectives in the treatment of hepatitis C. Antivir. Ther. 11, 267–271 Rustgi, V. et al. A phase 2 dose-escalation study of albuferon combined with ribavirin in non-responders to prior interferon-based therapy for chronic hepatitis C infection. Program and abstracts of the 41st Annual Meeting of the European Association for the Study of the Liver, 26–30 April 2006, Vienna, Austria (Abstract 113)
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Editors-in-Chief Raymond Baker – formerly University of Southampton, UK and Merck Sharp & Dohme, UK Eliot Ohlstein – GlaxoSmithKline, USA DRUG DISCOVERY
TODAY THERAPEUTIC
STRATEGIES
Infectious diseases
Beyond interferon and ribavirin: Antiviral therapies for hepatitis C virus Ann D. Kwong, Sarah Cowherd, Peter Mueller* Vertex Pharmaceuticals, Inc., 130 Waverly St, Cambridge, MA 02138, USA
Hepatitis C virus (HCV) infection in humans is associated with the progressive liver failure. A new stage in HCV therapy has emerged with the development of Specifically Targeted Antiviral Therapies for HCV
Section Editors: Gary Woodnutt – CovX, San Diego, USA. Paul-Henri Lambert – Centre of Vaccinology, University of Geneva, Switzerland.
(STAT-C) directly inhibiting viral replication. In combination with pegylated interferon, HCV protease inhibitors have the potential to transform patient care by increasing the cure rate, decreasing side effects and shortening the duration of treatment. In the future, we foresee a shift in the current treatment regiment towards all oral STAT-C inhibitors. Introduction Chronic viral infection with hepatitis C virus (HCV) is an epidemic with approximately 170 million infected individuals worldwide [1,2]. Persistent infection with HCV in patients is associated with serious liver diseases such as chronic hepatitis, fibrosis and hepatocellular carcinoma [3]. Indeed, the burden of liver disease associated with HCV infection is increasing and a major cause for liver transplants in the United States. Currently, no specifically targeted antiviral therapies for HCV are available. The current therapies consists of standard of care (SOC) consists of weekly injections of pegylated interferon plus daily oral dosing with ribavirin [2,4,5], neither of which do not specifically target HCV. The failure rate for achieving a sustained viral response (SVR) using SOC for patients infected with HCV genotype 1 in the United States, Europe, and Japan is 50%. The long duration of treatment (48 weeks) is particularly difficult for patients to tolerate given significant side *Corresponding author: P. Mueller ([email protected]) 1740-6773/$ ß 2006 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddstr.2006.06.008
effects such as fatigue, flu-like symptoms, depression and suicide associated with interferon treatment, and side affects such as hemolytic anemia associated with ribavirin treatment. In addition, there are several contraindications for taking SOC including pregnancy, depression, anemia and decompensated liver disease that severely limit access to treatment for many HCV patients. As a result, many patients are ineligible or reluctant to be treated. For example, upwards of 50% of HCV genotype 1 infected patients elect to delay or refuse treatment due to the unfavorable benefit/risk trade-off associated with currently available options.
New HCV therapies The goal of any new HCV antiviral therapy is to improve the efficacy and tolerability of the current therapies and reduce the number of contraindications for starting therapy to allow more infected individuals to be treated. Because HCV replication is dependent on both viral and cellular factors, anti-HCV agents could be designed to either be Specifically Targeted Antiviral Therapies for HCV (STAT-C) directly inhibiting viral replication, or Non-Specific Targeted Antiviral Therapies for HCV (non-STAT-Cs) indirectly affecting viral replication through a cellular target [6].
STAT-C targets for therapeutic intervention in the HCV life cycle As shown in Fig. 1, there are five major steps in the HCV life cycle [7] in which an essential activity for HCV replication 211
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Figure 1. Multiple points in the HCV life cycle where STAT-C inhibitors can intervene and break the chain of infection.Schematic depiction of five steps in HCV viral replication where inhibitors targeting a viral-specific function can act. Briefly, these steps include: (1). Entry and uncoating: Attachment of the infecting virus to a hepatocyte, penetration into the host cell, and release of the (+)-strand HCV RNA genome into the cytoplasm. (2). IRES-dependent polyprotein synthesis: Binding of host ribosomes to the internal ribosome entry site (IRES) in the HCV viral genome and translation of the HCV polyprotein. (3). NS34A protease polyprotein cleavage: The HCV polyprotein is proteolytically processed by host and viral proteases. The NS34A protease is responsible for processing all of the non-structural proteins, which are part of the HCV replication machinery of replicase. (4). NS5B RNA polymerase viral RNA synthesis: The HCV replicase synthesizes new infectious viral RNA through two steps: (i) the ( )-strand or intermediate copy of the viral genome is synthesized using the incoming RNA as a template and (ii) more (+)-strand progeny HCV RNA is synthesized using the ( )-strand RNA as a template. (5). Packaging and egress: Capsids are assembled into which progeny RNA is packaged, during egress from the cell through the Golgi apparatus; a membrane is acquired into which viral glycoproteins are inserted. When the progeny virus exits the cell, it can initiate a new round of infection.
can be inhibited. After HCV infects the host hepatocyte (step 1), the viral RNA must be unpackaged in the cytoplasm of the cell so that the operating instructions encoded in its genome can be activated. HCV is a positive (+)-stranded RNA virus, which means that the genome can be directly translated (step 2) using the HCV internal ribosomal entry site (IRES) to produce a single polyprotein chain containing all the structural and non-structural viral proteins required for replication. This viral polyprotein is then proteolytically cleaved (step 3) by host and viral proteases forming structural proteins for packaging progeny virus and the multi-protein replicase complex required for viral RNA replication [8–10]. In step 4, the replicase complex synthesizes negative ( )-stranded replicative HCV 212
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RNA intermediates from the genomic (+)-strand RNA template, which in turn are used as a template to synthesize more progeny HCV RNA. In step 5, progeny HCV RNA is packaged and matured into infectious virus as it egresses through the endoplasmic reticulum out of the cell. Several companies have preclinical research programs to identify molecules that will attack and disable the virus at each of the potential points for STAT-C and non-STAT-C intervention. Several excellent, comprehensive reviews of HCV inhibitors in research, preclinical and clinical development are available [4,11–14]. For brevity, as shown in Table 1, only the most advanced STAT and non-STAT programs in Phase 2 or 3 clinical trials will be discussed.
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Table 1. Small molecule HCV inhibitors in Phase 2/3 clinical development Non-STAT HCV inhibitors Compound description
STAT HCV inhibitors Structure
Company
Status
Compound description
Viramidine Ribavirin pro drug
Valeant
Phase 3
Isotoribine (ANA975) oligonucleotide Toll-like receptor-7 agonist
Anadys/ Novartis
ActilonTM (CPG10101) Toll-like receptor-9 agonist
Structure
Status
VX-950 Peptidomimetic active site protease inhibitor
Vertex
Phase 2
Phase 2
SCH 503034 Peptidomimetic active site protease inhibitor
ScheringPlough
Phase 2
Coley
Phase 2
Valopicitabine (NM-283) Ribonucleoside polymerase inhibitor
Idenix/ Novartis
Phase 2
Celgosivir (MX-3253) 6 O-butanoyl castanospermine alpha-glucosidase inhibitor
Migenix/ ScheringPlough
Phase 2
ISIS-14803 Anti-IRES antisense oligodeoxynucleotide
Isis/Elan
Phase 2
Ceplene Histamine dichloride
Maxim/ ScheringPlough
Phase 2
The table is divided into Specifically Targeted Antiviral Therapies for HCV (STAT-C) or Non-Specific Targeted Antiviral Therapies for HCV (non-STAT-Cs).
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Company
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Non-STAT-C inhibitors Ribavirin-like molecules The most advanced non-STAT inhibitor in clinical development is Viramidine, which is currently in Phase III trials. Viramidine is a prodrug of ribavirin, which was designed to target the ribavirin component of SOC to the liver through adenosine deaminase conversion to ribavirin [15,16]. Ribavirin is a guanosine analogue, whose mechanisms of action include inhibition of inosine monophosphate dehydrogenase (IMPDH), a key step in de novo guanine synthesis, which is required for viral replication (see recent review [12]). Although ribavirin significantly increases the SVR for the SOC regimen, it is also associated with hemolytic anemia. At the recent 41st Annual Meeting of the European Association for the Study of the Liver held from 26 to 30 April 2006 in Vienna, Austria (the 2006 EASL mtg), it was reported that Viramidine, in combination with PEG-IFN, met its safety endpoint and demonstrated improved erythrocyte-sparing properties compared with ribavirin. However, Viramidine failed to meet its efficacy endpoint of noninferiority to ribarivirin in SOC [17].
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administered twice a week by intravenous infusion for 2 months, a HCV viral load drop of 1.0–3.8 log10 was observed in some patients [22].
HCV polymerase inhibitors (step 4) The HCV NS5B RNA-dependent RNA polymerase is the last viral protein to be translated from the HCV genome. It must be proteolytically released from the viral polyprotein through the action of the NS34A protease to form an active replicase complex. Although the structure of NS5B has all of the canonical features of other polymerases, it is unique in the manner in which it encircles the active site via extensive interactions between the fingers and thumb polymerase subdomains [23–25]. In enzymatic assays, NS5B lacks template specificity for HCV RNA and has poor catalytic activity, which might reflect a requirement for additional viral and host proteins in the HCV replicase complex to ensure efficient replication of progeny viral RNA molecules. As shown in Fig. 2A, the availability of the NS5B crystal structure has facilitated the development of four classes of inhibitors targeted to four distinct binding sites – one nucleoside active site and three non-nucleoside sites [26].
Immune modulators Compounds targeting host-immunomodulation include Ceplene, a histamine dichloride in Phase 2 clinical trials [18] and Isotoribine and Actilon (CPG 10101), which are oligonucleotide agonists of TLR-7 and TLR-9, respectively. By virtue of its effect on TLR-9, Actilon leads to activation of dendritic cells and B cells to induce production of antiviral interferons and drive virus-specific memory immune responses to help clear HCV infection. In a 4-week monotherapy trial involving HCV genotype 1 infected patients injected with Actilon once a week; a mean 1.63 log10 reduction in HCV viral load was observed [13]. In a 12 week Phase 1b study in treatment-refractory HCV patients, the triple combination of Actilon plus SOC achieved a mean 3.3 log10 decrease in HCV RNA levels from baseline compared with a 2.3 log10 decrease in patients receiving SOC alone (P < 0.05) [19,20].
HCV maturation inhibitors (step 5) Celgosivir is an alpha-glucosidase 1 inhibitor in Phase 2 clinical trials; in a monotherapy study, a HCV viral load decrease of <1.0–2.6 log10 was observed. Celgosivir acts via inhibition of host-directed glycosylation of viral glycoproteins, which are attached to the viral envelope during the maturation phase of the virus life cycle [21].
STAT-C inhibitors IRES inhibitors (step 2) ISIS-14803 is a 20-base phosphorothioate antisense oligodeoxynucleotide STAT-C inhibitor that targets the IRESdependent translation of the HCV genome into the viral polyprotein. In a Phase 2 study in which ISIS-14803 was 214
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Valopicitabine (NM283)
NM283 is an orally bioavailable pro-drug of a novel ribonucleoside analog and is the most advanced HCV polymerase inhibitor in development. NM283 has provided proof of concept for active site polymerase inhibitors with 0.7 log10 reduction in viral titer in 22 days [27,28] NM283 appears to act in two ways: (i) it inhibits the viral polymerase directly and (ii) it is incorporated into growing strands of viral RNA, which terminates RNA chain extension. In a 12 week Phase IIb study, patients treated with NM283 plus PEG-IFN decreased their HCV viral load by 2.5–2.8 log 10 compared with patients on SOC who achieved a mean HCV viral load reduction of 1.9 log10 [29,30]. At the 2006 EASL mtg, 12-week treatment data were presented from a Phase IIb study in which naı¨ve genotype 1 patients who were treated with various regimens of 200 or 800 mg of NM383 in combination with PEG-IFN [31–33]. The 200 mg and 800 mg/day combination (combo) arms demonstrated comparable antiviral potency with a decrease in HCV RNA of 3.93 log10 and 4.26 log10 for the 200 mg/day and pooled 800 mg/day arm, respectively. The 200 mg/day combo arm was better tolerated and the amount of NM293 in the 800 mg combo had to be dose-reduced through a protocol amendment. Severe adverse events associated with dehydration were attributed to NM283 and 2/12 treatment discontinuations due to the adverse events occurred in the 200 mg combination arm.
NS34A protease inhibitors (step 3) HCV NS34A protease is essential for the enzymatic cleavage of the HCV polyprotein, an early event in viral replication.
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Figure 2. Binding sites of HCV polymerase and protease inhibiitorsStructure-based drug design-driven targets for novel HCV inhibitors. (A). Space-filling model of the crystal of NS5B RNA polymerase showing four classes of inhibitor binding sites. NAS = Nucleoside Active Site inhibitor binding site. NN-A,B,C = Non-nucleoside inhibitor class A, B and C. (B). Space-filling model of the crystal structure of NS3 protease catalytic domain (silver), complexed with a NS4A peptide co-factor (not visible) and a HCV polypeptide substrate derived for the NS5A-5B cleavage site (green).
In vitro studies have suggested a potential second mechanism of action wherein two members of the host innate response pathway (TRIF [34] and CARDIF [35]), can be cleaved by HCV NS34A protease, resulting in the loss of innate immunity. CARDIF is targeted to the mitochrondrial membrane through a signal in the C-terminal portion of the molecule. When CARDIF is cleaved by HCV protease, it is separated from the C-terminal mitochondrial anchoring domain and moves to the cytoplasm. Recent studies have revealed that CARDIF is cytoplasmically localized in the liver of patients chronically infected with HCV, which is the first evidence that inhibition of innate immunity by HCV protease might occur in patients [35]. To date, significantly more potent antiviral activity has been observed clinically with HCV protease inhibitor monotherapy than with HCV polymerase inhibitors. Whether this discrepancy is due to the combination of directly inhibiting viral replication and restoring innate immunity with a single protease inhibitor remains to be determined. The success of HIV protease inhibitors gave hope that structure-based drug design of HCV NS34A protease inhibitors could be quickly developed for oral, small molecule-based antiviral therapy [36,37]. However, as shown in Fig. 2B, the crystal structure of the HCV protease domain [38] contains a shallow hydrophobic substrate binding site (substrate is shown in green) which makes it difficult to design high affinity inhibitors. Thus, it is remarkable that development of HCV protease inhibitors has advanced such that they are now among the most promising of the new STAT-C inhibitors, with three compounds (BILN 2061, SCH 503034 and VX-950) showing substantial antiviral activity in clinical trials.
BILN 2061
Although BILN 2061 is currently not in active clinical development, it is worth mentioning because it was the first compound to demonstrate proof of concept for a HCV protease inhibitor in a clinical trial that expanded the boundary of what could be accomplished in treating HCV-infected patients. In this paradigm-shifting study, BILN 2061 achieved a mean viral load drop of 2.5 log10 in 2 days (Fig. 3A), which is similar to the drop in viral load of patients infected with HCV genotype 1 of 2.8 log10 with 28 days of SOC therapy [39]. SCH 503034 and VX-950
Both SCH 503034 and VX-950 are oral peptidomimetic inhibitors that were designed by Schering-Plough and Vertex Pharmaceuticals Incorporated, respectively. In Phase Ib monotherapy dose-ranging studies with patients infected with HCV genotype 1, a maximum median decrease in plasma HCV RNA of 2.05 log10 was obtained after 14 days of dosing with SCH 503034 [40,41]. A similar antiviral effect with VX-950 was observed during dosing with VX-950 in a Phase 1b monotherapy study; the maximum median decrease in viral load was 4.4 log10 IU/mL in a monotherapy dose-ranging study [42–44], or was 4.0 log10 IU/mL in the monotherapy arm of a VX-950/PEG-IFN combination study [45,46] as illustrated in Fig. 3C. The kinetics of HCV viral load reduction in dual combination clinical trials with PEG-IFN and SCH 503034 [40,42] or VX-950 [45,47] is shown in Fig. 3B and C, respectively. Because the pattern of suppression of HCV RNA with PEG-IFN alone differed between in the two studies, it should be noted that the form of PEG-IFN used in each trial was different; SCH 503034 was combined with PEG-IFN-a-2b (PEG-Intron) and VX-950 www.drugdiscoverytoday.com
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was combined with PEG-IFN-a-2a (Pegasys). PEG-IFN-a-2b induced a transient decline of 1 log10 which rebounded close to baseline before each subsequent dose of interferon (Fig. 3B). By contrast, PEG-IFN-a-2b treatment alone resulted in a similar magnitude of decline in viral load of 1.1 log10 IU/mL with no rebound (Fig. 3C). In combination with their respective forms of PEG-IFN, both SCH 503034 and VX-950 induced a steep first
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phase drop in HCV viral load during the first 2 days of dosing of 2.5 log10 and 3.5 log10 IU/mL, respectively. However, the HCV RNA level transiently rebounded with SCH 505034 plus PEG-IFN-a-2b in a manner similar to that observed in the PEGIFN-a-2b arm alone, and the median maximum decrease in viral load observed in this study was 2.8 log10 (Fig. 3B). By contrast, HCV RNA levels in the PEG-IFN plus VX-950 arm did
Figure 3. Comparison of patient viral load reduction curves for HCV protease inhibitors (A). BILN 2061 viral response curve from a 2-day monotherapy Phase 1 study. (B). SCH 505034 viral response curve from a 14-day Phase Ib combination study with PEG-IFN-a-2b. (C). VX-950 viral response curve from a 14-day Phase Ib combination study with PEG-IFN-a-2b.
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not rebound and continued to decline, resulting in a median maximum decrease of 5.5 log10 IU/mL (Fig. 3C). SCH 503034
In summary, SCH 503034 monotherapy in HCV genotype 1 patients induced a mean maximal decrease in plasma HCV RNA levels of 2.06 log10 in a 14-day Phase I dose-ranging study. In a second Phase 1 crossover study, the double combination of SCH 503034 plus PEG-IFN-a-2b produced a mean maximum viral load drop of 2.9 log10, compared with a drop of 1.1 log10 with PEG-IFN-a-2b alone (Fig. 3B). VX-950
VX-950 monotherapy induced a maximum median reduction in plasma HCV RNA of 4.0 and 4.4 log10 in two separate clinical trials with HCV genotype 1 patients; 1/8 patient’s HCV RNA level became undetectable (<10 IU/mL) after 14 days (2005 AASLD ref). In a 14-day Phase Ib double combination study, VX-950 combined with PEG-IFN a-2a produced an rapid median HCV viral load drop of 3 log10 on Day 2 and a maximum median decrease in plasma HCV RNA of 5.5 log10 on Day 14; 4/ 8 patient’s HCV RNA levels became undetectable (<10 IU/mL). By contrast, patients who received PEG-IFN a-2a alone for 14 days had a median plasma HCV RNA decrease of 1.0 log10 and no patient’s viral RNA levels became undetectable in 14 days. In a 28-day Phase II triple combination study in HCV genotype 1 patients with VX-950 plus PEG-IFN a-2a and ribavirin, all patients (n = 12) achieved undetectable plasma HCV RNA levels of <10 IU/mL. No patients showed evidence of viral breakthrough on the triple therapy arm [47]. One interesting observation from both protease inhibitor clinical trials was the normalization of ALT levels among patients receiving the active drug who had entered the trial with elevated ALT levels [40,43]. Because HCV is a hepatotrophic virus, HCV protease inhibitor drug discovery has intentionally focused on targeting inhibitors to the liver and achieving high liver: plasma ratios. Thus, it is reassuring that high liver concentrations of SCH 503034 and VX-950 did not result in overt liver toxicity, which is often associated with elevated ALT levels. It is intriguing and encouraging that there was a rapid normalization of ALT levels, given that they are thought to be owing to chronic infection with HCV.
Conclusions: toward a new treatment landscape Chronic HCV infection remains a pressing medical issue. Improvement of the tolerability and efficacy of the current standard of care is an important goal and several exciting compounds with varied mechanisms are in clinical and preclinical development. Numerous points in the viral replication cycle have been targeted, and can be broadly classed as molecules targeting viral proteins essential to HCV replication (STAT-Cs) or host cell factors which aid in the replication of the virus or support or supplement the body’s innate
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immunity defense mechanism to viral pathogens (nonSTATs). To date some of the most promising results, both in terms of tolerability and efficacy, have been obtained with STAT-Cs targeting the HCV protease and polymerase and these might provide the vehicle to move toward a new era in HCV therapy. The entry of these new therapies to the market is expected to have a dramatic impact on the HCV treatment landscape. A model for this process is shown in Fig. 4 and can be subdivided into three stages starting with the current era and moving forward in time: Stage 1 is the present era in which a combination of pegylated interferon or consensus interferon and ribavirin are the only approved treatment regimens. Attempts to improve current SOC and clinical practice are primarily focused on two approaches to increase tolerability and compliance. The first approach is aimed at improving the pharmacologic properties of the current SOC. Examples of this approach include (i) improving the potency of interferon, (ii) improving the pharmacokinetic properties of interferon (e.g. Albuferon or albumin-interferon-a-2b [48,49] and (iii) decreasing the hemolytic anemia associated with ribavirin (e.g. Viramidine). The second approach is focused on optimizing the use of the individual components of SOC. Examples include (i) weight-based dosing of ribavirin, (ii) prophylactic dosing with antidepressants before starting IFN-based therapy, (iii) dosing with erythropoietin to treat ribavirin-induced hemolytic anemia and (iv) individualizing treatment by decreasing the length of dosing for patients who achieve a rapid viral response (RVR). Stage 2 is the era we will enter in approximately 3 years (2009–2010) and it will probably be marked by a major transformation in HCV treatment paradigms. On the basis of the currently available early clinical data, the new therapies in Stage 2, which will have the biggest impact are HCV protease inhibitors in combination with PEG-IFN and ribavirin or with PEG-IFN alone. The rapid and substantially increased potency of the HCV protease/PEG-IFN combination is expected to have a higher cure rate than the current SOC and shorter treatment duration. The addition of potent STAT-C drugs to HCV therapy is probably going to significantly increase the number of patients who enter therapy, who are infected with HCV genotype 1 (the hardest group to treat with current SOC), those who are currently in the ‘watch and wait’ mode, who have mild liver disease and who are waiting for more efficacious and better tolerated therapy. A second significant transformation of the HCV treatment landscape will occur in Stage 3 when all oral STAT-C combination treatment regimens are approved which will probably eliminate interferon from HCV treatment regimens. Timelines are hard to predict, but Stage 3 has the potential to launch 6–8 years from now (2012–2014) and a plethora of new oral treatment options might become available. www.drugdiscoverytoday.com
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Figure 4. Impact of STAT-Cs on the treatment landscapeSchematic depiction of stages in HCV therapy with increasing numbers of patients treated over time. 1980–1990s IFN or PEG-IFN monotherapy 1998 Stage 1: Non-STAT-C, IFN-based combination therapies with ribavirin with low efficacy and low tolerability 2009 Stage 2: STAT-C combination therapy, especially protease and polymerase inhibitors in combination with PEG-IFN 2012 Stage 3: All oral STAT-C therapy Abbreviations: SVR, sustained viral response; Rx, treatment; RBV, ribavirin; wks, weeks.
For example, new second generation HCV protease and polymerase inhibitors are probably be launched with one or more of the following properties: improved potency against wildtype and resistant virus, lower pill burden and twice a day (BID) or once a day (QD) dosing. Combination treatment with two STAT-C inhibitors such as a protease and polymerase inhibitor is also probably to be approved. In addition to HCV protease and polymerase inhibitors, new STAT-Cs with different mechanisms of action might be approved in Stage 3. Examples include, but are not limited to (i) virus entry inhibitors such as receptor binding and fusion inhibitors, (ii) inhibitors of the HCV IRES function, (iii) HCV core protein capsid assembly inhibitors and (iv) HCV helicase inhibitors. Similar to changes, which took place in the historical development of new HIV treatment regiments, the availability of STAT-Cs with different mechanisms of actions might result in the evolution of HCV patient care towards alloral combinations of targeted therapies. The current move to improve and individualize treatment is based on monitoring patient response to therapy by measuring the patients HCV viral load in their plasma. Because the field is trending towards adjusting duration of treatment based on the time it takes a patient’s plasma viral load to become ‘undetectable’, a standard assay for ‘undetectability’ needs to be implemented. Thus, it is probable that new diagnostics will be developed with one or more of the following properties: increased sensitivity to detect low HCV RNA 218
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plasma levels, (ii) increased accuracy in HCV RNA measurements, (iii) faster turn around times for HCV viral load measurements which will allow monitoring of the patient’s viral response to treatment to occur in real time, making it possible to individualize treatment options and (iii) more accurate genotype assays.
Acknowledgements We would like to thank Govinda Rao for providing chemical structures and the HCV polymerase model, James Griffith for providing the HCV protease model, and Mark Namchuk, John Randle, Jennifer Jackson and Lisa Dixon and Dasˇa Lipovsˇek for editorial assistance.
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