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| The hepatitis C virus (HCV) epidemic continues to be a serious health threat, infecting over 170 million people worldwide (Lin et al 2005). Three to four million people are newly infected each year (Ressink et al. 2006). The infection is often asymptomatic in its early stages, but the majority of HCV-infected individuals develop chronic hepatitis over time, which eventually advances to liver scarring (cirrhosis), and liver cancer (hepatocellular carcinoma) (Moriishi and Matsuura 2007) figure citation. The epidemiology of the virus is not well understood, but within the United States the prevalence of the virus spiked in the 1960s. This spike can be attributed largely to blood-to-blood transmission among injecting drug users (IDUs), and the virus continues to prevail globally because of the widespread increase in needle sharing drug abuse (Simmonds 2000).
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| Since 1998, an antiviral drug that combines interferon alpha-2b and ribavirin has been marketed and used to treat HCV infections (Keeffe 2006). Interferons function by disturbing viral replication through immune response stimulation. More specifically, they activate immune cells, such as natural killer cells and macrophages; they increase recognition of infection by up-regulating antigen presentation to T lymphocytes; and they increase the ability of uninfected host cells to resist new infections (Hunt 2009). The drug’s other component, Ribavirin, is a prodrug which when metabolized resembles purine RNA nucleotides. In this form, it interferes with the RNA metabolism required for viral replication. The mechanism of interference is not well known, but some mechanisms have been proposed (Feld and Hoofnagle 2005). Side effects of the drug range from mild flulike symptoms to severe symptoms like hair loss, bloody stools, depression and suicidal tendencies (AHFS 2010). While the interferon/ribavirin combination drug sustained response rates are 54-56%, these results imply that 40-50% of patients do not have lasting improvements with treatment (Feld and Hoofnagle 2005). The development of a novel antiviral drug, telaprevir (VX-950), attempts to improve care for infected individuals.
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| Vertex, the pharmaceutical drug company that produced telaprevir, plans to submit a New Drug Application to the U.S. Food and Drug Administration in the fourth quarter of 2010. To date, the telaprevir clinical development program is the largest conducted for any investigational direct-acting antiviral hepatitis C therapy. In fact, more than 2,500 people with hepatitis C have received telaprevir-based regimens as part of Phase 2 studies and the Phase 3 ADVANCE, ILLUMINATE, and REALIZE studies, outlined in figure citation. The candidate drug, telaprevir, is an oral protease inhibitor (PI). It prevents the proper function of a serine protease, a unique HCV enzyme that is essential to viral replication (Vertex 2010).
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| To better understand the mechanism of action behind telaprevir, it is useful to investigate the HCV virus and its lifecycle. HCV is classified in the Hepacivirus genus within the Flaviviridae family. HCV has a positive strand RNA genome that is composed of a 5’-non-coding region (NCR), which includes an internal ribosome entry site (IRES), an open reading frame that encodes structural and non-structural proteins, and a 3’-NCR. The structural proteins, which form the virion, include the core protein and the envelope proteins, E1 and E2. The non-structural proteins include the p7 ion channel, the NS2-3 protease, the NS3 serine protease and RNA helicase, the NS4A polypeptide, the NS4B and NS5A proteins and the NS5B RNA-dependent RNA polymerase (RdRp) figure citation nrmicro1645-53.jpg.
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| The primary host cells of HCV are hepatocytes, liver cells. However, HCV infections have been reported in B cells, dendritic cells and other cell types. The receptor molecule to which HCV binds is unknown, but several candidate molecules have been proposed: CD81, a tetraspanin protein that is found on the surface of many cell types, including hepatocytes, the LDL receptor (LDLR), scavenger receptor class B type I (SR-BI), and claudin-1. After binding to the host cell receptor, HCV enters the cell by clathrin-mediated endocytosis, by passage through endosomal, low pH compartments and presumably endosomal membrane fusion. Next, the genome is released into the cytosol; very little is known about the uncoating process. Translation initiation occurs in a binary complex that forms between the IRES and the 40 S ribosomal subunit. This complex develops further to form full membrane-associated replication compartments. In the compartments, translation and protein processing take place. During the polyprotein processing, cellular and viral proteases process both mature structural and non-structural proteins. The structural proteins and the p7 polypeptide are processed by the endoplasmic reticulum (ER) signal peptidase, whereas the non-structural proteins are processed by two viral proteases, the NS2-3 protease, and the NS3-4A serine protease (Moradpour 2007). The NS3-4A serine protease is the target molecule of telaprevir.
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| NS3 is a multifunctional protein, with a serine protease located in the N-terminal one-third and an RNA helicase/NTPase located in the C-terminal two-thirds of the protein. The NS4A protein is a cofactor for the NS3 serine protease. Its central portion is integrated as an important part of the enzyme core, and its N-terminal portion is responsible for membrane association of the NS3-4A complex. Its catalytic triad is formed by His57, Asp81, and Ser139 figure citation nrmicro 1645-f4.jpg. It has recently been shown that the NS3-4A serine protease cleaves and thereby inactivates two crucial adaptor proteins in innate immune sensing, namely Triff77 (aka TICAM-1) and Cardif78 (aka MAVS79, IPS-1 and VISA81) (Moradpour 2007). These findings have major implications for the persistence and pathogenesis of HVC, and are highly significant in the development of protease inhibiting drugs.
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| However, the NS3-4A serine protease has an unusually shallow substrate-binding pocket and as a result requires extended interaction surfaces with the substrate. Determinants of substrate specificity for drug development include an acidic amino acid residue at the P6 position, and a p1 cysteine or threonine, and an amino acid residue with a small side chain (alanine or serine) at the P1’ positions. Despite these challenges, Vertex developed telaprevir.
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| As mentioned before, more than 2,500 people with genotype 1 hepatitis C who had not been treated for their disease previously or had been treated before but did not achieve a lasting viral response have been treated with some dosing regime of telaprevir. In the proceeding sections, I will review the findings of three clinical studies: Reesink et al., Forestier et al., and Hezode et al., each of whom evaluated some aspect (antiviral activity, pharmacokinetics, and/or the safety) of telaprevir treatments in individuals infected with HCV.
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| Reesink et al. performed a phase I, placebo-controlled, double-blind study that evaluated the antiviral activity, pharacokinetics, and safety of telaprevir (VX-950) alone. More specifically, the aim of the study was to evaluate the safety and tolerability of ascending multiple doses of VX-950 in patients with chronic hepatitis C. To be eligible patients: were male or female, between the ages of 18 and 65 years old, had body mass indexes (BMIs) between 18.5 and 29.0 kg/m2 (males) or 18.5 and 32.5 kg/m2 (females), had a HCV RNA level ≥1 X 105 IU/mL, had HCV genotype 1 (any subtype), and had an alanine aminotransferase (ALT) concentration ≤4 times the upper limit of normal.
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| Three panels of patients that met the aforementioned criteria were enrolled. Panels 1 and 3 comprised of 10 patients on VX-950 and 2 on placebo. Panel 2 comprised of 8 patients on VX-950 and 2 on placebo. Patients were randomly assigned to VX-950 or placebo, both were administered as an oral suspension. For 14 consecutive days, patients received either 450 mg (panel 1) or 750 mg (panel 2) VX-950 every 8 hours (q8h), 1250 mg (panel 3) VX-950 every 12 hours (q12h), or a matching placebo regimen (Reesink et al. 2006).
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| The antiviral activity of VX-950 was assessed by measuring plasma HCV RNA levels. The drug’s pharmacokinetics was assessed by measuring the concentration of VX-950 in the plasma. Patients were monitored for safety and tolerability at regular intervals from the start of dosing through the experiments competition. Safety assessments included physical examinations and vital signs, clinical laboratory tests, 12-lead digital electrocardiograms (ECG), and questioning about adverse side effects (Reesink et al. 2006).
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| Reesink et al. found rapid and substantial viral decline in patients treated with VX-950 for fourteen days: all patients treated with VX-950 had at least a 2-log10 decrease from baseline in HCV RNA. HCV RNA levels decreased steadily in the 450-g q8h and 1250-mg q12h groups until day seven. Between day 7 and 14, HCV RNA levels in these groups actually increased, demonstrating viral breakthrough. Reesink et al. hypothesized that this breakthrough was related to the selection of HCV variants with decreased sensitivity to VX-950.
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| The median HCV RNA value decreased through the entire dosing period in the 750-mg q8h group. Moreover, HCV RNA decreased below the limit of detection (10IU/mL) in 3 patients: 1 in the 450-mg q8h group and 2 in the 750-mg q8h group. Three additional patients had HCV RNA levels that were below the lower limit of quantification (30 IU/mL) at day 14 figure 1 0.gif reference from Reesink et al (Reesink et al 2006).
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| Regarding the pharmacokinetics of the drug, VX-950 accumulated on multiple dosing with a median accumulation index of 1.8. The initial rapid decline in HCV RNA was related to maximal exposure to VX-950, and the second phase of viral decline was sustained by trough concentrations of the drug (Reesink et al. 2006).
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| There were no severe or serious adverse side effects and no breaks in the dosing regime or discontinuations due to adverse events. The most frequent adverse events were headache, flatulence, diarrhea, frequent urination, dry mouth, fatigue, abdominal pain, nausea, back pain, dry skin, and common colds. No clinically significant laboratory results in hematology or clinical chemistry tests occurred in any of the patients. Furthermore, no patients had clinically significant changes in ECGs from the predosing baselines (Reesink et al. 2006).
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| Ultimately, VX-950 was effective in reducing viral load in a population in which 79% of patients had not responded to prior interferon based regimens (Reesink et al. 2006). Where interferon based treatments failed to achieve a lasting viral response, VX-950 was successful. Interestingly, clinicians and researchers have begun to study the effects of a combination treatment, part old interferon/ribavirin and part new VX-950. One such study was conducted by Forestier et al. who investigated the antiviral activity of telaprevir combined with peginterferon alpha-2a in patients with chronic hepatitis C.
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| Much of Forestier et al.’s study design and organization is the same as Reesink et al. The experiment’s participant criteria were entirely the same as Reesink et al., with the addition that only individuals who had not previously been treated for hepatitis C were eligible to participate. The study was placebo-controlled for telaprevir; peginterferon alfa-2a treatment was open-labeled. Twenty total patients were enrolled and randomized into three treatment panels: 4 patients received a placebo orally q8h for 14 days and peginterferon alfa-2a via subcutaneous injection once weekly for 2 weeks; 8 patients received telaprevir orally q8h for 14 days; and 8 patients received telaprevir orally q8h for 14 days and peginterferon alfa-2a once weekly for 2 weeks. The first telaprevir dose was a 1250-mg loading dose, and the following doses were 750-mg q8h. Subcutaneous injects of interferon alfa-2a were at a dose of 180µg on days 1 and 8.
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| Similar to the findings of Reesink et al., Forrestier et al. observed viral decline in two phases. Overall, patients treated with telaprevir and interferon alfa-2a had greater decreases in HCV RNA and more sustained declines in comparison to patients with telaprevir only treatments. The median change in HCV RNA from baseline to day 15 was -1.09 log10 in the placebo and peginterferon alfa-2a group and -3.99 log10 in the telaprevir group. The greatest decrease in HCV RNA was in the telaprevir and peginterferon alfa-2a combination group, from baseline to day 15 RNA was -5.49 log10.
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| The second phase of decline was more sustained in the telaprevir and peginterferon alfa-2a group than in the telaprevir alone group. In the telaprevir group, 4 patients had continued decline, 2 patients had viral plateau, and 2 had viral rebound whereas, all patients in the telaprevir and peginterferon alfa-2a group had continued decline during the entire drug dosing period. In the telaprevir group, 1 patient had undetectable HCV RNA levels at day 15. In comparison, 6 patients in the telaprevir and peginterferon alfa-2a group were below the lower limit of quantification in HCV RNA levels, and 4 patients had undetectable HCV RNA levels reference figure 2 from Forestier et al. nfig002.gif paper.
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| Taken together, the findings of Reesink et al. and Forestier et al. confirm that telaprevir efficiently decreases HCV replication, as measured by viral RNA, through protease inhibition. Forestier et al. adds to the discussion the notion of combing new and old antiviral treatments (telaprevir and interferon alfa-2a), and presents a convincing study that supports the administration of a combined treatment for even greater antiviral activity.
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