Improved understanding of HCV replication has allowed for the development of new therapeutic agents that target enzymes directly. HCV is a flavivirus with an RNA genome encoding a polyprotein. After HCV enters hepatocytes, translation takes place to produce the structural polyprotein which must then be cleaved into functional proteins.
Several non-structural proteins ( NS2-NS5 ) mediate these intracellular functions and have proven promising therapeutic targets for DAAs ( direct acting antivirals ).
Current HCV protease inhibitors
Two HCV protease inhibitors ( PIs ), Boceprevir ( Victrelis ) and Telaprevir ( Incivo, Incivek ), are now licensed for chronic HCV treatment.
The non-structural (NS)-3/4A HCV protease is responsible for cleaving the HCV viral polyprotein into mature proteins. Both drugs bind reversibly to the NS3 active site, blocking polyprotein cleavage and preventing HCV replication.
In addition to this direct antiviral action, inhibition of NS3 protease may also act to restore the hepatocyte Interferon-signalling pathways.
Both drugs were designed using genotype-1 HCV-specific in vitro systems, and have limited activity against other HCV genotypes.
When used as monotherapy, virological resistance develops rapidly to Boceprevir and Telaprevir, necessitating combination with Peg-Interferon / Ribavirin ( PEG-IFN/RBV ). The pivotal trials of Boceprevir or Telaprevir triple therapy demonstrated significantly increased rates of SVR compared with PEG-IFN/RBV in both treatment-naïve and treatment experienced HCV genotype-1 infected patients.
Novel HCV protease inhibitors
Beyond Telaprevir and Boceprevir, a number of new NS3A protease inhibitors are being developed in phase II/III trials. The next protease inhibitor to market will likely be TMC-435 ( Simeprevir; Olysio ), which is dosed once daily, offering a benefit over current generation NS3/4A protease inhibitors.
A phase II trial of treatment naïve, genotype-1 individuals used Simeprevir with PEG-IFN/RBV for 24 of 48 weeks total therapy, guided by HCV RNA at weeks 4 to 20. 68–76% of patients achieved RVR ( rapid virological response ), of whom 88–95% achieved SVR ( sustained virological response ). 79–86% of patients were eligible for short duration ( 24 weeks ) therapy. The control group in this study also had a high SVR response, so the overall virological response in this cohort may have been over-estimated. It had a favourable side effect profile, with similar rates of rash and anaemia compared with the control group.
MK-5712 is a potent second generation NS3 protease inhibitor in early stage development. It requires once daily dosing, and has efficacy against HCV genotypes 1–6 in vitro.
MK-5172 also has activity against a number of variants that are resistant to other protease inhibitors in development.
There are several other protease inhibitors in development. These new protease inhibitors will likely replace the first generation PIs due to their improved side effect profile and simplified use, regardless of any additional improvement in SVR.
HCV NS5B polymerase inhibitors
NS5B polymerase inhibitors can be classified as nucleoside inhibitors ( NI ) or non-nucleoside inhibitors ( NNIs ). Nucleoside inhibitors are potent and are active against all HCV genotypes, as the HCV catalytic site is conserved across genotypes. They have a good resistance profile and NI-resistant HCV variants have displayed very poor fitness to date. The most promising nucleoside inhibitor at present is GS-7977 ( Sofosbuvir; Sovaldi ) which has entered phase III development for genotype-1 HCV in combination with PEG-IFN/RBV.
Sofosbuvir has also entered phase III development as IFN-free treatment for genotype-2/-3 HCV.
Mericitabine ( RG7128 ) is a second nucleoside inhibitor in advanced clinical development. In one study, HCV treatment naive patients infected with HCV genotypes-1/-4 received response guided Mericitabine plus PEG-IFN/RBV or PEG-IFN/RBV alone for at least 24 weeks. Virological response 12 weeks post-therapy ( SVR12 ) was 76% in the intervention group, compared with 56% in the standard therapy group.
Its antiviral potency ( 91% RVR ) has been confirmed in other phase II studies.
Non-nucleoside inhibitors bind to allosteric sites around the active site of the NS5B enzyme, induce conformational changes and down-regulate the polymerase's activity. There are multiple non-nucleoside inhibitors that have entered clinical development, including Tegobuvir ( GS-9190 ) in phase II development, and others in earlier stages including Filibuvir and Silibinin.
The class-wide limitations of non-nucleoside inhibitors to date include their relatively weak potency and rapid emergence of resistance. They may have a role in combination DAA regimens.
HCV NS5A inhibitors
NS5A replication complex inhibitors are potent, pan-genotypic antivirals. Daclatasvir ( Daklinza ) is a potent NS5A inhibitor with efficacy in HCV genotype-1 treatment naïve and experienced patients.
In a phase II study of treatment naïve patients, Daclatasvir given in combination with PEG-IFN/RBV versus standard therapy had an SVR12 of 83–92% versus 25%, respectively.
Natural polymorphisms at the HCV NS5A gene conferring Daclatasvir resistance have been identified from gene bank studies and have been shown to be clinically relevant in vivo. Further study is underway to determine how these primary resistance mutations might affect the NS5A class.
HCV resistance associated with DAA therapy
The high replication rate and error-prone HCV polymerase give rise to naturally occurring resistance-associated variants ( RAVs ). In the setting of polymerase inhibitor monotherapy, RAVs are selected within days, leading to virological breakthrough.
Single nucleotide substitutions have been identified that are associated with resistance to all polymerase inhibitors in development, and the R155/A156 substitutions are cross-resistant for all polymerase inhibitors.
Combination with PEG-IFN/RBV can prevent mutants from emerging. Hence current polymerase inhibitors trials underway in phase III are using triple therapy with PEG-IFN/ RBV. ( Xagena )
Doyle JS et al, Br J Clin Pharmacol 2013; 75: 931–943