Resistance to anti-HCV protease inhibitors
Highlights
► The NS3 protease inhibitors telaprevir (TVR) and boceprevir (BOC) are now approved for the treatment of genotype 1 HCV in North America and Europe. ► Resistance variants are selected rapidly with monotherapy. ► Key resistance residues for TVR and BOC include R155, A156, V36 and T54. ► TVR and BOC are therefore used in combination with peginterferon-α and ribavirin (RBV). ► Interferon-free treatment regimens are in development.
Introduction
Chronic infection with the hepatitis C virus (HCV) is estimated to affect 130–170 million individuals worldwide [1]. Chronic hepatitis C causes liver failure and liver cancer, and is the leading cause of liver transplantation in the Western world. These complications may be prevented by curative antiviral therapy. Unfortunately the standard-of-care treatment for the past decade, peginterferon-α (pegIFN) and ribavirin (RBV), cures only 40–50% of patients with genotype 1 HCV, the most common genotype in North America and Europe. More effective therapy has been a clinical priority.
Great effort has been invested in developing novel therapies that directly target the HCV lifecycle, collectively termed direct acting antiviral agents (DAAs). These candidates now include direct inhibitors of the HCV non-structural (NS) 3/4a protease, the NS5B polymerase and the NS5a phosphoprotein. The most advanced class are the NS3 protease inhibitors (PIs), and the first two agents, telaprevir [2, 3] and boceprevir [4, 5], were approved as first line agents for the treatment of chronic genotype 1 HCV in 2011.
The development of telaprevir and boceprevir has been instructive. As for the treatment of HIV, monotherapy is complicated by the rapid emergence of resistant variants. This occurs as a result of the high replication rate of HCV, the error-prone nature of the replication cycle, and the lack of a viral proof-reading mechanism. In fact, most single-mutations and double-mutations are likely to occur spontaneously before treatment, existing as minor populations within the viral quasispecies. Treatment with NS3 inhibitors therefore remains reliant on combination with an effective pegIFN and RBV backbone. Not all individuals are responsive to pegIFN and RBV therapy however, placing them at high risk for DAA treatment failure owing to functional monotherapy. The clinical significance of the HCV resistance-associated variants that emerge in this setting is not yet known. It is therefore timely to review the current literature concerning resistance to HCV protease inhibitors, and to consider the important issues confronting the field.
Section snippets
The HCV replication strategy promotes the emergence of resistant variants
The rapid emergence of drug resistance occurs as a consequence of the HCV replication strategy. The virus replicates at a rapid rate to produce 1012 virion particles/day (approximately 100-fold more than HIV) [6]. The RNA polymerase has poor fidelity and lacks an exonucleolytic proof-reading mechanism. Replication is inherently error-prone and is estimated to have an error-rate of in the order of 10−3 to 10−5 mutations per nucleotide per genomic replication (corresponding to a natural
The HCV NS3/4a protease
The HCV NS3 protein is a multifunctional protein that plays an essential role in the HCV life cycle. The NS3 protein possesses two major activities. Within the N-terminal third (approximately 180 amino acids) of the protein resides a serine-type protease that mediates the cis-cleavage of the NS3/4A site and subsequent cleavage of NS4A/4B, NS4B/5A and NS5A/5B junctions to liberate the HCV non-structural (NS) proteins for the assembly of the HCV replication complex. Approximately 440 amino acids
Development of NS3 protease inhibitors
The essential role of the HCV NS3/4a protease in the HCV life cycle, and its enzymatic activity, made it a prime target for the development of antiviral drugs. The crystal structure of the NS3 helicase domain was resolved in the late 1990s [15, 16, 17, 18, 19]. The enzyme has an unusually shallow substrate-binding pocket, located on the surface of the protein. The shallow catalytic site made the development of inhibitors a difficult task, and the traditional approach of screening libraries of
Resistance to NS3 protease inhibitors
The shallow catalytic site of the HCV NS3 protease allows minor structural modifications to interfere with substrate binding, promoting resistance. Single amino acid changes have been identified for all protease inhibitors that are associated with reduced drug sensitivity in vitro. Therefore, protease inhibitors have a low genetic barrier to resistance.
Clinical significance of protease inhibitor resistance associated variants
The development of telaprevir and boceprevir has already meant a number of important lessons concerning drug resistance have been learnt.
Protease inhibitor monotherapy is clearly ineffective, owing to the rapid and universal selection of resistance associated variants within days [31]. The rapidity with which resistant variants are selected suggests that resistance associated variants are present before treatment is started. Indeed, mathematical modeling predicts that all possible single and
Conclusion
HCV NS3 protease inhibitors have now been approved for the treatment of genotype 1 HCV. They have a low genetic barrier to resistance and must be used in combination with pegIFN and RBV. Indeed, the IFN responsiveness of a patient, and the ability of the pegIFN and RBV backbone to suppress pre-existing resistance associated variants determine the efficacy of telaprevir/boceprevir regimens. In patients who fail treatment, reversion to wild-type HCV is observed over time, and there is not yet
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgement
We are grateful to Dr. Jason Roberts of the Victorian Infectious Diseases Reference Laboratory, for assistance provided during the preparation of this manuscript.
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Cited by (35)
Pan-NS3 protease inhibitors of hepatitis C virus based on an R<sup>3</sup>-elongated pyrazinone scaffold
2018, European Journal of Medicinal ChemistryCitation Excerpt :These medications consist of combinations of two or three DAAs, e.g. Harvoni™ (ledipasvir [16], an NS5A inhibitor and 2). Due to the error prone viral polymerase, a major challenge in anti-HCV drug discovery is the resistance-associated amino acid substitutions that emerge in the target protein under pressure of selective antiviral drugs [17,18]. For example, the amino acid substitutions R155K/T/Q confer resistance to all NS3 protease inhibitors approved and in clinical trials [19].
Protease Inhibitors Drug Resistance Mutations in Turkish Patients with Chronic Hepatitis C
2016, International Journal of Infectious DiseasesCitation Excerpt :On the other hand, when SMV, a second generation PI is used in combination with PEG-IFN/RBV similar SVR rates can be obtained. HCV has a high replication capacity (1010-12 virion/day) and its polymerase activity does not have error-correcting ability.2,8,15 Formation of variants due to mutations occurring during viral replication may lead to a decrease in the sensitivity to antiviral agents.16–18
Discovery of pyrazinone based compounds that potently inhibit the drug-resistant enzyme variant R155K of the hepatitis C virus NS3 protease
2016, Bioorganic and Medicinal ChemistryEfficacy and safety of simeprevir for chronic hepatitis virus C genotype 1 infection: A meta-analysis
2016, Clinics and Research in Hepatology and GastroenterologyCitation Excerpt :But treatment with some agents such as boceprevir and telaprevir is limited to increased rates of drug-related adverse events including anemia, rash and nausea [3–5]. Moreover, we may encounter relatively rapid appearance of drug resistance in patients who do not achieve SVR [6]. Simeprevir (TMC-435; Janssen and Medivir) is a direct-acting antiviral agent, which has been approved in Japan, North America and Europe for the treatment of patients with chronic HCV infection.
High-resolution genetic profile of viral genomes: Why it matters
2015, Current Opinion in VirologyCitation Excerpt :Studies have shown that patients often harbor preexisting drug resistant variants as minor variants within the population, which then can arise and rapidly dominate the population under a drug selection pressure [32]. Clinical observations indicate the emergence of drug resistance within days of initiating therapy with protease inhibitors, suggesting drug-resistant polymorphisms preexist in vivo [33]. This genetic plasticity drives the ongoing challenge of antiviral resistance in HCV therapy.
Vinylated linear P2 pyrimidinyloxyphenylglycine based inhibitors of the HCV NS3/4A protease and corresponding macrocycles
2014, Bioorganic and Medicinal ChemistryCitation Excerpt :A consequence of the development of direct-acting antiviral HCV NS3/4A protease inhibitors is the possible emergence of resistant mutants which is a common setback among viral diseases in general. In vitro and in vivo studies have revealed mutations leading to protease variants with amino acid substitutions particularly in areas which interact with the P2 region of the protease inhibitor, both in the product-based and electrophilic class of inhibitors.10,11,28,29 A common attribute among the inhibitors that have entered clinical trials so far, is that they carry a proline or proline mimic in the P2 position, the latter in the case of simeprevir (Fig. 1).