Viewpoint Cite This: Biochemistry XXXX, XXX, XXX−XXX
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Biochemical Basis of Vosevi, a New Treatment for Hepatitis C Published as part of the Biochemistry series “Biochemistry to Bedside” Ronald Besandre and Hung-wen Liu* Division of Chemical Biology and Medicinal Chemistry, College of Pharmacy, and Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States NS3/4A protease inhibitors (PIs) constitute the first class of DAAs that were approved for HCV treatment, the latest of which include structurally similar voxilaprevir, grazoprevir, and glecaprevir. These drugs bind reversibly to the NS3/4A protease active site and interfere with self-cleavage of the HCV polyprotein.3 Structurally, these three macrocyclic peptidomimetics include the privileged N-terminal cyclopropyl acylsulfondamide moiety and C4-heteroaryl-substituted proline core that is common to nearly all currently marketed PIs. The former moiety engages in extensive interactions with two residues of the catalytic triad: His57 and Ser39.3 The latter, via its heteroaryl substituent, interacts with additional residues outside the active site. Specific to these three PIs is a macrocycle that connects the quinoxaline substituent of the proline core to the cycloalkyl substituent of the C-terminal carbamate. The quinoxaline moiety has a unique binding mode that distinguishes it from other PIs that possess different heterocycles linked to the proline core. It interacts with catalytic triad residues Asp81 and His57 instead of Arg155, Ala156, and Asp168, three residues outside the active site that, when mutated, confer resistance to PIs.3 Despite this advantageous feature, all three of the newest PIs remain susceptible to the clinically relevant A156T mutation, which results in steric clashes between the larger side chain of threonine and the aforementioned macrocycle. Currently, the only approved nucleotide-based prodrug that inhibits the HCV NS5B RNA-dependent RNA polymerase is sofosbuvir, and it is essential to modern HCV treatment plans. NS5B inhibition by sofosbuvir is attributed to its 2′-C-methyl group, which is thought to promote chain termination via steric conflict with an incoming nucleotide. Crystal structures of the NS5B ternary complex with bound primer, template, and the active drug metabolite give credence to this mode of inhibition.4 In vitro studies have demonstrated that the S282T mutant can be generated in response to sofosbuvir treatment and confers resistance to the drug. However, this mutant is clinically uncommon. Reduced fitness and the required transversion mutation are cited as possible reasons why the S282T mutant is rarely seen in practice. Velpatasvir, also a component in recently approved Vosevi, is an inhibitor of HCV NS5A, a multifunctional, non-enzyme phosphoprotein that plays vital yet ill-defined roles in viral replication and virion assembly. Similar to other members of the NS5A inhibitor drug class, velpatasvir contains a central aromatic scaffold that is flanked by two dipeptidic moieties,
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he hepatitis C virus (HCV), which is comprised of seven distinct genotypes, is estimated to infect more than 185 million people worldwide.1 More than half of individuals exposed to HCV via percutaneous exposure to infected blood fail to clear the virus and proceed to a chronically infected state. Approximately one-third will develop progressive hepatic fibrosis leading to cirrhosis over the course of 20−30 years and eventually require liver transplantation. Chronic HCV infection is also a leading cause of hepatocellular carcinoma and is associated with a number of extrahepatic complications such as B-cell non-Hodgkin lymphomas and mixed cryoglobulinemia.1 The status of HCV as a significant global health problem has driven efforts to develop direct-acting antiviral (DAA) agents capable of achieving cure rates better than those of the interferon/ribavirin treatment established in the early 2000s. Just over the past three years alone, these efforts have been rewarded by a handful of U.S. Food and Drug Administration approvals for interferon-free, all-oral combination regimens: sofosbuvir/ledipasvir (Harvoni), ombitasvir/paritaprevir/ritonavir/dasabuvir (Viekira Pak), elbasvir/grazoprevir (Zepatier), sofosbuvir/velpatasvir (Epclusa), and, most recently, sofosbuvir/velpatasvir/voxilaprevir (Vosevi) and glecaprevir/pibrentasvir (Mavyret). Until as recently as 2010, the standard of care for hepatitis C was administration of ribavirin and pegylated interferon over a period of 24 or 48 weeks, depending on the HCV genotype. A sustained virologic response (SVR), that is, undetectable HCV RNA levels in the blood, was achieved in ∼50% of those afflicted with HCV genotype 1, the most prevalent genotype globally. Side effects of this treatment regimen include flu-like symptoms, neuropsychiatric disorders, myelosuppression, and hemolytic anemia, which often results in poor patient adherence. However, the advent of all-oral DAAs that abrogate the need for interferon has revolutionized the treatment landscape of HCV. In particular, Vosevi, one of the most recently approved combination therapies and the focus of this work, achieved SVR rates of 96−98% in two phase III clinical trials, POLARIS-1 and POLARIS-4, for previously treated HCV infection.2 Vosevi is a combination of sofosbuvir, an NS5B polymerase inhibitor, velpatasvir, an NS5A inhibitor, and voxilaprevir, an NS3/4A protease inhibitor (Figure 1). It is indicated as a retreatment option for adults with chronic HCV infection without cirrhosis or with mild, compensated cirrhosis. Specifically, it may be used under two conditions where a previous treatment regimen has failed: for HCV genotypes 1−6 if the previous regimen contained an NS5A inhibitor or for HCV genotype 1a or 3 if prior treatment included sofosbuvir but not an NS5A inhibitor.2 © XXXX American Chemical Society
Received: November 15, 2017
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DOI: 10.1021/acs.biochem.7b01157 Biochemistry XXXX, XXX, XXX−XXX
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Biochemistry
Figure 1. HCV open reading frame (ORF) of the resultant polyprotein and the structures of the small molecule drugs that comprise Vosevi. The core (C) protein and envelope glycoproteins, E1 and E2, comprise the structural components of the virus particle. Assembly of the virus also requires ion channel p7 and protease NS2. The remainder of the ORF consists of genes encoding nonstructural proteins that are necessary for viral RNA replication, three of which are currently targeted by DAAs: the NS3/4A protease, the NS5A phosphoprotein, and the NS5B RNA-dependent RNA polymerase. The three components of Vosevi target these three HCV proteins, as indicated by the inhibitory arrows. The arrows originating from the two proteases, NS2 and NS3, denote self-cleavage sites.
which results in a roughly symmetrical molecule.5 Its structure implies a binding mode involving dimers of NS5A. Co-crystal structures of inhibitor-bound NS5A are not currently available; however, NS5A dimers of domain I have been crystallized, providing some evidence of this mode of binding. Furthermore, several mutants that confer resistance to NS5A inhibitors have been mapped to domain I: residues 28−32, residues 54−58, and residues 92 and 93.5 The paucity of structural data and challenges in designing biochemical assays to probe NS5A function have complicated inhibitor design. In spite of these limitations, velpatasvir and other NS5A inhibitors, whose structures were optimized via phenotypic screening, display potency at picomolar levels in HCV replicon-based cell assays. The observed level of potency may be due to interference with NS5A function at multiple stages of the HCV life cycle. The HCV drug market has grown tremendously since the approval of the first DAAs in 2011. Interferon-based regimens, once the cornerstone of treating chronic HCV, have largely been superseded by more effective combination DAA therapies such as the recently approved Vosevi and Mavyret. With regard to the former, Vosevi has shown effectiveness in the retreatment of patients who possessed NS3 and/or NS5A resistance-associated substitutions (RAS) at baseline.2 In the future, it will be important to elucidate the molecular underpinnings of resistance to DAA therapy. This is especially relevant for NS5A, the least understood HCV drug target. To consider the structural effect of mutations on inhibitor potency, the experimental tractability of NS5A will need to be improved if drug design is to become more structure-based.
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Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Thrift, A., El-Serag, H., and Kanwal, F. (2016) Global epidemiology and burden of HCV infection and HCV-related disease. Nat. Rev. Gastroenterol. Hepatol. 14, 122−132. (2) Bourlière, M., Gordon, S., Flamm, S., Cooper, C., Ramji, A., Tong, M., Ravendhran, N., Vierling, J., Tran, T., Pianko, S., Bansal, M., de Lédinghen, V., Hyland, R., Stamm, L., Dvory-Sobol, H., Svarovskaia, E., Zhang, J., Huang, K., Subramanian, G., Brainard, D., McHutchison, J., Verna, E., Buggisch, P., Landis, C., Younes, Z., Curry, M., Strasser, S., Schiff, E., Reddy, K., Manns, M., Kowdley, K., and Zeuzem, S. (2017) Sofosbuvir, velpatasvir, and voxilaprevir for previously treated HCV infection. N. Engl. J. Med. 376, 2134−2146. (3) Götte, M., and Feld, J. (2016) Direct-acting antiviral agents for hepatitis C: structural and mechanistic insights. Nat. Rev. Gastroenterol. Hepatol. 13, 338−351. (4) Appleby, T., Perry, J., Murakami, E., Barauskas, O., Feng, J., Cho, A., Fox, D., Wetmore, D., McGrath, M., Ray, A., Sofia, M., Swaminathan, S., and Edwards, T. (2015) Structural basis for RNA replication by the hepatitis C virus polymerase. Science 347, 771−775. (5) Belema, M., Lopez, O., Bender, J., Romine, J., St. Laurent, D. R., Langley, D., Lemm, J., O’Boyle, D., Sun, J., Wang, C., Fridell, R., and Meanwell, N. (2014) Discovery and development of hepatitis C virus NS5A replication complex inhibitors. J. Med. Chem. 57, 1643−1672.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hung-wen Liu: 0000-0001-8953-4794 Funding
This work was supported by grants from the National Institutes of Health (GM035906 and GM040541) and the Welch Foundation (F-1511). B
DOI: 10.1021/acs.biochem.7b01157 Biochemistry XXXX, XXX, XXX−XXX
