Saturday, December 2, 2017

Mavyret: A Pan-Genotypic Combination Therapy for the Treatment of Hepatitis C Infection

Published as part of the Biochemistry series “Biochemistry to Bedside”
Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, Massachusetts 01605, United States
DOI: 10.1021/acs.biochem.7b01160
Publication Date (Web): December 1, 2017
Copyright © 2017 American Chemical Society

Mavyret: A Pan-Genotypic Combination Therapy for the Treatment of Hepatitis C Infection
Ashley N. Matthew, Nese Kurt Yilmaz, and Celia A. Schiffer

Hepatitis C virus (HCV), a virus that infects more than 180 million people worldwide, is the causative agent of chronic liver disease, which often progresses to fibrosis, liver cirrhosis, and hepatocellular carcinoma (HCC). According to the World Health Organization, almost half a million patients infected with HCV die each year from cirrhosis and HCC alone. In the last several years, treatment of HCV infections has been revolutionized by the development of small molecular inhibitors that target essential proteins encoded by the viral genome. These inhibitors, known as direct-acting antivirals (DAAs), have improved treatment option and outcomes and eliminated the need for interferon injections. However, the emergence of resistance-associated variants (RAVs) and high genetic variation among the six distinct genotypes of the virus have been presenting challenges, even leading to treatment failure.

Newer all-oral DAA combination regimens for HCV infection consist of inhibitors that target the NS3/4A, NS5A, and NS5B viral proteins. Of note, NS3/4A protease inhibitors have become a mainstay of treatment as most new therapies contain an inhibitor from this class. While highly effective against other genotypes, treatment of genotype 3 infections has been the most challenging, especially in patients who failed previous therapy or have cirrhosis. Recently, AbbVie received Food and Drug Administration (FDA) approval for one of the first pan-genotypic combination therapies, Mavyret, consisting of glecaprevir and pibrentasvir, an NS3/4A protease and an NS5A inhibitor, respectively (Figure 1). Given the excellent pan-genotypic response and safety profile in patients, Mavyret was approved for the treatment of genotypes 1–6 in patients without cirrhosis, or with compensated cirrhosis. In patients with non-cirrhotic chronic HCV who were treatment-naïve or had previously been treated with pegylated interferon or ribavirin, the sustained virological response (SVR) rate was 83–100% across all genotypes.(1) In treatment-naïve patients with compensated liver disease, 99% of patients achieved SVR with a 12-week course.(2) Mavyret was approved as an 8-week course for treatment-naïve patients without cirrhosis, shortening the previous standard of care by an additional 4 weeks.

One component of the Mavyret combination, pibrentasvir (ABT-530), has excellent potency across all HCV genotypes and retains potency against common RAVs. Pibrentasvir had EC50 values across genotypes ranging from 1.4 to 5 pM against the HCV replicon in antiviral assays.(3) Under the selective pressure of inhibitors, RAVs emerge at positions 28, 30, 31, and 93 in the NS5A protein. In fact, all current NS5A inhibitors are susceptible to mutations at Tyr93. In vitro studies indicate pibrentasvir also selects these mutations, including Y93H, that confer resistance to other NS5A inhibitors.(3) However, pibrentasvir maintained good potency against many single-site NS5A mutations, suggesting double or triple mutants need to emerge to confer high levels of resistance against this inhibitor.

The other component of Mavyret, glecaprevir (ABT-493), is a P2–P4 macrocyclic NS3/4A protease inhibitor with subnanomolar to low nanomolar activity against all genotypes, including genotype 3.(4) NS3/4A protease inhibitors are often susceptible to single-site mutations at residues Arg155, Ala156, and Asp168. Most if not all protease inhibitors are susceptible to mutations at Asp168, which are often present in patients who fail therapy with a protease inhibitor. Notably, this active site residue is not conserved in genotype 3 and is Gln168 instead, contributing to the natural resistance of genotype 3 to most treatments. While potent against 168 variations, including genotype 3, glecaprevir is highly susceptible to A156T and A156V mutations. We have shown that inhibitors containing P2–P4 macrocycles, as in glecaprevir, are susceptible to changes at Ala156, as substitutions with a larger side chain result in steric clash with the inhibitor’s macrocycle.(5) Luckily, mutations at Ala156 do not occur alone because of reduced replicative capacity; however, additional mutations could restore the enzymatic fitness, which can lead to clinically viable multi-mutant resistant variants.

Thus, both components of Mavyret have good resistance profiles against wild type genotypes and single-mutant variants of HCV. What needs to be considered is the emergence of double, triple, or other multi-mutant variants that may have high levels of resistance to one or both components of this combination. Such multi-mutant variants potentially pose a threat to the longevity and success of HCV treatment. There are already double- and triple-mutant variants that have been isolated from patients who failed therapy with previously FDA-approved combination therapies. Considering the similarity in the inhibitor scaffolds and modes of action, there is a danger that these variants may be cross-drug resistant and not respond to any current treatment option, including Mavyret. As new drugs and combinations are developed, it will be important to understand the mechanisms of resistance for these multi-mutant variants and incorporate those insights into drug design. Rather than concentrating all effort into inhibitors from the same class with highly similar scaffolds, diversifying the arsenal of DAAs and considering triple-combination therapy may be required to avoid cases of incurable HCV infection.

The approval of Mavyret dual-combination therapy marks another milestone in the treatment of HCV infections. There had been a major effort to develop an all-oral combination therapy with activity against all genotypes. With the approval of Mavyret, this goal has been met. The newer-generation inhibitors and various combinations provide treatment options for patients and improve SVR rates across all genotypes. For many cases, Mavyret has decreased the standard of care from 24 to 8 weeks. More importantly, treatment options for patients with compensated liver disease are now available. One major remaining concern is the possible emergence of drug resistance. The newer inhibitors have better activity against single-site RAVs, but highly resistant multi-mutant strains may become clinically relevant. Preventing the emergence and spread of cross-resistant variants and developing inhibitors with improved potency against such variants may be the next challenge.

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