Hepatitis C NS5B polymerase inhibitors: 4,4-Dialkyl-1-hydroxy-3-oxo-3,4-dihydronaphthalene-3-yl benzothiadiazine derivatives

Catalytic 1,2-dihydronaphthalene and E-aryl-diene synthesis via CoIII-Carbene radical and o-quinodimethane intermediates

Catalytic synthesis of substituted 1,2-dihydronaphthalenes via metalloradical activation of o-styryl N-tosyl hydrazones ((E)-2-(prop-1-en-1-yl)benzene-N-tosyl hydrazones) is presented, taking advantage of the intrinsic reactivity of a cobalt(iii)-carbene radical intermediate. The method has been successfully applied to a broad range of substrates with various R1 substituents at the aromatic ring, producing the desired ring products in good to excellent isolated yields for substrates with an R2 = COOEt substituent at the vinylic position (∼70-90%). Changing the R2 moiety from an ester to other substituents has a surprisingly large influence on the (isolated) yields. This behaviour is unexpected for a radical rebound ring-closure mechanism, and points to a mechanism proceeding via ortho-quinodimethane (o-QDM) intermediates. Furthermore, substrates with an alkyl substituent on the allylic position reacted to form E-aryl-dienes in excellent yields, rather than the expected 1,2-dihydronaphthalenes. This result, combined with the outcome of supporting DFT calculations, strongly points to the release of reactive o-QDM intermediates from the metal centre in all cases, which either undergo a 6π-cyclisation step to form the 1,2-dihydronaphthalenes, or a [1,7]-hydride shift to produce the E-aryl-dienes. Trapping experiments using TEMPO confirm the involvement of cobalt(iii)-carbene radical intermediates. EPR spectroscopic spin-trapping experiments using phenyl N-tert-butylnitrone (PBN) confirm the radical nature of the catalytic reaction.

In vitro phenotypic characterization of hepatitis C virus NS3 protease variants observed in clinical studies of telaprevir

Telaprevir is a linear, peptidomimetic small molecule that inhibits hepatitis C virus (HCV) replication by specifically inhibiting the NS3·4A protease. In phase 3 clinical studies, telaprevir in combination with peginterferon and ribavirin (PR) significantly improved sustained virologic response (SVR) rates in genotype 1 chronic HCV-infected patients compared with PR alone. In patients who do not achieve SVR after treatment with telaprevir-based regimens, variants with mutations in the NS3·4A protease region have been observed. Such variants can contribute to drug resistance and limit the efficacy of treatment. To gain a better understanding of the viral resistance profile, we conducted phenotypic characterization of the variants using HCV replicons carrying site-directed mutations. The most frequently observed (significantly enriched) telaprevir-resistant variants, V36A/M, T54A/S, R155K/T, and A156S, conferred lower-level resistance (3- to 25-fold), whereas A156T and V36M+R155K conferred higher-level resistance (>25-fold) to telaprevir. Rarely observed (not significantly enriched) variants included V36I/L and I132V, which did not confer resistance to telaprevir; V36C/G, R155G/I/M/S, V36A+T54A, V36L+R155K, T54S+R155K, and R155T+D168N, which conferred lower-level resistance to telaprevir; and A156F/N/V, V36A+R155K/T, V36M+R155T, V36A/M+A156T, T54A+A156S, T54S+A156S/T, and V36M+T54S+R155K, which conferred higher-level resistance to telaprevir. All telaprevir-resistant variants remained fully sensitive to alpha interferon, ribavirin, and HCV NS5B nucleoside and nonnucleoside polymerase inhibitors. In general, the replication capacity of telaprevir-resistant variants was lower than that of the wild-type replicon.

Design and Development of NS5B Polymerase Non‐nucleoside Inhibitors for the Treatment of Hepatitis C Virus Infection

The hepatitis C virus (HCV) infects an estimated 130–170 million people worldwide and is associated with life‐threatening liver diseases. The recent introduction of the first two HCV direct‐acting antivirals (DAAs) as a complement to the interferon/ribavirin standard of care has provided patients with improved outcomes. Still, 25–30% of subjects infected with genotype 1 HCV do not respond adequately to treatment owing to the emergence of resistant virus and many suffer from severe side effects. A paradigm shift towards the development of interferon‐free combinations of DAAs with complementary modes of action is currently taking place. Virally encoded proteins and enzymes have become the target of HCV drug discovery efforts and several promising new agents are currently being evaluated in the clinic for treatment of chronic HCV infection. The NS5B RNA‐dependent RNA polymerase is responsible for replication of viral RNA and plays a pivotal role in the virus life cycle. NS5B is undoubtedly the most druggable HCV target and is susceptible to several classes of allosteric inhibitors that bind to four distinct sites on the enzyme. This chapter describes successful strategies that have led to the discovery of HCV NS5B antivirals. It is divided according to allosteric sites and describes how each of the known families of inhibitors was discovered, characterized and optimized to provide clinical candidates. When available, the strategies adopted by medicinal chemists to optimize initial leads and address challenges and liabilities encountered on the path to candidate selection are described, along with reported clinical outcomes.

Recent advances in drug discovery of benzothiadiazine and related analogs as HCV NS5B polymerase inhibitors

Hepatitis C virus (HCV) is a major health burden, with an estimated 170 million chronically infected individuals worldwide, and a leading cause of liver transplantation. Patients are at increased risk of developing liver cirrhosis, hepatocellular carcinoma and even liver failure. In the past two decades, several approaches have been adopted to inhibit non-structural viral proteins. The RNA-dependent RNA polymerase (NS5B) of HCV is one of the attractive validated targets for development of new drugs to block HCV infection. In this review, we report the recent progress made towards identifying and developing benzothiadiazines as HCV NS5B polymerase inhibitors. The substituted benzothiadiazine class was identified by HTS in 2002 as an NS5B inhibitor. Further optimization and modification of the core has improved the potency and pharmacokinetic properties of substituted benzothiadiazines. Research on palm site-binding benzothiadiazine analogs and related derivatives and analogs is discussed in this article.

Non-nucleoside inhibitors of hepatitis C virus polymerase: current progress and future challenges

The current standard of care for hepatitis C virus (HCV) infection is a combination of PEGylated interferon and ribavirin, which offer limited efficacy and significant side effects. Novel HCV-specific inhibitors, including those directed at the viral polymerase, have become the focus of HCV drug-discovery efforts in the past decade. In addition to the active site targeted by traditional nucleoside inhibitors, at least four different allosteric-binding sites have been reported for the HCV polymerase, which offer ample opportunities for small-molecule inhibitors. In this review, we summarize the recent progress in the discovery of non-nucleoside HCV polymerase inhibitors with a focus on novel chemical matters, their clinical efficacy, safety and potential for combination therapy.

Novel HCV NS5B polymerase inhibitors derived from 4-(1',1'-dioxo-1',4'-dihydro-1'lambda(6)-benzo[1',2',4']thiadiazin-3'-yl)-5-hydroxy-2H-pyridazin-3-ones. Part 5: Exploration of pyridazinones containing 6-amino-substituents

The synthesis of 4-(1',1'-dioxo-1',4'-dihydro-1'lambda(6)-benzo[1',2',4']thiadiazin-3'-yl)-5-hydroxy-2H-pyridazin-3-ones bearing 6-amino substituents as potent inhibitors of the HCV RNA-dependent RNA polymerase (NS5B) is described. Several of these agents also display potent antiviral activity in cell culture experiments (EC(50)<0.10 microM). In vitro DMPK data (microsome t(1/2), Caco-2 P(app)) for many of the compounds are also disclosed, and a crystal structure of a representative inhibitor complexed with the NS5B protein is discussed.