Worldwide approximately 170 million people (or almost 3% of the global population) are chronically infected with HCV. Chronically infected patients are at increased risk of developing liver cirrhosis and hepatocellular carcinoma. 1 In Western countries, infection with HCV is the most common cause of liver transplantation. The current standard therapy for chronic hepatitis C consists of the combination of pegylated interferon alpha (IFN-␣) and ribavirin. This therapy is only effective in 50% to 60% of infected patients and is associated with serious side effects. 2 There is thus an urgent need for more selective, potent, and better-tolerated therapies for chronic hepatitis C.Most antiviral drugs that are currently in clinical trials are inhibitors of the viral polymerase or serine protease. The design of antiviral drugs that inhibit the function of the HCV protease and polymerase therefore appears to be logical. The first HCV nonstructural (NS)3/4A serine protease inhibitor to enter clinical trials was

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Cyclophilin inhibitors currently in clinical trials for hepatitis C virus (HCV) are all analogues of cyclosporine (CsA). Sanglifehrins are a group of naturally occurring cyclophilin binding polyketides that are structurally distinct from the cyclosporines and are produced by a microorganism amenable to biosynthetic engineering for lead optimization and large-scale production by fermentation. Preclinical characterization of the potential utility of this class of compounds for the treatment of HCV revealed that the natural sanglifehrins A to D are all more potent than CsA at disrupting formation of the NS5A-CypA, -CypB, and -CypD complexes and at inhibition of CypA, CypB, and CypD isomerase activity. In particular, sanglifehrin B (SfB) was 30-to 50-fold more potent at inhibiting the isomerase activity of all Cyps tested than CsA and was also shown to be a more potent inhibitor of the 1b subgenomic replicon (50% effective concentrations [EC 50 s] of 0.070 M and 0.16 M in Huh 5-2 and Huh 9-13 cells, respectively). Physicochemical and mouse pharmacokinetic analyses revealed low oral bioavailability (F < 4%) and low solubility (<25 M), although the half-lives (t 1/2 ) of SfA and SfB in mouse blood after intravenous (i.v.) dosing were long (t 1/2 > 5 h). These data demonstrate that naturally occurring sanglifehrins are suitable lead compounds for the development of novel analogues that are less immunosuppressive and that have improved metabolism and pharmacokinetic properties.Hepatitis C virus (HCV) is a positive-strand RNA virus and is the major cause of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma (39). The World Health Organization (WHO) estimates that there are more than 170 million chronic carriers of HCV, which is approximately 3% of the world population (28,29). In developed countries, 50 to 76% of all cases of liver cancer and two-thirds of all liver transplants are due to chronic HCV infection (37).The current standard of care (SOC) involves subcutaneous injections of pegylated alpha-interferon (peg-IFN-␣) and oral dosing of the nonspecific antiviral drug ribavirin for a period of 24 to 48 weeks (9). Overall, response rates to the SOC depend upon the genotype and the pretreatment HCV RNA levels. Patients with genotypes 2 and 3 are more responsive to the SOC than patients infected with genotype 1 (particularly 1b) (25, 31). A significant number of HCV patients do not respond adequately to the SOC or cannot tolerate it due to side effects, leading to poor compliance and a sustained virological response (SVR) of about 50% (31).Direct-acting antiviral (DAA) drugs are being developed to target viral proteins, such as HCV NS5B polymerase or NS3 protease (35). One concern with the increased use of virustargeted agents is that, as was observed with HIV treatment, drug resistance emerges (47). However, compounds that target human proteins (i.e., cyclophilins), rather than viral targets, are also being developed, which should have higher hurdles for viral breakthrough and incidence of resistance duri...