We have used a structure-based drug design approach to identify small molecule inhibitors of the hepatitis C virus (HCV) NS3⅐4A protease as potential candidates for new anti-HCV therapies. VX-950 is a potent NS3⅐4A protease inhibitor that was recently selected as a clinical development candidate for hepatitis C treatment. In this report, we describe in vitro resistance studies using a subgenomic replicon system to compare VX-950 with another HCV NS3⅐4A protease inhibitor, BILN 2061, for which the Phase I clinical trial results were reported recently. Distinct drug-resistant substitutions of a single amino acid were identified in the HCV NS3 serine protease domain for both inhibitors. The resistance conferred by these mutations was confirmed by characterization of the mutant enzymes and replicon cells that contain the single amino acid substitutions. It is estimated that 170 million patients worldwide and about 1% of the population in developed countries are chronically infected with hepatitis C virus (HCV) 1 (1). The majority of acute HCV infections become chronic, some of which progress toward liver cirrhosis or hepatocellular carcinoma (2, 3). The current standard of care is pegylated interferon ␣ in combination with ribavirin, which has a sustained viral response rate of 40 -50% in genotype 1 HCV-infected patients, which accounts for the majority of the hepatitis C population in the United States and Japan, and of 80 -90% in patients infected with genotype 2 or 3 HCV (4, 5) (for a review, see Ref. 6). Thus, more effective therapeutic drugs with fewer side effects and shorter treatment durations are needed for patients infected with HCV.HCV is an enveloped, single-stranded RNA virus with a 9.6-kb positive-polarity genome, which encodes a polyprotein precursor of about 3,000 amino acids. The HCV polyprotein is proteolytically processed by cellular and HCV proteases into at least 10 distinct products, in the order of NH 2 -C-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B-COOH (for a review, see Ref. 7). NS3 serine protease and helicase as well as NS5B RNA-dependent RNA polymerase are believed to be components of a replication complex responsible for viral RNA replication and have been shown to be essential for the HCV replication in chimpanzees (8). These HCV enzymes have been the major targets for the development of HCV-specific therapeutics during the past decade (for a review, see Ref. 9). However, successful discovery of a new HCV-specific drug candidate has been hampered by the lack of a robust, reproducible infectious virus cell culture system. The development of a HCV replicon system by Lohmann et al. (10) and subsequent optimization by several laboratories (11, 12) has enabled quantitative evaluation of the antiviral potency of HCV inhibitors.The HCV NS3⅐4A protease is responsible for cleavage at four sites within the HCV polyprotein to generate the N termini of the NS4A, NS4B, NS5A, and NS5B proteins (13-17). It has been shown that the central region (amino acids 21-30) of the 54-residue NS4A protein is essentia...

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Chronic hepatitis C has become one of the most common liver diseases and is estimated to affect 170 million patients worldwide and ϳ1% of the population in developed countries (1). In many patients, hepatitis C virus (HCV) 2 infection leads to liver cirrhosis or hepatocellular carcinoma (2, 3). The current standard of care, a 48-week treatment with pegylated interferon (IFN)-␣ in combination with ribavirin, has a sustained viral response rate of 40 -50% in the difficult-to-treat genotype 1 HCV-infected patients (Refs. 4 and 5; for a review, see Refs. 6 and 7), which accounts for the majority of the hepatitis C patient population in the developed countries. A more effective treatment with fewer side effects and shorter treatment durations is urgently needed for HCVinfected patients.HCV is an enveloped virus containing a single-stranded, positive polarity RNA that encodes a polyprotein precursor of ϳ3000 amino acids. The HCV polyprotein is proteolytically processed by cellular and viral proteases into at least 10 distinct products in the order of NH 2 -C-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B-COOH (for a review, see Ref. 8). The structural proteins are processed by host signal peptidases, whereas the nonstructural (NS) proteins are processed by two virally encoded proteases, the NS2⅐3 and NS3⅐4A proteases. The NS2⅐3 protease is responsible for the cleavage between the NS2 and NS3 proteins, whereas the NS3⅐4A serine protease is responsible for the release of the remaining four nonstructural proteins, NS4A, NS4B, NS5A, and NS5B (9 -13). The essentiality of the NS3⅐4A serine protease for viral replication has been demonstrated by the nonproductive infection following liver inoculation of chimpanzees with a genomic HCV RNA containing a mutation in the NS3 protease active site (14). It has been shown that the central region (amino acids 21-30) of the 54-residue NS4A protein is essential and sufficient for the enhancement of the proteolytic activity of the NS3 serine protease (15-19). The central region of NS4A forms a tight heterodimer with the NS3 protein (18), for which the first x-ray crystal structure was solved in 1996 (20). The NS3⅐4A serine protease has been one of the major targets for the development of HCV-specific therapeutics during the past decade (for a review, see Ref. 21). VX-950, a potent, small molecule, selective inhibitor of the HCV NS3⅐4A serine protease, was discovered using structurebased drug design techniques (22). Clinical proof of concept for HCV protease inhibitors (PIs) has been demonstrated by Boehringer Ingelheim and Vertex Pharmaceuticals Inc. using BILN 2061 (23) and VX-950, 3 respectively. Both compounds reduced HCV viral load in patients by ϳ2-3 log 10 in the first 3 days of dosing. In some patients treated with VX-950, the HCV viral load dropped by Ͼ4 log 10 to below the limit of detection (Ͻ10 IU/ml) during 14 days of dosing. 3Because of the error-prone nature of the viral reverse transcriptase of retroviruses or the RNA-dependent RNA polymerase of RNA viruses, drug resistance frequen...

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