X-ray crystal structures of two non-nucleoside analogue inhibitors bound to hepatitis C virus NS5B RNAdependent RNA polymerase have been determined to 2.0 and 2.9 Å resolution. These noncompetitive inhibitors bind to the same site on the protein, ϳ35 Å from the active site. The common features of binding include a large hydrophobic region and two hydrogen bonds between both oxygen atoms of a carboxylate group on the inhibitor and two main chain amide nitrogen atoms of Ser 476 and Tyr 477 on NS5B. The inhibitor-binding site lies at the base of the thumb domain, near its interface with the C-terminal extension of NS5B. The location of this inhibitor-binding site suggests that the binding of these inhibitors interferes with a conformational change essential for the activity of the polymerase. Hepatitis C virus (HCV)1 infects about 3% of the world's human population. HCV infection can develop into chronic hepatitis, which, in some cases, causes cirrhosis of the liver, eventually leading to hepatocellular carcinoma (1). There is no vaccine against HCV currently, and no generally effective therapy for all genotypes of HCV is available. At the present time, the use of recombinant interferon ␣-2a, ␣-2b, "consensus" interferon, and pegylated interferon ␣-2b either in monotherapy or in combination with ribavirin is the only approved therapy available (2). However, limited efficacy and some adverse side effects are associated with these therapies (3). Therefore, the development of HCV-specific antiviral agents is needed urgently.Extensive studies have been done to understand the structures and functions of the individual components of the HCVencoded polyprotein (structural proteins C, E1, and E2 and nonstructural proteins NS2, NS3, NS4A, NS4B, NS5A, and NS5B) (4 -6). Among them, NS2, NS3 protease and helicase, and NS5B RNA-dependent RNA polymerase are essential enzymes for the replication of HCV. The high resolution crystal structures of NS3 protease (7-9) and helicase domains (10, 11) and NS5B polymerase (12-14) have been determined by crystallographic methods in the past 5 years. These enzymes are potential targets for structure-based drug design. The inhibitors of NS3 protease and, in some cases, corresponding structures of NS3 protease/inhibitor complexes have been reported recently (15). In the case of HCV NS5B polymerase, both nucleoside and non-nucleoside inhibitors have been discovered in recent years (16). 3TC (2Ј-deoxy-3Ј-thiacytidine proprietary compound lamivudine) triphosphate has been reported to have a weak inhibitory effect with a 50% inhibitory concentration (IC 50 ) of 180 M (17), whereas numerous non-nucleoside compounds have been documented to possess relatively potent anti-NS5B activity. Examples include specific rhodanines and barbituric acid derivatives, many of which were found to exhibit anti-NS5B activity with IC 50 values below 1 M (18, 19). Classes of dihydroxypyrimidine carboxylic acids and diketoacid derivatives were claimed as well with IC 50 values within the submicromolar range for the latt...

We describe a novel nucleoside analog, 2'-deoxy-3'-thiacytidine (BCH-189), in which the 3' carbon of the ribose ring of 2'-deoxycytidine has been replaced by a sulfur atom. In MT-4 T cells, this compound had significant time- and dose-dependent antiviral activity against five different strains of human immunodeficiency virus type 1 (HIV-1) (mean 50% inhibitory dose, 0.73 microM); known 3'-azido-3'-deoxythymidine (AZT)-resistant HIV-1 variants did not exhibit cross-resistance to it. BCH-189 also suppressed HIV-1 replication in the U937 monocytoid cell line as well as in primary cultures of human peripheral blood mononuclear cells; in these latter systems, suppression was fuller and longer lasting than that induced by AZT. Moreover, BCH-189 was less toxic than AZT in cell culture. BCH-189 may be a promising drug for the treatment of HIV-1-associated disease.

The HCV NS5B RNA dependent RNA polymerase plays an essential role in viral replication. The discovery of a novel class of inhibitors based on an N,N-disubstituted phenylalanine scaffold and structure-activity relationships studies to improve potency are described.

The synthesis of a series of purine analogues of the acyclonucleoside compound A* (A-Star, 1) is described. Compounds in this series have been shown to have pronounced activity against herpesviruses. These compounds have been designated "the glycerosides". The glyceropurines are described in this report. Nucleotides have been constructed containing glyceroadenine (A*, compound 1). These nucleotides are resistant to degradation by phosphodiesterases. The compound A* is both a poor substrate and a poor inhibitor of adenosine deaminase.

Nucleoside reverse transcriptase inhibitors (NRTIs) are the backbone of highly active antiretroviral therapy (HAART) recommended for the treatment of human immunodeficiency virus (HIV) infection (7,20), and the use of NRTI-containing combination therapy has significantly decreased the morbidity and mortality associated with HIV disease in treated patients (18,19). NRTIs share a common mechanism of action. All undergo intracellular activation to the NRTI triphosphate (NRTI-TP) form, after which they compete with endogenous deoxynucleotide triphosphates for binding to the viral reverse transcriptase (RT) enzyme, and incorporation of the monophosphate (MP) into the nascent DNA. Since NRTIs lack a substituent capable of supporting further DNA elongation, the incorporation of the NRTI-MP results in the termination of chain elongation and inhibition of reverse transcription.Many patients whose viral replication is effectively controlled by combination antiretroviral therapy ultimately experience virologic failure because of the development of antiretroviral resistance (for reviews, see references 23 and 24). A 6-year survey of viral genotypes in France found that almost 80% of clinical HIV samples collected until 2002 had mutations conferring resistance to NRTIs (29). Primary infection with resistant strains is also being increasingly recognized as a clinical problem in some countries (10,14,26,31). NRTI resistance results from mutational changes within the RT gene. The resulting resistance mechanisms fall into two main categories (for reviews, see references 4 and 8). One group of RT mutations acts to increase the rate of RT-catalyzed phosphorolysis, i.e., the RT-catalyzed excision of the incorporated NRTI-MP from the chain-terminated DNA. These mutations include M41L, D67N, K70R, L210W, T215Y, and K219Q and are sometimes referred to collectively as thymidine analogue mutations (TAMs). The accumulation of these mutations confers high-level resistance to zidovudine and affects viral sensitivity to other NRTIs, including stavudine, tenofovir, and abacavir (4). The other mechanism by which mutations in RT can cause resistance to NRTIs is by altering the discrimination between deoxynucleoside triphosphate substrates and NRTI-TP inhibitors by the substrate binding site of RT. Examples of such mutations include the M184V mutation, which is found frequently in patients experiencing virologic failure during treatment with lamivudine-containing HAART (6,15). This mutation causes high-level resistance to lamivudine and (in combination with other mutations) reduces sensitivity to didanosine, zalcitabine, and abacavir (32). Other mutations