Search

NVR 3-778 is the first capsid assembly modulator (CAM) that has demonstrated antiviral activity in hepatitis B virus (HBV)-infected patients. NVR 3-778 inhibited the generation of infectious HBV DNA-containing virus particles with a mean antiviral 50% effective concentration (EC 50 ) of 0.40 µM in HepG2.2.15 cells. The antiviral profile of NVR 3-778 indicates pan-genotypic antiviral activity and a lack of cross-resistance with nucleos(t)ide inhibitors of HBV replication. The combination of NVR 3-778 with nucleos(t)ide analogs in vitro resulted in additive or synergistic antiviral activity. Mutations within the hydrophobic pocket at the dimer-dimer interface of the core protein could confer resistance to NVR 3-778, which is consistent with the ability of the compound to bind to core and to induce capsid assembly. By targeting core, NVR 3-778 inhibits pregenomic RNA encapsidation, viral replication, and the production of HBV DNA- and HBV RNA-containing particles. NVR 3-778 also inhibited de novo infection and viral replication in primary human hepatocytes with EC 50 values of 0.81 µM against HBV DNA and between 3.7 and 4.8 µM against the production of HBV antigens and intracellular HBV RNA. NVR 3-778 showed favorable pharmacokinetics and safety in animal species, allowing serum levels in excess of 100 µM to be achieved in mice and, thus, enabling efficacy studies in vivo The overall preclinical profile of NVR 3-778 predicts antiviral activity in vivo and supports its further evaluation for safety, pharmacokinetics, and antiviral activity in HBV-infected patients., (Copyright © 2018 Lam et al.)

The hepatitis B virus (HBV) core protein serves multiple essential functions in the viral life cycle, and antiviral agents that target the core protein are being developed. Capsid assembly modulators (CAMs) are compounds that target core and misdirect capsid assembly, resulting in the suppression of HBV replication and virion production. Besides HBV DNA, circulating HBV RNA has been detected in patient serum and can be associated with the treatment response. Here we studied the effect of HBV CAMs on the production of extracellular HBV RNA using infected HepaRG cells and primary human hepatocytes. Representative compounds from the sulfonamide carboxamide and heteroaryldihydropyrimidine series of CAMs were evaluated and compared to nucleos(t)ide analogs as inhibitors of the viral polymerase. The results showed that CAMs blocked extracellular HBV RNA with efficiencies similar to those with which they blocked pregenomic RNA (pgRNA) encapsidation, HBV DNA replication, and Dane particle production. Nucleos(t)ide analogs inhibited viral replication and virion production but not encapsidation or production of extracellular HBV RNA. Profiling of HBV RNA from both culture supernatants and patient serum showed that extracellular viral RNA consisted of pgRNA and spliced pgRNA variants with an internal deletion(s) but still retained the sequences at both the 5' and 3' ends. Similar variants were detected in the supernatants of infected cells with and without nucleos(t)ide analog treatment. Overall, our data demonstrate that HBV CAMs represent direct antiviral agents with a profile differentiated from that of nucleos(t)ide analogs, including the inhibition of extracellular pgRNA and spliced pgRNA., (Copyright © 2017 Lam et al.)

Resistance to the 2'-F-2'-C-methylguanosine monophosphate nucleotide hepatitis C virus (HCV) inhibitors PSI-352938 and PSI-353661 was associated with a combination of amino acid changes (changes of S to G at position 15 [S15G], C223H, and V321I) within the genotype 2a nonstructural protein 5B (NS5B), an RNA-dependent RNA polymerase. To understand the role of these residues in viral replication, we examined the effects of single and multiple point mutations on replication fitness and inhibition by a series of nucleotide analog inhibitors. An acidic residue at position 15 reduced replicon fitness, consistent with its proximity to the RNA template. A change of the residue at position 223 to an acidic or large residue reduced replicon fitness, consistent with its proposed proximity to the incoming nucleoside triphosphate (NTP). A change of the residue at position 321 to a charged residue was not tolerated, consistent with its position within a hydrophobic cavity. This triple resistance mutation was specific to both genotype 2a virus and 2'-F-2'-C-methylguanosine inhibitors. A crystal structure of the NS5B S15G/C223H/V321I mutant of the JFH-1 isolate exhibited rearrangement to a conformation potentially consistent with short primer-template RNA binding, which could suggest a mechanism of resistance accomplished through a change in the NS5B conformation, which was better tolerated by genotype 2a virus than by viruses of other genotypes., (Copyright © 2014, American Society for Microbiology. All Rights Reserved.)

PSI-352938 is a novel cyclic phosphate prodrug of β-D-2'-deoxy-2'-α-fluoro-2'-β-C-methylguanosine-5'-monophosphate with potent anti-HCV activity. In order to inhibit the NS5B RNA-dependent RNA polymerase, PSI-352938 must be metabolized to the active triphosphate form, PSI-352666. During in vitro incubations with PSI-352938, significantly larger amounts of PSI-352666 were formed in primary hepatocytes than in clone A hepatitis C virus (HCV) replicon cells. Metabolism and biochemical assays were performed to define the molecular mechanism of PSI-352938 activation. The first step, removal of the isopropyl group on the 3',5'-cyclic phosphate moiety, was found to be cytochrome P450 (CYP) 3A4 dependent, with other CYP isoforms unable to catalyze the reaction. The second step, opening of the cyclic phosphate ring, was catalyzed by phosphodiesterases (PDEs) 2A1, 5A, 9A, and 11A4, all known to be expressed in the liver. The role of these enzymes in the activation of PSI-352938 was confirmed in primary human hepatocytes, where prodrug activation was reduced by inhibitors of CYP3A4 and PDEs. The third step, removal of the O(6)-ethyl group on the nucleobase, was shown to be catalyzed by adenosine deaminase-like protein 1. The resulting monophosphate was consecutively phosphorylated to the diphosphate and to the triphosphate PSI-352666 by guanylate kinase 1 and nucleoside diphosphate kinase, respectively. In addition, formation of nucleoside metabolites was observed in primary hepatocytes, and ecto-5'-nucleotidase was able to dephosphorylate the monophosphate metabolites. Since CYP3A4 is highly expressed in the liver, the CYP3A4-dependent metabolism of PSI-352938 makes it an effective liver-targeted prodrug, in part accounting for the potent antiviral activity observed clinically.

PSI-7977, a prodrug of 2'-F-2'-C-methyluridine monophosphate, is the purified diastereoisomer of PSI-7851 and is currently being investigated in phase 3 clinical trials for the treatment of hepatitis C. In this study, we profiled the activity of PSI-7977 and its ability to select for resistance using a number of different replicon cells. Results showed that PSI-7977 was active against genotype (GT) 1a, 1b, and 2a (strain JFH-1) replicons and chimeric replicons containing GT 2a (strain J6), 2b, and 3a NS5B polymerase. Cross-resistance studies using GT 1b replicons confirmed that the S282T change conferred resistance to PSI-7977. Subsequently, we evaluated the ability of PSI-7977 to select for resistance using GT 1a, 1b, and 2a (JFH-1) replicon cells. S282T was the common mutation selected among all three genotypes, but while it conferred resistance to PSI-7977 in GT 1a and 1b, JFH-1 GT 2a S282T showed only a very modest shift in 50% effective concentration (EC(50)) for PSI-7977. Sequence analysis of the JFH-1 NS5B region indicated that additional amino acid changes were selected both prior to and after the emergence of S282T. These include T179A, M289L, I293L, M434T, and H479P. Residues 179, 289, and 293 are located within the finger and palm domains, while 434 and 479 are located on the surface of the thumb domain. Data from the JFH-1 replicon variants showed that amino acid changes within the finger and palm domains together with S282T were required to confer resistance to PSI-7977, while the mutations on the thumb domain serve to enhance the replication capacity of the S282T replicons.

PSI-352938 is a novel cyclic phosphate prodrug of β-D-2'-deoxy-2'-α-fluoro-2'-β-C-methylguanosine 5'-monophosphate that has potent activity against hepatitis C virus (HCV) in vitro. The studies described here characterize the in vitro anti-HCV activity of PSI-352938, alone and in combination with other inhibitors of HCV, and the cross-resistance profile of PSI-352938. The effective concentration required to achieve 50% inhibition for PSI-352938, determined using genotype 1a-, 1b-, and 2a-derived replicons stably expressed in the Lunet cell line, were 0.20, 0.13, and 0.14 μM, respectively. The active 5'-triphosphate metabolite, PSI-352666, inhibited recombinant NS5B polymerase from genotypes 1 to 4 with comparable 50% inhibitory concentrations. In contrast, PSI-352938 did not inhibit the replication of hepatitis B virus or human immunodeficiency virus in vitro. PSI-352666 did not significantly affect the activity of human DNA and RNA polymerases. PSI-352938 and its cyclic phosphate metabolites did not affect the cyclic GMP-mediated activation of protein kinase G. Clearance studies using replicon cells demonstrated that PSI-352938 cleared cells of HCV replicon RNA and prevented replicon rebound. An additive to synergistic effect was observed when PSI-352938 was combined with other classes of HCV inhibitors, including alpha interferon, ribavirin, NS3/4A inhibitors, an NS5A inhibitor, and nucleoside/nucleotide and nonnucleoside inhibitors. Cross-resistance studies showed that PSI-352938 remained fully active against replicons containing the S282T or the S96T/N142T amino acid alteration. Replicons that contain mutations conferring resistance to various classes of nonnucleoside inhibitors also remained sensitive to inhibition by PSI-352938. PSI-352938 is currently being evaluated in a phase I clinical study in genotype 1-infected individuals.

The hepatitis C virus (HCV) NS5B RNA polymerase facilitates the RNA synthesis step during the HCV replication cycle. Nucleoside analogs targeting the NS5B provide an attractive approach to treating HCV infections because of their high barrier to resistance and pan-genotype activity. PSI-7851, a pronucleotide of beta-D-2'-deoxy-2'-fluoro-2'-C-methyluridine-5'-monophosphate, is a highly active nucleotide analog inhibitor of HCV for which a phase 1b multiple ascending dose study of genotype 1-infected individuals was recently completed (M. Rodriguez-Torres, E. Lawitz, S. Flach, J. M. Denning, E. Albanis, W. T. Symonds, and M. M. Berry, Abstr. 60th Annu. Meet. Am. Assoc. Study Liver Dis., abstr. LB17, 2009). The studies described here characterize the in vitro antiviral activity and cytotoxicity profile of PSI-7851. The 50% effective concentration for PSI-7851 against the genotype 1b replicon was determined to be 0.075+/-0.050 microM (mean+/-standard deviation). PSI-7851 was similarly effective against replicons derived from genotypes 1a, 1b, and 2a and the genotype 1a and 2a infectious virus systems. The active triphosphate, PSI-7409, inhibited recombinant NS5B polymerases from genotypes 1 to 4 with comparable 50% inhibitory concentrations. PSI-7851 is a specific HCV inhibitor, as it lacks antiviral activity against other closely related and unrelated viruses. PSI-7409 also lacked any significant activity against cellular DNA and RNA polymerases. No cytotoxicity, mitochondrial toxicity, or bone marrow toxicity was associated with PSI-7851 at the highest concentration tested (100 microM). Cross-resistance studies using replicon mutants conferring resistance to modified nucleoside analogs showed that PSI-7851 was less active against the S282T replicon mutant, whereas cells expressing a replicon containing the S96T/N142T mutation remained fully susceptible to PSI-7851. Clearance studies using replicon cells demonstrated that PSI-7851 was able to clear cells of HCV replicon RNA and prevent viral rebound.

The development of effective therapies for hepatitis C virus (HCV) must take into account genetic variation among HCV strains. Response rates to interferon-based treatments, including the current preferred treatment of pegylated alpha interferon administered with ribavirin, are genotype specific. Of the numerous HCV inhibitors currently in development as antiviral drugs, nucleoside analogs that target the conserved NS5B active site seem to be quite effective against diverse HCV strains. To test this hypothesis, we examined the effects of a panel of nucleotide analogs, including ribavirin triphosphate (RTP) and several chain-terminating nucleoside triphosphates, on the activities of purified HCV NS5B polymerases derived from genotype 1a, 1b, and 2a strains. Unlike the genotype-specific effects on NS5B activity reported previously for nonnucleoside inhibitors (F. Pauwels, W. Mostmans, L. M. Quirynen, L. van der Helm, C. W. Boutton, A. S. Rueff, E. Cleiren, P. Raboisson, D. Surleraux, O. Nyanguile, and K. A. Simmen, J. Virol. 81:6909-6919, 2007), only minor differences in inhibition were observed among the various genotypes; thus, nucleoside analogs that are current drug candidates may be more promising for treatment of a broader variety of HCV strains. We also examined the effects of RTP on the HCV NS3 helicase/ATPase. As with the polymerase, only minor differences were observed among 1a-, 1b-, and 2a-derived enzymes. RTP did not inhibit the rate of NS3 helicase-catalyzed DNA unwinding but served instead as a substrate to fuel unwinding. NS3 added to RNA synthesis reactions relieved inhibition of the polymerase by RTP, presumably due to RTP hydrolysis. These results suggest that NS3 can limit the incorporation of ribavirin into viral RNA, thus reducing its inhibitory or mutagenic effects.