Frank Narjes

Mechanism of Action and Antiviral Activity of Benzimidazole-Based Allosteric Inhibitors of the Hepatitis C Virus RNA-Dependent RNA Polymerase

ABSTRACT The RNA-dependent RNA polymerase of hepatitis C virus (HCV) is the catalytic subunit of the viral RNA amplification machinery and is an appealing target for the development of new therapeutic agents against HCV infection. Nonnucleoside inhibitors based on a benzimidazole scaffold have been recently reported. Compounds of this class are efficient inhibitors of HCV RNA replication in cell culture, thus providing attractive candidates for further development. Here we report the detailed analysis of the mechanism of action of selected benzimidazole inhibitors. Kinetic data and binding experiments indicated that these compounds act as allosteric inhibitors that block the activity of the polymerase prior to the elongation step. Escape mutations that confer resistance to these compounds map to proline 495, a residue located on the surface of the polymerase thumb domain and away from the active site. Substitution of this residue is sufficient to make the HCV enzyme and replicons resistant to the inhibitors. Interestingly, proline 495 lies in a recently identified noncatalytic GTP-binding site, thus validating it as a potential allosteric site that can be targeted by small-molecule inhibitors of HCV polymerase.

Interdomain Communication in Hepatitis C Virus Polymerase Abolished by Small-Molecule Inhibitors Bound to a Novel Allosteric Site

The hepatitis C virus (HCV) polymerase is required for replication of the viral genome and is a key target for therapeutic intervention against HCV. We have determined the crystal structures of the HCV polymerase complexed with two indole-based allosteric inhibitors at 2.3- and 2.4-Å resolution. The structures show that these inhibitors bind to a site on the surface of the thumb domain. A cyclohexyl and phenyl ring substituents, bridged by an indole moiety, fill two closely spaced pockets, whereas a carboxylate substituent forms a salt bridge with an exposed arginine side chain. Interestingly, in the apoenzyme, the inhibitor binding site is occupied by a small α-helix at the tip of the N-terminal loop that connects the fingers and thumb domains. Thus, these molecules inhibit the enzyme by preventing formation of intramolecular contacts between these two domains and consequently precluding their coordinated movements during RNA synthesis. Our structures identify a novel mechanism by which a new class of allosteric inhibitors inhibits the HCV polymerase and open the way to the development of novel antiviral agents against this clinically relevant human pathogen.

Recent Progress in the Development of Inhibitors of the Hepatitis C Virus RNA-Dependent RNA Polymerase

The global prevalence of hepatitis C virus (HCV) infection and the serious consequences associated with the chronic state of the disease have become a worldwide health problem. A combination therapy comprising Interferon-alpha and Ribavirin represents the current standard treatment for chronic HCV infection, although it has demonstrated limited success and causes serious side effects. Promising alternative approaches toward the control of HCV infection include the development of small molecule inhibitors of viral enzymes interfering with the essential steps in the life cycle of the virus. In this review we will focus on inhibitors of the HCV-encoded NS5B RNA-dependent RNA polymerase (NS5B RdRp) which is essential for viral replication and has been recognized as a prime target for therapeutic intervention.

A designed P1 cysteine mimetic for covalent and non-covalent inhibitors of HCV NS3 protease

The difluoromethyl group was designed by computational chemistry methods as a mimetic of the canonical P1 cysteine thiol for inhibitors of the hepatitis C virus NS3 protease. This modification led to the development of competitive, non-covalent inhibitor 4 (K(i) 30 nM) and reversible covalent inhibitors (6, K(i) 0.5 nM; and 8 K*(i) 10 pM).

Inhibition of the Hepatitis C Virus NS3/4A Protease THE CRYSTAL STRUCTURES OF TWO PROTEASE-INHIBITOR COMPLEXES

The hepatitis C virus NS3 protein contains a serine protease domain with a chymotrypsin-like fold, which is a target for development of therapeutics. We report the crystal structures of this domain complexed with NS4A cofactor and with two potent, reversible covalent inhibitors spanning the P1–P4 residues. Both inhibitors bind in an extended backbone conformation, forming an anti-parallel β-sheet with one enzyme β-strand. The P1 residue contributes most to the binding energy, whereas P2–P4 side chains are partially solvent exposed. The structures do not show notable rearrangements of the active site upon inhibitor binding. These results are significant for the development of antivirals.

Potent Inhibitors of Subgenomic Hepatitis C Virus RNA Replication through Optimization of Indole-N-Acetamide Allosteric Inhibitors of the Viral NS5B Polymerase

Infections caused by hepatitis C virus (HCV) are a significant world health problem for which novel therapies are in urgent demand. Compounds that block replication of subgenomic HCV RNA in liver cells are of interest because of their demonstrated antiviral effect in the clinic. In followup to our recent report that indole-N-acetamides (e.g., 1) are potent allosteric inhibitors of the HCV NS5B polymerase enzyme, we describe here their optimization as cell-based inhibitors. The crystal structure of 1 bound to NS5B was a guide in the design of a two-dimensional compound array that highlighted that formally zwitterionic inhibitors have strong intracellular potency and that pregnane X receptor (PXR) activation (an undesired off-target activity) is linked to a structural feature of the inhibitor. Optimized analogues devoid of PXR activation (e.g., 55, EC(50) = 127 nM) retain strong cell-based efficacy under high serum conditions and show acceptable pharmacokinetics parameters in rat and dog.

Inhibitor binding induces active site stabilization of the HCV NS3 protein serine protease domain

Few structures of viral serine proteases, those encoded by the Sindbis and Semliki Forest viruses, hepatitis C virus (HCV) and cytomegalovirus, have been reported. In the life cycle of HCV a crucial role is played by a chymotrypsin‐like serine protease encoded at the N‐terminus of the viral NS3 protein, the solution structure of which we present here complexed with a covalently bound reversible inhibitor. Unexpectedly, the residue in the P2 position of the inhibitor induces an effective stabilization of the catalytic His–Asp hydrogen bond, by shielding that region of the protease from the solvent. This interaction appears crucial in the activation of the enzyme catalytic machinery and represents an unprecedented observation for this family of enzymes. Our data suggest that natural substrates of this serine protease could contribute to the enzyme activation by a similar induced‐fit mechanism. The high degree of similarity at the His–Asp catalytic site region between HCV NS3 and other viral serine proteases suggests that this behaviour could be a more general feature for this category of viral enzymes.

Allosteric Inhibition of the Hepatitis C Virus NS5B RNA Dependent RNA Polymerase

The human and monetary costs of chronic hepatitis C and the complications arising from this disease emphasize the urgency to find a treatment for Hepatitis C Virus (HCV) infected patients. The current standard of treatment for patients chronically infected with HCV is combination therapy with pegylated interferon plus ribavirin. Recently, viral enzymes have become the target of efforts to develop small molecule inhibitors interfering with the essential steps in the life cycle of the virus. Amongst these enzymes the HCV-encoded NS5B RNA-dependent RNA polymerase (NS5B RdRp) is essential for viral replication and has been recognized as a prime target for therapeutic intervention. Several distinct classes of inhibitors of NS5B RdRp have been disclosed in the literature, including active site inhibitors such as nucleosides and pyrophosphate mimetics, as well as non-nucleoside inhibitors. The latter, based on the success of allosteric inhibitors in the treatment of HIV infection, have been developed into compounds which show activity in the subgenomic cell-culture assay of HCV replication. This review provides an account of the recent developments in this field.

Discovery of (7R)-14-Cyclohexyl-7-{(2-(dimethylamino)ethyl)(methyl) amino}-7,8-dihydro-6H- indolo(1,2-e)(1,5)benzoxazocine-11-carboxylic Acid (MK-3281), a Potent and Orally Bioavailable Finger-Loop Inhibitor of the Hepatitis C Virus NS5B Polymerase †

Infections caused by hepatitis C virus (HCV) are a significant world health problem for which novel therapies are in urgent demand. The polymerase of HCV is responsible for the replication of viral genome and has been a prime target for drug discovery efforts. Here, we report on the further development of tetracyclic indole inhibitors, binding to an allosteric site on the thumb domain. Structure-activity relationship (SAR) studies around an indolo-benzoxazocine scaffold led to the identification of compound 33 (MK-3281), an inhibitor with good potency in the HCV subgenomic replication assay and attractive molecular properties suitable for a clinical candidate. The compound caused a consistent decrease in viremia in vivo using the chimeric mouse model of HCV infection.

Hepatitis C Virus Serine Protease Inhibitors: Current Progress and Future Challenges

Hepatitis C is a predominantly chronic viral infection, affecting 1-3percent of the world population. The causative agent, the hepatitis C virus (HCV), has a positive strand-RNA genome that is utilized, in infected cells, as an mRNA to drive the synthesis of a large polyprotein precursor. This precursor subsequently undergoes proteolytic maturation to generate all of the functional, both structural and nonstructural proteins necessary for viral replication and assembly. The proteolytic activity that is responsible for the generation of the mature viral polymerase as well as for most of the cleavages occurring in the nonstructural region of the polyprotein is expressed by the virus itself and is contained in its nonstructural protein 3 (NS3). Here, the N-terminal 180 amino acids form a chymotrypsin-like serine protease domain. Full activation of this protease is achieved only after complexation with another viral protein, the cofactor protein NS4A. Together, NS3 and NS4A form the active, heterodimeric serine protease that presently is the target of medicinal chemistry efforts aiming at the development of inhibitors with potential antiviral activity. We here review the recent progress in our understanding of the structure and function of the enzyme and in the development of selective and potent NS3 protease inhibitors.