Antiviral targets of human noroviruses.

Human noroviruses are major causative agents of sporadic and epidemic gastroenteritis both in children and adults. Currently there are no licensed therapeutic intervention measures either in terms of vaccines or drugs available for these highly contagious human pathogens. Genetic and antigenic diversity of these viruses, rapid emergence of new strains, and their ability to infect a broad population by using polymorphic histo-blood group antigens for cell attachment, pose significant challenges for the development of effective antiviral agents. Despite these impediments, there is progress in the design and development of therapeutic agents. These include capsid-based candidate vaccines, and potential antivirals either in the form of glycomimetics or designer antibodies that block HBGA binding, as well as those that target essential non-structural proteins such as the viral protease and RNA-dependent RNA polymerase. In addition to these classical approaches, recent studies suggest the possibility of interferons and targeting host cell factors as viable approaches to counter norovirus infection. This review provides a brief overview of this progress.

Synthesis, Activity and Structure-Activity Relationship of Noroviral Protease Inhibitors.

The protease of norovirus, an important human pathogen, is essential for the viral replication and, therefore, represents a potential drug target. A series of tripeptide-based inhibitors of the protease were designed, synthesized and tested, among which several potent inhibitors were identified with Ki values as low as 75 nM. The structure-activity relationships of these inhibitors are discussed.

Structural basis of substrate specificity and protease inhibition in Norwalk virus.

Norwalk virus (NV), the prototype human calicivirus, is the leading cause of nonbacterial acute gastroenteritis. The NV protease cleaves the polyprotein encoded by open reading frame 1 of the viral genome at five nonhomologous sites, releasing six nonstructural proteins that are essential for viral replication. The structural details of how NV protease recognizes multiple substrates are unclear. In our X-ray structure of an NV protease construct, we observed that the C-terminal tail, representing the native substrate positions P5 to P1, is inserted into the active site cleft of the neighboring protease molecule, providing atomic details of how NV protease recognizes a substrate. The crystallographic structure of NV protease with the C-terminal tail redesigned to mimic P4 to P1 of another substrate site provided further structural details on how the active site accommodates sequence variations in the substrates. Based on these structural analyses, substrate-based aldehyde inhibitors were synthesized and screened for inhibition potency. Crystallographic structures of the protease in complex with each of the three most potent inhibitors were determined. These structures showed concerted conformational changes in the S4 and S2 pockets of the protease to accommodate variations in the P4 and P2 residues of the substrate/inhibitor, which could be a mechanism for how the NV protease recognizes multiple sites in the polyprotein with differential affinities during virus replication. These structures further indicate that the mechanism of inhibition by these inhibitors involves covalent bond formation with the side chain of the conserved cysteine in the active site by nucleophilic addition, and such substrate-based aldehydes could be effective protease inhibitors.

Antibody responses to norovirus genogroup GI.1 and GII.4 proteases in volunteers administered Norwalk virus.

An assay was developed to detect antibodies against two norovirus proteases among participants in a Norwalk virus (GI.1) challenge study. Prechallenge seroprevalence was lower against the protease from the homologous GI.1 virus than against protease from a heterologous GII.4 strain. Seroresponses were detected for 14 of 19 (74%) infected persons.