Development of antiviral agents targeting the RNA polymerase of influenza virus

Abstract

The rapid mutability of influenza virus in conjunction with genomic reassortment between viral strains promotes the virus’ ability to evade vaccines and to become resistant to antiviral drugs. Therefore, novel anti-influenza therapeutics utilizing new targets and creative strategies are essential. The RNA-dependent RNA polymerase of the virus consists of PA, PB1, and PB2 subunits. Biological and structural investigations of the functional domains of these subunits have broadened the target reservoir for drug screening. With the wealth of knowledge from these studies, identification of small-molecule inhibitors that specifically disrupt the polymerase assembly or abrogate polymerase activities has emerged as an innovative and promising approach.

In an attempt to facilitate the discovery of antiviral agents that target viral polymerase, isolated functional domains such as the PA endonuclease domain, the PB2 cap-binding domain, and the PA-PB1 interaction domains were expressed as screening targets. Based on the biochemical and structural properties of individual targets, a variety of platforms were established for the effective screening of inhibitors, including systematic evolution of ligands by exponential enrichment (SELEX), fluorescence resonance energy transfer (FRET) assay, fluorescence polarization (FP) assay, and enzyme-linked immunosorbent assay (ELISA). The antiviral efficacies of selected inhibitors were examined in vitro and in vivo, followed by verification of their antiviral mechanisms. Clinical merits of selected inhibitors were further evaluated, focusing mainly on their cross-protection abilities among influenza virus subtypes and their potential synergetic antiviral effects when used in combination with other drugs.

A number of small-molecule compounds, i.e. PA-30, ANA-0, PB2-19, PAC-3 and ANA-1, together with the aptamer PAN-2, were identified as potent inhibitors against the replication of multiple subtypes of influenza A virus, including H1N1, H3N2, H5N1, H7N7, H7N9, and H9N2, in Madin-Darby canine kidney (MDCK) cell cultures. The intranasal administration of the identified compounds enhanced survival rates and reduced lung viral loads in BALB/c mice infected with H1N1 virus. The docking analyses predicted the compounds targeting PA or PB2 interacted with enzyme active sites to abolish endonuclease or cap-binding activity of the polymerase, whereas the compound targeting the PA-PB1 interaction likely induced configurational changes that impeded polymerase assembly. In addition, the combined treatment of zanamivir with the PA- or PB2-targeted compounds exerted synergistic antiviral effects in vitro.

This study underscores the medical importance of polymerase functional domains as druggable targets, which may be due to the fact that these targeted areas are not only highly conserved among virus subtypes but also key to viral fitness. The identified antivirals exhibit substantial promise for clinical applications and provide new additions to the arsenal of drugs that are already used for chemoprophylaxis and treatment of influenza. Importantly, the established screening platforms for PA endonuclease inhibitors, PB2 cap-binding inhibitors, and PA-PB1 interaction disrupters should advance the development of a category of anti-influenza drugs that target viral polymerase.