Development of an allosteric inhibitor class blocking RNA elongation by the respiratory syncytial virus polymerase complex.

Respiratory syncytial virus (RSV) represents a significant health threat to infants and to elderly or immunocompromised individuals. There are currently no vaccines available to prevent RSV infections, and disease management is largely limited to supportive care, making the identification and development of effective antiviral therapeutics against RSV a priority. To identify effective chemical scaffolds for managing RSV disease, we conducted a high-throughput anti-RSV screen of a 57,000-compound library. We identified a hit compound that specifically blocked activity of the RSV RNA-dependent RNA polymerase (RdRp) complex, initially with moderate low-micromolar potency. Mechanistic characterization in an in vitro RSV RdRp assay indicated that representatives of this compound class block elongation of RSV RNA products after initial extension by up to three nucleotides. Synthetic hit-to-lead exploration yielded an informative 3D quantitative structure-activity relationship (3D-QSAR) model and resulted in analogs with more than 20-fold improved potency and selectivity indices (SIs) of >1,000. However, first-generation leads exhibited limited water solubility and poor metabolic stability. A second optimization strategy informed by the 3D-QSAR model combined with in silico pharmacokinetics (PK) predictions yielded an advanced lead, AVG-233, that demonstrated nanomolar activity against both laboratory-adapted RSV strains and clinical RSV isolates. This anti-RSV activity extended to infection of established cell lines and primary human airway cells. PK profiling in mice revealed 34% oral bioavailability of AVG-233 and sustained high drug levels in the circulation after a single oral dose of 20 mg/kg. This promising first-in-class lead warrants further development as an anti-RSV drug.