Human respiratory syncytial virus (hRSV) is a highly contagious Paramyxovirus that infects most children by age two, generating an estimated 75,000-125,000 hospitalizations in the U.S. annually. hRSV is the most common cause of
bronchiolitis and
pneumonia among infants and children under 1year of age, with significant mortality among high-risk groups. A regulatory agency-approved
vaccine is not available, and existing prophylaxis and
therapies are limited to use in high-risk pediatric patients; thus additional
therapies are sorely needed. Here, we identify a series of
benzimidazole analogs that inhibit hRSV
infection in vitro with high potency, using a previously-reported high-throughput screening assay. The lead compound, SRI 29365 (1-[6-(2-furyl)[1,2,4]triazolo[3,4-b][1,3,4]thiadiazol-3-yl]methyl-1H-
benzimidazole), has an EC50 of 66μM and a selectivity >50. We identified additional compounds with varying potencies by testing commercially-available chemical analogs. Time-of-addition experiments indicated that SRI 29365 effectively inhibits viral replication only if present during the early stages of
viral infection. We isolated a virus with resistance to SRI 29365 and identified mutations in the transmembrane domain of the viral
G protein genomic sequence that suggested that the compound inhibits
G-protein mediated attachment of hRSV to cells. Additional experiments with multiple cell types indicated that SRI 29365
antiviral activity correlates with the binding of cell surface
heparin by full-length
G protein. Lastly, SRI 29365 did not reduce hRSV titers or morbidity/mortality in efficacy studies using a cotton rat model. Although SRI 29365 and analogs inhibit hRSV replication in vitro, this work suggests that the
G-protein may not be a valid
drug target in vivo.