Sisunatovir hydrochloride (RV521 HCl) is a potent, orally bioavailable small molecule inhibitor targeting the RSV fusion (F) protein. It demonstrates strong in vitro antiviral activity against both RSV-A and RSV-B clinical isolates, with IC50 values in the low nanomolar range. This inhibitor is a valuable research tool for studying RSV entry mechanisms and for preclinical antiviral development.