4'-C-(Chloromethyl)-2'-deoxy-2'-fluoro-cytidine 3',5'-Bis(2-methylpropanoate)-d3 is labelled 4'-C-(Chloromethyl)-2'-deoxy-2'-fluoro-cytidine 3',5'-Bis(2-methylpropanoate) (C371125), a first-in-class nucleoside RSV polymerase inhibitor that demonstrated excellent anti-RSV efficacy and safety in a phase 2 clinical RSV challenge study.