Figure 3. Label-free quantification of RSV F mediated fusion by TMC-353121.

(A) Schematic of the mode of action of the small molecule RSV fusion inhibitor TMC-353121. Upon activation of the F protein through receptor engagement, an elongated form exposing a trimeric helical bundle is formed. TMC-353121 (yellow) binds to the trimeric helical bundle, thus preventing the collapse of F into the hairpin post-fusion conformation, which drives membrane fusion. (B) TMC-353121 is a potent inhibitor of virus infection as demonstrated by PRNT (IC₅₀ of 5.8 nM). (C) Anti-fusion activity of TMC-353121 against transiently expressed recombinant F measured using cell impedance (blue). No effect on cell impedance of the compound was observed for cells transfected with a noncoding empty vector (eV, black). (D) A luciferase reporter based fusion assay demonstrates the anti-fusion activity of TMC-353121 against both live virus (red) and transiently expressed recombinant F protein (blue). (E) Antifusion activity of TMC-353121 against RSV can be measured using cell impedance (red). No change in cell impedance following treatment of control cells with the compound was observed (black).