Respiratory syncytial virus entry inhibitors targeting the F protein

Abstract

Human respiratory syncytial virus (RSV) is the main viral cause of respiratory tract infection in infants as well as some elderly and high-risk adults with chronic pulmonary disease and the severely immunocompromised. So far, no specific anti-RSV therapeutics or effective anti-RSV vaccines have been reported. Only one humanized monoclonal antibody, Palivizumab, has been approved for use in high-risk infants to prevent RSV infection. Ribavirin is the only drug licensed for therapy of RSV infection, but its clinical use is limited by its nonspecific anti-RSV activity, toxic effect, and relatively high cost. Therefore, development of novel effective anti-RSV therapeutics is urgently needed. The RSV envelope glycoprotein F plays an important role in RSV fusion with, and entry into, the host cell and, consequently, serves as an attractive target for developing RSV entry inhibitors. This article reviews advances made in studies of the structure and function of the F protein and the development of RSV entry inhibitors targeting it.

Figure 1

Structure of respiratory syncytial virus (RSV) F protein and RSV fusion/entry processes. (A) Schematic representation of RSV F protein. Proteolytic cleavage of the precursor F0 produces the F1 and F2 subunits. Signal peptide (SP), heptad-repeat C (HRC), furin cleavage site (FCS), 27-mer fragment (pep27), putative fusion peptide (FP), domain I and II, heptad-repeat A (HRA), heptad-repeat B (HRB), transmembrane (TM), and cytoplasm (CP) domains are indicated. (B) A model of RSV F protein-mediated membrane fusion. In the prefusion state, the FP is buried in the F protein. Once the G protein binds to its receptor(s) on the target cell, the F protein changes conformation into a long HRA helix, at the end of which is FP that inserts into the target cell membrane, and the three HRA domains form a coiled coil trimer (in red). Subsequently, the HRB helices (in green) associate with the HRA trimer to form 6-HB, pulling the cell membrane and viral membrane into close proximity for fusion.

Figure 2

High-throughput (HTS) assays for screening of RSV fusion inhibitors targeting HRA trimer of F protein. (A) 6-HB formation by binding FITC-labeled C35 (Fl-C35), as a tracer, to 5-HB that was coated on the well of the culture plate. (B) Inhibition of a test compound by competing with Fl-C35 in binding with 5-HB and blocking the 6-HB formation.

Figure 3

Chemical structures of small-molecule RSV entry inhibitors targeting F protein.