The recent outbreak of Middle East respiratory syndrome (MERS) coronavirus (MERS-CoV) infection has led to more than 800 laboratory-confirmed MERS cases with a high case fatality rate (∼35%), posing a serious threat to global public health and calling for the development of effective and safe therapeutic and prophylactic strategies to treat and prevent MERS-CoV infection. Here we discuss the most recent studies on the structure of the MERS-CoV spike protein and its role in virus binding and entry, and the development of MERS-CoV entry/fusion inhibitors targeting the S1 subunit, particularly the receptor-binding domain (RBD), and the S2 subunit, especially the HR1 region, of the MERS-CoV spike protein. We then look ahead to future applications of these viral entry/fusion inhibitors, either alone or in combination with specific and nonspecific MERS-CoV replication inhibitors, for the treatment and prevention of MERS-CoV infection.

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SUMMARY The H7N9 influenza virus causes high-mortality disease in humans but no effective therapeutics are available. Here we report a human monoclonal antibody, m826, that binds to H7 hemagglutinin (HA) and protects against H7N9 infection. m826 binds to H7N9 HA with subnanomolar affinity at acidic pH and 10-fold lower affinity at neutral pH. The high-resolution (1.9 Å) crystal structure of m826 complexed with H7N9 HA indicates that m826 binds an epitope that may be fully exposed upon pH-induced conformational changes in HA. m826 fully protects mice against lethal challenge with H7N9 virus through mechanisms likely involving antibody-dependent cell-mediated cytotoxicity (ADCC). Interestingly, immunogenetic analysis indicates that m826 is a germline antibody and m826-like sequences can be identified in H7N9-infected patients, healthy adults and newborn babies. These m826 properties offer a template for H7N9 vaccine immunogens, a promising candidate therapeutic, and a tool for exploring mechanisms of virus infection inhibition by antibodies.

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.

A s of 23 July 2014, 837 laboratory-confirmed cases of Middle East respiratory syndrome (MERS-CoV) infection, including 291 deaths, had been reported to the WHO (http://www.who.int /csr/disease/coronavirus_infections/en/), raising concerns about its pandemic potential and calling for the development of vaccines and therapeutics against MERS-CoV infection.We previously identified peptidic HIV-1 and severe acute respiratory syndrome coronavirus (SARS-CoV) fusion inhibitors (1, 2), which led to the development of MERS-CoV spike (S) protein-mediated cell-cell fusion and six-helix bundle (6-HB) formation assays. Using these assays, we identified a peptide from the MERS-CoV S protein HR2 region, termed HR2P, that inhibited 6-HB formation, cell-cell fusion, and MERS-CoV replication (3). To identify small-molecule MERS-CoV fusion inhibitors, we used a cell-cell fusion assay to screen 1,280 compounds from an FDAapproved drug library obtained from MicroSource Discovery Systems, Inc. (Gaylordsville, CT), but none of the compounds at 10 M could significantly inhibit MERS-CoV S-mediated membrane fusion.Most recently, de Wilde et al. (4) and Dyall et al. (5) used a cytopathogenic effect assay to screen several hundred compounds from FDA-approved drug libraries and identified a series of compounds that inhibit the replication of both MERS-CoV and SARSCoV in the low micromolar range. Although the mechanisms of action have not been defined, both groups suggested that some of the drugs, such as chlorpromazine (a clathrin-mediated endocytosis inhibitor), might block virus entry (4, 5).Coronavirus enters the target cell via endocytosis or plasma membrane fusion, while the latter is the main pathway for MERSCoV entry (3). To determine whether these reported MERS-CoV replication inhibitors also block virus entry via plasma membrane fusion, we tested 16 compounds with MERS-CoV replication-inhibiting activity available in the FDA-approved drug library from MicroSource and ribavirin and mycophenolic acid (Sigma-Aldrich), which were reported to inhibit MERS-CoV replication (6), for inhibitory activity on MERS-CoV S-mediated cell-cell fusion using HR2P as a control. A cell-cell fusion inhibition assay was performed as we described before (3). Briefly, Huh-7 cells were used as target cells and 293T cells, which simultaneously express MERS-CoV S protein and enhanced green fluorescent protein (293T/MERS/EGFP), were used as effector cells. 293T/MERS/ EGFP cells were cocultured with HR2P or compounds at graded concentrations (initial concentration of 40 M) for 30 min, added to Huh-7 cells, and then incubated at 37°C for 2 to 4 h (3). As expected, HR2P inhibited cell-cell fusion with an IC 50 (half-maximal inhibitory concentration) of ϳ1 M and effectively blocked 6-HB formation. In contrast, most of these compounds exhibited no significant inhibitory activity at 40 M, except for the three neurotransmitter inhibitors (chlorpromazine, promethazine, and fluphenazine), which were moderate inhibitors of cell-cell fusion with IC 50 s of about 20, 20,...