Two novel fusion inhibitors of human respiratory syncytial virus

Antiviral Rotenoids and Isoflavones Isolated from Millettia oblata ssp. teitensis

Three new (1-3) and six known rotenoids (5-10), along with three known isoflavones (11-13), were isolated from the leaves of Millettia oblata ssp. teitensis. A new glycosylated isoflavone (4), four known isoflavones (14-18), and one known chalcone (19) were isolated from the root wood extract of the same plant. The structures were elucidated by NMR and mass spectrometric analyses. The absolute configuration of the chiral compounds was established by a comparison of experimental ECD and VCD data with those calculated for the possible stereoisomers. This is the first report on the use of VCD to assign the absolute configuration of rotenoids. The crude leaves and root wood extracts displayed anti-RSV (human respiratory syncytial virus) activity with IC50 values of 0.7 and 3.4 μg/mL, respectively. Compounds 6, 8, 10, 11, and 14 showed anti-RSV activity with IC50 values of 0.4-10 μM, while compound 3 exhibited anti-HRV-2 (human rhinovirus 2) activity with an IC50 of 4.2 μM. Most of the compounds showed low cytotoxicity for laryngeal carcinoma (HEp-2) cells; however compounds 3, 11, and 14 exhibited low cytotoxicity also in primary lung fibroblasts. This is the first report on rotenoids showing antiviral activity against RSV and HRV viruses.

Antiviral iridoid glycosides from Clerodendrum myricoides

The methanol root extract of Clerodendrum myricoides (Hochst.) Vatke afforded two new (1, 2) and two known (3, 4) iridoid glycosides. The structures of the isolated compounds were established based on NMR, IR, UV and MS data analyses. The crude extract and the isolated constituents were assayed for antiviral activity against the human respiratory syncytial virus (RSV) in human laryngeal epidermoid carcinoma (HEp-2) cells. The crude extract inhibited RSV infectivity at EC₅₀ = 0.21 μg/ml, while it showed cytotoxicity against HEp-2 cells with CC₅₀ = 9 μg/ml. Compound 2 showed 43.2% virus inhibition at 100 μM, while compounds 1 as well as 3 and 4 had only weak antiviral and cytotoxic activities.

Pharmacological Characterization of TP0591816, a Novel Macrocyclic Respiratory Syncytial Virus Fusion Inhibitor with Antiviral Activity against F Protein Mutants

Human respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in early childhood. However, no vaccines have yet been approved for prevention of RSV infection, and the treatment options are limited. Therefore, development of effective and safe anti-RSV drugs is needed. In this study, we evaluated the antiviral activity and mechanism of action of a novel macrocyclic anti-RSV compound, TP0591816. TP0591816 showed significant antiviral activities against both subgroup A and subgroup B RSV, while exerting no cytotoxicity. Notably, the antiviral activity of TP0591816 was maintained against a known fusion inhibitor-resistant RSV strain with a mutation in the cysteine-rich region or in heptad repeat B. Results of a time-of-addition assay and a temperature shift assay indicated that TP0591816 inhibited fusion of RSV with the cell membrane during viral entry. In addition, TP0591816 added after cell infection also inhibited cell-cell fusion. A TP0591816-resistant virus strain selected by serial passage had an L141F mutation, but no mutation in the cysteine-rich region or in heptad repeat B in the fusion (F) protein. Treatment with TP0591816 reduced lung virus titers in a dose-dependent manner in a mouse model of RSV infection. Furthermore, the estimated effective dose of TP0591816 for use against F protein mutants was thought to be clinically realistic and potentially tolerable. Taken together, these findings suggest that TP0591816 is a promising novel candidate for the treatment of resistant RSV infection.

Small Molecule Inhibitors of Middle East Respiratory Syndrome Coronavirus Fusion by Targeting Cavities on Heptad Repeat Trimers

Middle East Respiratory Syndrome Coronavirus (MERS-CoV) is a newly emerging viral disease with fatal outcomes. However, no MERS-CoV-specific treatment is commercially available. Given the absence of previous structure-based drug discovery studies targeting MERS-CoV fusion proteins, this set of compounds is considered the first generation of MERS-CoV small molecule fusion inhibitors. After a virtual screening campaign of 1.56 million compounds followed by cell-cell fusion assay and MERS-CoV plaques inhibition assay, three new compounds were identified. Compound numbers 22, 73, and 74 showed IC₅₀ values of 12.6, 21.8, and 11.12 μM, respectively, and were most effective at the onset of spike-receptor interactions. The compounds exhibited safe profiles against Human embryonic kidney cells 293 at a concentration of 20 μM with no observed toxicity in Vero cells at 10 μM. The experimental results are accompanied with predicted favorable pharmacokinetic descriptors and drug-likeness parameters. In conclusion, this study provides the first generation of MERS-CoV fusion inhibitors with potencies in the low micromolar range.

A quantitative assessment of dynamical differences of RSV infections in vitro and in vivo

Experimental results in vitro and in animal models are used to guide researchers in testing vaccines or treatment in humans. However, viral kinetics are different in vitro, in animals, and in humans, so it is sometimes difficult to translate results from one system to another. In this study, we use a mathematical model to fit experimental data from multiple cycle respiratory syncytial virus (RSV) infections in vitro, in african green monkey (AGM), and in humans in order to quantitatively compare viral kinetics in the different systems. We find that there are differences in viral clearance rate, productively infectious cell lifespan, and eclipse phase duration between in vitro and in vivo systems and among different in vivo systems. We show that these differences in viral kinetics lead to different estimates of drug effectiveness of fusion inhibitors in vitro and in AGM than in humans.

Modeling of fusion inhibitor treatment of RSV in African green monkeys

Respiratory syncytial virus (RSV) is a respiratory infection that can cause serious illness, particularly in infants. In this study, we test four different model implementations for the effect of a fusion inhibitor, including one model that combines different drug effects, by fitting the models to data from a study of TMC353121 in African green monkeys. We use mathematical modeling to estimate the drug efficacy parameters, ε_max, the maximum efficacy of the drug, and EC₅₀, the drug concentration needed to achieve half the maximum effect. We find that if TMC353121 is having multiple effects on viral kinetics, more detailed data, using different treatment delays, is needed to detect this effect.

Construction and Characterization of a Recombinant Human Respiratory Syncytial Virus Encoding Enhanced Green Fluorescence Protein for Antiviral Drug Screening Assay

Human respiratory syncytial virus (RSV) is the single most important cause of lower respiratory tract disease in infants and young children and a major viral agent responsible for respiratory tract disease in immunosuppressed individuals and the elderly, but no vaccines and antiviral drugs are available. Herein the recombinant RSV (rRSV) encoding enhanced green fluorescence protein (EGFP, rRSV-EGFP) was constructed and the potential for screening anti-RSV drugs was investigated. The recombinant plasmid of pBRATm-rRSV-EGFP, containing T7 transcription cassette composed of T7 promoter, RSV antigenomic cDNA with EGFP gene, HDV ribozyme (δ), and T7 terminator in the order of 5' to 3', was constructed and cotransfected into BHK/T7-9 cells together with helper plasmids encoding N, P, L, and M2-1 gene, respectively. The rescued rRSV-EGFP was confirmed by increasing expression of EGFP over blind passages and by RT-PCR. rRSV-EGFP was comparable to the other two recombinant RSVs encoding red fluorescent protein (RFP, rRSV-RFP) or luciferase (Luc, rRSV-Luc) in the growth kinetic, and there was a difference in sensitivity between them for screening anti-RSV agents based on infection of HEp-2 cells. The EGFP-encoding rRSV has been constructed and rescued successfully and has the potential for high-throughput anti-RSV drug screening in vitro.

Applications of Replicating-Competent Reporter-Expressing Viruses in Diagnostic and Molecular Virology

Commonly used tests based on wild-type viruses, such as immunostaining, cannot meet the demands for rapid detection of viral replication, high-throughput screening for antivirals, as well as for tracking viral proteins or virus transport in real time. Notably, the development of replicating-competent reporter-expressing viruses (RCREVs) has provided an excellent option to detect directly viral replication without the use of secondary labeling, which represents a significant advance in virology. This article reviews the applications of RCREVs in diagnostic and molecular virology, including rapid neutralization tests, high-throughput screening systems, identification of viral receptors and virus-host interactions, dynamics of viral infections in vitro and in vivo, vaccination approaches and others. However, there remain various challenges associated with RCREVs, including pathogenicity alterations due to the insertion of a reporter gene, instability or loss of the reporter gene expression, or attenuation of reporter signals in vivo. Despite all these limitations, RCREVs have become powerful tools for both basic and applied virology with the development of new technologies for generating RCREVs, the inventions of novel reporters and the better understanding of regulation of viral replication.

DNA-AuNP networks on cell membranes as a protective barrier to inhibit viral attachment, entry and budding

Viral infections have caused numerous diseases and deaths worldwide. Due to the emergence of new viruses and frequent virus variation, conventional antiviral strategies that directly target viral or cellular proteins are limited because of the specificity, drug resistance and rapid clearance from the human body. Therefore, developing safe and potent antiviral agents with activity against viral infection at multiple points in the viral life cycle remains a major challenge. In this report, we propose a new modality to inhibit viral infection by fabricating DNA conjugated gold nanoparticle (DNA-AuNP) networks on cell membranes as a protective barrier. The DNA-AuNPs networks were found, via a plaque formation assay and viral titers, to have potent antiviral ability and protect host cells from human respiratory syncytial virus (RSV). Confocal immunofluorescence image analysis showed 80 ± 3.8% of viral attachment, 91.1 ± 0.9% of viral entry and 87.9 ± 2.8% of viral budding were inhibited by the DNA-AuNP networks, which were further confirmed by real-time fluorescence imaging of the RSV infection process. The antiviral activity of the networks may be attributed to steric effects, the disruption of membrane glycoproteins and limited fusion of cell membrane bilayers, all of which play important roles in viral infection. Therefore, our results suggest that the DNA-AuNP networks have not only prophylactic effects to inhibit virus attachment and entry, but also therapeutic effects to inhibit viral budding and cell-to-cell spread. More importantly, this proof-of-principle study provides a pathway for the development of a universal, broad-spectrum antiviral therapy.

GS-5806 Inhibits a Broad Range of Respiratory Syncytial Virus Clinical Isolates by Blocking the Virus-Cell Fusion Process

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in infants and young children. In addition, RSV causes significant morbidity and mortality in hospitalized elderly and immunocompromised patients. Currently, only palivizumab, a monoclonal antibody against the RSV fusion (F) protein, and inhaled ribavirin are approved for the prophylactic and therapeutic treatment of RSV, respectively. Therefore, there is a clinical need for safe and effective therapeutic agents for RSV infections. GS-5806, discovered via chemical optimization of a hit from a high-throughput antiviral-screening campaign, selectively inhibits a diverse set of 75 RSV subtype A and B clinical isolates (mean 50% effective concentration [EC50] = 0.43 nM). The compound maintained potency in primary human airway epithelial cells and exhibited low cytotoxicity in human cell lines and primary cell cultures (selectivity > 23,000-fold). Time-of-addition and temperature shift studies demonstrated that GS-5806 does not block RSV attachment to cells but interferes with virus entry. Follow-up experiments showed potent inhibition of RSV F-mediated cell-to-cell fusion. RSV A and B variants resistant to GS-5806, due to mutations in F protein (RSV A, L138F or F140L/N517I, and RSV B, F488L or F488S), were isolated and showed cross-resistance to other RSV fusion inhibitors, such as VP-14637, but remained fully sensitive to palivizumab and ribavirin. In summary, GS-5806 is a potent and selective RSV fusion inhibitor with antiviral activity against a diverse set of RSV clinical isolates. The compound is currently under clinical investigation for the treatment of RSV infection in pediatric, immunocompromised, and elderly patients.

Activity of Oral ALS-008176 in a Respiratory Syncytial Virus Challenge Study

BACKGROUND Respiratory syncytial virus (RSV) infection is a cause of substantial morbidity and mortality. There is no known effective therapy. METHODS We conducted a randomized, double-blind, clinical trial in healthy adults inoculated with RSV. Participants received the oral nucleoside analogue ALS-008176 or placebo 12 hours after confirmation of RSV infection or 6 days after inoculation. Treatment was administered every 12 hours for 5 days. Viral load, disease severity, resistance, and safety were measured throughout the 28-day study period, with measurement beginning before inoculation. The primary end point was the area under the curve (AUC) for viral load, which was assessed immediately before administration of the first dose through the 12th day after inoculation in participants infected with RSV. RESULTS A total of 62 participants received placebo or one of three ALS-008176 dosing regimens: 1 loading dose of 750 mg followed by 9 maintenance doses of 500 mg (group 1), 1 loading dose of 750 mg followed by 9 maintenance doses of 150 mg (group 2), or 10 doses of 375 mg (group 3). In the 35 infected participants (23 of whom were treated with ALS-008176), the AUCs for viral load for groups 1, 2, and 3 and the placebo group were 59.9, 73.7, 133.4, and 500.9 log10 plaque-forming-unit equivalents × hours per milliliter, respectively (P≤0.001). The time to nondetectability on polymerase-chain-reaction assay (P<0.001), the peak viral load (P≤0.001), the AUC for symptom score (P<0.05), and the AUC for mucus weight were lower in all groups receiving ALS-008176 than in the placebo group. Antiviral activity was greatest in the two groups that received a loading dose--viral clearance was accelerated (P≤0.05), and the AUC for viral load decreased by 85 to 88% as compared with the placebo group. Within this small trial, no viral rebound or resistance was identified. There were no serious adverse events, and there was no need for premature discontinuation of the study drug. CONCLUSIONS In this RSV challenge study, more rapid RSV clearance and a greater reduction of viral load, with accompanying improvements in the severity of clinical disease, were observed in the groups treated with ALS-008176 than in the placebo group. (Funded by Alios BioPharma; ClinicalTrials.gov number, NCT02094365.).

The Pneumovirinae fusion (F) protein: A common target for vaccines and antivirals

The Pneumovirinae fusion (F) protein mediates fusion of the virus and cell membrane, an essential step for entry of the viral genome in the cell cytoplasm and initiation of a new infectious cycle. Accordingly, potent inhibitors of virus infectivity have been found among antibodies and chemical compounds that target the Pneumovirinae F protein. Recent developments in structure-based vaccines have led to a deeper understanding of F protein antigenicity, unveiling new conformations and epitopes which should assist in development of efficacious vaccines. Similarly, structure-based studies of potent antiviral inhibitors have provided information about their mode of action and mechanisms of resistance. The advantages and disadvantages of the different options to battle against important pathogens, such as human respiratory syncytial virus (hRSV) and human metapneumovirus (hMPV) are summarized and critically discussed in this review.

Targeting membrane-bound viral RNA synthesis reveals potent inhibition of diverse coronaviruses including the middle East respiratory syndrome virus

Coronaviruses raise serious concerns as emerging zoonotic viruses without specific antiviral drugs available. Here we screened a collection of 16671 diverse compounds for anti-human coronavirus 229E activity and identified an inhibitor, designated K22, that specifically targets membrane-bound coronaviral RNA synthesis. K22 exerts most potent antiviral activity after virus entry during an early step of the viral life cycle. Specifically, the formation of double membrane vesicles (DMVs), a hallmark of coronavirus replication, was greatly impaired upon K22 treatment accompanied by near-complete inhibition of viral RNA synthesis. K22-resistant viruses contained substitutions in non-structural protein 6 (nsp6), a membrane-spanning integral component of the viral replication complex implicated in DMV formation, corroborating that K22 targets membrane bound viral RNA synthesis. Besides K22 resistance, the nsp6 mutants induced a reduced number of DMVs, displayed decreased specific infectivity, while RNA synthesis was not affected. Importantly, K22 inhibits a broad range of coronaviruses, including Middle East respiratory syndrome coronavirus (MERS–CoV), and efficient inhibition was achieved in primary human epithelia cultures representing the entry port of human coronavirus infection. Collectively, this study proposes an evolutionary conserved step in the life cycle of positive-stranded RNA viruses, the recruitment of cellular membranes for viral replication, as vulnerable and, most importantly, druggable target for antiviral intervention. We expect this mode of action to serve as a paradigm for the development of potent antiviral drugs to combat many animal and human virus infections.

Viruses that replicate in the host cell cytoplasm have evolved to employ host cell-derived membranes to compartmentalize genome replication and transcription. Specifically for positive-stranded RNA viruses, accumulating knowledge concerning the involvement, rearrangement and requirement of cellular membranes for RNA synthesis specify the establishment of the viral replicase complex at host cell-derived membranes as an evolutionary conserved and essential step in the early phase of the viral life cycle. Here we describe a small compound inhibitor of coronavirus replication that (i) specifically targets this membrane-bound RNA replication step and (ii) has broad antiviral activity against number of diverse coronaviruses including highly pathogenic SARS-CoV and MERS-CoV. Since resistance mutations appear in an integral membrane-spanning component of the coronavirus replicase complex, and since all positive stranded RNA viruses have very similar membrane-spanning or membrane-associated replicase components implicated in anchoring the viral replication complex to host cell-derived membranes, our data suggest that the membrane-bound replication step of the viral life cycle is a novel, vulnerable, and druggable target for antiviral intervention of a wide range of RNA virus infections.

Screening and evaluation of anti-respiratory syncytial virus compounds in cultured cells

Respiratory syncytial virus (RSV) is a highly contagious pathogen that infects mainly ciliated cells of respiratory epithelium and type 1 pneumocytes in the alveoli frequently causing serious respiratory disease in infants, elderly, and immunocompromised patients. At present, prevention/treatment of RSV infection is limited to the use of specific anti-RSV antibody or an aerosol formulation of ribavirin, a drug of suboptimal efficacy and low safety profile. There is an urgent need for development of novel anti-RSV drugs and virucides. Here we describe the cell culture-based methods used in our laboratory in identification of novel inhibitors of RSV including the P13 fusion inhibitor, and the PG545 virucide. Protocols for antiviral screening, evaluation of anti-RSV potency, and elucidation of mode of antiviral activity of test compounds are described.

Recent advances in diagnosis, prevention, and treatment of human respiratory syncytial virus

Human respiratory syncytial virus (RSV) is a common cause of respiratory infection in infants and the elderly, leading to significant morbidity and mortality. The interdisciplinary fields, especially biotechnology and nanotechnology, have facilitated the development of modern detection systems for RSV. Many anti-RSV compounds like fusion inhibitors and RNAi molecules have been successful in laboratory and clinical trials. But, currently, there are no effective drugs for RSV infection even after decades of research. Effective diagnosis can result in effective treatment, but the progress in both of these facets must be concurrent. The development in prevention and treatment measures for RSV is at appreciable pace, but the implementation into clinical practice still seems a challenge. This review attempts to present the promising diverse research approaches and advancements in the area of diagnosis, prevention, and treatment that contribute to RSV management.

Respiratory Syncytial Virus Fusion Inhibitors

Infections with the respiratory syncytical virus (RSV) are the leading cause of lower respiratory tract infections and a serious health concern in infants less than 2 years of age, the immunocompromised and the geriatric population. Numerous research programs directed at small‐molecule inhibitors of RSV have been initiated over the last 50 years. RSV inhibitors that target the fusion event have shown a lot of promise and are reviewed in this chapter. However, none of these programs have yet reached the market or late‐stage clinical development. Therefore, focus in this review is given to the challenges in the preclinical development phase and the ideal target product profile. The challenges in clinical development are also discussed, including the use of a new RSV challenge strain (Memphis 37), clinical trial design in immunosupressed patients, patients with chronic obstructive pulmonary disease (COPD) and chronic heart failure (CHF) and clinical trials in infants.

Hsp90 inhibitors exhibit resistance-free antiviral activity against respiratory syncytial virus

Respiratory syncytial virus (RSV) is a major cause of respiratory illness in young children, leading to significant morbidity and mortality worldwide. Despite its medical importance, no vaccine or effective therapeutic interventions are currently available. Therefore, there is a pressing need to identify novel antiviral drugs to combat RSV infections. Hsp90, a cellular protein-folding factor, has been shown to play an important role in the replication of numerous viruses. We here demonstrate that RSV requires Hsp90 for replication. Mechanistic studies reveal that inhibition of Hsp90 during RSV infection leads to the degradation of a viral protein similar in size to the RSV L protein, the viral RNA-dependent RNA polymerase, implicating it as an Hsp90 client protein. Accordingly, Hsp90 inhibitors exhibit antiviral activity against laboratory and clinical isolates of RSV in both immortalized as well as primary differentiated airway epithelial cells. Interestingly, we find a high barrier to the emergence of drug resistance to Hsp90 inhibitors, as extensive growth of RSV under conditions of Hsp90 inhibition did not yield mutants with reduced sensitivity to these drugs. Our results suggest that Hsp90 inhibitors may present attractive antiviral therapeutics for treatment of RSV infections and highlight the potential of chaperone inhibitors as antivirals exhibiting high barriers to development of drug resistance.

Respiratory syncytial virus entry inhibitors targeting the F protein

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.

Targeting RSV with vaccines and small molecule drugs

Respiratory syncytial virus (RSV) is the most significant cause of pediatric respiratory infections. Palivizumab (Synagis®), a humanized monoclonal antibody, has been used successfully for a number of years to prevent severe RSV disease in at-risk infants. However, despite intense efforts, there is no approved vaccine or small molecule drug for RSV. As an enveloped virus, RSV must fuse its envelope with the host cell membrane, which is accomplished through the actions of the fusion (F) glycoprotein, with attachment help from the G glycoprotein. Because of their integral role in initiation of infection and their accessibility outside the lipid bilayer, these proteins have been popular targets in the discovery and development of antiviral compounds and vaccines against RSV. This review examines advances in the development of antiviral compounds and vaccine candidates.

The Prophylaxis and Treatment with Antiviral Agents of Respiratory Syncytial Virus Infections

In this review, we consider recent advances in the discovery and development of antiviral agents for respiratory syncytial virus (RSV) infections. A background to the various manifestations of human RSV infection and current treatments is provided. The technical, clinical and commercial issues surrounding the development of such antiviral agents are discussed.

Progress in understanding and controlling respiratory syncytial virus: still crazy after all these years

Human respiratory syncytial virus (RSV) is a ubiquitous pathogen that infects everyone worldwide early in life and is a leading cause of severe lower respiratory tract disease in the pediatric population as well as in the elderly and in profoundly immunosuppressed individuals. RSV is an enveloped, nonsegmented negative-sense RNA virus that is classified in Family Paramyxoviridae and is one of its more complex members. Although the replicative cycle of RSV follows the general pattern of the Paramyxoviridae, it encodes additional proteins. Two of these (NS1 and NS2) inhibit the host type I and type III interferon (IFN) responses, among other functions, and another gene encodes two novel RNA synthesis factors (M2-1 and M2-2). The attachment (G) glycoprotein also exhibits unusual features, such as high sequence variability, extensive glycosylation, cytokine mimicry, and a shed form that helps the virus evade neutralizing antibodies. RSV is notable for being able to efficiently infect early in life, with the peak of hospitalization at 2-3 months of age. It also is notable for the ability to reinfect symptomatically throughout life without need for significant antigenic change, although immunity from prior infection reduces disease. It is widely thought that re-infection is due to an ability of RSV to inhibit or subvert the host immune response. Mechanisms of viral pathogenesis remain controversial. RSV is notable for a historic, tragic pediatric vaccine failure involving a formalin-inactivated virus preparation that was evaluated in the 1960s and that was poorly protective and paradoxically primed for enhanced RSV disease. RSV also is notable for the development of a successful strategy for passive immunoprophylaxis of high-risk infants using RSV-neutralizing antibodies. Vaccines and new antiviral drugs are in pre-clinical and clinical development, but controlling RSV remains a formidable challenge.

Potent anti-respiratory syncytial virus activity of a cholestanol-sulfated tetrasaccharide conjugate

A number of different viruses including respiratory syncytial virus (RSV) initiate infection of cells by binding to cell surface glycosaminoglycans and sulfated oligo- and polysaccharide mimetics of these receptors exhibit potent antiviral activity in cultured cells. We investigated whether the introduction of different lipophilic groups to the reducing end of sulfated oligosaccharides would modulate their anti-RSV activity. Our results demonstrate that the cholestanol-conjugated tetrasaccharide (PG545) exhibited ∼5- to 16-fold enhanced anti-RSV activity in cultured cells compared with unmodified sulfated oligosaccharides. Furthermore, PG545 displayed virus-inactivating (virucidal) activity, a feature absent in sulfated oligosaccharides. To inhibit RSV infectivity PG545 had to be present during the initial steps of viral infection of cells. The anti-RSV activity of PG545 was due to both partial inhibition of the virus attachment to cells and a more profound interference with some post-attachment steps as PG545 efficiently neutralized infectivity of the cell-adsorbed virus. The anti-RSV activity of PG545 was reduced when tested in the presence of human nasal secretions. Serial passages of RSV in the presence of increasing concentrations of PG545 selected for weakly resistant viral variants that comprised the F168S and the P180S amino acid substitutions in the viral G protein. Altogether we identified a novel and potent inhibitor of RSV, which unlike sulfated oligo- and polysaccharide compounds, could irreversibly inactivate RSV infectivity.