Continuing the discussion of vaccines and therapeutics, John DeVincenzo (University of Tennessee, Memphis, TN) opened this session by describing ALN-RSV01, an siRNA compound targeted toward the RSV N, P, and L proteins required for viral replication. This compound, given either before or following RSV infection in animal trials, was observed to reduce viral replication in the lungs of treated animals. Additionally, ALN-RSV01 was also found to be protective when different strains of RSV were administered. In a single-dose randomized double blind clinical trial, the first known trial in which siRNA targeting a virus has been tested in humans, ALN-RSV01 was safely tolerated. Current studies are underway to further study the efficacy of this compound.
As a means to test the effectiveness of potential RSV vaccines and antiviral therapeutics, DeVincenzo described a “standardized” experimental human RSV strain developed in his laboratory. This virus was shown to have similar characteristics to the naturally occurring RSV strain, and also provided consistently reproducible viral endpoints and disease symptoms. Importantly, this experimental strain did not generate any adverse effects. In summary, this experimental RSV strain could provide a universal and reliable standard to measure clinical trial efficacy.
Wenliang Zhang (University of Georgia, Athens, GA) examined how small interfering RNA prophylaxis affects the primary and memory immune responses to RSV infection. This study indicated that siRNA treatment targeting the RSV P gene reduces viral titers, disease pathogenesis, and total cellular infiltrate detected in the BAL fluid following RSV infection. Additionally, this therapy induced an enhanced memory CD4+ and CD8+ T-cell response upon RSV re-infection, characterized by heightened T-cell kinetics as well as an increase in their cytokine production. In summary, this study suggests that siRNA prophylaxis can enhance the generation of a potent memory response as well as mediate antiviral effects.
Leon de Waal (Erasmus Medical College, Rotterdam, The Netherlands) compared the efficiency of more traditional read-outs of disease severity with those obtained using a microarray analysis comparing mRNA expression profiles in RSV-infected BALB/c mice primed with either live RSV or with vaccinia virus constructs expressing the RSV F, G, or M2 proteins. His data indicate that microarray analysis can identify phenotypic-specific expression profiles consistent with those observed in the more traditional immunological or virological based assays. Also, by comparing mRNA expression profiles, certain individual genes were identified that may be important in the development of specific responses. In summary, these data provide a foundation for how future studies could be performed to analyze the host immune response to various candidate vaccines in high resolution.
Ann Falsey (University of Rochester, Rochester, NY) described a phase II clinical trial addressing (1) the immunogenicity of an RSV subunit vaccine containing the F, G, and M proteins with or without adjuvant in at-risk elderly populations, and (2) any potential interference that may arise when given concomitantly with a licensed influenza vaccine. This study observed that the RSV vaccine was well tolerated in patients and was moderately immunogenic. The non-adjuvanted formulation was superior to the adjuvant formulation as determined by increased post-vaccination neutralizing antibody titers. However, antibody titers in both vaccination groups returned to baseline by 1 year, necessitating yearly boosting. Importantly, this study showed that either RSV vaccine formulation did not interfere with the influenza humoral response.
In addition to having no licensed RSV or hMPV vaccines, there are also no widely utilized antiviral therapies against these viruses available as treatment options. Daniel Pevear (Novartis, Cambridge, MA) opened the discussion on therapeutics with a very informative presentation focusing first on the stages of drug development beginning with conception and continuing through FDA approval. Additionally, Pevear presented an overview of several anti-RSV compounds currently in various stages of pre-clinical and clinical development, including four inhibitors of viral entry and three inhibitors of viral replication.
Another potential RSV therapeutic discussed was an anti-inflammatory, experimental immunosuppressive agent, leflunomide. Melinda Dunn (The Ohio State University, Columbus, OH) presented data showing that the administration of this drug, either concurrently with RSV inoculation or 3 days post-infection, reduced viral load by 3–4 logs in the lungs of treated animals by an as yet undefined mechanism. Next, Wieslawa Olszewska (Imperial College London, U.K.) tested the in vivo efficacy of an RSV F protein inhibitor, TMC353121 (Tibotec, Mechelen, Belgium), given either prior to or after RSV infection in BALB/c mice. Both prophylactic and therapeutic administration of the drug was shown to reduce weight loss, viral replication, and inflammatory infiltrate in the BAL fluid. Overall, this compound had antiviral effects similar to those of palivizumab, a currently licensed monoclonal antibody specific for the fusion protein (F protein).
There were also several promising studies at this meeting discussing the development of new antiviral antibodies against RSV. For example, Johan Lantto (Symphogen A/S, Lyngby, Denmark) presented work involving recombinant fully human polyclonal antibodies against RSV collectively termed Sym003. This antibody cocktail was shown to have potent viral neutralizing activities, recognize a broad array of epitopes on both the RSV F and G proteins, and to potently reduce RSV replication in vivo. Sym003 also has a potential for blocking the immunomodulatory effects of the F and G proteins and reduce RSV-mediated pathogenesis.
Next, Subramaniam Krishnan (MedImmune, Gaithersburg, MD) described motavizumab, a humanized IgG1 monoclonal antibody derived through affinity maturation from palivizumab. In in vitro assays, it was shown that motavizumab has the potential to bind to monocytes thus influencing their production of various chemotactic agents, as well as promoting their clearance of virus-infected epithelium.
Interestingly, Geoffrey Toms (Newcastle University, Newcastle upon Tyne, U.K.) examined the relationship between viral passages and neutralization ability by anti-F antibodies. He showed that viral adaptation to cell culture selects for neutralization-susceptible quasi-species variants, with clones from earlier passages being more resistant to neutralization by the anti-F antibody palivizumab than clones isolated from later cell passages. However, upon sequence analysis, there were no consistent changes in the F gene or F-G intra-gene sequences between resistant and susceptible clones. In summary, these data suggest there is a RSV-specific product, which is lost upon adaptation to cell culture, that confers resistance to anti-F antibodies and may have broad implications in the future designs of therapeutics.