Palivizumab was given at a dose of 0.62, 1.25, 2.5 or 5.0 mg/kg one day prior to challenge. RSV F nanoparticle vaccine and palivizumab induced serum anti-RSV F IgG titers that were high and dose dependent (GMT = 12,998–310,439, GMTs = 4626–95,441, respectively; Fig. 4A). Similarly the levels of PCA were robust for all groups that received the adjuvanted RSV F vaccine. PCA titers in animals passively transferred with palivizumab were significantly lower and only observed at the 5 and 2.5 mg/kg doses (30, 16 μg/ml).

Cotton rats receiving 0.625 and 1.25 mg/kg palivizumab had PCA titers below the level of detection MK2206 (10 μg/ml) (Fig. 4B). Neutralizing antibodies to RSV-A and RSV-B were induced in a dose-dependent manner (Fig. 4C). Even the lowest dose of 0.003 μg RSV F vaccine induced significant levels of neutralizing antibody against both RSV-A Long and RSV-B 18537. Neutralizing titers in the 5.0 mg/kg palivizumab group were comparable to those induced in animals actively immunized with the lowest dose of 0.003 μg RSV F vaccine

(Fig. 4C and D). The in vivo protective efficacy of the RSV F nanoparticle vaccine was evaluated in direct comparison to palivizumab by measuring inhibition of viral replication in the lungs and nasal passages of cotton rats challenged with RSV-B 18537. Post-challenge lung virus titers (GMT) were just above the LOD in animals given the lowest dose of RSV F (0.003 μg) and were below the LOD in recipients of higher doses of RSV F vaccine ( Fig. 5A). The RSV lung virus titer was 4.5 log10 in the placebo group ( Fig. 5A). Palivizumab also reduced lung RSV titers to below the LOD, with others more KU-55933 purchase detectable virus in the lowest doses consistent to what has been

previously observed [34]. Reduction of RSV titers in the nasal passages was also observed in a dose dependent manner for both the RSV F vaccine and palivizumab, with relatively lower virus levels in the RSV F vaccine group in concert with the levels of neutralizing titers induced ( Fig. 5B). Thus, the RSV F vaccine was protective against non-homologous virus challenge in the upper and lower respiratory tract and appears to be a potent immunogen that provided protection via active immunization exceeding that seen with palivizumab, despite the use of very low doses of vaccine. A passive immunization-virus challenge study was done to compare the relative potency of the vaccine, as measured by the PCA assay, relative to palivizumab. Cotton rats received IM injections of a pooled cotton rat anti-RSV F serum that delivered PCA doses of 5.6, 1.6 or 0.6 mg/kg or a similar range of palivizumab at 5.0, 1.3 or 0.6 mg/kg one day prior to RSV challenge. At 24 h after administration of anti-RSV F antibodies, the levels of RSV F IgG antibodies were high and dose dependent for all the groups with the exception of the group that received normal cotton rat serum (Fig. 6C).