Modified thermoresponsive Poloxamer 407 and chitosan sol–gels as potential sustained-release vaccine delivery systems

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Thermoresponsive, particle-loaded, Poloxamer 407 (P407)–Pluronic-R® (25R4) or chitosan-methyl cellulose (MC) formulations were developed as single-dose, sustained release vaccines. The sol–gels, loaded either with a particulate vaccine (cubosomes) or soluble antigen (ovalbumin) and adjuvants (Quil A and monophosphoryl lipid A), were free-flowing liquids at room temperature and formed stable gels at physiological temperatures. Rheological results showed that both systems meet the criteria of being thermoresponsive gels. The P407–25R4 sol–gels did not significantly sustain the release of antigen in vivo while the chitosan-MC sol–gels sustained the release of antigen up to at least 14 days after administration. The chitosan-MC sol–gels stimulated both cellular and humoral responses. The inclusion of cubosomes in the sol–gels did not provide a definitive beneficial effect. Further analysis of the formulations with small-angle X-ray scattering (SAXS) revealed that while cubosomes were stable in chitosan-MC gels they were not stable in P407–25R4 formulations. The reason for the mixed response to cubosome-loaded vehicles requires more investigation, however it appears that the cubosomes did not facilitate synchronous vaccine release and may in fact retard release, reducing efficacy in some cases. From these results, chitosan-MC sol–gels show potential as sustained release vaccine delivery systems, as compared to the P407–25R4 system that had a limited ability to sustain antigen release.

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