Delayed Antigen Release from Dex-HEMA Microspheres
Once the freeze-dried microsphere product is rehydrated by reconstitution in an aqueous solution, hydrolysis of the carbonate
ester groups in the dex-HEMA will be initiated. This will increase the mesh size in the hydrogel network. The encapsulated
protein will be released when the mesh size exceeds the hydrodynamic diameter of the protein. The actual release profile of
the encapsulated protein is primarily determined by the matrix density.
It has been shown that the release of proteins and liposomes can be tailored from days to months by varying the main factors
influencing the matrix density, which are the relative number of HEMA groups in dex-HEMA (degree of substitution, DS) and
the initial water content of the microspheres.2,4,7,8 In Figure 6, the effects of different DS and initial water content on in vitro release profiles of a model protein, IgG, are presented.
Figure 6. The effect of dex-HEMA DS and initial water content on in vitro release of IgG. In vitro release of proteins is
measured in time during incubation at physiological conditions. Release is expressed as percent of the maximal released amount
Most other microsphere delivery technologies, such as the poly-lactic-co-glycolic acid (PLGA)–based delivery technology, comprise
process steps that expose the protein to potentially detrimental organic solvents. In contrast, the OctoVAX microsphere formulation
is a completely aqueous process, which limits the effects of manufacturing on the integrity of the encapsulated proteins.
Furthermore, because of the open matrix of the microspheres, hydrolytic degradation does not induce local acidification that
may affect the protein's activity. As part of our single-shot vaccine development program, we have recently shown in a mouse
model that the potency to induce an antibody response is comparable between encapsulated HBsAg and freely injected HBsAg (Figure
7). These data suggest that the HBsAg particle domains that induce these antibody responses are not affected by the formulation
process or by the release process.
Figure 7. Antibody titers induced by HBsAg administered as a protein solution, admixed with placebo microspheres (MSP), or
encapsulated in OctoVAX microspheres (MSP). Antibody titers at day 84 are represented as geometric mean titers per group of
10 animals in relation to an international reference standard.