Chitosan Microparticles as a Drug Delivery System for Protein Vaccines
Anne Mette Beier
The aim of the studies was to prepare and characterise chitosan microparticles and evaluate their ability to deliver parenterally administered protein vaccines. The goal was to prepare positively charged chitosan microparticles of a size below 5 mm which could be efficiently loaded with protein and which could induce effective antibody and CTL responses. The study was performed using a model protein, ovalbumin.
In the present study it has been shown that chitosan microparticles with suitable size and charge properties for uptake by APCs can be obtained by sulphate precipitation. Moreover, these microparticles can be efficiently loaded with the model protein ovalbumin. The structural integrity of the protein was retained after loading but unfortunately these microparticles did not seem very stable and had a high initial release. They were able to produce an antibody response comparable to Adjuphos and the antibody response was characterised as a Th2 type. The chitosan microparticles were unable to produce reproducible CTL responses.
Further investigations demonstrated that cross-linking of chitosan microparticles with glutaraldehyde can increase stability of the particles. After cross-linking an increase in the size and a small decrease in the zeta potential could be observed. The cross-linking process also influenced the loading of ovalbumin to the microparticles, however, efficient loading was still possible. The initial burst of ovalbumin decreased for cross-linked chitosan microparticles compared to uncross-linked chitosan microparticles. By cross-linking the protein to the surface of the particles, release from the particles was completely avoided in vitro. The cross-linked formulations induced a better antibody response and there was a clear tendency to switch antibody responses from Th2 toward a Th1 response. The chitosan microparticles generally failed to induce a potent CTL response in mice. Most likely, this happened due to lack of presentation on MHC class I molecules as a result of unsuitable particle formulations.
In general these studies indicate that chitosan microparticles are a
potential drug delivery system for induction of an antibody response but
not for CTL induction. By manipulating the chitosan formulation by cross-linking,
the stability could be increased and the immune response could be switched
from Th2 towards a mixed Th1/Th2 antibody response. This also has important
implications for using chitosan particles in drug delivery, vaccination
and using them as immunotherapeutics.