[Dextran microspheres containing tetanus toxoid as a potential delivery system for nasal immunization: in vitro and in vivo characterization]

ABSTRACT

Poor absorption of protein antigens across mucosal membranes necessitates the use of drug delivery systems. Pre-formed dextran microspheres [sephadex] possess a good potential for delivery of protein and peptide drugs to mucosal membranes. However they have rarely been used for mucosal immunization. Regards the advantages of mucosal over parenteral immunization and also the penetration enhancement potential of dextran microspheres, we examined their efficacy as mucosal antigen delivery system. Dextran microspheres loaded with tetanus toxoid [TT] were prepared by suspending microspheres in TT solution followed by freeze-drying. Size and morphological features of microspheres were studied by optical and electron microscopes. Encapsulation efficiency of TT was determined by micro-BCA protein assay. In vitro release study was performed in a model, simulating nasal cavity. Immunoreactivity of encapsulated antigen was assayed by ELISA. Gamma-scintigraphy [GS] was used for determination of mucoadhesion and clearance rate of microspheres, by spraying of technetiumlabelled microspheres to the nose of human volunteers. Local irritating potential of microspheres was studied by nasal administration to human nose. Microspheres with a mean diameter of about 150 micro m and a TT encapsulation efficiency of 20.3% [n=3] were obtained. Antigenicity of encapsulated TT was determined as 90.5+/-1.8% [n=3] that of the original TT. Release studies showed that almost 100% of TT was released in 1 h. GS studies showed that after 4 h, 36.9+/-3.4% [n=4] of microspheres were cleared from nasopharynx region of volunteers. Considering the suitable release characteristics, desirable preservation of the antigenic activity of TT, appropriate mucoadhesive properties and also safety of TT loaded Sephadex microspheres, these microspheres could be used as a proper mucosal antigen delivery system. However further studies are needed regarding the immune response induced by these systems in an animal model