Design and application of chitosan microspheres as oral and nasal vaccine carriers: an updated review

Figures & data

Figure 1 Structure of chitin and chitosan.

Table 1 Advantages and disadvantages of chitosan microspheres prepared by various methods

Method	Advantages	Disadvantages	References
Interaction with anions
Ionotropic gelation	–These processes are simple and mild and have the following advantages: (a) use physical crosslinking by electrostatic interaction instead of chemical crosslinking; (b) reduce the possible toxic side effects of using various chemicals or reagents; (c) better control of degradation kinetics	–Release of vaccine depends on various factors such as molecular weight, degree of deacetylation, and concentration of chitosan and/or vaccine. Control of these factors by applying these methods is sensitive to the preparation of chitosan microspheres	^Citation8,Citation11,Citation49
Emulsification and ionotropic gelation			^Citation50,Citation51
Complex coacervation			^Citation52,Citation54
Crosslinking with other chemicals
Emulsion crosslinking method	–Easy to control particle size –High drug loading efficiency –Controlled release with improved bioavailability	–Tedious process, uses harsh crosslinking agents –Crosslinking agent sometimes reacts with active agent –Complete removal of unreacted crosslinking agent is a challenge of this method	^Citation17,Citation55,Citation56
Multiple emulsion method	–Improves entrapment efficiency and loading content –Better morphological characteristics –Improves production yield	–Cannot avoid the use of organic solvent and crosslinking agent	^Citation12
Thermal crosslinking method	–Provides suitable particle size	–Controlling the temperature is crucial because entrapment efficiency and release of vaccine depends on a controlled temperature during the crosslinking process	^Citation57
Crosslinking with a naturally occurring agent	–Naturally occurring agents are used as crosslinkers, thus less toxic –Smaller particle size, low crystallinity, and good sphericity –Shows superior biocompatibility –Exhibits slow degradation rate compared to glutaraldehyde crosslinked chitosan microspheres	–The physical, mechanical, and thermal stability of the microspheres prepared by this method are not well established; more investigation needed	^Citation58^–^Citation60
Emulsion droplet coalescence method	–High loading efficiency –Smaller particle size	–Particle size depends on the degree of deacetylation of chitosan. The decreased degree of deacetylation increases particle size which in turn decreases drug content	^Citation61
Coacervation or precipitation method	–The process avoids the use of toxic organic solvents –Particle size and drug release can be controlled	–Partially protects the loaded active agent from nuclease degradation	^Citation63
Reverse micellar method	–Smaller particle size and narrow size distribution –Thermodynamically stable particle size with suitable polydispersity index	–A tedious preparation process with many steps	^Citation64
Sieving method	–A simple method which is devoid of tedious processes –Can be easily scaled up	–Irregular particle shape	^Citation65
Solvent evaporation method	–Good entrapment efficiency and particle morphology	–Controlling particle size depends on using agglomeration preventing agent –Particle size decreased with the use of an increased amount of this agent	^Citation66
Spray drying method	–A popular method to prepare powder formulation –Good drug stability, good entrapment efficiency, and prolonged drug release can be achieved	–Control of size depends on size of nozzle, spray flow rate, pressure inlet air temperature –Entrapment efficiency depends on the molecular weight of chitosan, ie, chitosan with low molecular weight provides better entrapment efficiency	^Citation67

Table 2 Chitosan-based carrier systems with functional properties for the delivery of (model) vaccines through oral and nasal routes

Chitosan-based carrier type	Delivery route	(Model) vaccines	Functional properties	References
Chitosan microparticles	Oral	Ovalbumin	–Targets Peyer’s patches for M-cell uptake	^Citation59,Citation68
Chitosan microparticles	Oral	Tetanus toxoid	–Strong systemic and local immune responses	^Citation69
Chitosan microparticles	Oral/ nasal	Diphtheria toxoid	–Enhancement of both systemic and local immune responses	^Citation70
Eudragit^®-coated chitosan microspheres	Oral	Ovalbumin	–A controlled release profile of drug from the microspheres toward Peyer’s patches –Induces proper immune stimulation	^Citation72
Thiolated Eudragit-coated chitosan microspheres	Oral	Bovine serum albumin	–Retains structural integrity of protein –Improvement of mucoadhesiveness and residual time at the target site	^Citation4
Chitosan microspheres (mixed with protease inhibitors and permeation enhancer)	Oral	Hepatitis B surface antigen	–Enhancement of antigen stability –Strategic potential against chronic hepatitis B	^Citation73
Albumin–chitosan mixed matrix microspheres	Oral	Typhoid Vi^® antigen	–Induction of antigen-specific systemic and mucosal immune response	^Citation74
Chitosan microspheres	Nasal	Bordetella bronchiseptica dermonecrotoxin	–Shows suitable, but with some aggregation, physicochemical properties –Enhances immune stimulating activity in vitro and in vivo	^Citation49
Pegylated chitosan microspheres	Nasal	B. bronchiseptica dermonecrotoxin	–Improves stability and avoids aggregation of the microspheres –Improvises immune stimulatory activity compared to chitosan microspheres alone	^Citation92
Mannosylated chitosan microspheres	Nasal	B. bronchiseptica dermonecrotoxin	–Specifically targets macrophages through the mannosylated moieties of mannose receptor on the cell surface –Increases immune stimulatory activity in vitro and in vivo through specific targeting and activation of macrophages	^Citation91
Chitosan microspheres or chitosan solution alone	Nasal	N/A	–No perceptible toxic effects –Increases bioadhesive properties	^Citation95,Citation96
Heat-labile toxin formulated chitosan or N-trimethyl chitosan microspheres	Nasal	LTK63 mutant of heat-labile toxin (as adjuvant)	–Induces high antigen-specific systemic and mucosal immune response	^Citation88,Citation97
Chitosan–DNA nanospheres	Nasal	DNA encoding Respiratory syncytial viral antigens	–Strong cell-mediated immune response	^Citation98
Antigen-loaded chitosan/ Pluronic^® F127 microparticles	Nasal	Tetanus toxoid, diphtheria toxoid, and anthrax recombinant protective antigen	–Stabilization of protein antigen by F127 –Antigen stabilization strongly enhances the systemic and mucosal immune response of chitosan/F127 than that of chitosan microparticles alone	^Citation105,Citation106

Figure 2 Scanning electron microscope photographs of CMs, BBD-loaded CMs, MCMs, and BBD-loaded MCMs (5000×).

Notes: Bar represents 5 μm. Reprinted from Biomaterials, 29(12). Jiang HL, Kang ML, Quan JS, et al. The potential of mannosylated chitosan microspheres to target macrophage mannose receptors in an adjuvant-delivery system for intranasal immunization, 1931–1939. Copyright 2008 with permission from Elsevier.⁹¹

Abbreviations: BBD, Bordetella bronchiseptica dermonecrotoxin; CM, chitosan microsphere; MCM, mannosylated chitosan microsphere.

Figure 3 Representative structure of thiolated chitosan: (A) general structure of thiolated chitosan modified by an –SH group (X: linker) and (B) chitosan-N-acetyl-cysteine (modification of chitosan at the D-glucosamine unit by N-acetyl-cysteine).

Figure 4 Schematic representation of functional interaction between TCMs and mucin in mucosal vaccine delivery.

Abbreviation: TCM, thiolated chitosan microsphere.

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