Chitosan-based vaccine adjuvants: incomplete characterization complicates preclinical and clinical evaluation

Abstract

A number of preclinical and clinical studies with chitosan-adjuvanted antigen- and DNA-based vaccines have been carried out. Various chitosans and their modifications, in different forms (solutions, powders, gels and particles), have been evaluated with various antigens administered via different routes. Chitosan is a generic name for a wide array of glucosamine-based substances derived from biological sources, and standardization is necessary. However, in most of the studies published to date, molecular weight, viscosity, deacetylation degree and/or purity level (especially endotoxins) are not provided for the initial chitosan substance and/or final formulation and the preparation procedure is not detailed. Evaluation of adjuvant properties is challenging, given that the only available data are insufficient to demonstrate immunogenicity for chitosans with characteristics within certain intervals to elucidate mechanisms of action or to exclude impurities as the active substance. These and other issues of chitosan-based vaccine adjuvants are summarized and a step-by-step evaluation approach for chitosan-based vaccine adjuvants is outlined.

Keywords:

• adjuvant
• chitosan
• clinical trials
• endotoxin
• standardization
• vaccine

Financial & competing interests disclosure

YM Vasiliev is an employee of a federal budget-based research institute (Russian Federation). The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Key issues

• Various chitosan-based vaccine adjuvants have been studied in the last few decades (solutions, powders, particles as well as chitosan-coated particles based on various polymers and chitosan-containing multicomponent formulations).

• Increase in immunogenicity and protective efficacy/effectiveness has been reported for chitosan-adjuvanted vaccines against various human and animal pathogens of bacterial and viral nature, and increase in transfection and protein expression has also been described for chitosan-adjuvanted DNA vaccines.

• Several clinical trials have been carried out with chitosan-based vaccine adjuvants.

• Chitosan is a generic name for a wide family of biopolymers based on randomly distributed N-acetylated and deacetylated glucosamines generally obtained by hydrolysis (deacetylation) of chitin, a component of exoskeleton of crabs, shrimp and fungi.

• In most of the studies of chitosan-based vaccine adjuvants published to date, the initial substance used and/or the final chitosan-based formulation is not characterized even at a basic level; information on molecular weight, viscosity and/or deacetylation degree as well as endotoxin level is not specified, and preparation procedure and methods used for chitosan characterization are also not detailed.

• Lack of description of chitosan characteristics despite hundreds of studies published to date makes evaluation of immunogenicity and mechanisms of action of these vaccine adjuvants very challenging, if not nearly impossible.

• Whether chitosan itself, chitosan with characteristics within certain intervals, chitosan solutions or particles, modifications of chitosan administered via an optimal route with a certain or any antigen (vaccine) or something entirely different including contaminants have immunological activity is largely unknown.

• A step-by-step evaluation of immunogenicity and mechanisms of action is urgently needed using:

  • – A minimal set of characteristics: molecular weight and viscosity (Mn, Mw by chromatography (size exclusion chromatography, gel-permeation chromatography), Mv by viscosimetry), deacetylation degree (nuclear magnetic resonance) and endotoxin level (limulus amebocyte lysate test).

  • – Detailed preparation procedure.

  • – Characterized initial chitosan substance and final chitosan-based formulations if significant modifications are made, and endotoxin level should always be confirmed in the final formulation.

  • – Different forms and modifications of chitosans with various characteristics starting from the most basic – solutions, powders, particles and then chitosan-coated particles and multicomponent formulations.

  • – Various administration routes for different chitosan-based formulations and vaccines against various pathogens.

  • – A number of other well-known vaccine adjuvants in direct comparative studies.