Characterisation and stability studies of a hydrophilic decapeptide in different adjuvant drug delivery systems: A comparative study of PLGA nanoparticles versus chitosan-dextran sulphate microparticles versus DOTAP-liposomes

Abstract

Poly[lactic-co-glycolide] (PLGA) nanoparticles, chitosan-dextran sulphate microparticles, and DOTAP-liposomes were prepared as vaccine adjuvants and drug carriers for a small hydrophilic model peptide, and their different physico-chemical properties (size, PDI, zeta-potential, pH-value and peptide loading) were investigated. The model peptide's encapsulation efficiency (EE) in PLGA particles amounted to 15%, for DOTAP-liposomes to 20% and for chitosan particles up to 90%. The structural appearance of the particles was visualized by SEM and TEM. The stability of the aqueous formulations and the corresponding lyophilisates was monitored for 12 weeks (stored at T = 2–8 °C). The freeze-drying process and the addition of an appropriate cryoprotective agent (sucrose) proved to be essential for all carrier systems. As a result of this study, three different peptide-loaded drug delivery systems with different properties were successfully manufactured and showed sufficient product stability of their freeze-dried formulations over 12 weeks of storage.

Introduction

Vaccines besides antigens, isotonic and stabilizing compounds may also consist of adjuvants. These adjuvants are used to enhance the body's immune response to an infection or to a foreign body without initiating a specific immune response against themselves. Vaccines without adjuvant are very often not sufficiently immunogenic and unable to induce a significant antibody level. The most potent and oldest known adjuvant, which even today is referred to as a “gold standard” in terms of high adjuvant activity, was developed by Freund in 1937. This emulsion of water and mineral oil containing inactivated mycobacteria is called Freund's complete adjuvant (FCA) and is not applicable for human use due to potent local dermal reactions. Hence, a great demand in vaccine research areas for potent and well-tolerated adjuvant systems exists, and, accordingly, many different adjuvant systems are evaluated. At present aluminium-based systems are the most widely used adjuvants (Gupta, 1998) and are licensed in Europe and USA for human use. Next to these two types of adjuvants polymeric particles, liposomes and other emulsion based systems (Montanide™ or MF59^®) raised an increasing interest in vaccine research (Schijns, 2006).

Along these lines, the polymers PLGA, chitosan and DOTAP lipid were selected in the present study as adjuvant carrier systems for a hydrophilic model peptide. These materials were chosen because of their variety in physico-chemical nature: PLGA [poly(d,l-lactide-co-glycolide)] represents one of the most frequently studied polymers with adjuvant activity. It is biodegradable and biocompatible, and therefore it is approved for human use in sutures or implants for sustained drug delivery (Frazza and Schmitt, 1971). The antigen, which is incorporated or adsorbed onto the PLGA's surface, can gradually be released from the matrix as a function of the used polymer's degradation rate. Hence, PLGA formulations act like depot reservoirs with concurrent protection of the encapsulated antigen (Peek et al., 2008). The potent immunological adjuvant effect of PLGA was successfully demonstrated by several scientists using a number of model antigens (O‘Hagan et al., 1998) such as diphtheria (Singh et al., 1991) or tetanus toxoid, and an immunological effect comparable to aluminium hydroxide was demonstrated (Men et al., 1995, Singh et al., 1991). Moreover, a growing interest in chitosan-based particulate drug delivery systems and as an adjuvant matrix arose over the past decade (Agnihotri et al., 2004, Arca et al., 2009, Fini and Orienti, 2003). Chitosan is a natural, biocompatible, nontoxic polysaccharide which is obtained by deacetylation of chitin, a product of marine crustacean shells. The structure of chitosan is similar to cellulose, and the presence of additional primary amine groups yield hydrophilic properties which are beneficial for pharmaceutical applications. For example, a charged antigen could easily interact with the polycationic chitosan molecule and consequently can be incorporated in the matrix or absorbed on its surface. Because of these structural benefits and its availability as low-cost resource, chitosan represents an alternative to the synthetic PLGA polymer, and an immunological effect of tetanus toxoid loaded chitosan microspheres comparable to PLGA microspheres was shown by Jaganathan et al. (2005). Additionally Gordon et al. (2008) supported chitosan's potent immunological effects by showing similar amounts of generated T-cells and antibody titres of a chitosan hydrogel formulation compared to the common aluminium based antigen system.

In addition to particulate adjuvant systems, especially liposomes have many advantages concerning their applications as vaccine adjuvants because of their structural character. The positively charged DOTAP lipid in addition to cholesterol was chosen for a liposomal adjuvant formulation in the present study. The positively charged ammonium group in the head region of the lipid enables an easy penetration through negative charged cell membranes, and therefore the drug could be transferred easily and more selectively into targeted cells, e.g. dendritic or antigen presenting cells (Nakanishi et al., 1997). Its potent immunological adjuvant effect was demonstrated by several scientists (Cui et al., 2004, Fujimura et al., 2006, Vangasseri et al., 2006). Moreover, the DOTAP lipid may feature cell-mediated immune response and anti-tumor activity (Chen et al., 2008).

Next to its immunity enhancing effect, the manufacturability and stability of the formulation represents a main challenge for the development of a suitable and potent adjuvant. The objective of this study was the development of three different adjuvant systems for a hydrophilic model peptide and its physico-chemical evaluation over an observation time of 12 weeks (stored at T = 2–8 °C). Aqueous dispersions as well as lyophilised powders of these adjuvants were investigated. The model peptide used was a linear and hydrophilic decapeptide, which could be successfully incorporated in all three carrier systems and its entrapped amount between the three systems was compared. As indicators of sufficient system stability, the morphology, size, PDI, zeta-potential, pH, and the determination of the incorporated peptide amount were selected. Sucrose was chosen as the cryoprotective agent for all lyophilised formulations and added in appropriate amounts to stabilize the freeze-dried products.