Polymer relationships during preparation of chitosan–alginate and poly-l-lysine–alginate nanospheres

Abstract

The preparation of chitosan–alginate nanospheres is described and their properties compared to the poly-l-lysine–alginate system. The mass ratio range of sodium alginate:CaCl₂:cationic polymer (poly-l-lysine [PLL] or chitosan) to prepare nanospheres was 100:17:10. This mass ratio ensured that the calcium alginate was maintained in the pre-gel phase and sufficient cationic polymer was present to form nanospheres. At low cationic polymer concentrations, nanospheres were not formed, whereas microspheres were formed at higher concentrations. The release of entrapped methylene blue from the nanospheres was directly proportional (R²=0.98) to the sodium chloride concentration in the dissolution medium. The sodium ions more efficiently displace PLL compared to chitosan; hence, the mass of drug released from the chitosan–alginate nanospheres is slow for equivalent sodium ion concentration. Isothermal titration calorimetry studies determined that the primary binding affinity between calcium and alginate was 1.33×10⁶/mole and entropically driven, whereas, the second binding affinity was weaker (1.03×10⁴/mole) and driven by both enthalpy and entropy. This binding was competitively inhibited by sodium ions. Similarly, the binding of PLL to calcium alginate pre-gel was electrostatic and competitively inhibited by sodium, although, the thermodynamic parameters for this interaction could not be determined.

Introduction

Alginates are random, anionic, linear, polymers consisting of varying ratios of guluronic and mannuronic acid units. Salts of alginate are formed when metal ions react with the guluronic or mannuronic acid residues. Alginate polymers have been widely used in numerous biomedical applications, including drug delivery systems, as they are biodegradable, biocompatible, and mucoadhesive. These delivery systems are formed when monovalent, water-soluble, alginate salts undergo an aqueous sol–gel transformation to water-insoluble salts due to the addition of divalent ions such as, calcium, strontium, and barium [1]. Although strontium and barium alginate forms stronger insoluble matrices, calcium alginate is commonly used and forms a matrix for various delivery systems including gels, films, beads, microparticles, and sponges [2], [3], [4], [5]. Calcium ions have unequal affinity for the guluronic and mannuronic acid units of alginate [1]. As a result, calcium ions initially react with repeating guluronic acid units to form an ‘egg-box’-shaped structure that stack upon each other [1], [6]. Additional calcium ions then interact with unreacted guluronic and mannuronic acid residues to form a calcium alginate complex [1], [7]. Hence, knowledge of the relative composition of guluronic to mannuronic acid residues within an alginate sample is necessary to predict the strength of the complex and can impact properties of the delivery system such as, drug release rate [6]. Alginates with a high guluronic acid content form more rigid, porous gels due to their orientation within the ‘egg-box’ structure. Conversely, gels with low guluronic acid content are more randomly packed and less porous [8].

Nanospheres are submicron particles containing entrapped drugs intended for enteral and parenteral administration that may prevent or minimize drug degradation and metabolism as well as, cellular efflux. Three previous reports describe the preparation of nanospheres using a combination of alginate and poly-l-lysine (PLL), a cationic, natural polymer [4], [9], [10]. These researchers proposed that the addition of cationic PLL is essential to form stable, uniform alginate nanospheres by enveloping the negatively charged calcium alginate complex. PLL is biodegradable and available in a range of molecular weights. Previous research has demonstrated that, as the molecular weight of PLL increased, the thickness of PLL–alginate beads and microspheres decreased [11], [12]. Further, Aynie et al. [10] demonstrated that the pre-gel state is essential to prepare PLL–alginate nanospheres.

Because PLL is toxic and immunogenic when injected, chitosan was investigated as an alternative cationic polymer for the preparation of nanospheres. No reports have been published describing the use of chitosan to formulate alginate-based nanospheres. Chitosan is biodegradable, less immunogenic than PLL, readily available, and has mucoadhesive properties. It is a linear, high molecular weight polysaccharide consisting of glucosamine and N-acetyl glucosamine units. Both alginate and chitosan polymers have been widely used in drug delivery [1], [3], [5], [6], [11], [13], [14], [15], [16], [17], [18], [19]. Chitosan and its derivatives may also be useful for the peroral delivery of peptides [20]. Moreover, the addition of chitosan increases the mechanical strength of alginate microparticles [17], [21].

Isothermal titration calorimetry (ITC) provides information regarding interactions between molecules in solution by measuring parameters such as, binding affinity (K_a) and the heat of binding (H). Thus, this technique was used to study the interactions between calcium and sodium alginate, as well as, PLL and calcium alginate in the pre-gel state. Understanding these interactions would assist in the formulation of alginate-based nanospheres and help explain drug release mechanisms. The binding interactions between alginate and polycations in the gel state was previously studied using adsorption isotherm and fluorescent labeling techniques [22], [23].

The purpose of this research was to: (i) investigate the effect of some formulation variables such as, polymer mass ratios and ionic charge of entrapped drug on particle size and polydispersity of PLL–alginate and chitosan–alginate nanospheres; (ii) prepare alginate–chitosan nanospheres; (iii) study the dependence of sodium chloride concentration on drug release from the nanospheres prepared in (ii), and (iv) use ITC to study the interaction between sodium alginate and calcium chloride, and also sodium alginate pre-gel and PLL. Future studies will use this knowledge to design alginate-particulate delivery systems that control the release by controlling the rate of exchange between calcium and sodium ions.