Preparation, characterization and in-vitro evaluation of chitosan-based smart hydrogels for controlled drug release : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand

Abstract

Controlled drug release enhances the safety, efficacy and reliability of drug therapy. Regulation of the drug release rate results in a reduction in the frequency of drug administration and should encourage patients to comply with dosing instructions. Hydrogels are crosslinked, three-dimensional hydrophilic polymers, which swell without dissolving when brought into contact with water or other biological fluids. The number of polymers suitable for the controlled release of viable therapeutics is quite limited because of inherent toxicity or lack of certain properties such as biodegradability. In this thesis, chitosan was chosen as the base polymer for the development of new hydrogels that can be tailored for use in the site-specific delivery of drugs to the gastrointestinal tract. Chitosan is a non-toxic and biodegradable polymer obtained through the alkaline deacetylation of natural chitin. The interesting characteristics of chitosan make it an ideal candidate for use in controlled drug release formulations. However, chitosan exhibits some shortcomings such as hydrophobicity and a high pH-dependency for its physical properties. Hence, it is very difficult to control drug release with chitosan itself because of the various pH values of the internal organs of the human body. This may negatively affect the human body because of drug under- or over-release. In a structured programme, some new chitosan-based hydrogels have been prepared for controlled drug release investigations by applying three main approaches to overcome the shortcomings of chitosan. The first approach was the incorporation of chitosan into interpenetrating polymer network hydrogels with either a hydrophilic polymer or with hydrophilic monomers treated to bring about in situ copolymerization in the presence of chitosan and a suitable crosslinking agent. The second approach was the chemical modification of chitosan by grafting of a suitable vinyl macromer such as poly(ethylene glycol)-diacrylate, then crosslinking this modified chitosan. The equilibrium swelling studies were carried out for the hydrogels prepared using these two approaches at 37 °C at pH 2.1 (simulated gastric fluid, SGF) and at pH 7.4 (simulated intestinal fluid, SIF). The swelling results showed a pH-responsive nature of these hydrogels. They attained higher swelling values in SGF than in SIF. 5-Fluorouracil (5-FU), an anti cancer drug, was entrapped as a model drug in all the hydrogels prepared using these two approaches. The in-vitro drug release studies were carried out at 37 °C in SGF and SIF. From the preliminary investigations of the prepared hydrogels, they may be customized and used to expand the utilization of these systems in drug delivery applications. In the third approach, chitosan was modified in such a fashion that the hydrogels produced were also pH-responsive but attained limited swelling in SGF and higher swelling in SIF. Hence, the resulting hydrogels could be tailored for utilization for intestine-targeted delivery of peptide and protein drugs with a potential protection of the drugs from the harsh acidity of the stomach. In this third approach the ionotropic gelation was used for the preparation of the hydrogels based on the modified chitosan with another natural polymer (sodium alginate) in the presence of a divalent ion. Bovine serum albumin (BSA) was entrapped as a model protein drug and the in-vitro drug release profiles were established at 37 °C in SGF and SIF. The results showed promising release profiles of BSA. However, this hydrogel study requires more effort to limit the swelling and consequently the loss of drug in the SGF, to act as an excellent candidate for intestine-specific delivery of peptide and protein drugs.