Application of chitosan and chitosan derivatives as biomaterials

Abstract

Chitosan is a linear polysaccharide composed of randomly distributed β-(1-4)-linked d-glucosamine and N-acetyl-d-glucosamine. It is one of the major cationic polymers and the second most abundant polysaccharides in nature. It is extensively used to the biomedical and the industrial fields. Specially, chitosan has been studied much in the field of gene therapy during the last decade for its biocompatibility and non-cytotoxicity. However, it has several problems such as solubility, low transfection efficiency, and low specialty on targeted disease. To solve these problems, various strategies have been reported to enhance them. This review briefly introduces various strategies of chitosan carrier.

Introduction

Gene therapy uses genetic materials (e.g., deoxyribonucleic acids (DNA) or ribonucleic acid (RNA)) as a pharmaceutical agent to treat various diseases. Gene therapy has the following three main mechanisms: (1) delivering missing genes, (2) replacing defective genes, and (3) gene silencing undesired gene expression [1]. Through these mechanisms, gene therapy treats a wide range of diseases. Consequently, the interest in gene therapy is increasing. Despite these advantages, use of genetic materials in gene therapy is limited due to rapid degradation by nuclease, large size, poor cellular uptake, high anionic charge density, and non-specificity [1], [2], [3], [4]. To overcome these problems, vectors are used for safe delivery of genetic materials in gene therapy. Generally, vectors can be classified into two types. One of them is a viral vector which is commonly used to deliver genetic material into the cells. The viral vectors such as retroviruses, lentiviruses, adenoviruses, and adeno-associated viruses are very effective in achieving high transfection efficiency; however, their availability for therapeutic use in the human body is limited because of immune responses, safety problems, high cost, and low transgenic size [5], [6], [7], [8], [9]. The other type is non-viral vectors which are preferred as safer alternatives to viral vectors for gene therapy. The viral vectors such as liposome, protein, and cationic polymer have many advantages including stability, safety, low immune response, and cell targeting properties [5], [10]. Thus in recent years, the interest in non-viral vector is increasing, and active research has been reported.

Cationic polymers are widely used as carriers for non-viral genetic materials delivery [11], [12], [13]. They can condense with genetic materials through electrostatic interaction to form polyplexes and facilitate the cellular uptake by cells [12], [13]. In addition, the amine group of polyplexes is quick on the uptake of the cell absorbing protons, facilitating the escape of the polyplexes from endosome or lysosome through a triggered osmotic swelling effect. [12], [14], [15].

Chitosan is a linear polysaccharide composed of randomly distributed β-(1-4)-linked d-glucosamine (deacetylated unit) and N-acetyl-d-glucosamine (acetylate unit) – a structure very similar to that of cellulose. As such chitosan is one of the major cationic polymers [16], [17]. It is obtained by the alkaline deacetylation of chitin, which is the second most abundant polysaccharides in nature after cellulose. Chitosan forms inter- and intra-molecular hydrogen bonding owing to amine and hydroxyl groups; therefore, it has a rigid crystalline structure [18]. Chitosan has a various bioactivities due to the abundant primary amino groups in the chitosan main chain. For this reasons, the chitosan is extensively used to the biomedical fields such as drug and/or gene delivery and the industrial fields such as water treatment (e.g. harmful algae control), heavy metal flocculants and functional foods [19].

Chitosan is soluble in an acid solution but insoluble at natural and alkaline pH values because of the pKa value of chitosan of about 6.5 [20]. The solubility of chitosan is significantly dependent on the degree of deacetylation (DDA). When DDA of chitosan is ≤40%, chitosan is soluble up to a pH of 9. Whereas DDA of chitosan is ≥80%, it is soluble only up to a pH of 6.5 [18]. Moreover, the molecular weight (MW) of chitosan and the ionic strength of the solution influence the solubility of chitosan. Reporting on the chemical properties of chitosan including cationic properties, Sanford pointed out that the high charge density at pH < 6.5 forms gels with polyanions, adheres to negatively charged surfaces, chelates certain transitional metals, and is readily susceptible to chemical modification [21]. During the last decade, chitosan has been extensively used as a gene carrier for gene therapy by applying the chemical properties described above. It has also been extensively studied as non-viral derived cationic natural polymers for a number of pharmaceutical and biomedical applications due to its biocompatibility, biodegradability to normal body constituents, non-toxic, hemostatic, bacteriostatic, fungistatic, spermicidal, anticancerogen, anticholesteremic properties easily susceptible to chemical modification [21], [22], [23], [24]. In addition, chitosan is tightly condensed with negatively charged genetic materials, protecting genetic materials against nuclease degradation due to cationic property as a positive charge [25], [26]. Chitosan/DNA polyplexes have been reported to transfect into various cell types (e.g., human embryonic kidney cells (HEK293) [13], cervical cancer cells (HeLa cell) [13], primary chondrocytes [27], Chinese hamster ovary cells (CHO-K1) [28], fibroblast cells (NIH 3T3) [29], and epithelioma papulosum cyprinid cells (EPC) [30]).

This review briefly introduces the strategies of chitosan and chitosan derivatives that have been reported as genetic materials delivery carrier in various gene therapies.