Nano-hybrid carboxymethyl-hexanoyl chitosan modified with (3-aminopropyl)triethoxysilane for camptothecin delivery

Abstract

Silane-modified amphiphilic chitosan was synthesized by anchoring a silane coupling agent, (3-aminopropyl)triethoxysilane, to a novel amphiphilic carboxymethyl-hexanoyl chitosan (CHC). The chemical structure of this new organic–inorganic hybrid molecule was characterized by FTIR and ¹³C-, ²⁹Si-nuclear magnetic resonance, while the structural evolution was examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and dynamic light scattering (DLS). Experimental results indicated a self-assembly behaviour of molecules into nanoparticles with a stable polygonal geometry, consisting of ordered silane layers of 6 nm in thickness. The self-assembly property was found to be influenced by chemical composition and concentration of silane incorporated, while the size can be varied by the amount of anchored silane. It was also demonstrated that such vesicle exhibited excellent cytocompatibility and cellular internalization capability in ARPE-19 cell line, and presented well-controlled encapsulation and release profiles for (S)-(+)-camptothecin. These unique properties render it as a potential drug delivery nanosystem.

Introduction

Many approaches for the preparation of hybrid compounds have been recently reported. Among them, the synthesis of composites through the combination of polysaccharides and silicon-based materials has been widely reported for various fields, such as enzyme immobilization, porous materials and electrochemical sensors (Gamys et al., 2010, Guo et al., 2008, Lei et al., 2011, Mullner et al., 2010, Murakami et al., 2010, Qi et al., 2010, Salmah et al., 2011, Tripathi and Shahi, 2008, Wang and Zhang, 2006, Yavuz et al., 2009). Furthermore, chitosan-based materials have been proposed for a range of biomedical applications (Hu et al., 2009, Jayakumar et al., 2010, Liu et al., 2008, Murakami et al., 2010, Muzzarelli, 1993, Muzzarelli, 2009, Wang and Zhang, 2006). Among those relevant reports, chitosan-based organic–inorganic hybrid molecules capable of self-assembling into well-organized architecture are rarely found. It is currently an important research objective to integrate diverse functions into a given nanoobject for advanced applications. This is particularly interesting in the area of biomedical and pharmaceutical industries. It would be highly desirable to take advantage of the biocompatibility of natural polymers, e.g., chitosan or its modified analogues, and combine this with the unique functions of inorganic components.

Nanostructures of pristine chitosan or its composite analogues have been widely reported for drug delivery practice, and a number of cross-linking agents have been proposed in order to stabilize the resulting nanoobjects upon dilution, for instance when administrated to the blood circulatory system. However, for most polymeric drug delivery nanoparticles reported in literature, burst release of drug has been frequently observed in the early-phase of the drug release, mainly due to swelling of the polymer and release of surface bound drug. This effect is even more pronounced for water-soluble polymers and most natural polymers. Thus, it is not unusual that polymeric drug nanocarriers suffer from uncontrollable loss or leakage upon delivery, making a final therapeutic dose practically un-predictable. Moreover, many water-soluble natural polymers exert less preference to encapsulate water-insoluble drugs, albeit some emulsification technologies or analogous skills have been developed to overcome the problem. It is highly desirable to modify a natural macromolecule, making it capable of undergoing self-assembly to effectively entrap water-insoluble drugs for subsequent controlled drug release while retaining the good biocompatibility of the template macromolecule.

Our earlier work has demonstrated a successful synthesis of a new type of chitosan, namely carboxymethyl-hexanoyl chitosan (CHC) (Liu et al., 2008), wherein the pristine chitosan was modified first by carboxymethylation, to increase the flexibility of chitosan molecular chains in water, followed by hydrophobic modification with hexanoyl groups, to create the resulting amphiphilic molecules. However, extensive swelling of the resulting CHC in aqueous solution caused early-phase burst release, the same scenario being observed in many chitosan-based drug carriers. Furthermore, the CHC molecules demonstrated encapsulation efficiencies for doxorubicin of about 50%, leaving room for improvement. Therefore, it was considered interesting to design an organic–inorganic hybrid molecule. The resulting hybrid molecule should be capable of self-assembly, have reduced swelling and improved optimal drug loading capacity. Most importantly, the inorganic component should assemble into dense shell-like structures with low permeability, which should provide sustained release as well as reduced burst release.

Here we report the synthesis of a hybrid organic–inorganic molecule through the use of the CHC chitosan as a starting organic matrix, which is further chemically modified using a coupling agent, (3-aminopropyl)triethoxysilane (APTES) to provide the inorganic component (silane).