Core-modified chitosan-based polymeric micelles for controlled release of doxorubicin

Abstract

Amphiphilic stearic acid-grafted chitosan oligosaccharide (CSO-SA) micelles have been shown a good drug delivery system by incorporating hydrophobic drugs into the core of the micelles. One of the problems associated with the use of CSO-SA micelles is disassociation or the initial burst drug release during the dilution of drug delivery system by body fluid. Herein, the core of CSO-SA micelles was modified by the physical solubilization of stearic acid (SA) to reduce the burst drug release and enhance the physical stability of CSO-SA micelles. The CSO-SA micelles had 27.4 ± 2.4 nm number average diameter, and indicated pH-sensitive properties. The micelle size and drug release rate from micelles increased with the decrease of pH value. After the incorporation of SA into CSO-SA micelles, the micelle size was increased, and the zeta potential was decreased. The extents of the increase in micelle size and the decrease of zeta potential related with the incorporated amount of SA. The in vitro drug release tests displayed the incorporation of SA into CSO-SA micelles could reduce the drug release from the micelles due to the enhanced hydrophobic interaction among SA, hydrophobic drug and hydrophobic segments of CSO-SA.

Introduction

Polymeric micelles, self-assemblies of amphiphilic graft or block copolymers, are currently recognized as one of the most promising formulations of anti-tumor drug deliveries, and several formulations have been intensively studied in clinical trials (Kataoka et al., 1992, Kataoka et al., 2001, Kwon and Okano, 1996, Aliabadi and Lavasanifar, 2006). It is well known that polymer micelles have unique core–shell architecture that composed of hydrophobic segments as internal core and hydrophilic segments as surrounding corona in aqueous medium. The hydrophobic core provides a loading space for water-insoluble drugs, whereas the modification of hydrophilic shell affects pharmacokinetic behavior (Huh et al., 2005, Van Nostrum, 2004, Hruby et al., 2005). Additionally, the nano-scaled polymer micelles exhibit many advantages for the use of drug delivery carriers, such as prolonged circulation; tumor localization by enhanced permeability and retention (EPR) effect (Maeda et al., 2000, Maeda, 2001); and the controlled drug release by using stimuli-sensitive copolymers (Liu et al., 2004, Yoo and Park, 2004, Li et al., 2006, Lee et al., 2005).

Polymeric micelles have been used as delivery carriers for extensive variety of therapy and diagnoses drugs (Rapoport, 2004, Nakanishi et al., 2001, Lee et al., 2005, Hruby et al., 2005, Kabanov et al., 2002, Gillies and Frechet, 2005, Huh et al., 2005, Nishiyama et al., 2001). For anti-cancer drugs such as doxorubicin (DOX), polymeric micelles had been extensively utilized for passive targeting to solid tumors. Those DOX formulations exhibited a number of attractive features and advantages over the other types of drug carriers. Some DOX-loaded micelles exhibited prolonged systemic circulation time and much lower cardiotoxicity than free DOX (Kabanov et al., 2002). The folate-targeted DOX-loaded micelles showed superior cytotoxitity in cultured folate receptor (+) tumor cells in vitro (Yoo and Park, 2004).

In most cases, anti-cancer drugs such as DOX are incorporated into the hydrophobic core of micelles by hydrophobic interaction (Yokoyama et al., 1996, Nishiyama et al., 2001) and/or electrostatic interaction (Kabanov et al., 1996). These interactions are weak between core-forming blocks and incorporated drugs. Because intravenous injections of micellar solution are associated with extreme dilutions by blood, polymer micelles are easily deformable and disassemble which result in the leakage of loaded drugs (Borovinskii and Khokhlov, 1998, Sens et al., 1996). There are evidences that polymeric micelles will release drugs fleetly in a period of hours (Kang et al., 2002, Yu et al., 1998). It is one limitation of micelles as drug delivery carriers.

A number of strategies are used to overcome the drug leakage limitation of polymer micelles. In order to control the drug release rate and reduce the burst release, drug had been chemically conjugated to polymer chains to improve the interaction between the drug and polymer (Kataoka et al., 2001). The shell of micelles had also been cross-linked to reduce the drug leakage in our previous research (Hu et al., 2006b). However, the former strategy was complicate and the drug was pharmacological inactive until the conjugate bond was hydrolyzed. The latter one would change the surfaces structure and properties of micelles, such as the quality of primary amine on the micelle surface, the zeta potential and the ability of cellular uptake. In our early studies, chitosan oligosaccharide (CSO, the low-molecular weight chitosan) was chosen as a main material to prepare drug delivery carrier, because it is a biodegradable, biocompatible, and low-toxic material (Zhang et al., 2003, Vishu Kumar et al., 2004, Liu and Yao, 2002, Andres and Martina, 1998, Janes et al., 2001). The amphiphilic stearic acid-grafted chitosan oligosaccharide (CSO-SA) polymer was synthesized. The CSO-SA could self-aggregate to form polymer micelles in aqueous solution, which was applied to the loading of bovine serum albumin, DNA and paclitaxel (Hu et al., 2006a, Hu et al., 2006b, Hu et al., 2006c). In order to reduce the leakage of drug in future in vivo transport process, the core modification of CSO-SA micelle was developed as an alternative approach. Herein, SA was physical solubilized into the core of CSO-SA micelles to get the core-modified micelles. The effect of incorporation of SA on the properties of CSO-SA micelles was investigated. DOX was then used as a model drug to incorporate into the micelles. The effect of incorporation of SA on the properties of drug-loaded CSO-SA micelles, such as micelle size, zeta potential, surface morphology, drug entrapment efficiency and the in vitro drug release behaviors, were further investigated in detail.