Elsevier

Polymer

Volume 52, Issue 21, 29 September 2011, Pages 4753-4759
Polymer

Synthesis and physicochemical characterization of amphiphilic block copolymers bearing acid-sensitive orthoester linkage as the drug carrier

https://doi.org/10.1016/j.polymer.2011.08.024Get rights and content

Abstract

Amphiphilic block copolymers bearing an acid-sensitive orthoester linkage, composed of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic poly(γ-benzyl L-glutamate) (PBLG), were prepared as the carrier capable of selectively releasing the hydrophobic drug at the mildly acidic condition. Diblock copolymers with various lengths of PBLG were synthesized via ring opening polymerization of benzyl glutamate NCA in the presence of the acid-labile PEG as a macroinitiator. Owing to their amphiphilicities, the copolymers formed spherical micelles in aqueous conditions, and their particle sizes (22–106 nm in diameter) were dependent on the block length of PBLG. These nanoparticles were stable in the physiological buffer (pH 7.4), whereas they were readily decomposed under the acidic condition. In particular, the block copolymer with a smaller hydrophobic portion was rapidly disassembled under the acidic condition. Doxorubicin (DOX), chosen as the model anti-cancer drug, was effectively encapsulated into the hydrophobic core of the micelles using the solvent casting method. The loading efficiency depended on the hydrophobic block length of the copolymer; i.e., the longer hydrophobic block allowed for loading of larger amounts of the drug. In vitro release studies demonstrated that DOX was slowly released from the pH-sensitive micelles in the physiological buffer (pH 7.4), whereas the release rate of DOX significantly increased under the acidic condition (pH 5.0). From the in vitro cytotoxicity test, it was found that DOX-loaded pH-sensitive micelles showed higher toxicity to SCC7 cancer cells than DOX-loaded micelles without the orthoester linker. These results suggest that the amphiphilic block copolymer bearing the orthoester linkage is useful for pH-triggered delivery of the hydrophobic drug.

Introduction

The biodegradable micelle, composed of an amphiphilic copolymer, has emerged as the promising nano-carrier for the delivery of chemotherapeutic agents [1], [2]. Particularly, polymeric micelles have distinctive characteristics as the anti-cancer drug carrier, including high thermodynamic stability, prolonged circulation in blood, and preferential accumulation into tumor tissue. This site-specific accumulation of the polymeric micelle is due to its high permeability to tumor tissue with leaky vasculature and its low clearance by defective lymphatic vessels, which is also called the enhanced permeation and retention (EPR) effect [1], [3], [4], [5], [6], [7]. However, for practical applications, the fine tuning of drug release in a controlled fashion is an interesting field of research in drug delivery systems. It is a challenge for micelles to remain stable in the blood circulation and to selectively release the drug after reaching the target site. To overcome this issue, stimuli-sensitive polymers have been developed because these micelles are able to release the drug in response to pathophysiological conditions. Stimuli-sensitive micelles are expected to reach tumor tissue via the EPR effect, be internalized into the tumor cells by endocytosis, and release the drug when exposed to a certain intracellular stimulus.

Numerous stimuli-sensitive micelles have been developed using biomaterials that are sensitive to various stimuli such as ultraviolet radiation [8], ultrasound waves [9], pH changes [10], [11], [12], [13], [14], [15], [16], and temperature conditions [17], [18]. Of the stimuli, pH provides opportunity for site-specific drug delivery because there is the difference in pH between normal tissues and pathophysiological sites. For instance, it is well known that a mildly acidic microenvironment is present in the interstitial spaces of tumor tissues, ascribed to high rates of glycolysis in cancer cells [19], [20], [21]. The low pH is also found in the intracellular compartments of the cells such as endosomes and lysosomes. In this regard, pH-sensitive micelles, which exhibit physicochemical changes at the mildly acidic condition, have been considered as promising carriers for extracellular and/or intracellular drug delivery.

In the present study, we aimed to develop acid-sensitive and biodegradable amphiphiles as the carrier of the anti-cancer drug, doxorubicin (DOX). The amphiphilic diblock copolymers, composed of poly(ethylene glycol) (PEG) and poly(γ-benzyl L-glutamate) (PBLG), were prepared by ring opening polymerization of benzyl glutamate N-carboxy anhydride (NCA) in the presence of a PEG macroinitiator bearing the pH-sensitive orthoester linkage (PEG-pH-NH2). By varying the feed ratio of the macroinitiator to the benzyl glutamate NCA, three different diblock copolymers (PEG-pH-PBLGs) were obtained. The physicochemical characteristics of the PEG-pH-PBLGs were determined using 1H NMR, dynamic light scattering (DLS), and transmission electron microscopy (TEM). In addition, the in vitro DOX release behaviors of the micelles were measured under two different pH conditions (pH 7.4 and pH 5.0).

Section snippets

Materials

α-Methoxy-ω-amino poly PEG (PEG-NH2) with a Mn of 5000 g/mol was purchased from Laysan Bio Inc. (Arab, AL, USA). Monomethoxy PEG (PEG-OH) with a Mn of 5000 g/mol, benzyl glutamate, triphosgene, 3-amino-1,2-propanediol, trimethylorthoformate, p-toluene sulfonic acid (p-TSA), and doxorubicin·hydrochloride (DOX·HCl) were purchased from Sigma–Aldrich Co. (St. Louis, MO, USA). Trifluoroethyl acetate was obtained from Tokyo Chemical Industries (Tokyo, Japan). The water used in the experiments was

Results and discussion

In an attempt to develop acid-sensitive micelles as the carrier of the anti-cancer drug, a pH-sensitive orthoester linkage was introduced between hydrophilic PEG and hydrophobic PBLG, as shown in Fig. 1. Since the orthoester linkage is cleavable in acidic conditions such as the interstitium of solid tumors and the intracellular compartments of cells [24], it was expected that the PEG-pH-PBLG micelles may have potential as carriers for anti-cancer drugs.

Conclusions

A series of PEG-pH-PBLG diblock copolymers that are cleavable at the mildly acidic condition were synthesized, and their physicochemical characteristics as the carrier of DOX were explored. These block copolymers formed stable micelles at physiological conditions and effectively encapsulated DOX in their hydrophobic inner cores. However, the PEG-pH-PBLG micelles were highly unstable at the mildly acidic condition, leading to disintegration of the micellar structure and precipitation.

Acknowledgments

This work was supported by the Basic Science Research Programs (20100027955 & 20100015804) of MEST, the National R&D Program for Cancer Control (0920120) and the Korea Healthcare Technology R&D Project (A101706-1001-0000200) of MW, and the BioImaging Research Center at GIST, Republic of Korea.

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