Tumoral acidic extracellular pH targeting of pH-responsive MPEG-poly(β-amino ester) block copolymer micelles for cancer therapy
Introduction
Many therapeutic anticancer drugs, while pharmacologically effective in cancer treatment, are limited in their clinical applications by serious toxicities. To overcome these obstacles, researchers have focused on the development of nano-sized anticancer drug carriers, which can improve therapeutic efficacy while also reducing unwanted side effects [1], [2], [3]. Among these carriers are polymeric micelles, typically 20 to 100 nm in diameter, which are formed by amphiphilic block copolymers under aqueous conditions [4], [5], [6]. Intravenously injected polymeric micelles exhibit prolonged circulation times by avoiding rapid renal clearance and unwanted uptake by the reticuloendothelial system (RES) [7], [8], resulting in enhanced permeability and retention (EPR) of tumor tissue, in which disorganized visualization and defective vascular architecture have developed [9], [10].
Stimuli-responsive polymeric micelles as nano-sized drug carriers have been considered for the controlled release of drug into tumor tissue, with temperature, ultrasound, and pH used to trigger drug release from polymeric micelles [11], [12], [13], [14], [15]. Among these stimuli, change of pH at tumor tissue is useful, because tumor tissues have a more acidic environment, due to lactic acid produced by hypoxia and by acidic intracellular organelles [16], [17], [18]. Therefore, various pH-responsive polymeric micelles have been used for rapid, acidic pH-triggered rapid anticancer drug release at tumor sites, with demicellization leading to drug release.
We have described the preparation of pH-responsive MPEG-poly(β-amino ester) polymeric micelles, which are composed of a hydrophilic MPEG shell and a hydrophobic pH-responsive poly(β-amino ester) core [19], [20]. These MPEG-poly(β-amino ester) block copolymers were synthesized by a Michael-type step polymerization and their pH-responsive physicochemical characteristics (micellization–demicellization behavior, critical micelle concentration, average micelle size, etc.) were modulated by controlling the molar ratio of the hydrophilic MPEG and pH-responsive poly(β-amino ester) moieties, because the latter is pH-responsive due to its tertiary amine, with a pKb of about 6.5 [21], [22]. In particular, these pH-responsive polymeric micelles showed sharp pH-dependent micellization–demicellization transitions at the acidic extracellular pH of tumor cells (pH 6.8–7.2). To test the usefulness of these micelles in anti-tumor treatment, we prepared doxorubicin (DOX)-loaded micelles and tested their pH responsiveness and in vivo anti-tumor activity in B16F10 tumor-bearing mice.
Section snippets
Materials
Analytical grade methyl ether poly(ethylene glycol) (MPEG, Mn = 4,850, determined by GPC), acryloyl chloride, hexane-1,6-diol diacrylate (HDD), 4,4′-trimethylene dipiperidine (TDP), triethylamine, anhydrous chloroform, anhydrous dichloromethane (DCM), and DOX were purchased from Sigma Chemical Co. (St. Louis, MO) and used without further purification.
Synthesis of MPEG-poly(β-amino ester) block copolymers
MPEG-poly(β-amino ester) block copolymer was synthesized as described previously [23], [24]. Briefly, MPEG (1 equiv.), dissolved in anhydrous DCM
Preparation of pH-responsive MPEG-poly(β-amino ester) block copolymer
To construct an appropriate pH-responsive block copolymer to form nano-sized polymeric micelles, we combined hydrophilic MPEG with poly(β-amino ester), since the latter is pH-sensitive due to its tertiary diamines, which have a pKb of about 6.5 [21], [22]. The pH-responsive MPEG-poly(β-amino ester) block copolymer was prepared by a Michael-type step polymerization of monoacrylated MPEG (0.1 equiv.), hexane-1,6-diol, acrylate (HDD, 1 equiv.), and 4,4′-trimethylene dipiperidine (TDP, 1.1 equiv.),
Conclusion
The anticancer drug DOX was efficiently loaded into pH-responsive polymeric micelles of MPEG-poly(β-amino ester) block copolymer. DOX molecules were rapidly released from these pH-responsive polymeric micelles and localized in the nuclei of cells under acidic conditions, indicating that these micelles have tumor targeting ability. In mice, these DOX-loaded, pH-responsive polymeric micelles showed noticeable antitumor efficacy, and enhanced survival rates, compared with free DOX. These results
Acknowledgments
This research was financially supported by the Ministry of Science and Technology (F104AA010003-06A0101-00310) in Korea, and the Collaborative R & D Program of KRCF.
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These authors contributed equally to this paper.