Biocompatible, pH-sensitive AB(2) Miktoarm Polymer-Based Polymersomes: Preparation, Characterization, and Acidic pH-Activated Nanostructural Transformation

Abstract

Motivated by the limitations of liposomal drug delivery systems, we designed a novel histidine-based AB(2)-miktoarm polymer (mPEG-b-(polyHis)(2)) equipped with a phospholipid-mimic structure, low cytotoxicity, and pH-sensitivity. Using "core-first" click chemistry and ring-opening polymerization, mPEG(2kDa)-b-(polyHis(29kDa))(2) was successfully synthesized with a narrow molecular weight distribution (1.14). In borate buffer (pH 9), the miktoarm polymer self-assembled to form a nano-sized polymersome with a hydrodynamic radius of 70.2 nm and a very narrow size polydispersity (0.05). At 4.2 µmol/mg polymer, mPEG(2kDa)-b-(polyHis(29kDa))(2) strongly buffered against acidification in the endolysosomal pH range and exhibited low cytotoxicity on a 5 d exposure. Below pH 7.4 the polymersome transitioned to cylindrical micelles, spherical micelles, and finally unimers as the pH was decreased. The pH-induced structural transition of mPEG(2kDa)-b-(polyHis(29kDa))(2) nanostructures may be caused by the increasing hydrophilic weight fraction of mPEG(2kDa)-b-(polyHis(29kDa))(2) and can help to disrupt the endosomal membrane through proton buffering and membrane fusion of mPEG(2kDa)-b-(polyHis(29kDa))(2). In addition, a hydrophilic model dye, 5(6)-carboxyfluorescein encapsulated into the aqueous lumen of the polymersome showed a slow, sustained release at pH 7.4 but greatly accelerated release below pH 6.8, indicating a desirable pH sensitivity of the system in the range of endosomal pH. Therefore, this polymersome that is based on a biocompatible histidine-based miktoarm polymer and undergoes acid-induced transformations could serve as a drug delivery vehicle for chemical and biological drugs.

Fig. 1

Synthetic procedure for synthesis of the 3-miktoarm star copolymer.

Fig. 2

¹H-NMR spectrum of mPEG-b-(polyHis)₂ in D₂O with 0.1 wt.% DCl.

Fig. 3

Acid-base titration of mPEG-b-(polyHis)₂ in 150 mM NaCl. The solid lines were drawn to estimate the buffering range and the pK_a value.

Fig. 4

(a) Angle dependence (DLS mode) and (b) a partial Zimm plot (SLS mode) of the mPEG-b-(polyHis)₂ nanostructure in borate buffer (0.3 mg/mL, pH 9.0). Solid lines represent linear fits to the data points.

Fig. 5

Micrograph of polymersomes observed by TEM and a schematic illustration of a unilamellar vesicle formed by the miktoarm star copolymer. The degree of polymerization (DP) for ethylene glycol (k) and histidine (l) in each arm as well as the weight fraction (f) of PEG are also indicated.

Fig. 6

DLS intensity-weighted apparent hydrodynamic radius (R_h,app) distribution at 90° for the polymer solution (0.3 mg/mL) at (a) pH 7.4, (b) pH 6.8, (c) pH 6.0, and (d) pH 5.0.

Fig. 7

TEM micrographs of the polymer nanostructures at (a) pH 7.4, (b) pH 6.8, (c) pH 6.0, and (d) pH 5.0.

Fig. 8

Variation in (a) the micropolarity and (b) the critical association value (CAC) as a function of pH for the 0.3 mg/mL polymer solution (mean ± standard deviation; n=3).

Fig. 9

In vitro release profiles of the encapsulated CF in the polymersome at different pH (mean ± standard deviation; n=3).

Fig. 10

Cytotoxicity of mPEG-b-(polyHis)₂ polymersomes and representative polycations against MCF7/ADR-RES and A2780/ADR cells after a 5 d incubation (mean ± standard error; n=6).