Biocompatible, pH-sensitive AB2 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 AB₂-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 per mg polymer, mPEG_2kDa-b-(polyHis_29kDa)₂ strongly buffered against acidification in the endolysosomal pH range and exhibited low cytotoxicity on 5 days 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.