Effects of cholesterol incorporation on the physicochemical, colloidal, and biological characteristics of pH-sensitive AB₂ miktoarm polymer-based polymersomes

In our previous study, a histidine-based AB₂ miktoarm polymer, methoxy poly(ethylene glycol)-b-poly(l-histidine)₂ (mPEG-b-(PolyHis)₂), was designed to construct pH-sensitive polymersomes that transform in acidic pH; the polymer self-assembles into a structure that mimics phospholipids. In this study, the polymersomes further imitated liposomes due to the incorporation of cholesterol (CL). The hydrodynamic radii of the polymersomes increased with increasing CLwt% (e.g., 70nm for 0wt% vs. 91nm for 1wt%), resulting in an increased capacity for encapsulating hydrophilic drugs (e.g., 0.92μL/mg for 0wt% vs. 1.42μL/mg for 1wt%). The CL incorporation enhanced the colloidal stability of the polymersomes in the presence of serum protein and retarded their payload release. However, CL-incorporating polymersomes still demonstrated accelerated release of a hydrophilic dye (e.g., 5(6)-carboxyfluorescein (CF)) below pH 6.8 without losing their desirable pH sensitivity. CF-loaded CL-incorporating polymersomes showed better cellular internalization than the hydrophilic CF, whereas doxorubicin (DOX)-loaded CL-incorporating polymersomes presented similar or somewhat lower anti-tumor effects than free hydrophobic DOX. The findings suggest that CL-incorporating mPEG-b-(PolyHis)₂-based polymersomes may have potential for intracellular drug delivery of chemical drugs due to their improved colloidal stability, lower drug loss during circulation, acidic pH-induced drug release, and endosomal disruption.