Acid-Disintegratable Polymersomes of pH-Responsive Amphiphilic Diblock Copolymers for Intracellular Drug Delivery

Supramolecular vesicles, also referred to as polymersomes, self-assembled from amphiphilic polymers capable of synchronically loading with both hydrophilic and hydrophobic payloads have shown promising potential in drug delivery application. Herein, we report the fabrication of pH-responsive polymersomes via supramolecular self-assembly of amphiphilic diblock copolymers, poly(ethylene oxide)-b-poly(2-((((5-methyl-2-(2,4,6-trimethoxyphenyl)-1,3-dioxan-5-yl)methoxy)carbonyl)amino)ethyl methacrylate) (PEO-b-PTTAMA), which were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization of a pH-responsive monomer (i.e., TTAMA) using a PEO-based macroRAFT agent. The resultant amphiphilic diblock copolymer then self-assembled into vesicles consisting of hydrophilic PEO coronas and pH-responsive hydrophobic bilayers, as confirmed by TEM and DLS measurements. The polymersomes containing cyclic benzylidene acetals in the hydrophobic bilayers were relatively stable under neutral pH, whereas they underwent hydrolysis with the liberation of hydrophobic 2,4,6-trimethoxybenzaldehyde and the simultaneous generation of hydrophilic diol moieties upon exposure to acidic pH milieu, which could be monitored by UV/vis spectroscopy, SEM, and TEM observations. By loading hydrophobic model drug (Nile red) as well as hydrophilic chemotherapeutic drug (doxorubicin hydrochloride, DOX·HCl) into the bilayer and aqueous interior of the polymersomes, the subsequent release of Nile red and DOX·HCl payloads was remarkably regulated by the solution pH values, and a lower pH value led to a faster drug release profile. In vitro experiment, observed by a confocal laser scanning microscope (CLSM), revealed that the pH-responsive polymersomes were easily taken up by HeLa cells and were primarily located in the acidic organelles after internalization, where the pH-responsive cyclic acetal moieties were hydrolyzed and the embedded payloads were therefore released, allowing for on-demand release of the encapsulants mediated by intracellular pH. In addition to small molecule chemotherapeutic drugs, biomacromolecules (alkaline phosphatase, ALP) can also be encapsulated into the aqueous lumen of the polymersomes. Significantly, the pH-triggered degradation of polymersomes could also regulate the release of encapsulated ALP, as confirmed by ALP-activated fluorogenic reaction.