Polymersomes with Endosomal pH-Induced Vesicle-to-Micelle Morphology Transition and a Potential Application for Controlled Doxorubicin Delivery

In order to obtain a novel, pH responsive polymersome system, a series of pH responsive block copolymers were synthesized via the reversible addition–fragmentation chain transfer (RAFT) polymerization of 3,4-dihydro-2H-pyran (DHP) protected 2-hydroxyethyl methacrylate (HEMA) (2-((tetrahydro-2H-pyran-2-yl)oxy)ethyl methacrylate (THP-HEMA)) and 2-(dimethylamino) ethyl methacrylate (DMAEMA) using p(THP-HEMA) as a macro chain transfer agent (mCTA). The degree of polymerization (DP) of the p(THP-HEMA) block was fixed to 35, whereas the DP of the p(DMAEMA) block was systematically varied from 21 to 50. In aqueous solution, the block copolymer with the shortest p(DMAEMA) block (DP = 21) self-assembled into vesicles, while the polymer with 30 units of p(DMAEMA) formed a mixture of micelles and vesicles. The polymer with the longest p(DMAEMA) block (DP = 50) formed exclusively micelles. The corresponding polymersomes exhibited a morphology transition from vesicles at neutral pH values to micelles upon lowering the pH value down to endosomal pH value as investigated by DLS and cryo-TEM. The capability of polymersomes to encapsulate both hydrophobic (e.g., Nile Red) and hydrophilic (e.g., doxorubicin hydrochloride (DOX·HCl)) cargos was verified by in vitro studies. Drug release studies demonstrated that the DOX·HCl release is significantly accelerated under acidic pH values compared to physiological conditions. Cytotoxicity studies revealed that DOX·HCl loaded polymersomes exhibited an efficient cell death comparable to free DOX·HCl. CLSM and flow cytometry studies showed that DOX·HCl loaded vesicles were easily taken up by L929 cells and were mainly located in the cytoplasm and cell nuclei.