Polymersomes from Dual Responsive Block Copolymers: Drug Encapsulation by Heating and Acid-Triggered Release

A series of well-defined thermoresponsive diblock copolymers (PEO₄₅-<i>b</i>-P<i>t</i>NEA_<i>n</i>, <i>n</i> = 22, 44, 63, 91, 172) were prepared by the atom transfer radical polymerization of <i>trans</i>-<i>N</i>-(2-ethoxy-1,3-dioxan-5-yl) acrylamide (<i>t</i>NEA) using a poly­(ethylene oxide) (PEO₄₅) macroinitiator. All copolymers are water-soluble at low temperature, but upon quickly heating to 37 °C, laser light scattering (LLS) and transmission electron microscopy (TEM) characterizations indicate that these copolymers self-assemble into aggregates with different morphologies depending on the chain length of P<i>t</i>NEA and the polymer concentration; the morphologies gradually evolved from spherical solid nanoparticles to a polymersome as the degree of polymerization (“<i>n</i>”) of P<i>t</i>NEA block increased from 22 to 172, with the formation of clusters with rod-like structure at the intermediate P<i>t</i>NEA length. Both the spherical nanoparticle and the polymersome are stable at physiological pH but susceptible to the mildly acidic medium. Acid-triggered hydrolysis behaviors of the aggregates were investigated by LLS, Nile red fluorescence, TEM, and ¹H NMR spectroscopy. The results revealed that the spherical nanoparticles formed from PEO₄₅-<i>b</i>-P<i>t</i>NEA₄₄ dissociated faster than the polymersomes of PEO₄₅-<i>b</i>-P<i>t</i>NEA₁₇₂, and both aggregates showed an enhanced hydrolysis under acidic conditions. Both the spherical nanoparticle and polymersome are able to efficiently load the hydrophobic doxorubicin (DOX), and water-soluble fluorescein isothiocyanate-lysozyme (FITC-Lys) can be conveniently encapsulated into the polymersome without using any organic solvent. Moreover, FITC-Lys and DOX could be coloaded in the polymersome. The drugs loaded either in the polymersome or in the spherical nanoparticle could be released by acid triggering. Finally, the DOX-loaded assemblies display concentration-dependent cytotoxicity to HepG2 cells, while the copolymers themselves are nontoxic