The crucial role of macromolecular engineering, drug encapsulation and dilution on the thermoresponsiveness of UCST diblock copolymer nanoparticles used for hyperthermia.

Poly(acrylamide-co-acrylonitrile) (P(AAm-co-AN)), an upper critical solution temperature (UCST)-type copolymer in water, was synthesized by reversible addition fragmentation chain transfer (RAFT) copolymerization and used as a macro-RAFT agent for the polymerization of oligo(ethylene glycol) methyl ether methacrylate (OEGMA) to yield amphiphilic diblock P(AAm-co-AN)-b-POEGMA copolymer. A series of copolymers with different AN content was obtained allowing to finely tune the UCST behavior (cloud point (T_t-UCST) from 35 to 78 °C). Addition of the POEGMA block did not modify the T_t-UCST regardless its Mn but provided a lower critical solution temperature behavior at high temperature. Nanoparticles were then formulated by the nanoprecipitation technique revealing that copolymers with higher T_t-UCST yield smaller, better-defined nanoparticles. Eventually, doxorubicin (Dox) was encapsulated into nanoparticles made from the copolymer having a T_t-UCST close to mild hyperthermia (~43 °C). Surprisingly, Dox encapsulation increased T_t-UCST and gave smaller nanoparticles as opposed to their unloaded counterparts. The dilution of the suspension also led to a decrease of Tt-UCST. No obvious hyperthermia effect was observed on the cytotoxicity of Dox-loaded nanoparticles. Our study highlighted the influence of macromolecular engineering, drug encapsulation and nanoparticle dilution on UCST behavior, important features often overlooked despite their crucial impact in the development of thermosensitive nanoscale drug delivery systems.