ROS-responsive amphiphilic block copolymer-drug conjugate: Design, synthesis and potential as an efficient drug delivery system via a positive feedback strategy

Reactive oxygen species (ROS)-responsive drug delivery systems for cancer treatment often suffer insufficient drug release. Herein, we developed a ROS-responsive amphiphilic block copolymer-drug conjugate, TA-CA-Prodrug, based on a positive feedback strategy to address this issue. Cinnamaldehyde (CA), a ROS-generation agent, was modified as a ROS-responsive linker in the copolymer to connect hydrophilic and hydrophobic segments and conjugate an anti-tumor drug (PTX) to the main copolymeric chain. A pH-sensitive moiety (DPA) was also incorporated to the copolymer. This amphiphilic prodrug self-assembled into micelles at a particle size of around 150 nm with a negative zeta potential. ROS-responsive release of PTX and CA was confirmed after TA-CA-Prodrug was incubated with H2O2, and the drug release rate was dependent on the concentration of H2O2. After endocytosis into tumor cells, TA-CA-Prodrug were mainly colocalized with mitochondria owing to the charge reversal after protonation of DPA moieties of TA-CA-Prodrug in an acidic intracellular environment (i.e., pH ≤ 6.2). A relatively high level of ROS around the mitochondria induced simultaneous release of CA and PTX from the prodrug. Then, ROS-generation was triggered by released CA, which in turn enhanced PTX release and PTX-mediated cell cycle arrest, thus resulting in remarkable apoptosis of tumor cells after treatment with TA-CA-Prodrug compared with control groups (TK-Prodrug and Prodrug). Due to a cascaded ROS-feedback strategy, i.v. injection of TA-CA-Prodrug into mice bearing 4 T1 tumors led to a greater tumor inhibition efficacy than control groups, and no obvious side effects were confirmed from negligible changes in body weight as well as H&E stained major organs of mice. Therefore, this block copolymer-drug conjugate could enhance intracellular ROS production for efficient drug release and augment its anti-tumor effect.