Design of cysteine-based self-assembling polymer drugs for anticancer chemotherapy

Reactive oxygen species (ROS) play essential roles in the body, such as the production of energy in oxidative phosphorylation and signal transduction for homeostasis. Redox balance in biological systems gradually collapses due to various environmental factors, including aging and disease, and induces oxidative stress in the body. None of the natural or synthetic antioxidants have been approved clinically, owing to their adverse effects. Herein, we developed _L-cysteine (Cys)-based polymer micelles as new self-assembling antioxidants to reduce the adverse effects of conventional antioxidants. Poly(ethylene glycol)-block-poly(L-cysteine) (PEG-block-PCys) was synthesized via anionic ring-opening polymerization. Because the free SH groups in the side chains of the PCys segment were protected by disulfide bonds, the obtained block copolymers were amphiphilic and formed polymer micelles (Nano^Cyss) of tens of nanometers in size in aqueous media. The stability of Nano^Cyss in the presence of bovine serum albumin (BSA) was increased by increasing the molecular weight (MW) of the PCys segments, which was analyzed using dynamic light scattering (DLS). The size and coagulation tendency of Nano^Cyss were also analyzed using DLS measurements by changing the pH and NaCl concentration. Nano^Cyss were confirmed to be less toxic both in vitro and in vivo than N-acetylcysteine (NAC) because of their size and biocompatible PEG surface layer. Intraperitoneal (i.p.) administration of Nano^Cyss to the tumor xenograft mouse model successfully suppressed tumor growth. Interestingly, this effect depended on the MW of the PCys segments.