Redox-active injectable gel using thermo-responsive nanoscale polyion complex flower micelle for noninvasive treatment of local inflammation.

Reactive oxygen species (ROS) scavengers have not been widely used for treatment of local inflammatory reactions such as arthritis and periodontal disease because they are rapidly eliminated from the inflamed site, which results in a low therapeutic effect. Therefore, to enhance the local retention time of ROS scavengers, we developed a redox-active injectable gel (RIG) system by using poly[4-(2,2,6,6-tetramethylpiperidine-N-oxyl)aminomethylstyrene]-b-poly(ethylene glycol)-b-poly[4-(2,2,6,6-tetramethylpiperidine-N-oxyl)aminomethylstyrene] (PMNT-PEG-PMNT) triblock copolymer, which possesses ROS scavenging nitroxide radicals as side chains of the PMNT segment. Cationic PMNT segment in PMNT-PEG-PMNT forms polyion complexes with anionic poly(acrylic acid) (PAAc) to form a flower-like micelle (ca. 79 nm), which exhibits in situ thermo-irreversible gelation under physiological conditions. We confirmed the prolonged site-specific retention time of RIG by performing in vivo noninvasive electron spin resonance imaging and quantitative evaluation. In contrast to low-molecular-weight nitroxide radical compounds that disappeared from injection sites in less than 1h after subcutaneous injection, 40% of the RIG remained even at 3 days. We also found that RIG inhibits neutrophil infiltration and cytokine production, which leads to suppression of hyperalgesia. These findings indicate the potential of RIG as an innovative approach for treatment of local inflammation.