Room-temperature synthesis of ultrasmall redox-responsive nanogels for tumor-microenvironment-triggered drug release and preliminary in vivo antitumor evaluation

Smart nanogels, combining hydrogel and nanomaterial features, are promising drug carriers for cancer therapy because they can release cargo in response to tumor-related conditions. This study presents an easy, rapid synthesis of exceptionally small nanogels. To our knowledge, this is the first report of redox-degradable, multi-stimuli-responsive nanogels with relatively uniform size distributions, exhibiting a Z-average hydrodynamic diameter of 19.1 ± 0.3 nm in the collapsed state at 65 °C and a dry-state TEM diameter of 26 ± 6 nm, produced by aqueous nanogel-forming polymerization conducted entirely at room temperature without surfactants or organic solvents. The nanogels were synthesized by semi-batch radical polymerization of N-isopropylacrylamide (NIPA) using the redox-sensitive cross-linker N, N′-diacryloyl-L-cystine disodium salt (DACS), prepared in-house. DACS introduced COO⁻ groups, providing pH sensitivity, colloidal stability, and the ability to bind epirubicin hydrochloride (EPB). DLS and TEM revealed GSH concentration-dependent changes in nanogel size and morphology consistent with reductively induced structural disintegration. Drug-release studies showed minimal EPB release at pH 7.4, whereas the highest release was observed at pH 5.0 in the presence of 40 mM GSH, a strongly reducing condition used to accelerate nanogel disintegration. The nanogels showed relatively high drug loading (~ 33%). EPB-loaded nanogels retained cytotoxic activity toward MCF-7 cancer cells while showing lower cytotoxicity than free EPB toward non-tumorigenic MCF-10 A cells. In vivo, the EPB-loaded nanogels produced promising but heterogeneous tumor-suppressive effects, accompanied by necrosis and fibrotic tissue remodeling. These findings provide preliminary evidence of antitumor activity, although spontaneous regression in some untreated animals and inter-animal variability indicate that further validation is required.