Injectable ECM-mimetic dynamic Schiff-base hydrogels deliver 5-aminosalicylic acid for immunomodulatory and alleviation of ferroptosis-associated injury of intervertebral disc degeneration.

Intervertebral disc degeneration (IDD) is driven by persistent inflammation, macrophage dysregulation, and ferroptosis-associated injury of nucleus pulposus cells (NPCs). Here, we developed an acid-responsive injectable hydrogel based on oxidized chondroitin sulfate and adipic acid dihydrazide-modified hyaluronic acid for sustained delivery of 5-aminosalicylic acid (5-ASA). Single-cell transcriptomic analysis and tissue validation revealed increased inflammatory macrophage infiltration and ferroptosis-associated alterations in degenerated discs. In vitro, 5-ASA preserved extracellular matrix homeostasis, reduced iron accumulation and MDA accumulation, and restored GPX4 expression in IL-1β-stimulated NPCs. It also suppressed LPS-induced M1-like macrophage polarization and promoted a reparative phenotype. Conditioned-medium and Transwell coculture experiments further showed that macrophage reprogramming contributed to the alleviation of ferroptosis-associated injury by 5-ASA in NPCs. NF-κB inhibition partially contributed to the protective effects of 5-ASA in NPCs and macrophages, while macrophage-conditioned medium was associated with altered NF-κB signaling in recipient NPCs. The resulting 5-ASA-loaded HACS hydrogel exhibited favorable injectability, porous architecture, acid-responsive degradation, sustained drug release, and good biocompatibility. In a rat IDD model, intradiscal administration of 5-ASA@HACS improved disc height and histological structure, preserved collagen II, reduced MMP13, suppressed p-p65, restored GPX4, and shifted macrophage polarization toward CD206-positive cells. These findings support 5-ASA@HACS as a biomaterial-based strategy for coordinated immunomodulatory and alleviation of ferroptosis-associated injury in IDD.