Gelatin-based NIR and reduction-responsive injectable hydrogels cross-linked through IEDDA click chemistry for drug delivery application

NIR and reduction-responsive gelatin hydrogels were designed for anti-tumor drug delivery application. Gelatin was functionalized with norbornene (Gel-Nb), followed by covalently cross-linking with a tetrazine (Tz)-based cross-linker (DSe-DPEG-DTz) possessing a redox-cleavable diselenide moiety. The resulting hydrogels were highly porous thanks to the N₂ gas produced during the inverse electron demand Diels Alder ‘‘click reaction’’ between Nb and Tz. The hydrogels exhibited enhanced drug loading efficiency (≈ 94%) and excellent swelling ratios. The hydrogel prepared from the Nb:Tz mol. ratio of 10:10 (GHG-C) showed a storage modulus of 1100 Pa with an elastic rheological property. The doxorubicin (DOX)-loaded hydrogels (AR1) released minimal amounts (26%) of DOX at a physiological condition (PBS, pH 7.4). On the contrary, a fast release of DOX was observed in a reducing environment, where > 95% of DOX was released from AR3 after 48 h. The DOX and indocyanine green (ICG) co-loaded hydrogels (AR6) showed a burst release of DOX (>60% after 12 h) upon NIR irradiation, followed by a sustained release of the drug. The combined stimuli of GSH and NIR showed ≈ 85% in half of the total time. Gel-Nb, the cross-linker, and GHG-C were essentially non-toxic to the tested cell lines. Furthermore, AR3 and AR6 restricted the metabolic activities of BT-20 cells after treatment with GSH and NIR irradiation, respectively.