Selective Radioprotection by the Fusion Antioxidant Enzyme <scp>GS1XR</scp> Via an <scp>MMP</scp> ‐2/9‐Cleavable Cell‐Penetrating Switch

Radiotherapy is central to cancer treatment but radiation-induced oxidative stress also damages normal tissues. To achieve selective radioprotection of normal tissues, we systematically evaluated, in vitro and in vivo, the antioxidant and radioprotective performance of a previously engineered fusion antioxidant enzyme, GS1XR (GST-SOD1-X-R9). GS1XR efficiently enters normal cells in low matrix metalloproteinases (MMP)-2/9 environments, scavenges radiation-induced reactive oxygen species (ROS), maintains the Nrf2 antioxidant pathway, suppresses apoptosis, and increases clonogenic survival. In contrast, in 3D tumor microenvironments with high MMP-2/9, cleavage of the X peptide removes R9, resulting in a loss of transmembrane capacity and a pronounced reduction in intracellular ROS scavenging. Animal studies further showed that GS1XR significantly alleviates whole-body irradiation-induced hematopoietic injury and, in tumor-bearing models receiving radiotherapy, does not compromise radiotherapy-mediated tumor control. Collectively, GS1XR couples microenvironment-responsive cell entry with enzymatic antioxidation to achieve selective radioprotection while preserving radiotherapy-mediated tumor control.