A Tumor-Microenvironment-Responsive Chemically Masked IL‑2 Prodrug Potentiates PD‑1 Checkpoint Blockade and Elicits Robust Antitumor Efficacy.

Interleukin-2 (IL-2) is a potent mediator of T-cell activation with significant potential for cancer immunotherapy, yet its clinical utility is severely constrained by its narrow therapeutic window. Here we report a chemically masked IL-2 prodrug (Cm-proIL2) that enables tumor-microenvironment-responsive cytokine activation through tumor-associated protease cleavage. Site-specific conjugation of a poly-(ethylene glycol) (PEG) moiety selectively masks peripheral receptor engagement, while PEG removal within the tumor microenvironment restores IL-2 receptor binding and reduces molecular size, thereby enhancing intratumoral lymphocyte penetration and effector T cell functionality. Building on this modular platform, we further engineered a PD-1-targeted nanobody fusion, PD1-Cm-proIL2, to enable the cis delivery of IL-2 activity to PD-1+ tumor-infiltrating T cells. PD1-Cm-proIL2 induces robust antitumor immunity, resulting in complete tumor regression and durable protection upon tumor rechallenge in a murine colorectal cancer model. Together, these findings demonstrate the feasibility of chemically masked, tumor-responsive cytokine activation as a strategy for improving the therapeutic index of IL-2-based immunotherapies.