Single-fraction radiotherapy with a bioactive depot of CD39 blockade eradicates malignant tumors.

The efficacy of radio-immunotherapy is generally limited by adaptive resistance mechanisms that create an immunosuppressive tumor microenvironment. The molecular drivers of this radio-resistance remain poorly defined. Here we identify the ectonucleotidase CD39 as a critical driver of radiotherapy (RT)-induced immunosuppression. We show that RT robustly upregulates CD39 on tumor cells and tumor-infiltrating immune cells, creating an adenosine-rich tumor microenvironment that promotes immune evasion. Pharmacological inhibition of CD39 with POM-1 reverses this immunosuppression, synergizing with RT to enhance anti-tumor immunity. Furthermore, we engineer an immuno-gel (POM-1@iGel) for sustained local CD39 inhibition and concurrent immune stimulation. A single intratumoral injection of POM-1@iGel in combination with single-fraction RT establishes durable systemic anti-tumor immunity with immunological memory and leads to complete tumor regression in a murine colorectal cancer model. These findings elucidate a critical mechanism of RT-induced immune escape and present a rational, scalable strategy to overcome RT-induced immunosuppression and boost radio-immunotherapy in solid tumors.