PDE5A and neoplasm: The clinical data support our prior hypothesis that irradiating some GBM tumors can enhance aberrant myelopoiesis to generate MDSC, while concurrent tadalafil during RT may suppress MDSC.8 Previous clinical trials have examined the immune-modulating effect of PDE5 inhibition to reduce MDSC in blood and the tumor microenvironment in head and neck cancer patients.16,24 Our study expands this finding, as we hypothesized that combined PDE5 inhibition with RT would be more effective, because RT induces MDSC production to drive immunosuppression and radioresistance.