PDE5A and lymphopenia: We also showed that the inhibition of MDSCs using tadalafil during cranial irradiation of the mice with orthotopic GBM tumors reduced T-cell lymphopenia and improved survival.8 Tadalafil is an FDA-approved phosphodiesterase-5 (PDE5) inhibitor for erectile dysfunction and pulmonary hypertension and has an excellent safety profile.9,10 Based on our preclinical data, we designed a prospective study to combine tadalafil with RT and TMZ for IDH-wildtype GBM.