PDE5A and neoplasm: Administration of ARG1 inhibitor nor-NOHA and phosphodiesterase 5 (PDE5) inhibitor Sildenafil after RT reduced ARG1 expression and MDSC accumulation, enhanced infiltration of CD8+ T cells in the tumor, significantly elevated IFN-γ secretion by CD8+ T cells and delayed tumor regeneration after irradiation 120.