Similar findings were also reported in another clinical trial (NCT00894413), in which tadalafil treatment in HNSCC patients augmented systemic and tumor-specific immunity, reduced peripheral MDSCs numbers, and decreased ARG1 and iNOS in total MDSCs.264 However, even though PDE5 inhibitors can induce enhanced CTL responses, such treatment alone is unlikely to eliminate tumors completely, and on this basis, a combination with other therapies is a rational choice. Here, ARG1 is linked to neoplasm.