PDCD1 and head and neck squamous cell carcinoma: Currently, a phase I/IIb clinical trial (NCT02521870) is also underway on the use of the TLR9 agonist SD-101 administered intratumorally in combination with pembrolizumab (anti-PD-1 inhibitor) in naïve patients with recurrent or metastatic previously untreated HNSCC.