MTOR and head and neck squamous cell carcinoma: Agents targeting the PI3K pathway in HNSCC are under investigation, such as the pan-PI3K pathway inhibitor Buparlisib/BKM120 (Bup) and the dual inhibitor targeting the catalytic sites of PI3K and Mechanistic target of rapamycin (MTOR), Omipalisib/GSK2126458 (Omi).