Phase 2 Study of Imprime PGG & Pembrolizumab in Subjects With Adv SCCHN Who Failed Pembro Monotherapy or Experiencing SD

Objective: To determine the Overall Response Rate (ORR) to Imprime PGG + pembrolizumab in subjects with advanced Squamous Cell Carcinoma of the Head and Neck (SCCHN) Safety: To characterize the safety of Imprime PGG + pembrolizumab given in combination Hypothesis: Restore (for subjects who have failed pembrolizumab mono therapy) or enhance (for subjects who actively experiencing SD) sensitivity to checkpoint inhibitors (CPI) by appropriate and effective stimulation of the subject's innate and adaptive immune systems by combining Imprime PGG with pembrolizumab. The study will require documenting at least 5 objective responses among the 38 subjects enrolled who have failed prior pembrolizumab monotherapy and at least 17 objective responses among the 49 subjects enrolled who are actively experiencing stable disease following at least 4 cycles (but no more than 8 cycles) of pembrolizumab monotherapy.