Background: NEO-201 is a humanized IgG1 mAb which binds to Core 1 and/or extended Core 1 O-glycans expressed by solid and hematological cancers. NEO-201 reacts against cancer tissues but not most normal tissues. NEO-201 can identify and kill immune suppressor cells (iSCs), such as regulatory T cells (Tregs), neutrophils, and granulocytic myeloid-derived suppressor cells (gMDSCs) in human PBMCs via antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). We observed a reduction in the percentage of circulating Tregs in subjects with stable disease (SD) following single-agent NEO-201 treatment in our Phase 1 study. Resistance to PD-1/PD-L1 blockade in solid cancers may be due to the activity of iSCs in the tumor microenvironment (TME). Based on this, we launched a Phase II study with a safety lead-in combining NEO-201 with pembrolizumab in adults who have progressed on prior checkpoint therapy. Methods: The recommended phase 2 dose (RP2D) of NEO-201 was determined to be 1.5 mg/kg IV every 2 weeks. Cohorts consisting of advanced non-small cell lung cancer (NSCLC), head and neck squamous cell carcinoma (HNSCC), uterine or cervical cancer who progressed during or after one front-line standard of care treatment, including chemotherapy, checkpoint therapy and/or targeted therapy were eligible. Three patients received NEO-201 at the RP2D with pembrolizumab 400 mg IV every 6 weeks (1 cycle) and were evaluated for safety 2 weeks following the 1^st cycle of the therapy. DLTs were defined as specific Grade 3 or 4 toxicities related to NEO-201. Results: Safety lead-in was successfully completed at RP2D with no DLTs. Four subjects were enrolled in the safety lead-in; one subject electively discontinued after the first infusion and was not evaluable for DLT. Of the 3 evaluable subjects, 1 subject with refractory cervical cancer remained on study with SD for 8.5 months, electively coming off study for surgical debulking. The other 2 subjects developed progressive disease. All subjects had grade 3-4 transient neutropenia, which was expected. Other common toxicities included grade 3-4 decreased white blood cells and lymphocytes, grade 3 anemia and grade 2 infusion reactions. Conclusions: NEO-201 has demonstrated the ability to bind and kill iSCs in vitro and this phenomenon was observed in the initial first in human clinical study. This Phase II study was designed to determine if NEO-201 can reactivate the effectiveness of checkpoint inhibitor therapy in subjects who previously progressed on check point therapy. The safety of combining NEO-201 with pembrolizumab has been successfully established and all 4 disease cohorts are now open to accrual with response rate as the primary endpoint. The maximum accrual on this study will be 126 patients. Clinical trial information: NCT03476681.