Background: GB1275 is a first-in-class CD11b modulator that reduced myeloid-derived suppressor cells (MDSCs) and tumor associated macrophages (TAMs) at the tumor site, repolarized M2 immunosuppressive TAMs to an M1 phenotype, and increased tumor infiltration of activated CD8+ T cells in preclinical models. When combined with an anti-PD-1 antibody or chemotherapy, these immunomodulatory effects translated into potent anti-tumor effects and prolonged survival in orthotopic PDAC models [Panni RZ, et al. Sci Transl Med. 2019 Jul 3;11(499)]. This ongoing first-in-human study consists of dose escalation of GB1275 monotherapy (Regimen A), GB1275 + pembrolizumab (Regimen B), and GB1275 + nab-paclitaxel + gemcitabine (Regimen C), followed by Phase 2 expansion in newly diagnosed metastatic pancreatic, MSS colorectal, and PD-L1-positive gastric/GEJ cancers. Here we report interim results of the dose escalation portion of the trial. Methods: The dose escalation phase is based on a standard oncology phase 1, 3+3 design. Cohorts of 3 to 6 patients (pts) with histologically confirmed locally advanced/metastatic pancreatic, esophageal, gastric, MSS colorectal, prostate, or breast cancer were sequentially assigned to ascending dose levels of GB1275 taken orally twice daily (BID) in 1 of 3 regimens: Regimen A was initiated first; Regimen B commenced after completion of the first two cohorts of Regimen A, and Regimen C will be initiated when Regimen A is completed. Dose escalation was based on assessment of safety including dose-limiting toxicity (DLT). Serial blood and tumor samples were collected for pharmacokinetic (PK) and biomarker analyses. Results: As of January 21, 2020, 13 pts were treated, with 3 each in Regimen A (GB1275 100mg, 200 mg and 400 mg BID) dose levels and 4 in Regimen B with GB1275 100 mg BID + pembrolizumab. No DLTs have been reported. GB1275 treatment-related adverse events were reported in 5 pts; all were Grade 1 in severity. Preliminary PK analyses showed a mean elimination half-life of ~7 hours. Reduction in peripheral MDSCs was observed in the majority of pts with serial samples. Biomarker analysis in serial tumor tissue is ongoing. Conclusions: Preliminary data show minimal treatment-related toxicities with the studied regimens. PK data support BID dosing. Dose escalation is ongoing. Updated data will be presented. Clinical trial information: NCT04060342.