Background: Therapeutic strategies to engage anti-tumor innate immunity are still underdeveloped. Imprime PGG (imprime), a pathogen-associated molecular pattern (PAMP), through pattern recognition receptors, successfully illicit a broad-based innate immune response in preclinical models against various cancers. We aimed to study safety and efficacy of imprime in combination with pembrolizumab in advanced stage non-small cell lung cancer (NSCLC)., Methods: We conducted an investigator-initiated, multi-institutional, single-arm, phase Ib/II trial in previously treated, advanced stage NSCLC patients. Primary endpoints were maximum-tolerated dose (MTD) of imprime for the phase Ib, and progression-free survival (PFS) for the phase II study (NCT03003468)., Results: All 33 eligible patients were included in the safety analysis. No dose-limiting toxicity was observed and the imprime dose of 4 mg/kg was determined as the MTD. Thirty patients treated at the MTD (phase Ib, 6; phase II, 24) were included in the efficacy analysis. Median length of follow-up was 10.8 months. Confirmed objective response rate was 10% [95% confidence interval (CI): 2-27%], with one complete and two partial responses. Median PFS was 2.6 months (95% CI: 1.4-7.0), and 6- and 12-month PFS rates were 37% and 17%, respectively. Median overall survival (OS) was 11.1 months, and 6- and 12-month OS rates were 75% and 46%. Univariate analysis was performed to assess the impact of age, sex, race, disease-stage, programmed death-ligand 1 (PD-L1) expression levels, and prior immunotherapy on PFS and OS. Of these, prior immunotherapy negatively influenced OS [hazard ratio (HR): 2.95, 95% CI: 1.21-7.24]. Overall, the combination was safe and tolerable., Conclusions: The combination of imprime and pembrolizumab is tolerable but did not improve the outcome of advanced stage NSCLC patients who previously progressed on anti-programmed death 1 (PD-1)/PD-L1 immunotherapies. Further investigation is needed to understand the effects of therapeutic PAMPs to mount a strong innate immune response against cancer., Competing Interests: Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-24-346/coif). M.F. serves as an unpaid editorial board member of Translational Lung Cancer Research from October 2023 to September 2025. All authors report that they had received institutional research funding from Merck and HiberCell to conduct the study. M.F. received institutional research funding from Amgen, Abbvie, Elicio, Incyte, Genmab, Merck, GSK, Pfizer, Astellas, BMS, Beigene, AZ, Seattle Genetics, Mirati, Genentech, Immunocore, Fate therapeutics, and Poseida; received speaker honoraria from ASCO, Iowa Oncology Society and OncLive and serves on the Pfizer, Mirati, Abbvie, Immunocore, AstraZeneca, Jazz, Immunocore advisory board. J.M. received institutional research funding from Biohaven, Daiichi-Sankyo, BMS, Beyond Spring Pharma, Celldex; serves as a consultant or on the advisory board of Janssen, BMS, Bioatla, AZ, Tekeda, Abbvie, Daiichi-Sankyo, Regernron, Sanofi, Jazz Pharma, and Mirati; and received honoraria for educational events from Tempus. A.S. received institutional research funding from Hutchison MediPharma, Merck, Inhibrx, Verastem Oncology, Turning Point Therapeutics, Gritstone, Bolt Therapeutics, BMS, Pfizer, Astellas, Oncologie, Macrogenics, Seattle Genetics, Amgen, Daiichi, Lilly, Jacobio. A.S. serves as a consultant or on the advisory board for Regeneron/Sanofi, Pfizer, guardant, Catalyst Pharmaceuticals and KLJ Associates. A.S. received honoraria for educational events from OncLive, OSCO/ASCO Direct, ACPMP, Cholangiocarcinoma Summit, Cholangiocarcinoma Foundation, and ASCO. She also participates in ACPMP Think Tank. M.M.P. serves as consultant for Center for Business Models in HealthCare and Prevent Cancer through MMP Consulting Services and received institutional research support from Coleman Foundation and American Lung Association. M.M.P. received travel grant from GO2 Foundation and participate in Prevent Cancer and NLCRT. The authors have no other conflicts of interest to declare., (2024 AME Publishing Company. All rights reserved.)