Abstract The tumor microenvironment (TME), or all cellular and molecular components in and around the tumor, has a well-documented impact on cancer cell proliferation, chemoresistance, and overall patient survival. However much of the research into the TME has been limited to the primary site. How the TME of metastases compares to the primary lesions remains poorly characterized, even though metastasis is responsible for 90% of all cancer deaths. Understanding these differences is critical to treating advanced cancer patients. To study this our lab examined the TME in 22 matched sets primary pancreatic tumors and liver metastasis obtained from end-stage pancreatic cancer patients via our rapid autopsy program. We used a multiplex-immunofluorescent technique that allows staining of a single slide with up to 35 antigens and characterized the presence, position and functional statues of cells that comprise the pancreatic TME. These included: tumor cells, vasculature, fibroblasts, macrophages, mast cells, basophils, eosinophils, B cells, T cells, and NK cells, along with multiple subtypes of these. Based on our preliminary data from this, we hypothesize that the organ determines the features of the TME. Analyzing primary tumors and metastases revealed that both the composition and spatial distribution of different cell types are distinct between the primary tumor and liver metastases. PAM clustering of the cellular featured grouped primary tumor with primary and metastasis with metastatis 87% of the time. As the TME has been shown to impact chemotherapeutic response, we further examined chemotherapeutic effect between sites. Comparing gemcitabine (standard of care) and untreated patients, we observed distinct site-based responses. In the primary tumors with gemcitabine treatment there was a significant increase in inflammatory cells (neutrophils and macrophages) in comparison to untreated patients, while in the liver gemcitabine induced an increase in desmoplasia (SMA). Combined this data indicates that the distinct TME’s exist within the same patient in an organ specific manner which can impact chemotherapeutic response. Understand these differences is therapeutically critical to patients whose therapeutic strategies involve targeting the TME to improve drug delivery, modulate immune suppression or otherwise modulate the microenvironment. Citation Format: Krysten E. Vance, Micheal A. Hollingsworth, Paul Grandgenett. Identifying site-specific disparities in the tumor microenvironment of primary and metastatic disease [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3760.