Document resource
Background Radiopharmaceutical therapy (RPT) can directly kill tumors or, at lower doses, modulate the tumor microenvironment (TME). The range of beta particles (millimeters) produces a field effect; however, alpha-particle range is limited to a few cell diameters. Thus, an understanding of the spatial distribution of radionuclides at the cellular level is critical for development of RPT but remains challenging. Imaging Mass Cytometry™ (IMC™) technology could provide a direct route for visualization of RPT deposition and concurrent profiling of the TME.Prostate stem cell antigen (PSCA), overexpressed in pancreatic cancers, can be targeted using an engineered antibody fragment (A2DM) labeled with beta- or alpha-emitting radionuclides (177Lu or 225Ac). These RPT agents inhibited growth of pancreatic KPC-hPSCA (human PSCA) tumors in a syngeneic hPSCA knock-in mouse. Attempts to detect the stable decay product of 177Lu (177Hf), were unsuccessful since therapeutic radionuclides are administered at trace levels.Here, A2DM was labeled at mass levels with 169Tm to provide a cold RPT analog to enable detection and localization by IMC. In parallel, spatial profiling of changes to the TME following low-dose of alpha- or beta-RPT was assessed by multiplex IMC at the tissue and cellular level.Methods A2DM was conjugated to 169Tm-DOTA to generate the cold RPT analog. KPC-hPSCA tumor-bearing mice received escalating protein doses of 169Tm-A2DM (10-100µg), and tumors harvested at 48 hr. For low-dose RPT, mice were administered 225Ac-A2DM or 177Lu-A2DM, and tumors collected at days 4, 7 and/or 14. Tissues were formalin-fixed and processed for IMC staining with a 33-marker panel.Results The 169Tm-labeled analog was visualized in tumor sections by IMC in a protein dose-dependent manner (figure 1). IMC confirmed high hPSCA expression across most tumors, with some heterogeneity. KPC-hPSCA tumors exhibited dense stroma and an immunosuppressive TME enriched in myeloid-derived suppressor cells (MDSCs: CD11b+ Ly6G+ Arginase+ iNOS+) and tumor-associated macrophages (TAMs: F4/80+ CD206+). RPT-treated tumors showed a reduction in MDSCs, suggesting alleviation of immunosuppression, particularly at day 7 following 225Ac-RPT and day 4 following 177Lu-RPT (figure 2).Conclusions IMC technology was successfully employed to visualize the spatial distribution of an RPT analog and to assess RPT-induced changes in the TME. A reduction in immunosuppressive cells was observed following low-dose alpha- or beta-RPT. Going forward, co-administration of RPT agents labeled with mass amounts of cold radiometal and trace therapeutic radionuclide should enable evaluation of the RPT microdistribution and concurrent spatial evaluation of radiation effects on the TME, facilitating development of effective RPT strategies.Ethics Approval Animal studies were conducted under a protocol approved by the City of Hope Institutional Animal Care and Use Committee; approval number 18118.Abstract 957 Figure 1Direct visualization of RPT analog. A) 177Lu-A2DM RPT and cold analog 169Tm-A2DM. B) Dose-dependent visualization of 169Tm-A2DM in KPC-hPSCA tumors. C) Co-visualization of 169Tm analog with CD31, hPSCA, and CD45 in tumors using Hyperion XTI tissue modeAbstract 957 Figure 2Reduction in MDSCs in α-RPT- and β-RPT-treated tumors. IMC shows reduced MDSCs (CD11b+ Arginase+) in A) KPC-hPSCA tumors treated with 150nCi/10µg 225Ac-A2DM at day 7 or B) 75µCi/10µg 177Lu-A2DM at day 4 compared to controls. Imaged on Hyperion XTi