Abstract PR01: Analysis of spatiotemporal phenotypic heterogeneity in chemoresistant triple negative breast cancer using imaging mass cytometry

Shifts in tumor phenotype in response to selective pressures (i.e. changing microenvironments, drug treatments) pose one of the biggest obstacles to successful cancer therapies. Phenotypically diverse tumor and stroma subpopulations, and interactions between them that alter tumor biology, represent unique signaling niches. We hypothesize that these localized neighborhoods of breast tumor cells possess specialized phenotypes that mediate chemoresistance and represent novel therapeutic vulnerabilities. In order to assess these potential phenotypes, we utilized imaging mass cytometry (IMC), a highly multiplexed imaging modality that allows simultaneous measurement of 30-40 antigens while retaining the spatial architecture of the cancer tissue. We have constructed an IMC antibody panel that combines markers for tissue architecture, tumor and stromal cell phenotyping, and signaling pathway activation. IMC was applied to a collection of patient-derived xenograft (PDX) models of triple negative breast cancer (TNBC). Our PDX collection was established from patient tumors obtained before and after neoadjuvant chemotherapy. IMC analysis of 18 PDX models representing eight patients revealed that stromal cell phenotypes were generally shared between all models, but tumor cell phenotypes were largely patient-specific. While every model was comprised primarily of a few major tumor cell phenotypes, we noted that each case also harbored several minor, unique populations, suggesting that specialized neighborhoods may exist within the tumor mass. Sequential PDX models showed a wide range of phenotypic responses after chemotherapy, ranging from stable tumor composition to widespread changes in tumor phenotypes. Importantly, comparison of multiple pre-/post-chemotherapy PDX pairs revealed spatially constrained MAPK activation emerging after treatment. To capture acute changes in tumor phenotype, we treated animals bearing PDX tumors with chemotherapy and evaluated tumors by IMC. As tumors regressed and then regrew, we identified novel phenotypic shifts, again including increased MAPK signaling localized to discrete neighborhoods, suggesting this phenomenon may be a common feature of chemoresistant TNBC. Taken together, our findings suggest that distinct tumor phenotypes arise following treatment. Ongoing work is examining the spatial arrangement of these cell types to determine their local niche compositions, and then correlating these findings with clinical features. Our goal is to determine whether these unique signaling niches functionally contribute to chemoresistance and if disruption of these niches enhances chemosensitivity. Citation Format: Amanda L. Rinkenbaugh, Vidya C. Sinha, Jiansu Shao, Xiaomei Zhang, Gloria V. Echeverria, W. Fraser Symmans, Stacy L. Moulder, Helen Piwnica-Worms. Analysis of spatiotemporal phenotypic heterogeneity in chemoresistant triple negative breast cancer using imaging mass cytometry [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PR01.