Background: Black women with residual estrogen receptor (ER)-positive breast cancer after neoadjuvant chemotherapy (NAC) have worse distant recurrence free survival (DRFS) compared to White women. Distant metastases result from motile breast cancer stem cells (BCSCs) disseminating through intravasation portals on blood vessels called tumor microenvironment of metastasis (TMEM) doorways. Each TMEM doorway is composed of an actin-regulatory protein (Mena) expressing cancer cell, a macrophage expressing high levels of receptor tyrosine kinase TIE2, and an angiopoietin-expressing endothelial cell. High TMEM doorway density predicts metastatic potential in patients with ER+/HER2- breast cancer. The function of TMEM doorways can be pharmacologically blocked using TIE2 inhibitors. We hypothesized that racial disparity in DRFS in patients with residual ER-positive disease post-NAC is due to difference in density of TMEM doorways, BCSCs, and motile cells in the residual breast tumor microenvironment (TME). Methods: We performed a retrospective, multi-institutional study of the TME in the residual disease post-NAC from 200 patients. Triple immunohistochemistry of formalin-fixed paraffin-embedded (FFPE) tissue sections was used to visualize TMEM doorways. Immunofluorescent staining of FFPE tissue sections for SOX9 and MenaINV was used to visualize BCSCs and motile cells, respectively. The density of TMEM doorways, BCSC and motile cells was determined by automated image analysis. The relationship between the density of TMEM doorways, stem and motile cells, and DRFS was examined using Kaplan Meier log-rank test. Comparisons were made for the entire cohort, and separately for the two most prevalent molecular subtypes: ER+/HER2- and triple negative (TN). The analysis included 200 samples of residual breast cancer post-NAC: 100 from Black and 100 from White women. Results: Black compared to White women were more likely to develop distant recurrence (49% vs 34%, p=0.04), to receive mastectomy (70% vs 51%, p=0.009), and to have TN subtype (35% vs 21%, p<0.0001). Median time-to-follow-up was 70 months (range 2.8-160.7), and was similar between Black and White patients (67.7 vs 71.0 months, p=0.7). There was no racial disparity in DRFS in the overall cohort or TN subtype, but there was a trend towards worse survival in Black women in the ER+/HER2- subtype (Log-Rank p=0.09). TMEM doorway density was higher in Black women in the entire cohort (p=0.001) and in the ER+/HER2- subtype (p=0.01), while there was no difference in the TN subtype. TMEM doorway density was prognostic for DRFS in all patients (Log-Rank p=0.009) and the ER+/HER2- subtype (Log-Rank p=0.001), but not in the TN subtype. There was a trend towards higher BCSC density in the entire cohort (p=0.06) and the ER+/HER2- subtype (p=0.09), but there was no difference in the TN subtype. There was no racial disparity in motile cell density in the entire cohort as well as in the two subtypes. BCSC density and motile cell density were not prognostic for DRFS. Conclusion: Worse DRFS in Black, compared to White, women with ER-positive residual breast cancer post-NAC may be due to differences in TMEM doorway and BCSC density. Further studies are necessary to determine if the outcome of patients with residual disease and high TMEM doorway density post-NAC could be improved by blocking TMEM doorway function with TIE2 inhibitors.

Citation Format: Gina Kim, Saeed Asiry, Isabelle Oktay, Yu Lin, Xianjun Ye, Esther Cheng, Nurfiza Ladak, John Condeelis, Esther Adler, Paula Ginter, Timothy D'Alfonso, David Entenberg, Maja Oktay. Racial disparity in post-neoadjuvant chemotherapy residual breast tumor microenvironment [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-08-36.