Abstract PD6-08: Early dynamics of tumor microenvironment in triple negative or ER-low breast cancer: updated analyses from window of opportunity (WOO) MEDIOLA trial of olaparib and durvalumab

Abstract Background We conducted MEDIOLA WOO trial with olaparib and durvalumab before standard neoadjuvant chemotherapy (NAC) in 54 stage II/III triple negative or ER-low breast cancer (NCT03594396) (Im SA et al., SABCS 2021, PD15-08; Koh J et al., SABCS 2024, PS12-01). Here, we present updated analyses. Methods Olaparib 300mg bid was given for 4 weeks with 1500 mg durvalumab (D15), followed by NAC. FDG PET and serial biopsies were taken at baseline (BL), 2 weeks (OT1), and 4 weeks (OT2). We conducted RAD51 foci assays on BL to assess functional homologous recombination deficiency (fHRD), and CT at OT2 for RECIST evaluation. At all timepoints, we performed PD-L1 (22C3) immunohistochemistry (IHC), RNAseq, multiplex IHC (mIHC), and artificial intelligence-guided segmentation (AI-S) of H&E images into tumor cells (TC) and inflammatory cells (IC). Metabolic (mRes) and pathologic (pRes) responses were defined as ≥ 30% reduction of SUVmax in PET and ≥50% reduction in TC. RNAseq analyses included gene set enrichment analysis (GSEA), and molecular subtyping by PAM50 and TNBC types into basal-like immune-activated (BLIA), basal-like immunosuppressed (BLIS), luminal androgen receptor (LAR), and mesenchymal (MES). Results Pathologic complete response (pCR) was achieved in 70.4%. At OT1, early mRes and pRes were observed in 44.4% and 63.0%, which were associated with fHRD status at BL (p < 0.001). pCR was associated with mRes, pRes, and RECIST responses at OT2 (p < 0.001, 0.043, and 0.027), while no such associations were observed with PD-L1 IHC or HRD status at BL. Longitudinal analyses on AI-S results showed gradual decreases in TC from BL to OT1 and OT2 (p < 0.01), while an increase in IC was noted at OT1 compared to BL (p < 0.001), which persisted into OT2. PAM50 subtypes at BL were predominantly Basal (64.8%), followed by LumB (13.0%), Normal-like (11.1%), HER2-enriched (9.3%), and LumA (1.9%). Over time, the proportions of Basal tumors shrank (40.7% at OT1; 14.8% at OT2), while those of LumA expanded (27.8% at OT1; 53.7% at OT2). Most tumors were BLIS (85.2%) at BL, with only 5.6% of BLIA; however, BLIS decreased to 68.5% at OT1, while BLIA increased to 25.9%. mIHC also confirmed the increased amounts of CD3+, CD4+, and CD8+ T-cells at OT1/OT2 compared to BL (p < 0.05), most pronouncedly on CD8+ subset (p < 0.0001). OT1 showed downregulations of MYC target, G2M checkpoint, and E2F target gene sets compared to BL (adj p < 0.001) by GSEA, possibly reflecting the effect of PARP inhibition. OT2 tumors showed significant enrichment of interferon-γ/α, IL2-STAT5, IL6-JAK-STAT3, and TNFα signaling pathways compared to BL (adj p < 0.001). When stratified by clinico-pathologic responses, the most striking differences were observed for pCR status, especially at OT1. Large transitions on PAM50 and TNBC subtypes at OT1 occurred nearly exclusively within those who achieved pCR. By mIHC, the boosting of CD8+ T-cells with immune checkpoint expression (PD1, LAG3, or TIM3) at OT1 was noted in those with pCR (p < 0.01), while negligible in non-pCR patients. Similar patterns were noted in other T-cell subsets, including CD4+ memory T-cells, resting/activated regulatory T-cells, activated CD8+ T-cells, CD8+ memory T-cells, CD4+CXCL13+, and CD8+CXCL13+ cells (p < 0.05). After a median follow-up duration of 57.5 (8.0-78.1) months, 5 patients relapsed, whose tumors showed upregulation of epithelial-mesenchymal transition signature at OT1 (adj p < 0.001), and G2M checkpoint, E2F, and MYC target gene sets at OT2 (adj p < 0.01). Conclusions Temporal tumor microenvironmental changes occurred as early as 2 weeks after olaparib, which was the most reflective of pCR. Our findings highlight the immune modulatory effects of PARP inhibition and the value of longitudinal biopsies for response prediction and prognostication. Citation Format: J. Koh, K. Lee, A. Min, H. Ryu, D. Lee, J. Kim, Y. Shin, Y. Kim, C. Park, D. Ho, H. Nguyen, W. Lee, H. Moon, W. Han, H. Lee, T. Kim, N. Cho, G. Cheon, S. Im. Early dynamics of tumor microenvironment in triple negative or ER-low breast cancer: updated analyses from window of opportunity (WOO) MEDIOLA trial of olaparib and durvalumab [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD6-08.