Abstract GS1-04: Tumor infiltrating lymphocytes (TILs) and pathologic complete response (pCR) in stage II/III HER2+ breast cancer treated with taxane, trastuzumab, and pertuzumab (THP): secondary results from the ECOG-ACRIN EA1181/CompassHER2 pCR tri

Abstract Background: The association of TILs with pathologic complete response (pCR) and survival in HER2+ breast cancer is not well established with several prior studies showing inconsistent results. In this secondary analysis of EA1181, we evaluated the association between stromal TILs (sTILs) and pCR rates. Methods: EA1181 (NCT04266249) enrolled patients (pts) with anatomic clinical stage II/III HER2+ breast cancer who preoperatively received 4 cycles of trastuzumab and pertuzumab with 12 weeks of paclitaxel or docetaxel q3w x 4 (THP), followed by surgery. sTILs density was assessed on full-face hematoxylin and eosin (H&E)-stained tumor biopsy sections. sTILs scores were analyzed per protocol as a continuous variable (every 10% increment) and as a categorical variable (<10% vs 10-60% vs >60% as per Denkert et al, 2018), and also with an exploratory 30% cutoff commonly used in TNBC. Cox proportional hazards models were used to examine the association between sTILs and pCR, adjusting for available baseline factors. The associations between sTILs (by category) and clinicopathologic characteristics were examined using Fishers’ exact test. Results: Among 2141 pts with HER2+ BC treated on EA1181, H&E slides were available for 1328 (62%). The study population evaluated for sTILs was comparable to the overall population. pCR rates were 44.5% overall, 64% for HER2+/ER- and 33% for HER2+/ER+ disease. sTILs distribution was: 623 (47%) <10%, 698 (52%) 10-60%, and 7 (0.5%) >60%, which led us to merge the latter two groups (705 [53%] had ≥10% sTILs). In univariable and multivariable analyses, increasing sTILs (as a continuous variable) were associated with increasing pCR rates in both HER2+/ER+ and HER2+/ER- disease (p<0.001; Table). sTILs analyzed as a categorical variable (<10% vs ≥10%) were significantly associated with pCR for all patients (combined) and for those with HER2+/ER+ disease. In exploratory analysis using a cutoff of <30% vs ≥30% sTILs, an association with pCR was also seen in univariable analysis for both HER2+/ER+ disease (p<0.001) and HER2+/ER- disease (p=0.045), though in multivariable analysis with clinical factors, sTILs >30% remained a significant predictor for pCR only in HER2+/ER+ disease. Additional exploratory analysis with other molecular biomarkers, intrinsic subtypes, and immune signatures will be presented. Conclusion: sTILs were associated with pCR after THP, further supporting the important role of immune mechanisms in HER2+ breast cancer, and highlighting a potentially robust predictive tool to assess pathologic response. Baseline sTILs could potentially inform the preoperative design of future trials of therapy optimalization. Association of baseline sTILs with recurrence free survival in EA1181 will be reported in the future. Citation Format: S. S. Badve, F. Zhao, N. Tung, Y. Gokmen-Polar, C. C. O'Sullivan, A. Prat, E. P. Winer, J. L. Wright, A. Recht, A. C. Weiss, J. A. Tjoe, S. M. Feldman, G. B. Rocque, M. Smith, N. Unni, S. Sardesai, S. Tang, S. Modi, W. J. Irvin, P. Villagrassa, C. Battelli, A. K. Krie, N. Bagegni, M. A. George, M. L. Telli, V. F. Borges, N. D’Abreo, P. Shah, K. D. Miller, A. H. Partridge, L. A. Carey, A. M. DeMichele, A. C. Wolff, ECOG-ACRIN.Tumor infiltrating lymphocytes (TILs) and pathologic complete response (pCR) in stage II/III HER2+ breast cancer treated with taxane, trastuzumab, and pertuzumab (THP): secondary results from the ECOG-ACRIN EA1181/CompassHER2 pCR trial [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 GS1-04.