Abstract Background: Pathological complete response (pCR) to neoadjuvant systemic therapy is a crucialprognostic indicator in early-stage triple negative breast cancer (eTNBC). In Vietnam, the immunecheckpoint inhibitor, pembrolizumab was approved for neoadjuvant regimens combined withchemotherapy for eTNBC in 2023. However, the lack of public health insurance coverage for thisnovel therapy creates a significant barrier, limiting access for the vast majority of Vietnamesepatients. This study aims to identify valuable factors that predict pCR achievement in eTNBCpatients receiving neoadjuvant treatment with pembrolizumab combined with chemotherapy inVietnam, thereby optimizing treatment strategies. Methods: This is a prospective, single-arm, single-center study conducted at the Vietnam NationalCancer Hospital from March 2023 to May 2025, involving 49 patients with early triple-negativebreast cancer (eTNBC) who received neoadjuvant chemotherapy with TC-AC combined withpembrolizumab. The primary endpoint was pathological complete response (pCR), which twoindependent pathologists assessed. Univariate and multivariate logistic regression analyses wereused to identify independent prognostic factors for pCR. Results: The majority of patients (98%) presented with Stage II or III disease. Median tumor sizewas 3.5 cm, and median tumor-infiltrating lymphocytes (TILs) were 35%. HER2-negative patientsconstituted 67.3% of the cohort, while 32.6% were HER2-low. The pCR rate in this study was67.3% (33/49). In multivariate logistic regression analysis, TILs percentage was identified as anindependent predictor of pCR achievement (OR: 1.14; 95% CI: 1.06–1.27; p=0.003). Whenstratified by TILs level, the pCR rate was 89.7% for patients with TILs 30%, 50% for TILsbetween 20% and 30%, and 12.5% for TILs 20%. HER2 expression status also significantlyinfluenced pCR achievement, with HER2-low patients exhibiting a lower pCR rate compared toHER2-negative patients (p=0.049). Conclusion: This study identifies TILs and HER2 expression as predictive factors for pCR. TILs,specifically, demonstrate potential as a biomarker to guide individualized treatment in eTNBC.This is particularly important as ICIs gain approval for eTNBC treatment in developing countrieslike Vietnam, underscoring the critical need for reliable predictive tools to optimize patientoutcomes. Citation Format: A. DINH, Y. Le, P. Han, U. Le, H. Hoang, H. Duong. Predictive Markers for Pathological Complete Response to Neoadjuvant Therapy in Early-Stage Triple-Negative Breast Cancer: A Single-Center Study in Vietnam 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 PS4-07-23.
Dinh et al. (2026) studied this question.