Abstract PS2-06-10: Pathologic Complete Response by Breast Cancer Molecular Subtype in a Colombian Cohort: Real-World Evidence from Latin America

Abstract Background: Pathologic complete response (pCR) after neoadjuvant therapy (NAT) is a validated surrogate for long-term outcomes in breast cancer. Real-world data from Latin America are limited. We aimed to evaluate the pCR rate across molecular subtypes and its association with survival outcomes. Methods: We conducted a retrospective study of 488 patients with stage I–III breast cancer who received NAT and surgery (2019-2024) at a tertiary center in Cali, Colombia. Data were extracted from electronic records. pCR was defined as ypT0/Tis ypN0. Molecular subtypes were assigned by immunohistochemistry and FISH; staging followed AJCC 8th edition. Most patients received standard NAT according to institutional protocols: AC–T in luminal and AC–TC more often in triple-negative cases. HER2-positive patients received chemotherapy with either single-agent trastuzumab or dual blockade. Non-standard regimens were used based on clinical judgment or comorbidities. Primary outcome was pCR; secondary outcomes included recurrence, disease-free survival (DFS), and overall survival (OS). Associations were assessed using chi-square, Wilcoxon rank-sum, and multivariable logistic regression. Kaplan–Meier and log-rank tests were used for survival analyses. Results: Median age was 52 years (IQR: 44-61). Most tumors were invasive ductal carcinoma (93%) and grade III. Molecular subtypes were: Luminal A (13%), Luminal B (30%), HER2-Luminal B (17%), HER2-enriched (14%), and triple-negative (26%). Clinical stage was II–III in 97%, 56% T1–T2 and 59% node-positive. The overall pCR rate was 34.6% (169/488). Rates varied significantly across molecular subtypes: 60.3% in HER2-enriched, 54.9% in HER2-Luminal B, 44.2% in triple-negative, 17% in Luminal B, and 1.6% in Luminal A (p 0.001). pCR was significantly more frequent in ER-negative (50.0% vs. 24.5%; p 0.001) and PR-negative tumors (49.1% vs. 21.0%; p 0.001). Among receptor-positive case, those with low ER and PR expression (1–10%) had higher pCR compared to high expression (10%) (60.0% vs. 22.7%, p = 0.001; and 37.1% vs. 18.8%, p = 0.016, respectively). HER2-positive tumors showed greater pCR than HER2-negative (57.3% vs. 24.6%; p 0.001), as did those with high Ki-67 20% (39.0% vs. 13.3%; p 0.001) and grade III histology (p 0.001). T1–T2 tumors had higher pCR than T3–T4 (39.5% vs. 28.5%; p = 0.012). No significant associations were found with nodal status (p = 0.690) or clinical stage (p = 0.484). pCR rates did not differ significantly by regimen within molecular subtypes. In triple-negative tumors, adding carboplatin showed no benefit (AC–TC vs. AC–T: 43.6% vs. 47.7%, p = 0.351). In HER2-positive disease, dual vs. single-agent anti-HER2 therapy yielded similar pCR (52.8% vs. 59.2%, p = 0.782). Recurrence was lower in the pCR group (4.7% vs. 12.9%, p = 0.005), particularly for distant metastases (2.96% vs. 9.1%, p = 0.011). Patients who achieved pCR had significantly improved DFS compared to those who did not (log-rank p = 0.047). OS showed a numerical but non-significant difference (1.2% vs. 3.1% mortality, log-rank p = 0.35). Conclusion: In this real-world Latin American cohort, pCR rates and their prognostic significance were consistent with international evidence. HER2-positive and triple-negative tumors showed the highest response, while Luminal subtypes had limited benefit from standard NAT. pCR was independently associated with more favorable DFS. These findings support its value as a surrogate endpoint and highlight the need for subtype-adapted strategies in the neoadjuvant setting, such as immunotherapy in triple-negative disease. Citation Format: M. A. Solis Velasco, V. S. Muñoz-Anacona, D. F. Penagos-Cabrera, L. A. Olave-Asprilla, A. D. Fernandez-Osorio, U. O. Cardona-Nuñez, M. Zuluaga-Zuluaga, V. L. Roman-Vasquez, A. J. Guerrero-Villota. Pathologic Complete Response by Breast Cancer Molecular Subtype in a Colombian Cohort: Real-World Evidence from Latin America 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 PS2-06-10.