Abstract PS4-10-19: Impact of Adjuvant Capecitabine in Triple Negative Breast Cancer Patients Undergoing Perioperative Pembrolizumab: A Real-World Brazilian Cohort

Abstract Triple-negative breast cancer (TNBC) is a heterogeneous and aggressive disease with limited effective treatment strategies. The addition of pembrolizumab to neoadjuvant chemotherapy (NACT) has reshaped standard care, as demonstrated in the KEYNOTE-522 trial. However, adjuvant capecitabine—previously shown to benefit patients with residual disease in the CREATE-X trial—was not permitted in KEYNOTE-522. This study aimed to evaluate the real-world impact of adjuvant capecitabine in TNBC patients receiving perioperative pembrolizumab in a resource-limited setting.We conducted a retrospective observational study using electronic medical records from a referral cancer center in Brazil. Eligible patients had non-metastatic triple-negative breast cancer (TNBC) with T≥2 cm tumors and/or axillary involvement, treated with neoadjuvant chemotherapy (NACT) between 2016 and 2024. Patients were grouped as follows: (1) NACT + pembrolizumab + adjuvant capecitabine (28.2%), (2) NACT + adjuvant capecitabine (33.3%), and (3) NACT alone (38.5%). The primary endpoint was event-free survival (EFS), defined as the time from diagnosis to recurrence or death. Cox regression was used for multivariable analysis. Intergroup comparisons were performed using appropriate statistical tests, and the Bonferroni correction was applied to adjust for multiple comparisons and reduce the risk of false-positive findings.In this cohort (n=39), the mean age was 46.6 years; 69.2% were premenopausal, and 92.3% had stage II-III disease. Invasive ductal carcinoma was the predominant histology. The most frequent regimen was dose-dense doxorubicin/cyclophosphamide followed by weekly paclitaxel and carboplatin (53.8%) regardless of treatment group. Clinical characteristics were well balanced. After a median follow-up of 58.6 months, pathologic complete response (pCR) was achieved in 51.4% of the cohort. Across all patients, no significant difference in EFS was observed between groups (p=0.316). However, among patients without pCR (Group 1: 44.4%, Group 2: 100%, Group 3: 6,7%) median EFS was notably higher in group 2 than group 1 (p=0.026). These real-world findings suggest that in TNBC patients with residual disease the benefit of perioperative immunotherapy may be mitigated by concurrent use of adjuvant capecitabine. This underscores the need for individualized post-neoadjuvant strategies, particularly in low-resource settings, where access to immunotherapy remains limited. Citation Format: M. O. Sá, P. C. Diniz, E. M. Lima, R. V. Guedes, V. A. Silva, I. E. Mendes, L. L. Campos, C. C. Avelar, C. B. Pirfo. Impact of Adjuvant Capecitabine in Triple Negative Breast Cancer Patients Undergoing Perioperative Pembrolizumab: A Real-World Brazilian Cohort 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 PS4-10-19.