Abstract PS5-04-28: Real-world safety and effectiveness of the Keynote-522 regimen for triple-negative breast cancer in a private oncology practice in Rio de janeiro, Brazil

Abstract Introduction: The KEYNOTE-522 trial established neoadjuvant chemo immunotherapy as the standard of care for stage II-III triple-negative breast cancer (TNBC). However, the regimen is associated with high rates of grade ≥3 adverse events, raising concerns in real-world settings where toxicity management may differ from clinical trial. Despite growing global adoption, real-world data on this approach outcomes —particularly in low- and middle-income countries—remain limited. Objectives: To evaluate the real-world incidence of serious adverse events (SAEs) during treatment, pathological complete response (pCR), and overall survival outcomes among patients with stage II-III TNBC receiving the KEYNOTE-522 regimen at a private oncology center in Rio de Janeiro, Brazil. Methods: A retrospective cohort study was conducted at a single private oncology center, including women with previously untreated stage II or III TNBC who received treatment in accordance with the KEYNOTE-522 protocol.Data were obtained through clinician-reported chart reviews of electronic medical records. SAEs were defined as treatment-related hospitalizations or deaths occurring during the neoadjuvant phase. Patients were followed by their medical oncology team from initiation of neoadjuvant therapy through the last documented clinical encounter.Descriptive statistics were used to summarize baseline characteristics. Multivariable logistic regression using the Stepwise Forward method was performed to identify factors associated with adverse outcomes. Overall survival was estimated using the Kaplan-Meier method. Pathological complete response was defined as the absence of invasive residual tumor in both the breast and axillary lymph nodes at the time of definitive surgery (ypT0/Tis ypN0). The study was approved by the local Institutional Review Board. Results: A total of 85 women were included, with a median age of 46 years (range: 26-78); most were white (84.7%). Clinical stage II was the most common at diagnosis (53.6%) and 64.9% of tumors classified as grade 3. Pathogenic germline mutations were identified in 27.1% of patients.Treatment discontinuation was observed in 24.7% of cases, and corticosteroids due to SAEs were used in 14.1% of cases. There was one treatment-related death and 16 hospitalizations due to toxicity, resulting in a SAEs rate of 18.8% and a mean treatment delay of 13.5 days (±9.24). In multivariable analysis, older age was significantly associated with higher risk of SAEs, with a 7% increase in risk per year of age (OR=1.07; 95% CI: 1.03-1.12; p=0.003). Patients diagnosed at stage III had a 3.6-fold increased risk of SAEs compared to those at stage II (OR=3.60; 95% CI: 1.01-12.81; p=0.048). A pathological complete response (pCR) was achieved in 53.6% of patients.The OS rate was 87%, with a median follow-up of 32.9 months (95% CI: 31-35).Conclusão: In this real-world cohort from a private oncology practice in Rio de Janeiro, Brazil, nearly one in five patients experienced SAEs during treatment with the KEYNOTE-522 regimen. Older age and stage III disease were independently associated with an increased risk of toxicity. These serious toxicities raise concerns, particularly in a curative-intent setting.Despite the toxicity burden, a pathological complete response (pCR) was achieved in 53.6% of patients, consistent with results from clinical trials. Overall survival (OS) reached 87% at a median follow-up of 33 months, highlighting the potential for favorable outcomes in routine clinical practice. Citation Format: A. C. Pires, A. Bergmann, G. Bretas, A. B. Filgueiras, E. Neves, D. L. Santos, A. C. Goncalves. Real-world safety and effectiveness of the Keynote-522 regimen for triple-negative breast cancer in a private oncology practice in Rio de janeiro, Brazil 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 PS5-04-28.