Abstract PS5-05-07: Real-world Patient Characteristics and Treatment Patterns with Pembrolizumab Among Patients with Early-Stage Triple-Negative Breast Cancer in the United States

Abstract Background: Pembrolizumab (pembro) was approved by the Food and Drug Administration in July 2021 to treat high-risk, early-stage triple-negative breast cancer (TNBC) based on the KEYNOTE-522 trial. To support post-launch assessments, this real-world study aimed to outline patient characteristics and treatment patterns in patients with Stage II-III TNBC in the US community setting. Methods: This retrospective study used electronic medical records accessed via the Syapse data platform to examine previously untreated adults diagnosed with Stage II-III TNBC from August 2021 to July 2023. We analyzed patient characteristics, surgery rates and types, treatment patterns and duration, and reasons for treatment discontinuation in the neoadjuvant and adjuvant settings. Results: 322 patients were identified with Stage II (61.3%) or Stage III (38.7%) TNBC and who had undergone definitive surgery. Mean age was 58 years (standard deviation SD: 15). 47% had an ECOG performance status of 0, 14% had an ECOG of 1, while 37% had unknown status. In the neoadjuvant setting, most patients (75%) received pembro + chemotherapy, 12% received chemotherapy alone and 12% received no neoadjuvant therapy. The most common therapy in patients receiving neoadjuvant pembro + chemotherapy was pembro + carboplatin + taxanes + anthracyclines (A) + cyclosphosphamide (C) (83%) while the most common therapy in patients receiving chemotherapy alone was taxanes + AC (50%). Patients receiving pembro + chemotherapy were more likely to undergo mastectomy (56% vs 38%) and less likely to undergo lumpectomy (44% vs. 63%). Among patients receiving adjuvant therapy in the cohort previously treated with neoadjuvant pembro + chemotherapy, the most frequently administered adjuvant regimens were pembro monotherapy (70%) and pembro + capecitabine (10%). On the other hand, capecitabine was the most commonly administered treatment (75%) among patients receiving adjuvant therapy in the cohort that received neoadjuvant chemotherapy without pembro. The median (95% CI) time on pembro in the neoadjuvant and adjuvant settings was 5.3 (5.1, 5.6) and 5.6 (5.6, 5.7) months, respectively. Comparatively, median time on chemotherapy was 4.2 (3.4, 4.9) and 5.2 (4.4, 5.7) months, respectively. The primary reasons for discontinuation among patients who had received neoadjuvant chemotherapy + pembro (n=236) were completion of planned therapy (72%) and intolerance/toxicity (24%). In the adjuvant pembro group (n=97), primary reasons for discontinuation were completion of therapy (78%) and intolerance/toxicity (8.2%). Conclusion: Findings show a high utilization of pembro in neoadjuvant and adjuvant early-stage TNBC, highlighting its role as standard of care. Treatment duration for pembro was consistent with that reported in clinical trials and previous real-world studies, supporting use in routine clinical practice. High treatment completion rates suggest that pembro is generally well tolerated, supporting its use as a viable treatment option across perioperative TNBC setting. Citation Format: A. Haiderali, Y. Sun, L. Ai, F. Beca. Real-world Patient Characteristics and Treatment Patterns with Pembrolizumab Among Patients with Early-Stage Triple-Negative Breast Cancer in the United States 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 PS5-05-07.