Abstract PS5-05-13: Pathologic complete response rates after neoadjuvant chemotherapy and pembrolizumab in patients with triple negative breast cancer treated at the Royal Marsden:A retrospective analysis

Abstract Background: Triple-negative breast cancer (TNBC) is associated with high rates of early recurrence and poor prognosis. Chemotherapy remains the mainstay of treatment for early-stage TNBC in the preoperative setting. The KEYNOTE-522 recently demonstrated that adding pembrolizumab to chemotherapy significantly increases pathological complete response (pCR) rates in TNBC. However, real-world efficacy data remains limited. This study aims to evaluate the pCR rates among TNBC patients treated with neoadjuvant pembrolizumab at our Institution. Methods: We retrospectively retrieved data on patients diagnosed with stage I-III TNBC receiving neoadjuvant pembrolizumab at The Royal Marsden between June 2021 and June 2024. We collected data on patient demographics, tumour characteristics, treatment, and surgical pathology. We used simple statistics to evaluate the rate of pCR (ypT0/is, ypN0) after neoadjuvant systemic therapy. Results: Seventy-eight patients were included in the analysis. The median age at diagnosis was 49 years (interquartile range: 40-58). Most patients had clinical stage IIA at presentation (n=32, 41.0%) and invasive ductal carcinoma (n=74, 94.9%). Most tumours were grade 2 (n=65, 83.3%). Fifty-eight patients (74.3%) were able to complete the full regimen of chemotherapy. Among 20 patients discontinuing the treatment earlier, toxicity was the main reason for treatment discontinuation in 10 (50.0%). Forty-eight patients (61.5%) completed 8 cycles of pembrolizumab before surgery. Pembrolizumab was discontinued in 20 (25.2%), most commonly due to severe immune-related adverse effects in 15 patients (75.0%). Surgery involved breast-conservation in 43 patients (55.1%) and mastectomy in 35 patients (44.9 %). Nodal surgeries performed included sentinel lymph node biopsy in 43 patients (55.1%), axillary nodal clearance in 17 patients (21.8%), and targeted axillary dissection in 16 patients (20.5%). Overall, 41 patients achieved a pathologic complete response (pCR) rate (52.6%). pCR was more frequent in earlier-stage tumours: ( 100.0% [n=2] in stage I, 58.5% [n=31] in stage II, and 34.8% [n=8] in stage III) and in higher-grade tumours (36.4% [n=4] of grade 1 tumours, 55.4% [n=36] in grade 2 tumours, and 50% [n=1] in grade 3 tumours). pCR was achieved by 40 patients with ductal tumours (97.6%) versus 1 (2.4%) patient with other histology types pCR was seen in 29 patients who completed the planned course of chemotherapy (50%) versus 12 patients who did not (60%), and in 26 patients who completed the planned 8 cycles of pembrolizumab (54.2%) versus 15 of those who did not (50%). However, no statistically significant association was found between pCR and clinical stage, tumour grade /histology, number of chemotherapy or number of pembrolizumab cycles received (p = 0.05). Conclusion: In our analysis, the addition of pembrolizumab to chemotherapy resulted in similar pCR rates compared with published evidence. These findings reinforce the role of immunotherapy in early TNBC.. Further analyses with longer follow-up are warranted to evaluate the impact of pCR on survival in the real world. Citation Format: C. A. Chigbo, K. Udeogu, F. Sheima, N. Chukwuma, A. Khan, S. Redana, S. Mcgrath, A. Ring, S. Johnston, N. Battisti. Pathologic complete response rates after neoadjuvant chemotherapy and pembrolizumab in patients with triple negative breast cancer treated at the Royal Marsden:A retrospective analysis [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 PS5-05-13.