Abstract 4544: Immunotherapy with chemotherapy provides superb pathologic complete response rate in chemotherapy-resistant metaplastic carcinoma of breast

Abstract Background: Metaplastic carcinoma of breast (MCB) is a rare subtype of breast cancer (BC) accounting for <1% of newly diagnosed BC. It is composed of co-existing sarcomatous and carcinomatous components. Because of its sarcomatous components, the response rate of chemotherapy in MCB is low both in early BC and advanced BC. The pathologic complete response rate (pCR%) after neoadjuvant chemotherapy in MCB is as low as 10%. Neoadjuvant immunotherapy plus chemotherapy is a new standard of care for early triple-negative BC providing improved pCR%, event-free survival, and overall survival. Pembrolizumab was also approved by the FDA in 2021 for neoadjuvant treatment in TNBC. We aim to examine the modern pCR% of immunotherapy and chemotherapy in MCB patients. Methods: MCB patients and treatment regimens, including neoadjuvant immunotherapy, were retrospectively selected from Flatiron Health, a nationwide electronic health record database. Demographic data and neoadjuvant regimens were collected. pCR% was calculated for patients treated with neoadjuvant chemotherapy versus neoadjuvant immunotherapy plus chemotherapy. Results: A total of 87 early stage MCB patients diagnosed between January. 2011 and May 2024 were included from the Flatiron Health. 20 and 5 of the MCB patients received neoadjuvant chemotherapy alone and neoadjuvant immunotherapy plus chemotherapy, respectively. 2 in the neoadjuvant chemotherapy group (pCR%=10%) and 3 in the neoadjuvant immunotherapy/ chemotherapy group (pCR%=60%) achieved pCR. The pCR% of neoadjuvant chemotherapy is as low as previously reported in MCB patients. Conclusion: Neoadjuvant immunotherapy and chemotherapy provide comparably high pCR% in MCB patients as compared to other TNBC patients. We are validating the findings from additional cohorts from Taiwan. The high pCR% makes immunotherapy and chemotherapy an attractive strategy that worth further investigation in MCB patients. Citation Format: I-Chun Chen, Shang-Hsuan Peng, Tsung-Che Wu, Ching-Hung Lin, Diana Hui-Min Lin, Li-Wen Huang, Chi-Han Cheng, Yen-Shen Lu. Immunotherapy with chemotherapy provides superb pathologic complete response rate in chemotherapy-resistant metaplastic carcinoma of breast [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4544.