Abstract PS1-09-18: Efficacy of antibody drug conjugates in patients with advanced metaplastic breast cancer: a multi-institutional retrospective cohort study

Abstract Introduction: Metaplastic breast cancer (MpBC) is a rare, aggressive subtype of breast cancer that has limited evidence-based data to guide treatment. Historically, MpBC has had limited response to chemotherapy; however, the response of MpBC to antibody-drug conjugates (ADCs) remains poorly characterized. Methods: We performed a retrospective cohort study of patients (pts) with pathologic diagnosis of MpBC who received ADC therapy as part of standard of care in the metastatic setting across 4 NCI-designated cancer centers. The primary endpoints were objective response rate (ORR) at time of best response to ADC all pts were biological females. Most pts had triple negative disease (n=39, 81%), 5 (10%) had HER2+ disease, 3 (6%) had hormone receptor positive disease 95% CI 0.72-2.49, p=0.89). Median rwPFS of pts receiving ADC in the second line was 2.6 months, compared to 2.3 months in those receiving non-ADC/chemotherapy (HR 0.56; 0.28-1.11; p=0.10). Toxicity resulted in dose reductions in 17 pts (35%), dose delays in 14 (29%), 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-09-18.