Citations

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 & real world PFS (rwPFS) of ADC compared to non-ADC therapy, defined as time from treatment start to time of progression or death. Results: We identified 48 pts with MpBC who received at least one ADC between 3/2016 and 10/2025. Median age was 54 years (range 31-77); 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 & 17 (35%) were confirmed HER2-low (IHC 1+ or 2+, ISH nonamplified). The most common MpBC subtypes were with squamous differentiation (n=17, 35%), chondroid (n=11, 23%), matrix-producing (n=11, 23%) & spindle cell differentiation (n=10, 21%). Ten pts (21%) had de novo metastatic disease & 38 (79%) had recurrent disease. Most common metastatic sites were lung (n=33, 69%) & liver (21, 44%). 37/38 (97%) patients had curative intent anthracycline & taxane, with median disease-free interval of 16 months (range 2-241). 18 pts (38%) received curative intent immune checkpoint inhibitor. Most pts received ADC in the first (n=19, 40%) or second line (n=18, 38%) with Sacituzumab govitecan being the most common (n=38, 79%) ADC, followed by Trastuzumab deruxtecan (n=9, 19%). 7 pts (15%) received a second ADC. ORR at time of best response was 38% (n=18), with 4 pts experiencing complete response (1 patient who received ADC in 1L, 3 in the 2L) & 14 experiencing partial response. 3 pts (6%) experienced stable disease, & 27 pts (56%) experienced progressive disease as best response. Median rwPFS of pts receiving ADC in the first line was 2.3 months, compared to 4.3 months in those receiving non-ADC/chemotherapy (HR 1.34; 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%), & ADC discontinuation in 2 (4%). Conclusion: In this multi-institutional cohort of pts with metastatic MpBC, ADC treatment yielded a 38% ORR, including four complete responses. However, rwPFS was poor overall & similar between ADC & non-ADC therapy. These findings highlight the need to develop new treatment strategies for MpBC, though a subset of pts may experience meaningful response to ADC. Citation Format: S. Jacob, S. Premji, S. Fisch, A. LeVee, S. Marion, D. Schmolze, J. Mortimer, L. Huppert, D. Idossa, A. Nayak, P. Wileyto, A. DeMichele, P. D. Shah. Efficacy of antibody drug conjugates in patients with advanced metaplastic breast cancer: a multi-institutional retrospective cohort study [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 PS1-09-18.

Abstract Background: Disseminated tumor cells (DTCs) in bone marrow detected after treatment may represent occult residual disease. We enumerated DTCs after neoadjuvant chemotherapy (NACT) in patients (pts) diagnosed with high-risk early stage breast cancer and examined the relationship of these cells with response and survival. Methods: I-SPY SURMOUNT is a sub-study of the I-SPY 2 TRIAL (NCT01042379). Pts enrolled on I-SPY 2, who signed consent for this sub-study, had bone marrow aspirates (BMA) collected after NACT at the time of surgery. DTCs were isolated and enumerated from BMA using immunomagnetic enrichment/flow cytometry (IE/FC). DTCs were defined as EPCAM-positive and CD45-negative nucleated cells. Samples were considered positive using a predetermined threshold of >4 DTCs per mL (Magbanua et al, unpublished data). Pathologic response was assessed using the residual cancer burden (RCB) method at local sites, and pts underwent standard adjuvant therapy if indicated and follow up for recurrence events and death. Relationship of DTCs with clinicopathologic variables was examined using Chi-squared test. Group means were compared using t tests. The log-rank test was used to compare survival curves. Results: A total of 73 patients were enrolled, 51 of whom had successful DTC assessment. The median DTC per mL was 4 (interquartile range 1.2-11.6). 24/51 (47%) were DTC-positive. Clinical characteristics by DTC status are shown in the table. DTC-positive pts were significantly younger (p=0.02) and had larger pretreatment tumors (longest diameter by magnetic resonance imaging) compared to DTC-negative pts (p=0.032). DTCs were not associated with receptor subtype. Thirty pts (41%) achieved a pathologic complete response (pCR). DTCs were not associated with pCR (p= 0.166); however, DTC-positive patients were significantly more likely to have residual cancer (RCB-II/III) after NACT compared to DTC-negative patients (OR 3.3, p=0.037). Median follow up of this cohort was 2.8 years (range: 0.9-4.8). Interim survival analysis showed that DTCs were not significantly correlated with EFS (p=0.6) or DRFS (p=0.41). Conclusions: Detection of DTCs at surgery after NACT is significantly more common in young patients, those with larger tumors, and those with residual disease at surgery. While these associations suggest higher risk for later recurrence, larger studies and longer follow up are necessary to determine if DTCs add prognostic value over pathologic evaluation alone for pts receiving NACT. Citation Format: Mark Jesus M Magbanua, Laura van 't Veer, Amy Clark, A. Jo Chien, Judy Boughey, Heather Han, Anne Wallace, Heather Beckwith, Minetta Liu, Christina Yau, E. Paul Wileyto, Lamorna Brown Swigart, Jane Perlmutter, Lauren Bayne, Shannon Deluca, Stephanie Yee, Erica Carpenter, Laura Esserman, John Park, Lewis Chodosh, Angela DeMichele. Outcomes associated with disseminated tumor cells at surgery after neoadjuvant chemotherapy in high-risk early stage breast cancer: The I-SPY SURMOUNT study [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS2-07.