Abstract PS08-03: Sequencing Antibody-Drug Conjugate after Antibody-Drug Conjugate in Metastatic Breast Cancer (A3 study): Multi-Institution Experience and Biomarker Analysis

Abstract Background: Antibody-drug conjugates (ADCs) improve survival in patients with metastatic breast cancer (MBC) and offer the potential for targeted delivery of highly potent therapy. Many patients are now candidates for multiple ADCs, but optimal strategies for sequencing are unknown. We previously reported on a single institution experience of patients receiving multiple ADCs for MBC (Abelman, ASCO 2023). Here we report a multi-institution update with biomarker analysis. Methods: We included all patients treated at three academic medical institutions who received multiple ADCs for MBC. Patients were included if they had hormone receptor positive, HER2-negative (HR+/HER2-) breast cancer or triple-negative breast cancer (TNBC); patients with HER2+ metastatic breast cancer were excluded. Clinical information was determined by chart review. The metric of “cross-resistance” to the second ADC was defined as patients with progressive disease on first restaging assessment or progression within 60 days of treatment initiation. Every subsequent ADC beyond the first was compared against the prior ADC for presence of identical “antibody target” and “payload”. Comparisons across ADCs were performed using Fisher’s exact test. Significance was determined to be a type I error less than 0.05. A subset of patients had available whole exome tissue sequencing through commercially available sequencing platforms (BostonGene and Caris) performed around the time of receipt of ADC. All sequencing reports were examined for presence of pathogenic variants, variants of uncertain significance, and currently undefined variants and fusions as defined by each sequencing platform. Results: 68 patients were identified who received two or more ADCs for metastatic HR+/HER2- breast cancer or TNBC from August 2014-June 2023. 30 patients (44.1%) had HR+/HER2- disease and 38 patients (55.9%) had TNBC; 50 patients (73.5%) had HER2-low disease. Median age at time of second ADC was 59.6 (range 29.9-88.6). Patients had received a median of 4 lines of treatment in the metastatic setting prior to initiation of the second ADC. At time of first restaging, cross-resistance was present in 38/64 evaluable cases (59.4%). When the antibody target of the latter ADC was the same as the prior, cross-resistance was present in 11/14 cases (78.5%) compared to 26/49 cases (53.1%) when the later ADC targeted a different tumor-associated antigen. Relatively similar patterns of cross-resistance were observed regardless of whether the later ADC contained an identical payload to prior (6/10 cases, 60%) versus a different payload (22/42, 52.4%). Sequencing information was available for 20 patients who received multiple ADCs with 15 unique reports performed at the time of resistance to ADC1, prior to initiation of ADC2, or after ADC2 if presence of cross-resistance. Variants in topoisomerase-I associated genes (TOP1, TOP2A, TOP3A, TOP3B) were identified in a subset of patients mediating cross-resistance to the second ADC with a topoisomerase-I inhibitor payload. Conclusions: In this multi-institution study, cross-resistance to the second ADC appears to be driven by the antibody target in some patients versus the payload in others, highlighting the heterogeneity of mechanisms related to ADC resistance. Tumor sequencing revealed candidate resistance mutations that may guide optimal sequencing for patients with MBC. Citation Format: Rachel Occhiogrosso Abelman, Laura Spring, Geoffrey Fell, Andrew Davis, Whitney Hensing, Phoebe Ryan, Neelima Vidula, Seth Wander, Arielle Medford, Jennifer Shin, Elizabeth Abraham, Steven Isakoff, Beverly Moy, Leif Ellisen, Aditya Bardia. Sequencing Antibody-Drug Conjugate after Antibody-Drug Conjugate in Metastatic Breast Cancer (A3 study): Multi-Institution Experience and Biomarker Analysis [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS08-03.