Abstract CT206: Efficacy and safety of SV-BR-1-GM after progression on ADC in metastatic breast cancer patients

Abstract

Abstract Background: Antibody-drug conjugate (ADC) sequencing raises concerns of cross-resistance due to their shared cytotoxic payloads. Consequently, there is a pressing need for treatments after ADCs, especially in heavily pretreated patients. Immunization with irradiated SV-BR-1-GM, a GM-CSF secreting, antigen-presenting immortalized breast cancer cell line, has emerged as a potential post ADC solution based on a different mechanism of action. This retrospective analysis aims to assess the overall efficacy and safety of SV-BR-1-GM in a subset of ADC treated patients. Methods: Post-hoc subset analysis of the ongoing randomized Phase 2 study identified 23 advanced MBC patients who failed prior ADC therapy. Bria-IMT™ (irradiated SV-BR-1-GM ~20x106 cells, intradermally 48-72 hours after cyclophosphamide (300 mg/m2), followed by interferon-alpha at the inoculation sites 2 days later) was administered q3wks with a CPI. Endpoints included PFS, OS and disease control rate (the proportion of patients with stable disease, partial or complete responses) using the KM method censored to the data-cut date prior to data lock. Adverse events were classified according to their severity and relationship to treatment regimen. Results: All pts were heavily pretreated (median 6 prior lines, range 3 - 13). Median age was 62 (41-83). 14 pts had HR+/HER2-, 5 TNBC, and 2 HR+/HER2+ MBC. The remaining pts were HR-/HER2low or HR-/HER2+. Prior ADCs included: 4 ado-trastuzumab emtansine, 14 trastuzumab deruxtecan, 13 sacituzumab govitecan; 8 pts received > 1 ADC. 7 pts had also received ≥ 1 prior CPI line. To date, pts received a median 3 cycles of Bria-IMT™ + CPI (2-7). Kaplan-Meier analysis revealed a median OS of 42 weeks with survival of 69% at 6 months. The subset experienced a median PFS of 74 days with 40% of pts demonstrating a Bria-IMT PFS (PFS2) similar to or > their prior line PFS (PFS1). 39% of pts experienced a PFS2 ≥ their most recent line of ADC PFS. The disease control rate was 40% among evaluable pts. 43% of pts had a reduction in cancer-associated macrophage-like cell (CAML) levels; median reduction was 36% (22-98%) after 1 cycle. There were no toxicity-related treatment discontinuations. 43% (10/23) reported grade 3 or 4 AEs, independent of study drug causality. No interstitial lung disease (ILD) reported. AEs included injection site reaction (39%), fatigue (26%), and nausea/vomiting (43%), mostly mild to moderate. One case of elevated lipase was the most clinically significant grade 4 AE. Conclusion: The retrospective analysis of the Bria-IMT™ regimen in ADC refractory MBC patients suggests a potential treatment option for this patient population. The absence of serious AEs, notably ILD, and no toxicity-related treatment discontinuations, underscores the regimen's favorable safety profile. Future studies are warranted to confirm these results and explore the potential of Bria-IMT™ in broader clinical settings. Citation Format: Chaitali Nangia, Carmen Calfa, Blaise Bayer, Mingjin Chang, William Williams, Giuseppe Del Priore, Charles Wiseman, Saranya Chumsri. Efficacy and safety of SV-BR-1-GM after progression on ADC in metastatic breast cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT206.