SV-BR-1-GM after progression on ADC in patients with metastatic breast cancer.

1087 Background: Antibody-drug conjugate (ADC) sequencing raises concerns of cross-resistance due to their shared targets and cytotoxic payloads. Consequently, treatments after ADCs in heavily pretreated MBC is uncertain. SV-BR-1-GM, a GM-CSF secreting, antigen-presenting immortalized breast cancer cell line, has emerged as a post ADC solution (1) based on a different MOA. We now report the results of SV-BR-1-GM in ADC treated patients. Methods: Post-hoc analysis of an ongoing randomized Phase 2 censored to the data-cut date prior to data lock. Bria-IMT (~20x106 SV-BR-1-GM cells, intradermally 48-72 hours after ctx (300 mg/m2), followed by interferon-alpha 2 days later) administered q3wks with a checkpoint inhibitor (CPI). Adverse events were classified according to their severity and relationship to treatment regimen. Results: Among 54 advanced MBC patients randomized, 23 who failed prior ADC therapy were identified. All were heavily pretreated (median 6 prior lines, range 3 – 13). Median age 62 (41-83). 14 pts had HR+/HER2-, 5 TNBC, and 2 HR+/HER2+ MBC. The remaining were HR-/HER2+ or HR-/HER2 low. 8 pts received > 1 ADC. Prior ADCs included: 4 ado-trastuzumab emtansine, 14 trastuzumab deruxtecan, 13 sacituzumab govitecan. 7 pts had also received ≥ 1 prior CPI line. To date, pts received a median 3 cycles of Bria-IMT™ + CPI (2-8). 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. 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. No interstitial lung disease (ILD) was reported. Conclusions: This subset analysis of the Bria-IMT regimen in ADC refractory MBC patients suggests a potential clinical benefit and treatment option for this patient population. The absence of serious AEs, notably interstitial lung disease (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 broad clinical settings of heavily pretreated contemporary MBC patients. 1. Kamaraju 2024 AACR. Clinical trial information: NCT03328026 .