Window of opportunity study of the Notch inhibitor AL101 in Notch1 activated adenoid cystic carcinoma (ACC): Biological effects and biomarker correlates.

6119 Background: ACC with NOTCH1 pathway activation (ACC-N1) is associated with aggressive clinical behavior and poor outcomes. AL101, an inhibitor of gamma secretase-mediated Notch signaling, previously demonstrated modest clinical activity in metastatic NOTCH -mutant ACC. We conducted a window-of-opportunity study to evaluate the biological effects of AL101 in ACC-N1 and to identify biomarkers to inform rational combination strategies. Methods: Patients with ACC-N1 received AL101 (4 mg weekly) for 4 to 8 weeks prior to surgery. Eligibility required Notch1 pathway activation by cleaved Notch (NICD1) immunohistochemistry (≥ 70% nuclear staining). Treatment-related adverse events (AEs) were assessed per CTCAE v5.0 and radiographic response per RECIST v1.1. Whole-exome sequencing and RNA sequencing were performed on baseline samples, with paired analysis (pre- and post-treatment) in 12 patients. This report focuses on biomarker analysis; clinical endpoints and feasibility were previously presented. Results: 13 patients were enrolled between Nov/21 and Dec/23; 8 were newly diagnosed. The median number of AL101 doses were 6 (range: 4–7), the most common primary site was maxillary sinus (n=4). There were no grade 3-5 AEs. One patient achieved a partial response (ORR 7.7%), 11 had stable disease (including 2 with >20% tumor shrinkage), and one had progression in a non-target lesion. Post-treatment NICD1 expression was not significantly reduced (p=0.8). Genomic profiling revealed NOTCH1 activating mutations in 9/13 tumors and MYB-NFIB fusions in 8/13, with co-occurrence in 6 patients. MYB-NFIB fusion with MYB overexpression, irrespective of NOTCH1 mutation status, was associated with tumor shrinkage (p=0.02). Higher baseline NOTCH signaling activity by RNA sequencing (NOTCH signaling signature) correlated with greater tumor shrinkage (p=0.025). Although NICD1 IHC levels did not significantly change, AL101 treatment resulted in significant downregulation of NOTCH signaling activity, and the magnitude of signature reduction correlated with tumor shrinkage (p=0.037). Post-treatment transcriptomic analysis demonstrated upregulation of potentially druggable oncogenic pathways, providing biologic rationale for future combination therapeutic strategies. Conclusions: In this first window-of-opportunity study in ACC, AL101 demonstrated biological target modulation. Importantly, MYB overexpression and NOTCH signaling activity rather than NICD1 modulation or NOTCH1 mutation correlated with tumor shrinkage. These findings provide translational insights and support biomarker-driven development of rational combination strategies in NOTCH1-activated ACC. Clinical trial information: NCT04973683 .