(P 271) PHARMACODYNAMICS AND MECHANISTIC IMPACTS OF FHD-609, A BROMODOMAIN-CONTAINING PROTEIN 9 DEGRADER, IN A PHASE 1 STUDY IN PATIENTS WITH ADVANCED SYNOVIAL SARCOMA OR SMARCB1-LOSS TUMORS

Objective: Bromodomain-containing protein 9 (BRD9) represents a potential selective vulnerability for tumors with specific alterations in the Brahma-associated factor (BAF) chromatin remodeling complex. Incorporation of the SS18-SSX oncoprotein into BAF complexes, or recurrent loss-of-function mutations in the core BAF subunit SMARCB1 drive tumorigenesis and may lead to dependence on non-canonical BAF (ncBAF) for survival. Because BRD9 is a complex-specific subunit of ncBAF, selective BRD9 degradation provides a potential therapeutic approach to treat these diseases. FHD-609 is a potent, selective, heterobifunctional degrader of BRD9 investigated in BRD9-dependent cancers, including synovial sarcoma and solid tumors primarily characterized by or with evidence of SMARCB1 loss. The pharmacodynamics of FHD-609 were assessed in a Phase 1 study in order to demonstrate proof-of-mechanism and to aid in the identification of the recommended Phase 2 dose(s). The downstream molecular and phenotypic impacts of prolonged BRD9 degradation in tumor tissue were also explored.

Methods: BRD9 protein levels were detected in patient tumor biopsies and peripheral blood mononuclear cells (PBMCs) using our previously reported immunohistochemistry (IHC) and flow cytometry methods, respectively. Biopsies were collected at screening and either Cycle 2 Day 15 (C2D15), C3D1 or end of treatment, while PBMCs were isolated from serial blood samples. Additional relevant biomarkers, including Ki67, c-Myc and SS18-SSX were also evaluated by IHC. Phenotypic impacts of FHD-609 treatment were assessed by histology, and transcriptional changes were explored by RNA sequencing.

Results: BRD9 levels were assessed in tumor biopsies from 8 different dose cohorts, ranging from 5 mg to 80 mg twice-weekly (BIW), as well as 80 mg and 120 mg once-weekly (QW). Dose-dependent BRD9 degradation was observed in tumor cells, with H-scores of 0 in several samples at doses of 40 mg BIW and above. In PBMCs, all dose levels of FHD-609 achieved approximately the same degree of BRD9 degradation at the earliest timepoint (30 minutes post-infusion). However, the duration of degradation was extended at higher dose levels.

The downstream impacts of BRD9 degradation in tumor tissue were also investigated. In several biopsies with BRD9 H-scores of 0, the percentage of tumor cells positive for the proliferation marker Ki67 was markedly reduced relative to screening. Histological changes were also observed, with more pronounced treatment effects generally occurring at higher dose levels. Notable findings included reduced cellularity, altered cellular morphology including small cell change, and altered tissue architecture in some cases. Finally, RNA sequencing of tumor biopsies revealed broad transcriptional impacts in response to FHD-609 treatment. Notably, gene set enrichment analysis identified dose-dependent downregulation of gene sets associated with oncogenic growth and proliferation.

Conclusion: In this Phase 1 dose escalation study, treatment with FHD-609 drove robust degradation of BRD9 in synovial sarcoma patient tumors. Complete degradation was observed in samples from QW and BIW cohorts, including several collected multiple days post-dose. BRD9 degradation was associated with histological treatment effects, and decreased markers of proliferation.