Preclinical efficacy of BRG1/BRM ATPase inhibitor in B lymphoblastic leukemia.

10030 Background: Acute lymphoblastic leukemia (ALL) is the most common blood cancer in children and young adults. While ALL has a high cure rate, approximately 20% of patients who initially respond to treatment relapse and prognosis post relapse remains poor. New therapeutic treatments are needed to reduce relapse rates in B-ALL pediatric patients and to minimize the toxic side effects seen with current therapies. We and others have previously shown that BRG1/BRM ATPase inhibitors have efficacy in targeting acute myeloid leukemia (AML) and one, FHD-286, is currently being evaluated for its use in AML in an ongoing clinical trial (NCT04891757). BRG1 and BRM are the ATP-dependent catalytic subunits of the SWI/SNF chromatin remodeling complex which has a critical role in controlling gene expression, cell growth and survival of AML cells. Our prior work revealed that BRG1/BRM ATPase inhibition leads to selective transient reduction of normal B cells. Further, data from Depmap also nominates SMARCA4, the gene encoding BRG1, as a dependency in B-ALL cell lines, leading us to hypothesize that BRG1/BRM inhibition may be an effective therapy for B-ALL. Methods: We treated B-ALL cell lines with varying genetic lesions ( KMT2A-rearranged: RS4;11, KOPN-8, SEM; ETV6::RUNX1 fusion: REH and DUX4-rearranged: Nalm6) with dose titrations of FHD-286 and measured cell growth over the course of six days. In addition, at 48- and 72-hours post-treatment, we assayed cells for viability, cell cycle kinetics, and apoptosis induction via flow cytometry to provide insight into FHD-286’s mechanism of action in B-ALL. Results: We found that FHD-286 treatment resulted in decreased cell growth in a dose-dependent manner across all B-ALL cell lines. While all cell lines were displayed reduced proliferation at nanomolar doses, sensitivity varied. Nalm6, REH and SEM cells were the most sensitive with low nanomolar IC50s (2-7nM). In contrast, two of the KMT2Ar B-ALL lines, RS4;11 and KOPN-8, required higher doses to achieve growth inhibition (40-70nM). To better understand the mechanism by which FHD-286 is affecting cell growth, we evaluate whether FHD-286 is targeting cell growth through the apoptotic pathway or through cell cycle arrest. Our preliminary data indicates cell growth inhibition is only slightly attributable to cell cycle arrest and apoptosis induction, leading us to explore terminal differentiation as a mechanism of slowed proliferation. Conclusions: Our results show FHD-286 can strongly inhibit B-ALL cell growth in vitro, indicating FHD-286 as a potential therapeutic agent to treat B-ALL. In vivo validation is ongoing. Further directions include defining the effect of FHD-286 on chromatin remodeling, gene expression, and exploring efficacy in combination therapy, which has been shown to increase FHD-286 efficacy against AML. Our data may ultimately justify inclusion of B-ALL patients in clinical studies of BRG1/BRM inhibitors.