Amx-883, a potent and selective degrader of BRD9 drives differentiation in acute myeloid leukaemia and shows synergistic efficacy in combination with venetoclax In Vivo and prevents the emergence of resistance to venetoclax in vitro.

Abstract Background: Acute Myeloid Leukaemia (AML) is commonly characterized by a differentiation block which prevents maturation and results in an accumulation of immature cells. Therapies which remove the differentiation block and allow maturation of AML blasts including ATRA, FLT-3 inhibitors and most recently Menin inhibitors have demonstrated clinical benefit in several sub-sets of AML. BRD9 is a subunit of the non-canonical BAF complex where it plays a key structural and functional role being linked to regulation of chromatin structure and maintaining genomic stability in AML. Recent data illustrates a key regulatory role for BRD9 in maintaining AML myeloid cell stemness, and the loss of BRD9 expression suppressing development and survival of leukemic blasts. Results and conclusions: AMX-883 is a novel, orally bioavailable potent and selective degrader of BRD9 with exquisite selectivity over all other bromodomain containing proteins, and proteome wide. AMX-883 time dependently increases the expression of key proteins associated with myeloid differentiation in MV4-11 cells. As described in our recent paper (doi.org/10.1101/2024.12.31.630899) Amphista degraders do not employ the commonly used E3 ligases (cereblon, VHL) but instead drives degradation via DCAF16 as a targeted glue. Picomolar potency degradation of BRD9 was observed across a panel of AML cell lines with an associated reduction in cell viability regardless of the mutational status of the cell lines, with minimal viability effects observed in normal peripheral blood mononuclear cells. Dose dependent induction of key cell surface differentiation markers including CD11b, CD86 and CD14 was observed in a sub-set of the AML cell line panel with activity observed in a diverse rank of cytogenetic backgrounds including TP53 mutant cell lines. Acute pharmacodynamic studies in subcutaneously implanted MV4-11 cells showed a robust time dependent degradation of BRD9 with favourable bioavailability and PK profile suitable for daily dosing. A 14-day pharmacodynamic study in a luciferase tagged MV4-11 disseminated model, confirmed dose dependent degradation of BRD9, associated with a time dependent induction of CD11b and CD86. Measurement of the total disease burden in the animals highlighted significant tumor reduction in treated animals compared to vehicle treated animals. Using disseminated patient derived xenograft models, AMX-883 significantly reduced leukemic burden in bone marrow and blood over the treatment period and resulted in a significant increase in survival compared to vehicle or the positive control compound, venetoclax. Combination of AMX-883 with venetoclax resulted in synergistic efficacy in MV4-11 and MOLM-13 cell lines. AMX-883 co-dosed with venetoclax and azacitidine resulted in synergy with significant cell death at therapeutically relevant concentrations. Combination benefit was also observed with a range of other commonly used AML therapies in the same cell lines. We investigated the role of BRD9 in the emergence of resistance to venetoclax by generating a MV4-11 resistant cell line (increasing concentration exposure over a 3-month period as measured by cell viability). Concurrently, we cultured MV4-11 cells with increasing concentrations of venetoclax in the presence of a fixed concentration of AMX-883, corresponding to BRD9 degradation of 90%. The cell line co-cultured with venetoclax and AMX-883 did not develop resistance to venetoclax and had comparable sensitivity to cells on their first exposure to venetoclax. Whilst the venetoclax resistant line exhibited a significant increase in the anti-apoptotic proteins MCL-1 and BCL-2, co-culture with AMX-883 in addition to venetoclax prevented this increased expression. Our data demonstrates the potent, selective BRD9 degrader AMX-883 induces significant differentiation and preclinical efficacy in AML cell lines and in primary AML cell lines. We show this oral degrader is well tolerated in a murine pre-clinical experiment with significant survival benefit and reduced disease burden compared to control treatments. Collectively these data provide compelling evidence of the significant role of BRD9 in AML. AMX-883 is progressing through preclinical evaluation with a clinical trial in relapsed refractory AML patients planned for 2026.