Bmf-500: An Orally Bioavailable Covalent Inhibitor of FLT3 with High Selectivity and Potent Antileukemic Activity in FLT3-Mutated AML

Abstract

Activating mutations of the FMS-like tyrosine kinase 3 (FLT3) are the most frequent genetic alteration in AML and are associated with poor prognosis. Though several FLT3 inhibitors have entered clinical trials and reached commercialization, adverse events and dose-limiting toxicities often restrict the therapeutic window and limit their long-term use. Such limitations impact the ability to achieve long-lasting response in patients and ultimately result in therapy-induced resistance. Exquisite potency combined with high selectivity and improved safety profile is expected to help improve tolerance and overall treatment outcomes of FLT3-targeted therapy. BMF-500 was designed as a highly potent and selective, covalent, small molecule inhibitor of FLT3, that binds irreversibly to a reactive cysteine in the kinase active site. BMF-500 is a picomolar inhibitor with markedly improved potency over gilteritinib, a reversible inhibitor of FLT3. BMF-500 selectively killed AML cells harboring FLT3 activating mutations, including MV4-11 and MOLM-13, and engineered cells expressing FLT3 internal tandem duplications (FLT3-ITD) and/or FLT3 tyrosine kinase domain (TKD) mutations. In ex vivo cultures, BMF-500 as a single agent induced potent growth inhibition of patient-derived AML cells harboring either FLT3-ITD or FLT3 non-ITD mutations. The potent covalent inhibition of FLT3 by BMF-500 manifested in effective and durable cellular response that was improved over gilteritinib. For example, a 3-hour exposure followed by wash-out of BMF-500 outperformed 4 days of continuous exposure to gilteritinib, at all concentrations tested. In cells harboring FLT3 activating mutations, BMF-500 induced dose-dependent inhibition of FLT3 phosphorylation and downstream signaling, including phospho-STAT5 and phospho-ERK. A 1-hour pulse treatment with BMF-500 was sufficient to achieve deep and durable target inhibition for greater than 24 hours, an effect not observed with gilteritinib under similar conditions. Profiling BMF-500 across a broad panel of kinases and key cell-surface receptors revealed high selectivity for FLT3 mutants and selectivity against cKit. Potent FLT3 inhibition and high selectivity of BMF-500 translated to sustained tumor regression and improved survival in both subcutaneous and disseminated xenograft models of mutant FLT3-driven AML. Orally administered BMF-500 was well tolerated over 4 weeks of dosing. Study results including efficacy and PD response will be presented. Collectively these data demonstrate BMF-500 to be a novel FLT3 inhibitor with best-in-class potential, given its efficacy, durability, and selectivity in comparison to existing FLT3 inhibitors.