Characterization of midostaurin as a dual inhibitor of FLT3 and SYK and potentiation of FLT3 inhibition against FLT3-ITD-driven leukemia harboring activated SYK kinase

Abstract

Oncogenic FLT3 kinase is a well-established target in acute myeloid leukemia (AML), leading to the development of both multi-targeted and selective FLT3 inhibitors. In patients with FLT3-ITD-positive AML, spleen tyrosine kinase (SYK) has been found to be activated and elevated, playing a crucial role in the transformation and maintenance of the leukemic clone. Moreover, overexpression of constitutively activated SYK can confer resistance to highly selective FLT3 tyrosine kinase inhibitors (TKIs). To date, the effectiveness of the multi-targeted FLT3 inhibitor midostaurin against cells with activated SYK has not been investigated in the context of leukemia, despite its recognition as a target in systemic mastocytosis.

We assessed midostaurin’s ability to inhibit activated SYK in FLT3-mutant AML cells, comparing it with inhibitors that provide dual SYK/FLT3 inhibition, targeted SYK inhibition, and targeted FLT3 inhibition. Our results indicate that dual FLT3/SYK inhibitors and FLT3-targeted drugs effectively kill FLT3-transformed cells, while SYK-targeted small molecules show limited efficacy. Notably, midostaurin and other dual FLT3/SYK inhibitors demonstrated greater anti-proliferative activity compared to targeted FLT3 inhibitors like crenolanib and quizartinib against cells co-expressing FLT3-ITD and activated SYK-TEL. Additionally, further suppression of SYK enhanced the effects of both dual FLT3/SYK inhibitors and targeted FLT3 inhibitors on FLT3-ITD-driven leukemia, regardless of the presence of activated SYK. These findings have significant implications for designing drug combination studies in mutant FLT3-positive patients and for developing future generations of Crenolanib FLT3 inhibitors.