A Novel Isoflavone, ME-344, Targets the Cytoskeleton in Acute Myeloid Leukemia

The isoflavone ME-344 is a potent anti-cancer agent with preclinical efficacy in solid tumors in vitro and in vivo. In a recently completed phase I clinical trial in patients with refractory solid tumors, ME-344 was well tolerated and clinical responses in refractory patients were observed. ME-344 has been shown to reduce mitochondrial ATP generation in a tumor selective manner, though other potential activities have not been fully defined. In addition, the preclinical efficacy of ME-344 in leukemia has not been established. Therefore, we investigated the anti-leukemic properties and the mechanism of action of ME-344.

We treated a panel of 7 leukemia cell lines with increasing concentrations of ME-344, and measured cell growth and viability. ME-344 was cytotoxic to the 7 leukemia cell lines with an IC₅₀ in the range of 70-260 nM. In addition, ME-344 induced preferential death in primary AML patient samples over normal hematopoietic cells. In an OCI-AML2 xenograft model, ME-344 reduced tumor growth by up to 95% of control without evidence of toxicity.

Mechanistically, as reported in studies in solid tumors, ME-344 increased mitochondrial ROS generation in leukemic cells. However, antioxidant treatment did not rescue cell death, suggesting that ME-344 has additional targets beyond the mitochondria. To identify additional targets of ME-344, we conducted haplo-insufficiency profiling in S.cerevisiae to identify genes whose heterozygous deletion confers increased sensitivity to ME-344. The top hits from the screen were genes involved in cytoskeletal organization. Therefore, we tested the effects of ME-344 on actin and tubulin polymerization in cell free assays. While ME-344 did not inhibit actin polymerization, it inhibited tubulin polymerization by interacting near the colchicine-binding site and at a site distinct from vinca alkaloids. Furthermore, cells resistant to tubulin inhibitors due to tubulin point mutations were also resistant to ME-344. Finally, we showed that ME-344 synergizes with vinblastine in leukemia cells. Thus, our study demonstrates that ME-344 displays preclinical efficacy in leukemia through a mechanism at least partly related to targeting tubulin polymerization.