Pharmacologic inhibition of STAT5 in AML

The transcription factors STAT5A and STAT5B are essential downstream mediators of many tyrosine kinases, particularly in hematopoietic cancers. As such, STAT5 is activated by FLT3-ITD, which is a constitutively active tyrosine kinase driving the pathogenesis of acute myeloid leukemia (AML). Since STAT5 is a critical mediator of diverse malignant properties of AML cells, direct targeting of STAT5 function is of significant clinical value. Here, we describe the novel small molecular weight inhibitor AC-4-130 that directly binds to the phosphotyrosine (pY)-binding pocket of the STAT5 SH2 domain, thereby disrupting STAT5 activation, dimerization, nuclear translocation, and STAT5-dependent induction of gene transcription. AC-4-130 substantially impaired the proliferation and clonogenic growth of human AML cell lines and primary FLT3-ITD+ AML patient cells in vitro and in vivo. Importantly, AC-4-130 synergistically increased the cytotoxicity of the JAK1/2 inhibitor Ruxolitinib and the p300/pCAF inhibitor Garcinol. In summary, we report the development and preclinical evaluation of a novel, potent STAT5 SH2 domain inhibitor that can efficiently block pathological levels of STAT5 activity in AML. The synergistic effects of AC-4-130 tyrosine kinase inhibitors as well as emerging treatment strategies provide new opportunities for combinatorial treatment of leukemia and potentially other cancers. Overall design: MV4-11 and MOLM-13 cells were treated in triplicates with 5 µM AC-4-130 or DMSO (Ctrl)