Abstract B249: Small molecule inhibitors of the Pim protein kinases block the growth of T‐acute lymphoblastic leukemias

The Pim protein kinases, first identified as a proviral integration site in c‐Myc overexpressor mice, are increased in multiple human hematopoietic neoplasms including myeloid leukemia, diffuse large B‐cell lymphoma, and T‐cell lymphoma. We have developed novel benzylidene‐thiazolidine‐2, 4‐diones ( J. Med. Chem. (2009) 52:74) inhibitors of Pim kinases that kill a wide range of both myeloid and lymphoid cell lines with precursor T‐cell lymphoblastic leukemia/lymphoma (pre T‐LBL/T‐ALL) being the most sensitive. The most potent members of this chemotype have IC 50 s of 13 nM for Pim‐1 and 2.3 M for Pim‐2 while some compounds in this chemotype demonstrated selectivity9s of more than 2500‐fold and 400‐fold for Pim‐1 or Pim‐2 respectively while other congeners had equivalent potency towards both isozymes. Of the 47 additional protein kinases tested, only DYRK1a was sensitive to these agents. Incubation of pre T‐LBL cells with one of these Pim inhibitors, SMI‐4a, induced G1 phase cell cycle arrest secondary to a dose dependent induction of p27 Kip1 and translocation of this protein to the nucleus. Additionally, SMI‐4a induced apoptosis in these leukemic cells through the mitochondrial pathway, and inhibited mTORC1 pathway based on decreases in phosphorylation of p70 S6K and 4E‐BP1, two substrates of this enzyme. Using immuno‐deficient animals, we demonstrate that treatment 5/7 days with 60 mg/kg twice daily by oral gavage of SMI‐4a inhibits subcutaneous growth of pre T‐LBL tumors by an average of 47.9% (p FLT3/ITD signaling pathway, we examined the activity of SMI‐4a with or without rapamycin in myeloid leukemic MV4–11 carrying both MLL‐AF4 and FLT3‐ITD . We find that these two agents are highly synergistic in culture. SMI‐4a alone inhibited growth 18% and rapamycin 40% but when combined 76% of the cell growth was blocked. Our results demonstrate that unique combinations of a potent Pim inhibitor, SMI‐4a, and small molecule blockade of either the mTORC1 or ERK pathways have promise as a novel combination strategy for the treatment of human leukemia. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B249.