Targeting the mammalian target of rapamycin (mTOR) is a promising strategy for cancer therapy. However, the mTOR kinase functions in two complexes, TORC1 and TORC2, neither of which is fully inhibited by the allosteric inhibitor rapamycin or analogs. We compared rapamycin with the active-site TORC1/2 inhibitor PP242, in acute leukemia models harboring the Philadelphia chromosome (Ph) translocation. We demonstrate that PP242, but not rapamycin, causes death of mouse and human leukemia cells. In vivo, PP242 delays leukemia onset and augments the effects of current front-line tyrosine kinase inhibitors, more effectively than rapamycin. Surprisingly, PP242 has much weaker effects than rapamycin on proliferation and function of normal lymphocytes. PI-103, a less selective TORC1/2 inhibitor that also targets phosphoinositide 3-kinase, is more immunosuppressive than PP242. These findings establish that Ph+ transformed cells are more sensitive than normal lymphocytes to selective TORC1/2 inhibitors, and support the development of such inhibitors for leukemia therapy.

The mechanistic target of rapamycin (mTOR) is a serine/threonine kinase whose activity contributes to leukemia proliferation and survival. Compounds targeting the mTOR active site inhibit rapamycin-resistant functions and have enhanced anti-cancer activity in mouse models. MLN0128 (formerly known as INK128) is a novel, orally active mTOR kinase inhibitor currently in clinical development. Here we evaluated MLN0128 in preclinical models of B-cell acute lymphoblastic leukemia (B-ALL). MLN0128 suppressed proliferation of B-ALL cell lines in vitro and reduced colony formation by primary human leukemia cells from adult and pediatric B-ALL patients. MLN0128 also boosted the efficacy of dasatinib in Philadelphia Chromosome-positive (Ph+) specimens. In a syngeneic mouse model of lymphoid BCR-ABL+ disease, daily oral dosing of MLN0128 rapidly cleared leukemic outgrowth. In primary xenografts of Ph+ B-ALL specimens, MLN0128 significantly enhanced the efficacy of dasatinib. In non-Ph B-ALL xenografts, single agent MLN0128 had a cytostatic effect that was most pronounced in mice with low disease burden. In all in vivo models, MLN0128 was well tolerated and did not suppress endogenous bone marrow proliferation. These findings support the rationale for clinical testing of MLN0128 in both adult and pediatric B-ALL and provide insight towards optimizing therapeutic efficacy of mTOR kinase inhibitors.

Growth factors and many oncogenes activate the lipid kinase phosphoinositide 3-kinase (PI3K), initiating a signaling cascade that includes the protein kinases AKT and target of rapamycin (TOR). The PI3K/AKT/TOR signaling pathway is a significant contributor to disease in various human cancers, including hematologic malignancies. Here we discuss different strategies to inhibit TOR for the treatment of leukemia, lymphoma, and myeloma. The TOR enzyme exists in two complexes in cells, TORC1 and TORC2. The majority of preclinical and clinical efforts to target TOR have involved using rapamycin and its analogs (rapalogs), which suppress TORC1 only partially and do not acutely inhibit TORC2. A new class of small molecules targeting the ATP-binding site of the TOR kinase, termed active-site TOR inhibitors (asTORi), achieves greater inhibition of both TOR complexes, resulting in broader suppression of the PI3K/AKT/TOR signaling network. Preclinical evidence suggests that asTORi have greater efficacy than rapalogs in Philadelphia chromosome-positive acute lymphoblastic leukemia and in T-cell lymphoma. These agents also show greater tolerability in animal models relative to rapalogs or inhibitors of PI3K. These findings encourage broader evaluation of asTORi efficacy in acute myeloid leukemia, B-cell lymphoma, myeloma, and other blood cancers. Clin Cancer Res; 16(22); 5374-80. ©2010 AACR.

2585 Pre-B Acute Lymphoblastic Leukemia (B-ALL) remains a significant cause of morbidity and mortality in both adults and children. Targeting signaling pathways that sustain B-ALL cell survival is a promising strategy to improve treatment outcomes. The target of rapamycin (TOR) is a serine/threonine kinase that integrates signals from oncoproteins and the microenvironment to support ALL cell proliferation and survival. A single TOR enzyme in cells forms two multiprotein complexes, TORC1 and TORC2, with distinct regulation and substrates. The classical TOR inhibitor rapamycin inhibits only some functions of TORC1 and does not acutely inhibit TORC2. A new generation of TOR inhibitors has been developed that inhibit all outputs of both TORC1 and TORC2. These ATP-competitive, TOR kinase inhibitors (TOR-KIs) have improved anti-cancer activity in models of both solid tumors and hematological malignancies. Previously we reported that the TOR-KI compound PP242 is effective in mouse models of Philadelphia Chromosome-positive (Ph+) B-ALL, with minimal toxicity to normal hematopoietic cells or immune function (MR Janes et al., Nat. Med. 2010; 16:205). Here we present promising results on the efficacy of a second generation TOR-KI, INK128, in both Ph+ and non-Ph B-ALL. INK128 is an orally active, highly selective TOR-KI that is currently in phase I clinical trials for solid tumors, multiple myeloma and Waldenstrom's Macroglobulinemia. We find that INK128 has similar biological effects as PP242 on Ph+ B-ALL cells, but these effects are apparent at 5-fold lower concentrations in vitro and 80-fold lower doses in mice. In methylcellulose assays using Ph+ and non-Ph B-ALL specimens, INK128 greatly reduces colony numbers and enhances the efficacy of standard-of-care drugs. Both adult and pediatric non-Ph B-ALL specimens are sensitive to growth suppression by INK128. INK128 also causes apoptosis in B-ALL cells cultured on stromal cells. Importantly, INK128 suppresses human Ph+ B-ALL xenograft expansion in mice at doses that do not impair normal hematopoietic cell proliferation. INK128 is also less immunosuppressive than rapamycin. Together these results provide rationale for testing INK128 and other TOR-KIs in clinical trials of B-ALL therapy. Disclosures: Fruman: Intellikine: Membership on an entity's Board of Directors or advisory committees, Research Funding. Janes:Intellikine: Employment. Martin:Intellikine: Employment. Ren:Intellikine: Employment. Liu:Intellikine: Employment. Rommel:Intellikine: Employment.