Summary This study investigated the cytotoxicity and mechanism of action of AS703026, a novel, selective, orally bioavailable MEK1/2 inhibitor, in human multiple myeloma (MM). AS703026 inhibited growth and survival of MM cells and cytokine‐induced osteoclast differentiation more potently (9‐ to 10‐fold) than AZD6244. Inhibition of proliferation induced by AS703026 was mediated by G0‐G1 cell cycle arrest and was accompanied by reduction of MAF oncogene expression. AS703026 further induced apoptosis via caspase 3 and Poly ADP ribose polymerase (PARP) cleavage in MM cells, both in the presence or absence of bone marrow stromal cells (BMSCs). Importantly, AS703026 sensitized MM cells to a broad spectrum of conventional (dexamethasone, melphalan), novel or emerging (lenalidomide, perifosine, bortezomib, rapamycin) anti‐MM therapies. Significant tumour growth reduction in AS703026‐ vs. vehicle‐treated mice bearing H929 MM xenograft tumours correlated with downregulated pERK1/2, induced PARP cleavage, and decreased microvessels in vivo. Moreover, AS703026 (<200 nmol/l) was cytotoxic against the majority of tumour cells tested from patients with relapsed and refractory MM (84%), regardless of mutational status of RAS and BRAF genes. Importantly, BMSC‐induced viability of MM patient cells was similarly blocked within the same dose range. Our results therefore support clinical evaluation of AS703026, alone or in combination with other anti‐MM agents, to improve patient outcome.

Several protein-tyrosine phosphatases (PTPs) have been implicated in the control of growth hormone receptor (GHR) signaling, but none have been shown to affect growth in vivo. We have applied a battery of molecular and cellular approaches to test a family-wide panel of PTPs for interference with GHR signaling. Among the subset of PTPs that showed activity in multiple readouts, we selected PTP-H1/PTPN3 for further in vivo studies and found that mice lacking the PTP-H1 catalytic domain show significantly enhanced growth over their wild type littermates. In addition, PTP-H1 mutant animals had enhanced plasma and liver mRNA expression of insulin-like growth factor 1, as well as increased bone density and mineral content. These observations point to a controlling role for PTP-H1 in modulating GHR signaling and systemic growth through insulin-like growth factor 1 secretion.

Triple-negative breast cancers (TNBC) have an aggressive phenotype with a relatively high rate of recurrence and poor overall survival. To date, there is no approved targeted therapy for TNBCs. Aurora kinases act as regulators of mammalian cell division. They are important for cell-cycle progression and are frequently overexpressed or mutated in human tumors, including breast cancer. In this study, we investigated the therapeutic potential of targeting Aurora kinases in preclinical models of human breast cancers using a paninhibitor of Aurora kinases, AS703569. In vitro, AS703569 was tested in 15 human breast cancer cell lines. TNBC cell lines were more sensitive to AS703569 than were other types of breast cancer cells. Inhibition of proliferation was associated with cell-cycle arrest, aneuploidy, and apoptosis. In vivo, AS703569 administered alone significantly inhibited tumor growth in seven of 11 patient-derived breast cancer xenografts. Treatment with AS703569 was associated with a decrease of phospho-histone H3 expression. Finally, AS703569 combined to doxorubicin-cyclophosphamide significantly inhibited in vivo tumor recurrence, suggesting that Aurora kinase inhibitors could be used both in monotherapy and in combination settings. In conclusion, these data indicate that targeting Aurora kinases could represent a new effective approach for TNBC treatment. Mol Cancer Ther; 11(12); 2693-703. Ó2012 AACR.

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3848 Poster Board III-784 The mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) signaling pathway plays a crucial role in the pathogenesis of human multiple myeloma (MM) by promoting interactions of MM cells with bone marrow stromal cells (BMSCs) that secrete cytokines and growth factors for MM cell growth, survival, and resistance to chemotherapeutic drugs. Accumulating studies have supported targeting this signaling pathway in MM. Here we investigate cytotoxicity of AS703026, a novel selective MEK1/2 inhibitor with highly oral bioavailability, in MM cell lines and patient MM cells and define its mechanisms of action. AS703026, more potently (∼9-10 fold) than AZD6244, inhibits growth and survival of MM cells and cytokine-induced osteoclast differentiation. It specifically blocks baseline and adhesion-induced pERK1/2, but not pSTAT3. Selective MEK1/2 inhibition by AS703026 led to a cessation of cell proliferation accompanied by G0-G1 cell cycle arrest, as shown by increased subG0 cells, and concurrently abolished S phase cells. AS703026 also reduced expression of c-maf oncogene in a time-dependent manner, suggesting a MEK1/2-dependent regulation of c-maf that may contribute MM cell growth inhibition. AS703026 further induced apoptosis in MM cells, as manifested by caspase 3 and PARP cleavages in a time-dependent manner. It blocked osteoclastogenesis in vitro, as measured by number of TRAP-positive multinuclear cells following culturing PBMCs with RANKL and M-CSF. Importantly, AS703026 sensitized drug-resistant MM cells to a broad spectrum of conventional (dexamethasone, melphalan), as well as novel or emerging (lenalidomide, perifosine, bortezomib, rapamycin) anti-MM therapies. Synergistic or additive cytotoxicity (combination index < 1) induced by these combinations was further validated by annexin-V/PI staining and flow cytometric analysis. Combining these agents led to a significantly increased apoptosis and cell death than AS703026 alone, confirming enhanced cytotoxicity against MM cells. In vivo studies demonstrate that treatment of MM cell line H929-bearing mice with AS703026 (n=4 at 30 mg/kg; n=6 at 15 mg/kg), but not vehicle alone (n=6), blocked MM tumor growth in a dose-dependent manner (p<0.008 at 30 mg/kg; p<0.02 at 15 mg/kg). Immunoblotting and immunohistochemistrical staining showed that AS703026-reduced tumor growth was associated with downregulated pERK1/2, induced PARP cleavage, and decreased microvessels in vivo. Moreover, AS703026 (<200 nM) triggered significant cytotoxicity against the majority of patients with relapsed and refractory MM (>84%, n=18), regardless mutation status of 3 RAS and BRAF genes. Bone marrow stromal cells-induced viability of MM patient cells is similarly blocked within the same dose range. Our results therefore strongly support clinical protocols evaluating AS703026, alone or with other anti-MM agents, to improve patient outcome in MM. Disclosures: Chauhan: Progenra, Inc: Consultancy. Richardson:Keryx Biopharmaceuticals: Honoraria. Clark:EMD Serono: Employment. Ogden:EMD Serono: Employment. Andreas:EMD Serono: Employment. Rastelli:EMD Serono: Employment. Anderson:Millennium Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau.