ALBERT

Description: Abstract 3007 Background and Rationale: SIRT1 belongs to the silent information regulator 2 (Sir2) family of proteins and functions as NAD+-dependent deacetylase. Previous studies showed that resveratrol, a polyphenolic SIRT1 activator, inhibits tumorigenesis in various solid tumor and hematologic malignancies, including human multiple myeloma (MM) cells. This notion led to the discovery and development of more potent and selective pharmacological activators of SIRT1 as potential anti-cancer therapeutics. In this context, a recent medicinal chemistry research using high-throughput screening, and mass spectrometry identified SRT1720, a small molecule activator of SIRT1 that is structurally distinct from resveratrol. Here, we examined the anti-tumor activity of SRT1720 in MM cells using in vitro and in vivo model systems. Methods and Model: We utilized MM.1S, MM.1R, RPMI-8226, U266, KMS12BM, H929, and INA-6 (an IL-6 dependent) human MM cell lines, as well as purified tumor cells from patients with MM relapsing after prior therapies including lenalidomide or bortezomib. Cell viability, proliferation, and apoptosis assays were performed using trypan blue, MTT, thymidine incorporation, and Annexin V staining. Signal transduction pathways were evaluated using immunoblot analysis, ELISA, and enzymology assays. Results: We first confirmed the functional specificity of SRT1720 against SIRT1 using different experimental strategies. First, we utilized Fluor de Lys Deacetylase Assay to measure whether SRT1720 affects the SIRT1 deacetylase enzymatic activity. Treatment of MM.1R and RPMI-8226 MM cells with SRT1720 markedly increased the deacetylating activity; conversely, pre-treatment of cells with nicotinamide (NAM) - an inhibitor of SIRT1 – significantly blocked SRT1720-triggered deacetylating activity. Second, immunoblot analysis using antibodies specific against acetylated p53 (a known substrate of SIRT1) showed a marked decrease in acetylated state of p53 in SRT1720-treated MM cells. These findings in MM cells confirm SIRT1 as a selective target of SRT1720. We next examined the efficacyof SRT1720 in MM cells. Treatment of MM cell lines and primary patient cells for 24h significantly decreased their viability (IC50 range 3–7 uM) (P 〈 0.005; n=3) without markedly affecting the viability of normal peripheral blood mononuclear cells, suggesting specific anti-MM activity and a favorable therapeutic index for SRT1720. SRT1720-triggered apoptosis was confirmed in MM.1R and RPMI-8226 cells, evidenced by a marked increase in Annexin V+ and PI- cell population (P 〈 0.001, n=3). Importantly, SRT1720 induced apoptosis in MM cells even in the presence of bone marrow stromal cells. Mechanistic studies showed that SRT1720-triggered apoptosis in MM cells is associated with 1) activation of caspase-8, caspase-9, caspase-3, and PARP; 2) activation of pATM, CHK2, endoplasmic reticulum stress molecules pEIF2, and BIP; as well as an increase in reactive oxygen species (ROS); 3) inhibition of MM cell growth and survival pathway via NF-kB; and 4) inhibition of VEGF-induced migration of MM cells and associated angiogenesis. Importantly, blockade of pATM using a biochemical inhibitor KU-5593 significantly attenuated SRT1720-induced MM cell death (P value 〈 0.002; n=2). These data suggest that SRT1720-induced MM cell apoptosis is predominantly mediated by an ATM-dependent apoptotic pathway. We next examined the in vivo efficacy of SRT1720 using a human plasmacytoma xenograft mouse model. Treatment of tumor-bearing mice with SRT1720 (200 mg/kg, 5 days a week for three weeks), but not vehicle alone, significantly (P 〈 0.008) inhibits MM tumor growth in these mice. Finally, the combination of SRT1720 with bortezomib or dexamethasone triggered synergistic anti-MM activity. Conclusions: These preclinical studies provide the rationale for novel therapeutics targeting SIRT1 to improve patient outcome in MM. Disclosures: Munshi: Millennium Pharmaceuticals: Honoraria, Speakers Bureau. Richardson:Celgene: Membership on an entity's Board of Directors or advisory committees; Millenium: Membership on an entity's Board of Directors or advisory committees. Anderson:Millennium Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau.

Description: Bortezomib therapy has proven successful for the treatment of relapsed, relapsed/refractory, and newly diagnosed multiple myeloma (MM). At present, bortezomib is available as an intravenous injection, and its prolonged treatment is associated with toxicity and development of drug resistance. Here we show that the novel proteasome inhibitor ONX 0912, a tripeptide epoxyketone, inhibits growth and induces apoptosis in MM cells resistant to conventional and bortezomib therapies. The anti-MM activity of ONX-0912 is associated with activation of caspase-8, caspase-9, caspase-3, and poly(ADP) ribose polymerase, as well as inhibition of migration of MM cells and angiogenesis. ONX 0912, like bortezomib, predominantly inhibits chymotrypsin-like activity of the proteasome and is distinct from bortezomib in its chemical structure. Importantly, ONX 0912 is orally bioactive. In animal tumor model studies, ONX 0912 significantly reduced tumor progression and prolonged survival. Immununostaining of MM tumors from ONX 0912–treated mice showed growth inhibition, apoptosis, and a decrease in associated angiogenesis. Finally, ONX 0912 enhances anti-MM activity of bortezomib, lenalidomide dexamethasone, or pan-histone deacetylase inhibitor. Taken together, our study provides the rationale for clinical protocols evaluating ONX 0912, either alone or in combination, to improve patient outcome in MM.

Description: Abstract 612 Background: Therapeutic targeting of Ubiquitin-Proteasome pathway is exemplified by the recent FDA approval of dipeptidyl boronic acid bortezomib first-in-class proteasome inhibitor for the treatment of multiple myeloma (MM). As with other agents, dose-limiting toxicities and the development of drug resistance limit its long-term utility. The β5, β1 and β2 catalytic subunits within the 26S proteasome mediate chymotrypsin-like, caspase-like, and trypsin-like activities, respectively. Importantly, these catalytic subunits have corresponding immunoproteasome components LMP-7, LMP-2 and multicatalytic endopeptidase complex subunit-1 (MECL-1), which regulate immune cell function and cytokine production; however, the role of the immunoproteasome in MM cells is still unclear. Recent studies have therefore focused on the discovery and development of small molecule inhibitors of the immunoproteasomes, which will both delineate the function of immunproteasomes and allow for specific therapeutic targeting of the UPS in order to reduce off-target activities and associated toxicities. Here, we examined PR-924, an LMP-7-selective peptide-ketoepoxide proteasome inhibitor related to carfilzomib. PR-924, like carfilzomib, contains a ketopoxide pharmacophore that covalently modifies proteasomal N-terminal threonine active sites. We examined the effects of PR-924 in MM cell lines and primary patient cells in vitro. To determine the in vivo efficacy of PR-924, we utilized two xenograft models of human MM in SCID mice, a subcutaneous tumor plasmacytoma model and the SCID-hu model, which best reflects the human MM-BM microenvironment in vivo. Methods and Model: We utilized MM.1S, MM.1R, RPMI-8226, U266, DOX40, KMS12, LR-5, OPM1, OPM2 and INA-6 (an IL-6 dependent) human MM cell lines, as well as purified tumor cells from patients with MM relapsing after prior therapies including lenalidomide or bortezomib. Cell viability and apoptosis assays were performed using Trypan blue, MTT and Annexin V staining. Immunoblot analysis was performed using antibodies to caspase-8, caspase-9, caspase-3, caspase-7, PARP, Bcl-2, BID, or GAPDH. For tumor xenograft studies, CB-17 SCID male mice (n = 10; 5 mice/each group) were subcutaneously inoculated with 5.0 × 106 MM.1S cells in 100 microliters of serum-free RPMI-1640 medium. When tumors were measurable (∼150 mm3) 2-3 weeks after MM cell injection, mice were injected IV with either PR-924 (6 mg/kg BW) or vehicle twice weekly. Mice were sacrificed when their tumors reached 〉2 cm3. In the SCID-hu model, 2 × 106 INA-6 cells were injected directly into human bone chips implanted subcutaneously in SCID mice (n=10: 5 mice/EA group), and MM cell growth was assessed by serial measurements of circulating levels of soluble human interleukin-6 receptor (shulIL6R) in mouse serum. Statistical significance of differences observed in PR-924 vs. vehicle treated mice was determined using a Student t test. Results: PR-924 significantly inhibits growth of all the MM cell lines in a time- and dose-dependent manner (IC50 range: 3-5 μM; P

Description: Abstract 1838 Background and Rationale: The alkylating drug melphalan is routinely used in clinical protocols for the treatment of multiple myeloma (MM). Importantly, clinical trials in MM have effectively utilized combination of melphalan with proteasome inhibitor bortezomib and prednisolone (VMP regimen) to reduce toxicity, overcome drug resistance and enhance cytotoxicity. These findings highlight the utility of conventional alkylating agent, and importantly, provide impetus to develop conventional agents based prodrugs with a potent cytotoxic activity. In this context, pharmacological screening of alkylating oligopeptides led to the identification of a novel melphalan-containing prodrug J1 (L-melphalanyl-p-L-fluoro phenylalanine ethyl ester) - a new molecular entity with a more distinct activity profile than melphalan (Gullbo J, et al., Anticancer Drugs 2003,14:617–24; Gullbo J, et al., Invest New Drugs 2004, 22:411–20; Wickstrom M, et al., Mol Cancer Ther 2007, 6:2409–17). J1 is rapidly incorporated into the tumor cells cytoplasm, followed by intracellular hydrolysis in part mediated by aminopeptidase N (APN), resulting in a 10-fold greater release of free intracellular melphalan than exposure to melphalan at the same molar concentration (Gullbo J, et al., J Drug Target 2003,11:355–63; Wickstrom et al., Biochem Pharmacol 2010, 79(9):1281-90). In vitro studies showed a greater cytotoxic potency of J1 versus melphalan against different human solid cancers; however, its effect in MM is undefined. In the present study, we examined the anti-tumor activity of J1 in MM cells using both in vitro and in vivo model systems. Methods and Models: We utilized MM.1S, MM.1R, RPMI-8226, melphalan-resistant derivative of RPMI-8226 (LR-5), KMS12BM, and INA-6 (an IL-6 dependent) human MM cell lines, as well as purified tumor cells from patients with MM relapsing after prior therapies including lenalidomide or bortezomib. Cell viability-, proliferation-, and apoptosis assays were performed using Trypan blue, MTT, thymidine incorporation, and Annexin V/Propidium iodide staining. Signal transduction pathways were evaluated using immunoblot analysis, ELISA, and enzymologic assays. Statistical significance of data was determined using Student t test. Results: As aminopeptidase N (APN) has been shown to play a key role in conversion of J1 into melphalan in solid tumors, we first examined both expression and enzymatic activity of APN in MM cells. Immunoblot analysis showed a high expression of APN in various MM cell lines. Similarly, colorimetric analysis of APN enzymatic activity using the APN substrate L-alanine-4-nitro-anilide demonstrated elevated APN activity in MM cells. Moreover, pre-treatment of MM cells with APN inhibitor Bestatin showed a moderate, but significant blockade of J1-induced cytotoxicity in MM cells (P 〈 0.05; n=3). We next examined the effects of J1 in MM cells. Treatment of MM cell lines and primary patient cells for 24h significantly decreased their viability (IC50 range 0.5 – 1.0 uM; P 〈 0.001; n=3) without markedly affecting the viability of normal peripheral blood mononuclear cells, suggesting specific anti-MM activity and a favorable therapeutic index forJ1. Of note, the IC50 range of melphalan for MM cell lines is 10–40 uM. J1-triggered apoptosis was confirmed in MM.1R and RPMI-8226 cells, evidenced by marked increase in Annexin V+ and PI-cell population (P 〈 0.001, n=3). Importantly, J1induced apoptosis in MM cells even in the presence of MM bone marrow stromal cells. Mechanistic studies showed that J1-triggered apoptosis in MM cells is associated with 1) activation of caspase-7, caspase-8, caspase-9, caspase-3, and PARP; 2) induction of phospho-c-Jun and phospho-JNK, p53, and p21; 3) release of mitochondrial apoptogenic protein cytochrome-c; 4) inhibition of VEGF-induced migration of MM cells and angiogenesis; and 5) induction of DNA damage response, evidenced by increase in phospho-histone H2AX. Pre-treatment of MM cells with pan-caspase inhibitor Z-VAD-fmk attenuated J1-triggered MM cell apoptosis (P value 〈 0.001; n=3). Finally, treatment of tumor-bearing mice with J1 (3 mg/kg, twice a week for 2 weeks), but not vehicle alone, significantly (P 〈 0.008) inhibits MM tumor growth in these mice. Conclusions: Our study provides the rationale for clinical protocols evaluating J1, either alone or in combination, to improve patient outcome in MM. Disclosures: Richardson: Celgene: Membership on an entity's Board of Directors or advisory committees; Millenium: Membership on an entity's Board of Directors or advisory committees. Munshi:Millennium Pharmaceuticals: Honoraria, Speakers Bureau. Spira:Oncopeptide AB: Employment, Equity Ownership. Anderson:Millennium Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau.

Description: Previous studies have established a role of vascular-disrupting agents as anti- cancer agents. Plinabulin is a novel vascular-disrupting agent that exhibits potent interruption of tumor blood flow because of the disruption of tumor vascular endothelial cells, resulting in tumor necrosis. In addition, plinabulin exerts a direct action on tumor cells, resulting in apoptosis. In the present study, we examined the anti–multiple myeloma (MM) activity of plinabulin. We show that low concentrations of plinabulin exhibit a potent antiangiogenic action on vascular endothelial cells. Importantly, plinabulin also induces apoptotic cell death in MM cell lines and tumor cells from patients with MM, associated with mitotic growth arrest. Plinabulin-induced apoptosis is mediated through activation of caspase-3, caspase-8, caspase-9, and poly(ADP-ribose) polymerase cleavage. Moreover, plinabulin triggered phosphorylation of stress response protein JNK, as a primary target, whereas blockade of JNK with a biochemical inhibitor or small interfering RNA strategy abrogated plinabulin-induced mitotic block or MM cell death. Finally, in vivo studies show that plinabulin was well tolerated and significantly inhibited tumor growth and prolonged survival in a human MM.1S plasmacytoma murine xenograft model. Our study therefore provides the rationale for clinical evaluation of plinabulin to improve patient outcome in MM.

Description: Abstract 2994 Despite recent advances with new drugs such as bortezomib, thalidomide and lenalidomide, multiple myeloma (MM) remains an incurable disease. Used as single agents, these compounds have shown marked antitumor activity, but the number of patients with relapsed and refractory disease remains high. Combination of these agents with other classes of novel drugs would offer great promise to improve patient outcome. AT9283 (Astex therapeutics, Cambridge UK) is a multi-targeted kinase inhibitor that inhibits Aurora A (AURKA), Aurora B (AURKB) and Janus Kinase (JAKs). AURKA and AURKB expression has been correlated with genetic instability and cellular proliferation in MM; therefore, Aurora kinases represent an attractive therapeutic target in MM. In addition the JAK/STAT pathway plays an important role in the survival and proliferation of MM cells. Blocking this pathway may therefore be critical for the survival of MM cells. AT9283 decreased both phospho-Histone H3 and the phosphorylation of Aurora A at Thr 288 in Nocodazole treated cells, suggesting the dual activity of AT9283 against AURKA and AURKB. Importantly, besides Aurora kinase inhibition, we observed that AT9283 inhibited STAT3 tyrosine phosphorylation within 30 minutes of treatment. The effect of AT9283 on pSTAT3 inhibition was further investigated by using U3A cells stably expressing a luciferase reporter gene under the control of a STAT-dependent promoter. AT9283 inhibited STAT3-dependent luciferase activity with an EC50 of approximately 0.125 μ M. Consistent with AT9283 induced cytotoxicity, genetic depletion of STAT3, AURKA or AURKB showed growth inhibition of MM cells, suggesting that AT9283-induced inhibition of these molecules is in part the underlying mechanism of MM cell growth inhibition. In vivo data using a xenograft mouse model of human MM show that mice treated with AT9283 demonstrated slower tumor growth compared to the control group (p=0.018) and prolongation in median overall survival (32 days in treated group versus 18 days in control group; p 〈 0.0001) without adverse effects. We next evaluated the activity of AT9283 in combination with established MM drugs and strong synergistic effect was found when AT9283 was combined with lenalidomide (Selleck Chemicals LLC, TX, USA) (Combination Index 〈 0.9). We hypothesized that the synergistic effect of this combination is due to the fact that the two drugs target different pathways and different phases of the cell cycle, thus augmenting their individual anti-myeloma activity. We examined MM cell cytotoxicity of the combination by using AT9283 and lenalidomide at concentrations lower than their maximal cytotoxic concentrations. Increasing doses of AT9283 (0 -0.125 μ M) were added to lenalidomide (0-2μ M) and a significant decrease in viability (as measured by MTT and cell growth as determined by 3H-TdR at 48 h) was observed with combined therapy compared to either agent alone. A significant increase (55.7%) in early and late apoptosis occurred after 72 hours of exposure of cells to combined therapy with associated caspase-8 and PARP cleavage. Combination treatment resulted in downregulation of pSTAT3 and pERK following 4 hours of treatment. Considering the role that the BM microenvironment plays in growth and survival of MM cells, we examined whether the combination of low dose AT9283 plus lenalidomide induced MM cell death in the context of the BM microenvironment. MM.1S cells were cultured with or without BMSCs in the presence or absence of AT9283, lenalidomide or in combination regimen. Combined therapy inhibited 3H-TdR uptake of MM.1S cells cultured in the presence of BMSCs. Interestingly, we observed that AT9283 plus lenalidomide downregulated the expression of the p-STAT3 and p-ERK when MM.1S cells were cultured with BMSCs, highlighting the role of this drug combination in overcoming the protective effect of BMSCs. These results provide the rationale for the clinical evaluation of AT9283 in combination with lenalidomide in MM patients. Disclosures: Squires: 3Astex Therapeutics Ltd: Employment. Anderson:MILLENNIUM: Consultancy; CELGENE: Consultancy; NOVARTIS: Consultancy; MERCK: Consultancy; ONYX: Consultancy; BMS: Consultancy. Raje:novartis: Consultancy; celgene: Consultancy; astra zeneca: Research Funding; acetylon: Research Funding.

Description: Abstract 3830 Poster Board III-766 Multiple myeloma (MM) is an incurable bone marrow derived plasma cell malignancy. Despite significant improvements in treating patients suffering from this disease, MM remains uniformly fatal due to intrinsic or acquired drug resistance. Thus, additional modalities for treating MM are required. Targeting cell cycle progression proteins provides such a novel treatment strategy. Here we assess the in vivo and in vitro anti-MM activity of MLN8237, a small molecule Aurora A kinase (AURKA) inhibitor. AURKA is a mitotic kinase that localizes to centrosomes and the proximal mitotic spindle, where it functions in mitotic spindle formation and in regulating chromatid congression and segregation. In MM, increased AURKA gene expression has been correlated with centrosome amplification and a worse prognosis; thus, inhibition of AURKA in MM may prove to be therapeutically beneficial. Here we show that AURKA protein is highly expressed in eight MM cell lines and primary patient MM cells. The affect of AURKA inhibition was examined using cytotoxicity (MTT viability) and proliferation (3[H]thymidine incorporation) assays after treatment of these cell lines and primary cells with MLN8237 (0.0001 μM – 4 μM) for 24, 48 and 72h Although there was no significant inhibition of cell viability and proliferation at 24h, a marked effect on both viability and proliferation occurred after 48 and 72h treatment at concentrations as low as 0.01 μM. Moreover, MLN8237 inhibits cell growth and proliferation of primary MM cells and cell lines even in the presence of bone marrow stromal cells (BMSCs) or cytokines IL-6 and IGF1. Similar experiments revealed that MLN8237 did not induce cytotoxicity in normal peripheral blood mononuclear cells (PBMCs) as measured by MTT assay, but did inhibit proliferation at 48 and 72h, as measured by the 3[H]thymidine incorporation assay. To delineate the mechanisms of cytotoxicity and growth inhibitory activity of MLN8237, apoptotic markers and cell cycle profiles were examined in both MM cell lines and primary MM cells. Annexin V and propidium iodide staining of MM cell lines cultured in the presence or absence of MLN8237 (1 μM) for 24, 48 and 72h demonstrated apoptosis, which was further confirmed by increased cleavage of PARP, capase-9, and caspase-3 by immunoblotting. In addition, MLN8237 upregulated p53-phospho (Ser 15) and tumor suppressor genes p21 and p27. Cell cycle analysis demonstrated that MLN8237 treatment induces an accumulation of tetraploid cells by abrogating G2/M progression. We next determined whether combining MLN8237 with conventional (melphalan, doxorubucin, dexamethasone) and other novel (VELCADE®) therapeutic agents elicited synergistic/additive anti-MM activity by isobologram analysis using CalcuSyn software. Combining MLN8237 with melphalan, dexamethasone, or VELCADE® induces synergistic/additive anti-MM activity against MM cell lines in vitro (p≤0.05, CI100 mm3), mice were treated with daily oral doses of vehicle alone or 7.5mg/kg, 15mg/kg, 30mg/kg MLN8237 for 21 days. Overall survival (defined as time between initiation of treatment and sacrifice or death) was compared in vehicle versus- MLN8237- treated mice by Kaplan-Meier method. Tumor burden was significantly reduced (p=0.02) and overall survival was significantly increased (p=0.02, log-rank test) in animals treated with 30mg/kg MLN8237. In vivo anti-MM effects of MLN8237 were further validated by performing TUNEL apoptosis-cell death assay in tumor tissues excised from control or treated animals. Importantly, a significant dose-related increase in apoptotic cells was observed in tumors from animals that received MLN8237 versus controls. These results suggest that MLN8237 represents a promising novel targeted therapy in MM. Disclosures: Ecsedy: Millennium Pharmaceutical: Employment. Munshi:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Richardson:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees. Anderson:Millennium: Research Funding; Novartis: Research Funding; Celgene: Research Funding.

Description: Abstract 3833 Poster Board III-769 Aurora kinases, a family of mitotic regulators, whose expression has been recently linked to genetic instability and cellular proliferation in several cancers including multiple myeloma (MM), are being studied as novel mitotic therapeutic targets. Aurora A plays a crucial role in centrosome separation and spindle assembly and is required for mitosis and bipolar mitotic spindle formation. Aurora B, a member of the chromosomal passenger complex, is required for chromosome segregation, spindle assembly checkpoint and cytokinesis. Both aurora kinase A and B are significantly overexpressed in MM cells prompting the investigation of aurora kinase inhibitors as a therapeutic strategy in MM. Here, we investigated the preclinical activity of a small molecule multi-targeted inhibitor, AT9283, with potent in vitro kinase activity against aurora A and B kinases (3 nM), JAK2 and 3 (at 1.2 and 1.1 nM, respectively) and Abl T315I (at 4 nM). Growth inhibitory effects of AT9283 on MM cell lines and patient derived cells was observed with IC50 values of 0.25μM -0.5 μM at 48 hours using a [3H]-thymidine incorporation assay. Cell cycle analysis following AT9283 treatment resulted in increased G2/M phase and polyploidy consistent with failed cytokinesis (associated with aurora kinase B inhibition) confirmed by immunofluorescence assay. This was followed by induction of apoptosis assessed by Annexin V+PI+ staining peaking at 48 - 72 hours with associated caspase-8/-9 cleavage. The cellular inhibition of aurora kinase activity by AT9283 was confirmed by evaluating the phosphorylation of histone H3 at serine-10, a direct downstream substrate of aurora B kinase. Pretreatment of MM.1S cells with nocodazole, known to induce maximal phosphorylation of histone H3 by causing an M-phase block, resulted in decreased levels of phosphorylated histone H3 after AT9283 treatment suggesting the role of aurora B kinase inhibition by AT9283. Importantly, in addition to aurora kinase inhibition, we observed that AT9283 also inhibited signal transducer and activator of transcription (STAT3) tyrosine phosphorylation in MM cells within 30 minutes of treatment. Janus Kinase (JAK)2/STAT3 pathway is one of the major signaling cascades activated by gp130 family member cytokines that promotes MM cell survival. The effect of AT9283 on pSTAT3 inhibition was further investigated by using U3A cells stably expressing a luciferase reporter gene under the control of a STAT-dependent promoter. AT9283 inhibited STAT3-dependent luciferase activity with an EC50 of approximately 0.125 μM suggesting that STAT3 was functionally inhibited by AT9283. Since MM cell lines with the constitutive STAT3 tyrosine phosphorylation were more sensitive to AT9283, ongoing studies are aimed at understanding whether AT9283-induced effects on the JAK/STAT pathway enhances the efficacy of the aurora kinase inhibition in the context of MM. Finally, in vivo data using a xenograft mouse model of human MM show that mice treated with AT9283 demonstrated slower tumor growth compared to the control group without adverse effects. Our results show pleiotropic effects of AT9283 in MM and warrant further study to determine its suitability for clinical evaluation in MM. Disclosures: Squires: Astex Therapeutics, Ldt: Employment. Yule:Astex Therapeutics Ldt: Employment. Anderson:Novartis, Millennium, Celgene: Consultancy, Honoraria, Research Funding. Raje:Celgene, Norvartis, Astrazeneca: Research Funding.