Multiple Myeloma Cell Lines MM1S Research Articles

Transcriptional Co-Activator P300 Promotes Extra-Medullary Disease Formation and Myeloma Disease Progression

The transcription co-activator P300 regulates gene expression by histone H3 lysine 27 acetylation (H3K27ac) of chromatin and transcriptional regulators. P300 has been suggested to control a variety of biological functions, including differentiation, cell-cycle control, proliferation, apoptosis, and DNA damage response. The clinicopathologic/prognostic importance of P300 in multiple myeloma (MM) is unknown to date. P300 has been linked to the induction of epithelial-to-mesenchymal transition (EMT) and metastasis in a variety of solid tumor malignancies. Like EMT in solid tumors, extra-medullary disease (EMD) in MM relates to a poor prognosis and resistance to treatment. Similarly, the molecular changes concerning EMD in MM are poorly understood, and the role of P300 in MM EMD is yet to be investigated. We have employed chemical and functional genomics to unravel the role of P300 in MM disease progression and EMD formation. Six human MM cell lines-MM1S, KMS-18, H929, U266, OPM-2, and AMO-1-were used to construct lentiviral shRNA-mediated P300 knockdown (K/D) cells. In addition, to analyzing the effect of time-dependent P300 degradation, we have used a proteolysis-targeting chimera (PROTAC) compound termed “JQAD1” that selectively targets P300 for degradation. Treatment with JQAD1 effectively reduced the survival with an IC 50 of 50 nM in MM1S cells treated for 72 h. Furthermore, we wanted to understand the mechanism of P300 degradation-mediated inhibition of cell proliferation. Cell cycle analysis revealed a 40% increase in G0/G1 phase cell cycle arrest following treatment with 50 nM JQAD1 for 72 h. Blocking CD138 receptors is known to enhance MM sensitivity to proteasome inhibitors and significantly reduce tumor size. Immuno-flowcytometry analysis revealed that loss of P300 reduces CD138 expression in MM cells, suggesting their susceptibility to proteasome inhibitors. Next, we wanted to understand if P300 plays a role in developing EMD in MM. Boyden chamber-based migration and invasion assays exhibited reduced migration and invasion of P300 K/D cells with respect to control cells. Furthermore, to understand the distribution and localization of P300 silenced MM cells in vivo, we have used immune deficient NSG mice and studied MM progression after intravenous tumoral injection. In vivo studies revealed that P300 silencing led to reduced MM liver, kidney homing, and EMD formation, as observed through bioluminescence imaging, histology, and flow cytometry analysis, respectively. Further investigations are currently underway to find novel interaction partners and potential targets of P300 in MM and to delineate its mechanism of action. In conclusion, our research indicates that P300 is critical for the development of EMD and the progression of MM. This could potentially serve as a new therapeutic target for MM relapse and dissemination.

The Spop/BRD4/c-Myc Axis Modulates IMiDs Sensitivity in Multiple Myeloma

The development of novel agents including immunomodulatory drugs (IMiDs) lenalidomide (Len) and pomalidomide (Pom) has led to improved patient outcomes in multiple myeloma (MM); however, MM patients inevitably experience relapse and drug resistance. The expression level of CRBN, CRBN-binding proteins, and CRL4 CRBN ubiquitin ligase is considered to be associated with IMiDs resistance. Clinical studies have shown that MM patients can exhibit IMiDs resistance even without CRBN abnormalities, suggesting the molecular mechanisms of intrinsic resistance to IMiDs are not fully understood. To delineate the molecular mechanisms underlying IMiDs resistance, we performed genome-wide knockout (KO) screening in IMiDs-sensitive MM cells. For in vitro work, we used the MM cell lines MM1S, and H929. We used sgRNA to KO SPOP and BRD4 in MM cells. After performing a CRISPR KO screen in MM cells, we discovered that in addition to CRBN and its associated genes, sgRNAs targeting SPOP (speckle-type POZ protein, a speckled zinc finger protein) were highly enriched after treatment with Poma. Importantly, SPOP KO MM cell lines acquired significant resistance to Pom and Len treatment. To examine whether SPOP KO-induced IMiDs resistance was CRBN-pathway dependent, we assessed CRBN and its downstream interacting protein levels. SPOP KO showed no effect on CRBN expression; moreover, IMiDs can still trigger IKZF1 and IKZF3 degradation, associated with the downregulation of IRF4, suggesting that SPOP mediates sensitivity to IMiDs in a mechanism independent of CRBN-IKZF1/3 axis.Compared to plasma cells from normal donors, we found that the expression levels of SPOP were significantly decreased in MGUS patients, smoldering myeloma patients, newly diagnosed MM patients, and refractory relapsed MM patients (p-values: 2.4e-2, 2.0e-4, 9.9e-3, 3.17e-6, respectively). SPOP is the adaptor protein of the Cullin3-RING ubiquitin ligase complex. Therefore, the function of SPOP is determined by the characteristics of the substrates it recruits. We performed immunoprecipitation with overexpressed SPOP in MM cells, followed by mass spectrometry analysis, and identified BRD4 as a candidate protein binding to SPOP in MM, which was confirmed by Co-IP. KO of SPOP led to an upregulation of BRD4 protein levels in MM cells. As a downstream regulator of BRD4, the oncogene c-Myc showed a significant increase in protein expression in IMiDs-resistant MM patients. Furthermore, the KO of SPOP notably enhanced c-Myc expression in MM cells, while the KO of BRD4 in SPOP KO MM cells decreaed c-Myc expression and restored MM cell sensitivity to IMiD treatment. These findings suggest that SPOP modulates c-Myc expression through BRD4 in a CRBN-IKZF1/3 independent manner, thereby mediating MM cell sensitivity to IMiDs. BRD4 plays a significant role in the development and progression of various tumors. The BRD4 inhibitor JQ1 has been shown to exhibit anti-tumor effects in multiple types of cancer. We found that combination treatment with JQ-1 and IMiDs reversed IMiDs resistance of SPOP KO MM cell lines and primary MM cells from relapsed patients. Our data show that SPOP is a CRBN-independent modulator of IMiDs sensitivity by regulating the BRD4/c-Myc axis and provides the preclinical rationale for combining IMiDs with BRD4 inhibitors to overcome IMiDs resistance and improve patient outcomes.

Abstract 3436: Novel multifunctional tetravalent CD38 NKp46 FLEX NK࣪ engagers actively target and kill multiple myeloma cells

Abstract Given that CD38 is a clinically validated target for NK cell mediated cytotoxicity in multiple myeloma, we sought to leverage our FLEX NKTrademark platform to create a NK engager antibody targeting CD38. FLEX NKTrademark is a proprietary platform for production of tetravalent multifunctional NK engager antibodies with a novel FLEX linker to allow for simultaneous binding of both the targeted cancer cells and NK cells. NK engagement and activation is mediated through a binder directed against the natural cytotoxicity receptor NKp46. With the FLEX NKTrademark scaffold, we created two novel tetravalent NK cell engagers targeting CD38, one with a wild type (WT) Fc and one with a mutant null Fc. Both CD38 NKp46 engagers showed dose dependent binding to CD38 expressing multiple myeloma cell lines MM1S and KMS11 and no binding to a CD38 knock out MM.1S cell line. Both engagers also bound multiple myeloma cell lines with ~ 2-fold higher mean fluorescence intensity than anti-CD38 monoclonal antibody (mAb)or daratumumab alone. Epitope mapping studies for our anti-CD38 mAb using alanine scanning mutagenesis showed that amino acid S274 on CD38, critical for binding to daratumumab, is not important for our anti-CD38 antibody binding to CD38, suggesting a distinct epitope detected by our antibody. Interestingly, the NKp46 monoclonal antibody alone without a functional Fc induced peripheral blood NK cell cytolysis of the multiple myeloma cells, consistent with a prior report that NKp46 plays a key role in NK-cell mediated killing of myeloma cells. Both CD38 NKp46 engagers showed further enhanced dose dependent NK cell redirected cytolysis and degranulation against multiple myeloma cells compared to anti-NKp46 monoclonal antibody. The CD38 NKp46 engagers demonstrated higher cytotoxicity than the anti-CD38 binder mAb or daratumumab alone. The CD38 NKp46 engager with the WT Fc is more potent in induction of TNF-α and IFN-ɣ production compared to daratumumab and the engager with the mutant Fc. No IL-1β or IL-6 was induced by the engagers or daratumumab. Daratumumab treatment resulted in NK cell fratricide, while no fratricide was observed with the CD38-NKp46 NK engager with the mutant Fc. In peripheral blood mononuclear cell hemato-toxicity studies depletion of monocytes and NK cells were observed with daratumumab but no depletion was observed with the CD38 NKp46 engager with the mutant Fc. These results suggest that the CD38 NKp46 engagers have a favorable NK cell engager profile for targeting CD38 expressing multiple myeloma that’s distinct from daratumumab. Citation Format: Liang Lin, Hao-Ming Chang, Cecilia Nakid, Stanley Frankel, Dennis Wu, Jean Kadouche, Daniel Teper, Ofer Mandelboim, Jean-Christophe Bories, Antonio Arulanandam, Wei Li. Novel multifunctional tetravalent CD38 NKp46 FLEX NK࣪ engagers actively target and kill multiple myeloma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3436.

Abstract 556: Novel anti-TACI single and dual-targeting CAR T cells overcome BCMA antigen loss in multiple myeloma

Abstract Anti-BCMA CAR T cell therapy has had success clinically against multiple myeloma, recently leading to FDA approval. However, the median progression free survival is 8.8 months, suggesting that targeting BCMA alone may not be sufficient. Reports of BCMA-negative relapse have occurred, highlighting the need for therapeutics that can overcome antigen loss. We have generated a novel antibody against a second target in multiple myeloma, transmembrane activator and CAML interactor (TACI). This antibody specifically recognizes TACI-positive cells (with either endogenous or exogenous expression) and has no recognition of non-B cell peripheral blood mononuclear cells. We designed novel second-generation CAR T cells based on the single chain variable fragment (scFv) from this antibody. We observe that anti-TACI CARs are cytotoxic in vitro at comparable levels to anti-BCMA CAR against multiple myeloma cell lines MM1S and RPMI-8226. Anti-TACI CARs are also functional in vivo and expand in peripheral blood of xenograft multiple myeloma models. We generated BCMA-knockout MM1S cells and observe that while anti-BCMA CAR treated mice have outgrowth of tumor, anti-TACI CAR treated animals retain anti-tumor activity. To overcome potential antigen loss of either BCMA or TACI, we designed tandem bispecific CAR T cells based on both single-targeting CAR designs. These BCMA and TACI dual-targeting CARs are functional in vitro and in vivo, even in the context of single antigen loss. We monitor the activation (CD69), exhaustion (PD-1, TIM-3, LAG-3), and memory (CCR7, CD45RA) phenotype over time to investigate the differences of single versus dual-targeting CAR T cells. We also study the ability of these CAR T cells to bind soluble antigen, as well as their binding avidity to tumor cells. We observe that the anti-TACI scFv more proximal to the CD3ζ signaling domain has a stronger effect on CAR functionality than when this scFv is more distal. Our studies provide a potential superior therapeutic option which remains efficacious in the context of BCMA antigen loss. Citation Format: Rebecca C. Larson, Ana Castano, Amanda A. Bouffard, Michael C. Kann, Andrea Schmidts, Kathleen M. Gallagher, Marcela V. Maus. Novel anti-TACI single and dual-targeting CAR T cells overcome BCMA antigen loss in multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 556.

E-Selectin Ligand Expression Increases with Progression of Myeloma and Induces Drug Resistance in a Murine Transplant Model, Which Is Overcome By the Glycomimetic E-Selectin Antagonist, GMI-1271

There is increasing evidence that E-selectin and its ligands play an important role in the progression of multiple myeloma (MM) and drug resistance. We reported that the sialyltransferase ST3GAL6 influences homing and survival in MM, and postulated that it may function in the synthesis of E-selectin ligands (Glavey et al Blood, 2014). We also found that a small subpopulation of cells (~ 5%) from MM cell lines express functional E-selectin ligands, which could be expanded under hypoxic conditions typical of the bone marrow (BM) microenvironment. These cells were identified by reactivity with an antibody (HECA452), which binds the same carbohydrate epitope required for binding to E-selectin. Rolling of MM1S cells on E-selectin was blocked by a small molecule glycomimetic antagonist to E-selectin (GMI-1271). Moreover, GMI-1271 significantly enhanced the anti-myeloma activity of bortezomib (BTZ) in an in vivo murine transplant model (Natoni et al Blood, 2014). We now extend these observations to obtain a more complete understanding of the role E-selectin plays in MM biology and chemotherapy resistance for its potential clinical relevance.The parental, heterogeneous MM cell lines MM1S and RPMI8226 (MM1Spar, RPMI8226par, respectively) were sequentially sorted to obtain cell lines highly enriched (>85% and 80%) for the expression of cell surface carbohydrates bound by HECA452, and designated MM1SHECA452 and RPMI8226HECA452. The cell lines could be passaged in vitro and were stable for enriched E-selectin ligand expression. In contrast to parental cells, both MM1SHECA452 and RPMI8226HECA452 showed strong binding to E-selectin in static adhesion assays. Both MM1SHECA452 and RPMI8226HECA452 exhibited strong rolling on E-selectin under shear stress. MM1Spar or RPMI8226par failed to roll well on E-selectin. The addition of GMI-1271 during culture conditions led to a marked reduction in adhesion of MM1SHECA452 and profoundly inhibited rolling on E-selectin of both HECA452 enriched MM cell lines.The significance of these in vitro findings was studied in vivo. MM1Spar or MM1SHECA452 cells were injected i.v. into SCID beige mice. Beginning 5 days post tumor injection, the survival impact of treatment with saline control, GMI-1271, BTZ or a combination of both was determined. Mice transplanted with MM1SHECA452 had more aggressive disease with significantly shorter survival compared to those transplanted with MM1Spar. In contrast to MM1Spar cells, mice engrafted with MM1SHECA452 demonstrated a marked resistance to BTZ treatment. Whereas GMI-1271 treatment alone had no impact on survival, the combination of GMI-1271 and BTZ led to a highly significant improvement in survival of MM1Spar engrafted mice (P=0.0363), and more importantly broke the resistance and restored the anti-myeloma activity of BTZ in MM1SHECA452 engrafted mice (P=0.0123) (figure 1). The number of peripheral blood (PB) human CD138+ cells was increased in MM1SHECA452-engrafted mice within 60 min following a single injection of GMI-1271, and persisted for at least 24 hours (2.37% v. 0.03%, p <0.001). This effect was consistent with GMI-1271 disrupting the tumor microenvironment and mobilizing MM1SHECA-452 cells from the BM niche.Given these findings we wished to see if samples from patients with MM express E-selectin ligands and whether higher levels are seen with disease progression. BM and/or PB were obtained following informed consent from patients with MM and plasma cells (CD38+/CD138+) were analyzed for E-selectin ligand expression by flow cytometry using the HECA452 antibody. To date all primary MM samples (n=25) contained HECA452-reactive cell populations (median 22%). A consistently higher proportion of circulating MM cells express HECA452 when compared with paired BM samples (n=14), with a median difference of 33% (Wilcoxon signed rank test, p=0.02). HECA452 expression of MM in PB was significantly higher (on average 40% higher) in samples taken at relapse vs. diagnosis, (unpaired t test, p = 0.0008)These data provide compelling evidence that E-selectin ligand bearing cells play an important role in disease progression and drug resistance in MM, and a strong rationale for clinical strategies incorporating GMI-1271 to improve patient outcome. [Display omitted] DisclosuresSmith:GlycoMimetics, Inc.: Employment. Locatelli-Hoops:GlycoMimetics, Inc.: Employment. Oliva:GlycoMimetics, Inc.: Employment. Fogler:GlycoMimetics, Inc.: Employment. Magnani:GlycoMimetics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. O'Dwyer:Celgene: Honoraria, Research Funding.

Citron Rho-Interacting Serine/Threonine kinase (CIT) Is a Novel Therapeutic Target in Multiple Myeloma Cells

Introduction: Citron rho-interacting serine/threonine kinase (CIT) is a serine/threonine kinase which is a key component of the midbody and is essential for cytokinesis. CIT localizes to the central spindle and midbody and functions to promote efficient cytokinesis. CIT knockdown may disrupt cytokinesis and therefore cell growth. CIT has been reported to be upregulated and important for growth of several cancers. However, the significance of CIT has not been investigated in the field of multiple myeloma (MM). We therefore dissected the role of CIT in MM growth in vitro and in vivo.Materials and methods: CIT gene expression in MM cells was compared to normal plasma cells using public-available gene expression profile (GEP) data set (GSE6477). Kaplan-Meier curve for MM patient survival between high and low CIT expressing patients were examined by using the GEP data set (GSE4581). Protein expression of CIT in MM cells was confirmed by proteomic analysis and immunohistochemistry. Knockdown of CIT was performed in MM cell lines MM1s and OPM2 using lentiviral shRNAs. CIT knockdown was confirmed by reduced CIT mRNA in comparison to a scrambled control. Differences in cell proliferation and cell cycle between CIT knockdown cells and scramble control were analyzed by using thymidine uptake and PI staining, respectively. Cytokinesis failure was analyzed by immunofluorescence using alpha-tubulin antibody and DAPI. shCIT OPM2 (n=7) and the scrambled control cells (n=8) were injected subcutaneously into SCID-Bg mice (5x106 cells/mouse) and were followed for tumor development and survival.Results: CIT expression was significantly higher in MM patients’ plasma cells compared to healthy donors in GEP (p=0.02), proteomic analysis and immunohistochemistry. Also CIT expression was higher in relapsed patients compared to newly diagnosed patients by GEP. MM patients with high CIT expression had significantly worse overall survival compared to low CIT expressing patients (p=0.04). CIT knockdown MM cell lines showed reduced cell proliferation and G2 cell cycle arrest by thymidine uptake and PI staining compared to the scrambled control. Significantly, large amount of multinucleated cells, which indicates cytokinesis failure, were observed in the CIT knockdown cells compared to scrambled control. Reduced tumor growth (p<0.001) and prolonged survival (p<0.001) was observed in CIT knockdown MM cell line injected mice.Conclusions: shRNA knockdown of CIT in MM cells induces G2 arrest leading to cytokinesis failure in vitro with reduced cell proliferation in vivo. Since MM cells have significantly higher expression of CIT compared to normal plasma cells, CIT represents a novel therapeutic target for MM. Studies are ongoing to develop drugs to target CIT for MM treatment. DisclosuresAnderson:Celgene: Consultancy; Sanofi-Aventis: Consultancy; Onyx: Consultancy; Acetylon: Scientific Founder, Scientific Founder Other; Oncoprep: Scientific Founder Other; Gilead Sciences: Consultancy. Ghobrial:Onyx: Advisory board Other; BMS: Advisory board, Advisory board Other, Research Funding; Noxxon: Research Funding; Sanofi: Research Funding.

Silencing The Sialyltransferase Gene ST3GAL6 Inhibits Adhesion and Migration Of Myeloma Cells In Vitro and Reduces The Homing and Proliferation Of Tumor Cells In Vivo

BackgroundGlycosylation is a stepwise procedure of covalent attachment of oligosaccharide chains to proteins or lipids, and alterations in this process, especially increased sialylation, have been associated with malignant transformation and metastasis. The adhesion and trafficking of multiple myeloma (MM) cells is strongly influenced by glycosylation and multiple myeloma cells express a variety of adhesion molecules, including selectin ligands and integrins, which are typically dependent on glycosylation for their function. We have previously reported that the sialyltransferase ST3GAL6 is up-regulated in plasma cells from MM patients and that increased expression is associated with inferior overall survival (OS) in MM gene expression profiling (GEP) datasets. The functional significance of increased sialylation of MM cells has not previously been reported. MethodsMM cell lines MM1s and RPMI-8226 were confirmed to have high expression levels of ST3GAL6 at the gene and protein level compared to healthy controls. Knockdown of ST3GAL6 was confirmed in MM cell lines RPMI-8226 and MM1s using lentiviral shRNAs targeting different regions in the ST3GAL6 mRNA. Specific ST3GAL6 knockdown was confirmed by reduced ST3GAL6 mRNA and protein expression in comparison to a scrambled control. In a calcein-AM fluorescence based adhesion assay we next evaluated the effects of ST3GAL6 knockdown on MM-cell adhesion to bone marrow stromal cells (BMSC's) and fibronectin coated plates. Migration to 30nM SDF1-α was assessed using transwell plates comparing ST3GAL6 knockdown cells to scrambled controls. The commercially available sialyltransferase inhibitor 3Fax-Neu5Ac was used to pre-treat MM cells in vitro prior to assessment of apoptosis by flow cytometry. shST3GAL6 MM1s cells positive for green fluorescent protein and luciferin (GFP-Luc+) were injected into tail veins of SCID-Bg mice (5x106 cells, n=5/group) and mice were followed weekly using bioluminescent imaging (BLI) for tumor development. Bone marrow homing of tumor cells was assessed using in vivoconfocal imaging of the skull vasculature (n=3/group). ResultsKnockdown of ST3GAL6 in MM cell lines resulted in a 50% reduction in cell surface staining with the monoclonal antibody HECA-452. This indicated reduced expression of cutaneous lymphocyte associated antigen (CLA), a carbohydrate domain shared by sialyl Lewis X (sLex) and sialyl Lewis a (sLea) antigens, confirming suppression of ST3GAL6 activity. There was a significant reduction in the ability of knockdown cells to adhere to BMSC's and fibronectin in-vitro compared to scrambled controls (P=0.016, 0.032 respectively). Migration ability of these cells in response to SDF1-α was also reduced (P=0.01). In vivo in a xenograft SCID-Bg mouse model shST3GAL6 cells demonstrated a reduced tumor burden as assessed by weekly BLI (P=0.017 at week 4). A consolidated map of the skull bone marrow niche in mice injected with shST3GAL6 MM1s GFP-Luc+ cells revealed a reduced homing ability of these cells in comparison to mice injected with scrambled control cells. Treatment of the MM cell lines MM1s and RPMI-8226 with a sialyltransferase inhibitor 3Fax-Neu5Ac resulted in almost complete elimination of cell surface sLex and/or sLea expression as determined by HECA-452 staining. Following pre-treatment with 3Fax-Neu5Ac, MM1S cells grown in co-culture with BMSC's cells showed increased sensitivity to Bortezomib compared to cells treated with bortezomib alone. ConclusionsshRNA knockdown of ST3GAL6 in MM cells significantly inhibits adhesion and migration in vitro with reduced homing and proliferation potential in vivo. In conjunction with the results of enzymatic inhibition this indicates that sialylation may play an important role in the malignant behavior of MM cells. Studies are ongoing to address the potential role of altered glycosylation in MM. Disclosures:Ghobrial:Onyx: Advisoryboard Other; BMS: Advisory board, Advisory board Other, Research Funding; Noxxon: Research Funding; Sanofi: Research Funding.

Preclinical Evaluation of AT219, a Small Molecule Inhibitor of MDM2 As an Anti-Myeloma Agent.

AbstractAbstract 2948 Background:Deletions and mutations in the tumor suppressor protein p53 are an uncommon observation in new multiple myeloma (MM) patients and are observed more commonly in patients with advanced disease. p53 deletion has been observed to correlate with poor overall and progression free survival in MM patients. Wild type p53 modulates the expression levels of a broad array of proteins involved in cell cycle progression, apoptosis ultimately leading to cell cycle arrest and apoptosis. p53 is negatively regulated by MDM2. MDM2 binds to and ubiquitinates p53 marking it for proteasomal degradation. In addition, MDM2 is a direct downstream regulator of p53. Targeting the p53-MDM2 interaction by developing agents that bind to the p53 binding motif of MDM2 and reactivating p53 has therefore been an active area of research. Here, we present results from our pre-clinical studies using AT219, a small molecule inhibitor that binds to MDM2 preventing its interaction with p53. Methods:AT219 was obtained from Ascenta Therapeutics. Stock solutions were made using DMSO and working stock solutions were made using RPMI 1640 media containing 10% fetal bovine serum (20% serum for primary patient cells) supplemented with L-Glutamine, penicillin, and streptomycin. Akt1/2 kinase inhibitor (Akti) was purchased from Sigma. MTT assay was performed to study drug induced cytotoxicity and thymidine uptake was used as a measure to study differences in proliferation. Flow cytometry using Annexin V-FITC and propidium iodide (PI) was used to measure drug induced apoptosis in cell lines and patient cells. In addition, apo-2.7 was also used to measure apoptosis in patient cells. Mitocapture and cytochrome-c assays were also performed to confirm the induction of apoptosis in MM cell lines. In order to study the mechanism of action of the drug, immunoblotting studies were performed on lysates made from cell lines incubated with the drug for various time points. Results:AT219 induced potent cytotoxicity in MM cell lines MM1S, MM1R and H929, all three expressing wild type p53 with IC50 values of 2.5–5μM. Similar effects were observed when the above mentioned cell lines were treated with AT219 and the inhibitory effect of proliferation of these cells were examined. When MM1S or H929 cells were cultured with bone marrow stromal cells (BMSCs) derived from MM patients or with one of the three tumor promoting cytokines implicated in MM (IL6, IGF or VEGF) and treated with AT219, the drug was able to inhibit the proliferation of both cell lines to similar extents as observed when cultured independently without BMSCs or the cytokines. The increase in cytotoxicity was found to be due to cells undergoing apoptosis as observed when MM1S or H929 cells were cultured with AT219 and % apoptotic cells were measured as measured by annexin/PI, mitocapture and cytochrome c assays. AT219 was also observed to induce more potent apoptosis in primary cells obtained from new MM patients with wild type p53 than in cells obtained from relapsed MM patients with wild type p53. AT219 clearly upregulated p53 as observed by performing immunoblots after treatment with the drug in MM1S and H929 cells. In addition, MDM2 and p21 were also found to be significantly upregulated and Bax was slightly upregulated post drug treatment. Bcl2, Mcl1 and Xiap levels were down regulated. In MM1S cells AT219 treatment resulted in a slight down regulation of pAkt (Ser 473). However, in H929 cells we observed a transient upregulation of pAkt following AT219 treatment. This prompted us to test AT219 in combination with Akti on MM cell lines. Our results on both MM1S and H929 cells using AT219 in combination with Akti demonstrated synergy. We are currently testing this combination in primary cells drawn from MM patients with both wild type p53 and those with p53 deletions and mutations. Conclusions:Our studies validate the anti-MM activity of AT219 in MM patients with wild type p53. In addition to using AT219 in combination with Akti, we are testing AT219 in combination with existing anti- MM chemotherapeutic agents. Interesting results from our studies will form the basis for clinical evaluation of AT219 as a single agent or in combination with an Akt inhibitor or other agents in MM patients. Disclosures:Kumar:Celgene: Consultancy, Research Funding; Merck: Consultancy, Honoraria; Millennium Pharmaceuticals, Inc.: Research Funding; Novartis: Research Funding; Genzyme: Research Funding; Cephalon: Research Funding.

Frequent Dysregulation of Fc Gamma Receptor IIb (Fc γRIIb) and Immunoreceptor Tyrosine-Based Inhibitory Motif (ITIM) Signaling Occurs in Multiple Myeloma (MM) Patients

Abstract 2917The inhibitory Fc receptor, Fc γRIIb, is expressed on plasma cells, controls their persistence in the bone marrow (BM) and their ability to produce serum Ig. Activation of Fc γRIIb leads to the phosphorylation of ITIM and recruitment of SH2-containing inositol 5’-phosphatase (SHIP) in plasma cells. Immunoreceptor tyrosine-based activation motif (ITAM) and ITIM provide the basis for two opposing signaling modules that duel for control of plasma cell activation. Fc γRIIb-mediated SHIP phosphorylation activates downstream ITAM or ITIM signaling. To determine whether multiple myeloma (MM) cells express Fc γRIIb, we performed immunohistochemical staining on bone marrow mononuclear cells from MM patients and controls. We found that not only CD20+ B cells expressed Fc γRIIb but more importantly CD138+ cells from MM patients also showed expression of this receptor. Next, we examined whether Fc γRIIb was present and expressed in CD138+ primary MM cells purified from fresh MM BM and the MM cell lines MM1s, RPMI8226, and U266 using PCR and RT-PCR on DNA and mRNA, respectively. We focused on the transmembrane domain of the Fc γRIIb gene with four primers from different parts of this domain since this portion plays a critical role in this receptor’s function. The MM cell lines expressed different amounts of Fc γRIIb. Notably, we found that 17% (5/30) of MM patients showed absence of Fc γRIIb both using RT-PCR for mRNA and PCR for DNA. Moreover, use of these same primers on nonmalignant PBMCs from the MM patients also showed absence of this gene in the same five patients. As a result of these findings, we are currently sequencing Fc γRIIb in MM patients to determine if additional patients show mutational changes that affect the function of this receptor. We also further determined SHIP-1 phosphorylation using Western blot analysis since this protein mediates downstream signaling of Fc γRIIb. Following stimulation with Fc complexes, phosphorylation of SHIP-1 was markedly reduced in MM tumor cells compared to normal CD20+ B cells. Interestingly, the patients with missing Fc γRIIb expressed higher levels of SHIP-1 gene expression compared to patients with normal Fc γRIIb expression. We investigated the IgG-binding ability of MM patients (n=33) and normal donors (n=33) to Fc γRIIb. Each serum sample was incubated with cells from MHC1, a cell line that specifically expresses Fc γRIIb but not Fc γRI and Fc γRIIa. The results showed MM patients’ serum IgG have much lower Fc γRIIb-binding ability than normal human IgG (P<0.05) by using both flow cytometric and immunofluorescence assays. Our findings suggest that the monoclonal protein produced by MM patients has a very low Fc γRIIb-binding ability and is incapable of signaling through the inhibitory ITIM pathway. Germline loss of Fc γRIIb in MM patients with variation in its expression level and its downstream signaling molecule SHIP and its phosphorylation as well as the inability of MM IgG to bind cells containing this receptor is a potential new mechanism that contributes to the uncontrolled growth of MM. Disclosures:Berenson:Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; Millennium Pharmaceuticals, Inc.: Consultancy, Honoraria, Research Funding, Speakers Bureau; Onyx Pharmaceuticals: Consultancy, Honoraria, Research Funding, Speakers Bureau; Celgene: Consultancy, Honoraria, Research Funding, Speakers Bureau; Medtronic: Consultancy, Honoraria, Research Funding, Speakers Bureau; Merck: Research Funding; Genentech: Research Funding.