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Introduction: Regulatory T (Treg) cells, an immunosuppressive subset of CD4+ T cells characterized by the expression of the master transcription factor forkhead box protein P3 (FOXP3), are a component of the immune system with essential roles in maintaining self-tolerance. Treg cells which are indispensable for preventing autoimmunity, also suppress effective tumor immunity (Togashi et al. Nat Rev Clin Oncol 2019) Treg cells abundantly infiltrate into tumor tissues, present in the tumor microenvironment where they promote tumor development and progression by dampening antitumor immune responses. The abundantly infiltrate of Treg cells into tumor tissues is often associated with poor prognosis in cancer patients (Tanaka et al Eur. J. Immunol. 2019). The chemokine receptor CXCR4 and its ligand, stromal cell-derived factor-1 (SDF-1/CXCL12) are critically involved in immune cell trafficking. CXCR4 overexpression, which has been identified in multiple cancer types, also supports cancer metastasis, recurrence and therapeutic resistance. More importantly, CXCR4 was shown to enhance tumor immune evasion by recruiting Treg (Santagata et al. Oncotarget. 2017) Objective: To study the effect of the high affinity CXCR4 antagonist, BL-8040, on the biology of Treg cells. To study how BL-8040 affects the ability of these cells to penetrate into the tumors, their migratory ability, their survival and also the differentiation of naive T cells towards Treg. Methods:C57BL/6 mice bearing LivMet pancreatic tumors and control mice were used for in-vivo study. In-vitro study was done with CD4 +CD25 hiFOXP3 + (Treg) cells which were isolated from fresh whole blood. CD4 +CD25 - cells were served as T conventional cells (Tconv). Differentiation of Treg cells was done from Naïve CD4+ T cells which were isolated from cord blood and stimulated with anti-CD3/CD28, TGF-b, IL-2 with or without BL-8040 for 6 days. Results: When mice bearing pancreatic cancer were treated with BL-8040, we found a significant reduction in the number of infiltrating Treg into the tumor. Following treatment with BL-8040 there was no alteration in the number of Treg in the blood neither in control mice nor in mice bearing tumors. To further understand the mechanism by which BL-8040 effected Treg cells we isolated Treg and Tconv cells and found that Treg cells express lower level of CXCR4, as compared to Tconv (Figure1). Further to, when we compared their motility, we found that Treg migration less efficiently towards CXCL12. Despite this, BL8040 more efficiently suppressed CXCL12 induced migration of Treg compared to Tconv. 100 nM of BL8040 was found to inhibits the migration of 82 % from the Treg compared to only 56.6% of Tconv cells (Figure 2). CXCR4 involves classical pathways of cell survival. In order to study the role of CXCR4 in the viability of Treg, we incubated Treg and Tconv cells in the presence of BL-8040 for 24 hr. We found that Treg cells are more sensitive to BL-8040 treatment with 19.2% of cell death compared to only 3.5% of Tconv cell death (Figure 3). Treg are one of the lineages of T helper (Th) cells which differentiated from naïve CD4 T cells. We found that BL-8040 inhibits the differentiation of naive CD4 T cells toward Treg. 10uM of BL-8040 shows a 5-fold inhibition in Treg differentiation from naïve CD4 T cells (Figure 4). Conclusions: In this work we show that the CXCR4 antagonist, BL-8040, can act as an immunomodulator by affecting the biology of regulatory T cells. BL8040 reduce the amount of infiltrating Treg into the tumors, impaired the migratory capacity of Treg toward CXCL12 and induces their cell death. Furthermore, BL-8040 was found to inhibit the differentiation of naïve CD4 T cells toward Treg. Taking all these together, BL-8040 may therefore improve the anticancer immune response, without impairing the activity of Tconv and thus can potentially serve as an effective immunomodulatory agent for cancer. Figure 1 Figure 1. Disclosures Abraham: Biokine Therapeutics: Current Employment. Vainstein: BioLineRx LTD: Current Employment. Shemesh-Darvish: BioLineRx LTD: Current Employment. Sorani: BioLineRx LTD: Current Employment. Eizenberg: Biokine Therapeutics Ltd: Current Employment. Peled: Biokine Therapeutics Ltd: Current Employment; Gamida Cell: Research Funding.

Introduction: Lifelong blood cell production is dependent on rare hematopoietic stem cells (HSCs) to perpetually replenish mature cells via a series of lineage-restricted intermediates. The bulk of HSCs are CD34+, however, most CD34+ cells are lineage-restricted progenitors and HSCs remain rare. HSCs can be enriched further based on CD45RA, Thy1 (CD90), and CD38 expression. Loss of CD90 expression was proposed to be sufficient to separate CD34+CD38−CD45RA− CD90+ HSCs from CD34+CD38−CD45RA− CD90- multipotent progenitors (MPPs). Recently, it was demonstrated that the expression CD49f is a specific HSC marker. Single CD49f(+) cells were highly efficient in generating long-term multilineage grafts, and the loss of CD49f expression identified transiently engrafting MPPs. Results: CD34+ cells were purified from BL-8040 and G-CSF mobilized grafts and stained for CD38, CD45RA, CD90, and CD49f. The percentage of CD34+CD38- hematopoietic stem and progenitors was similar in both grafts (Figure 1A). However, whereas 23.2 % of BL-8040 mobilized CD34+ CD38- cells did not express CD45RA; only 1.6% of G-CSF mobilized CD34+ CD38- cells did not express CD45RA (Figure 1B). The percentages of CD34+CD38−CD45RA−CD49f+CD90+/-, CD34+CD38−CD45RA−CD49f+ CD90+, and CD34+CD38−CD45RA− CD90+ HPCS were increased significantly by 45, 25 and 12 -fold in the BL-8040 graft compared to G-CSF graft derived CD34+CD38- cells (Figure 1C). To assess the long-term engraftment potential of the BL-8040 mobilized CD34+ cells, engraftment was allowed for 4, 8, and 22 weeks after transplantation. Successful and robust long-term human engraftment of CD45+ and CD45+CD34+ cells was observed at week 22 (Figure 2A, 2B). The % of human CD45 cells remained stable in the BM whereas the percentage of CD45 cells in the blood and spleen increased at week 22 (Figure 2C). At 4 weeks, human CD3+CD4+ T cells were only observed at a low percentage in the spleen but not in the BM, whereas no significant percentage of CD3+CD8+ cells were found neither in the BM nor in the spleen (Figure 2D, 2E). 22 weeks after transplantation, the percentage of human CD3+CD4+ and CD3+CD8+ T cells was significantly increased in the spleen (30% vs. 5%, respectively) and to much lower levels in the BM (Figure 2D, 2E). Furthermore, successful and robust long-term human engraftment of secondary recipient was observed 14 weeks following the second transplantation (Figure 2A and 2B). Conclusion: In association with the high percentage of HPCs in the BL-8040-derived graft, we found a robust myeloid and lymphoid long-term engraftment (week 22) of BL-8040 mobilized human CD34+ cells in NSG mice. The ability of BL-8040 to collect high numbers of HPCs may be beneficial for a variety of HPCs dependent therapeutics. Disclosures Abraham: Biokine: Employment. Oberkovitz: BiolineRx: Employment. Eizenberg: Biokine: Employment. Vainstein: BiolineRx: Employment. Benami: BiolineRx: Employment. Golan: BiolineRx: Employment. Or: Bioline: Consultancy. Peled: Biokine: Consultancy; Biosight: Consultancy.

Background: Acute Myeloid Leukemia (AML) is a heterogeneous group of diseases characterized by uncontrolled proliferation and survival of hematopoietic stem and progenitor cells. The chemokine CXCL12 and its receptor CXCR4 are key players in the survival, bone marrow (BM) retention and the maintenance of AML blasts in their stemness state. CXCR4 overexpression is associated with poor prognosis in AML patients. Signaling activated through CXCR4 was shown to be detrimental by increasing survival of tumor cells and promoting resistance to therapy. Objective: To study the effect of the CXCR4-antagonist, BL-8040, on the survival of human AML blasts and to investigate the molecular mechanism by which inhibition of CXCR4 signaling leads to leukemia cell death. Methods: Human AML cell lines and human primary AML samples were used for in vitro studies. The in-vivo effect of BL-8040 was tested using the MV4-11, U-937, THP-1 cells and human primary AML cells engrafted in NOD scid gamma (NSG) mice. Results: We found that BL-8040 directly induced apoptosis of AML cells both in FLT3-ITD and FLT3-WT AML, in-vitro and in-vivo. BL-8040 treatment triggered mobilization of AML blasts from their protective BM microenvironment and induced their terminal differentiation, in-vitro and in-vivo. The apoptosis of AML cells induced by BL-8040 was attributed to miR-15a/miR-16-1 up-regulation resulting in down-regulation of their target genes BCL-2, MCL-1 and cyclin-D1. The increase in miR-15a/miR-16-1 levels directly induced AML cell death. Moreover, CXCR4 blockade by BL-8040 also inhibited survival signals by the ERK/AKT kinases enhancing the apoptosis effect. Survival of AML cells was found to be dependent on BCL-2 as demonstrated by the ability of the BCL-2 inhibitor, ABT-199, to induce dose dependent apoptosis in vitro. It was reported that the MCL-1 protein plays a key role in acquiring resistance to ABT-199. We found that BL-8040 synergizes with ABT-199 in inducing AML cell death. This could be attributed to the reduction of both, AKT/ERK and MCL-1 levels, by treatment with BL-8040. In addition, BL-8040 synergizes with the FLT3 inhibitor AC220 in the induction of AML cell death both in-vivo and in-vitro. The combined treatment of BL-8040 and AC220 was found to prolong survival and reduce minimal residual disease in-vivo. Interestingly, the combined treatment was also associated with a significant reduction in the expression of BCL-2 and ERK signaling. Conclusions: BL-8040 can be a potential therapeutic option in AML by targeting not only AML anchorage in the BM but also AML survival and differentiation. Our results demonstrate that BL-8040 in AML regulates the expression of miR-15a/16-1 and their target genes BCL-2, MCL-1 and cyclin-D1. Furthermore, these results indicate that the CXCR4 antagonist, BL-8040 may tip the balance toward cell death by down- regulating survival signals through miR-15a/16-1 pathway and inhibition of the ERK/AKT survival signaling cascade in AML cells. Our results provide rational for combination of BL-8040 with ABT-199 to overcome potential acquired resistance to ABT-199 in AML patients. The synergistic effect of BL-8040 with AC220 could provide a rational basis for the combination of BL-8040 with FLT3 inhibitors in FLT3-ITD AML patient population. Figure 1. Figure 1. Figure 2. Figure 2. Figure 3. Figure 3. Disclosures Abraham: Biokine Therapeutics Ltd: Employment. Bulvik:Biokine Therapeutics Ltd: Employment. Wald:Biokine Therapeutics Ltd: Employment. Eizenberg:Biokine Therapeutics Ltd: Employment. Pereg:BioLineRx Ltd: Employment. Peled:Biokine Therapeutics Ltd: Consultancy, Employment.

Background: Persistence of minimal residual disease (MRD) after induction, consolidation and salvage therapies is an independent poor prognostic marker in AML. The bone marrow (BM) microenvironment is a protective niche that promotes chemotherapy resistance of leukemia cells. This residual subset of chemotherapy-resistant cells present during remission eventually contributes to the leukemia relapse/persistence. Adhesion of AML cells to BM stroma is dependent on the CXCR4 receptor binding to its ligand CXCL12. High levels of CXCR4 expression correlate with poor survival in AML. It is postulated that blocking the CXCL12/CXCR4 axis will dislodge AML cells from the protective microenvironment and, when combined with chemotherapy, may improve the response to therapy. BL-8040 (BKT140) is a short peptide which inhibits the binding of CXCR4 to CXCL12, resulting in mobilization of leukemic blasts from the BM to the peripheral blood (PB). BL-8040 has strong affinity for the CXCR4 receptor with long receptor occupancy and as a single agent induces differentiation and apoptosis of leukemia cells in preclinical models. A clinical trial assessing the safety and efficacy of BL-8040 in combination with cytarabine (Ara-C) for the treatment of relapsed/refractory AML patients was recently completed (NCT01838395). Here we report results from correlative studies done on samples from patients who participated in this trial. Objective: To assess BL-8040 single agent anti leukemia effects in relapsed/refractory AML. Method: Patients received a once daily SC dose of BL-8040 monotherapy on days 1-2 followed by the same dose of BL-8040 plus Ara-C (1.5g/m2 for patients ≥60; 3g/m2 for patients Results: 45 patients (including 3 receiving compassionate use BL-8040) with a median age of 61 yrs were treated. The majority of patients had poor risk disease with 24% secondary AML and 17% with prior allogeneic stem-cell transplantation. Most patients were heavily pretreated with 19% relapsing after a short first remission (CR1 ≤12 months), 17% had ≥2 relapses and 45% refractory to 1-2 inductions. The CR+CRi rate was 38% in subjects receiving BL-8040 dose ≥1.0 mg/kg (n=39). Ongoing follow-up (FU) of responding patients (expansion phase; N=19) shows median EFS of 9.3 months (range 4.3-12.8 months). At the time of this analysis 10/19 patients are alive with a FU range of 0.96 - 12.8 months. FACS analysis revealed that BL-8040 monotherapy triggered an average 4.3-fold increase of immature AML blasts (CD45+Low/CD34+/CD117+/HLA-DR+) in PB (i.e., mobilization). Response to treatment was associated with more robust mobilization of AML blasts following BL-8040 monotherapy (responders 9.5 vs non-responders 1.7 fold median). Preferential mobilization of AML blasts over normal cells (4.7 fold vs. 1.4 fold, respectively) was confirmed by FISH analysis in a subset of patients with informative cytogenetic abnormalities. Induction of granulocytic differentiation of immature leukemia progenitors (2.5 fold) in the BM following BL-8040 monotherapy was noted. Finally, induction of AML blasts apoptosis (40% increase) by BL-8040 alone was evident by FACS in day 3 (pre-araC) bone marrow biopsies. Conclusions: The data demonstrate that sustained blockade of the CXCR4-CXCL12 axis with BL-8040 is safe and well tolerated and when given in combination with Ara-C improves the response rate achieved historically with Ara-C alone. In addition, treatment with BL-8040 as a single agent rapidly and efficiently induces mobilization, differentiation and cell death of AML blasts. This selective effect on chemotherapy-resistant cells may be translated into reduction of residual disease arguing for incorporation into front-line trials and such studies are ongoing. Disclosures Foran: Millennium Pharmaceuticals, Inc.: Research Funding; medscape: Honoraria; karyopharm: Honoraria; pfizer: Honoraria; novartis: Honoraria; boehringer ingelheim: Research Funding; agios: Research Funding; Cellerant: Research Funding. DiPersio:Incyte Corporation: Research Funding. Peled:Biokine Therapeutics: Employment. Abraham:Biokine Therapeutics Ltd: Employment. Pereg:BioLineRx Ltd: Employment. Vainstein:BioLineRx Ltd.: Employment. Oberkovitz:BioLineRx Ltd.: Employment. Aharon:BioLineRx Ltd.: Employment. Cortes:ARIAD: Consultancy, Research Funding; BMS: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; Teva: Research Funding.

Introduction: Multiple myeloma (MM) is an incurable hematological malignancy characterized by proliferation of malignant plasma cells in the bone marrow (BM). Interactions between MM cells and BM milieu facilitate disease progression and therapy resistance. Chemokine receptor CXCR4 and its cognate ligand CXCL12 are implicated in these processes and are associated with poor prognosis. Sphingosine-1-phosphate (S1P) pathway is involved in cancer progression, including oncogenesis, cell survival and cell migration, therefore representing an attractive target for anti-cancer therapy. FTY720 (fingolimod) is a modulator of S1P signaling system that exhibit immunosuppressive and anti-cancer properties. The role of S1P system and FTY720 modulator in MM is less defined. The aim of this study was to explore the functional consequences of possible cross-talk between the CXCR4/CXCL12 and the S1P axes in MM cells and to evaluate the effect of S1P targeting with FTY720 as potential anti-MM therapeutic strategy. Results: The partners of the S1P pathway (S1P receptor 1 and sphingosine kinase 1 (SPHK1)) and CXCL12 chemokine were found to be co-expressed in MM cell lines and primary BM samples from MM patients. Increased mRNA levels of SPHK1 and CXCL12 were detected in MM BM samples (n=24) comparing to BM from healthy donors (n=7) (p Interestingly, suppressive potential of FTY720 negatively correlated with CXCR4 expression on MM cells. Enforced expression of CXCR4 reduced the sensitivity to FTY720, whereas silencing of endogenous CXCL12 increased the sensitivity of MM cells to FTY720-mediated cell death. These results suggested the CXCR4 axis to be directly regulated by S1P pathway. In support, we have found that FTY720 treatment significantly reduced CXCR4-dependent MM cell adhesion to fibronectin and abrogated MM migration toward CXCL12. Activation of signaling pathways, such as MAPK and Akt, in response to CXCL12 stimulation was also fully blocked by FTY720 pre-treatment. In addition to functional suppression, FTY720 directly and profoundly reduced CXCR4 cell-surface levels in a dose-dependent manner. Importantly, none of the suppressive effects of FTY720 (neither apoptosis, nor migration or adhesion inhibition) were dependent on protein phosphatase 2A (PP2A) activation, suggesting alternative mechanism of action. To further investigate down-stream molecular machinery involved in FTY720-mediated CXCR4 targeting in MM cells, the intra-cellular levels of different signaling mediators were evaluated. We identified the mTOR pathway to be regulated by CXCR4 and targeted by FTY720. FTY720 treatment suppressed mTOR signaling in MM cells, as demonstrated by de-phosphorylation of p70S6K and S6. Forced expression of CXCR4 and interaction with BM stromal cells antagonized with FTY720-mediated apoptosis and prevented FTY720-induced S6 de-phosphorylation. While, combination of FTY720 with mTOR inhibitor RAD001 resulted in significantly increased cell death, effectively abrogating CXCR4- and stroma-dependent resistance to FTY720 and suppressing mTOR signaling in MM cells. Finally, in a recently developed novel xenograft model of CXCR4-dependent systemic MM with BM involvement, in vivo FTY720 effectively reduced tumor burden in two third of the treated mice, decreasing both the levels of M protein in blood and the number of MM cells in BM. Conclusions: Taken together, our findings demonstrate cross talk between S1P and CXCR4/CXCL12 signaling pathways that may be of importance for MM cell survival and localization of the MM cells in CXCL12-expressing protective niches in the BM. Moreover, this is, to our knowledge, the first evidence that CXCR4 can be directly targeted with FTY720 modulator, thus restricting the tumor-promoting activities of S1P and CXCR4/CXCL12 axes. In addition, mTOR pathway was recognized as down-stream molecular partner being involved in FTY720-mediated anti-myeloma activities. Combining FTY720 with mTOR inhibitors may thus serve as promising novel therapeutic strategy in MM. Disclosures Peled: BioLineRx: Research Funding.

Background The chemokine CXCL12 and its receptor CXCR4 are key players in mediating the interactions between the bone marrow (BM) microenvironment and Acute Myeloid Leukemia (AML) cells. CXCL12, which is constitutively secreted from the BM stroma and AML cells, is critical for the survival and retention of AML cells within the BM. CXCR4 expression is associated with poor prognosis in AML patients with or without a mutated FLT3 gene. Antagonists to CXCR4 inhibit migration of AML cells, induce mobilization of AML cells into the circulation and enhance anti-leukemic effects of chemotherapy in mice models. The hypothesis that CXCL12/CXCR4 interactions contribute to the resistance of AML cells to signal transduction inhibitor, and chemotherapy-induced apoptosis, is currently being tested in a series of clinical trials in humans. Methods In the present study, the effect of the high affinity CXCR4 antagonist BL-8040 (BKT140) and AMD3100 (Mozobil) alone and in combination with ARA-C or the FLT-3 inhibitor AC220 on the survival and proliferation of AML cells in-vitro was examined. In the in-vitro study HL60 (FLT3-WT), MV4-11 (FLT3-ITD) cell lines and human primary AML cells from patients with FLT3-ITD mutations and FLT3-WT genewere used. Cells were incubated for 48 hrs in the presence of BL-8040 (8µM-20µM), AMD3100 (20µM), ARA-C (10-200 ng/ml) and AC220 (0.5-50nM). The level of viable cells, percentage of apoptosis and cell cycle were evaluated by FACS using propidium iodide and 7-AAD. In-vivo, we used NOD scid gamma (NSG) mice engrafted with human primary AML blasts and explored the effects of single injection of BL-8040 on the mobilization and survival of the blasts in the blood and the BM of the engrafted mice. Results In-vitro, treatment of MV4-11 cells (FLT3-ITD) with BL-8040, unlike treatment with AMD3100, directly inhibited cell growth by 35% and increased cell death by 39%. Furthermore, in-vitro, treatment of primary AML cells (FLT3-ITD) with BL-8040 directly inhibited cell growth by 28-47% and increased cell death by 75-100%. A combination of BL-8040 with AC220 or ARA-C further increased the apoptotic effect of these agents achieving a 96% reduction in cell viability and inducing cell death by 70- 90% of AML cells. When we studied the in-vitro effect of these agents on FLT3-WT cells (HL-60 cell line and primary AML cells), we found that BL-8040 inhibits cell growth by 16-50%. Unlike the FLT3-ITD cells, in the FLT3-WTcells we did not observe additive effects on cell growth for the combined treatments of BL-8040 with AC220. The combined treatment of BL-8040 with ARA-C was found to further increase the percentage of AML cell death. Moreover, BL-8040 decreases the percent of cycling cells by reducing the number of cells in G2/M+S phase while increasing the number of apoptotic cells in sub-G0 phase. It is interesting to mention that the migration of all tested AML cells toward CXCL12 was entirely inhibited by BL-8040. In-vivo, we found that a single injection of BL-8040 (100μg/mice) into NSG mice engrafted with human AML cells, induces rapid mobilization of AML cells to the periphery within 4 hrs after injection (an 8 fold increase from the control). When mice were administered with 5 consecutive injections of BL-8040 (400μg/mice), a reduction in the number of AML blasts in the blood was observed (40-60% reduction) with induction of AML cell apoptosis within the BM and in the blood by 10-20-fold, compared to the control. Conclusions The CXCR4 antagonist BL-8040 was found to rapidly and efficiently induces cell death of AML cells both in-vitro and in-vivo. These results suggest potential therapeutic advantages of BL-8040 in both FLT3-positive and negative AML patients by targeting not only AML anchorage in the BM but AML survival as well. Furthermore, it could provide a rational basis for BL-8040 therapy in combination with ARA-C and the FLT3 inhibitor AC220. Disclosures: Eizenberg: Biokine: Employment. Pereg:BioLineRx LTD: Employment. Klapper:BioLineRx LTD: Employment. Abraham:Biokine: Employment.

Abstract 3735 Background: Chronic myeloid leukemia (CML) is myeloproliferative disease of hematopoietic stem cells that is driven by the constitutively active oncogenic tyrosine kinase BCR-ABL. Inhibition of BCR-ABL activity by tyrosine kinase inhibitors (TKIs) such as imatinib mesylate, revolutionized the treatment of CML and remains a major therapeutic strategy for CML patients. However, existence of quiescent leukemic stem cells (LSCs) that are resistant to TKIs and may be responsible for CML resistance and recurrence continues to pose challenges for CML cure. Thus, novel therapies that eradicate LSCs are in need. The bone marrow (BM) microenvironment is believed to have a role in protecting LSCs and CML cells from TKIs-induced apoptosis. Chemokine receptor CXCR4 and its ligand CXCL12 have a key role in the trafficking and retention of normal and leukemic SCs in the BM niche. Although, CXCR4 signaling is impaired in CML cells, it can be elevated by the imatinib treatment and may induce stroma-promoted chemo-resistance. Therefore, CXCR4 inhibition may antagonize the survival and spread of CML and LSC cells and may restore their sensitivity to TKIs in the BM microenvironment context. Methods: In order to study the role of CXCR4 in CML progression, we over-expressed CXCR4 in K562 cells using a lentiviral vector and generated a cell line with high and stable CXCR4 expression. Results: CXCL12 stimulation did not significantly affect the migration of K562 cells in vitro. In contrast, elevated CXCR4 increased the in vitro survival and proliferation of K562 cells in response to CXCL12 treatment, suggesting an important role of CXCR4/CXCL12 axis in CML growth. In accordance, we found that in vitro treatment with CXCR4 antagonist BKT140 (8 μM) directly inhibited the cell growth by 40–60% and induced cell death of three CML cell lines tested (K562, KCL22 and LAMA84). Combination of BKT140 (8 μM) with IC50 concentrations of imatinib (0.2–0.3 μM) significantly increased the anti-CML apoptotic effect in achieving 90–95% reduction in cell viability in vitro (p Conclusions: Taken together, our data indicate the importance of CXCR4/CXCL12 axis in CML growth and CML-BM stroma interaction. CXCR4 inhibition with BKT140 antagonist efficiently synergized with imatinib both in vitro as well as in vivo, over-coming the protective effect of the BM stroma. These results provide the rational basis for CXCR4-targeted therapy in combination with TKI to override drug resistance and suppress residual disease. Disclosures: No relevant conflicts of interest to declare.

Abstract 952 Background: B-cell Non Hodgkin lymphoma (NHL) represents the most common malignant lymphoid neoplasm, with heterogeneous pathophysiological and clinical presentation. It may show systemic, nodal or extra nodal localization in various sites, including bone marrow (BM). Although anti-CD20 antibody rituximab significantly improved the outcome of NHL patients, the relapsed/refractory rates are still high. Chemokine receptor CXCR4 and its ligand CXCL12 are critically involved in the survival and trafficking of normal and malignant B lymphocytes. Taking together with the fact that interaction of malignant B cells with stromal cells via CXCR4/CXCL12 signaling may provide chemo-resistance, we hypothesized that blockade of CXCR4 may antagonize the survival and spreading of lymphoma cells and restore their chemo-sensitivity. Results: The effect of CXCR4 antagonist BKT140 on lymphoma cell growth and rituximab-induced cytotoxicity was tested in vitro and in vivo. Inhibition of CXCR4 with BKT140 in CD20-expressing lymphoma cell lines (BL-2, BJAB, Raji, Ramos, SUDHL4, OCI-Ly7) and primary lymphoma cells from patients with BM involvement resulted in significant inhibition of cell growth and in the induction of cell death, respectively. Combination of BKT140 with rituximab significantly enhanced the cytotoxic effect (apoptosis) against the lymphoma cells in a dose-dependent manner (p Conclusions: Our results demonstrate potent anti-lymphoma effect of CXCR4-specific antagonist BKT140 in vitro and in vivo. The BKT140-mediated anti-lymphoma effect synergizes with that of Rituximab. Moreover, BKT140 effectively targets lymphoma cells in the bone marrow microenvironment, overcoming the stroma-induced resistance to rituximab. These findings suggest the possible interaction between CD20 and CXCR4 pathways in NHL, and provide the scientific basis for the development of novel combined CXCR4-targeted therapies for refractory NHL. Disclosures: No relevant conflicts of interest to declare.

Abstract 2260 Background: BKT140 is a high affinity CXCR4 inhibitor with an extended K off-rate. Pre-clinical studies in animal models with BKT140 showed a robust mobilization of white blood cells (WBC) and hematopoietic stem cells (HSC). Furthermore, BKT140 also showed a direct anti-tumor effect against human-derived multiple myeloma (MM), lymphoma and primary leukemia cells and cell lines in vitro and in vivo, causing significant apoptosis. Aims: To assess BKT140 toxicity (primary endpoints), the mobilization capacity of CD34+ hematopoietic progenitors and CD138 MM cells, and pharmacokinetic (PK) and pharmacodynamic (PD) (secondary endpoints). Methods: 16 MM patients in first CR/PR were included in a phase I/IIa study, in which escalating doses of BKT140 (30, 100,300,900 μg/kg) were administered together with a high-dose cyclophosphamide (Cy) (2 g/m2) and G-CSF (5 μg/Kg) for stem cell mobilization. G-CSF was started on day 5 post Cy and BKT140 was injected subcutaneously (SC) once on day 10. Toxicity, PK, and mobilization capacity (assessed by serial measurements of number of WBC and CD34+ and CD138+ cells) were measured pre- and post BKT140 administration. Results: BKT140 was well tolerated at all doses and none of the patients developed grade II-IV toxicity. BKT140 was rapidly absorbed with no observed lag time, with peak plasma concentrations occurring 0.5h after administration. Clearance was rapid, with a median terminal half-life of 0.69h. BKT140 administration resulted in a significant dose-dependent increase in the number of peripheral blood neutrophils, monocytes, lymphocytes, and CD34+ cells compared to the G-CSF/Cy individual patient baseline. The maximum increase in the number of WBC from baseline was observed within 8h following BKT140 injection, 2.5-, and 3.0-, 4.1- and 4.8-fold, for the 4 BKT140 doses, respectively. Furthermore, BKT140 administration resulted in a significant increase in the mean absolute PB CD34+ cells mobilized (6.6, 7.5, 11.2 and 20.6 ×106/kg) for the 4 BKT140 administered doses, respectively. Moreover, the number of aphaeresis was reduced from 2.25 procedures at the first two BKT140 doses to 1.25 and 1 aphaeresis at the highest BKT140 doses, respectively. An increase in the number of CD138+ cells was observed in 6 out of 6 pts that had CD138+ cells in their blood and were treated with lower doses of BKT140 (30 and 100 μg/kg). Interestingly, in pts that were treated with the highest doses of BKT140 (300 and 900 μg/kg) a reduced number of CD138+ cells was observed in 3 out of 7 pts that had CD138+ cells in their blood, whereas in 4 pts, an increase in the number of CD138+ cells was shown. Three pts who did not have CD138+ cells in their blood were not affected by BKT140. The BKT140 mobilized grafts were used for AutoSCT following 200 mg/m2 melphalan conditioning. Pts received an average of 5.3×106 CD34+ cells/kg. All transplanted pts rapidly engrafted (n=15). The median day for neutrophil (>500/mm3) and platelet (>20,000/mm3, >50,000/mm3,) was on day 11 (range, 0–13), day 11 (range, 0–14), and day 14 (range, 0–23), respectively. Conclusions: The current data suggests that BKT140 can safely be added to G-CSF-based harvesting regimens, can increase CD34+ cell mobilization and reduce the number of collection days. Furthermore, due to its ability to release MM cells from the bone marrow and stimulate their cell death, additional studies are warranted to further evaluate the effect of BKT140 as an anti-MM agent. Disclosures: Nagler: Biokine Therapeutics Ltd: Consultancy. Abraham: Biokine Therapeutics Ltd: Employment, Equity Ownership, Patents & Royalties. Wald: Biokine Therapeutics Ltd: Employment. Shaw: Biokine Therapeutics Ltd: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Eizenberg: Biokine Therapeutics Ltd: Employment, Equity Ownership, Patents & Royalties. Peled: Biokine Therapeutics Ltd: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding.

Abstract 3857 Poster Board III-793 The chemokine receptor CXCR4 and its ligand CXCL12 are involved in the development and progression of a diverse number of hematological malignancies, including leukemia, lymphoma and multiple myeloma (MM). Binding CXCL12 to CXCR4 activates a variety of intracellular signal transduction pathways and effector molecules that regulate cell chemotaxis, adhesion, survival, apoptosis and proliferation. It was previously shown that CXCR4 signaling can directly induce caspase-independent cell apoptosis through the interaction with the HIV gp120 envelope protein. In the present study we investigated the effect of CXCR4 specific antagonists 4F-benzoyl-TN14003 (T140) and AMD3100 on the survival and proliferation of different human hematological cancer cells. Here, we demonstrate that T140, but not AMD3100, exhibits preferential cytotoxicity towards malignant cells of hematopoietic origin, as compared to primary normal cells or solid prostate and breast tumor cells. The in vitro treatment with T140, but not with AMD3100, significantly decreased the number of viable chronic myeloid leukemia K562 cells, acute T cell leukemia Jurkat cells, acute promyelocytic leukemia NB4 and HL60 cells, and four different MM cell lines (U266, NCI-H929, RPMI8226 and ARH77), demonstrating the highest sensitivity to T140 (p Disclosures: No relevant conflicts of interest to declare.