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Cancer-related anemia is present in more than 60% of newly diagnosed cancer patients and is associated with substantial morbidity and high medical costs. Drugs that enhance erythropoiesis are urgently required to decrease transfusion rates and improve quality of life. Clinical studies have observed an unexpected improvement in hemoglobin and RBC transfusion-independence in patients with acute myeloid leukemia (AML) treated with the isocitrate dehydrogenase 2 (IDH2) mutant-specific inhibitor enasidenib, leading to improved quality of life without a reduction in AML disease burden. Here, we demonstrate that enasidenib enhanced human erythroid differentiation of hematopoietic progenitors. The phenomenon was not observed with other IDH1/2 inhibitors and occurred in IDH2-deficient CRISPR-engineered progenitors independently of D-2-hydroxyglutarate. The effect of enasidenib on hematopoietic progenitors was mediated by protoporphyrin accumulation, driving heme production and erythroid differentiation in committed CD71(+) progenitors rather than hematopoietic stem cells. Our results position enasidenib as a promising therapeutic agent for improvement of anemia and provide the basis for a clinical trial using enasidenib to decrease transfusion dependence in a wide array of clinical contexts.

Treatment with hypomethylating agents (HMAs) azacitidine or decitabine is the current standard of care for high risk myelodysplastic syndromes (MDSs) but is associated with low rates of response. The limited number of treatment options for patients with high risk MDS highlights a need for new therapeutic options. Venetoclax is an inhibitor of the BCL-2 protein which, when combined with an HMA, has shown high response rates in unfit and previously untreated acute myeloid leukemia. We performed a retrospective study of high risk MDS patients receiving combination HMA plus venetoclax in order to determine their effectiveness in this context. We show that in our cohort, the combination results in high response rates but is associated with a high frequency of myelosuppression. These data highlight the efficacy of combination HMA plus venetoclax in high risk MDS, warranting further prospective evaluation in clinical trials.

The 90 kDa Ribosomal S6 Kinase (RSK) drives cell proliferation and survival in cancers, although its oncogenic mechanism has not been well characterized. Phosphorylated level of RSK (T573) was increased in acute myeloid leukemia (AML) patients and associated with poor survival. To examine the role of RSK in AML, we analyzed apoptosis and the cell cycle profile following treatment with BI-D1870, a potent inhibitor of RSK. BI-D1870 treatment increased the G2/M population and induced apoptosis in AML cell lines and patient AML cells. Characterization of mitotic phases showed that the metaphase/anaphase transition was significantly inhibited by BI-D1870. BI-D1870 treatment impeded the association of activator CDC20 with APC/C, but increased binding of inhibitor MAD2 to CDC20, preventing mitotic exit. Moreover, the inactivation of spindle assembly checkpoint or MAD2 knockdown released cells from BI-D1870-induced metaphase arrest. Therefore, we investigated whether BI-D1870 potentiates the anti-leukemic activity of vincristine by targeting mitotic exit. Combination treatment of BI-D1870 and vincristine synergistically increased mitotic arrest and apoptosis in acute leukemia cells. These data show that BI-D1870 induces apoptosis of AML cells alone and in combination with vincristine through blocking mitotic exit, providing a novel approach to overcoming vincristine resistance in AML cells.

Most patients with acute myeloid leukemia (AML) die from complications arising from cytopenias resulting from bone marrow (BM) failure. The common presumption among physicians is that AML-induced BM failure is primarily due to overcrowding, yet BM failure is observed even with low burden of disease. Here, we use large clinical data sets to show the lack of correlation between BM blast burden and degree of cytopenias at the time of diagnosis. We develop a splenectomized xenograft model to demonstrate that transplantation of human primary AML into immunocompromised mice recapitulates the human disease course by induction of BM failure via depletion of mouse hematopoietic stem and progenitor populations. Using unbiased approaches, we show that AML-elaborated IL-6 acts to block erythroid differentiation at the proerythroblast stage and that blocking antibodies against human IL-6 can improve AML-induced anemia and prolong overall survival, suggesting a potential therapeutic approach.

CREB binding protein (CBP) and p300 are critical regulators of hematopoiesis through both their transcriptional coactivator and acetyltransferase activities. Loss or mutation of CBP/p300 results in hematologic deficiencies in proliferation and differentiation as well as disruption of hematopoietic stem cell renewal and the microenvironment. Aberrant lysine acetylation mediated by CBP/p300 has recently been implicated in the genesis of multiple hematologic cancers. Understanding the effects of disrupting the acetyltransferase activity of CBP/p300 could pave the way for new therapeutic approaches to treat patients with these diseases.

The transcription factor CREB (cAMP Response Element Binding Protein) is overexpressed in the majority of acute myeloid leukemia (AML) patients, and this is associated with a worse prognosis. Previous work revealed that CREB overexpression augmented AML cell growth, while CREB knockdown disrupted key AML cell functions in vitro. In contrast, CREB knockdown had no effect on long-term hematopoietic stem cell activity in mouse transduction/transplantation assays. Together, these studies position CREB as a promising drug target for AML. To test this concept, a small molecule inhibitor of CREB, XX-650-23, was developed. This molecule blocks a critical interaction between CREB and its required co-activator CBP (CREB Binding Protein), leading to disruption of CREB-driven gene expression. Inhibition of CBP-CREB interaction induced apoptosis and cell cycle arrest in AML cells, and prolonged survival in vivo in mice injected with human AML cells. XX-650-23 had little toxicity on normal human hematopoietic cells and tissues in mice. To understand the mechanism of XX-650-23, we performed RNA-seq, ChIP-seq and Cytometry Time of Flight with human AML cells. Our results demonstrate that small molecule inhibition of CBP-CREB interaction mostly affects apoptotic, cell cycle, and survival pathways, which may represent a novel approach for AML therapy.

Although loss-of-function RUNX1 mutations are commonly found in hematopoietic malignancies, how RUNX1 functions during hematopoietic and leukemic development is unclear. Using a CRISPR/AAV6 system to target the RUNX1 locus in human CD34+ hematopoietic stem and progenitor cells (HSPCs), we show that RUNX1 deficiency causes monocytic skew at the expense of erythro-megakaryocytic potential and stem cell activity, including a severe in vivo stem cell competitive defect. RNA-seq and ATAC-seq review that these effects are mediated by broad upregulation of PU.1 and NFKB transcriptional programs; downregulation of GATA1- and TAL1-dependent erythro-megakaryocytic differentiation; and downregulation of cell cycle programs mediated by MYC and E2F. Treatment with IL-3 rescues RUNX1-deficient cell proliferative and stem cell defects. Together, these results show that RUNX1 controls transcription factor activity and cytokine signaling, and loss of RUNX1 causes monocytic skewing and hypersensitivity to IL-3-dependent expansion. We are currently studying how the IL-3 selects for inflammation-sensitive RUNX1 deficient cells and whether targeting IL-3 signaling may be a viable therapeutic in the prevention or treatment of RUNX1 mutant malignancies.

// Bryan Mitton 1 , Katie Hsu 1 , Ritika Dutta 1 , Bruce C. Tiu 1 , Nick Cox 2 , Kevin G. McLure 1 , Hee-Don Chae 1 , Mark Smith 2 , Elizabeth A. Eklund 3 , David E. Solow-Cordero 4, * , Kathleen M. Sakamoto 1, * 1 Department of Pediatrics, Stanford University, Stanford, CA, USA 2 Medicinal Chemistry Knowledge Center, Stanford ChEM-H, Stanford, CA, USA 3 Division of Hematology/Oncology, Department of Medicine, Northwestern University Feinberg School of Medicine, Chicago, IL, USA 4 High-Throughput Bioscience Center, Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, CA, USA * These authors have contributed equally and share senior authorship Correspondence to: Kathleen M. Sakamoto, e-mail: kmsakamo@stanford.edu Keywords: small molecule screen, novel therapeutics, CREB Received: September 25, 2015 Accepted: January 13, 2016 Published: January 30, 2016 ABSTRACT The transcription factor CREB (cAMP Response Element Binding Protein) is an important determinant in the growth of Acute Myeloid Leukemia (AML) cells. CREB overexpression increases AML cell growth by driving the expression of key regulators of apoptosis and the cell cycle. Conversely, CREB knockdown inhibits proliferation and survival of AML cells but not normal hematopoietic cells. Thus, CREB represents a promising drug target for the treatment of AML, which carries a poor prognosis. In this study, we performed a high-throughput small molecule screen to identify compounds that disrupt CREB function in AML cells. We screened ~114,000 candidate compounds from Stanford University’s small molecule library, and identified 5 molecules that inhibit CREB function at micromolar concentrations, but are non-toxic to normal hematopoietic cells. This study suggests that targeting CREB function using small molecules could provide alternative approaches to treat AML.

e18362 Background: Financial toxicity (FT) is a major cause of distress for cancer patients. We have previously reported that FT was high among patients at a county hospital and a tertiary academic center. In this study, we sought to compare factors associated with FT at these institutions. Methods: Patients with clinic visits were asked to fill out an 11-item validated FT survey, the COST tool, and demographic questions from 10/2017 - 2/2018 at the county hospital and 7/2015 - 10/2018 at the academic center. We used a logistic regression model to analyze demographic factors associated with FT. Results: Mean COST score was significantly higher at the county hospital (28.7) compared to the academic center (20.4) (p < 0.001). Distribution of cancer stage was similar between the institutions. Patients at the county hospital were more likely to have an annual household income (AHI) < $25K (p < 0.001), and be insured by Medi-cal (p < 0.001). Patients at the academic center were more likely to be White (p < 0.001). Monthly OOP spending was more likely to be > $500 at the academic center (p < 0.001). Conclusions: Overall, FT was higher at the county hospital as compared to the academic center. FT at both institutions was associated with higher OOP spending and lower AHI. However, variations in FT among other demographic factors may reflect differences in care delivery systems and patient populations. Our study demonstrates that demographic groups at risk for FT differs between institutions suggesting the factors that contribute financial toxicity are complex and variable. Thus local assessments of FT may be necessary for individual health systems to develop targeted interventions to improve FT among their patients.[Table: see text]

Acute Myeloid Leukemia (AML) is an aggressive cancer resulting in severe cytopenias related to bone marrow (BM) failure. The common assumption for AML-induced BM failure is overcrowding due to clonal expansion of immature myeloid blasts, leading to failure of normal hematopoiesis. However, in a cohort of 293 AML patients, we found that disease burden (% of blasts determined on diagnostic BM aspirate) did not predict severity of cytopenia (hemoglobin rs=-0.053; p=0.49; WBC rs=-0.030, p=0.70; platelet rs=0.091, p=0.026), strongly arguing against simple crowding as the main mechanism underlying AML-induced BM failure. Thus, the goal of our study is to identify novel mechanism(s) associated with AML-induced BM failure, potentially enabling development of new therapies to improve AML management and reverse morbidity. Conventional xenograft models of human AML do not typically exhibit cytopenias, making them unsuitable to study AML-induced BM failure. We speculated that in the mouse, increased splenic extramedullary hematopoiesis compensated for failed intramedullary hematopoiesis due to AML. To test this hypothesis, we performed surgical splenectomy on NSG mice prior to their transplantation with human AML. Strikingly, splenectomized NSG mice engrafted with primary human AML at a 30-70% disease burden (n=8 for primary AML samples, n=5-10 for each group of splenectomized NSG mice) developed leukopenia and severe anemia compared to sham-operated AML-engrafted controls. AML-engrafted splenectomized NSG mice showed early mortality compared to AML-engrafted NSG mice with intact spleens (10.1 weeks vs. 34.2 weeks p Utilizing our model, splenectomized NSG mice engrafted with human AML demonstrated depletion of normal hematopoietic progenitors (HSPCs) including HSCs (11.1 fold p To explore mechanisms by which AML blasts inhibit normal HSPCs, we generated conditioned media (CM) from patient-derived AML blasts, and found that AML-CM suppressed BFU-E colony formation from normal HSPCs (3.1-5.1 fold). Furthermore, in a direct co-culture system with AML blasts and CD34+ HSPCs, AML blasts inhibited erythroid differentiation from the CFU-E to normoblast stage by 81%. Removing CD34+ cells from the AML co-culture allowed cells to resume differentiation to the proerythroblast stage. These experiments demonstrate that AML imparts a differentiation blockade along the MEP-proerythroblast axis in a cell non-autonomous, reversible fashion. Using cytokine array analysis, we identified elevated IL-6 levels in AML sample-derived CM (n=10, 2841±766.4 pg/ml, 7.80 fold increase, p=0.03) compared to CD34+-derived CM (n=5, 364±36.0 pg/ml). Increased IL-6 was also found in BM aspirates from human AML engrafted splenectomized NSG mice (715±125pg/ml, p=0.001) compared to mice engrafted with normal CD34+ HSPCs (undetectable). Thus, we hypothesized that IL-6 produced by AML blasts acts as a paracrine factor to suppress erythropoiesis. Consistent with this hypothesis, an IL-6 neutralizing antibody reversed the inhibition of BFU-E formation imparted by AML-CM. Furthermore, the addition of recombinant IL-6 to liquid cultures of erythroid differentiation resulted in a 23% reduction in proerythroblasts. Together, our data suggests that (1) overcrowding is not the primary mechanism resulting in BM failure in AML; (2) AML blasts play a previously unrecognized role in imparting a differentiation blockade along the MEP-proerythroblast axis, resulting in progressive anemia; and (3) this differentiation blockade is at least partially attributable to IL-6 secreted from AML blasts as a paracrine factor. Disclosures No relevant conflicts of interest to declare.

The 90 kDa Ribosomal S6 Kinase (RSK), downstream of the ERK signaling pathway, has recently been implicated in a wide variety of cancers, ranging from lung cancer to medulloblastoma, as a driver of cancer cell proliferation and survival. However, its role in Acute Myeloid Leukemia (AML) remains unknown. Thus, the goal of this study was to characterize RSK-dependent signaling pathways in AML, with the overall hypothesis that disruption of this pathway represents a potential strategy for the treatment of AML. The RSK family consists of four gene isoforms, RSK1-4 (RPS6KA1 (RSK1), RPS6KA2 (RSK3), RPS6KA3 (RSK2), RPS6KA4 (RSK4). Knockdown (KD) of RSK1 by shRNA in HL-60 and KG-1 cell lines resulted in reduced AML cell growth in vitro. NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice were injected with 2x106 HL-60 or KG-1 RSK1KD cells and vector control transduced cells in order to investigate the effects of RSK1 KD on AML cell growth and survival in vivo. Mice injected with RSK1 KD cells exhibited prolonged survival by 17 and 21 days respectively for HL-60 and KG-1 cell induced disease (p=0.0023 and 0.0018 respectively). These data indicate that RSK1 knockdown inhibits leukemia progression, and RSK1 is required for maximal proliferation of AML cells in vivo. Pharmacological inhibition of total RSK (RSK1-4) by the small molecule inhibitor BI-D1870 reduced AML cell growth and induced cell death in both AML cell lines and patient samples after treatment for 48 hours. The IC50 for growth inhibition was 1.8 uM for MOLM-13, 1.6 uM for MV-4-11, and 1.9 uM for HL-60 cells. In methylcellulose colony assays, normal hematopoietic stem and progenitor cell proliferation was not affected by RSK inhibition up to a concentration of 15 uM, establishing an approximately 10-fold therapeutic index. To elucidate the mechanism by which RSK inhibition suppresses AML proliferation, we performed cell cycle analysis with HL-60 cells. RSK inhibition by BI-D1870 resulted in delayed S-phase progression and accumulation of cells in late S-phase with increased pH2AX, cPARP, and CDK2/Cyclin A expression, as measured by flow cytometry. These data indicate that inhibition of RSK leads to activation of DNA damage pathways and arrest in S-phase, resulting in apoptosis. Inhibition of CDK activity rescued S-phase arrest, demonstrating that activation and dysregulation of CDK are crucial mediators of RSK inhibitor-induced S-phase arrest. In summary, this is the first study to demonstrate that RSK plays an important role in maintaining AML cell survival and proliferation and to position RSK as a promising target for treatment of AML. Disclosures No relevant conflicts of interest to declare.

CREB (cAMP Response Element Binding Protein) is a nuclear transcription factor that plays a critical role in regulating myeloid cell proliferation and differentiation. CREB is overexpressed in Acute Myeloid Leukemia (AML) cells from the majority of AML patients at diagnosis, and CREB overexpression is associated with a poor prognosis.Transgenic mice overexpressing CREB in myeloid cells develop myelodysplasia/myeloproliferative neoplasms. CREB also cooperates with other oncogenes, such as Sox4, to induce transformation to AML. Knockdown of CREB inhibits AML proliferation but does not affect normal hematopoietic stem cell activity, establishing the crucial role of CREB in AML cell growth and survival. In vitro, CREB overexpression leads to increased resistance to apoptosis in AML cells. Thus, we hypothesized that increased CREB expression confers chemoresistance, as this may represent one reason that patients with high CREB levels have worse prognoses and relapse following therapy. Previous studies have demonstrated that chemotherapy resistance can result from increased DNA damage repair activity, but CREB has never been implicated in these DNA damage repair processes, nor has CREB even been described as an important transcriptional regulator of DNA damage repair genes. The goal of this study was to characterize whether CREB expression confers chemoresistance through regulation of DNA repair genes in AML cells. Firstly, we established that CREB expression levels correlate with chemoresistance by treating KG-1 cells engineered to express lower and higher levels of CREB with etoposide and doxorubicin, both chemotherapy drugs used to treat AML. Cells with CREB overexpression had increased viability compared to CREB knockdown cells after treatment with both chemotherapies at a range of concentrations. To investigate the underlying mechanism, we performed CREB chromatin immunoprecipitation and RNA-seq following small molecule CREB inhibition to identify the sets of genes that are regulated by CREB in AML cells and whose expression levels are sensitive to CREB inhibition. Out of 88 DNA damage repair genes found to be CREB-bound, 41 exhibited at least a 2-fold change in expression after CREB inhibition. qPCR was performed to determine whether the expression of DNA damage repair genes were proportional to CREB levels. Transcription of ATM, ATR, RAD54L, and RAD51, genes important in sensing and repairing DNA damage, were coordinately regulated with CREB expression. ATM, ATR, RAD54L, and RAD51 were reduced by approximately 42.0%±0.1%, 44.8%±0.1%, 40.2%±0.1%, and 27.9%±0.1% respectively in CREB knockdown cells (p≤0.05). Reduced expression of these genes also had a functional consequence. CREB knockdown cells initiated a lesser DNA damage repair response in response to etoposide treatment, as determined by measured phospho-H2AX levels, compared to wild-type CREB-expressing cells. Conversely, cells with CREB overexpression exhibited the strongest DNA damage repair response following etoposide treatment. Taken together, these data demonstrate that CREB overexpression has a protective effect against DNA damage and confers chemoresistance, likely through upregulation of DNA damage repair genes. Future studies will seek to determine if small molecule inhibition of CREB can reduce the transcription of DNA damage repair genes and thus sensitize AML cells to chemotherapeutic agents. Disclosures No relevant conflicts of interest to declare.

CREB (cAMP Response Element Binding Protein) is a nuclear transcription factor that plays a critical role in the pathogenesis of Acute Myeloid Leukemia (AML). CREB is overexpressed in the majority of AML patients, and this is associated with a poor prognosis. CREB overexpression leads to increased AML cell proliferation and resistance to apoptosis in vitro. For CREB to be transcriptionally active, however, it must first be phosphorylated at Serine 133. Previous work has suggested that Ribosomal S6 Kinase (pp90rsk or RSK) is the primary kinase responsible for growth factor-induced phosphorylation of CREB, and that RSK is activated downstream of the Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) and Granulocyte-Colony Stimulating Factor (G-CSF) in AML cells. The overall role and regulation of RSK in AML cells, however, remains unknown. Thus, the goal of this study was to characterize the RSK-CREB signaling pathway in AML, with the overall hypothesis that disruption of this pathway represents a potential therapeutic strategy for the treatment of AML. We report that of the four known isoforms of RSK, RSK1 and RSK2 appear to be the predominant subtypes expressed in AML cells. To identify additional upstream pathways responsible for activation of these isoforms in AML cells, we performed cytokine stimulation experiments. Granulocyte-Colony Stimulating Factor (G-CSF), Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF), Thrombopoietin (TPO), and Interleukin 3 (IL-3) were all capable of stimulating phosphorylation and activation of RSK in KG-1 and HL-60 cells, implicating that multiple signaling pathways converge on RSK. G-CSF and GM-CSF had the greatest effects, stimulating increased phosphorylation and activation of RSK by 2.8 and 2.6 fold, respectively. Using shRNA technology, we then generated AML cell lines (HL-60 and KG-1) in which the expression of each isoform was ‘knocked-down’ to examine whether these two isoforms play unique roles in AML cells. Interestingly, RSK1 appears to be the isoform primarily responsible for phosphorylating CREB downstream of the G-CSF receptor. We demonstrate that G-CSF treatment of RSK1 knockdown cells did not induce an increase in CREB phosphorylation, and baseline CREB phosphorylation was also significantly decreased in these cells. Previous data had shown that blockade of total RSK activity using the non-selective RSK inhibitor BI-D1870 induced cell death in both AML cell lines and primary AML patient samples. RSK1 knockdown in HL-60 cells sensitized them to this agent (IC50 1.2 microM vs 3.3 microM), while the sensitivity of RSK2 knockdown cells was unchanged. Finally, since targets of RSK also include regulators of apoptosis (BAD) and cellular stress signaling pathways (IkB), we examined the effects of inhibiting RSK on the phosphorylation of these proteins. Levels of phosphorylated CREB and BAD decreased by 50% in HL-60 cells after 2 hours of treatment with the RSK inhibitor, suggesting that this treatment induces apoptosis. In summary, targeting the RSK-CREB signaling axis may represent a novel therapeutic strategy for AML. Future experiments will further define the role of RSK in proliferation and survival of AML cells and normal hematopoietic cells. Disclosures No relevant conflicts of interest to declare.

CREB (cAMP Response-Element Binding Protein) is a nuclear transcription factor critical for hematopoietic cell proliferation, differentiation, and survival. We previously demonstrated that 60% of patients with Acute Myelogenous Leukemia (AML) overexpress CREB in leukemic blasts, and CREB overexpression in these patients was associated with an increased risk of relapse and decreased event-free survival. Previous studies have suggested that CREB may play an important role in the regulation of apoptosis in a wide variety of cancers. Specifically, CREB has been shown to up-regulate members the anti-apoptotic protein family such as Bcl-2, Bcl-XL and Mcl-1, leading to chemotherapy resistance in vitro. CREB-mediated resistance to apoptosis may underlie the increased rate of relapse and poor survival of AML patients with CREB overexpression. Thus, we hypothesized that targeted inhibition of CREB in AML cells would promote AML cell apoptosis. To test this hypothesis, we developed a small-molecule inhibitor of CREB function, XX-650-23. This molecule disrupts the interaction between CREB and its binding partner CBP (CREB-Binding Protein), which is required for full activation of CREB-mediated gene transcription. Treatment of primary AML patient bone marrow samples with XX-650-23 induced apoptosis and cell death at a dose of 2 uM. The degree of apoptosis varied with the expression level of CREB in primary AML cells tested. Higher CREB levels correlated with higher sensitivity to XX-650-23. In non-leukemic primary patient bone marrow samples, CREB levels were very low, and XX-650-23 did not induce apoptosis in these cells. AML cell lines (KG-1 and HL-60) also underwent apoptosis following CREB inhibition, in proportion to CREB expression level. CREB knockdown or overexpression in KG-1 cells decreased and increased susceptibility to apoptosis, respectively. Mechanistically, the onset of apoptosis in AML cells occurred simultaneously with down-regulation of Bcl-2, a validated CREB-regulated gene. Inhibition of Bcl-2 function using the specific Bcl-2 inhibitor ABT-737 (100 nM) induced apoptosis similar to XX-650-23, indicating that Bcl-2 inhibition alone is sufficient to cause apoptosis. Thus, targeted inhibition of CREB results in Bcl-2 downregulation and is sufficient to induce apoptosis in AML cells. Proteomic analysis using Mass Cytometry-Time of Flight (CyTOF) revealed that one compensatory cellular response to CREB inhibition is increased phosphorylation of CREB. This phosphorylation decreased in the presence of BI-D1870, a specific inhibitor of the pp90RSK kinase (RSK), but not by pharmacologic inhibition of the p38 or ERK kinases, using SB202190 or U0126, respectively. We therefore examined the role of pp90RSK in the regulation of apoptosis in AML cells. Pharmacologic inhibition of RSK independently lead to AML cell apoptosis (BI-D1870, IC50=3.3 uM), in part due to blockade of CREB phosphorylation. In summary, our data provide the first evidence that inhibition of CREB, or its chief activator RSK, is sufficient to induce apoptosis in AML cells. Current work focuses on defining CREB target genes mediating XX-650-23 response using chromatin-immunoprecipitation with massively parallel DNA sequencing (ChIP-Seq), and defining the RSK kinome in AML cells using 2-dimensional gel phosphoprotein profiling. These studies will more fully define the role of the RSK-CREB signaling axis in AML proliferation, survival, and apoptosis. Disclosures: No relevant conflicts of interest to declare.