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Therapy-related acute myeloid leukemia and myelodysplastic syndromes (t-AML/t-MDS) are secondary hematologic malignancies associated with poor prognosis, warranting insights into their predisposing conditions and cells of origin. We identified patients with myeloma who developed t-AML/t-MDS and analyzed their stem and progenitor cells collected years before the onset of secondary disease. We demonstrate that aberrant stem cells with high CD123 expression can be detected long before the onset of overt leukemia. Rigorous sorting, followed by targeted sequencing, resulted in ultradeep functional depth of sequencing and revealed preexisting mutant hematopoietic stem cell (HSC) clones, mainly harboring TP53 mutations, that became the dominant population at the time of leukemic presentation. Taken together, these data show that HSCs can act as reservoirs for leukemia-initiating cells many years before the onset of myeloid leukemia.

The second-generation thrombopoietin (TPO) receptor agonists eltrombopag and romiplostim are potent activators of megakaryopoiesis and represent a growing treatment option for patients with thrombocytopenic hematological disorders. Both TPO receptor agonists have been approved worldwide for the treatment of children and adults with chronic immune thrombocytopenia. In the EU and USA, eltrombopag is approved for the treatment of patients with severe aplastic anemia who have had an insufficient response to immunosuppressive therapy and in the USA for the first-line treatment of severe aplastic anemia in combination with immunosuppressive therapy. Eltrombopag has also shown efficacy in several other disease settings, for example, chemotherapy-induced thrombocytopenia, selected inherited thrombocytopenias, and myelodysplastic syndromes. While both TPO receptor agonists stimulate TPO receptor signaling and enhance megakaryopoiesis, their vastly different biochemical structures bestow upon them markedly different molecular and functional properties. Here, we review and discuss results from preclinical and clinical studies on the functional and molecular mechanisms of action of this new class of drug.

Even though HIV is associated with worse prognosis in many malignancies, the clinical course of myelodysplastic syndrome (MDS) in HIV + patients has not been well studied. Determining the clinical presentation and outcomes of MDS in these patients would be important for future diagnostic strategies, as anemia and other cytopenias are commonly seen in HIV + patients. Unique data mining software was used to identify cases of MDS or AML in adult patients who were also HIV + at Albert Einstein/Montefiore Medical Center between 1 January 2003 and 1 January 2017. Using Chi-Square and Fisher's exact test, characteristics of the HIV + MDS patients were compared to 135 HIV - MDS patients from the same institution diagnosed between 1997 and 2011. Fourteen biopsy proven MDS patients were identified with concomitant HIV. HIV + MDS patients presented at a younger age (59 vs. 71 yrs

Thrombocytopenia remains a challenge in myeloid malignancies, needing safer and more effective therapies. JNJ-26366821, a pegylated synthetic peptide thrombopoietin (TPO) mimetic not homologous to endogenous TPO, has an in-vitro EC50 of 0.2 ng/mL for the TPO receptor and dose dependently elevates platelets in volunteers. We demonstrate that JNJ-26366821 increases megakaryocytic differentiation and megakaryocytic colony formation in healthy controls and samples from myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). JNJ-26366821 had no effect on proliferation of malignant myeloid cell lines at doses up to 1000 ng/mL and malignant patient-derived mononuclear cells showed no increased cell growth or leukemic colony formation capacity at concentrations between 0.2 ng/mL and 10 ng/mL. Furthermore, JNJ-26366821 did not enhance in-vivo engraftment of leukemic cells in an AML xenotransplantation murine model. Our results show that JNJ-26366821 stimulates megakaryopoiesis without causing proliferation of the malignant myeloid clones in MDS/AML and provides the rationale for clinical testing of JNJ-26366821 in myeloid malignancies.

Introduction: Lenalidomide (LEN) is approved for treatment of Myelodysplastic Syndrome (MDS) with chromosome 5q deletion (del 5q) as it has shown to induce disease remission and transfusion independence in close to 65% of this population. For patients (pts) without del 5q, LEN has shown to reduce transfusion needs in one-quarter of pts, but its use is limited by significant thrombocytopenia, and pts with platelet counts (PLTs) Methods: Pts aged ≥18 years with low/int risk MDS per IPSS, or non-proliferative chronic myelomonocytic leukemia (CMML), with bone marrow blasts Results: 51 pts were accrued, with 44 evaluable pts. Of evaluable pts, most were male (70%), median age was 71 years (range 34-93). 42 (95%) pts had MDS and 2 (5%) CMML. 24 (55%) pts were low-risk per IPSS-R score. 28 (64%) pts were treatment-naïve and 16 (36%) had received ≥1lines of treatment, including erythropoiesis stimulating agents. 24 (55%) pts were assigned to arm A and 20 (45%) pts to arm B. ELT/LEN was well tolerated. Non-hematological grade 3-4 treatment-related adverse events were few, including G3 hyperbilirubinemia (7%), G3 transaminitis (2%), G3 diarrhea (2%), G3 arthralgias (2%). 2 pts had major bleeding events; there were 2 deaths, one attributable to pneumonia and one to gallbladder cancer. 1 of 31 pts who received ELT had a reversible increase in peripheral blasts while on treatment, and 1 pt had development of marrow fibrosis after 6 years of ELT. ITT analysis of total population (51) revealed an objective response (ORR) of 35%; ORR of 32% (9/28) in arm A (PLTs ≥50,000),and 39% (9/23) in arm B (PLTs 17 pts received ELT alone, among these ORR was 41% (7/17), with 29% achieving bilineage responses, 1 CR. 13 pts received LEN alone, ORR was 46% (6/13), of these 100% achieved RBC-TI. 14 pts received ELT/LEN combination, ORR was 36%, 2 (14%) achieved bilineage responses and 2 (14%) CR. There were 2 HIV-positive MDS pts, both achieved sustained CRs, one with ELT alone and one with ELT/LEN combination. Mean time to response was 9.4 wks (range 6-12.4) for ELT alone, 10.9 wks (range 2.4-16) for LEN alone, and 9.9 wks (range 2-20) for ELT/LEN combination. Mean duration of response was 102 wks for ELT (range 8-ongoing), 63 wks (range 25-ongoing) for LEN, and 66 wks (range 8.3-ongoing) for ELT/LEN as of last follow-up. Conclusions: Treatment with ELT/LEN demonstrates good efficacy with ORR of 40.9%, response durability and an acceptable safety profile for evaluable pts with low/int risk MDS. ELT can lead to single agent responses with an acceptable safety profile and sizable proportion of multilineage responses. One patient developed bone marrow fibrosis and one patient had a transient increase in blasts, hence, undermining these pre-existing safety concerns. Lastly, using our study schema 32% of pts were able to initiate or continue LEN achieving a 36% ORR whereas in prior studies these pts would have been excluded. Figure 1 Figure 1. Disclosures Yacoub: Agios: Membership on an entity's Board of Directors or advisory committees; Acceleron Pharma: Membership on an entity's Board of Directors or advisory committees; CTI Biopharma: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Incyte: Speakers Bureau. Gritsman: iOnctura: Research Funding. Shastri: Kymera Therapeutics: Research Funding; Onclive: Honoraria; GLC: Consultancy; Guidepoint: Consultancy. Verma: Celgene: Consultancy; Acceleron: Consultancy; Novartis: Consultancy; Stelexis: Consultancy, Current equity holder in publicly-traded company; Eli Lilly: Research Funding; Curis: Research Funding; Medpacto: Research Funding; Incyte: Research Funding; Stelexis: Current equity holder in publicly-traded company; Throws Exception: Current equity holder in publicly-traded company; GSK: Research Funding; BMS: Research Funding. OffLabel Disclosure: Lenalidomide has shown to be effective achieving RBC Transfusion Independence in Myelodysplastic Syndromes without deletion 5q Eltrombopag is a TPO-R agonist, its purpose in this trial is to evaluate if it would increase platelet counts in patients with Myelodisplastic Syndrome.

Thrombocytopenia defined as platelet counts of Several studies evaluating the use of thrombopoietin receptor agonists (TPO-RA) for the clinical management of thrombocytopenia have shown promising clinical results. TPO-RA eltrombopag (EP) which has been effective as a single agent to raise platelet counts in MDS, but failed to stimulate platelet production in a phase III placebo-controlled clinical study when used in combination with AZA (NCT02158936). Here, we assessed the molecular and cellular mechanisms of AZA contributing to thrombocytopenia and interfering with TPO-RA mediated rescue. Our results demonstrate that AZA mediates the rapid induction of dsRNAs and activation of interferon type I (IFN-I) signaling in various hematopoietic cells, including stem and progenitor cells of healthy donors and MDS/AML. This engagement of IFN-I/STAT1/SOCS1 signaling resulted in significant inhibition of megakaryocytic progenitor growth and differentiation, independently of DNA hypomethylation and endogenous retroviruses. We show that the inhibitory effects of AZA on megakaryopoeisis can be counteracted through inhibition of IFN-I signaling, SOCS1 activation or P38 MAPK activity. Our findings provide evidence intercepting inhibition of TPO-R signaling by AZA-induced innate immune IFN-I pathway and thrombocytopenia.

Purpose: Overactivation of TGF-β signaling is observed in myelodysplastic syndromes (MDS) and is associated with dysplastic hematopoietic differentiation. Galunisertib, a first-in-class oral inhibitor of the TGF-β receptor type 1 kinase (ALK5) has shown effectiveness in preclinical models of MDS and acceptable toxicity in phase I studies of solid malignancies. Patients and Methods: A phase II multicenter study of galunisertib was conducted in patients with very low-, low-, or intermediate-risk MDS by the Revised International Prognostic Scoring System criteria with hemoglobin ≤ 10.0 g/dL. Patients received oral galunisertib 150 mg twice daily for 14 days on/14 days off. Results: Ten of 41 evaluable patients (24.4%; 95% confidence interval, 12.4–40.3) achieved hematologic improvement erythroid response by International Working Group (IWG) 2006 criteria. A total of 18 of 41 patients (43.9%) achieved erythroid response as per IWG 2000 criteria. Nine of 28 (32.1%) of transfusion-dependent patients had hematologic improvement. A total of 18 of 41 (44%) patients had a significant reduction in fatigue. Overall median duration of response was 90 days in all patients. Rigorous stem and progenitor flow cytometry showed that patients with an early stem cell differentiation block were more likely to respond to galunisertib. The most common treatment-emergent adverse events were grade 1 or 2 in 20 (49%) of 41 patients, including any-grade fatigue (8/41, 20%), diarrhea (7/41, 17%), pyrexia (5/41, 12%), and vomiting (5/41, 12%). Conclusions: In summary, galunisertib treatment has an acceptable safety profile and was associated with hematologic improvements in lower- and intermediate-risk MDS, with responses in heavily transfusion-dependent patients and in those with signs of an early stem cell differentiation block.

Thrombocytopenia is commonly seen in myelodysplastic syndrome (MDS) patients, and bleeding complications are a major cause of morbidity and mortality. Thrombocytopenia is an independent factor for decreased survival and has been incorporated in newer prognostic scoring systems. The mechanisms of thrombocytopenia are multifactorial and involve a differentiation block of megakaryocytic progenitor cells, leading to dysplastic, hypolobated and microscopic appearing megakaryocytes or increased apoptosis of megakaryocytes and their precursors. Dysregulated thrombopoietin (TPO) signaling and increased platelet destruction through immune or nonimmune mechanisms are frequently observed in MDS. The clinical management of patients with low platelet counts remains challenging and approved chemotherapeutic agents such as lenalidomide and azacytidine can also lead to a transient worsening of thrombocytopenia. Platelet transfusion is the only supportive treatment option currently available for clinically significant thrombocytopenia. The TPO receptor agonists romiplostim and eltrombopag have shown clinical activity in clinical trials in MDS. In addition to thrombopoietic effects, eltrombopag can inhibit leukemic cell proliferation via TPO receptor-independent effects. Other approaches such as treatment with cytokines, immunomodulating drugs and signal transduction inhibitors have shown limited activity in selected groups of MDS patients. Combination trials of approved agents with TPO agonists are ongoing and hold promise for this important clinical problem.

Acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) are associated with disease-initiating stem cells that are not eliminated by conventional therapies. Novel therapeutic targets against preleukemic stem cells need to be identified for potentially curative strategies. We conducted parallel transcriptional analysis of highly fractionated stem and progenitor populations in MDS, AML, and control samples and found interleukin 8 (IL8) to be consistently overexpressed in patient samples. The receptor for IL8, CXCR2, was also significantly increased in MDS CD34(+) cells from a large clinical cohort and was predictive of increased transfusion dependence. High CXCR2 expression was also an adverse prognostic factor in The Cancer Genome Atlas AML cohort, further pointing to the critical role of the IL8-CXCR2 axis in AML/MDS. Functionally, CXCR2 inhibition by knockdown and pharmacologic approaches led to a significant reduction in proliferation in several leukemic cell lines and primary MDS/AML samples via induction of G0/G1 cell cycle arrest. Importantly, inhibition of CXCR2 selectively inhibited immature hematopoietic stem cells from MDS/AML samples without an effect on healthy controls. CXCR2 knockdown also impaired leukemic growth in vivo. Together, these studies demonstrate that the IL8 receptor CXCR2 is an adverse prognostic factor in MDS/AML and is a potential therapeutic target against immature leukemic stem cell-enriched cell fractions in MDS and AML.

Lenalidomide (Len) is clinically indicated in myelodysplastic syndrome (MDS) but its use is limited by significant thrombocytopenia. Eltrombopag (EP) is a thrombopoietin receptor agonist that can stimulate platelet production and has shown preclinical efficacy in inhibiting leukemic cell proliferation. Thus, we determined the preclinical efficacy and safety of combining Len and EP in acute myelogenous leukemia (AML) and MDS. We found that single agent treatment of leukemia and lymphoma cell lines with EP and Len showed differential sensitivities to either agent. Combination therapy did not result in reversal of anti-malignant effects on these cells. Furthermore, the combination of Len and EP resulted in significant inhibitory effects on growth of leukemic colonies in the majority of primary MDS and AML samples. Most importantly, EP was able to reverse the anti-megakaryopoietic effects of Len in primary MDS patient samples. These results provide a preclinical rationale for the use of this combination in MDS and AML.

Transforming growth factor beta is an important regulator of hematopoiesis and its overactivation has been shown to occur in myelodysplastic syndromes. Even though TGF-beta leads to activation of the transcription factor SMAD2, the exact gene expression program and chromatin changes stimulated in human hematopoiesis are not well studied. We generated primary erythroid progenitors from human CD34+ stem cells and assessed genome occupancy of SMAD2 upon TGF-beta stimulation by Chip-seq analysis. We observed widespread occupancy of important stem cell and differentiation regulators with approximately 7000 binding regions. Parallel RNA-seq identified gene expression changes that are associated with smad2 binding at respective promoters and enhancers. Genes involved in stem cell maintenance as well as erythroid differentiation were prominently altered by TGF-beta and contained prominent SMAD2 peaks in hematopoietic progenitors. Next, to determine the relative contribution of various TGF-beta ligands in bone marrow failure, we analyzed a large cohort of MDS serum samples and found TGF-beta 1 (TGF-b1) to be the predominantly elevated ligand in 30% samples. Exposure to MDS patient serum led to erythroid differentiation blocks in primary hematopoietic stem cells in clonogenic cultures. Finally, we analyzed the effect of constitutive secretion of TGF-b1 in a transgenic mouse containing TGF-b1 gene driven by an albumin promoter. These mice constitutively secrete TGF-b1 and become cytopenic and show marrow histological changes that mimic human MDS. We observed stem cell expansion and block at the stage of common myeloid progenitor (CMP) in these mice; findings that are similar to stem and progenitors alterations seen in human MDS patients. To inhibit TGF-b1 signaling we next used a novel ALK5 (TGF-beta receptor I kinase) inhibitor that has been shown to be safe for human use. Vactosertib is specific small molecule inhibitor of ALK5 with a IC50 between 1 x 10-8 M and 1 x 10-9 M and has been shown to have an acceptable safety profile in phase I trial in solid tumors (NCT02160106). Vactosertib was able to inhibit smad2 phosphorylation and activation in a variety of hematopoietic cell lines. Vactosertib treatment led to reversal of hematopoietic alterations upon exposure to MDS serum in clonogenic assays. Vactosertib dosing in vivo also led to increased blood counts in TGF/alb transgenic mice and resulted in normalization of stem and progenitor changes. In conclusion, we demonstrate increased TGF-b1 levels in subset of MDS samples. We also determined TGF-b1 stimulated smad2 binding in important regulatory regions of the genome in primary human erythroid progenitors. We also demonstrate the preclinical efficacy of novel TGF-beta receptor 1 kinase inhibitor in vitro and in vivo. These studies provide preclinical rationale for using this ALK5 inhibitor in trials. Disclosures Huh: MedPacto Inc: Employment. Hwang:MedPacto Inc: Employment. Seong-Jin:MedPacto Inc: Employment. Will:Novartis Pharmaceuticals: Research Funding. Steidl:BayerHealthcare: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; GlaxoSmithKline: Research Funding; Celgene: Consultancy; Aileron Therapeutics: Consultancy, Research Funding; Stelexis Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Other: Scientific Co-Founder; Pieries Pharmaceuticals: Consultancy. Verma:Stelexis: Equity Ownership, Honoraria; Acceleron: Honoraria; Celgene: Honoraria; BMS: Research Funding; Janssen: Research Funding.

Introduction The World Trade Center (WTC) disaster exposed first responders to high levels of aerosolized carcinogens (Lioy et. al. Env. Health Perspect 2002). Clonal hematopoiesis is associated with exposure to smoking and genotoxic stimuli (Jaiswal et. al. NEJM 2014; Genovese et. al. NEJM 2015). We sought to determine its incidence in WTC-exposed first responders. We also assessed the effect of WTC particulate matter (WTC-PM) on genome integrity in vitro, and in murine studies. Methods Deep targeted sequencing was performed on blood collected from 481 first responders (429 WTC-exposed firefighters, 52 WTC-exposed emergency medical service workers) and 52 non-exposed first responders. Samples were analyzed for 237 genes mutated in hematologic malignancies and interpreted using reference databases. Non synonymous somatic mutations were annotated and analyzed. Results In the WTC-exposed cohort, 57 individuals with 66 somatic mutations of expected pathogenic potential were identified (overall prevalence 11.9%). In the non-exposed cohort, only one pathogenic mutation was found in the IDH2 gene (overall prevalence 1.9%). There was a strong association between increasing age and prevalence of mutations in the WTC-exposed cohort (Fig 1A). DNMT3A (16/66), TET2 (7/66), SF3B1 and SRSF2 (3/66 each) were the most common genes identified in the WTC-exposed cohort (Fig 1B). Median VAF was 12% and missense mutations were most frequent alteration. Aging, smoking, DNA repair and alkylating agent exposure related mutational signatures were observed with a cytosine to thymine (C→T) transition being most common. Next, we assessed the effect of WTC-PM on genome integrity and replication in vitro. WTC-PM that was collected in the first three days after 9/11 was used in concentrations mimicking exposure levels. Lymphocytes exposed to WTC-PM demonstrated a significant increase in phosphorylated H2AX foci accumulation, suggesting a DNA damage response (Fig 2). Since common fragile sites (CFSs) detect basal levels of stress in the cell, and activate DNA damage response (DDR), we profiled DNA replication dynamics at CFS-FRA16D at very high resolution using the single molecule analysis of replicated DNA (SMARD) assay. Treatment with WTC-PM significantly altered replication at two common fragile sites (regions 1 and 2 of FRA16D, Fig 3A) with replication pausing being observed at multiple sites (Fig 3B-I, white rectangles). Striking increase in replication initiation was seen, characterized as dormant origins activated to rescue replication pausing (Fig 3E, J). These alterations were accompanied by a corresponding increase in replication speed, conditions that lead to DNA replication errors and mutagenesis (Fig 3F, K). Next, we treated mice with WTC-PM via the oropharyngeal route to mimic first responder exposures, and then harvested and analyzed their bone marrow compartments. Significant expansion of hematopoietic stem cells (Kit+, Sca1+, Lineage-ve, KSL) was seen in WTC-PM treated mice (Fig 4A,B). Whole genome sequencing of sorted stem cells showed a significant increase in non-synonymous SNPs, deletions and indels in the WTC-PM treated samples when compared to control (Fig 4C-E). These genomic alterations were found to occur at low VAF throughout the whole genome, demonstrating widespread genotoxic effects of WTC-PM on hematopoietic stem cells in vivo (Fig 4F). Discussion We report a high burden of mutations in 11.9% (57/481) WTC-exposed first responders compared to the non-exposed cohort (1.9%, 1/52). The frequency of the somatic mutations was many fold higher than in previous studies (Jaiswal et. al. NEJM 2014; Genovese et. al. NEJM, 2015). In the 50-59 year age group, 10% of WTC-exposed individuals carried somatic mutations, compared to the frequency of 2.5% reported by Jaiswal et. al. for the same age group. Despite deeper sequencing performed in our study, the median VAF in our study was 12%, indicating that the difference in technique did not bias our study towards increased detection of small, subclinical clones when compared to previous studies. Furthermore, we demonstrate that WTC-PM can perturb DNA replication and increased genomic instability in vivo, potentially leading to higher burden of clonal hematopoiesis in WTC-exposed first responders. These results demonstrate adverse environmental exposures can be associated with a high rate of clonal hematopoiesis. Disclosures Landgren: Sanofi: Membership on an entity's Board of Directors or advisory committees; Adaptive: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Merck: Other: IDMC; Theradex: Other: IDMC; Abbvie: Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Membership on an entity's Board of Directors or advisory committees. Fletcher:Genoptix/Neogenomics: Employment. Ebert:Broad Institute: Other: Contributor to a patent filing on this technology that is held by the Broad Institute.; Celgene: Research Funding; Deerfield: Research Funding. Steidl:GlaxoSmithKline: Research Funding; Celgene: Consultancy; Aileron Therapeutics: Consultancy, Research Funding; Stelexis Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Other: Scientific Co-Founder; Pieries Pharmaceuticals: Consultancy; BayerHealthcare: Consultancy, Research Funding; Novartis: Consultancy, Research Funding. Will:Novartis Pharmaceuticals: Research Funding. Verma:Stelexis: Equity Ownership, Honoraria; Acceleron: Honoraria; Celgene: Honoraria; BMS: Research Funding; Janssen: Research Funding.

e18527 Background: In patients (pts) with AML and MDS, thrombocytopenia is a source of morbidity/mortality and there is a need for safer treatment options. RWJ-800088 (RWJ) is a novel pegylated thrombopoietin (TPO) mimetic peptide that increases megakaryopoiesis preclinically and in humans. Before using a TPO mimetic agent in AML and MDS pts, it is important to evaluate if the agent has potential stimulatory effects on malignant myeloid cells. To assess whether RWJ is likely to stimulate malignant myeloid cells, we evaluated its effects in vitro on primary human samples from healthy controls and AML/MDS pts. Methods: We studied the effect of various RWJ doses on proliferation of AML/MDS cell lines; clonogenic capacity for leukemic colony forming units (CFU) and megakaryocytic CFU (CFU-Mk) of mononuclear cells (MNC) from controls and several MDS and AML pts; and expression of differentiation markers in MNC cultures. Results: RWJ treatment had no effect on proliferation of THP1, MOLM13, MV411 and MDSL cell lines at doses up to 1000 ng/ml. A slight increase in proliferation of CMK, a megakaryocytic cell line, was seen at doses > 5xEC50 (mean 12%, p = 0.03). Primary clonogenic assays from 18 AML/MDS pts did not show increased growth of leukemic CFU at RWJ concentrations up to 10 ng/ml and RWJ had a nonsignificant inhibitory effect in 4 pts. Notably, RWJ significantly increased normal CFU-Mk formation from MNC in 6 of 8 AML and all MDS pts (mean 2.6-fold, p = 0.02), as well as from healthy CD34+ stem cells and healthy MNC controls (mean 1.7-fold, p = 0.03). FACS analysis of primary cultures treated with RWJ showed no increase in the malignant blast compartment, no change in differentiation markers of granulocyte/monocyte or erythrocyte lineages but a significant increase in the number of CD41a+ megakaryocytic cells (mean 1.8-fold, p < 0.01). RWJ treatment increased phosphorylation of STAT5 but not STAT3, demonstrating on target stimulation of the TPO receptor in hematopoietic cells. Conclusions: Our results show that the novel TPO mimetic RWJ stimulates megakaryopoiesis without causing proliferation of the malignant myeloid clone in MDS and AML.

While the TP53 tumor suppressor gene is mutated in more than 50% of human tumors, in Acute Myeloid Leukemia (AML) TP53 mutations are rare, occurring in less than 10% of cases. Yet, functional inactivation of wild-type p53 due to non-mutational abnormalities occurs frequently in AML and other hematological malignancies. A major mechanism of p53 inactivation results from the overexpression of its endogenous inhibitors MDMX (also known as MDM4, HDMX, and HDM4) and MDM2 which are frequently overexpressed in various p53 wild-type human cancers, including AML. Strikingly, MDMX has been reported to be overexpressed in over 92% of AML cases, while MDM2 overexpression is less frequent. Pharmacological disruption of both these interactions has long been sought after as an attractive strategy to fully restore p53-dependent tumor suppressor activity in AML and other cancers with wild-type p53. Nonetheless, selective targeting of this pathway has thus far been limited to MDM2-only small molecule inhibitors which lack affinity for MDMX. ALRN-6924 is an optimized alpha helical p53 stapled peptide and first-in-class dual MDMX/MDM2 inhibitor which has recently entered phase I/II clinical testing (NCT02264613, NCT02909972) in solid tumors and lymphomas with, thus far, excellent tolerability and objective responses as single agent1. The goal of our study was to evaluate the molecular, cellular, and biochemical mechanisms of action of ALRN-6924 in AML. We used biochemical affinity studies as well as single molecule FISH and live single cell imaging to assess MDMX/MDM2 binding as well as p21 transactivation by p53 in response to ALRN-6924. Effects on cellular proliferation, apoptosis, DNA repair, cell cycle, clonogenic capacity, and serial replating were determined using AML cell lines and primary human AML patients' cells, including in leukemic stem (CD34+ CD38-) and progenitor (CD34+CD38+) cells. Genome-wide molecular effects were determined by RNA seq. P53 activity in a patient undergoing treatment with ALRN-6924 was measured by intracellular staining of p53 and its target gene p21 in CD34+ cells by flow cytometry. Furthermore, we evaluated ALRN-6924 activity in a xeno-transplantation model of human AML in NSG mice. We found that MDMX is significantly overexpressed in highly fractionated leukemic stem (Lin-CD34+CD38-CD90-) and progenitor (Lin-CD34+CD38+CD123+CD45+) cells in AML patients compared to identically sorted, age-matched healthy controls (p Our study provides insight into the molecular, cell biological, and in vivo effects of pharmacological dual MDMX/MDM2 inhibition in AML, which may have important implications for other MDMX/MDM2-related cancers. Our findings provide proof-of-concept for ALRN-6924 as a novel therapeutic in p53-wildtype AML, and provide a rationale for its further preclinical and clinical development in AML and other cancers. Furthermore, the success of targeting p53 raises the intriguing prospect that the same development path is possible for other helix-in-groove targets, and may thus pave the way for a new class of targeted therapeutics. 1 ASCO 2017 annual meeting: J Clin Oncol 35, 2017 (suppl; abstr 2505) Disclosures Guerlavais: Aileron: Employment. Annis: Aileron Therapeutics: Employment. Will: Novartis Pharmaceuticals: Consultancy, Research Funding. Aivado: Aileron: Employment. Steidl: Novartis: Research Funding; Celgene: Consultancy; Bayer Healthcare: Consultancy; GlaxoSmithKline: Research Funding; Aileron Therapeutics: Consultancy, Research Funding.

Introduction: While the presence of HIV portends a poor prognosis in many malignancies, its clinical association with MDS and AML has not been well studied, and is known primarily from small case reports and case series. We evaluated the prevalence and clinical course of unexplained cytopenias and clinically proven MDS/AML in a large cohort of HIV-positive (HIV+) patients (pts) seen at a single center, and compared their clinical course to MDS/AML in non-HIV (HIV-) pts. Methods: Data from 15,120 HIV+ pts seen between 1/1/1997 to 6/1/2016 was analyzed for hematologic parameters, HIV status (CD4 count, HIV viral load), chronic disease status and nutrient deficiencies. Anemia was defined as hemoglobin Results: Out of 13,277 HIV+ pts that had evaluable blood counts, 1075 pts (8.1%) had cytopenias affecting more than one lineage. 912 pts had bilineage cytopenias and 163 pts had a pancytopenia. This was significantly higher than the frequency of bi/trilineage cytopenias in a non-HIV elderly population that was recently analyzed; 8.1% in HIV+ pts vs 0.9% in non-HIV pts, p 250 and HIV viral load We also identified 10 MDS and 9 AML pts in our HIV+ cohort. Mean CD4 count for these cases was 340 cells/mm3, with only 4 pts < 200 cells/mm3. Median time to MDS or AML diagnosis from date of HIV diagnosis was 15.6 and 13.7 yrs respectively. All pts were receiving anti-retroviral therapy at the time of MDS or AML diagnosis. Only 3 MDS pts and 1 AML patient had a history of prior exposure to chemotherapy. Clinical characteristics of the HIV+ MDS pts (n=10) were compared to HIV- MDS pts (n=135) from the same institution diagnosed between 1997-2011. HIV+ MDS pts presented at a younger age (59.5 vs 70.6 yrs, p = 0.001) and also had significantly higher risk (Int-2/High) disease by IPSS (87.5% vs 17.8%, p Conclusions: Anemia occurs in most HIV pts at some point in their course, however, bicytopenia and pancytopenia are generally thought to be uncommon, especially in pts with well-controlled disease. We demonstrated in our cohort an unexpectedly high prevalence of unexplained bi/trilieneage cytopenias, not associated with advanced HIV, chronic disease or nutrient deficiencies. This finding suggests the presence of an independent, HIV-specific, mechanism of hematopoietic suppression, acting either at the level of the stem cell, or the bone marrow milieu. We also report on the largest cohort of HIV+ MDS/AML cases in the literature and demonstrate that HIV+ pts who develop MDS and AML do so at a younger age, present with more advanced disease that is resistant to standard therapy, and have worse overall survival than a control cohort of HIV- pts. These data present evidence that suggests that MDS should be considered as a diagnosis early in HIV. Appropriate risk stratification and new therapeutic approaches are needed. Kaplan-Meier curve comparing overall survival in HIV infected and non-HIV infected MDS patients. Kaplan-Meier curve comparing overall survival in HIV infected and non-HIV infected MDS patients. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Activation of MAPK signaling is characteristic of many cancers, and approximately 15% of AML patients carry activating RAS mutations. The PI3K pathway is also constitutively activated in AML. Both pathways interact with each other extensively and provide compensatory signaling when one pathway is inhibited. It has been demonstrated in vitro that concurrent inhibition of both pathways is effective in blocking the proliferation of AML cell lines through an immediate decrease in pAkt and pErk, leading to inhibition of pS6 (Gritsman et al, J Clin Invest, 2014, 124(4):1794-1809). The combination of alpelisib (BYL-719), which inhibits PI3K (p110alpha), and binimetinib (MEK-162), which inhibits MEK (MAPK pathway), was tested in various tumor types in the Phase Ib Clinical Trial CMEK162x2109. We present here the clinical data and correlative phosphoprotein analysis of 6 patients with relapsed refractory RAS-mutated AML treated at Massachusetts General Hospital on Extension Arm 4A of this trial. Entry to this trial was restricted to adult patients with RAS-mutated AML previously treated with 1 or 2 prior chemotherapy regimens or for whom there was no known effective therapy. Patients received 200mg QD of alpelisib and 45mg BID of binimetinib concurrently and continuously with adjustments for toxicities. Hematologic toxicity could not be determined in these patients with active AML causing myelosuppression. There were responses in blood and/or bone marrow (BM) in 3 patients, but none made partial response criteria by IWG guidelines, mainly due to platelet counts 50% reduction in a third patient. A rise in absolute neutrophil count was seen in 3 patients, 2 from below 100/ul to above 500/ul, and in 1 from 5,320/ul to 12,797/ul during the first month. BM partial responses were seen in 2 patients, with blast percentages dropping from 39% to 10% in 1 patient and from 12% to 5% in another. We analyzed the effects of this drug combination on signaling targets by collecting peripheral blood samples on day 1 pre-treatment and at 6 and 24 hours post-initiation of treatment. In 4 patients, we performed analysis of the phosphorylation of Akt, ribosomal protein S6, Erk, STAT5, and STAT3 on timed peripheral blood samples by immunoblotting. In all 4 cases at baseline, pAkt, pErk, and pS6 were detectable, while pSTAT5 and pSTAT3 levels were variable. In 3 of 4 cases, we observed a transient decrease in pAkt at 6 hours, but then a rebound at 24 hours. In 3 of 4 cases, we observed a lack of sustained pS6 inhibition. We observed sustained pErk inhibition at 24 hours in 2 cases. One patient who had blood blast clearance, improvement in ANC, and a drop in bone marrow blasts, showed strong inhibition of pAkt and pERK at 6 hours, although pS6 did not decrease. In another case we performed phospho-flow cytometry on timed whole blood samples. We observed an increase in pAkt, pS6, and pErk from baseline at 24 hours, both in CD34+38+blasts, and in the primitive CD34+38- cells. This second patient showed no clinical benefit from the treatment in terms of blood blast count, ANC, or platelets. We also performed immunohistochemistry for pS6, pErk, pAkt, pSTAT5, pSTAT3, p-eIF4E, and Caspase 3 on BM sections obtained at diagnosis and at one month and two months post-initiation of treatment, when available. These studies generally showed either persistence or an increase in the pS6 and p-eIF4E signals, both indicators of mTORC1 activity, in post-treatment BM samples. Levels of pAkt, pErk, pSTAT5 and pSTAT3 were highly variable in the post-treatment bone marrow samples. In conclusion, we demonstrated some initial target inhibition with the concurrent use of alpelisib and binimetinib in a subset of patients. However, inhibition of late downstream targets was not sufficiently sustained to achieve consistent clinical benefit in our patients with RAS-mutated AML. While the strategy of concurrent inhibition of various critical signaling pathways remains interesting, sustained inhibition of downstream signaling may require a different dosing schedule of the two drugs. Given the incomplete inhibition of mTORC1 targets pS6 and p-eIF4E in most cases, the addition of a third agent to inhibit pathways causing cross-activation downstream of mTORC1 may be required. Disclosures Weinstock: Novartis: Consultancy, Research Funding. Fathi:Bexalata: Other: Advisory Board participation; Merck: Other: Advisory Board participation; Celgene: Consultancy, Research Funding; Seattle Genetics: Consultancy, Other: Advisory Board participation, Research Funding; Agios Pharmaceuticals: Other: Advisory Board participation.

Thrombocytopenia is a frequent symptom and clinical challenge in patients with myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Eltrombopag is a small molecule thrombopoietin receptor agonist that might be a new option to treat thrombocytopenia in these diseases, provided that it does not stimulate malignant hematopoiesis. In this work, we studied the effects of Eltrombopag on proliferation, apoptosis, differentiation, colony formation, and malignant self-renewal of bone marrow mononuclear cells of patients with AML and MDS. Malignant bone marrow mononuclear cells did not show increased proliferation, or increased clonogenic capacity at concentrations of Eltrombopag ranging from 0.1 to 30 μg/mL. On the contrary, we observed a moderate, statistically nonsignificant (P = .18), decrease of numbers of malignant cells (mean, 56%; SD, 28%). Eltrombopag neither led to increased 5-bromo-2-deoxyuridine incorporation, decreased apoptosis, an increase of malignant self-renewal, nor enhanced in vivo engraftment in xenotransplantations. Furthermore, we found that Eltrombopag was capable of increasing megakaryocytic differentiation and formation of normal megakaryocytic colonies in patients with AML and MDS. These results provide a preclinical rationale for further testing of Eltrombopag for treatment of thrombocytopenia in AML and MDS.

Recurring deletions of chromosome 7 and 7q [−7/del(7q)] occur in myelodysplastic syndromes and acute myeloid leukemia (AML) and are associated with poor prognosis. However, the identity of functionally relevant tumor suppressors on 7q remains unclear. Using RNAi and CRISPR/Cas9 approaches, we show that an ~50% reduction in gene dosage of the mixed lineage leukemia 3 (MLL3) gene, located on 7q36.1, cooperates with other events occurring in −7/del(7q) AMLs to promote leukemogenesis. Mll3 suppression impairs the differentiation of HSPC. Interestingly, Mll3-suppressed leukemias, like human −7/del(7q) AMLs, are refractory to conventional chemotherapy but sensitive to the BET inhibitor JQ1. Thus, our mouse model functionally validates MLL3 as a haploinsufficient 7q tumor suppressor and suggests a therapeutic option for this aggressive disease.

To determine which of the Lean tools will be implemented at the Argentinian site, Gary asks Britta to compile an overview of the tools that she feels will be most relevant to UCo overall. Britta will present these to Alejandro and his team and agree on the techniques that they think will have the greatest value and benefit to the site.