Citations

Introduction: Elevated pro-inflammatory cytokines are a hallmark feature of myeloproliferative neoplasms (MPNs). The pro-inflammatory cytokine interleukin-8 (IL8) is increased in patients with myelofibrosis (MF) and correlates with adverse outcome, including overall survival. Previously, the Levine/Fang labs identified increased IL8 secretion from individual CD34+ stem cells in a subset of MF patients. The role of IL8 and its cognate receptors CXCR1/2 in MF pathogenesis has not been delineated. Methods: Single-cell cytokine assays were performed on isolated CD34+ cells from 60 clinically annotated MPN patients (20 MF, 20 PV, 20 ET) using a previously described micro-chip platform (Kleppe et al, Can Disc 2013). 10 healthy donors (CD34+ cells from hip replacements) were used as controls. Integrated RNA-Seq and Assay for Transposase-Accessible Chromatin followed by next-generation sequencing (ATAC-Seq) was performed on CD34+ cells from MPN patients with and without expanded IL8 secreting clones for gene expression/chromatin accessibility analysis. To model the role of IL8-CXCR2 on fibrosis in vivo, the human MPLW515L transplant model (hMPLW515L) of MF was used. Specifically, wild-type (WT) murine bone marrow (Creneg-Cxcr2f/f; Cxcr2WT) or marrow lacking the CXCR2 receptor (VavCre-Cxcr2f/f; Cxcr2KO)were retrovirally infected with MSCV-hMPLW515L-IRES-GFP and transplanted into lethally irradiated WT recipient mice and monitored for disease. Blood counts, chimerism, and flow cytometry were assayed. Moribund mice were sacrificed and assayed for grade reticulin fibrosis and overall survival. Results: Single-cell cytokine assays confirmed an increased proportion of IL8-secreting CD34+ cells in MF patients (40%) in comparison to other MPN sub-types (10% PV/0% ET) (Figure 1A). MF patients with expanded IL8 secreting clones (defined as >50% of total CD34+ cells) had increased leukocytosis (p<0.0001), larger spleen sizes (p=0.0004), greater prevalence of constitutional symptoms (p=0.0084), and higher-grade reticulin fibrosis in marrow (Figure 1B) in comparison to MF patients without prevalent IL8 clones. IHC confirmed increased IL8 expression in marrow biopsies from 8/15 MF patients in comparison to 0/4 normal controls (Figure 1C), and high IL8 expression was also observed in MF splenic megakaryocytes (MKs) as well as in splenic stromal/endothelial cells not seen in normal spleen (Figure 1D). Integrated RNA-Seq/ATAC-Seq analysis of IL8-high MF patients confirmed up-regulation of IL8-CXCR2 signaling and enrichment in pro-inflammatory pathways (i.e TNFa, NFkB, etc) by GSEA, as well as increased expression/accessibility of pro-inflammatory genes S100A8 and S100A9-previously implicated in fibrosis development. Flow analysis of IL8-high MF CD34+ cells revealed enhanced surface expression of CXCR2 and its analog CXCR1, such that MF was characterized by increased IL8 ligand and receptor expression (Figure 1E) and coincided with enhanced NFkB pathway activity (Figure 1F). Consistent with this, colony forming assays of cultured MF CD34+ cells revealed enhanced colony output when cultured with IL8 compared to WT CD34+ cells-an effect ameliorated by co-treatment with the CXCR1/2 antagonist Reparixin (Figure 1G). In vivo, hMPLW515L adoptive transplant with Cxcr2KO hematopoietic donor cells demonstrated improved leukocytosis, thrombocytosis (Figure 2A) and splenomegaly in comparison to Cxcr2WT hMPLW515L recipient mice. Pathologic analysis revealed a reduction in reticulin fibrosis in bone marrow (Figure 2B) and spleen, translating into an improvement in overall survival (Figure 2C). Notably, a significant reduction in dysplastic MKs-a hallmark feature of MF-was also observed in Cxcr2KO hMPLW515L mice (Figure 2D) supporting a role for CXCR2 signaling in MK proliferation. Conclusion: IL8 secreting clones are associated with increased symptom severity and fibrosis grade in MF. Gene expression of MF CD34+ IL8 secreting clones shows up-regulation of inflammatory genes S100A8/A9, implicated in myofibroblast proliferation. Cxcr2 KO abrogates fibrosis formation and prolongs survival in the hMPLW515L model, and CXCR1/2 inhibition impairs colony forming capacity of MF CD34+ cells. These data suggest pharmacologic inhibition of this pathway should be investigated as potential therapy in MF and in PV/ET patients at high risk of fibrotic transformation. Disclosures Fan: IsoPlexis: Current Employment, Current equity holder in private company; Singleron Biotechnologies: Current Employment, Current equity holder in private company. Levine:Morphosys: Consultancy; Prelude Therapeutics: Research Funding; Qiagen: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Gilead: Honoraria; Loxo: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy; Amgen: Honoraria; Astellas: Consultancy; Imago: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; C4 Therapeutics: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Isoplexis: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Honoraria, Research Funding; Roche: Consultancy, Honoraria, Research Funding; Lilly: Consultancy, Honoraria; Janssen: Consultancy. Hoffman:Protagonist: Consultancy; Abbvie: Membership on an entity's Board of Directors or advisory committees; Dompe: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; Forbius: Consultancy.

AbstractActivation of the P53 pathway through inhibition of MDM2 using nutlins has shown clinical promise in the treatment of solid tumors and hematologic malignancies. There is concern, however, that nutlin therapy might stimulate the emergence or expansion of TP53-mutated subclones. We recently published the results of a phase 1 trial of idasanutlin in patients with polycythemia vera (PV) that revealed tolerability and clinical activity. Here, we present data indicating that idasanutlin therapy is associated with expansion of TP53 mutant subclones. End-of-study sequencing of patients found that 5 patients in this trial harbored 12 TP53 mutations; however, only 1 patient had been previously identified as having a TP53 mutation at baseline. To identify the origin of these mutations, further analysis of raw sequencing data of baseline samples was performed and revealed that a subset of these mutations was present at baseline and expanded during treatment with idasanutlin. Follow-up samples were obtained from 4 of 5 patients in this cohort, and we observed that after cessation of idasanutlin, the variant allele frequency (VAF) of 8 of 9 TP53 mutations decreased. Furthermore, disease progression to myelofibrosis or myeloproliferative neoplasm blast phase was not observed in any of these patients after 19- to 32-month observation. These data suggest that idasanutlin treatment may promote transient TP53 mutant clonal expansion. A larger study geared toward high-resolution detection of low VAF mutations is required to explore whether patients acquire de novo TP53 mutations after idasanutlin therapy.

Introduction: FDA approval of JAK2-inhibitors for myelofibrosis (MF) has changed the landscape for treatment introducing possible increased complexity of clinical trial designs in MF. Many patients with MF will have insufficient response or intolerance to ruxolitinib, opening an opportunity for testing of novel agents alone or in combination. Novel trial designs exist for early stage (phase 1 and 2) studies. The Myeloproliferative Neoplasms Research Consortium (MPN-RC) is an NCI-funded academic group of laboratory and clinical scientists working to develop and evaluate treatments that will improve the survival of patients with MPNs focused on early stage trials in MF. This review was undertaken to understand the landscape of MF clinical trials and improve our approach to trial design. Methods: Studies with "myelofibrosis" as a condition were abstracted from clinicaltrials.gov using an application programming interface from 2010-2019 and reviewed for phase, trial design features, and endpoints. Trial design review captured such features as presence of dose escalation and approach (rule-based versus model-based/assisted) in the phase 1 setting. Observational and behavioral based studies were excluded. A total of 165 treatment interventional studies including MF patients (either MF as the primary cohort, or MF along with other MPN, myelodysplastic syndrome, or hematologic malignancies related cohorts) were reviewed. Primary and secondary endpoints were reviewed for quality of life, symptoms, or other patient-reported outcomes (PRO). Results: A median of 16.5 (range 13-20) trials were initiated per year; 92 (56%) phase 2, 54 (33%) phase 1; 17 (10%) phase 3 and 2 (1%) phase 4, with 77 (47%) industry sponsored (Figure 1). Among phase 1 trials, 41/54 (76%) included dose escalation, the majority employing a standard 3+3 design (or other rule-based design) or not specifically detailing the dose escalation schema. Less than 5 dose escalation trials were explicitly described as model-based or model-assisted. Almost half of the phase 1 trials (23/54; 43%) included combination treatment, with 17/23 (74%) using ruxolitinib or fedratinib. Among phase 2 trials, 82 (89%) were non-randomized and 10 (11%) randomized trials; 35/92 (38%) involved combination treatment and 57 (62%) single agent. Median actual/target enrollment for the phase 3 trials (n=16) was 192 patients (range 49-2233 patients); only 2 phase 3 trials were > 500 patients. Median number of total outcomes was 6.0 (range 1-28) across all studies with a median of 5.5 (range 1-28) for 2010-2014 versus 7.0 (range 1-22) for 2015-2019 (p=0.09). Overall 63/165 (38%) studies included at least one PRO endpoint; 15 (28%) phase 1, 33 (36%) phase 2 and 14 (82%) phase 3. Inclusion of PRO endpoints for years 2010-2014 was similar to studies started in years 2015-2019 [31/82 (38%) and 32/83 (39%)]. Industry sponsored studies included PRO endpoints more frequently [46/77 (60%) versus 17/88 (19%); p<0.001] than non-industry sponsored trials but this is primarily due to a higher proportion of phase 3/4 trials (n=17) being conducted by industry versus non-industry (n=2) (Figure 2). Conclusions: For phase 1 trials in MF, few dose-escalation trials included model-based or model-assisted methods. These designs are encouraged in order to maximize therapeutic benefits in MF, a rare cancer. Clinical trial designs in myelofibrosis have shifted in recent years as evidenced by an increase in number of endpoints, likely to accommodate new challenges in the post JAK2-inhibitors approval era. Use of patient-reported outcomes including quality of life and symptom assessment was high in the phase 3 setting, and continued use should be encouraged as endpoint assessments in MF clinical trial designs. Disclosures Mascarenhas: Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Mesa:LaJolla Pharma: Consultancy; Novartis: Consultancy; Sierra Onc: Consultancy; Abbvie: Research Funding; Celgene: Research Funding; CTI: Research Funding; Genetech: Research Funding; Incyte: Research Funding; Promedior: Research Funding; Samus: Research Funding. Hoffman:Dompe: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Forbius: Consultancy; Protagonist: Consultancy.

Introduction Essential thrombocythemia (ET) and polycythemia vera (PV) patients suffer from various symptoms that worsen quality of life (QOL), yet serial data on symptom changes resulting from therapy are sparse in the literature. Patient questionnaires from 2 large multicenter trials (MPN-RC 111, 112) were used to assess change in symptom burden and QOL over 12 months and impact of baseline symptom burden on subsequent change in ET / PV patients on hydroxyurea (HU) or pegylated interferon alfa-2a (PEG). Methods Trials MPN-RC 111 was a single-arm, open-label, phase II trial evaluating response to PEG in high-risk ET / PV patients with HU resistance/intolerance or splanchnic vein thrombosis (SVT; NCT01259817). MPN-RC 112 was a randomized, open-label, phase III trial comparing response to PEG versus HU in cytoreductive therapy naïve high-risk ET / PV patients diagnosed < 5 years ago (NCT01258856). Measures Patients reported disease-related symptoms via the validated Myeloproliferative Neoplasms Symptom Assessment Form (MPN-SAF), QOL via the European Organisation for the Research and Treatment of Cancer Core QOL Questionnaire (EORTC QLQ-C30), and (if applicable) PEG-related symptoms (flu-like symptoms, injection site irritation, blurry vision, vision change, flushing) at baseline, 3, 6, 9, and 12 months. Analysis Mixed models assessed mean changes from baseline in the MPN-SAF Total Symptom Score (TSS), MPN-SAF items, QOL, and PEG-related symptoms in MPN-RC 111, 112 PEG, and 112 HU patients. Mixed models also assessed the impact of baseline symptom burden (high [TSS ≥ 20] versus low) on subsequent change in PEG (MPN-RC 111 and 112) and HU patients. Results Patients Of the 135 enrolled MPN-RC 111 patients, 20 with SVT and 1 with no questionnaires were excluded. Of the remaining 114, 64 (56%) / 50 (44%) had ET / PV. Patients were 51% / 48% female. Median age was 65 / 64 years, and median time since diagnosis was 38 / 55 months. 31% / 22% had prior thrombosis, and 19% / 56% had splenomegaly. Of the 168 enrolled MPN-RC 112 patients (82 PEG, 86 HU), 2 with no questionnaires were excluded. Of the remaining 166, 79 (48%) / 87 (52%) had ET / PV. Patients were 50% / 33% female. Median age was 60 / 62 years, and median time since diagnosis was 3 / 3 months. 25% / 29% had prior thrombosis, and 11% / 37% had splenomegaly. Symptoms Questionnaire completion rates ranged from 90 - 99%, 87 - 100%, and 75 - 96% for on-treatment MPN-RC 111, 112 PEG, and 112 HU patients. At baseline, TSS (0 [absent] - 100 [worst imaginable]) and QOL (0 [very poor] - 100 [excellent]) means (SDs) were 19.5 (18.4) and 71.6 (20.1) for MPN-RC 111, 17.0 (13.6) and 67.9 (24.3) for MPN-RC 112 PEG, and 14.6 (11.4) and 73.8 (18.8) for MPN-RC 112 HU patients. On average, MPN-RC 111 patients had significant improvement of TSS, fatigue, abdominal pain, abdominal discomfort, dizziness, numbness, night sweats, and fever; MPN-RC 112 PEG patients had significant worsening of fever; and MPN-RC 112 HU patients had significant worsening of inactivity (no mean changes indicating improvement were observed). PEG patients had significant worsening of PEG-related symptoms. The greatest improvements occurred in the 46 (40%), 27 (33%), and 23 (28%) MPN-RC 111, 112 PEG, and 112 HU patients with high baseline symptom burden. On average, PEG patients with high baseline symptom burden had significant improvement of TSS, fatigue, early satiety, abdominal pain, abdominal discomfort, inactivity, headache, concentration, dizziness, numbness, insomnia, cough, night sweats, itching, bone pain, fever, weight loss, and QOL, while those with low baseline symptom burden had significant worsening of TSS, early satiety, headache, itching, and bone pain. On average, HU patients with high baseline symptom burden had significant improvement of TSS, early satiety, abdominal discomfort, headache, dizziness, numbness, insomnia, itching, and weight loss, while those with low baseline symptom burden had significant worsening of TSS, early satiety, abdominal discomfort, inactivity, concentration, and sexual desire/function (Figures 1 and 2). Conclusions Although no statistical comparisons were made across trials, overall improvements were seen in MPN-RC 111 but not 112. Patients with high baseline symptom burden experienced the greatest improvements in symptom burden and QOL during treatment with PEG or HU, which may explain the improvements seen in the more advanced patients in MPN-RC 111 compared to 112. Disclosures Mascarenhas: Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Yacoub:Dynavax: Current equity holder in publicly-traded company; Ardelyx: Current equity holder in publicly-traded company; Cara Therapeutics: Current equity holder in publicly-traded company; Hylapharm: Current equity holder in private company; Incyte: Speakers Bureau; Agios: Honoraria, Speakers Bureau; Novartis: Speakers Bureau; Roche: Other: Support of parent study and funding of editorial support. Hoffman:Protagonist: Consultancy; Forbius: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Dompe: Research Funding. Silver:PharmaEssentia: Speakers Bureau. Mesa:Bristol Myers Squibb: Research Funding; Incyte: Research Funding; AbbVie: Research Funding; Samus Therapeutics: Research Funding; Genentech: Research Funding; CTI BioPharma: Research Funding; Promedior: Research Funding; Sierra Oncology: Consultancy; LaJolla Pharmaceutical Company: Consultancy; Novartis: Consultancy.

Preclinical Rationale: Myelofibrosis (MF) is a chronic myeloproliferative neoplasm for which there are limited therapies. TGFβ plays a pivotal role in the pathobiology of MF by not only promoting bone marrow fibrosis (BMF) and collagen deposition, but also by enhancing the dormancy of normal but not MF hematopoietic stem cells (HSCs). TGFβ has also previously been reported to inhibit normal megakaryocyte (MK) production (Bruno et al Blood 1998). TGFβ1 promotes the synthesis of collagen by normal human mesenchymal stromal cells (MSCs) and activates the TGFβ receptor I/SMAD pathway as well as non-canonical TGFβ pathways. We generated MKs from MF subject mononuclear cells (MNCs) and showed that they elaborated significantly greater levels of TGFβ1 than TGFβ2/3 TGFβ1 treatment reduced the numbers of hematopoietic colonies generated by normal but not MF MNCs. Treatment of MSCs with AVID200, a potent TGFβ1/3 protein trap, significantly decreased MSC proliferation, phosphorylation of SMAD2, and collagen expression. Robust expression of pSMAD2 was observed in the absence of exogenous TGFβ in normal donor or MF-MKs, Addition of AVID200 to -MKs decreased pSMAD2 without affecting total SMAD2/3, indicating that AVID200 blocks the effects of autocrine TGFβ produced by MKs and led to increased numbers of MKs. Moreover, treatment of primary MF MNCs with AVID200 led to increased numbers of progenitor cells with wild type JAK2 and a reduction of mutated colonies. AVID200 blocked TGFβ1-induced p57Kip2 expression and SMAD2 activation by MF MNCs allowing the normal progenitor cells to preferentially cycle, proliferate, and form hematopoietic colonies. Clinical Trial Design: Based on these findings, a phase 1 trial of AVID200 is ongoing in INT-2/high risk MF subjects resistant or intolerant to ruxolitinib; baseline platelet count of ≥ 25 x 109/L, and grade 2/3 BMF. Subjects received intravenous AVID200 (Lots A and B) in dose cohorts of 180 mg/m2 (A), 550 mg/m2 (A), 180 mg/m2 (B) on Day 1 of a 21 day cycle. Cohorts of 3 subjects with a target toxicity rate of 30% were enrolled to estimate the maximum tolerated dose (MTD). A modified toxicity probability interval design was used. Response was assessed by IWG/ELN criteria after 6 cycles of AVID200. Subjects attaining at least a CI or SD with a decrease in BMF by ≥1 grade, continued AVID200. Clinical Trial Results: 10 subjects were enrolled (1 withdrew before receiving treatment) and 9 were treated with AVID200 and were evaluable for DLT assessment [Table1]. Median time after ruxolitinib discontinuation was 3.5 months (0.5-12.2). No DLTs were observed. Grade 3/4 AEs (regardless of attribution) were observed in 6 (66.7%) subjects. Grade 3/4 non-hematologic AEs observed were epistaxis (1, 11.1%), extraocular muscle paresis (1, 11.1%), fatigue (1, 11.1%) and rash (1, 11.1%). Grade 3/4 hematologic AEs were anemia (3, 33.3%) and thrombocytopenia (2, 22.2%) [Table 2]. The median number of cycles received was 5.7 (range 0 - 12). 5 subjects received 6+ cycles and were evaluable. CI occurred in 2 subjects [anemia, spleen and TSS (n=1); TSS (n=1)] 1 of which is still being treated, 2 subjects had SD, 1 subject with 21% blasts prior to study treatment had progressive MPN-BP. 4 subjects failed to reach response evaluation after 6 cycles, 2 had PD due to increasing splenomegaly, 1 subject received an allogeneic transplant and 1 is still being treated [Cycle 2]. The median platelet count at baseline was 114 (range: 42-290) and 159 after cycle 6 [Figure 1]. Maximum changes in platelets from baseline was +64% [range -73%, 169%] in all subjects. 7 subjects had an increase in platelets from baseline during treatment. 2 subjects normalized their platelet count from thrombocytopenic levels. The effect of AVID200 on BMF is currently being examined. 2 subjects remain on treatment. Conclusions: AVID200 a TGFβ1/3 protein trap is well tolerated in advanced MF subjects. Clinical responses were observed at the 550 mg dose and the expansion efficacy cohorts at doses 2 and 3 are enrolling 12 additional subjects. Furthermore, AVID200 therapy improved thrombocytopenia in MF subjects which may be due to AVID200 inhibiting the effects of TGFβ1 on normal MKpoiesis. Updated subject safety and efficacy data along with correlative data will be presented. Disclosures Mascarenhas: Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Kuykendall:Blueprint Medicines: Research Funding; BMS: Research Funding; Incyte: Research Funding; Novartis: Research Funding. Komrokji:Jazz: Honoraria, Speakers Bureau; Abbvie: Honoraria; Agios: Speakers Bureau; BMS: Honoraria, Speakers Bureau; Geron: Honoraria; Incyte: Honoraria; Acceleron: Honoraria; Novartis: Honoraria. Gerds:Gilead Sciences: Research Funding; Imago Biosciences: Research Funding; Sierra Oncology: Research Funding; Celgene: Consultancy, Research Funding; Roche/Genentech: Research Funding; CTI Biopharma: Consultancy, Research Funding; Apexx Oncology: Consultancy; AstraZeneca/MedImmune: Consultancy; Pfizer: Research Funding; Incyte Corporation: Consultancy, Research Funding. Migliaccio:Novartis: Research Funding. O'Connor-McCourt:Forbius: Current Employment. Tremblay:Forius: Current Employment. Nadler:Forbius: Consultancy; Nadler Pharma Associates: Current Employment; Symphogen: Consultancy; Iksuda Therapeutics: Consultancy; Tessa Therapeutics: Consultancy. Mesa:Celgene: Research Funding; Genetech: Research Funding; Samus: Research Funding; Promedior: Research Funding; CTI: Research Funding; LaJolla Pharma: Consultancy; Incyte: Research Funding; Sierra Onc: Consultancy; Abbvie: Research Funding; Novartis: Consultancy. Hoffman:Forbius: Consultancy; Abbvie: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Dompe: Research Funding; Protagonist: Consultancy.

Nearly 20% of patients with myelofibrosis progress to blast phase disease; an aggressive form of acute myeloid leukemia. Although previous studies have implicated loss of TP53 or JARID2 in progression, by and large the genetic events that lead to conversion to blast phase remain unknown. To identify genes whose loss drives progression, we performed a focused CRISPR/Cas9 screen in which murine Jak2V617F bone marrow cells expressing Cas9 were transduced with two separate sgRNA libraries of known tumor suppressor genes and subjected to colony replating assays. Transduction of one of the two libraries led to serial replating and enhanced self-renewal of the Jak2V617F cells. Subsequent DNA sequencing revealed enrichment of all four guides targeting STK11, the gene that encodes LKB1 which regulates a number of key cellular pathways including energy utilization by activation of AMPK. To confirm that loss of Stk11 is the event that leads to increased clonogenicity, we collected cells from Jak2V617F/Vav-Cre+ and control Vav-Cre+ mice and induced Stk11 knockout by electroporating Cas9-Stk11 sgRNA ribonucleoprotein complexes. Consistent with the screening results, only Jak2V617F Vav-Cre+ cells with Cas9-Stk11 sgRNA showed serial replating. To determine whether Stk11 is required for growth of cells with a different driver of enhanced JAK/STAT signaling, we doubly transduced Stk11 homozygous floxed bone marrow cells with MPLW515L-mCherry and Cre-GFP to delete Stk11. As expected, cells with both MPLW515L and Cre recombinase showed enhanced self-renewal, while singly infected control cells failed to replate. These results demonstrate that activation of JAK/STAT signaling can overcome the requirement for Stk11 in normal hematopoiesis and suggest that STK11 loss may be a strong driver of malignant transformation in combination with enhanced JAK-STAT signaling. We next investigated the mechanism by which loss of STK11 cooperates with enhanced JAK/STAT signaling to promote leukemia. RNA-sequencing of wild-type, Stk11+/+/ MPLW515L, and Stk11-/-/MPLW515L hematopoietic cells revealed enrichment of a number of pathways related to hypoxia, oxidative phosphorylation and mitochondrial translation in cells lacking LKB1. Western blot assays confirmed activation of mTOR signaling as well as HIF1a stabilization and pathway activation, both of which have been reported to lie downstream of LKB1 loss. We also performed a number of studies to determine the relevance of reduced LKB1 expression to leukemic progression. First, we induced deletion of Stk11 in mice that were transplanted with HSPCs expressing MPLW515L after development of the MPN phenotype. Loss of Stk11 caused a rapid lethality that was associated with enhanced bone marrow fibrosis and osteosclerosis. We also observed accumulation of leukemic blasts in small clusters consistent with AML transformation arising in the spent phase MPN. Additionally, we deleted STK11 by CRISPR/Cas9 in primary MPN patient samples and monitored their engraftment in immunocompromised mice. We observed enhanced engraftment and increased reticulin fibrosis and osteosclerosis in mice that received the STK11 edited cells compared to those with non-targeted sgRNA. Third, we compared the expression of STK11 in paired blast and chronic phase myelofibrosis patient samples by RT-PCR. Consistent with the hypothesis that loss of STK11 facilitates leukemia, we found that its expression was decreased by more than 50% in five of seven paired post-MPN AML patient samples, with two having STK11 levels below 20%. We further validated downregulation of LKB1 by immunohistochemistry on paired chronic and blast phase MPN specimens and observed little staining in the blast phase specimens. Finally, to further show that the mechanism of in vitro enhanced self-renewal is related to leukemia progression, we stained the paired marrows for HIF1a and saw a dramatic increase in staining at the AML phase. We also analyzed RNA-seq data of paired chronic versus blast phase MPNs specimens and observed that there is a strong congruence of enriched pathways that are associated with the in vitro mouse HSPC phenotype and the human blast phase progression, such as oxidative phosphorylation and hypoxia. Together, our study demonstrates that loss of LKB1/STK11 promotes transformation of cells with activated JAK/STAT signaling and that STK11 is a prominent candidate tumor suppressor gene in post-MPN AML. Disclosures Gurbuxani: UpToDate: Honoraria. Hoffman:Dompe: Research Funding; Protagonist: Consultancy; Abbvie: Membership on an entity's Board of Directors or advisory committees; Forbius: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees. Levine:Astellas: Consultancy; Amgen: Honoraria; Gilead: Honoraria; Qiagen: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Morphosys: Consultancy; Novartis: Consultancy; Prelude Therapeutics: Research Funding; Loxo: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Imago: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; C4 Therapeutics: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Isoplexis: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Honoraria, Research Funding; Roche: Consultancy, Honoraria, Research Funding; Lilly: Consultancy, Honoraria; Janssen: Consultancy. Rampal:Galecto: Consultancy; Incyte: Consultancy, Research Funding; Constellation: Research Funding; Stemline: Consultancy, Research Funding; Celgene: Consultancy; Jazz Pharmaceuticals: Consultancy; CTI Biopharma: Consultancy; Abbvie: Consultancy; Pharmaessentia: Consultancy; Promedior: Consultancy; Blueprint: Consultancy.

Abstract Myeloproliferative neoplasms (MPN) that have evolved into accelerated or blast phase disease (MPN-AP/BP) have poor outcomes with limited treatment options and therefore represent an urgent unmet need. We have previously demonstrated in a multicenter, phase 1 trial conducted through the Myeloproliferative Neoplasms Research Consortium that the combination of ruxolitinib and decitabine is safe and tolerable and is associated with a favorable overall survival (OS). In this phase 2 trial, 25 patients with MPN-AP/BP were treated at the recommended phase 2 dose of ruxolitinib 25 mg twice daily for the induction cycle followed by 10 mg twice daily for subsequent cycles in combination with decitabine 20 mg/m2 for 5 consecutive days in a 28-day cycle. Nineteen patients died during the study follow-up. The median OS for all patients on study was 9.5 months (95% confidence interval, 4.3-12.0). Overall response rate (complete remission + incomplete platelet recovery + partial remission) was 11/25 (44%) and response was not associated with improved survival. We conclude that the combination of decitabine and ruxolitinib was well tolerated, demonstrated favorable OS, and represents a therapeutic option for this high-risk patient population. This trial was registered at www.clinicaltrials.gov as #NCT02076191.

Treatment of polycythemia vera (PV) with the Murine Double Minute 2 (MDM2) antagonist, idasanutlin, in a phase 1 trial was reported by our group to be well tolerated with a high overall response rate (Mascarenhas et al, Blood. 2019 Jun 5). A global, phase 2 trial is currently underway evaluating idasanutlin in hydroxyurea (HU) resistant/intolerant PV patients (NCT03287245). MDM2, a negative regulator of TP53 is upregulated in PV CD34+ cells and inhibition of MDM2 targets PV hematopoietic stem/progenitor cells (HSPC) (Lu et al, Blood. 2014;124(5):771-90). Additional trials of MDM2 antagonists have shown promise, however, there is concern that these agents have the potential to induce TP53 mutations or promote expansion of TP53 mutated clones. Resistance to MDM2 inhibitors has been evaluated in solid tumor cell lines and attributed to either the emergence of de novoTP53 mutations or the selection of TP53 mutated clones. (Michaelis et al, Cell Death Dis. 2011;2:e243; Skalniak et al, Cancers. 2018;10(11)). The effect of MDM2 inhibition on TP53 mutant clones is of particular interest in myeloproliferative neoplasms (MPNs). TP53 mutations have been reported with a low allele burden in ~15% of chronic MPN patients (Kubesova et al, Leukemia. 2018;32(2):450-61), however, TP53 loss of heterozygosity and rapid expansion of TP53 mutant clones is associated with transformation to blast phase (Lundberg et al, Blood 2014,123:2220-8). As reported, in the idasanutlin PV trial, 1/13 patients were identified to have a baseline pathogenic TP53 mutation in hematopoietic cells (VAF 5.5%), using a deep sequencing assay with a limit of detection (LOD) of VAF 0.5%. This patient was a non-responder to idasanutlin and upon treatment had an increasing JAK2V617F and TP53 mutant VAF. End of study hematopoietic cell specimens of all patients were deep sequenced (LOD 0.1%) and revealed that 4 additional patients harbored detectable TP53 mutations after idasanutlin treatment with VAF ranging from 1-12%. In each sample, 1-5 unique TP53 mutations were identified, all within the hotspot domain of the TP53 gene. Deeper sequencing of baseline and follow-up samples revealed these mutations were present at a subclonal level (VAF 0.1-5.5%) and increased over time, indicating that treatment with the MDM2 antagonist promoted expansion of already existing TP53 mutant clones (Table 1, Figure 1). None of the patients who lacked a TP53 mutation at baseline developed a TP53 mutant clone with idasanutlin treatment. There was no clear association of presence of TP53 mutations with prior HU, anagrelide or interferon exposure. There has been careful monitoring of patients to determine whether the expanding TP53 clone has clinical ramifications. Patients were on study for a median of 54 weeks (23-131). The only patient who exemplified resistance to idasanutlin was the single patient with a high burden TP53 mutation (37%). All other patients were taken off study due to patient choice/toxicity. Furthermore, all TP53 mutant and non-mutant patients have had stable disease with no evidence of progression to MF or AML. Sequencing of 2 patients post-discontinuation of idasanutlin revealed that the VAF of the TP53 mutant clones decreased since the agent was discontinued. Updated patient molecular data post-treatment discontinuation will be reported at the meeting. To investigate whether idasanutlin induces de novo TP53 mutations in PV myeloid cells we performed long term HSPC cultures. Mononuclear cells from 6 distinct PV patients were treated continuously with idasanutlin (500 nM) over ~6 weeks and DNA from both treated and untreated colonies were analyzed using next generation sequencing with a LOD of 2% VAF and no TP53 mutations were detected. The combined in vitro and clinical data reveals that treatment with an MDM2 antagonist is not associated with the emergence of de novoTP53 mutations but rather the expansion of prior existing TP53 clones. This does not appear to have clinical repercussions, however, close monitoring of these patients is essential. We recommend that patients be screened for TP53 mutations prior to treatment with an MDM2 antagonist and that if present the TP53 mutant VAF be followed during their treatment course. Resistance to MDM2 inhibition is likely dependent on the TP53 mutant VAF and further studies will need to clarify the ideal dosing schedule of MDM2 antagonists and/or combinatorial therapy to prohibit TP53 mutant clonal expansion. Disclosures Houldsworth: Cancer Genertics: Other: stock in; Sema4: Employment. Rossi:Sema4: Employment. Kiladjian:AOP Orphan: Honoraria, Research Funding; Celgene: Consultancy; Novartis: Honoraria, Research Funding. Rampal:Agios, Apexx, Blueprint Medicines, Celgene, Constellation, and Jazz: Consultancy; Constellation, Incyte, and Stemline Therapeutics: Research Funding. Mascarenhas:Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Research Funding; Roche: Consultancy, Research Funding; Merck: Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; CTI Biopharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Research Funding; Promedior: Research Funding; Merus: Research Funding; Pharmaessentia: Consultancy, Membership on an entity's Board of Directors or advisory committees. Hoffman:Merus: Research Funding.

Abstract Introduction: Approximately 75% of myelofibrosis (MF) patients develop anemia during evolution of the disease process predicting decreased survival. Previous studies exploring the effect of danazol in the treatment of anemia in MF demonstrate responses in anemia of 30-55%. Ruxolitinib has demonstrated improvement in MF related splenomegaly, symptom burden, and even survival yet improvements in cytopenias are uncommon. We designed a phase II multicenter pilot study to evaluate the efficacy and tolerability of combination therapy with ruxolitinib and danazol in MF patients with anemia. Methods: This is a pre-planned interim analysis of a Simon optimum two-stage phase II trial designed to include a minimum of 10 and a maximum of 27 patients. Participants received ruxolitinib 10mg (plat >75 x10x9) BID or 5mg (plat <75 x10 x9) BID with danazol 200mg orally TID. Dose escalation was allowed after completion of 28 days for lack of response or for disease progression. Patients without progression were continued for 6 cycles at 56 days each. Treatment modifications were based on adverse events including thrombocytopenia, leukopenia, and elevation in creatinine and transaminases. Treatment responses were evaluated by the IWG-MRT response criteria (Blood 2013). Patient reported outcome questionnaires (MPN-SAF and EORTC QLQ-C30) were administered at baseline, prior to treatment cycles, and at study discontinuation. Results: Patients: Ten of the 12 evaluable patients enrolled (median age 70.5, range 57-78, M:F ratio 4:1) are included in this analysis. Eight patients had primary MF (PMF), 1 had post essential thrombocytosis (ET) MF, and 1 had post polycythemia vera (PV) MF. Jak2 V617F mutation was positive in 30%. At the time of enrollment 40% had received an erythrocyte transfusion in the last month and all were DIPSS Int-2 or higher. Median baseline hemoglobin was 9.0 g/dL (range 8.3 - 12.4). Median baseline platelet level was 172 x10 (9)/L (range 56 - 346). Most (90%) had received prior therapy and many were refractory to multiple lines of therapy prior to enrollment. Three (30%) patients had previously participated in a JAK inhibitor clinical trial. Tolerability: Among 6 patients who have completed treatment, median duration of treatment was 39 days (range 24-287) with treatment discontinuation due to progression of disease in 2 patients, allogeneic stem cell transplant in 1, alternative treatment in 1, unrelated adverse event (hyponatremia) in 1, and unrelated death (leukemic transformation with intracranial hemorrhage) in 1. Hematologic Grade 3 or > adverse events included anemia (60%), neutropenia (20%), and leukopenia (10%). Non-hematologic Grade 3 or > adverse events included electrolyte abnormalities (20%), edema (10%), infection (10%), and intracranial hemorrhage (10%). Efficacy: Treatment response per IWG-MRT response criteria included stable disease (SD) in 8/10 (80%), clinical improvement in 1/10 (10%), and progressive disease (PD) in 1 (10%). The median change in hemoglobin and platelet count from baseline was -0.05 g/dL (range -0.5 - 1.8) and 28.5 x10(9)/L (range -143 – 118) respectively, however 4/10 (40%) and 7/10 (70%) had improvement at some point during treatment in level of anemia and/or thrombocytopenia. The median follow-up time was 2 months (range 0.9-9.4). Among 6 patients with a baseline and at least one post-baseline MPN-SAF TSS, 2/6 (30%) of patients had at least a 10-point (clinically meaningful) improvement, with 1 of the 2 achieving the stricter 50% improvement from baseline. Responses might have been confounded due to impact of prior therapies. Conclusions: On interim analysis, ruxolitinib and danazol combination therapy demonstrates modest additional clinical benefit, however in a group of exceedingly high risk MF patients with significant anemia and prior single agent JAK inhibitor failures. Preliminary data suggests improvement in anemia and/or thrombocytopenia in some treated patients however only one clinical improvement per IWG-MRT criteria was observed. Combination therapy was well tolerated with no major incremental toxicity attributable to the addition of danazol. Maturation of data is needed to fully evaluate efficacy of ruxolitinib and danazol combination therapy prior to continuation of accrual. Disclosures Mascarenhas: Incyte Corporation: Consultancy, Research Funding; Novartis Pharmaceuticals Corporation: Research Funding. Reeder:Millennium, Celgene, Novartis: Research Funding. Tibes:Seattle Genetics, Inc.: Research Funding. Mesa:Incyte: Research Funding; Novartis: Consultancy; Gilead: Research Funding; Genentech: Research Funding; Promedior: Research Funding; Ns Pharma: Research Funding; Celgene: Research Funding; Lilly: Research Funding; Cti: Research Funding.

Abstract We conducted a two-stage Simon phase II, multi-center study in two groups: related and unrelated donor transplants. Based on the results of a previously conducted prospective study (MPD-RC 101, Rondelli et al, Blood, 2014) demonstrating relatively poor outcome in the unrelated donor arm due to high non-relapse mortality and graft failure (GF), we sought to improve this noted disparate outcome. The hypothesis driving this trial was that pre-transplant JAK1/2 inhibition would result in an increase in performance status, reduction in splenomegaly and inflammatory cytokine expression leading to improved HCT outcome. In the unrelated arm in the first 11 patients, the trial was to be terminated if 6 or more failures (GF or death due to any cause by 100 days post-transplant) were observed. In the related arm in the first 11 patients, the trial was to be terminated if 3 or more failures were observed. Key eligibility criteria included diagnosis of primary MF or post-PV MF or post-ET MF, age 18-70 years, and DIPSS int-2/high risk disease or int-1 risk disease with at least one additional risk factor. Patients could have received up to 6 months of ruxolitinib prior to enrollment provided that there was no evidence of loss of response to ruxolitinib during this period. Ruxolitinib was given for 56 days followed by a 4-day taper ending day -6. The conditioning regimen utilized was IV fludarabine 40 mg/m2 IV and IV busulfan 2.0 mg/KBW IV once daily for 4 days from days -5 to -2. GVHD prophylaxis consisted of cyclosporine or tacrolimus and methotrexate. Patients undergoing unrelated donor HCT also received low dose thymoglobulin (0.5 mg/KBW on day -3/2.0 mg/KBW on day -2 & day -1). The study has enrolled 21 patients (related, 7; unrelated, 14) with a median age of 59 years (range 39-70). 17 (81%) patients had palpable splenomegaly with median spleen size of 11 cm, and 16 patients had int-2/high risk disease. Of the 21 patients, 19 patients received transplantation (related 5/7 [71%], unrelated 14/14 [100%] underwent transplant). Two patients in the related arm did not proceed with transplantation as one progressed to acute myeloid leukemia (baseline blast count 14%) and another experienced sudden death of unknown cause the day after starting ruxolitinib. In addition, there was one GF in the related arm. Due to these 3 failures in the related arm, further enrollment was stopped in this arm per protocol criteria. In the unrelated arm, there were 4 failures (Table 1), and the study met the protocol criteria for further enrollment. Mean percent reduction from baseline to day -9 in palpable spleen length below the left costal margin was 30% (p=0.04) in 12 patients with palpable spleen. 5/12 (42%) had any reduction, and 4/12 (33%) had ≥50% reduction. From baseline to best score on day -9 or day -5, 11/19 (58%) patients with data had any improvement in MPN-SAF Total Symptom Score, and 7/19 (37%) had ≥50% reduction. Of the 19 patients undergoing transplant, ruxolitinib was tapered off successfully in all patients with no significant withdrawal syndrome or delay in transplant noted. In the pre-transplant phase, most common Grade 3 or higher toxicity considered to be related to ruxolitinib was anemia (6/21, 29%), and one patient had legionella pneumonia deemed not related to ruxolitinib. Median time to neutrophil recovery (> 0.5 x 109/L) and platelet recovery (>20 x 109/L) was 19 and 19.5 days. By day 100, 7/19 (37%) experienced Grade 2-4 acute GVHD (G2 in 5, G3 in 2). In the first 30 days post HCT, most common Grade 3 or higher toxicity was febrile neutropenia seen in 5/19 (26%) patients. In all 21 patients from time of registration with median follow-up of 5.8 months, 6-mo overall survival was 75% (95% CI 44-90%). For the 19 transplant recipients, 6-mo OS was 83% (95% CI 55-94%) from date of transplant. Conclusions: Stage 1 trial results highlight important observations. First, ruxolitinib may not be suitable for all myelofibrosis patients pre-transplant, eg, alternative approaches might be more appropriate in the context of accelerating disease. Second, we demonstrated a safe approach for tapering ruxolitinib prior to transplant which allowed patients to commence conditioning with reduced spleen size and symptoms. Third, pre-transplant ruxolitinib does not appear to have any adverse impact on early post-HCT outcomes following unrelated donor transplant, although Stage 2 enrollment will be required to better assess the potential benefits of this approach. Disclosures Gupta: Novartis: Consultancy, Honoraria, Research Funding; Incyte: Consultancy, Research Funding. Yacoub:Incyte: Consultancy, Honoraria, Speakers Bureau; Seattle Genetics: Consultancy, Honoraria, Speakers Bureau; Alexion: Honoraria. Mesa:Promedior: Research Funding; Celgene: Research Funding; CTI: Research Funding; Gilead: Research Funding; Incyte: Research Funding; Galena: Consultancy; Ariad: Consultancy; Novartis: Consultancy. Mead:Novartis: Honoraria, Research Funding, Speakers Bureau. Mascarenhas:Novartis: Other: DSMB , Research Funding; Roche: Research Funding; Promedior: Research Funding; Janssen: Research Funding; CTI Biopharma: Research Funding; Incyte: Other: Clinical Trial Steereing Committee, Research Funding.