Kiladjian, Jean‐Jacques

Abstract Introduction: Thrombocytopenia, a hallmark of cytopenic myelofibrosis (MF), is associated with poor survival and quality of life impairment. Patients with MF and moderate or severe thrombocytopenia (platelet counts <100x10 9/L, <50x10 9/L) tend to have high symptom burden as measured by the Total Symptom Score (TSS), driven largely by physical functioning symptoms (Mesa R et al, ASH 2021 abstract in submission). Pacritinib, an investigational JAK2/IRAK1 inhibitor, was studied in patients with platelet counts ≤100x10 9/L in the PERSIST-2 trial. Unlike the pivotal studies upon which available JAK1/2 inhibitors were approved that relied on a modified TSS version that excluded 'tiredness' from the response analysis, PERSIST-2 included 'tiredness' as part of TSS and found response rates of 25% for pacritinib vs. 14% for best available therapy (BAT), P=0.08. Here, we retrospectively analyzed TSS on PERSIST-2 using the modified scoring system. In addition, we evaluated the impact of pacritinib and BAT, including ruxolitinib, on MF symptoms including 'tiredness' and 'inactivity'. Methods: All PERSIST-2 patients in the intention-to-treat population randomized at least 22 weeks prior to the end of the study were included. PERSIST-2 randomized patients 1:1:1 to pacritinib 200mg BID, pacritinib 400mg daily, or BAT (including ruxolitinib). The modified TSS score was calculated as the sum of individual symptom scores for early satiety, abdominal discomfort, rib pain, night sweats, itching, and bone pain ('tiredness' and 'inactivity' were not included), and response was defined as a ≥50% reduction in score from baseline to week 24. Symptoms were categorized as physical function-related (tiredness, inactivity), spleen-related (satiety, abdominal discomfort, rib pain), or cytokine-related (sweats, itching, bone pain). The Wilcoxon Rank Sum test was used to compare differences in scores. Results: The analysis included 74 patients randomized to pacritinib 200mg BID and 72 to BAT; an additional 75 were randomized to pacritinib 400mg QD (dose no longer in development). At baseline, 39% had moderate and 46% had severe thrombocytopenia. Ruxolitinib was selected as BAT in 45% of patients with moderate and 38% with severe thrombocytopenia. The co-primary endpoint for the study - TSS response rate in pooled pacritinib arms versus BAT - showed a significant difference using the modified TSS: 30% vs. 14%, P=0.008. Similarly, the modified TSS response rate was higher for patients treated with pacritinib 200mg BID compared to BAT (35% vs. 14%, P=0.004, Figure 1a). Among patients on the BAT arm, modified TSS response rates were modestly higher for patients who received ruxolitinib prior to week 24 (19% [n=6/32]) compared to those who did not (10% [n=4/40]), though both were lower than response rates with pacritinib. Patients on the pacritinib 200mg BID arm experienced greater percentage reductions in individual myelofibrosis symptoms between baseline and week 24 compared to BAT, both in patients with moderate and severe thrombocytopenia (Figure 1b, 1c). For physical function symptoms, severity was reduced more on pacritinib 200mg BID compared to BAT by week 24 (median reduction in tiredness: 30% vs. 13%, P=0.0261; inactivity: 22% vs. 7%, P=0.099). Among patients with severe thrombocytopenia, symptom improvement was minimal on BAT (Figure 1b). Conclusion: The symptom benefit of pacritinib in patients with moderate and severe thrombocytopenia is demonstrated in the PERSIST-2 study. A significant reduction in symptoms was observed in patients treated with pacritinib 200mg BID, the dose being studied in an ongoing Phase 3 trial (PACIFICA), compared to BAT. Furthermore, the modified TSS response rate of 35% in patients treated with pacritinib 200mg BID is comparable to that reported for approved JAK1/2 inhibitors in patients with higher platelet counts, suggesting the potential for pacritinib to address the unmet need of patients with cytopenic myelofibrosis. Figure 1 Figure 1. Disclosures Palmer: CTI BioPharma: Consultancy, Research Funding; Protagonist: Consultancy, Research Funding; Incyte: Research Funding; PharmaEssentia: Research Funding; Sierra Oncology: Consultancy, Research Funding. Mesa: Incyte Corporation: Consultancy, Research Funding; Samus: Research Funding; Sierra Oncology: Consultancy, Research Funding; CTI: Research Funding; Novartis: Consultancy; Celgene: Research Funding; Pharma: Consultancy; La Jolla Pharma: Consultancy; Constellation Pharmaceuticals: Consultancy, Research Funding; CTI: Research Funding; Abbvie: Research Funding; Gilead: Research Funding; Genentech: Research Funding; Promedior: Research Funding; AOP: Consultancy. Oh: Abbvie: Membership on an entity's Board of Directors or advisory committees; Blueprint Medicines: Membership on an entity's Board of Directors or advisory committees; Celgene Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Constellation: Membership on an entity's Board of Directors or advisory committees; CTI Biopharma: Membership on an entity's Board of Directors or advisory committees; Disc Medicine: Membership on an entity's Board of Directors or advisory committees; Geron: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees; Kartos Therapeutics: Membership on an entity's Board of Directors or advisory committees; PharamaEssentia: Membership on an entity's Board of Directors or advisory committees; Sierra Oncology: Membership on an entity's Board of Directors or advisory committees. Rampal: Sierra Oncology: Consultancy; Novartis: Consultancy; BMS/Celgene: Consultancy; Stemline: Consultancy, Research Funding; Jazz Pharmaceuticals: Consultancy; Constellation: Research Funding; Kartos: Consultancy; Disc Medicine: Consultancy; Blueprint: Consultancy; Pharmaessentia: Consultancy; Incyte: Consultancy, Research Funding; Memorial Sloan Kettering: Current Employment; Abbvie: Consultancy; CTI: Consultancy. Buckley: CTI Biopharm: Current Employment. Roman-Torres: CTI Biopharm: Current Employment. Verstovsek: Protagonist Therapeutics: Research Funding; Genentech: Research Funding; Gilead: Research Funding; Incyte Corporation: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; NS Pharma: Research Funding; AstraZeneca: Research Funding; Blueprint Medicines Corp: Research Funding; CTI BioPharma: Research Funding; Promedior: Research Funding; Ital Pharma: Research Funding; PharmaEssentia: Research Funding; Roche: Research Funding; Novartis: Consultancy, Research Funding; Sierra Oncology: Consultancy, Research Funding; Constellation: Consultancy; Pragmatist: Consultancy. Mascarenhas: Sierra Oncology: Consultancy, Membership on an entity's Board of Directors or advisory committees; Merck: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Merus: Research Funding; Geron: Consultancy; Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees; PharmaEssentia: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Forbius: Research Funding; Genentech/Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Geron: Consultancy, Research Funding; Constellation: Consultancy, Membership on an entity's Board of Directors or advisory committees; Promedior: Consultancy, Membership on an entity's Board of Directors or advisory committees; Kartos: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene/BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Prelude: Consultancy; Galecto: Consultancy; CTI Biopharm: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding. Vannucchi: AbbVie: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte Corporation: Honoraria, Membership on an entity's Board of Directors or advisory committees. Kiladjian: CTI BioPharma: Membership on an entity's Board of Directors or advisory committees, Research Funding; AP Orphan: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; PharmaEssentia: Membership on an entity's Board of Directors or advisory committees. Harrison: Galacteo: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Promedior: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Constellation Pharmaceuticals: Research Funding; Incyte Corporation: Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Keros: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Shire: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Abbvie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AOP Orphan Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sierra Oncology: Honoraria; Gilead Sciences: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Geron: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; CTI BioPharma: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Bose: Novartis: Honoraria; NS Pharma: Research Funding; Pfizer: Research Funding; Constellation Pharmaceuticals: Research Funding; Blueprint Medicines: Honoraria, Research Funding; Kartos Therapeutics: Honoraria, Research Funding; Incyte Corporation: Honoraria, Research Funding; Promedior: Research Funding; CTI BioPharma: Honoraria, Research Funding; Sierra Oncology: Honoraria; Astellas: Research Funding; BMS: Honoraria, Research Funding; Celgene Corporation: Honoraria, Research Funding. Craig: CTI BioPharma: Current Employment. Gerds: PharmaEssentia Corporation: Consultancy; CTI BioPharma: Research Funding; Sierra Oncology: Consultancy; AbbVie: Consultancy; Celgene/Bristol Myers Squibb: Consultancy; Constellation: Consultancy; Novartis: Consultancy.

e19040 Background: Clinical trials investigating JAK1/JAK2 inhibitors for myelofibrosis (MF) subjects have measured symptom improvement as a minimum 50% reduction in total symptom score (TSS) at the end of a 24-week treatment period. This landmark analysis is based on post-baseline score changes from Weeks 21 (W21) to 24 and requires vastly different absolute TSS improvements for patients with very high or low baseline (BL) TSS to reach responder status. A phase 3 clinical study, SIMPLIFY-1, randomized 432 intermediate and high risk JAK inhibitor (JAKi) naive MF patients 1:1 to momelotinib (MMB) or ruxolitinib (RUX). Non-inferiority on the MPN-SAF TSS response rate endpoint at W24 was not met (MMB: 28% vs RUX: 42%); however, improvement in each of the 7 TSS items was similar for MMB vs RUX. To understand the discrepancy, we applied item analysis and mixed effect models for repeated measures (MMRM) to SIMPLIFY-1. Methods: Analyses were conducted in the intention-to-treat (ITT) population and in a symptomatic subset (selected as subjects with BL TSS ≥ 10). The distributions of TSS items were examined at BL and shift in scores at W24 (health state shifts) were assessed. GEE models were used to estimate item-level odds ratios using multiple predictive imputations for missing data. MMRM compared mean change in TSS from BL to W24 using data from all visits. The meaningful change threshold (MCT) was determined using Patient Global Impression of Change. Results: BL scores across items were heterogenous in the MMB and RUX groups; the proportion of subjects with no or mild symptoms (0–3 on a 0-10 point scale) ranged from 44% (tiredness) to 81% (itching). Distributions of BL scores were different across arms with 6 out of 7 items in the MMB arm reporting more severe or very severe symptoms (scores of 7-10) at BL. Despite the imbalance in BL scores, item-level health state shifts showed similar improvements for MMB and RUX. Categorical responder analysis showed no significant differences on any items. Odds ratios for each between-group comparison ranged from 0.74 to 1.20. MMRM mean TSS change at W24 was 6.35 (MMB) vs 7.87 (RUX) in the ITT and 8.80 (MMB) vs 10.46 (RUX) in the symptomatic subset. Mean TSS were near the within-subject MCT of 8 points in the ITT and exceeded the MCT in the symptomatic subset. The between-group difference was 1.52 (95% CI: (0.196, 2.847)) in the ITT and 1.67 (95% CI: -0.134, 3.468) in the symptomatic subset. Conclusions: Comparable item health state shifts at W24 and similar improvements in mean TSS as shown by MMRM, with a minimal between-group difference of 1.52 on the 70 point scale in context of an 8-point MCT suggest MMB provides clinically relevant and comparable symptom improvements to RUX; these analyses require further validation in independent data sets. Imbalance in BL symptom scores in MMB subjects may have contributed to the inability to demonstrate non-inferiority in TSS response rate at W24. Clinical trial information: NCT01969838.

7046 Background: Momelotinib (MMB) is a potent JAK1, JAK2 and ACVR1 inhibitor with clinical activity against the hallmark features of myelofibrosis (MF), namely anemia, constitutional symptoms and splenomegaly, across the continuum of JAKi naïve or previously JAKi treated intermediate/high risk MF patients as demonstrated in the previously conducted Phase 3 SIMPLIFY-1 & -2 clinical trials (S1, S2). S1 enrolled JAKi-naïve patients with MF (n = 432) double-blind randomized 1:1 to MMB or ruxolitinib (RUX). S2 enrolled patients with MF with hematological toxicity during prior RUX therapy (n = 156) randomized 2:1 to open-label MMB or best available therapy (BAT; consisting of RUX in 88% of patients). In both trials, following the 24-week randomized treatment (RT) period, patients could continue MMB (MMB→MMB) and those randomized to RUX/BAT could cross-over to MMB (RUX/BAT→MMB) for extended treatment (ET). Previously published data from the SIMPLIFY studies demonstrate robust overall survival (OS) for MMB-treated patients in S1 and S2 (median not reached and 34.3 months, respectively) with a maximum follow up of approximately 5 years and median of 2.9 years in S1 and 2.3 years in S2. Methods: OS data for patients receiving MMB in S1 and S2 are reported here for subgroups defined by Week 24 (W24) transfusion independence (TI) responders vs non-responders, and also other efficacy endpoints. Survival was estimated using KM analysis with descriptive log-rank tests for comparison applied (all p-values are descriptive). Results: As previously reported, W24 TI rates were higher in the MMB arms of S1 (67% vs 49%) and S2 (43% vs 21%). In S1, W24 TI responders in the MMB group show an OS advantage, with median OS not reached and 3-year survival of 80% (HR = 0.30; p = 0.0001) compared to MMB TI non-responders. Similarly in S2, W24 TI responders in the MMB group show a trend toward better OS compared to TI non-responders (HR = 0.57; p = 0.0652). The HRs in S1 for MMB responders vs non-responders for W24 SRR and TSS were 0.59 (p = 0.0904) and 0.65 (p = 0.1657), respectively. Alternative analyses using OS defined from W24 demonstrated consistent results. Conclusions: These new analyses suggest JAKi naïve patients receiving MMB who maintain or achieve TI at W24 have favorable OS compared to MMB TI non-responders, with a similar trend observed in S2. These findings are consistent with anemia and transfusion dependency being key predictors of shortened OS in MF and suggest that TI response at W24 may become a surrogate for clinical benefit, supporting the clinical relevance of MMB’s differentiated pro-erythropoietic ACVR1 inhibition. Clinical trial information: NCT01969838.

Background: The COVID pandemic has resulted in significant changes many aspects of daily living. To understand the impact that COVID-19 has had on the myeloproliferative neoplasm (MPN) patient population, we conducted an internet based survey. Methods: Survey: This survey was hosted Mayo Clinic's secured REDCap system for online surveys with a link to the survey alongside a brief description posted via the www.mpnqol.com website, as well as other MPN organizational partners. Survey responses were completely anonymous. Questions included MPN-Total symptom score (TSS), NCCN distress thermometer (NCCN-DT), questions regarding impact on medical care, and questions from CDC COVID-19 community survey question bank regarding the impact of social distancing as well as changes in health behaviors. Distress thermometer, MPN-TSS, and/or questions related to the impact of COVID-19 on MPN treatment were analyzed by: MPN diagnosis, among those with ET, PV, or MF diagnoses; medication status; stay at home order; community spread; and country. Associations were tested using Kruskal-Wallis or Wilcoxon rank sum tests (for continuous variables) or Fisher Exact tests (for categorical variables). Analysis: Results: Patient Demographics (Table 1): 1560 people responded to the survey, 1217 were eligible for analysis. Median age was 62 (range: 21-93), and 298 (24.6%) respondents were male. There were respondents from USA, Australia, Canada, Netherlands, Ireland, and UK. 233 patients (19.2%) have myelofibrosis, 419 (34.6%) polycythemia vera and 543 (44.8%) essential thrombocythemia. At the time of the COVID outbreak, 1026 (84.3%) were on MPN directed medical therapy, including ruxolitinib (15.3%), interferon (14.7%), hydroxyurea (42.7%) and ASA (47.7%). 165 (13.6%) respondents received COVID testing, of which 5 had positive tests. Impact on MPN Care (Table 2): We sought to understand how patient's clinical care changed. Over half respondents who spoke to their MPN doctor had a telemedicine visit after COVID19 (57.1%). 422 (36.5%) patients spaced out visits, of which 99 (22.7%) felt there were consequences. A change in therapy due to COVID-1 occurred in only 5.4% of patients. MPN Symptom Burden and QoL: Data captured on the NCCN-DT had a median of 4 (0-10). MPN-SAF-TSS composite score was collected in 1150 respondents, median score was 26 (0-90). These scores are higher than those previously reported. Pandemic Impact on Lifestyle (Table 1): 595 (49.5%) of patients report living in a community where there is significant COVID-19 spread. 946 (78.9%) reported that COVID-19 has impacted their day to day life. 198 (17.2%) of patients agreed, or strongly agreed that COVID had a significant impact on their finances. 799 (67.8%) had a stay at home order. Of those who quarantined (112), the median duration was 30d (1-120). The majority of people increased hand-washing, and cleaning habits. 954 (81.7%) respondents reported wearing masks in public. 908 (76.8%) reported increased stress from social distancing. The majority of respondents report using healthy coping habits, such as reaching out to friends/loved ones, breathing and relaxation, as well as healthy diet. Less than 15% of patients report unhealthy coping strategies, such as use of opioids, benzodiazepines, alcohol, or cannabis Factors associated with response: There were no differences in responses based on type of MPN. If a patient was on medication, they were more likely to have spoken to their provider (p<0.001). If there was a stay at home order in place, there was a higher MPN-TSS score (p<0.001). If the respondent lived in an area of high community spread, they had a higher NCCN-DT score (p<0.001). Patients in the USA had a higher NCCN-DT score (p=0.001), were more likely to stretch out the duration of time between visits (p<0.001) and less likely to have a telemedicine visit (p<0.001). Although fewer respondents in the USA thought COVID-19 was a serious disease (p=0.02), a higher percentage of respondents wore masks (p <0.001). Conclusions: In our survey of MPN patients, there were many changes noted in clinical care. The use of telemedicine was common, and at least a third had significant changes in their care. Most experienced increased stress, however, employed healthy coping strategies. Only a minority of patients had a COVID test, and only 5 were positive, further data will be needed to understand the impact of COVID infection. Figure 1 Disclosures Harrison: Roche: Honoraria; Incyte Corporation: Speakers Bureau; Celgene: Honoraria, Research Funding, Speakers Bureau; Sierra Oncology: Honoraria; Promedior: Honoraria; Shire: Honoraria, Speakers Bureau; AOP Orphan Pharmaceuticals: Honoraria; Novartis: Honoraria, Research Funding, Speakers Bureau; Janssen: Speakers Bureau; Gilead Sciences: Honoraria, Speakers Bureau; CTI Biopharma Corp: Honoraria, Speakers Bureau. Pemmaraju:Affymetrix: Other: Grant Support, Research Funding; Plexxikon: Research Funding; Incyte Corporation: Honoraria; Roche Diagnostics: Honoraria; Blueprint Medicines: Honoraria; Novartis: Honoraria, Research Funding; SagerStrong Foundation: Other: Grant Support; DAVA Oncology: Honoraria; Samus Therapeutics: Research Funding; Cellectis: Research Funding; Daiichi Sankyo: Research Funding; Celgene: Honoraria; AbbVie: Honoraria, Research Funding; Stemline Therapeutics: Honoraria, Research Funding; MustangBio: Honoraria; LFB Biotechnologies: Honoraria; Pacylex Pharmaceuticals: Consultancy. Vannucchi:Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Incyte: Membership on an entity's Board of Directors or advisory committees; Blueprint: Membership on an entity's Board of Directors or advisory committees; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AbbVie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Gupta:Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Sierra Oncology: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bristol MyersSquibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Research Funding; Pfizer: Consultancy. 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). Shimoda:Kyowa Hakko Kirin Co., Ltd.: Research Funding; Pfizer Inc.: Research Funding; Otsuka Pharmaceutical: Research Funding; Celgene: Honoraria; Perseus Proteomics: Research Funding; PharmaEssentia Japan: Research Funding; AbbVie Inc.: Research Funding; Astellas Pharma: Research Funding; Merck & Co.: Research Funding; CHUGAI PHARMACEUTICAL CO., LTD.: Research Funding; Bristol-Myers Squibb: Honoraria; Takeda Pharmaceutical Company: Honoraria; Novartis: Honoraria, Research Funding; Shire plc: Honoraria; Asahi Kasei Medical: Research Funding; Japanese Society of Hematology: Research Funding; The Shinnihon Foundation of Advanced Medical Treatment Research: Research Funding. Kiladjian:AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees. Komatsu:AbbVie: Other: member of safety assessment committee in M13-834 clinical trial.; Otsuka Pharmaceutical Co., Ltd., Shire Japan KK, Novartis Pharma KK, PharmaEssentia Japan KK, Fuso Pharmaceutical Industries, Ltd., Fujifilm Wako Pure Chemical Corporation, Chugai Pharmaceutical Co., Ltd., Kyowa Hakko Kirin Co., Ltd., Takeda Pharmaceutica: Research Funding; Meiji Seika Pharma Co., Ltd.: Patents & Royalties: PCT/JP2020/008434, Research Funding; PPMX: Consultancy, Research Funding; Otsuka Pharmaceutical Co., Ltd., PharmaEssentia Japan KK, AbbVie GK, Celgene KK, Novartis Pharma KK, Shire Japan KK, Japan Tobacco Inc: Consultancy; Takeda Pharmaceutical Co., Ltd, Novartis Pharma KK, Shire Japan KK: Speakers Bureau. Abello:Takeda: Honoraria, Research Funding; Novartis: Consultancy, Honoraria; Amgen: Consultancy, Research Funding; Dr. Reddy's: Consultancy, Research Funding; Abbvie: Consultancy, Research Funding. Gomez-Almaguer:Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AbbVie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AstraZeneca: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Pfizer: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Scherber:Incyte Corporation: Current Employment, Current equity holder in publicly-traded company. Mauro:Novartis: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding; Bristol-Myers Squibb: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding; Sun Pharma/SPARC: Research Funding; Takeda: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding; Pfizer: Consultancy, Honoraria, Other: Travel, Accommodation, Expenses, Research Funding. Mesa:Samus Therapeutics: Research Funding; Novartis: Consultancy; Genentech: Research Funding; Incyte: Research Funding; Bristol Myers Squibb: Research Funding; AbbVie: Research Funding; CTI BioPharma: Research Funding; Sierra Oncology: Consultancy; LaJolla Pharmaceutical Company: Consultancy; Promedior: Research Funding.

Myelofibrosis (MF) is characterized by progressive bone marrow (BM) fibrosis resulting from aberrant megakaryopoeisis and expression of pro-inflammatory cytokines. These two processes, heavily influenced by bromodomain and extraterminal domain (BET)-mediated gene regulation, lead to myeloproliferation and cytopenias. CPI-0610 is a potent, selective and unique BET inhibitor under investigation in MF patients as monotherapy or in combination with ruxolitinib in the MANIFEST trial (NCT02158858). In ruxolitinib naïve and experienced MF patients, treatment with CPI-0610 monotherapy or in combination with ruxolitinib not only reduced spleen volume and symptoms, but also improved hemoglobin levels and overcame transfusion dependency in a subset of patients. To evaluate the effects of CPI-0610 on BM biology, correlative analyses were conducted using patient samples collected from the MANIFEST trial. Pre-treatment and post-treatment BM biopsy samples were available from 78 evaluable patients, with reticulin staining conducted and evaluated by local pathologists available for 34 pre-treatment and 24-week post-treatment pairs of BM biopsy samples. Improvement in reticulin staining of at least one grade was observed in 32% (11/34) of patients. Central pathology review of reticulin staining is ongoing and will be presented. Additional bone marrow biopsy pairs collected pre-treatment and 24-week post-treatment for exploratory histopathological assessments were available from a total of 23 unselected MANIFEST patients at the time of the abstract submission. To assess changes in erythroid and megakaryocytic lineages, H&E and immunohistochemistry (IHC) staining of CD71 and CD61 were conducted centrally. Semi-quantitative analysis revealed an overt increase in erythroid lineage formation in 61% (14/23) of patients. Patients with CD71 improvement in BM IHC generally showed a greater increase in median hemoglobin levels than those without improvement. Tight clusters of megakaryocytes (MK), characteristic in MF BM, were observed at baseline in all cases and were decreased/absent after treatment in 52% (12/23) patients. In addition, decreased MK number to <5/high powered field was also observed in 39% (9/23) of patients. Overall improvement in MK histotopography was observed in 65% of patients (15/23). Central histopathology review with IHC staining, including reticulin staining, will be conducted on additional samples and will be available for presentation at the conference. To further dissect the underlying mechanism of these relative improvements in bone marrow composition and histotopography induced by CPI-0610, CD34+ hematopoietic stem cells were isolated from peripheral blood collected from multiple MF patients at baseline to evaluate the impact of CPI-0610 on MK and erythroid differentiation in vitro. When CD34+ cells from MF patients were treated with CPI-0610 in erythroid differentiation conditions in the presence of SCF, IL3 and EPO, a dose-dependent increase in more mature erythroid cell populations was observed. However, under the same conditions, ruxolitinib suppressed erythroid differentiation, which is consistent with ruxolitinib inhibition of EPO signaling and the anemia caused by ruxolitinib treatment in MF patients. Suppressive effects of ruxolitinib on erythroid differentiation were partially rescued by CPI-0610 in a dose-dependent manner. CPI-0610 treatment of CD34+ cells from MF patients in MK differentiating conditions in the presence of SCF, IL6, IL9 and TPO resulted in a dose-dependent decrease in proliferation of CD34+ cells and an increase in the ratio of late MK (CD34-/CD41a+/CD42b+) and early MK (CD34+/CD41a+/CD42b+). This CPI-0610-induced dose-dependent maturation of aberrant immature MK cells may play a role in reducing MF disease manifestations. Molecular studies to further our understanding of the underlying mechanisms of CPI-0610 treatment are ongoing. Taken together, these paired BM biopsy and in vitro myeloid maturation results demonstrated an effect of CPI-0610 in promoting erythroid and MK differentiation. These results may partially explain CPI-0610's clinical effects in MF patients, including rising hemoglobin, reduced transfusion dependency and reduction in spleen volume and symptoms. Disclosures Mertz: Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Keller:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Meyer:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Zavidij:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Cui:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Verstovsek:Roche: Research Funding; Gilead: Research Funding; Promedior: Research Funding; Sierra Oncology: Consultancy, Research Funding; Protagonist Therapeutics: Research Funding; AstraZeneca: Research Funding; Genentech: Research Funding; Novartis: Consultancy, Research Funding; Blueprint Medicines Corp: Research Funding; Celgene: Consultancy, Research Funding; CTI Biopharma Corp: Research Funding; PharmaEssentia: Research Funding; Incyte Corporation: Consultancy, Research Funding; ItalPharma: Research Funding; NS Pharma: Research Funding. Gupta:Sierra Oncology: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bristol MyersSquibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy; Incyte: Honoraria, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Talpaz:IMAGO: Consultancy; Takeda: Research Funding; BMS: Membership on an entity's Board of Directors or advisory committees; Constellation Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Novartis: Research Funding. Harrison:Promedior: Honoraria; AOP Orphan Pharmaceuticals: Honoraria; Roche: Honoraria; Incyte Corporation: Speakers Bureau; Gilead Sciences: Honoraria, Speakers Bureau; CTI Biopharma Corp: Honoraria, Speakers Bureau; Shire: Honoraria, Speakers Bureau; Janssen: Speakers Bureau; Novartis: Honoraria, Research Funding, Speakers Bureau; Celgene: Honoraria, Research Funding, Speakers Bureau; Sierra Oncology: Honoraria. Mead:CTI: Consultancy; Gilead: Consultancy; Novartis: Consultancy, Honoraria, Other: travel, accommodations, expenses, Research Funding, Speakers Bureau; Celgene/BMS: Consultancy, Honoraria, Other: travel, accommodations, expenses, Research Funding; Abbvie: Consultancy. Mesa:Novartis: Consultancy; CTI BioPharma: Research Funding; Bristol Myers Squibb: Research Funding; Sierra Oncology: Consultancy; Genentech: Research Funding; Samus Therapeutics: Research Funding; LaJolla Pharmaceutical Company: Consultancy; Incyte: Research Funding; Promedior: Research Funding; AbbVie: Research Funding. 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). Senderowicz:Constellation Pharmaceuticals: Consultancy, Current equity holder in publicly-traded company, Ended employment in the past 24 months; Puma Biotechnology: Membership on an entity's Board of Directors or advisory committees. Colak:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Shao:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Luptakova:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Humphrey:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Wang:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Trojer:Constellation Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company.

Introduction: MF is a clonal stem cell disorder characterized by progressive bone marrow failure, extramedullary hematopoiesis, and debilitating constitutional symptoms. Approximately 60% of pts develop anemia within a year of diagnosis; pts who are transfusion dependent have worse survival and quality-of-life outcomes (Tefferi A, et al. Mayo Clin Proc 2012;87:25-33). Treatment for MF-associated anemia remains critically unaddressed, with no approved therapies available. Luspatercept is a first-in-class erythroid maturation agent that has been shown to increase hemoglobin and reduce transfusion burden in pts with myelodysplastic syndromes (Fenaux P, et al. N Engl J Med 2020;382:140-51) and β-thalassemia (Cappellini MD, et al. N Engl J Med 2020;382:1219-31). Pts have also achieved multiple response episodes with luspatercept (Fenaux P, et al. Blood 2019;134:841). We previously reported findings from the ongoing open-label, phase 2 ACE-536-MF-001 trial evaluating luspatercept in pts with MF-associated anemia (Gerds AT, et al. Blood 2019;134:557). Here we report updated study results, focusing on response in pts requiring RBC transfusions. Methods: 79 pts with MF and anemia were enrolled (Figure). Of these, 43 were requiring RBC transfusions, defined as 6-12 U/84 d prior to treatment, or 4-12 U/84 d for the 3 expansion cohort pts. Pts in Cohort 3B received a stable ruxolitinib (RUX) dose for ≥ 16 wks (40 wks for expansion cohort pts), whereas Cohort 2 pts did not (Figure). Pts in Cohort 3B received a median RUX dose of 20 mg/d (range 5-40) for a median of 73 wks (range 29-359) prior to treatment. Pts received luspatercept every 21 d at a starting dose of 1.0 mg/kg (1.33 mg/kg for 3 expansion cohort pts in Cohort 3B), with doses escalating up to 1.75 mg/kg; 81% and 50% of pts in Cohorts 2 and 3B escalated to 1.75 mg/kg. Primary endpoint for pts receiving transfusions was RBC transfusion-independence (RBC-TI) for ≥ 12 consecutive wks within the first 24 wks on study. However, as responses were observed that did not fall within the 24-wk response window, additional assessments were conducted for the entire treatment duration for RBC-TI and ≥ 50% decrease in RBC transfusions (minimum 4 RBC U decrease) from baseline. Intent-to-treat data were analyzed as of March 29, 2020. Results: Pts in Cohorts 2 and 3B received a median of 8 cycles of luspatercept (range 1-39), for a median (mean) duration of 24 (31) and 23 (39) wks, respectively. Pts in Cohorts 2 and 3B received a median of 2.7 RBC U/28 d (range 1-5) and 2.7 RBC U/28 d (range 2-4), respectively. For the primary endpoint, 2/21 (10%) and 6/22 (27%) pts receiving transfusions in Cohorts 2 and 3B achieved RBC-TI over any consecutive 12 wks during the first 24 wks. Median time to start of first RBC-TI response period in Cohorts 2 and 3B was 1.5 days and 37 days; median duration of response was 49 wks (range 16-82) and 42 wks (range 12-111), respectively. When assessing the entire treatment period, 4/21 (19%) and 8/22 (36%) pts in Cohorts 2 and 3B achieved RBC-TI ≥ 12 wks, respectively. 25% of the RBC-TI responders in both cohorts experienced 2 separate episodes of RBC-TI ≥ 12 wks. Median cumulative duration of all individual ≥ 12-wk response episodes of RBC-TI was 59 wks (range 24-82) in Cohort 2 and 55 wks (range 12-116) in Cohort 3B. 8/21 (38%) and 10/22 (46%) pts in Cohorts 2 and 3B achieved a ≥ 50% reduction in RBC transfusion burden (minimum ≥ 4 U reduction) over 12 wks; of these, 38% and 20% of responders in Cohorts 2 and 3B experienced 2 separate ≥ 12-wk response episodes, and 1 pt (13%) in Cohort 2 experienced 3 separate responses. 4/15 (27%) and 8/14 (57%) pts in Cohorts 2 and 3B who reached 24 wks of treatment achieved clinical benefit and therefore continued luspatercept on an ongoing basis. Safety is reported for all 79 pts on study. 5 (6%) pts had grade 3-4 treatment-related adverse events (AEs); these AEs were diarrhea (n = 2), dehydration (n = 1), and hypertension (n = 3). There were no treatment-related deaths. Treatment-related AEs (any grade) occurring in ≥ 5% of pts were hypertension (13%), bone pain (9%), and diarrhea (5%). 8 (10%) pts had ≥ 1 AE leading to discontinuation. 16 (20%) pts remain on treatment. Conclusions: These longer-term findings suggest durable activity of luspatercept in pts with MF-associated anemia. In addition, a quarter of pts receiving transfusions achieved more than one ≥ 12-wk episode of RBC-TI response with luspatercept. The incidence of grade 3-4 AEs with luspatercept was low. Disclosures Gerds: Roche/Genentech: Research Funding; Celgene: Consultancy, Research Funding; Sierra Oncology: Research Funding; AstraZeneca/MedImmune: Consultancy; Incyte Corporation: Consultancy, Research Funding; Pfizer: Research Funding; Apexx Oncology: Consultancy; CTI Biopharma: Consultancy, Research Funding; Gilead Sciences: Research Funding; Imago Biosciences: Research Funding. Vannucchi:Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Blueprint: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Incyte: Membership on an entity's Board of Directors or advisory committees. Passamonti:Novartis: Speakers Bureau; BMS: Speakers Bureau; Roche: Other: Support of parent study and funding of editorial support. Kremyanskaya:Constellation Pharmaceuticals: Research Funding; Incyte Corporation: Research Funding; Bristol Myers Squibb: Research Funding; Protagonist Therapeutics: Consultancy, Research Funding; Astex Pharmaceuticals: Research Funding. Gotlib:CTI Biopharma: Research Funding; Kartos: Consultancy, Honoraria, Research Funding; Incyte: Consultancy, Honoraria, Research Funding; Roche: Honoraria, Research Funding; BMS: Research Funding. McCaul:BMS: Speakers Bureau; Seattle Genetics: Speakers Bureau; Karyopharm: Speakers Bureau; Incyte: Speakers Bureau. Ribrag:argenX: Research Funding; pharmamar: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; servier: Consultancy; nanostring: Honoraria, Membership on an entity's Board of Directors or advisory committees; gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees; AZD: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other; Infinity: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; MSD: Honoraria, Membership on an entity's Board of Directors or advisory committees; gustave roussy comprehensive cancer center: Current Employment; EPZ: Honoraria, Membership on an entity's Board of Directors or advisory committees; epizyme (EPZ): Research Funding. Mead:Novartis: Consultancy, Honoraria, Other: travel, accommodations, expenses, Research Funding, Speakers Bureau; Celgene/BMS: Consultancy, Honoraria, Other: travel, accommodations, expenses, Research Funding; Abbvie: Consultancy; CTI: Consultancy; Gilead: Consultancy. Harrison:Roche: Honoraria; Gilead Sciences: Honoraria, Speakers Bureau; Incyte Corporation: Speakers Bureau; Janssen: Speakers Bureau; CTI Biopharma Corp: Honoraria, Speakers Bureau; Sierra Oncology: Honoraria; Celgene: Honoraria, Research Funding, Speakers Bureau; Shire: Honoraria, Speakers Bureau; Novartis: Honoraria, Research Funding, Speakers Bureau; AOP Orphan Pharmaceuticals: Honoraria; Promedior: Honoraria. Mesa:Sierra Oncology: Consultancy; LaJolla Pharmaceutical Company: Consultancy; Novartis: Consultancy; Incyte: Research Funding; CTI BioPharma: Research Funding; Genentech: Research Funding; AbbVie: Research Funding; Promedior: Research Funding; Bristol Myers Squibb: Research Funding; Samus Therapeutics: Research Funding. Kiladjian:AbbVie: Membership on an entity's Board of Directors or advisory committees; AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Barosi:Bristol Myers Squibb: Consultancy. Gerike:Bristol Myers Squibb: Current Employment, Current equity holder in publicly-traded company. Shetty:BMS: Current Employment, Current equity holder in publicly-traded company. Pariseau:Bristol Myers Squibb: Current Employment. Miranda:Bristol Myers Squibb: Current Employment. Schwickart:Bristol Myers Squibb: Current Employment, Current equity holder in publicly-traded company. Giuseppi:BMS: Current Employment, Current equity holder in publicly-traded company. Zhang:Bristol Myers Squibb: Current Employment, Current equity holder in publicly-traded company. Backstrom:Acceleron Pharma: Current Employment, Current equity holder in publicly-traded company; BMS: Current equity holder in publicly-traded company. Verstovsek:AstraZeneca: Research Funding; Roche: Research Funding; CTI Biopharma Corp: Research Funding; Genentech: Research Funding; Incyte Corporation: Consultancy, Research Funding; NS Pharma: Research Funding; Novartis: Consultancy, Research Funding; PharmaEssentia: Research Funding; ItalPharma: Research Funding; Sierra Oncology: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; Blueprint Medicines Corp: Research Funding; Promedior: Research Funding; Gilead: Research Funding; Protagonist Therapeutics: Research Funding.

Abstract PAC203 is a randomized dose-finding study of pacritinib, an oral JAK2/IRAK1 inhibitor, in patients with advanced myelofibrosis who are intolerant of or resistant to ruxolitinib. Patients were randomized 1:1:1 to pacritinib 100 mg once per day, 100 mg twice per day, or 200 mg twice per day. Enhanced eligibility criteria, monitoring, and dose modifications were implemented to mitigate risk of cardiac and hemorrhagic events. Efficacy was based on ≥35% spleen volume response (SVR) and ≥50% reduction in the 7-component total symptom score (TSS) through week 24. Of 161 patients, 73% were intolerant of and 76% had become resistant to ruxolitinib; 50% met criteria for both. Severe thrombocytopenia (platelet count <50 × 103/μL) was present in 44%. SVR rates were highest with 200 mg twice per day (100 mg once per day, 0%; 100 mg twice per day, 1.8%; 200 mg twice per day, 9.3%), particularly among patients with baseline platelet counts <50 × 103/μL (17%; 4 of 24). Although TSS response rate was similar across doses (100 mg once per day, 7.7%; 100 mg twice per day, 7.3%; 200 mg twice per day, 7.4%), median percent reduction in TSS suggested a dose-response relationship (–3%, −16%, and −27%, respectively). Pharmacokinetic and pharmacodynamic modeling based on all available data showed greatest SVR and TSS reduction at 200 mg twice per day compared with lower doses. Common adverse events were gastrointestinal events, thrombocytopenia, and anemia. There was no excess of grade ≥3 hemorrhagic or cardiac events at 200 mg twice per day. Pacritinib 200 mg twice per day demonstrated clinical activity and an acceptable safety profile and was selected as the recommended dose for a pivotal phase 3 study in patients with myelofibrosis and severe thrombocytopenia. This trial was registered at www.clinicaltrials.gov as #NCT03165734.

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.

TPS6641 Background: Hydroxyurea is widely used to treat patients with high-risk PV and ET. However, some patients are either clinically resistant to hydroxyurea or have unacceptable side effects, supporting the need for active therapeutic options in this setting. SAR302503 is an orally administered selective JAK-2 inhibitor that reduced symptom burden and provided a durable clinical benefit, including a reduction in the JAK2V617F allele burden, with an acceptable safety profile in a Phase 1/2 trial of patients with primary myelofibrosis (MF), post-PV MF, or post-ET MF (J Clin Oncol 2011;29:789; Pardanani, ASH 2011; Abs 3838). Based on the results in MF, in October 2011 we initiated a Phase 2 trial to evaluate SAR302503 for the treatment of patients with PV and ET who are resistant or intolerant to hydroxyurea (NCT01420783; ARD12042). Methods: Eligibility criteria include age of at least 18 years, ECOG PS 0–2, and hydroxyurea resistant PV or ET as defined by European LeukemiaNet Consensus Criteria (Blood 2009;113:4829). Patients are being randomized (1:1:1) to receive 100, 200, or 400 mg of SAR302503 orally once daily in consecutive 28-day cycles for at least 8 cycles in the absence of progression or unacceptable toxicity. The primary end points will be assessed after the completion of 8 cycles and are: for PV, the proportion of patients maintaining a hematocrit below 45% without phlebotomy for at least 3 months; for ET, the proportion of patients with a platelet count lower than 400 × 109/L for at least 3 months. Secondary objectives include safety, response rate, pharmacokinetics, pharmacodynamics as measured by JAK2V617F allele burden and STAT3 phosphorylation inhibition, assessment of disease-related symptoms, and quality of life. Enrollment of the planned total of 90 patients is ongoing at approximately 45 sites globally. Twelve patients have been enrolled as of January 2012.

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.