Verstovšek, Srđan

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.

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.

Study Rationale: At present there are no effective therapies for patients with primary or secondary myelofibrosis (MF) after the failure of the JAK inhibitory class of therapies. Furthermore, patients with monocytosis, thrombocytopenia, accelerated phase, or high-risk genetic features fare poorly with JAK inhibitors (JAKi), as intolerance, resistance, and/or ineligibility often limit treatment options. Median overall survival in most studies post-JAKi demonstrate poor outcomes (14-28 months) among patients with MF. Consequently, there is a considerable need in identifying new therapeutic targets beyond JAK-STAT inhibition. One potential cancer target is CD123, aberrantly expressed on leukemia stem cells and aberrant/malignant microenvironmental plasmacytoid dendritic cells (pDC) in certain myeloid cancers including MF. Tagraxofusp (TAG), a CD123-targeted therapy FDA-approved for patients with blastic plasmacytoid dendritic cell neoplasm, a pDC-derived malignancy, has demonstrated preclinical proof-of-concept in MF. We now report the updated results of an ongoing study of TAG monotherapy in patients with relapsed/refractory MF, or who were unable to tolerate currently approved JAKi. (NCT02268253) Experimental Design: A multicenter Phase 1/2 trial with TAG administered as a daily IV infusion at 7, 9, or 12 mcg/kg on days 1-3 every 21 days (cycle 1-4), 28 days (cycles 5-7), and 42 days (cycles 8+) in stage 1, and 12 mcg/kg every 21 days (C1-4), 28 days (C5+) in the currently ongoing stage 2. Results: Thirty-five patients (pts) with a median age 69 years (range: 54-87) have been treated thus far: 26% had baseline monocytosis and 69% had baseline thrombocytopenia, which impacts optimal dosing of JAKi, including 37% with platelets <50x109/L including 7 pts with platelets ≤20; 75% had baseline palpable splenomegaly. Baseline genomic analysis in 35 pts revealed: JAK2 V617F (n=21); CALR (n=3); MPL (n=2); 7 of 21 pts who had comprehensive NGS, had an ASXL1 mutation. Most common treatment-related adverse events (incidence ≥15%) include hypoalbuminemia (23%), headache (17%), and elevated alanine aminotransferase (17%). Notably, capillary leak syndrome (CLS) was reported in 3 pts (1 was grade 4). Twenty-one pts (60%) achieved stable disease (SD) by IWG-MRT revised 2013 criteria, including 4 who had substantial clinical benefits; SD was noted in 7 of 9 (78%) with monocytosis, 7 of 13 (54%) with baseline platelets ≤50x109/L, including 3 of 7 (43%) with baseline platelets ≤20x109/L, and 2 of 3 (67%) pts with accelerated phase. At week 24, spleen responses by palpation were observed in 10 (45%), including 2 with >50% spleen reduction; 12 of 26 (46%) had symptom burden reduction, including 3 pts who met the IWG-MRT criteria. There have been three deaths, all deemed to be unrelated to study drug, occurring in cycles 1, 6, 13, respectively. The median overall survival of our cohort was 31 months (range: 0.6-42 months). Conclusions: Interim results demonstrate clinical efficacy and safety of TAG monotherapy in JAKi relapse/refractory/intolerant MF patients, including those associated with recognized poor-risk biological and clinical features, such as monocytosis. Some patients have experienced clinical benefit, including reduction in splenomegaly and improvement in quality of life and enrollment continues. Disclosures Pemmaraju: Celgene: Honoraria; DAVA Oncology: Honoraria; Roche Diagnostics: Honoraria; Blueprint Medicines: Honoraria; AbbVie: Honoraria, Research Funding; MustangBio: Honoraria; SagerStrong Foundation: Other: Grant Support; Pacylex Pharmaceuticals: Consultancy; Novartis: Honoraria, Research Funding; LFB Biotechnologies: Honoraria; Stemline Therapeutics: Honoraria, Research Funding; Plexxikon: Research Funding; Cellectis: Research Funding; Incyte Corporation: Honoraria; Affymetrix: Other: Grant Support, Research Funding; Daiichi Sankyo: Research Funding; Samus Therapeutics: Research Funding. Gupta:Sierra Oncology: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol MyersSquibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy; Incyte: Honoraria, Research Funding. Ali:Incyte Corporation: Consultancy. Yacoub: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; 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. Wang:PTC Therapeutics: Consultancy; Stemline: Speakers Bureau; Macrogenics: Consultancy; Astellas: Consultancy; Genentech: Consultancy; Pfizer: Speakers Bureau; Abbvie: Consultancy; Jazz Pharmaceuticals: Consultancy; Bristol Meyers Squibb (Celgene): Consultancy. Lee:Roche Molecular Systems: Consultancy; Jazz: Consultancy; AstraZeneca: Consultancy; Helsinn: Other: Member -DSMB; BMS: Consultancy. Schiller:Pfizer: Current equity holder in publicly-traded company, Research Funding; Regimmune: Research Funding; Samus: Research Funding; Sangamo: Research Funding; Tolero: Research Funding; Trovagene: Research Funding; Kaiser Permanente: Consultancy; Johnson & Johnson: Current equity holder in publicly-traded company; Jazz Pharmaceuticals: Research Funding; Geron: Research Funding; Genentech-Roche: Research Funding; Gamida: Research Funding; FujiFilm: Research Funding; Forma: Research Funding; Bristol-Myers Squibb: Current equity holder in publicly-traded company, Research Funding; DeltaFly: Research Funding; Deciphera: Research Funding; Daiichi Sankyo: Research Funding; Cyclacel: Research Funding; Constellation: Research Funding; Celator: Research Funding; Astellas Pharma: Honoraria, Research Funding; Ariad: Research Funding; Actinium: Research Funding; Abbvie: Research Funding; Stemline: Speakers Bureau; Gilead: Speakers Bureau; Agios: Consultancy, Research Funding, Speakers Bureau; Sanofi: Speakers Bureau; Amgen: Consultancy, Current equity holder in publicly-traded company, Research Funding, Speakers Bureau; AstraZeneca: Consultancy; Incyte: Consultancy, Research Funding, Speakers Bureau; Novartis: Consultancy, Research Funding; Ono Pharma: Consultancy; Celgene: Research Funding, Speakers Bureau; Karyopharm: Research Funding; Kite Pharma: Research Funding; Mateon: Research Funding; MedImmune: Research Funding; Onconova: Research Funding. Brooks:Stemline: Current Employment. Rupprecht:Stemline: Current Employment. Talpaz:Takeda: Research Funding; IMAGO: Consultancy; Constellation Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Novartis: Research Funding. Mughal:Stemline: Current Employment. Verstovsek:PharmaEssentia: Research Funding; AstraZeneca: Research Funding; ItalPharma: Research Funding; Incyte Corporation: Consultancy, Research Funding; Gilead: Research Funding; Promedior: Research Funding; CTI Biopharma Corp: Research Funding; Genentech: Research Funding; Blueprint Medicines Corp: Research Funding; Novartis: Consultancy, Research Funding; Sierra Oncology: Consultancy, Research Funding; Roche: Research Funding; NS Pharma: Research Funding; Celgene: Consultancy, Research Funding; Protagonist Therapeutics: Research Funding.

Abstract Momelotinib (MMB) is a JAK1/2 and ACVR1 inhibitor with demonstrated clinical activity in all 3 hallmarks of myelofibrosis (MF): anemia, constitutional symptoms, and splenomegaly. In this phase 2 open-label translational biology study (NCT02515630) of 41 transfusion-dependent patients with MF, we explored mechanisms underlying the favorable activity of MMB on MF-associated iron-restricted anemia, including its impact on serum hepcidin levels, and markers of iron storage and availability, erythropoiesis, and inflammation. A transfusion-independent response (TI-R), defined as red blood cell transfusion independence (TI) ≥12 weeks at any time on study, occurred in 17 patients (41%; 95% confidence interval [CI], 26%-58%), including 14 patients (34%; 95% CI, 20%-51%) who achieved TI-R by week 24. In addition, 78% of TI nonresponse (TI-NR) patients achieved a ≥50% decrease in transfusion requirement for ≥8 weeks. Adverse events (AEs) were consistent with previous studies of MMB in MF, with cough, diarrhea, and nausea as the most common. Twenty-one patients experienced grade ≥3 AEs, most commonly anemia and neutropenia. Consistent with preclinical data, daily MMB treatment led to an acute and persistent decrease in blood hepcidin associated with increased iron availability and markers of erythropoiesis. Baseline characteristics associated with TI-R were lower inflammation and hepcidin as well as increased markers of erythropoiesis and bone marrow function. Overall, the study demonstrates that MMB treatment decreases hepcidin in conjunction with improving iron metabolism and erythropoiesis, suggesting a mechanistic explanation for the reduced transfusion dependency observed in transfusion-dependent MF patients treated with MMB, thereby addressing the key unmet medical need in the MF population.

7083 Background: Patients with Myelofibrosis (MF) suffer from significant fatigue, constitutional symptoms and splenomegaly (Mesa et. al. Cancer 2007) not improved by current therapy. The 6MWT measures functional status in cardio-pulmonary clinical trials. We undertook a prospective feasibility study of the 6MWT in MF patients for use in therapeutic clinical trials. Methods: MF patients (able to walk, normal vitals, not on a JAK2 inhibitor) were eligible. Baseline features and the MF symptom assessment form (MFSAF: Mesa et. al. Blood 2008) was administered. The 6MWT was administered in standardized fashion (American Thoracic Society: observed laps of a 30 - 35 meter long course, indoors, level, without encouragement), and then repeated for further validation. Results: Safety of the 6MWT: Fifty five MF patients were enrolled with only 2 patients (3%) stopping the 6MWT early secondary to fatigue. There were no adverse events attributable to undergoing the 6MWT in test subjects. Results of the 6MWT: Current results of our ongoing study demonstrate compared to age matched healthy volunteer controls, males with MF walk approximately 90 meters less and females with MF walk approximately 60 meters less (Table). Correlation of 6MWT Results to Patient Reported Symptoms (MFSAF): Results from the MFSAF demonstrated the fatigue (as assessed by the brief fatigue inventory (Medoza et. al. Cancer 1999) embedded in the MFSAF) showed patients with a higher BFI (i.e., more fatigued) had more impairment in the 6MWT than those with low BFI scores. Symptomatic burden as assessed by the MFSAF were typical for MF, correlation of individual symptom severity with 6MWT will require a larger trial for adequate power. Conclusions: The 6MWT can be safely performed in un-selected MF patients, and demonstrates a functional deficit compared to published controls. Validation of the ability of the 6MWT to measure functional improvements in MF patients as a response to novel therapy trials is planned. [Table: see text] No significant financial relationships to disclose.