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Background: Pacritinib is a JAK2/IRAK1/ACVR1 inhibitor that is approved in the United States for the treatment of patients with myelofibrosis who have a platelet count < 50 × 109/L. Phase 3 clinical studies of pacritinib included patients across a wide range of baseline platelet and hemoglobin levels., Patients and Methods: In order to assess the impact of baseline blood counts on pacritinib efficacy, an analysis of efficacy outcomes by baseline platelet and hemoglobin levels was performed using data pooled from 2 Phase 3 studies of pacritinib in patients with MF (PERSIST-1 and PERSIST-2)., Results: Of 276 patients evaluable for spleen response, spleen volume reduction occurred consistently across platelet subgroups (< 100 × 109/L or ≥ 100 × 109/L) and hemoglobin subgroups (< 8 g/dL, ≥ 8 to < 10 g/dL, or > 10 g/dL), with no diminution in treatment effect in patients with severe thrombocytopenia or anemia. Among 159 patients evaluable for symptoms response, improvement in total symptom score (TTS) was similar across platelet subgroups. A ≥ 50% improvement of TSS occurred more frequently in patients with baseline hemoglobin < 8 g/dL compared with those with baseline hemoglobin ≥ 8 to < 10 g/dL or > 10 g/dL. Patients with baseline hemoglobin < 8 g/dL also experienced improved hemoglobin sustained over 24 weeks, whereas subgroups with less severe anemia had stable hemoglobin levels over time. Symptom improvement as assessed using the Patient Global Impression of Change instrument was generally consistent across platelet and hemoglobin subgroups., Conclusion: Pacritinib demonstrates consistent efficacy in patients with MF regardless of baseline platelet and hemoglobin counts., Competing Interests: Disclosure Nico Gagelmann: has consulted for Morphosys and Kite/Gilead; and has received travel support from Kite/Gilead, Janssen, and Neovii. Prithviraj Bose: has consulted for or received honoraria from AbbVie, Blueprint, BMS, Cogent, CTI BioPharma Corp., a Sobi company, Disc Medicine, GSK, Incyte, Ionis, Jubilant, Karyopharm, Morphic, MorphoSys, Novartis, PharmaEssentia, and Sumitomo; and has received research funding from Blueprint, BMS, Cogent, CTI BioPharma Corp., a Sobi company, DISC Medicine, Geron, Incyte, Ionis, Janssen, Kartos, Karyopharm, MorphoSys, Sumitomo, and Telios. Vikas Gupta: has consulted for AbbVie, BMS-Celgene, CTI BioPharma Corp., a Sobi company, GSK, Imago, Incyte, Karyopharm, MorphoSys, Novartis, Pfizer, Roche; has received research funding from Novartis, Incyte; has participated in advisory boards for AbbVie, BMS- Celgene, Constellation, CTI BioPharma Corp., a Sobi company, GSK, Incyte, Novartis, Pfizer. Donal P. McLornan: has received speakers fees from AbbVie, BMS/Celgene, Jazz Pharmaceuticals and Novartis; has received research funding from BMS/Celgene and Jazz Pharmaceuticals; and has participated on advisory boards and received travel assistance from Jazz Pharmaceuticals. Pankit Vachhani: has received honoraria from AbbVie, Amgen, Blueprint Medicines, Cogent Biosciences, Incyte, CTI BioPharma., a Sobi company, Daiichi Sankyo, GSK, Karyopharm, Novartis, Pfizer, Genetech, Inc., Servier, Stemline, MorphoSys, and LAVA Therapeutics; and has participated in speaker's bureaus for Incyte, CTI BioPharma Corp., a Sobi company, and Blueprint Medicines. Hafia Kathrin Al-Ali: has received research funding from BMS and Incyte; has participated on advisory boards and/or received honoraria: AbbVie, AOP Pharma, Blueprint, BMS, GSK, Otsuka, and Novartis; and has received travel assistance from AbbVie, BMS and Alexion. Haris Ali: has received research/ grant support from Incyte, has consulted for Karyopharm, GSK, and PharmaEssentia, and has participated in speakers bureaus for BluePrint and BMS. Philipp Treskes: consults for Sobi Inc. Sarah Buckley: is employed by Sobi Inc. and has received payment of unvested equity awards from CTI BioPharma Corp., a Sobi company, following its acquisition in June 2023 by the Swedish Orphan Biovitrum AB (publ). Karisse Roman-Torres: is employed by Sobi Inc. and has received payment of unvested equity awards from CTI BioPharma Corp., a Sobi company, following its acquisition in June 2023 by the Swedish Orphan Biovitrum AB (publ). Bart Scott: has participated on the data and safety monitoring board for Nektar and Johnson and Johnson; has participated in advisory panels for BMS, Celgene, Jazz Pharmaceuticals, and Novartis; has consulted for Alexion, Celgene, BMS, and Incyte; has received honoraria from Celgene, and BMS; and has received research funding from BMS and Novartis., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

In patients with cytopenic myelofibrosis, treatment with the JAK2/IRAK1 inhibitor pacritinib was associated with anemia benefit in the phase 3 PERSIST-2 study. The impact of pacritinib on transfusion independence (TI) has not been previously described, nor has the mechanism by which pacritinib improves anemia been elucidated. Because it has been previously postulated that inhibition of activin A receptor, type 1 (ACVR1)/activin receptor-like kinase-2 improves anemia in patients with myelofibrosis via suppression of hepcidin production, we assessed the relative inhibitory potency of pacritinib compared with other JAK2 inhibitors against ACVR1. Pacritinib inhibited ACVR1 with greater potency (half-maximal inhibitory concentration [IC50] = 16.7 nM; Cmax:IC50 = 12.7) than momelotinib (IC50 = 52.5 nM; Cmax:IC50 = 3.2), fedratinib (IC50 = 273 nM; Cmax:IC50 = 1.0), or ruxolitinib (IC50 > 1000; Cmax:IC50 < 0.01). Pacritinib's inhibitory activity against ACVR1 was corroborated via inhibition of downstream SMAD signaling in conjunction with marked suppression of hepcidin production. Among patients on PERSIST-2 who were not transfusion independent at baseline based on Gale criteria, a significantly greater proportion achieved TI on pacritinib compared with those treated on best available therapy (37% vs 7%, P = .001), and significantly more had a ≥50% reduction in transfusion burden (49% vs 9%, P < .0001). These data indicate that the anemia benefit of the JAK2/IRAK1 inhibitor pacritinib may be a function of potent ACVR1 inhibition., (© 2023 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

Thrombocytopenia is common in patients with myelofibrosis (MF) and is a well-established adverse prognostic factor. Both of the approved Janus kinase (JAK) inhibitors, ruxolitinib and fedratinib, can worsen thrombocytopenia and have not been evaluated in patients with severe thrombocytopenia (platelet counts <50×109/L). Pacritinib, a novel JAK2/interleukin-1 receptor-associated kinase 1 inhibitor, has been studied in two phase III trials (PERSIST-1 and PERSIST- 2), both of which enrolled patients with MF and severe thrombocytopenia. In order to better characterize treatment outcomes for this population with advanced disease, we present a retrospective analysis of efficacy and safety data in the 189 patients with severe thrombocytopenia treated in the PERSIST studies. The proportion of patients in the pacritinib group meeting efficacy endpoints was greater than in the BAT group for ≥35% spleen volume reduction (23% vs. 2%, P=0.0007), ≥50% modified Total Symptom Score reduction (25% vs. 8%, P=0.044), and self-reported symptom benefit ("much" or "very much" improved; 25% vs. 8%, P=0.016) at the primary analysis time point (week 24). The adverse event profile of pacritinib was manageable, and dose modification was rarely required. There was no excess in bleeding or death in pacritinib-treated patients. These results indicate that pacritinib is a promising treatment for patients with MF who lack safe and effective therapeutic options due to severe thrombocytopenia.

Myelofibrosis (MF) has heterogeneous clinical manifestations, with some patients exhibiting a myelodepletive phenotype characterized by cytopenias and an absent or low JAK2V617F allele burden. Ruxolitinib may be less effective in these patients. We assessed the efficacy of pacritinib, a JAK2/IRAK1 inhibitor, in MF patients with low JAK2V617F allele burden. In this post hoc analysis of the PERSIST-1 and -2 trials, patients with MF randomized to pacritinib or best available therapy (BAT) were stratified by JAK2V617F allele burden quartile for spleen response of ≥35% and improvement in total symptom score of ≥50%. Five hundred thirty-six patients were included. Patients with lower JAK2V617F allele burden had smaller baseline spleens and lower hemoglobin and platelet counts as compared with higher allele burden patients. Among pacritinib-treated patients, spleen responses were observed across all JAK2V617F allele burden quartiles and in JAK2V617F- disease. No spleen responses were observed among BAT-treated patients with allele burden ≤50% or JAK2V617F- disease. The intention-to-treat response rate was significantly higher on the pacritinib arm for JAK2V617F- disease (23.0% vs 0%; P = .033), and for the lowest allele burden quartiles (0%-25%: 20.9% vs 0%, P < .001; 25%-50%: 15.4% vs 0%, P = .020). There were significantly more symptom responders with pacritinib vs BAT in the 0% to 25% and 25% to 50% cohorts. Pacritinib treatment led to superior spleen and symptom burden reduction compared with BAT in patients with absent or low JAK2V617F allele burden, suggesting that pacritinib may be uniquely suited for patients with myelodepletive MF., (© 2020 by The American Society of Hematology.)

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., (© 2020 by The American Society of Hematology.)