Platelet Response in Pacritinib-Treated Patients with Cytopenic Myelofibrosis: A Retrospective Analysis of PERSIST-2 and PAC203 Studies

• Published in: Blood 2023-11, Vol.142 (Supplement 1), p.4554-4554
• Main Authors: Vachhani, Pankit, Yacoub, Abdulraheem, Traer, Elie, Benajiba, Lina, Passamonti, Francesco, Kishtagari, Ashwin, Akhtari, Mojtaba, McCloskey, James, Buckley, Sarah, Suthar, Purvi, Roman-Torres, Karisse, Mascarenhas, John
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2023-178725

Background: Myelofibrosis (MF) is a myeloid malignancy characterized by clonal hematopoisis, bone marrow fibrosis (BMF) and ineffective extramedullary hematopoiesis, resulting in progressive cytopenias. Thrombocytopenia is both prognostic of poor outcomes and predictive of treatment intolerance with the JAK1/2 inhibitor ruxolitinib, which exacerbates cytopenias. Pacritinib is a JAK1-sparing inhibitor of JAK2/IRAK1/ACVR1 that can be administered at full dose to patients regardless of baseline platelet count (PLT). While prior studies have shown that pacritinib is associated with PLT stability in most patients, PLT improvement has not been described outside of a recently published case report ( Yacoub A, et al. JCO Precis Oncol. 2023;7:e2200523). Here, we report rates of hematologic improvement in PLTs (HI-P) with pacritinib across two clinical trials. Methods: Patients with baseline PLT ≤100x10 9/L on the pacritinib 200 mg BID arms of the phase 3 PERSIST-2 study (randomized ≥12 weeks prior to study termination) or the phase 2 dose-finding PAC203 study were included. HI-P was defined per International Working Group (IWG) criteria (baseline PLT 20x10 9/L and by at least 100%; baseline PLT 20-100 x 10 9/L: absolute increase of ≥30x10 9/L) in the absence of PLT transfusions, sustained over any 8 weeks while on treatment. Efficacy outcomes (reduction in spleen volume, symptom score, and BMF) were compared between HI-P responders vs non-responders (those who did not meet IWG criteria). Rates of PLT improvement were also analyzed on the best available therapy (BAT) arm of the PERSIST-2 study over the treatment period (end of study treatment). Results: Of 117 patients randomized to pacritinib (75 from PERSIST-2, 42 from PAC203), 16% (n=19) experienced HI-P on study (as defined in methods). Additionally, 14 of the 19 HI-P patients had sustained platelet improvement over ≥12 weeks). By contrast, only 5% (4/77) of patients on BAT achieved HI-P response prior to end of study treatment. HI-P responders on pacritinib compared to non-responders, had numerically higher median baseline PLT count (63 vs 45x10 9/L) and hemoglobin (Hb; 10.3 vs 8.8 g/dL). A similar percentage had prior (