Pacritinib For Bone Marrow Fibrosis In Patients With Myelofibrosis Who Have Thrombocytopenia
Key claim: The PACRIMYEL phase II trial tests whether pacritinib’s dual inhibition of JAK2 and IRAK1 (targeting JAK-STAT and NF-κB pathways simultaneously) can reduce bone marrow fibrosis and improve cytopenias in myelofibrosis patients with thrombocytopenia (platelets 50–120 × 10⁹/L).
Abstract
We hypothesize that pacritinib leads to modification of the myelofibrosis (MF) disease phenotype, especially related to BM fibrosis and cytopenias; due potentially to its dual effect as an inhibitor of the JAK and NFκB pathways, through its targets JAK2 and IRAK1 respectively, leading to a decrease of inflammatory cytokines and/or effects on stem/progenitor populations restoring hematopoiesis New evidence suggests that blocking simultaneously the JAK/STAT and NF-κB pathways might have a beneficial effect on aspects that only inhibition of the JAK pathway cannot achieve: partial recovery of BM histology and
PACRIMYEL is a multicenter, open-label, single arm, phase II, exploratory study including patients with MF and platelet count between 50 - 120 x 109/L. Clinic visits will occur on weeks 4, 8, 12, 24, 36 and 52 during the first year and every 12 weeks during the second year of the treatment, and pacritinib will be dispensed at every visit to the clinic.
Bone fibrosis will be assessed by biopsy and MRI imaging [mDixon Quant "(Philips), IDEAL IQ (General Electric) or qDixon (Siemens)] on weeks 24 and 52 after the first dose of study treatment. Splenomegaly and SVR (Splenic Volume Reduction) will be assessed by physical exam and MRI imaging on weeks 24 and 52 after the first dose of study treatment if splenomegaly at diagnosis. Same MRI to evaluate BM imaging will be used to measure spleen volume. Additionally, spleen size will be assessed by physical exam during the routine clinic visits. All patients should complete all efficacy assessments through Week 52, including patients who stop study treatment or have protocol-defined progressive disease prior to Week 24 and 52, unless the patient withdraws consent or dies. For patients who discontinue treatment before disease assessments on week 24 and week 52 for other reasons different than protocol-based progression of the disease (i.e. toxicity), and with no recent disease / fibrosis assessment (last BM biopsy > 12 weeks), disease and fibrosis assessments will be performed by the end of treatment visit. The trial includes the assessment of safety (AEs, comorbidities) throughout the study period at every visit.
Patient-reported symptoms through MPN-SAF TSS 2.0 will be collected screening, baseline (C1D1), and on Week 12, Week 24, Week 36, Week 52 and in 12-weeks intervals during the second year. Blood samples for translational research will be collected at screening and at week 24 for determination of cytokines.
Why this matters
PACRIMYEL is the first prospective trial designed specifically to test whether pacritinib’s dual JAK2/IRAK1 inhibition can drive bone marrow fibrosis regression — a disease-modifying endpoint distinct from the symptom/spleen endpoints used in prior pacritinib trials. By targeting the moderately thrombocytopenic PMF subpopulation (platelets 50–120 × 10⁹/L), it occupies a niche between the severe-thrombocytopenia Phase 3 pacritinib study and standard ruxolitinib-eligible patients, sharpening subtype-specific natural-history understanding of fibrosis evolution under NF-κB/JAK-STAT co-blockade.