MPN-343: The BET Inhibitor Pelabresib Decreases Inflammatory Cytokines, Improves Bone Marrow Fibrosis and Function, and Demonstrates Clinical Response Irrespective of Mutation Status in Myelofibrosis Patients in the Phase 2 MANIFEST Trial

Context: Pelabresib (CPI-0610) is a potent, selective BET bromodomain inhibitor under investigation in the ongoing Phase 2 MANIFEST trial, given as monotherapy to ruxolitinib (rux)-intolerant/refractory myelofibrosis (MF) pts in arm 1, as an “add-on” to rux in MF pts who have suboptimal/lost response to rux in arm 2, and as combination therapy with rux in JAKi-naive MF pts in arm 3. Objective: We assessed activity of pelabresib, including changes in inflammatory cytokines (Ck) in blood, improvement of bone marrow (BM) biology, including fibrosis (BMF), erythroid (Ery) progenitor and megakaryocyte (MK) numbers and clustering, and impact of mutational status on clinical response. Results: Rapid PD response was demonstrated by median blood IL8 mRNA level reduced to 55% of baseline (BL) levels 4 hours post-1st dose of pelabresib in 102 pts, with similar reduction across arms. Elevated blood Ck levels were observed at BL across arms. Reduction of NF-κB and non-NF-κB regulated inflammatory Ck was observed on cycle 1 day 14 (C1D14) across arms and maintained through C9D1 (24 weeks). BMF grading was assessed by local pathologists for BL and post-treatment (most at 24 weeks) biopsies from 116 evaluable pts. Relative BMF improvement of ≥1 grade was observed in 33% (38/116) of all pts, with 21% (6/29) in arm 1, 41% (16/39) in arm 2, and 33% (16/48) in arm 3. BMF grade worsening was observed in only 6% (7/116) pts. Exploratory analyses of Ery and MK lineages by immunohistochemical staining for CD71 and CD61, respectively, on BM biopsy pairs (BL and week 24) revealed an increase in Ery progenitors in 59% (22/37) of pts. Tight clusters of MK, characteristic in MF BM, were observed at BL, and improvement in MK numbers and/or clustering was observed in 65% (24/37) pts. Mutation analysis revealed similar profiles at BL across arms. SVR35 at 24 weeks was similar, regardless of mutational status for each of the 14 genes analyzed. Conclusions: Our data demonstrate a robust PD effect of pelabresib in MF pts, indicate broad clinical responses irrespective of mutational status, and suggest the disease-modifying potential of pelabresib by improving BM fibrosis and function.