Abstract 7184: Novel role of INCB057643, a bromodomain and extra-terminal (BET) protein inhibitor, in myeloid cell regulation and immunosuppressive tumor environment remodeling in myelofibrosis (MF)

Abstract Dysregulated epigenetic gene expression is a cancer hallmark. BET proteins can bind to acetylated chromatin to regulate transcription factor activity (eg, NF-kB), leading to overproduction of inflammatory cytokines (Kleppe. Cancer Cell. 2018). The BET inhibitor (BETi), INCB057643, has been assessed in MF and other cancers (NCT02711137; NCT04279847 [Study 103]). Although NFkB inhibition is proposed as one of the key mechanisms of BETi-mediated activity in MF, the mechanism of BETi is unresolved. This study assessed cellular and molecular mechanisms of BETi in MF. Total RNAseq was performed on whole blood from commercial MF samples treated ex vivo with INCB057643 and samples from patients (pts) enrolled in Study 103 (dose escalation, monotherapy). Differential gene expression (DGE) and gene set enrichment analysis (GSEA) were performed. Plasma biomarkers were analyzed using HuMAP 2.0 (IQVIATM panel). The impact of INCB057643 on ex vivo differentiation of pt-derived CD34+ cells expressing MPN driver mutations was tested at clinical doses. INCB057643 inhibited pathogenic megakaryopoiesis by reducing megakaryocyte (MK) differentiation and proinflammatory cytokine levels, including neutrophil (etc) chemo-attractants, IL-8 and ENA-78. Although IL-8 was produced at similar levels by wild-type and mutant MK progenitors, ENA-78 was produced only by mutant cells. Both cytokines were inhibited by INCB057643. Contrasting with MK, the proportion of erythroid cells was increased. RNAseq of clinical samples showed INCB057643 dose-dependent effects on gene expression. DGE analysis showed differential expression of epigenetic genes, and genes involved in platelet biogenesis; both are known BET functions. DGE analysis of a myeloid gene set representing monocytes, macrophages, dendritic cells, and neutrophils showed dose-dependent decreases in functional genes (MGST1, IDO2) and immunosuppressive protumorigenic genes (MARCO, SIGLEC11), but representative T- or B-cell gene expression was either unchanged or elevated (PRF1). Consistent with DGE, INCB057643 inhibited proinflammatory cytokine production (eg, RANTES, IL-18 produced by myeloid cells) as well as ENA-78, suggesting reduced inflammatory cell trafficking in bone marrow. Increased expression of MYC is associated with MPN progression; DGE and GSEA showed that hallmark MYC targets signatures were reduced by INCB057643. In summary, INCB057643 decreased malignant MK precursors that drive MF. Although data confirmed INCB057643 regulates epigenetic gene expression globally, selective inhibition of myeloid and MK cells, but not other lineages, was observed. INCB057643 inhibited myeloid cells by downregulating immunosuppressive receptors and proinflammatory cytokines, leading to tumor microenvironment remodeling and reduced inflammation. Citation Format: Gaurang Trivedi, Pat Feldman, Natalia Pardo, Erin Crowgey, Brittney Wass, Hamza Celik, Betty Lamothe, Diana Alvarez Arias. Novel role of INCB057643, a bromodomain and extra-terminal (BET) protein inhibitor, in myeloid cell regulation and immunosuppressive tumor environment remodeling in myelofibrosis (MF) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 7184.