Abstract: Pelabresib (CPI-0610) is a novel, potent bromodomain and extraterminal domain (BET) inhibitor currently in advanced clinical development for the treatment of myelofibrosis (MF). While Janus kinase (JAK) inhibitors remain the cornerstone of MF therapy, they are limited by their inability to significantly modify disease progression and their propensity to exacerbate cytopenias. Pelabresib addresses these unmet clinical needs through epigenetic modulation, specifically by inhibiting BET proteins that act as epigenetic "readers" controlling the transcription of key oncoproteins and inflammatory cytokines. Preclinical and clinical data, notably from the MANIFEST and MANIFEST-2 trials, demonstrate that pelabresib—both as a monotherapy and in combination with the JAK inhibitor ruxolitinib—can induce meaningful spleen and symptom responses, improve anemia, reduce bone marrow fibrosis, and decrease mutant allele burden. This review synthesizes the current literature on pelabresib, detailing its pharmacological activity, molecular mechanism of action, current limitations, and future perspectives in the management of myelofibrosis.
1. Introduction
Myelofibrosis (MF) is a clonal myeloproliferative neoplasm characterized by bone marrow fibrosis, extramedullary hematopoiesis, splenomegaly, and debilitating constitutional symptoms such as fever, night sweats, and weight loss [1]. The pathogenesis of MF is heavily driven by an overactive JAK-STAT signaling pathway, aberrant cytokine production, and a defective bone marrow microenvironment [1][4]. Currently, Janus kinase (JAK) inhibitors, such as ruxolitinib and fedratinib, are the primary approved therapies for MF and have been highly successful in reducing spleen volume and symptom burden [1][2].
Despite their clinical utility, JAK inhibitors have significant limitations. They do not substantially impact the natural history of the disease, meaning their "disease-modifying" effects—such as the reversal of bone marrow fibrosis or significant reduction in mutant allele burden—are limited [2]. Furthermore, many patients are ineligible for or must discontinue JAK inhibitor therapy due to treatment-emergent cytopenias (anemia and thrombocytopenia), and patients who become refractory to JAK inhibition face a dismal overall survival [1][5]. Consequently, there is a critical need for non-JAK inhibitor-based therapies. Pelabresib (CPI-0610) has emerged as a leading next-generation therapeutic candidate. As an epigenetic modulator targeting BET proteins, pelabresib is being extensively evaluated to overcome the limitations of current standard-of-care therapies [1][2].
2. Pharmacological Activity
The clinical efficacy of pelabresib has been primarily evaluated in the comprehensive phase 2 MANIFEST study, which investigated the drug across distinct MF patient populations [1].
Monotherapy in JAKi Refractory/Intolerant Patients: In a cohort of patients refractory or intolerant to JAK inhibitors, pelabresib monotherapy demonstrated notable activity, particularly in ameliorating anemia. Among transfusion-dependent patients, 21.4% to 23.8% converted to transfusion independence [1][2]. Furthermore, in non-transfusion-dependent patients with baseline hemoglobin <10 g/dL, 57.9% achieved a sustained ≥1.5 g/dL improvement in hemoglobin [2]. Spleen volume reduction of ≥35% (SVR35) and total symptom score reduction of ≥50% (TSS50) were achieved in 23.8% and 47.4% of evaluable patients, respectively, in one cohort [1], though another specific refractory cohort reported disappointing SVR35 (0%) and TSS50 (8.3%) rates [2].
Add-on Therapy: When used as an "add-on" therapy for patients experiencing a sub-optimal response to ruxolitinib, pelabresib converted 34.4% of transfusion-dependent patients to transfusion independence. In this setting, 20.8% of patients achieved SVR35 and 46.2% achieved TSS50 at week 24 [1].
Upfront Combination Therapy: The most compelling pharmacological activity for pelabresib has been observed in the upfront setting for JAK inhibitor-naive patients. When combined with ruxolitinib, 67% to 68% of patients achieved SVR35, and 56% to 57% achieved TSS50 [1][4]. Importantly, this combination demonstrated significant disease-modifying potential, with 28% to 33% of patients experiencing at least a 1-grade improvement in bone marrow fibrosis and 29.5% showing a >25% reduction in the JAK2V617F-mutant allele fraction [1][4].
These robust phase 2 results led to the initiation of the phase 3 MANIFEST-2 trial (NCT04603495), a randomized, double-blind, placebo-controlled study evaluating pelabresib plus ruxolitinib versus ruxolitinib alone in JAK inhibitor-naive patients [3][5][6]. Interim data from MANIFEST-2 confirm positive clinical outcomes and good tolerability, supporting a potential paradigm shift toward frontline combination therapy [4].
3. Molecular Mechanism of Action
Pelabresib (CPI-0610) is a potent inhibitor of the bromodomain and extraterminal domain (BET) family of proteins [1]. BET proteins function as epigenetic "readers" that localize to discrete chromosomal locations and recruit regulatory complexes to control the transcription of multiple key oncoproteins, including nuclear factor kappa B (NF-κB), c-Myc, and B-cell lymphoma-2 (BCL-2) [1][2].
In the context of myelofibrosis, the inhibition of the BET protein BRD4 leads to the attenuation of the NF-κB pathway, which plays a pivotal role in driving the pro-inflammatory state characteristic of the disease [1]. By inhibiting BET proteins, pelabresib significantly dampens the inflammatory response of bone marrow-derived macrophages, suppressing the expression of critical inflammatory cytokines such as interleukin (IL)-6, IFN-b1, IL-1b, IL-12a, CXCL9, and CCL12 [1]. Additionally, it is hypothesized that BET inhibition can modify megakaryocyte differentiation and proliferation, which are central to the development of bone marrow fibrosis in MF [4].
Preclinical models, including MPLW515L-driven murine models of MF, have demonstrated that BET inhibition abrogates NF-κB signaling and reduces inflammatory cytokine production. Furthermore, the combination of a BET inhibitor with a JAK inhibitor yields synergistic effects, leading to a profound reduction in inflammatory cytokine levels, splenomegaly, and bone marrow fibrosis both in vitro and in vivo [1][2].
4. Structure-Activity Relationship (SAR)
The provided literature focuses extensively on the clinical efficacy, translational biology, and molecular targets of pelabresib (CPI-0610) within the context of myelofibrosis. However, the specific chemical structure, functional group modifications, and detailed Structure-Activity Relationship (SAR) data for pelabresib are not described in the provided texts. Its pharmacological profile is defined by its classification as a pan-BET inhibitor that selectively antagonizes bromodomain epigenetic readers to modulate gene transcription [1][2].
5. Current Limitations
While pelabresib represents a significant advancement, its use is associated with certain limitations and toxicities. The most common treatment-emergent adverse events (TEAEs) observed with pelabresib therapy are hematological. In monotherapy trials, thrombocytopenia occurred in 25.6% of patients (with 14% experiencing grade 3) and anemia occurred in 11.6% (with 9.3% experiencing grade 3) [1]. When used in combination with ruxolitinib in the upfront setting, anemia and thrombocytopenia were observed in 33% and 32% of patients, respectively, reaching grade 3/4 severity in 29% and 8% of cases [1].
Additionally, the efficacy of pelabresib as a monotherapy in heavily pretreated or highly refractory populations has shown limitations. In a specific cohort of patients refractory to or intolerant of JAK inhibitors, the rates of significant spleen volume reduction (≥35% SVR) and symptom reduction (≥50% TSS) were reported as 0% and 8.3%, respectively, which was considered disappointing and suggests that pelabresib may be most effective when utilized in combination regimens rather than as a standalone salvage therapy [2].
6. Future Perspectives
The future of pelabresib in the treatment of myelofibrosis is highly promising, particularly in the frontline setting. The developmental path forward for this agent is firmly rooted in combination strategies, specifically alongside ruxolitinib [2]. The ongoing phase 3 MANIFEST-2 trial is expected to provide definitive data on the efficacy and safety of this combination in JAK inhibitor-naive patients [1][4][5].
If the phase 3 data confirm the robust spleen and symptom responses, alongside the disease-modifying effects seen in phase 2 (such as the reduction of bone marrow fibrosis and mutant allele burden), pelabresib could drive a paradigm shift in MF management [4]. Moving away from sequential monotherapies, the future treatment landscape for MF is likely to mirror that of other hematologic malignancies, relying on rationally designed combination therapies to not only control symptoms but also alter the natural history and progression of the disease [1][6].
7. References
