Pelabresib plus ruxolitinib for JAK inhibitor-naive myelofibrosis: a randomized phase 3 trial

Abstract

Janus kinase (JAK) inhibitors provide limited depth and durability of response in myelofibrosis. We evaluated pelabresib-a bromodomain and extraterminal domain (BET) inhibitor-plus ruxolitinib (a JAK inhibitor) compared with placebo plus ruxolitinib as first-line therapy. In this phase 3 study (MANIFEST-2), JAK inhibitor-naive patients with myelofibrosis were randomized 1:1 to pelabresib 125 mg once daily (QD; 50-175 mg QD permitted) for 14 days followed by a 7-day break (21-day cycle), or to placebo in combination with ruxolitinib 10 or 15 mg twice daily (BID; 5 mg QD-25 mg BID permitted). Primary endpoint was reduction in spleen volume of ≥35% from baseline at week 24. Key secondary endpoints were absolute change in total symptom score (TSS) and TSS50 response (≥50% reduction in TSS from baseline at week 24). The primary endpoint was met in 65.9% of patients randomized to pelabresib-ruxolitinib (n = 214) versus 35.2% to placebo-ruxolitinib (n = 216) (difference, 30.4%; 95% confidence interval (CI), 21.6, 39.3; P < 0.001). Absolute change in TSS was -15.99 versus -14.05 (difference, -1.94; 95% CI, -3.92, 0.04; P = 0.0545) and TSS50 was achieved in 52.3% versus 46.3% (difference, 6.0%; 95 CI, -3.5, 15.5) with pelabresib-ruxolitinib versus placebo-ruxolitinib. Exploratory analyses of proinflammatory cytokine amounts and bone marrow morphology showed greater improvement with the combination. Thrombocytopenia and anemia were the most common treatment-emergent adverse events, occurring in 52.8% (13.2% grade ≥3) versus 37.4% (6.1% grade ≥3) and 44.8% (23.1% grade ≥3) versus 55.1% (36.5% grade ≥3), respectively. Pelabresib in combination with ruxolitinib is well tolerated, improves signs of underlying myelofibrosis pathobiology and provides substantial clinical benefit over standard-of-care JAK inhibitor monotherapy. ClinicalTrials.gov identifier: NCT04603495 .

Fig. 1. Patient disposition.

The study opened for enrollment in November 2020; the first patient received their initial treatment on 22 April 2021, and the last patient was enrolled on 2 March 2023. Percentages reported are based on the number of patients randomized (intent-to-treat set). ^aOther (noncompliance/protocol violation (n = 2 in each arm) or withdrawal of consent (n = 11 in the pelabresib–ruxolitinib arm, and n = 4 in the placebo–ruxolitinib arm)). ^bTreatment ongoing as of 31 August 2023.

Fig. 2. Splenic response.

a, Percentage change in spleen volume from baseline at week 24, overlaid with the proportion of patients with spleen response (defined as a ≥35% reduction in spleen volume from baseline, by central read). Patients without week 24 change from baseline assessment are not shown and were considered nonresponders for spleen response. Difference between treatment arms was compared by stratified Cochran–Mantel–Haenszel test (performed two-sided Cochran–Mantel–Haenszel test at the alpha level 5%). The exact P value is 1.64 × 10⁻¹⁰. b, Kaplan–Meier time-to-event estimate for spleen response.

Fig. 3. Symptom response.

a, Absolute change in TSS score from baseline at week 24; patients without week 24 data are not shown. Change from baseline was determined by ANCOVA model using multiple imputation. Least square mean difference was determined from the ANCOVA model using baseline DIPSS, baseline platelet count and baseline spleen volume as factors and baseline TSS as covariate, performed two-sided Cochran–Mantel–Haenszel test at the alpha level 5%. b, Percentage change in TSS score from baseline at week 24, overlaid with the proportion of patients experiencing a TSS50 response. Patients without baseline and week 24 data are not shown. Difference between treatment arms was compared by stratified Cochran–Mantel–Haenszel test (performed two-sided Cochran–Mantel–Haenszel test at the alpha level 5%; weighted 95% CI adjusted across strata).

Fig. 4. Hemoglobin change over time.

Graph shows the mean change in hemoglobin over time in the overall study population and in the subgroup of patients with anemia (defined as patients with a baseline hemoglobin of <10 g dl⁻¹). Hemoglobin response is defined as a mean increase in hemoglobin from baseline of ≥1.5 g dl⁻¹ in the absence of transfusions during the previous 12 weeks (baseline hemoglobin defined as the last assessment before or on cycle 1 day 1, regardless of blood transfusions). Patients who received transfusions are included in these data. Data are presented as mean ± 95% CI. In accordance with preplanned hierarchical statistical testing, for these data, we report descriptive statistical analyses only.

Extended Data Fig. 1. MANIFEST-2 study design.

*Patients enrolled in the placebo plus ruxolitinib arm could cross over to the pelabresib plus ruxolitinib arm if progressive splenomegaly occurred at or after 24 weeks of treatment (defined as enlargement of spleen volume by at least 25% compared with baseline); there were no crossovers as of 31 August 2023. ^†The starting dose for pelabresib was 125 mg QD and protocol-defined dose modifications based on AEs and treatment response allowed a dose range between 50 mg and 175 mg QD. ^‡Ruxolitinib was started at 10 mg BID (baseline platelet count 100–200 × 109/L) or 15 mg BID (baseline platelet count >200 × 109/L) with a mandatory dose increase by 5 mg BID after one cycle and a maximum dose of 25 mg BID as per the label. AE, adverse event; BID, twice daily; CT, computed tomography; DIPSS, Dynamic International Prognostic Scoring System; ET, essential thrombocythemia; Int-1, Intermediate-1; Int-2, Intermediate-2; MRI, magnetic resonance imaging; MFSAF, Myelofibrosis Symptom Assessment Form; PV, polycythemia vera; QD, once daily; TSS, total symptom score; TSS50, ≥50% TSS reduction from baseline.

Extended Data Fig. 2. Spleen (central read) response and symptom score at week 24 according to pre-defined subgroups.

A) Shows spleen response (defined as ≥35% reduction in spleen volume), according to subgroups. (B) Shows the mean absolute change in TSS from baseline to week 24, according to subgroups. (C) Shows the TSS50 response rate at week 24, according to subgroups. Data are presented as mean value ± 95% CI. CI, confidence interval; DIPSS, Dynamic International Prognostic Scoring System; LS, least squares; MF, myelofibrosis; PET-MF, post-essential thrombocythemia myelofibrosis; PMF, primary myelofibrosis; PPV-MF, post-polycythemia vera myelofibrosis; Resp, number of responders; TSS, total symptom score; TSS50, ≥50% TSS reduction from baseline.

Extended Data Fig. 3. Duration of spleen and TSS50 responses.

(A) Shows Kaplan–Meier estimate of duration of spleen response by central read, defined as the time from first spleen response ( ≥ 35% reduction in spleen volume) until a spleen volume reduction of <35% from baseline and an increase of >25% from nadir was first documented. Median duration of response using Kaplan–Meier was not estimable in either treatment arm at the data cut-off. The median duration of follow-up of splenic response was 27.143 weeks (95% CI: 24.143, 36.143) in the pelabresib–ruxolitinib arm and 25.429 weeks (95% CI: 24.143, 35.857) in the placebo–ruxolitinib arm. (B) Shows Kaplan–Meier estimate of the duration of symptom response, defined as the time from onset of TSS50 response until the time at which a < 50% reduction in TSS from baseline and an increase of ≥25% from nadir is first observed. These data are not mature. TSS, total symptom score; TSS50, ≥50% TSS reduction from baseline.

Extended Data Fig. 4. Dual spleen and symptom responses.

SVR35, ≥35% reduction in spleen volume; TSS50, ≥50% reduction in total symptom score.

Extended Data Fig. 5. Pro-inflammatory cytokines, bone marrow morphology, and erythrocyte progenitors.

For exploratory endpoints, we report descriptive statistical analyses only. (A) shows the mean change in inflammatory cytokines from baseline at week 24. Data are presented as mean change from baseline ± 95% CI. *NF-κB – set included B2M, CRP, CD40-L, hepcidin, IL-6, IL-12p40, MIP-1 beta, MPIF-1, RANTES, TNFR2, TNF, and VCAM-1. (B) shows the secondary endpoint change in reticulin fibrosis grade (graded according to the WHO classification) by central read at week 24. Percentages are based on the number of patients with bone marrow assessments in the intent-to-treat population at each timepoint; non-evaluable patients are those who are ongoing study treatment and have not yet reached that timepoint or opted out of bone marrow assessment. (C) shows the change in megakaryocytes (CD61 + ) density from baseline in the bone marrow at week 24, overlayed with individual observations. Density assessed by digital pathology. The horizontal line within the boxplot indicates the mean. The lower and upper ends of the box represent the 25th and 75th percentiles, respectively. The boxplot whiskers indicate the Tukey interval. (D) shows examples of immunohistochemistry stainings for megakaryocytes (CD61 + ) representative for observed mean changes of megakaryocyte densities in samples from a single patient randomized to pelabresib–ruxolitinib and a single patient randomized to placebo-ruxolitinib, respectively. There were 117 patients in the pelabresib–ruxolitinib arm and 129 patients in the placebo-ruxolitinib arm with megakaryocyte density data. Immunohistochemistry staining was performed, and digital images analyzed by an automated quantitative cell-specific detection analysis. (E) shows the percentage change in erythrocyte progenitors (CD71+ cells) in the bone marrow according to treatment arm and red blood cell transfusions, overlayed with individual observations. Red blood cell transfusions refer to the number of patients who received any red blood cell transfusion between week 20 and week 24 on treatment. The horizontal line within the boxplots indicates the mean. The lower and upper ends of the box represent the 25th and 75th percentiles, respectively. The boxplot whiskers indicate the Tukey interval. The y-axis encompasses the 99 percentiles of all values. B2M, beta-2 microglobulin; CI, confidence interval; CRP, C-reactive protein; CD40-L, CD40 ligand; IL, interleukin; MIP, macrophage inflammatory protein; MPIF, myeloid progenitor inhibitory factor; NF-κB, Nuclear factor kappa B; RANTES, regulated upon activation, normal T-cell expressed and secreted; TNF, tumor necrosis factor; TNFR, TNF receptor; VCAM, vascular cell adhesion molecule; WHO, World Health Organization.

Extended Data Fig. 6. Change in pro-inflammatory cytokine levels according to spleen response ( ≥ 35% reduction in spleen volume).

Figure shows the mean relative difference in inflammatory cytokine changes from baseline at week 24 in patients with spleen response compared to patients without spleen response. Data are presented as mean relative difference ± 95% CI. For exploratory endpoints, we report descriptive statistical analyses only. *NF-κB set included B2M, CRP, CD40-L, hepcidin, IL-6, IL-12p40, MIP-1 beta, MPIF-1, RANTES, TNFR2, TNF, VCAM-1. B2M, beta-2 microglobulin; CI, confidence interval; CRP, C-reactive protein; IL, interleukin; MIP, macrophage inflammatory protein; MPIF, myeloid progenitor inhibitory factor; NF-κB, nuclear factor kappa B; RANTES, regulated upon activation, normal T cell expressed and secreted; TNF, tumor necrosis factor; TNFR, TNF receptor; VCAM, vascular cell adhesion protein.

Extended Data Fig. 7. Impact of driver and high-molecular risk mutations at baseline on clinical response rates at week 24.

Data are presented as mean value ± 95% CI. *HMR mutations include ASXL1, EZH2, IDH1, IDH2, SRSF2, and U2AF1 mutations. ^†Clopper–Pearson 95% CI. ASXL1, ASXL transcriptional regulator 1; CALR, calreticulin; CI, confidence interval; EZH2, enhancer of zeste 2 polycomb repressive complex 2 subunit; HMR, high molecular risk; IDH1/2, isocitrate dehydrogenase 1/2; JAK2, Janus kinase 2; MPL, MPL proto-oncogene, thrombopoietin receptor; SRSF2, serine and arginine rich splicing factor 2; SVR35, ≥35% reduction in spleen volume from baseline; U2AF1, U2 small nuclear RNA auxiliary factor 1; WT, wild type.