BackgroundMyelofibrosis (MF) is characterized by splenomegaly, MF-associated symptoms, cytopenias (eg, anemia), and impairment of the bone marrow (BM) microenvironment (including fibrosis). Pelabresib (CPI-0610; PELA) is an investigational, oral, small molecule drug that inhibits BET proteins and subsequent BET-mediated gene expression involved in MF pathogenesis. The Phase 3 MANIFEST-2 study (NCT04603495) met its primary endpoint, showing a statistically significant higher proportion of patients (pts) with ≥35% reduction in spleen volume (SVR35) from baseline (BL) at Week (Wk) 24 with PELA+ruxolitinib (RUX) vs placebo (PBO)+RUX (p<0.001) in Janus kinase inhibitor-naïve pts with MF. PELA+RUX also showed a trend toward improved total symptom score (TSS) from BL at Wk 24, as well as improvements in multiple measures of anemia (eg, hemoglobin [Hb] response) and in the BM microenvironment, vs PBO+RUX (Rampal R, et al. Presented at ASH 2023 [Oral 628]).AimTo present follow-up efficacy and safety outcomes at Wk 48 from the Phase 3 MANIFEST-2 study.MethodsEligible pts had a DIPSS ≥intermediate-1 risk, platelet count ≥100 × 109/L, spleen volume ≥450 cm3, ≥2 symptoms with an average score ≥3 or TSS ≥10 by MF Symptom Assessment Form v4.0, peripheral blast count <5%, and ECOG PS ≤2. Pts were randomized 1:1. PELA or PBO was administered once daily for 14 consecutive days of 21-day cycles in combination with RUX, which was administered twice daily for 21-day cycles. Primary endpoint was SVR35 at Wk 24. Key secondary endpoints were absolute change in TSS and ≥50% reduction in TSS from BL (TSS50) at Wk 24. Other prespecified endpoints included SVR35 at Wk 48, absolute change in TSS and TSS50 at Wk 48, Hb response (≥1.5 g/dL mean increase from BL without transfusions in the prior 12 wks), BM fibrosis (BMF), and safety. Mutation profiles were assessed by next-generation sequencing. Informed consent was obtained from all pts.ResultsAs of March 29, 2024, all pts assessed had been followed for ≥48 wks; 58.9% (126/214) and 62.0% (134/216) of pts continued on double-blind treatment in the PELA+RUX and PBO+RUX arms, respectively. Preliminary data are presented here; analyses are ongoing and data points are subject to change. At Wk 48, 56.5% (121/214) vs 37.5% (81/216) of pts had SVR35 responses in the PELA+RUX vs PBO+RUX arms, showing sustained benefit beyond Wk 24. SVR35 responders at any time were 82.2% (176/214) vs 58.3% (126/216) in the PELA+RUX vs PBO+RUX arm; loss of SVR35 response was observed in 13.1% (23/176) vs 19.8% (25/126) of pts in the PELA+RUX vs PBO+RUX arm. At Wk 48, least squares mean (standard error) absolute change in TSS was −16.24 (1.133) vs −14.11 (1.085) in the PELA+RUX vs PBO+RUX arms, with a greater difference in absolute TSS observed between treatment arms at Wk 24 and Wk 48 for pts with higher symptom burden at BL. TSS50 response at Wk 48 was 45.3% (97/214) vs 39.4% (85/216) with PELA+RUX vs PBO+RUX. Dual SVR35 and TSS50 response was observed in 36.0% (77/214) vs 19.0% (41/216) of pts in the PELA+RUX vs PBO+RUX arms at Wk 48. Hb response was observed in 12.6% (27/214; 95% confidence interval [CI], 8.17-17.07) vs 6.9% (15/216; 95% CI, 3.55-10.33) of pts in the PELA+RUX vs PBO+RUX arms, with differences between arms in mean Hb levels maintained at Wk 48; in pts with anemia (Hb BL <10 g/dL), Hb response was observed in 17.9% (12/67; 95% CI, 8.73-27.09) vs 14.1% (10/71; 95% CI, 5.99-22.18) of pts. The trend of higher rate of BMF improvement of ≥1 grade in the PELA+RUX arm vs the PBO+RUX arm continued to be observed at Wk 48. Of 426 pts evaluated for safety, ≥1 treatment-emergent adverse event (TEAE) was reported in 97.6% vs 96.7% of pts in the PELA+RUX vs PBO+RUX arms; Grade ≥3 events were reported in 56.6% vs 62.1% of pts. Rates of the most common TEAEs (≥10%) at Wk 48 in the PELA+RUX vs PBO+RUX arms were similar to rates at Wk 24. Updated efficacy and safety results, including data on BMF, mutational profile, and leukemic transformation, will be presented.ConclusionAt Wk 48, PELA+RUX continued to show improvements in spleen volume, TSS, multiple measures of anemia, and the BM microenvironment vs PBO+RUX, impacting the four hallmarks of MF. These data suggest that PELA+RUX could lead to more profound and sustained responses in pts with MF vs PBO+RUX.