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

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 .