Background: MMB, an oral JAK1/2 and ACVR1/ALK2 inhibitor, showed clinical activity on MF symptoms, RBC transfusion requirements (anemia), and spleen volume in the SIMPLIFY trials, including in MF patients (pts) with thrombocytopenia. MOMENTUM is a pivotal phase 3 study of symptomatic and anemic MF pts previously treated with a JAK inhibitor (JAKi) testing MMB vs DAN. This analysis evaluated MOMENTUM pts with baseline (BL) platelet counts (PLT) ≤150 x 10⁹/L. Methods: Eligibility: Primary or post-ET/PV MF; DIPSS high risk, Int-2, or Int-1; MF Symptom Assessment Form Total Symptom Score (MFSAF TSS) ≥10; Hgb <10 g/dL; prior JAKi for ≥90 days, or ≥28 days if RBC transfusions ≥4 units in 8 weeks (wks) or Gr 3/4 thrombocytopenia, anemia, or hematoma; palpable spleen ≥5 cm; PLT ≥25 x 10⁹/L. JAKi taper and washout was ≥21 days. Randomization: 2:1 to MMB 200 mg QD plus DAN placebo or DAN 600 mg QD plus MMB placebo for 24 wks. Primary endpoint: TSS response (≥50% reduction from BL) rate at wk 24. Key secondary endpoints, assessed sequentially at wk 24: RBC transfusion independence (TI) rate, splenic response rate (SRR; ≥25% reduction in volume from BL), change from BL in TSS, SRR (≥35% reduction from BL) and rate of zero transfusions since BL. Results: 60 (74%) of 81 MMB pts and 25 (58%) of 43 DAN pts with BL PLT ≤150 x 10⁹/L completed the 24-week randomized treatment (RT) phase. Median BL TSS were 29 (MMB) and 24 (DAN), Hgb were 7.9 (MMB) and 8.0 (DAN) g/dL, and PLT were 67 x 10⁹/L (MMB) and 64 x 10⁹/L (DAN). Prior JAKi was ruxolitinib in 124 pts (100%) and fedratinib in 6 pts (5%). Efficacy results are in Table. These results are consistent with the overall ITT analysis set (N=195). Most common Gr ≥3 TEAEs in the RT phase were thrombocytopenia (MMB, 31%; DAN, 16%) and anemia (MMB, 7%; DAN, 14%); Gr ≥3 bleeding events occurred in 9% of MMB and 5% of DAN pts. TEAEs led to study drug discontinuation in 15% of MMB and 19% of DAN pts in RT phase. A trend toward improved OS up to wk 24 was seen with MMB vs DAN [HR (95% CI)=0.490 (0.195, 1.235)]. Additional analyses of pts with BL PLT <100 x 10⁹/L (N=100) and BL PLT <50 x 10⁹/L (N=31) show similar treatment effects of MMB vs DAN. Conclusions: In thrombocytopenic MF pts who were symptomatic and anemic, MMB was superior to DAN for symptom responses, transfusion requirements, and spleen responses and showed comparable safety and favorable survival. MMB may address a critical unmet need in thrombocytopenic MF pts. Clinical trial information: NCT04173494.

*Least-squares mean from mixed model for repeated measures.