Background: PAC is a potent JAK2 inhibitor without significant JAK1 inhibition with minimal myelosuppression in early-phase studies in MF. Methods: The efficacy and safety of daily oral PAC was compared to BAT (2:1 randomization stratified for risk and platelet count). The 1⁰ endpoint was the proportion of ITT patients (pts) achieving ≥ 35% spleen volume reduction (SVR) at week (wk) 24 by centrally reviewed MRI or CT. Secondary endpoints included the proportion achieving ≥ 50% reduction in total symptom score (TSS) at wk 24 using the MPN Symptom Assessment Form. Results: Patients:327 were enrolled (PAC:220, BAT:107), 62% with 1⁰ MF. Median time from diagnosis was 1.12 years (PAC 0.99, BAT 1.60): 32% and 15% had a platelet counts < 100,000/µL or <50,000/ µL; 75% were JAK2V617F positive. Efficacy: The median duration of treatment was 16.2 months PAC and 5.9 months BAT. Sixty-two percent of BAT patients received active disease directed therapy. The SVR rates at week 24 were 19.1% for PAC vs. 4.7% for BAT (p=0.0003) in ITT and 25% vs. 5.9% (p=0.0001) in the evaluable population. 79% of BAT patients crossed over to PAC; 21% had achieved a >35% reduction in spleen volume at data cutoff. TSS composite V1 + V2 response rates were 24.5%for PAC vs. 6.5% for BAT (p<0.0001) by ITT, and were 40.9% vs. 9.9% in evaluable pts (p<0.0001). Efficacy with baseline cytopenias: In pts with <100,000 and <50,000 platelets/μ/L, the SVR rates were 16.7% for PAC vs. 0% for BAT (p=0.009), and 22.9% vs. 0% (p=0.045) by ITT and 23.5% vs. 0% (p=0.007) and 33.3% vs. 0% (p=0.037) in evaluable pts. In RBC transfusion dependent pts, 25.7% of PAC pts became RBC independent vs. 0% of BAT pts (p=0.043). Safety: The most common adverse events (AE) for PAC were diarrhea, nausea, and vomiting; (grade 3 were <5%, <1%, <1% respectively). Hematologic AEs were similar between PAC and BAT. Conclusions: This study demonstrated PAC was well tolerated and induced significant and sustained SVR and symptom control even in patients with severe thrombocytopenia. PAC therapy resulted in RBC transfusion independence in a significant proportion of pts. Clinical trial information: NCT01773187