Pacritinib (SB1518) is a Janus kinase 2 (JAK2), JAK2(V617F), and Fms-like 
tyrosine kinase 3 inhibitor that does not inhibit JAK1. It demonstrated a 
favorable safety profile with promising efficacy in phase 1 studies in patients 
with primary and secondary myelofibrosis (MF). This multicenter phase 2 study 
further characterized the safety and efficacy of pacritinib in the treatment of 
patients with MF. Eligible patients had clinical splenomegaly poorly controlled 
with standard therapies or were newly diagnosed with intermediate- or high-risk 
Lille score. Patients with any degree of cytopenia were eligible. Thirty-five 
patients were enrolled. At entry, 40% had hemoglobin <10 g/dL and 43% had 
platelets <100 000× 10(9)/L. Up to week 24, 8 of 26 evaluable patients (31%) 
achieved a ≥35% decrease in spleen volume determined by magnetic resonance 
imaging and 14 of 33 (42%) attained a ≥50% reduction in spleen size by physical 
examination. Median MF symptom improvement was ≥50% for all symptoms except 
fatigue. Grade 1 or 2 diarrhea (69%) and nausea (49%) were the most common 
treatment-emergent adverse events. The study drug was discontinued in 9 patients 
(26%) due to adverse events (4 severe). Pacritinib is an active agent in 
patients with MF, offering a potential treatment option for patients with 
preexisting anemia and thrombocytopenia. This trial was registered at 
www.clinicaltrials.gov as #NCT00745550.