JAK2 and myelofibrosis: Also, in a phase II trial in patients with myelofibrosis, a reduction in spleen size was observed in some patients by week 12, however, gastrointestinal adverse events were very common (50%) and resulted in the study discontinuation.19, 28, 32 Pacritinib (SB1518) is another dual JAK2/FLT3 inhibitor that has been approved for the treatment of intermediate- and high-risk myelofibrosis.