Pacritinib

Background: Myelodysplastic syndrome (MDS) and myelodysplastic/myeloproliferative neoplasm (MDS/MPN) are blood disorders that can cause serious complications in children and adults. MDS and MDS/MPN can also progress to acute myeloid leukemia. Treatments for these disorders are risky and not always effective. Better treatments are needed. Objective: To test a study drug (pacritinib) in adults and children with MDS or MDS/MPN. Eligibility: Children (aged 12 to 17 years) and adults (aged 18 years and older) with MDS or MDS/MPN. Design: Participants will be screened. They will have a physical exam with blood tests. They will have tests of their heart function. They may have a bone marrow biopsy: An area over the hip will be numbed; a needle will be inserted to remove a sample of soft tissue from inside the hipbone. Pacritinib is a capsule taken by mouth. All participants will take the study drug 2 times a day, every day, in 28-day cycles. They will write down the date and time they take each capsule. Doctors will assign varying dosages of the drug to different participants. Participants will have clinic visits each week during cycle 1; every 2 weeks during cycle 2; and gradually increasing to every 3 months after cycle 13. Treatment will continue for up to 8 years. Bone marrow biopsies, heart tests, and other tests will be repeated at intervals throughout the study. Participants will also fill out questionnaires about their quality of life, the symptoms of their disease, and other topics.