Pacritinib Before Transplant for Myeloproliferative Neoplasms (MPN)

Study Drug Administration: If you are found to be eligible to take part in this study, you will take pacritinib at about the same time each day by mouth, 2 times each day. Your doctor will tell you when to start and stop taking pacritinib. You may be able to take the drug for about 2-6 months depending on how you tolerate the drug and when your transplant date is. If you do not receive your transplant, you may be able to continue taking the study drug as long as the doctor thinks it is in your best interest. You must swallow the capsules whole with a glass (about 8 ounces) of water. Do not open, break, or chew the capsules. If you vomit or miss a dose of pacritinib, take your next dose of pacritinib at your regular time. Do not "make up" a missed or vomited dose. You will be given a study drug diary to write down what time you take each dose of pacritinib. You need to bring the study drug diary, any leftover study drug, and any empty study drug containers with you to each study visit. The dose of pacritinib you receive may be lowered or stopped, if the doctor thinks it is needed. About 21 days after your last dose of pacritinib, you will given standard of care drugs and you will have an allogeneic stem cell transplant. Your doctor will explain this treatment and the stem cell transplant to you in more detail. You will be required to sign a separate consent form. Study Visits: One (1) time each month: * You will have a physical exam. * Blood (about 2 teaspoons) will be drawn for routine tests and to check your kidney and liver function. * You will have an electrocardiogram (EKG -- a test that measures the electrical activity of the heart). On Day 14 (+/- 2 days) of of Cycle 1, blood (about 2 teaspoons) will be drawn for routine tests and to check your kidney and liver function. You can have this blood drawn at a local lab or clinic that is closer to your home. The results will be sent to the study doctor at MD Anderson. During Week 2 of Cycle 1, a member of the study staff will call to ask you about any symptoms you may be having. This call should last about 5-10 minutes. Length of Study: You will be on study for up to 1 year after the transplant. You may be taken off study early if the disease gets worse, if you have any intolerable side effects, of if you are unable to follow study directions. Your participation on this study will be over after about 1 year of follow-up tests. End-of-Study Visit: Within about 7 days after your last dose of pacritinib, but before your stem cell transplant: * You will have a physical exam and an ultrasound, MRI, or CT scan of your abdomen to measure your liver and spleen. * You will have an EKG. Before your transplant, you will have a bone marrow biopsy/aspiration to check the status of the disease. Follow-Up Tests: You will have follow-up visits at about 1, 3, 6, and 12 months after the transplant: * You will complete 3 questionnaires about your symptoms and quality of life. It should take about 20-30 minutes to complete the questionnaires. * At Month 3, you will have a physical exam and an ultrasound, MRI, or CT scan of your abdomen to measure your liver and spleen. This will be repeated at Month 12, if your doctor thinks it is needed. * At Months 3 and 12, you will have a bone marrow biopsy/aspiration to check the status of the disease. This is an investigational study. Pacritinib is not FDA approved or commercially available. It is currently being used for research purposes only. The study doctor can explain how the study drug is designed to work. Up to 40 participants will be enrolled in this study. All will take part at MD Anderson.