The purpose of this clinical trial is to compare drug combinations to learn which drugs work best to prevent graft-versus-host-disease (GVHD) in people who have received a stem cell transplant. The source of stem cells is from someone who is not related and has a different blood cell type than the study participant. The researchers will compare the new drug combination to a standard drug combination. They will also learn about the safety of each drug combination.

Participants will:

* Receive the standard or new drug combination after transplant
* Visit the doctor's office for check-ups and tests after transplant that are routine for most transplant patients
* Take surveys about physical and emotional well-being
* Give blood and stool samples.

This is a multi-center, randomized, double-blinded, placebo controlled trial.

This phase II trial compares the effect of ASTX727 in combination with iadademstat to ASTX727 alone in treating patients with accelerated or blast phase Philadelphia chromosome negative myeloproliferative neoplasms (MPNs). ASTX727 is a combination of two drugs, cedazuridine and decitabine. Cedazuridine is in a class of medications called cytidine deaminase inhibitors. It prevents the breakdown of decitabine, making it more available in the body so that decitabine will have a greater effect. Decitabine is in a class of medications called hypomethylation agents. It works by helping the bone marrow produce normal blood cells and by killing abnormal cells in the bone marrow. Iadademstat may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Giving ASTX727 in combination with iadademstat may be more effective than ASTX727 alone in treating patients with accelerated or blast phase Philadelphia chromosome negative MPNs.

The purpose of this study is to assess Tacrolimus/Methotrexate/Ruxolitinib versus Post-Transplant Cyclophosphamide/Tacrolimus/Mycophenolate Mofetil in Non-Myeloablative/Reduced Intensity Conditioning Allogeneic Peripheral Blood Stem Cell Transplantation