A Platform Protocol to Investigate Post-Transplant Cyclophosphamide-Based Graft-Versus-Host Disease Prophylaxis in Patients With Hematologic Malignancies Undergoing Mismatched Unrelated Donor Peripheral Blood Stem Cell Transplantation

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.