The purpose of this study is to assess the initial tolerability, safety and recommended phase 2 dose of vedolizumab intravenous (IV) administered for GvHD prophylaxis along with standard GvHD prophylaxis therapy (in participants undergoing allogeneic hematopoietic stem cell transplantation [allo-HSCT]).

The drug being tested in this study is called Vedolizumab. Vedolizumab (also called MLN0002) is approved for the treatment of adult participants with moderately to severely active ulcerative colitis (UC) and Crohn's disease (CD) who achieved an inadequate response, had a loss of response, or were intolerant to conventional and/or biologic treatments. This study will look at the tolerability and pharmacokinetics of Vedolizumab in participants undergoing allo-HSCT when added to standard GvHD prophylaxis (tacrolimus plus short-term methotrexate) for the prevention of acute GvHD (major complication in allo-HSCT).

The study will enroll approximately 36 participants. Participants will be assigned to different dose-escalating cohorts in order to find out the recommended phase 2 dose (RP2D) of the study:

• Cohort 1: Vedolizumab 75 mg
• Cohort 2: Vedolizumab 300 mg
• Cohort 3: Vedolizumab Dose 1

All participants have to receive 1 injection of Vedolizumab on Day -1 before allo-HSCT and on Days 13 and 42 after allo-HSCT. If none of the participants receiving vedolizumab at 75 mg experience dose-limiting toxicities (DLTs), dose escalation will continue to 300 mg on Day -1 before allo-HSCT and on Days +13 and +42 after allo-HSCT. If the first 3 participants at 300 mg tolerate the treatment without experiencing DLTs, then the decision on whether to increase the vedolizumab IV dose in the next cohort will be guided by the PK results.

Cohorts will be escalated in same manner until the identification of RP2D. The cohort at that dose level may be expanded to include approximately 18 additional participants undergoing myeloablative conditioning or reduced-intensity conditioning (RIC) and receiving either related or unrelated allo-HSCT for the treatment of hematologic malignancies or myeloproliferative neoplasms. This group of participants will allow the further assessment of the tolerability and clinical activity of vedolizumab.

This multi-center trial will be conducted in the United States. The overall time to participate in this study will be approximately 2 years. Following the treatment period, participants who remain in remission will be followed for development of acute and chronic GvHD and safety during clinic visits at 4, 5, 6, 9, and 12 months after allo-HSCT or until death or withdrawal of consent or termination of the study by the sponsor. Participants who complete the study will attend a 12-month follow-up visit. Patients who have been discontinued from treatment will attend an end of treatment visit 30 to 40 days after the last dose of study drug using all study procedures outlined for the 12-month follow-up visit.

Inclusion Criteria:

1. Is undergoing matched or single-antigen mismatched unrelated-donor myeloablative transplant for the treatment of acute lymphoblastic leukemia (ALL) or acute myeloid leukemia (AML); Is less than or equal to (<=) 60 years of age.

   For the cohort after RP2D

2. Is undergoing matched or single-antigen mismatched related or unrelated-donor transplant and receiving myeloablative conditioning or RIC for the treatment of hematologic malignancies or myeloproliferative neoplasms; Is less than or equal to (<=) 75 years of age.

Exclusion Criteria:

1. Has received prior allogeneic transplants or who are planned to undergo umbilical cord blood transplant, receive ex vivo T-cell-depleted hematopoietic stem cells (HSCs), received any in vivo T-cell depleting antibodies, or non-myeloablative conditioning.
2. Has active cerebral/meningeal disease, active cytomegalovirus (CMV) colitis, or signs and symptoms of progressive multifocal leukoencephalopathy (PML) or any history of PML.
3. Is undergoing transplant for the treatment of nonmalignant hematological disorders (for example: aplastic anemia, sickle cell anemia, thalassemias, Fanconi anemia).

Takeda makes patient-level, de-identified data sets and associated documents available after applicable marketing approvals and commercial availability have been received, an opportunity for the primary publication of the research has been allowed, and other criteria have been met as set forth in Takeda's Data Sharing Policy (see www.TakedaClinicalTrials.com/Approach for details). To obtain access, researchers must submit a legitimate academic research proposal for adjudication by an independent review panel, who will review the scientific merit of the research and the requestor's qualifications and conflict of interest that can result in potential bias. Once approved, qualified researchers who sign a data sharing agreement are provided access to these data in a secure research environment.

The population of participants evaluable for vedolizumab safety was defined as all participants who received any amount of vedolizumab intravenously.

TEAEs are adverse events that started after the first dose of study drug and no more than Week 18 after the last dose of study drug

At each visit the investigator had to document any occurrence of adverse events and abnormal laboratory findings. Any event spontaneously reported by the participant or observed by the investigator was recorded, irrespective of the relation to study treatment.