Comparing Cyclophosphamide and Abatacept With Standard of Care Treatment Following Stem Cell Transplantation

The experimental GVHD prophylaxis arm consists of cyclophosphamide and abatacept. Cyclophosphamide induces apoptosis of activated T cells and abatacept (CTLA4Ig) blocks activation of T cells by inhibiting the co-stimulatory signal.

Compared to the standard-of-care control arm, the experimental arm is much more convenient and expected to be associated with fewer toxicities.

In addition there is a great theoretical potential for immunological synergy between cyclophosphamide and abatacept for inducing post-transplant immunologic tolerance that clinically might translate into less GVHD without increase in relapse Patients will be randomized 1:1 to the experimental vs the standard of care arm. Randomization will be done prior to the use of any conditional therapy.

The two arms will be stratified by disease (acute leukemia vs others) and donor type (MRD vs MUD/MUD vs Haplo) in an effort to keep them balanced.

The conditioning regimen for both arms will be mainly Busulfan/Fludarabine (A Total Body Irradiation based conditioning regimen will be allowed for diseases such as ALL)

The GVHD prophylaxis regimen on the experimental arm will consist of high dose Cyclophosphamide on Days +3 and +4 followed by abatacept for 6 months.

The GVHD prophylaxis regimen on the standard of care arm will consist of methotrexate on Days +1,+3, +6 and +11 and tacrolimus for patients with a 10/10 matched related or unrelated donor and of high dose cyclophosphamide on Days +3 and +4 followed by tacrolimus and mycophenolate for patients with a haploidentical donor.