Safety Study of Bone Marrow Transplant Using Mismatched Tissue Followed by Chemotherapy

Allogeneic blood or marrow transplantation (alloBMT), following either marrow-ablative or nonmyeloablative conditioning, is a potentially curative treatment for a variety of hematologic malignancies and non-malignant hematologic disorders. Of all the potential sources of allografts, transplantation of stem cells from a human leukocyte antigen (HLA)-matched sibling has generally produced the best overall outcomes, i.e. overall and progression-free survival. Unfortunately, only about a third of candidates for alloBMT have HLA-matched siblings. For patients who lack HLA-matched siblings, there are three alternative sources of stem cells for alloBMT: 1) volunteer unrelated donors; 2) umbilical cord blood; and 3) partially HLA-mismatched, or haploidentical, related donors. Since any patient shares exactly one HLA haplotype with each biological parent or child and half of siblings, an eligible haploidentical donor can be identified rapidly in nearly all cases. However, haploidentical BMT has been associated with significant risks of graft rejection and severe GVHD, which are manifestations of excessive alloreactivity by host and donor T cells, respectively. The risk of severe GVHD may be reduced in intensively conditioned recipients of grafts that have been rigorously depleted of mature T cells or selectively depleted of alloreactive T cells, but the risks of serious infection and death from prolonged immune compromise in these patients remains high. Cyclophosphamide(Cy) is a highly immunosuppressive antineoplastic agent that has an established role in conditioning for alloBMT. Typically, the drug is administered prior to transplantation to prevent graft rejection by suppressing the host immune system. However, pre-transplantation conditioning with Cy increases the risk of GVHD following allogeneic T cell infusion in mouse models. In contrast, administration of a properly timed, high dose of Cy after BMT inhibits both graft rejection and GVHD.