Partially HLA Mismatched (Haploidentical) Allogeneic Bone Marrow Transplantation

Allogeneic stem cell transplantation is a potentially curative treatment for patients with hematologic malignancies such as leukemia, lymphoma and myeloma who are at high risk of relapse. As well as the potential to deliver high doses of chemotherapy or radiation this procedure affords the benefit of an immunologic weapon against disease in the form of the graft versus tumor effect. The major variables affecting the outcome of allogeneic transplant include: patient selection (age and comorbidities); disease status at the time of transplantation (remission vs. active disease); type of donor (HLA matched vs. mismatched or related vs. unrelated); type of conditioning regimen; source of stem cells (bone marrow vs. peripheral blood). Recent advances in the field of stem cell transplant have substantially lowered transplant related morbidity and mortality. The availability of stem cell transplant as a treatment modality is dependent upon the availability of a suitable donor. Best outcomes are thought to occur in HLA matched sibling donors. The chance of a sibling being HLA matched is approximately 25%. Despite the development of large worldwide donor registries the likelihood of finding an HLA matched unrelated donor is 60-70% at best and drops to 10% in some ethnic minorities. In addition the process of identifying, confirming and harvesting an unrelated donor is cumbersome and time consuming at a time when the patient must proceed immediately to transplant (time from initiation of search to identification of an unrelated donor identification takes a median of 49 days). Therefore the development of alternative sources of hematopoietic stem cells is an area of immense interest to many investigators. Alternative sources include cord blood and HLA haploidentical donors. Cord blood has been an attractive source permitting immediate availability and possibly a lower rate of graft versus host disease (GVHD). However delay in engraftment, particularly after myeloablative conditioning, remains a significant disadvantage. Haploidentical transplants carry some of the same advantages with virtually all patients having immediate access to a suitable and willing donor in the form of a partially HLA matched family member. Furthermore the number of potential donors allows for donor selection based upon factors such as age, gender, KIR alloreactivity. Finally the donor is readily available for future cellular therapies such as donor lymphocyte infusion. Early attempts at haploidentical transplantation were hampered by high rates of graft failure and severe graft versus host disease. Recent advances in graft versus host disease prophylaxis with post transplant high dose cyclophosphamide (Cy) have overcome these barriers to a large degree. Published studies have shown that HLA-haploidentical bone marrow transplant (BMT) after non-myeloablative conditioning and using 2 doses of post-transplantation Cy followed by tacrolimus and mycophenolate mofetil (MMF) is a well-tolerated procedure. However, the major cause of treatment failure in this high-risk population was early relapse. As conditioning intensity has been clearly linked to rates of relapse in multiple diseases, it is postulated that utilizing conditioning with higher anti-tumor potential will lead to a lower relapse rate. Given the advances in GVHD prophylaxis with post-transplantation Cy, reduced intensity conditioning with Fludarabine, Busulfan and total body irradiation combined with high-dose post-transplantation Cy is the platform for this study. The toxicities of this reduced intensity conditioning regimen are not expected to differ substantially from previous data incorporating post-transplantation Cy. However, this regimen may have higher anti-tumor potential resulting in a decreased relapse rate.