Our ongoing nonmyeloablative allogeneic peripheral blood stem cell (PBSC) transplant protocol (03-H-0170) for patients with severe sickle cell disease (SCD) and B-thalassemia from HLA-matched family donors has excellent results thus far. Our long term leukocyte engraftment rate is 85-90% with the same disease-free survival. None of the engrafted patients had acute sickle-related events, significant toxicity associated with the conditioning regimen, or any evidence of graft versus host disease (GVHD).
While these results rival the transplant outcomes from low risk transplant patients with B-thalassemia, there are areas for improvement. The first is the 10-15% graft rejection rate, where a majority of these individuals were male donor and female recipient pairs. Another limitation is the significant delay in donor red cell engraftment in one recipient who had pre-existing allo-antibody to donor red cells from previous transfusions. Also we have excluded another group of individuals with preformed antibodies, recipients having major ABO incompatibility to the donors.
To overcome these limitations (and reduce the transplant failure rate) in this new protocol, we will continue our nonmyeloablative approach in the patients with SCD and B-thalassemia with HLA-matched family donors, but using an increased intensity regimen in a subset considered at high risk for transplant failure. This modified regimen consists of pentostatin and oral cyclophosphamide, which we hypothesize will reduce both the T cells that mediate leukocyte rejection and the B/plasma cells that produce anti-donor erythrocyte antibodies. The main transplant backbone will remain as alemtuzumab, low dose total body irradiation of 300 cGy, and sirolimus; the transplant graft will remain as unmanipulated G-CSF mobilized, T-cell replete, PBSC product for hematopoietic and lymphoid reconstitution.
The primary endpoint of this study is the percentage/number of patients who have sustained donor type hemoglobin at 1 year post transplant for male donors female recipients. The primary endpoint for those with pre-existing antibodies is the presence of donor red cells with reticulocytes greater than or equal to30 k/uL at 2 years post-transplant. Other endpoints include the toxicity of the pentostatin-cyclophosphamide regimen, the degree of donor-host chimerism necessary for long-term graft survival and disease amelioration, incidence of acute and chronic GVHD, incidence of graft rejection, transplant-related morbidity, as well as disease-free and overall survival. Since SCD and B-thalassemia are non-malignant disorders of red cells, severe GVHD, lack of donor erythrocyte (prolonged donor red cell aplasia), or graft rejection is collectively considered transplant failure.