A Novel Allogeneic Transplant Conditioning Regimen Designed for Tolerance Induction in Patients with Severe Sickle Cell Disease.

Allogeneic transplantation of hematopoietic stem cells remains the only curative approach for patients with sickle cell disease (SCD), yet procedural toxicities and graft-versus-host disease limit this approach to children. We chose a low-dose radiation approach utilizing rapamycin based upon its unique ability to promote T cell tolerance even when T cells are stimulated in the presence of costimulation (Powell, et al, J Immunol. 1999). We confirmed this approach in vivo in a murine bone marrow transplantation model comparing a short course of conventional immunosuppression with cyclosporine to that with rapamycin, with long-term, high-level chimerism attained only in mice treated with rapamycin (Powell et al., manuscript submitted). We have now begun accrual to an IRB approved clinical trial testing this approach in adults with SCD and herein report the results in our first two subjects. Protocol entry criteria include those previously reported (Walters, et al, NEJM, 1996) with the addition of pulmonary hypertension defined as a tricuspid-regurgitant jet velocity (TRV) > 2.5 m/s (Gladwin et al, NEJM, 2004). Additionally, patients must have failed a 6-month course of hydroxyurea. The conditioning regimen consists of a single radiation dose of 300cGy, alemtuzumab (1mg/kg total), and oral rapamycin targeting trough levels between 10–20 ng/ml. The graft consists of unmanipulated mobilized peripheral blood progenitors obtained from an HLA-matched sibling. The first patient is a 24-year-old female referred due to recurrent transient ischemic attacks and strokes despite chronic exchange transfusions. Her initial evaluation was notable for evidence of significant hemolysis with a total bilirubin of 9.8 mg/dl, an LDH 1,172 units/L (range 113–226), an absolute reticulocyte count of 318,000/uL and an undetectable haptoglobin. Cardiac doppler-echocardiogram revealed a TRV of 3.7 m/s, consistent with severe pulmonary hypertension. The conditioning was well tolerated and the patient did not require parenteral antibiotics or nutritional support. Assessment of donor chimerism, measured by microsatellite PCR and hemoglobin electrophoresis, revealed an early peak of myeloid chimerism which has stabilized at approximately 60%, with lower levels of lymphoid chimerism at approximately 5–10% now more than 300 days post-transplant. Hemoglobin levels stabilized at 11–12 g/dL, without detectable sickle hemoglobin, now allowing for therapeutic phlebotomy. Remarkably, the LDH and TRV fell concurrently toward the normal range. Patient 2 is a 26 year old male with frequent crises requiring hospitalization twice monthly. At one month post-transplant, myeloid chimerism is 97%, with lymphoid chimerism currently unmeasurable due to lymphopenia, and similar to that seen in patient 1 at the same time point. Patient 2 also required neither parenteral antibiotics nor nutritional support. Finally, neither patient to date has developed any symptoms or signs of either acute or chronic GVHD. Thus with two patients accrued to date, we have demonstrated the ability of allogeneic HSC transplantation to achieve mixed hematopoietic chimerism without the development of GVHD and for the first time, established the reversibility of the associated pulmonary hypertension.