A Pilot Study of Reduced Toxicity Conditioning (RTC) with Busulfan (Bu), Fludarabine (Flu), and Alemtuzumab Followed by Allogeneic Stem Cell Transplantation (AlloSCT) to Induce Sustained Mixed Donor Chimerism In Patients with Symptomatic Sickle Cell Disease (SCD)

Abstract 3504

The only known curative therapy for patients with SCD is a matched family donor AlloSCT (Walters et al, NEJM, 1996). Complications after myeloablative AlloSCT are numerous and include death, graft versus host disease (GVHD), and infections, making reduced toxicity conditioning regimens more appealing. A major obstacle allowing most SCD patients to undergo SCT is the lack of an unaffected, HLA-matched identical sibling (Bhatia et al, BMT, 2008). Umbilical cord blood (UCB) has been proven to be an excellent alternate donor source that can safely reconstitute hematopoiesis after AlloSCT in pediatric recipients with non-malignant conditions (Cairo et al, BBMT, 2008). In this study, we report the results of a RTC regimen followed by an AlloSCT from matched family and unrelated UCB donors in selected patients with symptomatic SCD. Between August 27, 2004 and March 2, 2010, 18 patients (14M:2F) with symptomatic SCD (HbSS=10, HbSC=4, HbSßThal=4) underwent an AlloSCT. Indications for SCT included: ACS (n=8), CVA (n=2), multiple VOC (n=9), splenic sequestration (n=6) and retinopathy (n=1). Conditioning was Bu (4mg/kg × 4d < 4 yrs and 12.8mg/kg × 4d > 4 yrs), Flu (30mg/m² × 6d), and Alemtuzumab (2mg/m² × 1d, 6mg/m² × 2d, and 20mg/m² × 2d). Median age was 6.29 yrs(1.3–19.2). Median follow-up was 26 months. Donor sources: 8-6/6 HLA-matched sibling bone marrow (BM), 2–6/6 sibling UCB, 1–6/6, 4–5/6 and 3–4/6 unrelated UCB. Donors were HbAA (n=13), HbAC (n=1), HbAS(n=3), or ßThal trait (n=1). Sibling BM recipients received a median total nucleated cell (TNC) dose/kg of 7.16 × 10⁸ (range 2.27–11.31) and a median CD34 cell dose/kg of 5.38 × 10⁶ (range 1.50–6.83). Recipients of related or unrelated UCB received a median TNC dose/kg of 4.35 × 10⁷ (range 3.40–9.10) and a median CD34 cell dose/kg of 2.21 × 10⁵ (range 0.60–7.94). All received tacrolimus and mycophenolate mofetil as GVHD prophylaxis (Bhatia/Cairo et al, BBMT, 2009) and phenytoin or keppra as seizure prophylaxis for 180 days post SCT. Patients receiving sibling donor AlloSCT had a median time to neutrophil engraftment of 16 days (range 13–41). Among evaluable unrelated UCBT recipients, median time to neutrophil engraftment was 34 days (range 27–47). Four patients (all unrelated UCBT) experienced primary graft failure at day +60 post-SCT secondary to CMV (n=2), adenovirus, and low Bu Css levels with early withdrawal of MMF. Patients with sibling donors had a median time to platelet engraftment of 26 days (range 17–75). Of evaluable unrelated UCB recipients, median time to platelet engraftment was 54 days (range 43–70). Patients achieved mean whole blood donor chimerism of 71.0, 79.6, 85.7, 92.9, 90.7% and 93.2% at days 30, 60, 100, 180, 365 and 730 post-transplant, respectively. Mean erythroid (CD71) donor chimerism at these time points was 78.1, 81.1, 86.6, 90.5, 87.9% and 94.0%, respectively. Patients with non-sickle cell trait donors had median HbS levels of 0% at 1 yr (n=8) and 2 yrs (n=3) post-SCT; those with sickle cell trait donors had median HbS levels of 38.3% at 1 yr (n=3) and 39% at 2yrs (n=3) post-SCT. Among evaluable patients, the Kaplan-Meier probability of grade II-IV acute GVHD is 33.3% and of chronic GVHD is 8.3%. Among the ten donor-recipient pairs in which at least one member of the pair had CMV positivity, four experienced CMV reactivation on days +13, +26, +30 and +32 (BM: n=2, unrelated UCB: n=2). Of the four patients with primary graft failure (all unrelated UCBT recipients), one resumed chronic transfusions, two died of complications from disseminated CMV and adenoviral infections, and one received a second myeloablative SCT 1 yr after initial SCT and died of fungal sepsis. Among all patients, the Kaplan-Meier probability of OS is 81.8% (100% with sibling donors, 62.5% with UCB donors) and of EFS is 77.8% (100% with sibling donors, 50% with UCB donors) with none of these patients showing any evidence of SCD symptomatology post-SCT. The longest follow-up is 2134 days. In summary, we report the largest experience of RTC and matched family AlloSCT in selected children with symptomatic SCD with persistent long-term donor chimerism and absence of SCD symptoms or progression of disease. However, with the high incidence of viral infections and toxic deaths following unrelated UCBT, it is too premature to extend this treatment option to those SCD patients lacking a sibling donor.