Reduced toxicity, myeloablative HLA-haploidentical hematopoietic stem cell transplantation with post-transplantation cyclophosphamide for sickle cell disease

Allogeneic hematopoietic stem cell transplantation (HSCT) offers the possibility of cure for sickle cell disease (SCD) patients. Unfortunately, the probability of finding an HLA-matched donor for SCD patients is low. HSCT from HLA-haploidentical donors using reduced intensity conditioning, unmanipulated bone marrow and post-transplantation cyclophosphamide (ptCy) has resulted in negligible toxicity but high rates of graft rejection. We hypothesized that combining ptCy with a myeloablative reduced toxicity conditioning including serotherapy to increase immune ablation would allow for better engraftment. In a pilot approach, we treated three patients with SCD (5, 8, and 20 years old) lacking a matched donor. All patients had severe disease-related complications despite standard treatment. They received unmanipulated bone marrow from parental HLA-haploidentical donors. Conditioning consisted of alemtuzumab 0.2 mg/kg/day on days −9 and −8, fludarabine 30 mg/m²/day on days −7 to −3, treosulfan 14 g/m²/day on days −7 to −5, thiotepa 2 × 5 mg/kg/day on day −4, and cyclophosphamide 14.5 mg/kg/day on days −3 and −2. GVHD prophylaxis was performed using cyclophosphamide 2 × 50 mg/kg on days +3 and +4 and mycophenolate mofetil, tacrolimus from day +5. After a follow-up of 11, 14, and 30 months, all three patients are alive and well, off immunosuppression, and without symptoms of SCD. One patient experienced mild skin GVHD grade I, none showed chronic GVHD. Asymptomatic CMV reactivation was seen in two patients. HLA-haploidentical HSCT can extend the donor pool for patients with SCD. Whether intensification of the conditioning regimen and intensive immunosuppression leads to improvement in engraftment rates while still allowing a favorable toxicity profile deserves further investigation.