Alternative Donor Hematopoietic Stem Cell Transplantation for Sickle Cell Disease in Europe

Hematopoietic stem cell transplant (HSCT) from an HLA identical sibling is a well-established curative therapy for sickle cell disease (SCD). HSCT from an unrelated donor is a treatment option, but the likelihood of finding a donor varies according to ethnicity and results are still limited. HLA haploidentical relatives can be alternatively used but, to date, only small series of patients have been described. We report outcomes of patients (pts) transplanted with related haploidentical (Haplo) or unrelated (UD) donors grafts and reported to EBMT/EUROCORD databases.

Sixty four pts transplanted in 22 EBMT centers between 1991 and 2017 were retrospectively analyzed. Pts were described according to the donor type: haploidentical (n=40) and unrelated (n=24) [adult UD n=19; cord blood (CB) n=5]. The objective of the study was to describe alternative donor transplants for SCD in Europe without performing comparison analyses due to the size and heterogeneity of the groups. Primary endpoint was 3-year overall survival (OS).

Median follow-up (FU) was 28 months (range: 1.6-156) [29.5 months (range: 2.1 - 133.5) for Haplo and 24.6 (range: 1.6 - 156) for UD]. Median age at HSCT was 14.2 years (range: 3-31.7) in Haplo and 11.8 (range: 2.1-42.8) in UD, with a predominance of children (<16 years) in both groups (23/40 and 19/24, respectively). Before HSCT, 68% of overall pts were treated with hydroxyurea and 62% received more than 20 red blood cell (RBC) units. RBC alloimmunization occurred in 14% of transfused pts.

In both groups, vaso-occlusive crisis and cerebral vasculopathy were the most frequent SCD complications and the main indications for HSCT. Other complications were acute chest syndrome (44%), liver disease (31%) and infection (23%). In Haplo, median year of transplant was 2014 (range: 1991-2017) and in UD 2011 (range: 2004-2015).

In Haplo, two major protocols were used: (1) post -transplant cyclophosphamide (PTCY) with G-CSF primed bone marrow (BM) and a fludarabine+ cyclophosphamide+thiotepa+2Gy TBI conditioning regimen [16 pts and 2 centers performing most (n=13) of the transplants]; (2) a protocol (performed in 2 centers) consisting in the use of G-CSF mobilized peripheral blood stem cells (PBSC) with ex-vivo B and T cell depletion (BT depleted) (15 pts) and a fludarabine+thiotepa+ treosulfan conditioning regimen (14/15 pts). Haplo donors were most frequently the parents [mother (50%), father (29%), brother (14%) and cousin (7%)]. ATG was used in 95% of transplants and the most frequent combination for graft versus host disease (GvHD) prophylaxis was mycophenolate mofetil (MMF)+sirolimus in PTCY and MMF+ cyclosporine A (CSA) in BT depleted.

In UD, graft source distribution was 14 BM, 5 PBSC and 5 CB. Conditioning regimens were mainly myeloablative (83%) with fludarabine+thiotepa+ treosulfan in 54% of HSCT. ATG was used in 87% and campath in 9% of transplants; GvHD prophylaxis was CSA and methotrexate in 50%.

Neutrophil engraftment at 60 days was 95±4% in Haplo and 84±8% in adult UD, after a median engraftment time of 18 and 22 days, respectively. In Haplo, 7 pts experienced graft failure (3 primary and 4 late), of those 3 had a second allogeneic transplant and were alive at last FU, at 16, 16 and 63 months respectively; 1 patient died after rescue with autologous transplant and 3 were alive after autologous reconstitution. In adult UD, 3 pts had a primary and 1 a late graft failure, none of them had a second transplant and were all alive at last FU, at 2, 13, 28, 118 months respectively.

Grade II-IV acute GvHD at 100 days was 25±7% in Haplo and 21±9% in adult UD; acute GvHD grade III-IV was observed in 3 pts in Haplo (none in BT depleted) and 2 pts in adult UD. Chronic GvHD was observed in 10 pts in Haplo (5 extensive, 3 of these in PTCY) and 3 pts in adult UD (2 extensive).

OS at 3 years was 88±4%; being 89±5% in Haplo (88±8% for PTCY, 92±8% for BT depleted) and 94±5% in adult UD. 3-year event free survival was 58±7%; in detail, 60±9% in Haplo (56±12% for PTCY, 68±13% for BT depleted) and 60±12% in adult UD. Overall, 8 pts died (5 Haplo and 3 UD) due to infections or GVHD.

Among the 5 pts receiving CB transplant 3 are alive (1 of which after graft failure and a second allogeneic transplant).

Conclusion: This preliminary analysis shows that, despite an acceptable OS, rejection and chronic GvHD are still of concern; therefore alternative donor transplants for SCD should be performed in experienced centers with prospective clinical trials.