Fludarabine/Treosulfan/Thiotepa/ATG Conditioning Leads to Earlier Engraftment and Higher Donor Chimerism Than Busulfan/Cyclophosphamide, and Enables the Use of Mismatched and Unrelated Donors for Transplantation in Haemoglobinopathies
Authors: O'Boyle Farah , Killeen Natalie , Valganon Mikel , Chakravorty Subarna , Karnik Leena , New Helen , Foroni Letizia , Szydlo Richard , Roberts Irene , de la Fuente Josu 
Published: October 2019

Abstract
Abstract 3234
Haemopoietic stem cell transplantation is the only proven curative treatment available for haemoglobinopathies. From 2000 to 2010 severty-four consecutive transplants were conditioned with busulfan 14 mg/kg, cyclophosphamide 200 mg/kg and alemtuzumab (Bu/Cy) achieving a DFS of 94.5%. In order to reduce busulfan toxicity, minimise mixed chimerism and enable the use of related mismatched and unrelated donors, the conditioning regimen was modified: fludarabine 160 mg/m2, treosulfan 42 g/m2, thiotepa 10 mg/kg and ATG (Thymoglobulin) 7.5 mg/kg or 11.25 mg/kg (FTTA). 18 patients [13 with β thalassaemia major and 5 with sickle cell disease, median age 6.5 years (2 – 16)] received a related transplant: fifteen 10/10 matched sibling, one 10/10 matched related and two 9/10 mismatched sibling. The source of stem cells was BM in 16 patients and mixed cord blood and BM in 2 with a median cell dose of 3.85 ×108 TNC.kg (range 1.50 – 6.87). Median follow-up was 6.7 months (2.40 – 20). Two patients received an unrelated transplant (α thalassaemia major, 12 years, 10/10 matched BM, 2.4 ×108 TNC/kg, survival +5 months; sickle cell disease, 7 years, 9/10 matched PBSC, 6 ×106CD34+/kg, survival +1.8 months). Endogenous haemopoiesis was suppressed with hypertransfusions. GvHD prophylaxis: ciclosporin and MMF. Patients with thalassaemia were Pesaro class I or II. Patients with sickle cell disease were transplanted for stroke or recurrent vaso-oclusive crisis and/or acute chest syndrome not responding to hydroxycarbamide.
All patients are alive with transfusion-independence and donor haematological values (DFS 100%, OS 100%). Acute GvHD ≥grade 2 occurred in 1 related patient (5.6%) and both unrelated patients; chronic limited GvHD in 2 related patients (11.1%), all with resolution following steroid treatment. Neutrophil engraftment was achieved at a median of 12 days (9 – 21) in comparison to 19.5 days (12 – 28 days) with the previous protocol. Chimerism studies in whole blood demonstrated that donor haemopoiesis was higher for FTTA v Bu/Cy at all-time points (day +28: 100% >95% for FTTA v 81.4% >95%, 16% >90–95% and 2.4% >50–80%; day +90: 91.6% >95% and 8.3% >90–95% v 67.1% >95%, 12.8% >90–95%, 12.8% >80–90% and 12.8% >50–80%; day +150: 50% >95%, 37.5% >90–95% and 12.5% >50–80% v 55.8% >95%, 10.8% >90–95%, 14.7% >80–90%, 8.8% >50–80%, 2.9% >30–50% and 2.9% >20–30%). Donor T-cell lymphopotiesis is FTTA was day +28: 100% (68 – 100), day +90: 98.5% (55–100) and day +150: 98% (73–100). In conclusion, FTTA leads to earlier engraftment and higher donor chimerism than Bu/Cy, no graft failure, and enables the use of mismatched and unrelated donors for transplantation in haemoglobinopathies, whilst the incidence of GvHD is low.

Disclosures:
No relevant conflicts of interest to declare.