Fludarabine/Treosulfan/Thiotepa/ATG Conditioning for Related Transplantation in β Thalassaemia Major Leads to Early and Sustained Engraftment with Low Incidence of VOD and GvHD

Haemopoietic stem cell transplantation is a well-established treatment modality for β thalassaemia. The challenge is to minimise graft failure due to the expanded haemopoietic compartment whilst reducing the risk VOD due to the effects of iron load. From 2011 to 2016 thirty-five consecutive sibling transplants were conditioned with fludarabine 160 mg/m², treosulfan 42 g/m², thiotepa 10 mg/kg and ATG (Thymoglobulin) 7.5 mg/kg (FTTA). Endogenous haemopoiesis was suppressed pre-transplantation with hypertransfusions for a minimum of 8 weeks. GvHD prophylaxis was provided with ciclosporin and MMF. The median age was 5 years (2 - 16). The source of stem cells was BM in 30 patients and mixed cord blood and BM in 5 patients. The median cell dose was 4.17 x10⁸ TNC/kg (range 0.23 - 8.51, including patients having red cell depletion for major ABO incompatibility) and 7.28 x 10⁶ CD34+/kg (range 2.05 - 14.43) for the patients receiving bone marrow only; 1.64 x 10⁸ TNC/kg (range 1.22 - 3.9) and 4 x 1 0⁶ CD34+/kg (range 2.06 - 8.79) for BM and 3.8 x 10⁷ TNC/kg (range 0.8 - 7.32) and 0.53 x 10⁶CD34+/kg (range 0.06 - 1.8) for cord blood for the patients receiving a combination of cord blood and bone marrow. The median survival was 21.1 months (0.4-50.7). Patients were Pesaro class I or II (Pesaro class III patients were intensively chelated pre-transplantation to reduce the stage of fibrosis and return to class I or II). All patients were evaluated with liver biopsy pre-transplantation and defibrotide prophylaxis given if Ishak stage ≥ 3.

All patients engrafted and achieved evidence of donor haemopoiesis on day +28, and all but one patient achieved transfusion-independence and donor haematological values. Two patients had secondary graft failure: one patient had progressive reduction of donor haemopoiesis with reestablishment of ineffective thalassaemic haemopoiesis and transfusion dependence on day +313, another patient had reestablishment of ineffective thalassaemic and myelodysplastic haemopoiesis with 7 deletion on day +532; both patients established normal haemopoiesis following a second BMT. There was one death (2.9%), on day +89 due to idiopathic pneumonia syndrome. Acute GvHD ≥ grade 2 occurred in 4 patients (11.4%). Chronic limited GvHD occurred in 5 patients (14.3%) and extensive in 1 patient (2.9%). Chronic GvHD was only present at 18 months in one patient (2.9%). VOD occurred in 2 patients (5.7%, days +7 and +9 respectively) and responded to standard measures and defibrotide treatment. The median neutrophil engraftment was 11 days (range 9 to 21). The median platelet engraftment >20 x10⁹/L was 28 days (range 16 to 73) and >50 x10⁹/L was 32 days (range 19 to 73). The median time to cessation of immunosuppression was 160 days (105-309).

Chimerism studies on day +28 demonstrated 97.1% in whole blood (WB) and 76.9% in T cells (T) >95%, 2.9% WB and 3.8% T >90-95%, 0% WB and 19.2% T >50-89%, and 0% WB and 4.8% T <50%; day +90: 81.3% WB and 56.7% T >95%, 3.1% WB and 13.3% T >90-95%, 12.5% WB and 23.3% T >50-89%, 3.1% WB and 6.7% T <50%; day +180: 48.1% WB and 42.3% T >95%, 33.3% WB and 19.2% T >90-95%, 11.1% WB and 38.5% T >50-89%, 7.4% WB and 0% T <50%; and day +365: 56% WB and 41.7% T >95%, 8% WB and 20.8% T >90-95%, 16% WB and 20.8% T >50-89%, 20% WB and 16.7% T <50%.

In conclusion, the 3 year's overall survival was 97.1% and the disease-free survival 89.2%. Hence, FTTA leads to early and sustained engraftment with low rate of graft failure, and minimal occurrence of VOD and IPS, whilst the incidence of GvHD is low, though there is a high rate of mixed chimerism, the large majority which is stable. Further improvements should focus on minimising unstable cases.