Fludarabine/Treosulfan/Thiotepa/ATG Conditioning for Related Transplantation in b Thalassaemia Major Leads to Early and Sustained Engraftment with Low Incidence of VOD and GvHD but a High Rate of Long-Term Mixed Chimerism

Haemopoietic stem cell transplantation is a well-established treatment modality for b 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-nine 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 - 19). The source of stem cells was BM in 33 patients and mixed cord blood and BM in 6 patients. The median cell dose was 3.88 x10⁸ TNC/kg (range 0.23 - 8.51, including patients having red cell depletion for major ABO incompatibility) and 7.19 x 10⁶ CD34+/kg (range 2.05 - 14.43) for the patients receiving bone marrow only; 1.55 x 10⁸ TNC/kg (range 1.27 - 3.9) and 3.32 x 10⁶ CD34+/kg (range 2.06 - 8.79) for BM and 2.47 x 10⁷ TNC/kg (range 0.8 - 7.32) and 0.44 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 25.8 months (2.6-65.9). 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 two patients 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 clonal haemopoiesis with 7del on day +532; both patients established normal haemopoiesis following a second BMT. There was one death (2.7%), on day +89 due to idiopathic pneumonia syndrome. Acute GvHD ≥ grade 2 occurred in 5 patients (12.8%). Chronic limited GvHD occurred in 6 patients (15.4%) and extensive in 2 patients (5.1%). Chronic GvHD was only present at 18 months in one patient (2.7%). VOD occurred in one patient (2.7%, day +9) 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 27 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 97.4% in whole blood (WB) and 76.7% in T cells (T) >95%, 2.6% WB and 3.3% T >90-95%, 0% WB and 20% T >50-89%; day +90: 83.8% WB and 57.1% T >95%, 2.7% WB and 14.3% T >90-95%, 10.8% WB and 22.9% T >50-89%, 2.7% WB and 5.7% T <50%; day +180: 50% WB and 44.8% T >95%, 30% WB and 17.2% T >90-95%, 10% WB and 37.9% T >50-89%, 10% WB and 0% T <50%; and day +365: 55.6% WB and 42.3% T >95%, 7.4% WB and 19.2% T >90-95%, 14.8% WB and 18.2% T >50-89%, 22.2% WB and 19.2% T <50%.

In conclusion, the 3 years' overall survival was 97.4% and the disease-free survival 90.3%. Following a second procedure for the two cases suffering secondary graft failure, the disease-free survival increased to 97.4%. 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 to minimise unstable cases will focus in changes to serotherapy.