Alternative Donor Transplantation Using Intensified Conditioning, T-Cell-Replete HLA-Haploidentical Grafts and Post-Transplantation Cyclophosphamide in the Treatment of High-Risk and Advanced ALL: Feasibility and Outcome

Unmanipulated, T-cell-replete (TCR) HLA-haploidentical hematopoietic stem cell transplantation (haplo-HSCT) using post-transplantation cyclophosphamide (PTCY) for selective in vivo T-cell depletion of allo-reactive T-cells has become a valuable treatment alternative in patients with various hematologic disorders who lack a conventional donor or need timely transplant due to aggressive disease. However, as of yet few data are available for the treatment of ALL with this approach, particularly in relapsed and refractory disease.

To evaluate the outcome of TCR haplo-HSCT utilizing PTCY in the context of intensified conditioning in patients with high-risk, relapsed and refractory ALL, we retrospectively analysed 29 patients (B-ALL n=27, T-ALL n=2; 12 male; median age 42 years: range 8-68) transplanted between 2010 and 2015 in five German transplant centers including one pediatric. Disease was active (relapsed/refractory) in 14 patients (48%) while others were in first (35%) or subsequent remission (17%). Eleven patients (38%) had relapsed after a previous allogeneic transplantation. Patients not in CR received cytoreductive chemotherapy prior to the conditioning regimen, according to the "sequential therapy" concept (Schmid C et al., JCO 2005; Tischer J et al, Ann Hematol 2013). Conditioning was TBI-based in 15 adults consisting of fludarabine and +/- cyclophosphamide plus either 12 Gy TBI or 8 Gy TBI applied in patients older than 55 years; children (n=3) received 10-12 Gy TBI plus etoposide. In adults with relapse after a first allogeneic transplantation conditioning was drug-based replacing TBI with treosulfan (3 x 10-12 g/m²) and etoposide. Post-grafting immunosuppression was high-dose cyclophosphamide, tacrolimus or cyclosporine A (n=4) and MMF in all patients. 27/29 patients engrafted, while 2 patients died early in aplasia. No primary graft rejection was observed. Acute GvHD grade II-IV occurred in 7 patients (24%); no patient developed grade IV acute GvHD. Mostly mild to moderate chronic GvHD was observed in 6 patients (21%), and CI of chronic GvHD at 2 years was 25%. No patient died due to GvHD. Severe toxicity (grade III-IV) was observed in 12 patients (41%), most commonly mucositis (n=34%), diarrhoea (31%), hyperbilirubinemia and transient elevation of transaminases (28%) and hemorrhagic cystitis (21%). CMV reactivated in 11/23 patients at risk and EBV in 3 while no patient developed CMV disease or PTLD. Proven invasive aspergillosis was diagnosed in 2, probable invasive aspergillosis in 5 patients. One-year non-relapse mortality (NRM) was 10%. Patients grafted with active disease experienced a one-year NRM of 14%. One-year relapse incidence was 35%. After a median follow up of 31 months (range 7.1-53.6), estimated one-year and three-year overall and relapse-free survival was 72/49% and 55/41%, respectively.

In summary, intensification of the preparative regimen is well tolerated in the setting of TCR haplo-HSCT using PTCY as GvHD prophylaxis with low GVHD rates and NRM in patients with high-risk, relapsed and refractory ALL, while providing effective anti-leukemic activity. Results are at least comparable to HLA-matched transplantation. Thus, we suggest that the donor pool can be safely expanded in patients with high-risk and advanced ALL who lack a conventional donor, and that unrelated donor transplantation might be challenged in patients suffering from aggressive disease in the future.