Fludarabine and Treosulfan Conditioning Is Feasible and Leads to High OS and Low NRM
Authors: Bernardi Chiara , Morisset Stéphane , Pradier Amandine , Mamez Anne-Claire , Giannotti Federica , Morin Sarah , Masouridi Levrat Stavroula , Roth-Guepin Gabrielle , D'Aveni Maud , Kicki Céline , Campidelli Arnaud , Simonetta Federico , Pagliuca Simona , Chalandon Yves , Rubio Marie-Thérèse 
Published: December 2024

Optimization of conditioning regimens is a cornerstone of hematopoietic stem cell transplant medicine. Over time, various combinations of chemotherapy, immunosuppressants, and radiation-based regimens have been proposed for both malignant and non-malignant hematological disorders. The advent of reduced intensity conditioning (RIC) regimens has popularized the use of fludarabine and busulfan (FB2) for treating numerous diseases. However, busulfan remains associated with a notable toxicity mainly due to issues related to its pharmacokinetics (PK). An emerging alternative, fludarabine combined with low doses of treosulfan (FT10) has demonstrated excellent toxicity profiles and outcomes in randomized clinical trials. Specifically, treosulfan, offers three main advantages over busulfan: it has a more stable and predictable linear PK profile, causes lower organ toxicity; and has more convenient dosing schedule. Real-life data are needed to confirm the transplant-outcomes benefits of treosulfan-based conditioning regimens. Additionally, there is limited information on immune reconstitution following allogeneic hematopoietic stem cell transplantation (HSCT) with FT10, a factor that could further elucidate the regimen's impact on outcomes.

Here we report clinical and immune reconstitution data of a bi-institutional cohort of adult patients who underwent a matched-related or -unrelated or mismatched unrelated HSCT with peripheral blood stem cells for an hematological disorder after FT10 conditioning regimen (Fludarabine 30mg/m2 for 5 days and treosulfan 10 g/m2 for 3 days) associated with rabbit anti-T lymphocyte globulin (ATLG Neovii, 10 mg/kg for 1 - 3 days), between 2019 and 2023.

Overall 89 patients were included in this study (47 with AML, 21 with MDS, 8 with lymphoma, 13 with other hematological diseases). Graft-versus-host disease (GvHD) prophylaxis consisted of short course methotrexate (15 mg/m2 on day +1, and 10 mg/m2 on days +3 and +6) in combination with tacrolimus or cyclosporine in 52 and 3 patients, while cyclosporine and MMF were used in 30 patients, and calcineurine inhibitors alone were administered to 4 patients.

Median follow-up among survivors was 15,87 (range 1,28-52,7) months. The observed 2-year probability of survival (OS) was 83,05% (95% CI 74,37 -92,75%) and the progression-free survival (PFS) at 2 years was 70,49% (95% CI 60,83-81,69%). The 2 years cumulative incidence (CI) of relapse was 27,26% (95% CI 16,98-37,55%) while non relapse mortality (NRM) was 2,25% (95% CI 0-5,34%). The CI of grade II-IV acute GvHD at day +100 was 17,08% (95% CI 9,15-25%) while the 2-year CI of moderate/severe chronic GvHD was 18,05% (95% CI 7,13-28,97%).

We observed the immune reconstitution of lymphocyte subtypes (CD4, CD8, B cells) at different timepoints after HSCT. At day 30 the medians were notably low with a gradual yet significant increase in lymphocyte subset over time. By the 3-month, the median CD4 T cell count had risen to 79 cells/µl (0-791), CD8 T cells to 156 cells/µl (0-2008) and B cells to 64 cells/µl (0-1041). This upward trend continued at 6 months and then at 1 year. At the 1-year milestone, the immune recovery was more pronounced, although still not fully normalized. The median CD4 cell count stood at 139 cells/µl (0-571), CD8 T cells at 246 cells/µl (0-1833) and B cells at 77 cells/µl (0-2950). Notably at 3 months post transplant, 68% of patients (54 out of 79) had CD4 cell counts exceeding 50 cells/µl. The recovery to more robust levels was slower, with only 20% (17 out of 83) reaching CD4 counts above 200 cells/µl by 6 months, and 28% (19 out of 67) achieving this threshold by 1 year.

In our series, the preparative regimen based on FT10 and ATLG appear safe. Comparing the results to the ones described in the literature, FT10 is associated to an high OS and a very low NRM. FT10 is a valid cost-effective alternative to busulfan based regimen conditioning before HSCT. Our results show for the first time the kinetics of immune reconstitution of major adaptive immune cell subsets after HSCT with an early CD4 and CD8 T cells reconstitution at 3 months. Larger retrospective multicenter studies and prospective clinical trials comparing the immune reconstitution of treosulfan and busulfan-based regimen are needed in order to elucidate the relation between FT10, immune reconstitution and clinical outcome.