Prognostic Impact of Hematopoietic Recovery After Fludarabine, IV Busulfan and Antithymocyte Globulins (FB2 regimen) Reduced-Intensity Conditioning Regimen (RIC) Allogeneic Stem Cell Transplantation (allo-SCT)

RIC regimens are increasingly used prior to allo-SCT. The FB2 regimen (Fludarabine 120–150 mg/m² + IV Busulfan 6.4 mg/Kg + ATG Thymoglobuline 5mg/Kg) is currently the most widely used RIC regimen in many European centres. This retrospective analysis aimed to assess the hematopoietic and immune recovery in a homogeneously treated cohort of 53 patients (males: n=33; median age: 59 years (range: 22–70)) who received the FB2 regimen between January 2007 and October 2010 in our department.

Diagnoses were as follow: AML n=23; ALL n=1; biphenotypic leukemia n=1; lymphoma n=16; myelodysplastic syndrome n=9; multiple myeloma n=3. Nineteen patients (36%) had received a prior autologous SCT. The majority of patients (n=40, 75.5%) were transplanted in complete remission. Thirty patients received a graft from a matched sibling donor (56.5%). All patients, but one (who received unmanipulated bone marrow) received G-CSF-mobilized PBSCs. GVHD prophylaxis consisted of cyclosporine (CsA) alone in patients transplanted with an HLA-identical sibling, and CsA+ mycophenolate mofetyl in other cases. None of the patients received G-CSF during aplasia following transplant while nine patients received erythropoietin before day+100.

Engraftment was achieved in 96% of patients (n=51). Median times for neutrophils (n=51) and platelets (n=22) recovery were 17 days (range: 0–39) and 10 days (range: 4–186), respectively. The majority of patients (n=31, 58%) did not receive platelet support during aplasia. The cumulative incidences of grade II-IV and grade III-IV acute GVHD were 30% and 15%, respectively, while overall incidence of chronic extensive GVHD was 33%.

With a median follow-up of 19 months (range: 2–53), the 2-year OS, DFS, relapse incidence, and NRM were 63%, 59.5%, 35% and 6%, respectively. In univariate analysis, when regarding pre-transplant factors associated with outcome, the only factor correlated with a significantly higher 2-year OS and DFS was a higher total circulating lymphocytes count at transplant (> 730/mm³) (OS: 81.5% vs 43.2%, p=0.01; DFS: 73.2% vs 45.5%, p=0.03). Regarding post-transplant factors, we found that higher recovery of leukocytes (>5000/mm³) (2-year OS: 78% vs 46%, p=0.007; 2-year DFS: 70% vs 48%, p=0.08), neutrophils (>3230/mm³) (2-year OS: 76% vs 50%, p=0.02; 2-year DFS: 67.5% vs 52.0%, p=0.09), and monocytes (>590/mm³) (2-year OS: 80% vs 47%, p=0.004; 2-year DFS: 75% vs 42%, p=0.007) at day+30 post-transplant were the most significant factors associated with outcome. In multivariate analysis, the only independent factors associated with a significantly higher OS and DFS were a better immune status at transplant (lymphocytes count >730/mm³; HR 0.22; 95%CI: 0.08–0.63, p=0.005; and HR: 0.29; 95%CI: 0.12–0.71, p=0.006, respectively) and a higher monocytes count at day+30 post-transplant (>590/mm³) (HR: 0.24; 95%CI: 0.08–0.66, p=0.006; and HR: 0.28; 95%CI: 0.11– 0.68, p=0.005; respectively).

These results suggest that hematopoietic status and recovery before and after FB2 RIC allo-SCT can be significant predictors of outcome. This paves the way for future studies aiming to closely monitor the kinetics of immune recovery after RIC allo-SCT and to evaluate the impact of growth factors and other immunostimulatory cytokines in the setting of RIC allo-SCT.

No relevant conflicts of interest to declare.