Flow Cytometry-Based Pre-Transplant Minimal Residual Disease Detection Strongly Impacts on the Outcome of Haploidentical Transplantation with Αβ T Cell Depletion in Pediatric Acute Lymphoblastic Leukemia

Abstract

Introduction Allogeneic hematopoietic stem cell transplantation (HSCT) is indicated for patients with relapsed or refractory acute lymphoblastic leukemia. Patients with persistence of minimal residual disease (MRD) before HCT are at increased risk of disease relapse. Multiparameter flow cytometry (MFC) is the most commonly used method of MRD detection in clinical practice. This study aimed to evaluate MRD status before HCT on outcome of ALL patients receiving allogeneic HSCT from haploidentical donors with TCRαβ+/CD19+ depletion of the graft. Materials and methods A total of 120 pts with ALL (T-lineage ALL (T-ALL)- 37, B cell precursor (BCP)-ALL-83, 45 female, 75 male, median age 8.7 years (0.5-20) underwent allogeneic HSCT between June 2013 and June 2019. All pts received Haplo graft and were in morphologic remission. Disease status at transplant was CR1 in 35 pts, CR2 in 68 pts and CR>2 in 17 pts. Transplantation in CR1 was performed according to risk stratification scheme in the current institutional ALL protocol (Moscow-Berlin 2008, 2015). MRD detection in the bone marrow prior to НSСТ was performed in all pts by MFC according the AIEOP BFM FLOW Network SOP. MRD negativity was defined as <0.001% of all bone marrow nucleated cells. Seventy-nine patients were MRD negative before HSCT, 41 were MRD-positive. The median MRD level (among MRD-positive patients) prior HCT was 0.025%. Thirty (25%) pts received treosulfan-based myeloablative preparative regimen, while TBI-based regimen was used in 90 (75%) pts. Two regimens of GvHD prophylaxis were used. Regimen 1 (n=27): thymoglobulin 5mg/kg, rituximab 200 mg/m2 and bortezomib on day +2, +5; regimen 2 (n=93): tocilizumab at 8 mg/kg on day -1 and post-transplant bortezomib, 89 pts receive additional abatacept at 10 mg/kg on day +2, +7, +14, +28. TCR αβ+/CD19+ depletion of HSCT with CliniMACS technology was implemented in all cases. The median dose of CD34+ cells was 9.3 x106/kg (range 4.3-19.8), αβ T cells - 30x103/kg (range 1-361). Median time of follow-up for survivors was 1.6 years (range: 0.13 - 4.8). Results Primary engraftment was achieved in 116 of 120 pts (3 pts died before engraftment due to septic events, one relapsed early), the median time to neutrophil and platelet recovery was 13 and 14 days, respectively. All engrafted pts had verified morphologic remission and achieved sustained complete donor chimerism by day +30, seven of them had detectable MRD (5 of them were MRD-positive before HCT). Transplant-related mortality was 5 % (95% CI: 2-11). The cumulative incidence (CI) of relapse at 1.5 years was 25%(95%CI:18-35) for the whole cohort. Among patients, who had MRD-negative remission prior to HSCT, CI of relapse was 14 % (95%CI:8-26) with median time of relapse of 0.54 months, as compared to MRD-positive cohort, with CI of relapse of 44 % (95%CI:31-63) with median time to relapse 0.29 months, p=0.0004. pEFS (event=death or relapse) was 70% (95%CI: 61-78) for the whole cohort, in MRD (-) group pEFS was 79% (95%CI: 68-88), as compared to 56%(95%CI:40-72) in the MRD(+) group, p=0.025. CI of relapse in BCP-ALL pts, who had MRD-negative remission prior to HCT was 12 %(95%CI:6-25), in MRD(+) group 49% (95%CI:33-72), p=0.0002, in T-ALL pts in MRD (-) and MRD (+) groups CI of relapse was 17 % (95%CI:6-51) and 38 % (95%CI:20-76), respectively, p=0.14. Conclusion These results suggest that MRD detection by multiparameter flow cytometry prior to HSCT is a highly significant prognostic factor in the setting of haploidentical HSCT on the platform of ab T cell depletion. We expect that further improvement of the outcome can be achieved based on the combination of current safe haplo HSCT platform and novel targeted immunotherapy approaches. Figure Disclosures Maschan: Miltenyi Biotec: Other: lecture fee.