Impact of Minimal Residual Disease, As Detected by Multiparametric Flow Cytometry Pre and Post Transplantation, On Outcome of Myeloablative Hematopoietic Cell Transplantation for Acute Lymphoblastic Leukemia
Authors: Bar Merav , Wood Brent L. , Radich Jerald P. , Doney Kristine C. , Appelbaum Frederick R. , Gooley Ted A. 
Published: October 2019

Abstract
Abstract 592
Relapse occurs in approximately 30% of patients with acute lymphoblastic leukemia (ALL) after allogeneic hematopoietic transplantation (allo-HCT), with relapse rate of more than 60% in high-risk patients. The outcome of relapsed patients is usually poor despite salvage therapies. To improve the outcome of ALL patients after HCT identification of markers to predict impending relapse is necessary. In this study we retrospectively evaluated the prognostic value of minimal residual disease (MRD) monitoring by seven-color multiparametric flow cytometry (MFC) in 159 patients with ALL, who underwent myeloablative allo-HCT in our center between April 2006 and March 2011. Sixty two patients were younger than 18 years at time of transplant, 90 patients were in CR1, 57 patients were in CR2, and 12 patients were in ≥CR3 at time of transplant. 133 patients had B-ALL, 24 patients had T-ALL, and 2 patients had biphenotypic leukemia. Among the B-ALL patients 34 patients had Ph+ disease. Seven-color MFC was performed on bone marrow aspirates before and after HCT. MRD was defined as a cell population showing deviation from normal antigen expression patterns compared with normal or regenerating marrow, with total blast count lower than 5%. Overt hematologic relapse was identified as total blast count ≥5%.
153 patients had pre-HCT flow data within 50 days before transplant. Among these patients MRD pre-HCT increased the risk of relapse (HR=3.17; 95% CI, 1.60–6.28; p=.002) and mortality (HR=2.39; 95% CI, 1.46–3.90, p=.0005) relative to the risk without MRD before HCT. These results were qualitatively unchanged after adjusting for the presence of abnormal cytogenetics or positive BCR-ABL by molecular testing. One year estimates of relapse for patients without and with MRD pre-HCT were 12% and 27%, respectively, and one year estimates of overall survival (OS) were 75% and 44% respectively. Three years estimated relapse were 16% and 33% and 3 years estimated OS were 68% and 40% for patients without and with MRD pre-HCT respectively (Figures 1, 2).
137 patients had “day-28” flow data, where MFC was done between day 17 and day 40 post-transplant. Among patients who had not relapsed by the time of their day-28 MFC, the risk of relapse was higher among patients with positive day-28 MFC compared to those with a negative day-28 MFC (HR=3.55; 95 CI,1.58–8.02; p=.002). Patients with MRD at day 28 were also at increased risk of subsequent mortality compared to those without MRD at day 28 (HR=2.47; 95% CI, 1.23–4.96; p=.01). These results were qualitatively unchanged after adjusting for the presence of abnormal cytogenetics. One year estimates of relapse for patients without and with MRD post-HCT were 15% and 39%, respectively, and 1-year estimates of OS were 76% and 41%, respectively. Three years estimated relapse were 19% and 45% and 3 years estimated OS were 70% and 33% for patients without and with MRD post-HCT, respectively.
Modeling post-HCT flow values as a time-dependent covariate also showed an increased risk of relapse (HR=5.27; 95% CI, 2.16–12.87, p=.0003) and death (HR=3.21; 95% CI, 1.58–6.53, p=.001) among patients who had MRD compared to those who did not. These associations remained qualitatively the same after adjusting for presence of pre-HCT MRD (HR=6.04 for relapse; HR=3.53 for overall mortality).
These data suggest that pre- or post-HCT MRD, as detected by seven-color MFC, is associated with increased risk of relapse and death after myeloablative HCT for ALL.
Figure 1. Cumulative incidence of relapse after HCT based on MRD status before HCT Figure 1. Cumulative incidence of relapse after HCT based on MRD status before HCT
Figure 2. Overall survival after HCT based on MRD status before HCT Figure 2. Overall survival after HCT based on MRD status before HCT

Disclosures:
No relevant conflicts of interest to declare.