Minimal Residual Disease Monitoring by Four Color Flow Cytometry before Second Induction Is an Independent Prognostic Factor in Childhood Acute Myeloid Leukemia - AML-BFM MRD Study Group.

Monitoring of minimal residual disease (MRD) by flow cytometry in childhood acute myeloid leukemia (AML) is still a topic of discussion in terms of the optimal time of analysis and the best antibody combinations. Here we report the largest prospective international series of MRD monitoring (395 samples) in children with AML (n=124).

All children have been treated according to the AML-BFM 98 study. Based upon a CD33/CD34 backbone, a wide panel of antibodies, independent from the initial immunophenotype, was applied at up to four defined time points during treatment (MRD1: day 15, MRD2: before 2^nd induction, MRD3: before 3^rd block, MRD4 before 4^th block). For each of the 12 leukemia associated immunophenotypes (LAIP) and time points, a threshold level based upon a previous retrospective analysis of 149 other children with AML as well as analysis of normal bone marrow was determined.

Regarding all four time points there is a statistically significant difference in the 3-year event free survival (EFS) in those children presenting with MRD positivity at ≥ 3 time points (p_EFS 68% vs. 33%, p=0.01). The MRD level at time point 2 has turned out to have the most significant predictive value for 3-year EFS: 66% (n=104) vs. 35% (n=17), p_logrank=0.006. Regarding the standard risk group (defined by FAB subtype [M1, M2± Auer rods, M3, M4eo], favorable cytogenetics and morphologically determined treatment response at day 15) only, it is possible to identify a subgroup with poor outcome (20% [n=5] vs. 88% [n=40], p_logrank <0.0001). In accordance with this, a multivariate analysis proved the MRD2 as a significant prognostic factor (p_Chi²=0.023) independent from the known risk factors like FAB subtype, cytogenetics or morphological blast percentage at day 15. As a conclusion, (A) MRD monitoring before second induction has the same predictve value as examining MRD level at four different time points during intensive chemotherapy and (B) the immunologic analysis of MRD before second induction is particulary suitable for risk group stratification and therapeutic consequences in addition to the known risk factors.