Very Early/Early Relapses of ALL Show Unexpected Changes of Clonal Markers and High Heterogeneity in Initial and Relapse Treatment Response: ALL-BFM 2000 and ALL-REZ BFM 96/2002.

Abstract 2612

Poster Board II-588

Minimal residual disease (MRD)-measurements have been integrated into treatment protocols for childhood acute lymphoblastic leukemia (ALL) using T-cell-receptor(TCR)/immunoglobulin(IG) gene rearrangements. Additionally TCR/IG gene rearrangements can be applied to describe clonal relations and identify cell populations which undergo clonal evolution and selection during and after treatment. Patterns of TCR/IG gene rearrangements might differ between initial and relapse diagnosis and are interesting and very informative in order to describe both the quantitative extent at diagnosis and the kinetics during treatment of main- and subpopulations. In order to minimize the chance of a false negative MRD-result knowledge about the stability of TCR/IG-markers is important. Therefore, stability of TCR/IG-markers between initial and relapse diagnsosis was assessed in previous studies. These studies recommended to use two markers besides considering gene-locus and clonality profile. However, it has never been validated prospectively whether the TCR/IG-markers actually chosen for MRD-quantification and risk-group stratification are sufficient for clinically and biologically reliable MRD-monitoring during frontline treatment. We have aimed to compare marker profiles between initial and relapse diagnosis by a systematic approach in order to evaluate markers acutally chosen for initial MRD-quantification. Further, MRD-response to induction of initial and relapse treatment have been compared. One main objective of our study was to gain new insights in clonal heterogeneity, resistance and clonal evolution. We have performed a prospective study including patients treated according to the frontline-trial ALL-BFM 2000 and who suffered a relapse. The interim analysis included finally 45 patients showing a median time to relapse of 1.5 years (range 0.6 – 3.8). Therefore, mainly (73%) very early (during first 18 months after initial diagnosis) and early (between 18 and 30 months) relapses were included in the study. In 33% of patients (15/45) all markers identified at initial diagnosis remained stable at relapse diagnosis and no additional marker was gained at relapse. The remaining 67% of patients showed at least one clonal change. In 38% (17/45) of patients at least one gain of a TCR/IG gene rearrangement was seen; in 53% (24/45) of patients loss at relapse and in 16% (7/45) V-V-replacement or Vd2-Ja ongoing rearrangement product was seen at relapse. Considering markers actually chosen for initial MRD-quantification, in 62% of patients both markers remained stable, in 27% at least one, in 11% no marker. Early on-therapy relapses showed the same proportion of patients with a marker-gain, than later on-/off-therapy relapses. A backtracking-analysis of markers gained revealed the presence of nearly all markers (17/19: 90%) at initial diagnosis at different quantitative levels independently of time point of relapse. Paired initial and relapse molecular response to induction varied remarkably. Only 5 of 24 patients included in this analysis showed similar response (1 poor, 4 intermediate, 3 good responders) which demonstrates the unexpected high heterogeneity. It is interesting to note that complete identical markers for MRD-monitoring during initial and relapse treatment were only chosen in half of the patients (12/24). These observations challenges the classical and still existing opinion that early relapses derive from resistant leukemia because resistant MRD was not eliminated by polychemotherapy. Based on our data and data from other studies comparing genetic markers in paired samples between initial and relapse diagnosis, the extrapolation of an algorithm for MRD-marker choice is not possible and would be too artificial at the moment. From the biological point of view, the practical restriction of using only one or two markers for MRD quantification at several time points during early and later phases of the treatment should not be recommended. Despite the higher proportion of very early/early relapses in this interim analysis we observed a high heterogeneity of marker-profiles between initial and relapsed diagnosis and molecular response to treatment at both disease stages.