[Minimal residual disease analysis in acute lymphoblastic leukemia of childhood within the framework of COALL Study: results of an induction therapy without asparaginase].

UNLABELLED: The detection of minimal residual disease (MRD) is a major prognostic factor for treatment in acute lymphoblastic leukemia (ALL) of childhood. Several groups showed the predictive value of MRD after 5 weeks of chemotherapy (at the end of induction therapy). Patients with more than 1 leukemic cells in 100 cells (> or = 10(-2)) at this time-point have a significantly higher relapse rate. The MRD measurement has been shown to be an independent prognostic factor at several time points in the BFM study (ALL-BFM 90) as well as in the EORTC study. The aim of our investigations was the detection of MRD at the end of induction therapy within the COALL studies which is different from the above studies. In the COALL studies, therapy starts with a 1 week DNR prephase (24 h infusion on day one) and i.th. MTX. Induction therapy consisted of 3 drugs over a period of 4 weeks (Prednisolone, Vincristine and Daunorubicin), asparaginase is given later in consolidation. At the end of induction therapy, bone marrow was obtained for cytomorphologic and molecular analysis.

PATIENTS AND METHODS: We investigated bone marrow samples from 76 patients. All patients were in morphologic remission at the end. of induction therapy. For MRD analysis, DNA was isolated from bone marrow mononuclear cells. Clonal T-cell-receptor (TCR) or immunoglobulin gene (IgH) rearrangements were identified by PCR. Monoclonal products were either sequenced directly (TCR) or after excision from high resolution agarose gels. Subsequently patient-specific oligonucleotides for allele-specific PCR were generated. PCR analysis was performed with 1 microgram DNA for each reaction within a semiquantitative matter. This method reached sensitivities down to 10(-5).

RESULTS: Eighty-four percent of the analysed samples were MRD positive at the end of induction therapy. 20 out of 76 patient samples (26%) were highly positive (> or = 10(-2)), 28 patients had levels of about 10(-3) (37%), 16 had levels around 10(-4) (21%) and 12 patients had no detectable residual cells (16%). All analysed 15 T-ALL patients had detectable residual disease at this timepoint. Until now, 5/20 patients with very high MRD level at the end of induction therapy suffered a relapse.

DISCUSSION: Patients with very high MRD level at the end of induction therapy showed an elevated risk of relapse, but the predictive value is much poorer than for example in the BFM 90 MRD-study. We suggest, that a high MRD level at this timepoint results from a different induction therapy compared to the BFM 90 study. In the COALL studies asparaginase is given only after induction therapy to decrease the risk of thrombosis. We would like to conclude that this differences were compensated later during therapy as the event free survival of both studies is similar. In conclusion, an optimal information from MRD studies is strongly associated with the given therapy. Therefore we initiated an additional MRD time-point after the first chemotherapy block in consolidation.