Prospective Evaluation of MRD-Kinetics in 274 Children with High-Risk ALL Treated in Trial ALL-BFM 2000: Insights into Development of Resistance and Impact on Further Refinement of Treatment Stratification Strategies.

In trial ALL-BFM 2000, high-risk (HR) acute lymphoblastic leukemia (ALL) is defined by inadequate initial response to induction treatment [poor prednisone response on treatment day eight, non remission on treatment day 33, and/or a high load of minimal residual disease (MRD, ≥10E-3) after 12 weeks of treatment (TP2)] and/or by cytogenetics [t(4;11 or t(9;22)]. Between August 1999 and November 2006, 494 (15%) out of 3255 study patients were stratified into the HR branch of trial ALL-BFM 2000. 431 (87%) of these HR patients underwent successful MRD monitoring at TP2 with 274 (56%) patients having received additional extensive prospective MRD monitoring subsequent to TP2. Patients with an indication for stem cell transplantation (SCT) and a suitable donor were scheduled for SCT within six weeks after the third HR block of the intensive consolidation phase. The estimated 4-years event-free-survival (4y-pEFS) for the entire HR group was 68%+/−3%, estimated survival was 74%+/−3%. Patients with MRD load of ≤10E-4 at TP2 (n=231) had a 4y-pEFS of 82%+/−3%, patients with MRD levels of 10E-3 at TP2 (n=84) had a 4y-pEFS of 74%+/−6%, and patients with MRD of >10E-3 at TP2 (n=116) had a 4y-pEFS of 35%+/−5%. MRD-kinetics subsequent to TP2 revealed, that 85% of all patients with an MRD level of 10E-3 at TP2 continued to decrease their load below 10E-3 during the pulsatile intensive consolidation phase, whereas this was observed in only 35% of patients with an MRD level of >10E-3 at TP2. The 4y-pEFS of patients with an MRD load persisting at 10E-2 after application of three intensive HR blocks after TP2 was 0% if no SCT was performed, and 33%+/−11% after SCT in CR1. Our data reflect that extensive MRD measurements in HR-ALL patients allow a dynamic insight into the development of resistance, and serve as valuable tool for further clinical treatment adjustment. “MRD non-response” after three BFM HR blocks identifies a patient group in urgent need of alternative treatment elements, closely monitored by MRD, before going into SCT in CR1.