Augmented Therapy Based on PCR-MRD Levels of Children with Acute Lymphoblastic Leukemia: A Report from the Japanese Children's Cancer and Leukemia Study Group ALL 2000 MRD Pilot Study

Abstract

Many studies have shown the presence of minimal residual disease (MRD) following therapy for childhood acute lymphoblastic leukemia (ALL) to be an important prognostic marker. We have also shown a significant relationship between survival outcomes in patients enrolled in the previous ALL 911 study and molecular MRD levels 5 weeks (time point 1, TP1) and 12 weeks (TP2) following the initiation of chemotherapy (Leukaemia and Lymphoma 2002; 43: 1001). The aim of this study was to evaluate if polymerase chain reaction (PCR)-based MRD assay is sufficiently dependable for tailoring therapy, and if augmented therapy can reduce MRD levels to those associated with a favourable outcome. The subjects were under 18 years of age, and had newly diagnosed precursor B or T-cell ALL. Patients below one year old and those with t(9;22) were excluded. Written informed consent was obtained from patients or their legal guardians. The ALL 941-based protocol (45thASH, San Diego, 2003) utilized PCR-based MRD assay using immunoglobulin & T-cell receptor gene rearrangements. MRD was detected by nested PCR, with screening of rearrangements using multiplex PCR primers as described previously (Leukaemia and Lymphoma 2002; 43: 1001). Patients were initially stratified into 3 risk groups (in ascending order: SR, HR, and HHR) according to leukocyte count and age at time of diagnosis. The MRD+/+ patients with levels ≥ 10−3 at both TP1 and TP2 received augmented therapy 14 weeks after initiation, and the remainder continued to receive the initial risk-adapted protocols. A total of 311 patients with a median age of 5.3 years (range 1.0–16.8) were eligible for this study. There were 4 (1.3%) non-responders and no deaths in induction. Of the 307 patients stratified, 169 (55%) were SR, 107 (35%) were HR, and 31 (10%) were HHR. The 2nd stratification by MRD level at TP2 was possible for 72.3% (222/307; insufficient DNA=28; missing time-points=25; no marker=32). Out of the 222 patients stratified, 125 (56.3%) were MRD−/−, 58 (26.1%) were MRD+/−, and 38 (17.4%) were MRD+/+. At the point of analysis, the median follow-up time was 63 months (range 33–89). The overall 5-year event–free survival (EFS) rate of the 307 patients was 80.1% (SE 2.5), higher than the EFS of the ALL941 study, which was 76.2% (SE 2.1) (p=0.167). The 5-year EFS rates according to the 1st stratification were 85.5% (SE 4) for SR, 76.1% (SE 4.5) for HR, and 64.6% (SE 9.2) for HHR, while the equivalent rates for the 2nd stratification were 87.0% (SE 3.1) for MRD−/−, 75.5% (SE 7.7) for MRD+/−, and 75.3% (SE 6.4) for MRD+/+. From the 95 patients whose MRD levels were measured at 5 consecutive points from TP1 to TP5 (5, 12, 18, 24, and 30 weeks after the start of therapy), 21 subjects with MRD+/+ received an augmented chemotherapy, and MRD levels became undetectable in 9 patients at TP3, 5 patients at TP4, and 4 patients at TP5. The corresponding cumulative 5-year relapse rates of those patients were 11%, 50%, and 50%, respectively. Thus, negative MRD status at TP3, but not at TP4 or TP5, seems to be associated with a favourable outcome. Our results confirm the strong performance of MRD-based treatment interaction in a multi-institutional study without adversely affecting the outcome in childhood ALL. Moreover, present findings suggest that an augmented therapy could reduce MRD to levels associated with a favourable outcome. To improve the applicability and accuracy of MRD assay, new MRD-PCR targets and RQ-PCR-based MRD detection are needed in subsequent studies.[Acknowledgment: This study was partly supported by grants from the Children's Cancer Association of Japan (CCAJ)].