Clinical Implication of BCR/ABL Fusion Transcript Monitoring in Addition to Ig/TCR Gene Rearrangement-Based Minimal Residual Disease in Philadelphia Chromosome-Positive Childhood Acute Lymphoblastic Leukemia

Background: Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL) accounts for about 3% of pediatric ALL and has a poor prognosis. Advances of treatment due to the tyrosine kinase inhibitor imatinib have improved the cure rates. According to recent guidelines in the amended European intergroup trial on Ph+ ALL (EsPhALL), patients with rapid minimal residual disease (MRD) response and negativity during further treatment are no longer eligible for allogeneic stem cell transplantation (alloSCT). This down-grading of therapy in a circumscribed patient cohort with favorable prognosis is a desirable development as stem cell transplantation still implies a considerable risk of toxicity. These guidelines refer to MRD by immunoglobulin/T-cell receptor (Ig/TCR) gene rearrangements and do not consider monitoring of the BCR/ABL fusion transcript as long as informative results for Ig/TCR MRD are available. However, discrepancies between the results of the two methods occur. This complicates the decision on alloSCT indication if Ig/TCR MRD becomes negative while the BCR/ABL fusion transcript remains detectable.

Objectives/Methods: We therefore evaluated the prognostic relevance of this specific combination of findings, i.e. the continuous negativity for Ig/TCR MRD and persistently positive results for BCR/ABL after the second intensive consolidation block or later, in 16 pediatric patients with Ph+ ALL. They were identified among 139 German and Austrian Ph+ patients treated in the ALL-BFM 2000 or EsPhALL trial from August 1, 1999 to July 31, 2013. Twelve out of the 16 patients received imatinib in first-line treatment intermittently as previously described in the EsPhALL protocol (Biondi A et al Lancet Oncol. 2012) or continuously as recommended by the amended EsPhALL protocol.

Results: Eight of the 16 identified patients received an alloSCT in first complete remission (1^st CR), whereas the remaining eight patients were treated with chemotherapy only. Of the eight patients with alloSCT, seven are in first continuous complete remission (1^st CCR) with median EFS of 7.6 years, one patient died after second relapse. In the group of eight patients without alloSCT three are in 1^st CCR with a median EFS of 2.6 years, four patients are in 2^nd CR after relapse (3/4 had alloSCT in 2^nd CR, median EFS 4.7 years), and one patient with Down syndrome died of an infectious complication. Remarkably, two patients of the latter group (both with M-BCR) showed a protracted increase of BCR/ABL copy numbers over several years with neither morphological signs of relapse nor Ig/TCR MRD based reappearance. One of them eventually suffered a relapse 5 years after diagnosis, one is still in 1^st CCR with EFS of 5.2 years.

Conclusion: The data suggest that patients with Ig/TCR MRD negativity and persistently detectable BCR/ABL fusion transcript have a high risk of relapse when treated with chemotherapy only and may benefit from alloSCT. However, patient numbers are currently too small to deduce recommendations from this observation. Further investigation of a larger cohort with longer follow-up is needed to confirm the prognostic importance of BCR/ABL fusion transcript monitoring in addition to Ig/TCR MRD, especially considering a potential additional impact of the recently implemented continuous imatinib treatment. One additional patient would have met the diagnostic inclusion criteria of this analysis. He had an extensive increase of BCR/ABL fusion transcript at the end of maintenance treatment while being in morphological remission and negative for Ig/TCR MRD. This patient proved to be BCR/ABL positive in granulocytes revealing a chronic myeloid leukemia (CML) misdiagnosed as ALL during initial blast crisis. This indicates that an underlying CML might be taken into consideration also in other patients of the analyzed cohort. In consequence, BCR/ABL in granulocytes is now tested in all newly diagnosed Ph+ ALL patients in Germany to ensure the differentiation of BCR/ABL positive ALL vs. CML in blast crisis.