Philadelphia chromosome positive acute lymphoblastic leukaemia (Ph+ALL) occurs in 2–5% of paediatric ALL and is historically associated with a poor prognosis. Although 80–90% of children achieve remission, their event-free-survival (EFS) with conventional chemotherapy prior to tyrosine kinase inhibitors (TKI) was poor, with a 7-year EFS rate of 32% (Arico et al, 2010). The addition of imatinib as monotherapy appeared promising in initial treatment of adults with Ph+ALL, despite a high rate of relapse (Druker et al, 2001). Many relapsed adults on imatinib monotherapy were found to have a resistant mutation within the kinase domain of BCR-ABL1 (Jones et al, 2008). Other studies have shown that TKI's, such as imatinib or dasatinib, as monotherapy can select for TKI resistant clones, which may then be overcome by the addition of cytotoxic chemotherapy in the mouse model (Boulos et al, 2011).
The Children's Oncology Group (COG) clinical trial, AALL0031, used imatinib (340 mg/m2/d) in conjunction with intensive chemotherapy to treat children and adolescents with Ph+ALL (Schultz et al, 2009). This dosage is equivalent to approximately 600 mg/d in adults and was well tolerated with minimal additional side effects as compared to the identical chemotherapy arm without imatinib. AALL0031 differed from adult protocols in several aspects: use of drug combinations not common in adult protocols, intensive dosing of imatinib that was given continuously for the majority of 2.5 years and no continuation of TKI after completion of therapy. Three-year EFS on this treatment was 84% (more than double those of patients treated in the pre-imatinib era). Thus far, it remains unknown whether patients that relapse following this treatment approach have recurred due to development of imatinib resistance.
A 2-year-old male with Ph+ALL and initial white blood cell count of 117 × 109/l was initially treated with a standard four-drug induction of vincristine, asparginase, doxorubicin, and prednisone. At presentation he showed no evidence of extramedullary disease. He achieved complete morphological and cytogenetic remission at the end of induction. He then received post-induction therapy according to COG AALL0031 cohort 5 (not on study). His therapy included the intensive systemic regimen with central nervous system (CNS)-directed therapy without cranial radiation. Twenty-four months into treatment he presented with headaches and mental status changes caused by a CNS relapse. BCR-ABL1 sequence analysis of his cerebrospinal fluid (CSF) blasts identified a guanine substitution for adenine, producing the missense mutation methionine 244 to valine (M244V) (Fig 1A). Concomitant bone marrow aspiration showed no leukaemia by morphology, flow cytometry or by fluorescent in situ hybridization. However, sequence analysis of the marrow sample identified the same mutation found in his CSF. BCR-ABL1 sequencing of the bone marrow specimen from initial diagnosis identified no mutation (Fig 1A).
A biological correlate study to AALL0031 was developed to determine whether or not BCR-ABL1 kinase domain mutations were present in medullary relapse samples from Ph+ALL patients. COG AALL0031 enrolled 93 patients with Ph+ALL aged 1–21 years from 2002 to 2006 (Schultz et al, 2009). From this study, nine relapsed bone marrow samples were available for sequence analysis (Table 1). Eight of the nine samples from imatinib-treated patients showed no BCR-ABL1 kinase domain mutation (Table 1). One sample, from a patient who relapsed 15 months after diagnosis, carried the histidine 396 to proline (H396P) mutation (Fig 1B). A bone marrow sample from initial diagnosis of this child identified no mutation (Fig 1B).
|Case||Time to relapsea||Imatinib exposure||BCR-ABL1|
|Presented caseb||24 months||>1 year||M244V|
|1||26 months||<1 year (BMT)c||WT|
|2||10 months||<1 year||WT|
|4||12 months||<1 year||WT|
|5||23 months||<1 year||WT|
|6||19 months||<1 year (BMT)c||WT|
|7||39 months||>1 year||WT|
|8||45 months||>1 year||WT|
|9||15 months||>1 year||H396P|
These results further validate that BCR-ABL1 kinase domain mutations can occur after treatment of Ph+ALL with imatinib and intensive multiple chemotherapeutic agents. From these 10 samples we identified two resistant mutations from patients who received imatinib and combination chemotherapy for more than 1 year. This mutation rate appears to be less than previously published in adults treated with imatinib monotherapy (Jones et al, 2008) (15 of 17 relapsed patients) or with hyperCVAD (hyperfractionated cyclophosphamide, vincristine, doxorubicin, dexamethasone, methotrexate, cytarabine) combination therapy (Ravandi et al, 2010) where mutations were observed in three of five relapsed patients.
Neither mutation was detected in samples obtained at diagnosis suggesting that the vast majority of the leukaemic cells did not have the mutation. This does not preclude the concept of a low level of mutations at diagnosis, as previously shown (Pfeifer et al, 2007). M244V and H396 mutations have been shown to be more resistant to imatinib but both have been shown to be sensitive to second generation TKI's, such as nilotinib and dasatinib (O'Hare et al, 2005). Treatment with dasatinib has been shown to overcome H396R resistance in CML (Talpaz et al, 2006).
Our results are the first to describe BCR-ABL1 kinase domain mutations in paediatric patients with Ph+ALL treated with intensive chemotherapy and imatinib. We are also the first to report an imatinib-resistant BCR-ABL1 kinase mutation from a CNS recurrence in a paediatric patient. It has been previously shown that imatinib has low penetrance into the CNS, which implies that selective pressure occurred systemically followed by expansion in the sanctuary of the CNS.
The two mutations identified here are predicted to be sensitive to second generation TKIs, suggesting that these TKIs may be effective reinduction therapy for relapse following treatment of Ph+ALL with chemotherapy and imatinib. Importantly, dasatinib penetrates the CNS, a property that may help to decrease the risk of CNS recurrence in Ph+ALL. These next generation TKI's may further decrease relapse rates when used for initial therapy of Ph+ALL. Many patients who experience a relapse with combination chemotherapy and a TKI do not appear to carry a TKI-resistant mutation, suggesting that other BCR-ABL1 independent pathways play critical roles in leukaemia cell survival. Other tyrosine kinases, such as HCK, FGR, and LYN, are essential for Ph+ALL transformation (Hu et al, 2004). Therefore, less selective inhibitors like dasatinib may play an important role in salvage therapy for these patients. The current and planned COG Ph+ALL trials combine dasatinib rather than imatinib with intensive chemotherapy. Future studies will address whether dasatinib-resistant BCR-ABL1 mutants develop in patients who relapse on these studies.