Exploring the Pre-B receptor checkpoint as a therapeutic target in acute lymphoblasic leukaemia

Abstract

Acute lymphoblastic leukaemia (ALL) is a clonal disorder of developing lymphocytes and is the most common malignancy in children and adolescents. While cure rates are high, treatment is associated with significant morbidity and relapsed ALL remains one of the leading causes of cancer‐related deaths in children. New, less toxic therapies are clearly needed for refractory ALL. There are number of lines of evidence to suggest that ALL cells hijack component of Pre‐B cell receptor signalling and that this dependency may be amenable to therapeutic exploitation. There are a number of tyrosine kinase inhibitors (TKIs) targeting Pre‐BCR signalling that are showing great promise in the clinic for other leukaemic subtypes which warrant preclinical evaluation in childhood ALL. These include CAL‐101 (PI3K‐δ inhibitor), Ibrutinib (BTK inhibitor), Fostamatinib (SYK inhibitor) and Dasatinib (BCR‐ABL/SRC inhibitor). TKIs were evaluated in ALL cells, including cell lines and patient derived xenograft samples (PDX) from 15 predominantly high risk/relapse primary ALLs. ALL cell lines were generally resistant to all drugs but modest sensitivity was seen to the active form of fostamatinib, R406 and dasatinib in Pre‐BCR+ cells. CAL‐101 and dasatinib were shown to be cytostatic, while ibrutinib and R406 were associated with cell cycle arrest and induction of apoptosis. Pharmacodynamic assessments using phospho‐flow cytometry and western blotting showed inhibition of the relevant targets at the GI50 concentrations. PDX ALL cells were generally more sensitive than the cell lines; CAL‐101 (2 from 14 PDXs); ibrutinib (3 from 14 PDXs); R406 (6 from 15 PDXs) and dasatinib (4 from 15 PDXs). Pre‐BCR+ ALL cells were more likely to be sensitive to dasatinib and fostamatinib. Some Pre‐BCR‐ ALL were also sensitive to some TKIs, although predictive biomarkers remain to be established. Significant synergism was seen after co‐treatment of the TKIs with the glucocorticoid (GC), dexamethasone. This was most marked for the dexamethasone and dasatinib combination and significantly potentiated G1 arrest and apoptosis in both GC sensitive and resistant ALL cells. Synergism was associated with a significant increase in expression of the GR target, GILZ and enhanced induction of proapoptotic BIM. In vivo preclinical confirmation of these data may offer new therapies for refractory ALL.