Neil Shah and colleagues have now investigated the mechanisms behind this observation. Using phosphorylation of the BCR–ABL1 substrate CRKL as a surrogate marker of BCR–ABL1 inhibition, the authors showed that, in patients treated with dasatinib, CRKL phosphorylation decreased 4 hours after dosing but was restored by 8 hours. They modelled this in vitro using K562 CML cells treated with concentrations of imatinib and dasatinib that were chosen to reflect clinically effective doses achieved in patients. When the cells were assessed 48 hours after treatment, about 90% of cells treated with dasatinib for only 1 hour were dead; 24 hours of treatment with imatinib was required for the same level of cytotoxicity. However, treatment of K562 cells with eight different concentrations of either dasatinib or imatinib for times ranging from 20 minutes to 48 hours established a clear concentration-dependent effect whereby transient high doses and continuous low doses induced equivalent cytotoxicity. Phosphorylation of BCR–ABL1 was indeed transient after short-term high-dose therapy with either imatinib or dasatinib, and studies with BCR–ABL1-negative myeloid leukaemia cell lines and K562 cells expressing BCR–ABL1 carrying the T315I mutation indicated that off-target effects were not likely to be responsible for the observed cytotoxicity.
How does transient inhibition of BCR–ABL1 induce cell death? The authors investigated the kinetics of induction of apoptotic pathways and found identical irreversible induction of the pro-apoptotic BH3-only protein BIM (also known as BCL2L11) in K562 cells after either transient potent or prolonged BCR–ABL1 inhibition. Conversely, BIM induction was fully reversible when the cells were treated transiently with lower doses.
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