Tyrosine kinase inhibitor insensitivity of non‐cycling CD34+ human acute myeloid leukaemia cells with FMS‐like tyrosine kinase 3 mutations

Summary The efficacy of tyrosine kinase (TK) inhibitors on non‐cycling acute myeloid leukaemia (AML) cells, previously shown to have potent tumourigenic potential, is unknown. This pilot study describes the first attempt to characterize non‐cycling cells from a small series of human FMS‐like tyrosine kinase 3 (FLT3) mutation positive samples. CD34+ AML cells from patients with FLT3 mutation positive AML were cultured on murine stroma. In expansion cultures, non‐cycling cells were found to retain CD34+ expression in contrast to dividing cells. Leukaemic gene rearrangements could be detected in non‐cycling cells, indicating their leukaemic origin. Significantly, the FLT3‐internal tandem duplication (ITD) mutation was found in the non‐cycling fraction of four out of five cases. Exposure to the FLT3‐directed inhibitor TKI258 clearly inhibited the growth of AML CD34+ cells in short‐term cultures and colony‐forming unit assays. Crucially, non‐cycling cells were not eradicated, with the exception of one case, which exhibited exquisite sensitivity to the compound. Moreover, in longer‐term cultures, TKI258‐treated non‐cycling cells showed no growth impairment compared to treatment‐naive non‐cycling cells. These findings suggest that non‐cycling cells in AML may constitute a disease reservoir that is resistant to TK inhibition. Further studies with a larger sample size and other inhibitors are warranted.