Medical Research Society

Chronic myeloid leukemia (CML), a hematopoietic stem cell (HSC) disorder, cannot be eradicated by conventional chemotherapy or the tyrosine kinase inhibitor (TKI) imatinib mesylate (IM). This disease persistence results from a population of quiescent CML HSC that are also not effectively targeted by the second generation BCR-ABL TKIs, dasatinib (DAS) or nilotinib in vitro. To target CML stem/progenitor cells we investigated BMS214662 (BMS), a cytotoxic farnesyltransferase inhibitor (FTI), previously reported to kill non-proliferating tumour cells, which has anti-leukaemic activity in acute myeloid leukaemia. We assessed the efficacy of BMS alone and in combination with IM or DAS in primary CD34 CML cells in vitro using a CFSE-based flow cytometry method to track cell division, caspase-3 activity, TUNEL and TMRE to measure apoptosis and long term cultureinitiating cell (LTC-IC) assays and an in vivo K562 xenograft model to assess stem cell response. IM or DAS alone reversibly arrested proliferation of CML stem/progenitor cells without inducing apoptosis. In contrast, BMS, alone or in combination with IM or DAS, potently induced apoptosis of both proliferating and quiescent CML stem/progenitor cells with <1% recovery of Philadelphia positive LTC-IC (P=0.033). Normal stem/progenitor cells were relatively spared by BMS, suggesting selectivity for leukaemic stem/progenitor cells. In vivo, BMS synergised with DAS to eradicate K562 xenografts, whereas either drug alone was ineffective. The ability to induce selective apoptosis of leukaemic stem/progenitor cells was unique to BMS and not seen with a structurally similar FTI, BMS-225975. BMS was cytotoxic against CML blast crisis stem/progenitor cells, particularly in combination with a TKI and equally effective in cell lines harbouring wild-type versus mutant BCR-ABL, including the T315I mutation. This is the first report of an agent with activity in resistant and blast crisis CML that selectively kills CML stem cells through apoptosis and offers potential for the eradication of chronic phase CML. Y2