Quiescent Chronic Myelogenous Leukemia (CML) Cells Are Resistant to BCR-ABL Inhibitors but Preferentially Sensitive to BMS-214662, a Farnesyltransferase Inhibitor (FTI) with Unique Quiescent-Cell Selective Cytotoxicity.

Abstract

The major concern in the treatment of CML is resistance to the approved agent imatinib mesylate at all stages of disease, most commonly due to mutations in BCR-ABL (but other mechanisms have also been identified). Experimental agents such as dasatinib (BMS-354825), a novel, oral kinase inhibitor that targets BCR-ABL and SRC kinases, or AMN107, which targets BCR-ABL but not SRC, were designed to address all or parts of these mechanisms and are currently under clinical testing. A second concern in CML is persistence of BCR-ABL-positive cells or ‘residual disease' in the majority of patients on imatinib therapy, including those with complete cytogenetic responses. Bone marrow studies reveal that the residual disease resides at least in part in the primitive CD34+ progenitor compartment, suggesting that imatinib may not be effective against these cell populations (Bhatia et al, Blood 101:4701, 2003). Moreover, several imatinib-resistant ABL kinase domain mutations have been detected in CD34+/BCR-ABL+ progenitors (Chu et al, Blood 105:2093, 2005), a scenario for eventual disease relapse. A hallmark of CD34+ primitive CML progenitors is quiescence (Elrick et al, Blood 105:1862, 2005). We hypothesized that BCR-ABL inhibitors like imatinib may not be effective in killing CML cells in this non-proliferative state. This was tested by comparing cytotoxicity of imatinib or dasatinib in proliferating K562 cells and in cells forced into quiescence by nutrient depletion. Proliferating K562 cells were effectively killed by imatinib (IC50 250–500 nM) and dasatinib (IC50 <1.00 nM). However, cells in quiescent cultures were far more resistant (imatinib IC50 >5000 nM; dasatinib IC50 >12 nM), suggesting that these inhibitors may be less effective in eradicating quiescent CD34+ progenitors. BMS-214662 is a FTI in Phase I clinical development. Unlike many other FTI, BMS-214662 exhibits potent cytotoxic activity against a variety of human tumor cells, and uniquely, its cytotoxicity is highly selective against non-proliferating cancer cells of epithelial origin (Lee et al, Proceedings of the AACR 42:260s, 2001). We now demonstrate similar selectivity in K562 CML cells. BMS-214662 was 68-fold more potent in killing quiescent (IC50 = 0.7 uM) than proliferating K562 cells (IC50 = 47.5 uM). Because BCR-ABL inhibitors and BMS-214662 target distinct cell populations (proliferating vs quiescent), there may be a positive therapeutic interaction when these agents are used in combination. In vitro studies in quiescent K562 cultures demonstrated that the combination of BMS-214662 and dasatinib, at concentrations readily achievable in the clinic, produced supra-additive cytotoxicity (% cell kill: dasatinib alone = 0%, BMS-214662 alone = 21%, combination = 71%). In vivo studies in K562 xenografts implanted SC in mice also showed that the combination of BMS-214662 and dasatinib produced a superior anti-leukemic activity than either dasatinib alone (P=0.0157) or BMS-214662 alone (P=0.0002). These results highlight the potential utility of BMS-214662 for targeting the quiescent progenitor compartment which, in combination with targeted agents such as dasatinib, address both BCR-ABL-dependent and -independent mechanisms of resistance, and may produce more durable responses and suppress the emergence of resistance.