CML is a myeloproliferative disease that originates in a haemopoietic stem cell as a result of the t(9;22) translocation, giving rise to the Philadelphia chromosome and bcr-abl oncoprotein. The disease starts in chronic phase, but as a result of genomic instability, it progresses over time to accelerated phase and then to blast crisis, becoming increasingly resistant to therapy. Bcr-abl is a constitutively active tyrosine kinase that has been targeted by tyrosine kinase inhibitors (TKIs), including imatinib (IM), nilotinib and dasatinib. We have developed various flow cytometry techniques to enable us to isolate candidate CML stem cells from chronic phase patients at diagnosis that efflux Hoechst dye, express CD34, lack CD38 and are cytokine non-responsive in culture over periods of up to 12 days in growth factors. These stem cells have been shown to regenerate bcr-abl positive haemopoiesis in immunocompromised mice upon transplantation(1). We previously demonstrated that IM was anti-proliferative for CML stem cells but did not induce apoptosis(2). Clinical experience now confirms that IM may not target CML stem cells in vivo with few patients achieving complete molecular remission and relapse occurring rapidly upon drug withdrawal. Our recent efforts have focused on understanding why CML stem cells are resistant to IM and on trying to find novel ways to induce apoptosis of this population. We have shown that CML stem cells express very high levels of functional wild type bcr-abl – no kinase domain mutations have been detected in the stem cell population. Dasatinib, a more potent multi-targeted TKI than IM, inhibits bcr-abl activity more efficiently than IM but still does not induce apoptosis of the stem cell population (3). Most recently we have tested a number of novel drug combinations and found that farnesyltransferase inhibitors have activity against CML (4). BMS-214662 is the most effective of these and induces apoptosis of phenotypically and functionally defined CML stem cells in vitro, as a single agent and in combination with IM or dasatinib. The effect against CML stem cells is selective with little effect on normal stem cells. The drug is also effective against blast crisis CML stem cells and equally effective against wild type and mutant bcr-abl, including the most resistant mutant T315I. In association with apoptosis there is activation of caspase-8 and caspase-3, inhibition of the MAPK pathway, IAP-1, NF-KB and iNOS. Furthermore, BMS-214662 synergises with MEK1/2 inhibitors, suggesting a second mechanism other that RAS inhibition for induction of apoptosis. Our intentions are now to explore the activity of BMS-214662 in other cancer stem cell disorders and to move this pre-clinical work to a clinical trial combining dasatinib with BMS-214662 in CML.