Dual-specific Src and Abl kinase inhibitors, PP1 and CGP76030, inhibit growth and survival of cells expressing imatinib mesylate-resistant Bcr-Abl kinases

Overcoming BCR::ABL1 dependent and independent survival mechanisms in chronic myeloid leukaemia using a multi-kinase targeting approach

BACKGROUND

Despite improved patient outcome using tyrosine kinase inhibitors (TKIs), chronic myeloid leukaemia (CML) patients require life-long treatment due to leukaemic stem cell (LSC) persistence. LSCs reside in the bone marrow (BM) niche, which they modify to their advantage. The BM provides oncogene-independent signals to aid LSC cell survival and quiescence. The bone-morphogenetic pathway (BMP) is one pathway identified to be highly deregulated in CML, with high levels of BMP ligands detected in the BM, accompanied by CML stem and progenitor cells overexpressing BMP type 1 receptors- activin-like kinases (ALKs), especially in TKI resistant patients. Saracatinib (SC), a SRC/ABL1 dual inhibitor, inhibits the growth of CML cells resistant to the TKI imatinib (IM). Recent studies indicate that SC is also a potent ALK inhibitor and BMP antagonist. Here we investigate the efficacy of SC in overcoming CML BCR::ABL1 dependent and independent signals mediated by the BM niche both in 2D and 3D culture.

METHODS

CML cells (K562 cell line and CML CD34+ primary cells) were treated with single or combination treatments of: IM, SC and the BMP receptors inhibitor dorsomorphin (DOR), with or without BMP4 stimulation in 2D (suspension) and 3D co-culture on HS5 stroma cell line and mesenchymal stem cells in AggreWell and microfluidic devices. Flow cytometry was performed to investigate apoptosis, cell cycle progression and proliferation, alongside colony assays following treatment. Proteins changes were validated by immunoblotting and transcriptional changes by Fluidigm multiplex qPCR.

RESULTS

By targeting the BMP pathway, using specific inhibitors against ALKs in combination with SRC and ABL TKIs, we show an increase in apoptosis, altered cell cycle regulation, fewer cell divisions, and reduced numbers of CD34+ cells. Impairment of long-term proliferation and differentiation potential after combinatorial treatment also occurred.

CONCLUSION

BMP signalling pathway is important for CML cell survival. Targeting SRC, ABL and ALK kinases is more effective than ABL inhibition alone, the combination efficacy importantly being demonstrated in both 2D and 3D cell cultures highlighting the need for combinatorial therapies in contrast to standard of care single agents. Our study provides justification to target multiple kinases in CML to combat LSC persistence.

Natural course and biology of CML

Chronic myeloid leukaemia (CML) is a myeloproliferative disorder arising in the haemopoietic stem cell (HSC) compartment. This disease is characterised by a reciprocal t(9;22) chromosomal translocation, resulting in the formation of the Philadelphia (Ph) chromosome containing the BCR-ABL1 gene. As such, diagnosis and monitoring of disease involves detection of BCR-ABL1. It is the BCR-ABL1 protein, in particular its constitutively active tyrosine kinase activity, that forges the pathogenesis of CML. This aberrant kinase signalling activates downstream targets that reprogram the cell to cause uncontrolled proliferation and results in myeloid hyperplasia and 'indolent' symptoms of chronic phase (CP) CML. Without successful intervention, the disease will progress into blast crisis (BC), resembling an acute leukaemia. This advanced disease stage takes on an aggressive phenotype and is almost always fatal. The cell biology of CML is also centred on BCR-ABL1. The presence of BCR-ABL1 can explain virtually all the cellular features of the leukaemia (enhanced cell growth, inhibition of apoptosis, altered cell adhesion, growth factor independence, impaired genomic surveillance and differentiation). This article provides an overview of the clinical and cell biology of CML, and highlights key findings and unanswered questions essential for understanding this disease.

In silico identification of novel kinase inhibitors targeting wild-type and T315I mutant ABL1 from FDA-approved drugs

The constitutively active fusion protein BCR-ABL1 is the major cause of chronic myeloid leukemia (CML), and selective inhibition of ABL1 is a promising approach for the treatment of CML. Reported drugs worked well in clinical practice, such as imatinib, dasatinib, nilotinib and bosutinib. However, resistance arises due to ABL1 mutation in patients, especially the T315I gate-keeper mutation. Thus, wide spectrum drugs targeting ABL1 are urgently needed. In order to screen potential drugs targeting wild-type ABL1 and T315I mutant ABL1, 1408 FDA approved small molecule drugs were subjected to molecular docking. With subsequent molecular dynamic (MD) simulation and MM/GBSA binding free energy calculation and energy decomposition, we identified chlorhexidine and sorafenib as potential "new use" drugs targeting wild-type ABL1, while nicergoline and plerixafor targeted T315I ABL1. Meanwhile, we also found that residues located in the ATP-binding site and A-loop motif played key roles in drug discovery towards ABL1. These findings may not only serve as a paradigm for the repositioning of existing approved drugs, but also instill new vitality to ABL1-targeted anti-CML therapeutics.

Current concepts in pediatric Philadelphia chromosome-positive acute lymphoblastic leukemia

The t(9;22)(q34;q11) or Philadelphia chromosome creates a BCR-ABL1 fusion gene encoding for a chimeric BCR-ABL1 protein. It is present in 3-4% of pediatric acute lymphoblastic leukemia (Ph(+) ALL), and about 25% of adult ALL cases. Prior to the advent of tyrosine kinase inhibitors (TKI), Ph(+) ALL was associated with a very poor prognosis despite the use of intensive chemotherapy and frequently hematopoietic stem-cell transplantation (HSCT) in first remission. The development of TKIs revolutionized the therapy of Ph(+) ALL. Addition of the first generation ABL1 class TKI imatinib to intensive chemotherapy dramatically increased the survival for children with Ph(+) ALL and established that many patients can be cured without HSCT. In parallel, the mechanistic understanding of Ph(+) ALL expanded exponentially through careful mapping of pathways downstream of BCR-ABL1, the discovery of mutations in master regulators of B-cell development such as IKZF1 (Ikaros), PAX5, and early B-cell factor (EBF), the recognition of the complex clonal architecture of Ph(+) ALL, and the delineation of genomic, epigenetic, and signaling abnormalities contributing to relapse and resistance. Still, many important basic and clinical questions remain unanswered. Current clinical trials are testing second generation TKIs in patients with newly diagnosed Ph(+) ALL. Neither the optimal duration of therapy nor the optimal chemotherapy backbone are currently defined. The role of HSCT in first remission and post-transplant TKI therapy also require further study. In addition, it will be crucial to continue to dig deeper into understanding Ph(+) ALL at a mechanistic level, and translate findings into complementary targeted approaches. Expanding targeted therapies hold great promise to decrease toxicity and improve survival in this high-risk disease, which provides a paradigm for how targeted therapies can be incorporated into treatment of other high-risk leukemias.

Targeting protein kinases in cancer therapy: a success?

The fundamental role of kinases in cancer progression has promoted the development of a plethora of therapeutic inhibitors. Despite the promise of effective treatment with little associated toxicity, the clinical experience with these agents has been mixed. This review will summarize recent advances made in the development of kinase inhibitors to highlight emerging issues and the strategies by which they being addressed.

JAK of all trades: JAK2-STAT5 as novel therapeutic targets in BCR-ABL1+ chronic myeloid leukemia

The transcription factor signal transducers and activators of transcription 5 (STAT5) has an important and unique role in Breakpoint Cluster Region - Abelson 1 (BCR-ABL1)-driven neoplasias. STAT5 is an essential component in the signaling network that maintains the survival and growth of chronic myeloid leukemia (CML) cells. In contrast, the function of the prototypical upstream kinase of STAT5, the Janus kinase JAK2, in CML is still under debate. Although there is widespread agreement that JAK2 is part of the signaling network downstream of BCR-ABL1, it is unclear whether and under what circumstances JAK2 inhibitors may be beneficial for CML patients. Recent studies in murine models have cast doubt on the importance of JAK2 in CML maintenance. Nevertheless, JAK2 has been proposed to have a central role in the cytokine signaling machinery that allows the survival of CML stem cells in the presence of BCR-ABL1 tyrosine kinase inhibitors. In this review, we summarize the current debate and provide an overview of the arguments on both sides of the fence. We present recent evidence showing that CML stem cells do not depend on BCR-ABL1 kinase activity but require the continuous support of the hematopoietic niche and its distinct cytokine environment and suggest that it has the potential to resolve the dispute.

Src family kinases mediate cytoplasmic retention of activated STAT5 in BCR-ABL-positive cells

Persistent activation of the Abl tyrosine kinase in the BCR-ABL fusion protein is the major cause of chronic myeloid leukemia (CML). Among many other substrates BCR-ABL phosphorylates STAT5 and Src family kinases (SFK). Activated pSTAT5 is essential for initial transformation and maintenance of the disease. Cytokine-induced phosphorylation on tyrosine 694 typically leads to nuclear accumulation of pSTAT5 and target gene expression. We verified that in BCR-ABL-positive progenitor cells from a CML patient and in K562 cells pSTAT5 is cytoplasmic. However, upon ectopic expression of BCR-ABL p210 in non-myeloid cells, co-transfected STAT5A is phosphorylated on Y694 and localized in the nucleus arguing for an additional factor mediating cytoplasmic retention in CML cells. Expression of the SFK v-Src, Hck or Lyn together with STAT5A results in phosphorylation on Y694 and cytoplasmic retention. Upon coexpression of BCR-ABL and individual SFK the cytoplasmic retention of activated STAT5A mediated by v-Src and Hck but not Lyn is dominant over nuclear translocation induced by BCR-ABL. Cytoplasmic retention depends on the kinase activity of SFK and is mediated through the interaction of the SH2 domain of STAT5A with the SFK. Interestingly, nuclear accumulation of STAT5A as a result of activation by FLT3-ITD, an oncogene found in acute myeloid leukemia, cannot be prevented by coexpression of SFK. Importantly, inhibition of SFK in K562 cells restored nuclear accumulation of pSTAT5A, enhanced STAT5 target gene expression and increased colony formation. Thus, SFK mediate cytoplasmic retention of pSTAT5A in BCR-ABL-positive cells. Cytoplasmic pSTAT5A in CML cells might balance the controversial functions of STAT5 in cellular senescence and differentiation versus G1/S progression and survival.

Functions of the Lyn tyrosine kinase in health and disease

Src family kinases such as Lyn are important signaling intermediaries, relaying and modulating different inputs to regulate various outputs, such as proliferation, differentiation, apoptosis, migration and metabolism. Intriguingly, Lyn can mediate both positive and negative signaling processes within the same or different cellular contexts. This duality is exemplified by the B-cell defect in Lyn-/- mice in which Lyn is essential for negative regulation of the B-cell receptor; conversely, B-cells expressing a dominant active mutant of Lyn (Lynup/up) have elevated activities of positive regulators of the B-cell receptor due to this hyperactive kinase. Lyn has well-established functions in most haematopoietic cells, viz. progenitors via influencing c-kit signaling, through to mature cell receptor/integrin signaling, e.g. erythrocytes, platelets, mast cells and macrophages. Consequently, there is an important role for this kinase in regulating hematopoietic abnormalities. Lyn is an important regulator of autoimmune diseases such as asthma and psoriasis, due to its profound ability to influence immune cell signaling. Lyn has also been found to be important for maintaining the leukemic phenotype of many different liquid cancers including acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML) and B-cell lymphocytic leukaemia (BCLL). Lyn is also expressed in some solid tumors and here too it is establishing itself as a potential therapeutic target for prostate, glioblastoma, colon and more aggressive subtypes of breast cancer. LAY To relay information, a cell uses enzymes that put molecular markers on specific proteins so they interact with other proteins or move to specific parts of the cell to have particular functions. A protein called Lyn is one of these enzymes that regulate information transfer within cells to modulate cell growth, survival and movement. Depending on which type of cell and the source of the information input, Lyn can positively or negatively regulate the information output. This ability of Lyn to be able to both turn on and turn off the relay of information inside cells makes it difficult to fully understand its precise function in each specific circumstance. Lyn has important functions for cells involved in blood development, including different while blood cells as well as red blood cells, and in particular for the immune cells that produce antibodies (B-cells), as exemplified by the major B-cell abnormalities that mice with mutations in the Lyn gene display. Certain types of leukaemia and lymphoma appear to have too much Lyn activity that in part causes the characteristics of these diseases, suggesting it may be a good target to develop new anti-leukaemia drugs. Furthermore, some specific types, and even specific subtypes, of solid cancers, e.g. prostate, brain and breast cancer can also have abnormal regulation of Lyn. Consequently, targeting this protein in these cancers could also prove to be beneficial.

Tyrosine kinase inhibitors as potential drugs for B-cell lymphoid malignancies and autoimmune disorders

INTRODUCTION

In the last few years, several tyrosine kinase inhibitors (TKIs) have been synthesized and become available for preclinical studies and clinical trials. This article summarizes recent achievements in the mechanism of action, pharmacological properties, and clinical activity and toxicity, as well as the emerging role of TKIs in lymphoid malignancies, allergic diseases, and autoimmune disorders.

AREAS COVERED

A literature review was conducted of the MEDLINE database PubMed for articles in English. Publications from 2000 through January 2012 were scrutinized. The search terms used were Bruton's tyrosine kinase (Btk) inhibitors, PCI-32765, GDC-0834, LFM-A13, AVL-101, AVL-292, spleen tyrosine kinase (Syk) inhibitors, R343, R406, R112, R788, fostamatinib, BAY-61-3606, C-61, piceatannol, Lyn, imatinib, nilotinib, bafetinib, dasatinib, GDC-0834, PP2, SU6656 in conjunction with lymphoid malignancy, NHL, CLL, autoimmune disease, allergic disease, asthma, and rheumatoid arthritis. Conference proceedings from the previous 5 years of the American Society of Hematology, European Hematology Association, American Society of Clinical Oncology, and ACR/ARHP Annual Scientific Meetings were searched manually. Additional relevant publications were obtained by reviewing the references from the chosen articles.

EXPERT OPINION

The use of TKIs, especially inhibitors of Btk, Syk, and Lyn, is a promising new strategy for targeted treatment of B-cell lymphoid malignancies, autoimmune disorders and allergic diseases. However, definitive data from ongoing and future clinical trials will aid in better defining the status of TKIs in the treatment of these disorders.

Targeting Lyn tyrosine kinase through protein fusions encompassing motifs of Cbp (Csk-binding protein) and the SOCS box of SOCS1

The tyrosine kinase Lyn is involved in oncogenic signalling in several leukaemias and solid tumours, and we have previously identified a pathway centred on Cbp [Csk (C-terminal Src kinase)-binding protein] that mediates both enzymatic inactivation, as well as proteasomal degradation of Lyn via phosphorylation-dependent recruitment of Csk (responsible for phosphorylating the inhibitory C-terminal tyrosine of Lyn) and SOCS1 (suppressor of cytokine signalling 1; an E3 ubiquitin ligase). In the present study we show that fusing specific functional motifs of Cbp and domains of SOCS1 together generates a novel molecule capable of directing the proteasomal degradation of Lyn. We have characterized the binding of pY (phospho-tyrosine) motifs of Cbp to SFK (Src-family kinase) SH2 (Src homology 2) domains, identifying those with high affinity and specificity for the SH2 domain of Lyn and that are preferred substrates of active Lyn. We then fused them to the SB (SOCS box) of SOCS1 to facilitate interaction with the ubiquitination-promoting elongin B/C complex. As an eGFP (enhanced green fluorescent protein) fusion, these proteins can direct the polyubiquitination and proteasomal degradation of active Lyn. Expressing this fusion protein in DU145 cancer cells (but not LNCaP or MCF-7 cells), that require Lyn signalling for survival, promotes loss of Lyn, loss of caspase 3, appearance of an apoptotic morphology and failure to survive/expand. These findings show how functional domains of Cbp and SOCS1 can be fused together to generate molecules capable of inhibiting the growth of cancer cells that express high levels of active Lyn.

Breast cancer patients' clinical outcome measures are associated with Src kinase family member expression

Background:

This study determined mRNA expression levels for Src kinase family (SFK) members in breast tissue specimens and assessed protein expression levels of prominent SFK members in invasive breast cancer to establish associations with clinical outcome. Ki67 was investigated to determine association between SFK members and proliferation.

Methods:

The mRNA expression levels were assessed for eight SFK members by quantitative real-time PCR. Immunohistochemistry was performed for c-Src, Lyn, Lck and Ki67.

Results:

mRNA expression was quantified in all tissue samples. SRC and LYN were the most highly expressed in malignant tissue. LCK was more highly expressed in oestrogen receptor (ER)-negative, compared with ER-positive tumours. High cytoplasmic Src kinase protein expression was significantly associated with decreased disease-specific survival. Lyn was not associated with survival at any cellular location. High membrane Lck expression was significantly associated with improved survival. Ki67 expression correlated with tumour grade and nuclear c-Src, but was not associated with survival.

Conclusions:

All eight SFK members were expressed in different breast tissues. Src kinase was highest expressed in breast cancer and had a negative impact on disease-specific survival. Membrane expression of Lck was associated with improved clinical outcome. High expression of Src kinase correlated with high proliferation.

Design, synthesis and evaluation of (E)-alpha-benzylthio chalcones as novel inhibitors of BCR-ABL kinase

Novel (E)-alpha-benzylthio chalcones are reported with preliminary in vitro activity data indicating that several of them are potent inhibitors (comparable to imatinib, the reference compound) of BCR-ABL phosphorylation in leukemic K562 cells, known to express high levels of BCR-ABL. The ability of such compounds to significantly inhibit K562 cell proliferation suggests that this scaffold could be a promising lead for the development of anticancer agents that are able to block BCR-ABL phosphorylation in leukemic cells.

Activity of FB2, a novel dual Abl/Src tyrosine kinase inhibitor, against imatinib-resistant chronic myeloid leukemia in vivo and in vitro

FB2 is a novel Abl/Src dual tyrosine kinase inhibitor which is designed to overcome imatinib resistance. Besides imatinib-sensitive cell lines (K562), FB2 significantly inhibited the growth of imatinib-resistant cell lines of different resistance mechanisms (K562/G5.0 and K562/G01), and decreased the expression of autophosphorylation of Bcr/Abl, c-Src and Lyn kinases on them. It also inhibited the proliferation of Src over activated cells DU145 and MDA-MB-231. Furthermore, FB2 potently prolonged the survival time of non-obese diabetic/severe combined immunodeficient mice harboured K562/G5.0 cells. These results indicated that FB2, an Abl/Src dual tyrosine kinase inhibitor, is a promising candidate for imatinib-resistant CML and Src over activated cancer.

Understanding the role of mutations in therapeutic decision making for chronic myeloid leukemia

In patients with chronic myeloid leukemia (CML) resistant to imatinib, resistance is commonly associated with mutations in the BCR-ABL protein. Approximately 85% to 90% of resistance-associated mutations occur within the ABL kinase domain, and confer resistance either directly, by blocking imatinib binding, or indirectly, by altering the conformation of BCR-ABL. The degree of resistance depends on the mutation, with some remaining sensitive to imatinib. Imatinib dose escalation may overcome resistance in some of these patients or therapy can be switched to the second-generation tyrosine kinase inhibitors (TKIs) nilotinib or dasatinib. The long-term efficacy of second-generation TKIs may also be related to specific BCR-ABL mutations, with the T315I mutant remaining resistant to all currently available TKIs. Other treatments, including investigational agents, may be options for patients with this mutation. The choice of therapy should be guided by multiple factors, including mutational analysis, disease phase, patient characteristics, and the safety profile of the agents.

Targeted treatment of imatinib-resistant chronic myeloid leukemia: Focus on dasatinib

The efficacy of imatinib in chronic myeloid leukemia has been remarkable, but the development of resistance and the persistence of minimal residual disease have dampened the initial enthusiasm for this much heralded 'magic bullet'. Much progress has been made in elucidating the mechanisms which underlie imatinib resistance. The most common cause of such drug resistance is the selection of leukemic clones with point mutations in the Abl kinase domain leading to amino acid substitutions which prevent the appropriate binding of the drug. Other mechanisms include genomic amplification of BCR-ABL and modulation of drug efflux or influx transporters. Dasatinib is a multi-target kinase inhibitor which has increased potency and is able to inhibit most Bcr-Abl mutant cell lines. Clinical trials of dasatinib in imatinib-resistant and -intolerant chronic myeloid leukemia and Philadelphia chromosome-positive acute lymphoid leukemia have shown that it is effective and well tolerated. In this review, we will discuss the pre-clinical development of dasatinib, the clinical trial data demonstrating its efficacy and tolerability and highlight certain aspects of its toxicity profile and mechanisms of resistance.

The effect of the dual Src/Abl kinase inhibitor AZD0530 on Philadelphia positive leukaemia cell lines

Background

Imatinib mesylate, a selective inhibitor of Abl tyrosine kinase, is efficacious in treating chronic myeloid leukaemia (CML) and Ph+ acute lymphoblastic leukaemia (ALL). However, most advanced-phase CML and Ph+ ALL patients relapse on Imatinib therapy. Several mechanisms of refractoriness have been reported, including the activation of the Src-family kinases (SFK). Here, we investigated the biological effect of the new specific dual Src/Abl kinase inhibitor AZD0530 on Ph+ leukaemic cells.

Methods

Cell lines used included BV173 (CML in myeloid blast crisis), SEM t(4;11), Ba/F3 (IL-3 dependent murine pro B), p185^Bcr-Abl infected Ba/F3 cells, p185^Bcr-Abl mutant infected Ba/F3 cells, SupB15 (Ph+ ALL) and Imatinib resistant SupB15 (RTSupB15) (Ph+ ALL) cells. Cells were exposed to AZD0530 and Imatinib. Cell proliferation, apoptosis, survival and signalling pathways were assessed by dye exclusion, flow cytometry and Western blotting respectively.

Results

AZD0530 specifically inhibited the growth of, and induced apoptosis in CML and Ph+ ALL cells in a dose dependent manner, but showed only marginal effects on Ph- ALL cells. Resistance to Imatinib due to the mutation Y253F in p185^Bcr-Abl was overcome by AZD0530. Combination of AZD0530 and Imatinib showed an additive inhibitory effect on the proliferation of CML BV173 cells but not on Ph+ ALL SupB15 cells. An ongoing transphosphorylation was demonstrated between SFKs and Bcr-Abl. AZD0530 significantly down-regulated the activation of survival signalling pathways in Ph+ cells, resistant or sensitive to Imatinib, with the exception of the RTSupB15.

Conclusion

Our results indicate that AZD0530 targets both Src and Bcr-Abl kinase activity and reduces the leukaemic maintenance by Bcr-Abl.

Targeting Src signaling in metastatic bone disease

Src is a tyrosine kinase involved in the regulation of a range of cellular processes including proliferation, adhesion, motility and survival. In addition, it is a key regulator of bone metabolism. Src has been implicated in the pathogenesis of a number of cancers, and has been found to be overexpressed in breast, prostate, colorectal, pancreatic and nonsmall-cell lung tumors. There is also evidence that aberrant Src signaling may contribute to the increased osteoclastic activity associated with bone metastases. Bone metastases frequently occur in cancer patients with advanced disease. The metastasized cells disrupt normal bone remodeling pathways resulting in the release of growth factors that further promote tumor growth. Thus, a cycle of metastatic bone destruction is initiated, leading to compromised skeletal integrity and substantially reduced quality of life. Because of the role of Src in both cancer development and in bone metabolism, it may provide a therapeutic target for patients with bone metastases.

Crystal structures of the Lyn protein tyrosine kinase domain in its Apo- and inhibitor-bound state

The Src-family protein-tyrosine kinase (PTK) Lyn is the most important Src-family kinase in B cells, having both inhibitory and stimulatory activity that is dependent on the receptor, ligand, and developmental context of the B cell. An important role for Lyn has been reported in acute myeloid leukemia and chronic myeloid leukemia, as well as certain solid tumors. Although several Src-family inhibitors are available, the development of Lyn-specific inhibitors, or inhibitors with reduced off-target activity to Lyn, has been hampered by the lack of structural data on the Lyn kinase. Here we report the crystal structure of the non-liganded form of Lyn kinase domain, as well as in complex with three different inhibitors: the ATP analogue AMP-PNP; the pan Src kinase inhibitor PP2; and the BCR-Abl/Src-family inhibitor Dasatinib. The Lyn kinase domain was determined in its "active" conformation, but in the unphosphorylated state. All three inhibitors are bound at the ATP-binding site, with PP2 and Dasatinib extending into a hydrophobic pocket deep in the substrate cleft, thereby providing a basis for the Src-specific inhibition. Analysis of sequence and structural differences around the active site region of the Src-family PTKs were evident. Accordingly, our data provide valuable information for the further development of therapeutics targeting Lyn and the important Src-family of kinases.

Transient inhibition of ATM kinase is sufficient to enhance cellular sensitivity to ionizing radiation

In response to DNA damage, the ATM protein kinase activates signal transduction pathways essential for coordinating cell cycle progression with DNA repair. In the human disease ataxia-telangiectasia, mutation of the ATM gene results in multiple cellular defects, including enhanced sensitivity to ionizing radiation (IR). This phenotype highlights ATM as a potential target for novel inhibitors that could be used to enhance tumor cell sensitivity to radiotherapy. A targeted compound library was screened for potential inhibitors of the ATM kinase, and CP466722 was identified. The compound is nontoxic and does not inhibit phosphatidylinositol 3-kinase (PI3K) or PI3K-like protein kinase family members in cells. CP466722 inhibited cellular ATM-dependent phosphorylation events and disruption of ATM function resulted in characteristic cell cycle checkpoint defects. Inhibition of cellular ATM kinase activity was rapidly and completely reversed by removing CP466722. Interestingly, clonogenic survival assays showed that transient inhibition of ATM is sufficient to sensitize cells to IR and suggests that therapeutic radiosensitization may only require ATM inhibition for short periods of time. The ability of CP466722 to rapidly and reversibly regulate ATM activity provides a new tool to ask questions about ATM function that could not easily be addressed using genetic models or RNA interference technologies.

Stem cell and kinase activity-independent pathway in resistance of leukaemia to BCR-ABL kinase inhibitors

BCR-ABL tyrosine kinase inhibitors, such as imatinib (Gleevec) are highly effective in treating human Philadelphia chromosome-positive (Ph⁺) chronic myeloid leukaemia (CML) in chronic phase but not in terminal acute phase; acquired drug resistance caused mainly by the development of BCR-ABL kinase domain mutations prevents cure of the leukaemia. In addition, imatinib is ineffective in treating Ph⁺ B-cell acute lymphoblastic leukaemia (B-ALL) and CML blast crisis, even in the absence of the kinase domain mutations. This type of drug resistance that is unrelated to BCR-ABL kinase domain mutations is caused by the insensitivity of leukaemic stem cells to kinase inhibitors such as imatinib and dasatinib, and by activation of a newly-identified signalling pathway involving SRC kinases that are independent of BCR-ABL kinase activity for activation. This SRC pathway is essential for leukaemic cells to survive imatinib treatment and for CML transition to lymphoid blast crisis. Apart from BCR-ABL and SRC kinases, stem cell pathways must also be targeted for curative therapy of Ph⁺ leukaemia.

The kinase inhibitor dasatinib induces apoptosis in chronic lymphocytic leukemia cells in vitro with preference for a subgroup of patients with unmutated IgVH genes

Src family kinases (SFKs) were described to be overexpressed in chronic lymphocytic leukemia (CLL). We wished to examine the effects of the Src and Abl kinase inhibitor dasatinib on the intracellular signaling and survival of CLL cells. Dasa-tinib showed a dose- and time-dependent reduction of global tyrosine phosphorylation and of activating phosphotyrosine levels of SFKs. Treatment with 100 nM dasatinib led to decreased levels of the activated, phosphorylated forms of Akt, Erk1/2, and p38, and induced PARP cleavage through caspase activity. In Mec1 and JVM-3 cell lines, dasatinib increased p53 protein levels and inhibited proliferation. In freshly isolated CLL cells, dasatinib reduced the expression of Mcl-1 and Bcl-x(L). Combination of 5 microM dasatinib and fludarabine increased the apoptosis induction of each by approximately 50%. In 15 primary CLL samples, cells with unmutated immunoglobulin variable heavy chain (IgV(H)) genes were more sensitive to dasatinib than those with mutated IgV(H) genes (P = .002). In summary, dasatinib shows potent inhibitory effects on the survival of CLL cells in vitro, most prominently in samples obtained from patients with unfavorable prognostic features.

Src-family kinases in the development and therapy of Philadelphia chromosome-positive chronic myeloid leukemia and acute lymphoblastic leukemia

The BCR-ABL kinase inhibitor imatinib has shown significant efficacy in chronic myeloid leukemia (CML) and is the standard front-line therapy for patients in chronic phase. However, a substantial number of patients are either primarily refractory or acquire resistance to imatinib. While a number of mechanisms are known to confer resistance to imatinib, increasing evidence has demonstrated a role for BCR-ABL–independent pathways. The Src-family kinases (SFKs) are one such pathway and have been implicated in imatinib resistance. Additionally, these kinases are key to the progression of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL). The dual SFK/BCR-ABL inhibitor dasatinib is now clinically available and has markedly greater potency compared with imatinib against native BCR-ABL and the majority of imatinib resistant BCR-ABL mutants. Therefore, this agent, as well as other dual SFK/BCR-ABL inhibitors under development, could provide added therapeutic advantages by overcoming both BCR-ABL– dependent (i.e., BCR-ABL mutations) and – independent forms of imatinib resistance and delaying transition to advanced phase disease. In this review, we discuss the preclinical and clinical evidence demonstrating the involvement of SFKs in imatinib resistance and the progression of CML and Ph+ ALL, as well as the potential role of dual SFK/BCR-ABL inhibition in the management of these diseases.

Part II: management of resistance to imatinib in chronic myeloid leukaemia

Updated findings from a randomised comparison of imatinib versus previous standard treatment in the treatment of newly diagnosed chronic myeloid leukaemia suggest that this first-generation tyrosine-kinase inhibitor can induce excellent long-term responses in most patients. However, a small proportion of patients will not respond or will lose previous responses, and, for these patients, alternative treatments are needed. This review is the second of two parts: the first part provided a review of the mechanisms underlying resistance to imatinib and this second part will discuss the management of patients who are resistant to imatinib by reviewing the many new drugs being introduced into clinical practice and suggesting strategies for decision making.

The role of Src in prostate cancer

The Src family kinases (SFKs) are the largest family of nonreceptor protein tyrosine kinases and are responsible for signal transduction during many cellular activities, including differentiation, adhesion, and migration. Aberrant Src/SFK activity has been widely implicated in cancer development. Several lines of evidence indicate a role for SFKs in the development of prostate cancer, e.g. SFK overexpression in prostate cancer cell lines and tissues and reduced cancer cell proliferation, invasion, and migration following Src inhibition. In particular, Src may be involved in androgen-independent growth during advanced stages of disease. Src signaling is also a key pathway during normal and dysregulated bone functioning, and bone metastases are responsible for substantial morbidity in advanced prostate cancer. Src/SFK inhibition therefore represents a potentially useful therapeutic strategy for patients with various stages of prostate cancer. To date, four Src inhibitors have reached clinical trials. Of these, the broadest range of in vitro prostate cancer data are available for dasatinib, which inhibits several SFKs as well as other tyrosine kinases. Src inhibitors may be specifically evaluated in prostate cancer clinical trials in the near future.

Role of p21(WAF1/CIP1) as an attenuator of both proliferative and drug-induced apoptotic signals in BCR-ABL-transformed hematopoietic cells

The constitutive tyrosine kinase activity of the BCR-ABL fusion protein plays a crucial role in the pathogenesis of chronic myeloid leukemia and promotes growth factor-independent survival of hematopoietic cells. In 32D cells, expression levels of retrovirally transduced BCR-ABL were positively correlated with the levels of the cell cycle regulator protein p21, and this upregulation of p21 expression depended on the kinase activity of BCR-ABL. To assess the role of p21 on BCR-ABL-positive hematopoietic cells, we compared proliferation and drug-induced apoptosis in bone marrow (BM) cells from wild-type and p21 knockout mice after retroviral transfer of the BCR-ABL fusion gene. As compared with wild-type cells, p21 knockout cells showed increased proliferation, suggesting that p21 acted as an attenuator of BCR-ABL-mediated cell proliferation. In marked contrast, deletion of p21 promoted apoptosis induction by imatinib and taxol in BCR-ABL-transformed BM cells. These findings demonstrate that p21 has a dual function in BCR-ABL-transformed murine BM cells: It attenuates the effects of two apparently opposed phenomena such as BCR-ABL-mediated cell proliferation and drug-induced apoptosis. This dual function of p21 calls for a cautious evaluation of the suitability of p21 as a secondary target in anticancer therapy.

Constitutive overexpression of P-glycoprotein, rather than breast cancer resistance protein or organic cation transporter 1, contributes to acquisition of imatinib-resistance in K562 cells

PURPOSE

The purpose of this study was to investigate the contribution of drug transporters in acquired imatinib-resistance. Specifically, we focused on the efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), and an influx transporter, organic cation transporter 1 (OCT1).

MATERIALS AND METHODS

We established imatinib-resistant K562 cells (K562/IM). Real-time PCR or Western blot analyses were performed to examine the mRNA or protein levels. Alamar blue method was used in the cytotoxicity assay. The transport activities and intracellular imatinib levels were measured by flow cytometry and HPLC, respectively.

RESULTS

K562/IM displayed a 47-fold increase in resistance to imatinib over the parent K562 cells. Both P-gp and BCRP were overexpressed in K562/IM relative to K562. Furthermore, the intracellular imatinib level was markedly reduced in K562/IM. Interestingly, cyclosporin A, a P-gp inhibitor, but not fumitremorgin C, a BCRP inhibitor, restored both imatinib-sensitivity and the intracellular imatinib level. By contrast, no significant difference in the expression and function of OCT1 was observed between K562/IM and K562.

CONCLUSIONS

P-gp, rather than BCRP or OCT1, is partially responsible for the development of imatinib-resistance due to constitutive and functional overexpression, leading to reduced intracellular accumulation of imatinib in K562/IM.

Flying under the radar: the new wave of BCR-ABL inhibitors

The introduction of the BCR-ABL kinase inhibitor imatinib mesylate (Gleevec; Novartis) revolutionized the treatment of chronic myeloid leukaemia (CML). However, most patients with CML receiving imatinib still harbour molecular residual disease and some develop resistance associated with ABL kinase domain mutations. The second-generation BCR-ABL inhibitors nilotinib (Tasigna; Novartis) and dasatinib (Sprycel; Bristol-Myers Squibb) have shown significant activity after imatinib failure in clinical trials, but still face similar obstacles to imatinib, including negligible activity against the frequent BCR-ABL T315I mutation and modest effects in advanced phases of CML. Various medicinal chemistry efforts, in part aided by structural studies of the ABL kinase-imatinib complex have resulted in the synthesis of a new generation of BCR-ABL inhibitors, some of which have shown encouraging preliminary activity in clinical trials, including against T315I mutants. Here, we discuss these emerging therapies, which have the potential to improve the outcome of patients with CML.

The design and preliminary structure-activity relationship studies of benzotriazines as potent inhibitors of Abl and Abl-T315I enzymes

We describe the design, synthesis and structure-activity relationship studies in optimizing a series of benzotriazine compounds as potent inhibitors of both Abl and Abl-T315I enzymes. The design includes targeting of an acid functional residue on the alphaC-helix that is available only upon kinase activation. This designed interaction provides an advantage in overcoming the challenges arising from the T315I mutation of Abl and transforms poor (ca. 10 microM) inhibitors into those with low nM potency.

Roots of imatinib resistance: A question of self-renewal?

The BCR-ABL-fusion gene is critical for the development of chronic myeloid leukemia (CML) and BCR-ABL positive acute lymphatic leukemia (Ph+ ALL). Blocking BCR-ABL by the ABL tyrosine kinase inhibitor imatinib mesylate (IM, Gleevec) is clinically highly efficient. Treatment response is unfortunately compromised by the emergence of IM resistance, which is regularly seen in accelerated and blastic phase of CML (CML-AP/BP) and in Ph+ ALL. BCR-ABL kinase domain mutations are then considered the causative mechanism of IM resistance, because 50-60% of the IM resistant patients harbour such mutations. In contrast, IM resistance arises very rarely in patients that are treated with IM in early chronic phase of CML. This implies that BCR-ABL independent factors such as the cellular context of BCR-ABL expression and stage of disease decisively control the evolution of IM resistance. In line with this, novel Abl-kinase inhibitors such as dasatinib (DA) or nilotinib (NI) - although capable of inhibiting most of the BCR/-BL kinase mutants - still often fail to overcome resistance and do mostly not induce durable cytogenetic responses in IM resistant CML-AP/BC and Ph+ ALL patients. On the basis of available evidence it is proposed here that alternative genetic aberrations, which synergize with BCR-ABL to enable leukemic self-renewal are of causal importance for the evolution of clinical kinase inhibitor resistance. Kinase mutations may in turn reflect clonal variants of cells that emerge on the basis of an already existing IM resistant and self-renewing leukemic cell population. This model has clinical implications as it implies that even highly potent Abl-kinase inhibition can not target the genetic basis of IM resistance and will also not resolve the problem of Abl-kinase inhibitor resistance.

Novel Abl kinase inhibitors in chronic myeloid leukemia in blastic phase and Philadelphia chromosome-positive acute lymphoblastic leukemia

Chronic myeloid leukemia (CML) is characterized by the presence of the Philadelphia chromosome, which is associated with a balanced translocation involving chromosomes 9 and 22 to produce a fusion gene (bcr-abl) that gives rise to a constitutively activated Abl tyrosine kinase. This kinase led to the discovery of several small-molecule inhibitors, imatinib being the first and most successful of these. Resistance to imatinib results in some patients from Abl kinase point mutations. Overcoming imatinib resistance represents one of the biggest challenges facing clinicians in the modern management of CML. In this review, we discuss the current understanding of CML pathophysiology and mechanisms of imatinib resistance and how advancing this knowledge has led to the design of novel therapies in the area of blastic phase CML and Philadelphia chromosome-positive acute lymphoblastic leukemia with previous imatinib failure.

Oncogenic signaling: new insights and controversies from chronic myeloid leukemia

Chronic myeloid leukemia (CML), which is caused by the BCR–ABL fusion tyrosine kinase, is one of the most intensively studied human cancers. ABL kinase inhibitors have been spectacularly successful in treating CML, but disease persistence and acquired drug resistance can prevent eradication and cure of the leukemia. The development of better therapies will depend on a full understanding of signaling pathways in CML, facilitated by model studies using mutant mice.

Inhibition of Bcr-Abl Phosphorylation and Induction of Apoptosis by Pyrazolo[3,4-d]pyrimidines in Human Leukemia Cells

A series of pyrazolo[3,4-d]pyrimidines, previously found to be Src inhibitors, was tested for their ability to inhibit proliferation of three Bcr-Abl-positive human leukemia cell lines (K-562, KU-812, and MEG-01), on the basis of the experimental evidence that various Src inhibitors are also active against Bcr-Abl kinase (the so called dual Src/Abl inhibitors). They reduce Bcr-Abl tyrosine phosphorylation and promote apoptosis of the Bcr-Abl-expressing cells. A cell-free enzymatic assay on isolated c-Abl confirmed that such compounds directly inhibit Abl activity. Finally, molecular modeling simulations were also performed to hypothesize the binding mode of the compounds into the Abl binding site.

Inhibition of IL-6-dependent growth of myeloma cells by an acidic peptide repressing the gp130-mediated activation of Src family kinases

An acidic domain (AD) of gp130 was previously found to interact with the Src family kinase (SFK) Hck. Here, the influence of myristoylated peptides derived from this AD was assessed in the mouse myeloma cell line, 7TD1. The IL-6-dependent growth of 7TD1 cells was reduced by approximately 75%, if 100 microM of myristoylated 18mer peptide (18AD) was included in the growth medium, but was unaffected by a control peptide with scrambled sequence (18sc). A similar differential inhibition by peptides 18AD and 18sc was observed for the erythropoietin-dependent growth of BaF-EH cells expressing chimeric erythropoietin receptor-gp130 and human Hck and for the human myeloma cell line INA-6. While the peptide 18AD concentration inhibiting 50% was approximately 30 microM in 7TD1 and BaF-EH cells, peptide 18AD did not significantly inhibit growth of IL-6-independent MM1.S myeloma and OKT1 hybridoma cells or of BaF-EH cells supplied with IL-3. Treatment with 100 microM peptide 18AD caused the same degree or 60% of apoptosis induction as IL-6 deprivation in 7TD1 or INA-6 cells, respectively. Co-immunoprecipitation experiments revealed that peptide 18AD interfered with the association of Hck and gp130 in 7TD1 lysates in a concentration-dependent manner. IL-6-treatment of INA-6 cells induced the kinase activities of Fyn, Lyn and Hck, but not Src, and the IL-6-induced SFK activities were inhibited by peptide 18AD. Expression in 7TD1 cells of a kinase-inactive Hck mutant (K269R) elicited a dominant-negative effect on cell number increases providing further evidence that SFKs are required for gp130 signalling in myeloma cells.

Src kinase signaling in leukaemia

Role of Src kinases in acute lymphoblastic leukaemia has been recently demonstrated in leukaemia mouse model. Retained activation of Src kinases by the BCR-ABL oncoprotein in leukaemic cells following inhibition of BCR-ABL kinase activity by imatinib indicates that Src activation by BCR-ABL is independent of BCR-ABL kinase activity and provides an explanation for reduced effectiveness of the BCR-ABL kinase activity inhibitors in Philadelphia chromosome-positive acute lymphoblastic leukaemia. Simultaneous inhibition of kinase activity of both BCR-ABL and Src kinases results in long-term survival of mice with acute lymphoblastic leukaemia. Leukaemic stem cells exist in acute lymphoblastic leukaemia, and complete eradication of this group of cells would provide a curative therapy for this disease.

Tyrphostins and other tyrosine kinase inhibitors

The development of tyrosine phosphorylation inhibitors has transformed the approach to cancer therapy and is likely to affect other fields of medicine. In spite of the conservation among protein tyrosine kinases (PTKs), one can develop small molecules that block the activity of a narrow spectrum of PTKs and that exhibit much less toxicity than the currently used chemotherapeutic agents. In this review, we discuss principles for inhibiting specific PTKs. We discuss (a) the birth of the concept of generating targeted, nontoxic signal transduction inhibitors, (b) the potential of substrate-competitive versus the more common ATP-competitive PTK inhibitors, (c) the combination of PTK inhibitors with other signal transduction inhibitors to induce apoptosis-the best way to induce the demise of the cancer cell, and (d) the potential to utilize PTK inhibitors/tyrphostins to attenuate nonmalignant pathological conditions, such as immune disorders, tissue rejection, and restenosis.

Activation of a novel Bcr/Abl destruction pathway by WP1130 induces apoptosis of chronic myelogenous leukemia cells

Imatinib mesylate (Gleevec) is effective therapy against Philadelphia chromosome-positive leukemia, but resistance develops in all phases of the disease. Bcr/Abl point mutations and other alterations reduce the kinase inhibitory activity of imatinib mesylate; thus, agents that target Bcr/Abl through unique mechanisms may be needed. Here we describe the activity of WP1130, a small molecule that specifically and rapidly down-regulates both wild-type and mutant Bcr/Abl protein without affecting bcr/abl gene expression in chronic myelogenous leukemia (CML) cells. Loss of Bcr/Abl protein correlated with the onset of apoptosis and reduced phosphorylation of Bcr/Abl substrates. WP1130 did not affect Hsp90/Hsp70 ratios within the cells and did not require the participation of the proteasomal pathway for loss of Bcr/Abl protein. WP1130 was more effective in reducing leukemic versus normal hematopoietic colony formation and strongly inhibited colony formation of cells derived from patients with T315I mutant Bcr/Abl-expressing CML in blast crisis. WP1130 suppressed the growth of K562 heterotransplanted tumors as well as both wild-type Bcr/Abl and T315I mutant Bcr/Abl-expressing BaF/3 cells transplanted into nude mice. Collectively, our results demonstrate that WP1130 reduces wild-type and T315I mutant Bcr/Abl protein levels in CML cells through a unique mechanism and may be useful in treating CML.

New Targeted Therapies for Chronic Myelogenous Leukemia: Opportunities to Overcome Imatinib Resistance

The advent of tyrosine kinase inhibitors (TKIs) has ushered in a new era in the management of chronic myelogenous leukemia (CML). Imatinib, the first TKI to be approved for the treatment of CML and the current standard first-line therapy, has significantly improved the prognosis of patients with CML. Nevertheless, a minority of patients in chronic-phase CML and even more patients with advanced-phase disease demonstrate resistance to imatinib or develop resistance during treatment. In 40% to 50% of cases, this is attributed to the development of mutations that impair the ability of imatinib to bind to and inhibit the constitutively active Bcr-Abl kinase. Consequently, researchers have developed novel, more potent TKIs that can overcome not only Bcr-Abl-dependent mechanisms of resistance, but also those that are Bcr-Abl-independent. These include: dasatinib, a potent dual Bcr-Abl and Src inhibitor; nilotinib, a selective, potent Bcr-Abl inhibitor; bosutinib (SKI-606) and INNO-406 (NS-187), which are both Src-Abl inhibitors; and others. Combination therapy is also being explored concurrently using agents that affect a variety of oncogenic pathways and immune modulation. Herein, we review some of these strategies, particularly those for which clinical data are currently available.

Overcoming drug resistance in chronic myeloid leukemia

PURPOSE OF REVIEW

Despite the excellent clinical results with imatinib in chronic myeloid leukemia, most patients have minimal residual disease and others will develop resistance and may eventually progress. Thus there is a need for developing approaches to overcome and prevent resistance to imatinib.

RECENT FINDINGS

Several new agents have been developed with significant activity in imatinib-resistant chronic myeloid leukemia. A second generation of more potent tyrosine kinase inhibitors, some with dual activity against Abl and Src, have shown very impressive results. Other agents, such as hypomethylating agents, farnesyl transferase inhibitors and homoharringtonine, have also shown preclinical and clinical promise. The use of vaccines as a way of providing an immunomodulatory approach to chronic myeloid leukemia is starting to develop as a major strategy to achieve eradication of the disease.

SUMMARY

Multiple effective agents are being developed to overcome resistance to imatinib. The challenge for the future is to incorporate them into effective strategies that can eliminate the disease and cure all patients with chronic myeloid leukemia.

Tyrosine 311 is phosphorylated by c-Abl and promotes the apoptotic effect of PKCdelta in glioma cells

In this study we characterized the phosphorylation of tyrosine 311 and its role in the apoptotic function of PKCdelta in glioma cells. We found that c-Abl phosphorylated PKCdelta on tyrosine 311 in response to H2O2 and that this phosphorylation contributed to the apoptotic effect of H2O2. In contrast, Src, Lyn, and Yes were not involved in the phosphorylation of tyrosine 311 by H2O2. A phosphomimetic PKCdelta mutant, in which tyrosine 311 was mutated to glutamic acid (PKCdeltaY311E), induced a large degree of cell apoptosis. Overexpression of the PKCdeltaY311E mutant induced the phosphorylation of p38 and inhibition of p38 abolished the apoptotic effect of the PKCdelta mutant. These results suggest an important role of tyrosine 311 in the apoptotic function of PKCdelta and implicate c-Abl as the kinase that phosphorylates this tyrosine.

Novel tyrosine kinase inhibitors in chronic myelogenous leukemia

PURPOSE OF REVIEW

The successful introduction of the tyrosine kinase inhibitors has initiated a new era in the management of chronic myeloid leukemia.

RECENT FINDINGS

Imatinib therapy has significantly improved prognosis of chronic myeloid leukemia. A minority of patients with chronic-phase disease (4% annually) and considerably more in advanced stages develop resistance. This is attributed, in 40-50% of cases, to the development of BCR-ABL (breakpoint cluster region/Abelson oncogene) tyrosine kinase domain mutations that impair imatinib binding. This has led to the development of more potent novel tyrosine kinase inhibitors that can overcome both BCR-ABL-dependent and BCR-ABL-independent mechanisms of resistance. Preliminary results of phase I and II trials with dasatinib and nilotinib have provided promising data that may reduce disease progression and potentially prevent acquired resistance to the tyrosine kinase inhibitors.

SUMMARY

Novel tyrosine kinase inhibitors with more potent and selective Bcr-Abl inhibition and with multitargeted inhibition of Bcr-Abl and Src family kinases are promising and may further improve prognosis in chronic myeloid leukemia.

Targeting multiple kinase pathways in leukemic progenitors and stem cells is essential for improved treatment of Ph+ leukemia in mice

It is generally believed that shutting down the kinase activity of BCR-ABL by imatinib will completely inhibit its functions, leading to inactivation of its downstream signaling pathways and cure of the disease. Imatinib is highly effective at treating human Philadelphia chromosome-positive (Ph(+)) chronic myeloid leukemia (CML) in chronic phase but not Ph(+) B cell acute lymphoblastic leukemia (B-ALL) and CML blast crisis. We find that SRC kinases activated by BCR-ABL remain fully active in imatinib-treated mouse leukemic cells, suggesting that imatinib does not inactivate all BCR-ABL-activated signaling pathways. This SRC pathway is essential for leukemic cells to survive imatinib treatment and for CML transition to lymphoid blast crisis. Inhibition of both SRC and BCR-ABL kinase activities by dasatinib affords complete B-ALL remission. However, curing B-ALL and CML mice requires killing leukemic stem cells insensitive to both imatinib and dasatinib. Besides BCR-ABL and SRC kinases, stem cell pathways must be targeted for curative therapy of Ph(+) leukemia.