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

Nilotinib and MEK inhibitors induce synthetic lethality through paradoxical activation of RAF in drug-resistant chronic myeloid leukemia

We show that imatinib, nilotinib, and dasatinib possess weak off-target activity against RAF and, therefore, drive paradoxical activation of BRAF and CRAF in a RAS-dependent manner. Critically, because RAS is activated by BCR-ABL, in drug-resistant chronic myeloid leukemia (CML) cells, RAS activity persists in the presence of these drugs, driving paradoxical activation of BRAF, CRAF, MEK, and ERK, and leading to an unexpected dependency on the pathway. Consequently, nilotinib synergizes with MEK inhibitors to kill drug-resistant CML cells and block tumor growth in mice. Thus, we show that imatinib, nilotinib, and dasatinib drive paradoxical RAF/MEK/ERK pathway activation and have uncovered a synthetic lethal interaction that can be used to kill drug-resistant CML cells in vitro and in vivo.

Pharmacological targets in the ubiquitin system offer new ways of treating cancer, neurodegenerative disorders and infectious diseases

Recent advances in the development and discovery of pharmacological interventions within the ubiquitin-proteasome system (UPS) have uncovered an enormous potential for possible novel treatments of neurodegenerative disease, cancer, immunological disorder and microbial infection. Interference with proteasome activity, although initially considered unlikely to be exploitable clinically, has already proved to be very effective against haematological malignancies, and more specific derivatives that target subsets of proteasomes are emerging. Recent small-molecule screens have revealed inhibitors against ubiquitin-conjugating and -deconjugating enzymes, many of which have been evaluated for their potential use as therapeutics, either as single agents or in synergy with other drugs. Here, we discuss recent advances in the characterisation of novel UPS modulators (in particular, inhibitors of ubiquitin-conjugating and -deconjugating enzymes) and how they pave the way towards new therapeutic approaches for the treatment of proteotoxic disease, cancer and microbial infection.

Destabilization of Bcr-Abl/Jak2 Network by a Jak2/Abl Kinase Inhibitor ON044580 Overcomes Drug Resistance in Blast Crisis Chronic Myelogenous Leukemia (CML)

Bcr-Abl is the predominant therapeutic target in chronic myeloid leukemia (CML), and tyrosine kinase inhibitors (TKIs) that inhibit Bcr-Abl have been successful in treating CML. With progression of CML disease especially in blast crisis stage, cells from CML patients become resistant to imatinib mesylate (IM) and other TKIs, resulting in relapse. Because Bcr-Abl is known to drive multiple signaling pathways, the study of the regulation of stability of Bcr-Abl in IM-resistant CML cells is a critical issue as a possible therapeutic strategy. Here, we report that a new dual-kinase chemical inhibitor, ON044580, induced apoptosis of Bcr-Abl+ IM-sensitive, IM-resistant cells, including the gatekeeper Bcr-Abl mutant, T315I, and also cells from blast crisis patients. In addition, IM-resistant K562-R cells, cells from blast crisis CML patients, and all IM-resistant cell lines tested had reduced ability to form colonies in soft agar in the presence of 0.5 µM ON044580. In in vitro kinase assays, ON044580 inhibited the recombinant Jak2 and Abl kinase activities when the respective Jak2 and Abl peptides were used as substrates. Incubation of the Bcr-Abl+ cells with ON044580 rapidly reduced the levels of the Bcr-Abl protein and also reduced the expression of HSP90 and its client protein levels. Lysates of Bcr-Abl+ cell lines were found to contain a large signaling network complex composed of Bcr-Abl, Jak2, HSP90, and its client proteins as detected by a gel filtration column chromatography, which was rapidly disrupted by ON044580. Therefore, targeting Jak2 and Bcr-Abl kinases is an effective way to destabilize Bcr-Abl and its network complex, which leads to the onset of apoptosis in IM-sensitive and IM-resistant Bcr-Abl+ cells. This inhibitory strategy has potential to manage all types of drug-resistant CML cells, especially at the terminal blast crisis stage of CML, where TKIs are not clinically useful.

Tyrphostin-like compounds with ubiquitin modulatory activity as possible therapeutic agents for multiple myeloma

With the goal of developing small molecules as novel regulators of signal transduction and apoptosis, a series of tyrphostin-like compounds were synthesized and screened for their activity against MM-1 (multiple myeloma) cells and other cell lines representing this malignancy. Synthesis was completed in solution-phase initially and then adopted to solid-phase for generating a more diverse set of compounds. A positive correlation was noted between compounds capable of inducing apoptosis and their modulation of protein ubiquitination. Further analysis suggested that ubiquitin modulation occurs through inhibition of cellular deubiquitinase activity. Bulky groups on the sidechain near the α,β-unsaturated ketone caused a complete loss of activity, whereas cyclization on the opposite side was tolerated. Theoretical calculations at the B3LYP/LACV3P(∗∗) level were completed on each molecule, and the resulting molecular orbitals and Fukui reactivity values for C(β) carbon were utilized in developing a model to explain the compound activity.

Differential nuclear staining assay for high-throughput screening to identify cytotoxic compounds

As large quantities of novel synthetic molecules continue to be generated there is a challenge to identify therapeutic agents with cytotoxic activity. Here we introduce a Differential Nuclear Staining (DNS) assay adapted to live-cell imaging for high throughput screening (HTS) that utilizes two fluorescent DNA intercalators, Hoechst 33342 and Propidium iodide (PI). Since Hoechst can readily cross cell membranes to stain DNA of living and dead cells, it was used to label the total number of cells. In contrast, PI only enters cells with compromised plasma membranes, thus selectively labeling dead cells. The DNS assay was successfully validated by utilizing well known cytotoxic agents with fast or slow cytotoxic activities. The assay was found to be suitable for HTS with Z' factors ranging from 0.86 to 0.60 for 96 and 384-well formats, respectively. Furthermore, besides plate-to-plate reproducibility, assay quality performance was evaluated by determining ratios of signal-to-noise and signal-to-background, as well as coefficient of variation, which resulted in adequate values and validated the assay for HTS initiatives. As proof of concept, eighty structurally diverse compounds from a small molecule library were screened in a 96-well plate format using the DNS assay. Using this DNS assay, six hits with cytotoxic properties were identified and all of them were also successfully identified by using the commercially available MTS assay (CellTiter 96® Cell Proliferation Assay). In addition, the DNS and a flow cytometry assay were used to validate the activity of the cytotoxic compounds. The DNS assay was also used to generate dose-response curves and to obtain CC50 values. The results indicate that the DNS assay is reliable and robust and suitable for primary and secondary screens of compounds with potential cytotoxic activity.

A novel small molecule deubiquitinase inhibitor blocks Jak2 signaling through Jak2 ubiquitination

AG490 is a tyrosine kinase inhibitor with activity against Jak2 and apoptotic activity in specific leukemias. Due to its weak kinase inhibitory activity and poor pharmacology, we conducted a cell-based screen for derivatives with improved Jak2 inhibition and activity in animals. Two hits emerged from an initial small chemical library screen, and more detailed structure-activity relationship studies led to the development of WP1130 with 50-fold greater activity in suppressing Jak2-dependent cytokine signaling than AG490. However, WP1130 did not directly suppress Jak2 kinase activity, but mediated Jak2 ubiquitination resulting in its trafficking through HDAC6 to perinuclear aggresomes without cytokine stimulation or SOCS-1 induction. Jak2 primarily contained K63-linked ubiquitin polymers, and mutation of this lysine blocked Jak2 ubiquitination and mobilization in WP1130-treated cells. Further analysis demonstrated that WP1130, but not AG490, acts as a deubiquitinating enzyme (DUB) inhibitor, possibly through a Michael addition reaction. We conclude that chemical modification of AG490 resulted in development of a DUB inhibitor with activity against a DUB capable of modulating Jak2 ubiquitination, trafficking and signal transduction.

Ubiquitin-specific proteases as cancer drug targets

Ubiquitin-specific proteases are deubiquitinating enzymes involved in the removal of ubiquitin from specific protein substrates resulting in protein salvage from proteasome degradation, regulation of protein localization or activation. DNA alteration and overexpression in different cancer types, as well as involvement in many cancer-associated pathways, make ubiquitin-specific proteases attractive for the cancer drug discovery purposes. Their proteolytic function associated to available structural biology data reinforce their potential for pharmacological interference. Here, we review this class of enzymes as cancer drug targets in terms of validation and druggability.

Chronic myeloid leukemia stem cells and developing therapies

Chronic myeloid leukemia therapy has remarkably improved with the use of frontline BCR-ABL kinase inhibitors such that newly diagnosed patients have minimal disease manifestations or progression. Effective control of disease may also set the stage for eventual 'cure' of this leukemia. However, the existence of Philadelphia chromosome-positive leukemic cells that are unaffected by BCR-ABL inhibition represents a major barrier that may delay or prevent curative therapy with the current approaches. The most commonly reported mechanism of resistance to tyrosine kinase inhibitor-based therapies involves BCR-ABL gene mutations and amplification, but these changes may not be solely responsible for disease relapse when inhibitor-based therapies are curtailed. Therefore new targets may need to be defined before significant advancement in curative therapies is possible. Emerging evidence suggests that persistence of chronic myeloid leukemia stem cells or acquisition of stem cell-like characteristics prevents complete elimination of chronic myeloid leukemia by tyrosine kinase inhibition alone. This review focuses on several recently emerging concepts regarding the existence and characteristics of chronic myeloid leukemia stem cells. Definitions based on human primary cells and animal model studies are highlighted as are the potential signaling pathways associated with disease repopulating cells. Finally, several recently defined therapeutic targets and active compounds that have emerged from stem cell studies are described. Our goal is to provide an unbiased report on the current state of discovery within the chronic myeloid leukemia stem cell field and to orient the reader to emerging therapeutic targets and strategies that may lead to elimination of this leukemia.

p53 stabilization induces apoptosis in chronic myeloid leukemia blast crisis cells

Philadelphia chromosome positive chronic myeloid leukemia has a progressive course starting in a benign phase and terminating in a blastic phase. In this study, we show that human homolog double minute 2 (HDM2) inhibition, with MI-219-a novel compound, and consequently p53 stabilization induce chronic myeloid leukemia (CML) blast crisis cells to undergo apoptosis regardless of the presence of the T315I mutation in the BCR-ABL kinase domain. The response to MI-219 is associated with the downregulation of c-Myc and the induction of p21(WAF1). The p53 target and pro-apoptotic proteins PUMA, Noxa and Bax are induced, whereas full length Bid protein decreases with increased activity of pro-apoptotic cleaved Bid, and decrease of Mcl-1 is observed by increased caspase activity. CD95/FAS (FAS antigen) receptor is also induced by MI-219, indicating that both intrinsic and extrinsic apoptotic responses are transcriptionally induced. In addition, p53 protein accumulates in the mitochondrial fraction of treated cells involved in transcription-independent induction of apoptosis. We conclude that HDM-2 inhibition with MI-219 effectively induces p53-dependent apoptosis in most blast crisis CML cells, with or without BCR-ABL mutation(s).

Bcr-Abl ubiquitination and Usp9x inhibition block kinase signaling and promote CML cell apoptosis

Although chronic myelogenous leukemia (CML) is effectively controlled by Bcr-Abl kinase inhibitors, resistance to inhibitors, progressive disease, and incomplete eradication of Bcr-Abl-expressing cells are concerns for the long-term control and suppression of this disease. We describe a novel approach to targeting key proteins in CML cells with a ubiquitin-cycle inhibitor, WP1130. Bcr-Abl is rapidly modified with K63-linked ubiquitin polymers in WP1130-treated CML cells, resulting in its accumulation in aggresomes, where is it unable to conduct signal transduction. Induction of apoptosis because of aggresomal compartmentalization of Bcr-Abl was observed in both imatinib-sensitive and -resistant cells. WP1130, but not Bcr-Abl kinase inhibitors, directly inhibits Usp9x deubiquitinase activity, resulting in the down-regulation of the prosurvival protein Mcl-1 and facilitating apoptosis. These results demonstrate that ubiquitin-cycle inhibition represents a novel and effective approach to blocking Bcr-Abl kinase signaling and reducing Mcl-1 levels to engage CML cell apoptosis. This approach may be a therapeutic option for kinase inhibitor-resistant CML patients.

Molecular mechanisms of leukemia-associated protein degradation

Chemical biology, using small molecules as probes to study the cellular signaling network, has developed rapidly in recent years. The interaction between chemistry and biology not only provides new insight into the understanding of cellular activities, but also generates new lead compounds for the treatment of diseases. Transcription factors and kinases such as retinoic acid receptor-alpha (RARα), acute myeloid leukemia 1 (AML1), CAAT/enhancer-binding protein α (C/EBPα), c-myc, and c-abl play important roles in the differentiation of hematopoietic stem/progenitor cells. Abnormalities in these proteins may cause the dysregulation of hematopoiesis and even the occurrence of leukemia. Ubiquitin-mediated protein degradation represents a critical mechanism in regulating the cellular levels and functions of these proteins. Thus, targeting protein degradation has been emerging as an important strategy to conquer malignant diseases. In this review, we will summarize the recent advances in the understanding of the roles of protein degradation in leukemia, with an emphasis on the mechanisms revealed by small molecules.

Deubiquitinase inhibition by small-molecule WP1130 triggers aggresome formation and tumor cell apoptosis

Recent evidence suggests that several deubiquitinases (DUB) are overexpressed or activated in tumor cells and many contribute to the transformed phenotype. Agents with DUB inhibitory activity may therefore have therapeutic value. In this study, we describe the mechanism of action of WP1130, a small molecule derived from a compound with Janus-activated kinase 2 (JAK2) kinase inhibitory activity. WP1130 induces rapid accumulation of polyubiquitinated (K48/K63-linked) proteins into juxtanuclear aggresomes, without affecting 20S proteasome activity. WP1130 acts as a partly selective DUB inhibitor, directly inhibiting DUB activity of USP9x, USP5, USP14, and UCH37, which are known to regulate survival protein stability and 26S proteasome function. WP1130-mediated inhibition of tumor-activated DUBs results in downregulation of antiapoptotic and upregulation of proapoptotic proteins, such as MCL-1 and p53. Our results show that chemical modification of a previously described JAK2 inhibitor results in the unexpected discovery of a novel DUB inhibitor with a unique antitumor mechanism.

NCI First International Workshop on The Biology, Prevention and Treatment of Relapse after Allogeneic Hematopoietic Cell Transplantation: report from the committee on prevention of relapse following allogeneic cell transplantation for hematologic malignancies

Prevention of relapse after allogeneic hematopoietic stem cell transplantation is the most likely approach to improve survival of patients treated for hematologic malignancies. Herein we review the limits of currently available transplant therapies and the innovative strategies being developed to overcome resistance to therapy or to fill therapeutic modalities not currently available. These novel strategies include nonimmunologic therapies, such as targeted preparative regimens and posttransplant drug therapy, as well as immunologic interventions, including graft engineering, donor lymphocyte infusions, T cell engineering, vaccination, and dendritic cell-based approaches. Several aspects of the biology of the malignant cells as well as the host have been identified that obviate success of even these newer strategies. To maximize the potential for success, we recommend pursuing research to develop additional targeted therapies to be used in the preparative regimen or as maintenance posttransplant, better characterize the T cell and dendritic cells subsets involved in graft-versus-host disease and the graft-versus-leukemia/tumor effect, identify strategies for timing immunologic or nonimmunologic therapies to eliminate the noncycling cancer stem cell, identify more targets for immunotherapies, develop new vaccines that will not be limited by HLA, and develop methods to identify populations at very high risk for relapse to accelerate clinical development and avoid toxicity in patients not at risk for relapse.

Degrasyn potentiates the antitumor effects of bortezomib in mantle cell lymphoma cells in vitro and in vivo: therapeutic implications

Mantle cell lymphoma (MCL) is an aggressive histotype of B-cell non-Hodgkin lymphoma that has increased in incidence over the past few decades and is incurable, usually poorly responsive to standard chemotherapy combinations, and associated with poor prognoses. Discovering new therapeutic agents with low toxicity that produce better outcomes in patients with MCL is an ongoing challenge. Recent studies showed that degrasyn, a novel small-molecule inhibitor of the Janus kinase/signal transducer and activation of transcription (JAK/STAT) pathway, exerts antitumor activity in lymphoid tumors by inhibiting key growth and survival signaling (JAK/STAT) pathways. In the present study, we found that treatment of both typical and blastoid-variant MCL cells with degrasyn in combination with bortezomib resulted in synergistic growth inhibition and apoptosis induction in vitro. The apoptosis in these cells was correlated with the downregulation of constitutive NF-kappaB and phosphorylated STAT3 activation, leading to the inhibition of c-Myc, cyclin D1, and bcl-2 protein expression and the upregulation of bax protein expression. In vivo, degrasyn and bortezomib interacted to synergistically prevent tumor development and prolong survival durations in a xenotransplant severe combined immunodeficient mouse model of MCL. These findings suggest that agents such as degrasyn that can pharmacologically target constitutively expressed NF-kappaB and STAT3 in MCL cells may be useful therapeutic agents for MCL when administered together with bortezomib.

Triptolide induces cell death independent of cellular responses to imatinib in blast crisis chronic myelogenous leukemia cells including quiescent CD34+ primitive progenitor cells

The advent of Bcr-Abl tyrosine kinase inhibitors (TKI) has revolutionized the treatment of chronic myelogenous leukemia (CML). However, resistance evolves due to BCR-ABL mutations and other mechanisms. Furthermore, patients with blast crisis CML are less responsive and quiescent CML stem cells are insensitive to these inhibitors. We found that triptolide, a diterpenoid, at nanomolar concentrations, promoted equally significant death of KBM5 cells, a cell line derived from a Bcr-Abl-bearing blast crisis CML patient and KBM5STI571 cells, an imatinib-resistant KBM5 subline bearing the T315I mutation. Similarly, Ba/F3 cells harboring mutated BCR-ABL were as sensitive as Ba/F3Bcr-Abl(p210wt) cells to triptolide. Importantly, triptolide induced apoptosis in primary samples from blast crisis CML patients, who showed resistance to Bcr-Abl TKIs in vivo, with less toxicity to normal cells. Triptolide decreased X-linked inhibitor of apoptosis protein, Mcl-1, and Bcr-Abl protein levels in K562, KBM5, and KBM5STI571 cells and in cells from blast crisis CML patients. It sensitized KBM5, but not KBM5STI571, cells to imatinib. More importantly, triptolide also induced death of quiescent CD34(+) CML progenitor cells, a major problem in the therapy of CML with TKIs. Collectively, these results suggest that triptolide potently induces blast crisis CML cell death independent of the cellular responses to Bcr-Abl TKIs, suggesting that triptolide could eradicate residual quiescent CML progenitor cells in TKI-treated patients and benefit TKI-resistant blast crisis CML patients.

Hsp90 cleavage by an oxidative stress leads to its client proteins degradation and cancer cell death

The heat shock protein 90 (Hsp90) plays a crucial role in the stability of several proteins that are essential for malignant transformation. Hsp90 is therefore an interesting therapeutic target for cancer therapy. In this paper, we investigated whether an oxidative stress generated during ascorbate-driven menadione redox cycling (ascorbate/menadione), affects Hsp90 leading to the degradation of some critical proteins and cell death. Unlike 17-AAG, which inhibits Hsp90 but enhances Hsp70 levels, ascorbate/menadione-treated cells present an additional Hsp90 protein band of about 70kDa as shown by Western blot analysis, suggesting Hsp90 cleavage. This Hsp90 cleavage seems to be a selective phenomenon since it was observed in a large panel of cancer cell lines but not in non-transformed cells. Antibodies raised against either the N-terminus or the C-terminus domains of Hsp90 suggest that the site of cleavage should be located at its N-terminal part. Furthermore, antibodies raised against either the alpha- or the beta-Hsp90 isoform show that Hsp90beta is cleaved while the alpha isoform is down-regulated. We have further shown that different Hsp90 client proteins like Bcr-Abl (a chimerical protein expressed in K562 leukemia cells), RIP and Akt, were degraded when K562 cells were exposed to an oxidative stress. Both Hsp90 cleavage and Bcr-Abl degradation were observed by incubating K562 cells with another H(2)O(2)-generating system (glucose/glucose oxidase) and by incubating KU812 cells (another leukemia cell line) with ascorbate/menadione. Due to the major role of Hsp90 in stabilizing oncogenic and mutated proteins, these results may have potential clinical applications.

Therapy options in imatinib failures

Chronic myelogenous leukemia (CML) is defined by the presence of the constitutively active tyrosine kinase breakpoint cluster region/Abelson (Bcr-Abl), which activates numerous signal transduction pathways leading to uncontrolled cell proliferation. The development of the Bcr-Abl-targeted imatinib represents a paradigm shift in the treatment of CML, because treatment with imatinib resulted in significantly better patient outcome, response rates, and overall survival compared with previous standards. Despite this advance, not all patients benefit from imatinib because of resistance and intolerance. Resistance to imatinib can develop from a number of mechanisms that can be defined as Bcr-Abl-dependent (e.g., most commonly resulting from point mutations in the Abl kinase domain) and Bcr-Abl-independent mechanisms (including the constitutive activation of downstream signaling molecules, e.g., Src family kinases), which could result in the activation of the pathway regardless of Bcr-Abl inhibition. Clearly, new treatment approaches are required for patients resistant to or intolerant of imatinib, which can be dose escalated in patients who demonstrate resistance. This does not result in long-term responses. Hematopoietic stem cell transplantation is limited by the availability of matched donors and the potential for morbidity. Dasatinib, a dual Bcr-Abl/Src kinase inhibitor, has shown efficacy against all imatinib-resistant Bcr-Abl mutations except for T315I. A large trial program showed that dasatinib is effective in patients previously exposed to imatinib and has a manageable safety profile in all phases of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia, resulting in its approval. Nilotinib, an analogue of imatinib, also has demonstrated activity in a similar patient population. These agents and less clinically advanced strategies are discussed in this review.

WP1066, a novel JAK2 inhibitor, suppresses proliferation and induces apoptosis in erythroid human cells carrying the JAK2 V617F mutation

PURPOSE

The discovery of an activating somatic mutation in codon 617 of the gene encoding the Janus kinase (JAK)-2 (JAK2 V617F) in patients with myeloproliferative disorders has opened new avenues for the development of targeted therapies for these malignancies. However, no effective JAK2 inhibitors are currently available for clinical use.

EXPERIMENTAL DESIGN

We investigated the activity of (E)-3(6-bromopyridin-2-yl)-2-cyano-N-(S0-1phenylethyl)acrylamide (WP1066), a novel analogue of the JAK2 inhibitor AG490, in JAK2 V617F-positive erythroleukemia HEL cells and in blood cells from patients with polycythemia vera.

RESULTS

We found that WP1066 significantly inhibited JAK2 and its downstream signal transducer and activator of transcription-3, signal transducer and activator of transcription-5, and extracellular signal-regulated kinase-1/2 pathways in a dose- and time-dependent manner. As a result, WP1066 concentrations in the low micromolar range induced time- and dose-dependent antiproliferative and proapoptotic effects in HEL cells. As expected, WP1066 inhibited the proliferation of peripheral blood hematopoietic progenitors of patients with polycythemia vera carrying the JAK2 V617F mutation in a dose-dependent manner.

CONCLUSIONS

Our data suggest that WP1066 is active both in vitro and ex vivo and should be further developed for the treatment of neoplasms expressing the JAK2 V617F mutation.

Overcoming kinase resistance in chronic myeloid leukemia

Imatinib is a small-molecule inhibitor of BCR-ABL tyrosine kinase activity, with proven efficacy and tolerability. Despite imatinib's activity, the development of resistance, whether BCR-ABL dependent or independent, is a concern. BCR-ABL-dependent resistance is commonly a result of mutations in the BCR-ABL gene, which can induce a structural predisposition towards the active conformation of the protein, resulting in a shift in the equilibrium of BCR-ABL from inactive, which imatinib binds, to active, which imatinib is unable to bind. BCR-ABL gene amplification may play a role in the development of imatinib resistance in patients with CML. There are a number of BCR-ABL-independent mechanisms of imatinib resistance, including the efflux protein multidrug resistance protein-1, of which imatinib is a substrate. Another mechanism may be the development of alternative pathways of disease progression, leading to less reliance on BCR-ABL; indeed, the SRC family tyrosine kinases LYN and HCK have been frequently implicated in treatment resistance and progression of CML. Clearly, imatinib resistance requires the development of other treatment options. Dasatinib, with increased binding potency (325-fold greater potency than imatinib for wild-type BCR-ABL), inhibition of both the active and inactive formation of BCR-ABL, and targeting of SRC family kinases, is the only agent approved for the treatment of patients with imatinib-resistant or -intolerant CML and Ph+ ALL. Dasatinib is highly active in all phases of these diseases, and is active in the majority of imatinib-resistant mutations, with the exception of T315I. The development of agents that effectively inhibit T315I mutations suggests that future treatment options will include combination therapy.

Getting to the stem of chronic myeloid leukaemia

Tyrosine kinase inhibitor (TKI) therapy for chronic myeloid leukaemia (CML) is the consummate success story for targeted therapy, yet relapse is a nearly inevitable consequence of cessation or interruption of therapy. Primitive TKI-refractory CML stem cells are the likely source of these relapses, as they provide sanctuary for the Philadelphia chromosome. In advanced disease, their progressively anaplastic progeny ultimately maintain CML independently of the CML haematopoietic stem cell (HSC). Interestingly, there are at least two distinct cell types capable of self-renewal in different phases of CML: first, a primitive HSC with BCR-ABL mutation, which maintains the more indolent chronic-phase disease and, second, a coexisting mutated progenitor cell which acquires stem cell characteristics responsible for rapid cell expansion in advanced disease.

Lyn regulates BCR-ABL and Gab2 tyrosine phosphorylation and c-Cbl protein stability in imatinib-resistant chronic myelogenous leukemia cells

Lyn kinase functions as a regulator of imatinib sensitivity in chronic myelogenous leukemia (CML) cells through an unknown mechanism. In patients who fail imatinib therapy but have no detectable BCR-ABL kinase mutation, we detected persistently activated Lyn kinase. In imatinib-resistant CML cells and patients, Lyn activation is BCR-ABL independent, it is complexed with the Gab2 and c-Cbl adapter/scaffold proteins, and it mediates persistent Gab2 and BCR-ABL tyrosine phosphorylation in the presence or absence of imatinib. Lyn silencing or inhibition is necessary to suppress Gab2 and BCR-ABL phosphorylation and to recover imatinib activity. Lyn also negatively regulates c-Cbl stability, whereas c-Cbl tyrosine phosphorylation is mediated by BCR-ABL. These results suggest that Lyn exists as a component of the BCR-ABL signaling complex and, in cells with high Lyn expression or activation, BCR-ABL kinase inhibition alone (imatinib) is not sufficient to fully disengage BCR-ABL-mediated signaling and suggests that BCR-ABL and Lyn kinase inhibition are needed to prevent or treat this form of imatinib resistance.