Search

Objective: BCR ABL oncogene encodes the BCR-ABL chimeric protein, which is a constitutively activated non-receptor tyrosine kinase. The BCR-ABL oncoprotein is a key molecular basis for the pathogenesis of chronic myeloid leukemia (CML) via activation of several downstream signaling pathways including JAK/STAT pathway. Development of leukemia involves constitutive activation of signaling molecules including, JAK2, STAT3, STAT5A and STAT5B. Thymoquinone (TQ) is a bioactive constituent of Nigella sativa that has shown anticancer properties in various cancers. The present study aimed to evaluate the effect of TQ on the expression of BCR ABL, JAK2, STAT3, STAT5A and STAT5B genes and their consequences on the cell proliferation and apoptosis in K562 CML cells., Methods: BCR-ABL positive K562 CML cells were treated with TQ. Cytotoxicity was determined by Trypan blue exclusion assay. Apoptosis assay was performed by annexin V-FITC/PI staining assay and analyzed by flow cytometry. Transcription levels of BCR ABL, JAK2, STAT3, STAT5A and STAT5B genes were evaluated by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Protein levels of JAK2 and STAT5 were determined by Jess Assay analysis., Results: TQ markedly decreased the cell proliferation and induced apoptosis in K562 cells (P < 0.001) in a concentration dependent manner. TQ caused a significant decrease in the transcriptional levels of BCR ABL, JAK2, STAT3, STAT5A and STAT5B genes (P < 0.001). TQ induced a significant decrease in JAK2 and STAT5 protein levels (P < 0.001)., Conclusion: our results indicated that TQ inhibited cell growth of K562 cells via downregulation of BCR ABL/ JAK2/STAT3 and STAT5 signaling and reducing JAK2 and STAT5 protein levels.

Nanotechnology has demonstrated great promise for the development of more effective and safer cancer therapies. We recently developed a highly selective inhibitor of BCR-ABL fusion tyrosine kinase for chronic myeloid leukemia (CML). However, the poor drug-like properties were hurdles to its further clinical development. Herein, we re-investigate it by conjugating an amphiphilic polymer and self-assembling into a nanoparticle (NP) with a high loading (~10.3%). The formulation greatly improved its solubility and drastically extended its circulation half-life from ~5.3 to ~117 h (>20-fold). In the 150 days long-term engraftment model experiment, long intravenous dosing intervals of the NPs (every 4 or 8 days) exhibited much better survival and negligible toxicities as compared to daily oral administration of the inhibitor. Moreover, the NPs showed excellent inhibition of tumor growth in the subcutaneous xenograft model. All results suggest that the ultra-long circulating pro-drug NP may provide an effective and safe therapeutic strategy for BCR-ABL-positive CML., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Imatinib-resistance is a significant concern for Bcr-Abl-positive chronic myelogenous leukemia (CML) treatment. Emodin, the predominant compound of traditional medicine rhubarb, was reported to inhibit the multidrug resistance by downregulating P-glycoprotein of K562/ADM cells with overexpression of P-glycoprotein in our previous studies. In the present study, we found that emodin can be a potential inhibitor for the imatinib-resistance in K562/G01 cells which are the imatinib-resistant subcellular line of human chronic myelogenous leukemia cells with overexpression of breakpoint cluster region-abelson (Bcr-Abl) oncoprotein. Emodin greatly enhanced cell sensitivity to imatinib, suppressed resistant cell proliferation and increased potentiated apoptosis induced by imatinib in K562/G01 cells. After treatment of emodin and imatinib together, the levels of p-Bcr-Abl and Bcr-Abl were significantly downregulated. Moreover, Bcr-Abl important downstream target, STAT5 and its phosphorylation were affected. Furthermore, the expression of Bcr-Abl and signal transducers and activators of transcription 5 (STAT5) related molecules, including c-MYC, MCL-1, poly(ADP-ribose)polymerase (PARP), Bcl-2 and caspase-3, were changed. Emodin also decreased Src expression and its phosphorylation. More importantly, emodin simultaneously targeted both the ATP-binding and allosteric sites on Bcr-Abl by molecular docking, with higher affinity with the myristoyl-binding site for enhanced Bcr-Abl kinase inhibition. Overall, these data indicated emodin might be an effective therapeutic agent for inhibiting resistance to imatinib in CML treatment.

Herein we described the synthesis of siRNA-NES (nuclear export signal) peptide conjugates by solid phase fragment coupling and the application of them to silencing of bcr/abl chimeric gene in human chronic myelogenous leukemia cell line K562. Two types of siRNA-NES conjugates were prepared, and both sense strands at 5' ends were covalently linked to a NES peptide derived from TFIIIA and HIV-1 REV, respectively. Significant enhancement of silencing efficiency was observed for both of them. siRNA-TFIIIA NES conjugate suppressed the expression of BCR/ABL gene to 8.3% at 200 nM and 11.6% at 50 nM, and siRNA-HIV-1REV NES conjugate suppressed to 4.0% at 200 nM and 6.3% at 50 nM, whereas native siRNA suppressed to 36.3% at 200 nM and 30.2% at 50 nM. We could also show complex of siRNA-NES conjugate and designed amphiphilic peptide peptideβ7 could be taken up into cells with no cytotoxicity and showed excellent silencing efficiency. We believe that the complex siRNA-NES conjugate and peptideβ7 is a promising candidate for in vivo use and therapeutic applications.

Polyphyllin D, a compound derived from Paris polyphylla rhizoma, demonstrated strong anticancer activities in a previous study. Our results demonstrated that polyphyllin D exerts a growth inhibitory effect by inducing apoptosis and differentiation in the human erythroleukemia cell line K562. Polyphyllin D induced apoptosis via the mitochondrial apoptotic pathway, as evidenced by the decreased Bcl-2 and Bcr/Abl expression levels, the disruption of MMP and increased Bax, cytochrome c and cleaved-caspase-3 levels. At a low dose, polyphyllin D increased CD14 expression on the surface of K562 cells and induced cells to differentiate into monocytes or mature macrophages. These data suggest that polyphyllin D has the potential to be a potent therapeutic agent for treating human chronic myelogenous leukemia., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Imatinib, a multitargeted tyrosine kinase inhibitor, exhibits potent anticancer activity against leukemia harboring the Bcr-Abl oncogene and some solid tumors overexpressing c-kit and PDGFR. However, its clinical efficacy is severely compromised by the emergence of resistance primarily due to acquired mutations in the Bcr-Abl kinase domain. In this study, we showed that combination of imatinib with platinum (Pt)-based anticancer agents, including cisplatin and oxaliplatin, exhibited synergistic cytotoxic effect specifically in Bcr-Abl+ human chronic myeloid leukemia cell line K562 but not in Bcr-Abl- RPMI8226 cells. Importantly, the synergistic effect was also found to circumvent imatinib resistance in an imatinib-selected resistant subline K562 ima1.0. The combination treatment increased apoptosis and DNA damage. Mechanistic study revealed that increased inhibition of Bcr-Abl and downstream ERK phosphorylation by the drug combination may contribute to the synergistic effect., (Copyright © 2015 Japanese Pharmacological Society. Production and hosting by Elsevier B.V. All rights reserved.)

Imatinib mesylate is a tyrosine kinase inhibitor with selectivity for abelson tyrosine-protein kinase 1 (c-Abl), breakpoint cluster region (Bcr)-Abl fusion protein (Bcr-Abl), mast/stem cell growth factor receptor Kit (c-Kit), and platelet-derived growth factor receptor (PDGFR). Previous studies demonstrated that imatinib in the low micromolar range exerted antiproliferative effects on neuroblastoma cell lines. However, although neuroblastoma cells express c-Kit and PDGFR, the imatinib concentrations required to achieve significant growth inhibitory effects (≥ 10 μM) are substantially higher than those required for inhibition of ligand-induced phosphorylation of wild type c-Kit and PDGFR (≤ 1 μM), suggesting that additional mechanisms are responsible for the antitumor activity of imatinib on these cells. In this study, we show that treatment of neuroblastoma cell lines with 1-15 μM imatinib resulted in a dose dependent inhibition of 5-bromo-2'-deoxyuridine (BrdU) incorporation into newly synthesized DNA. The antiproliferative effect of imatinib was dependent on the upregulation of the cyclin-dependent kinase (CDK) inhibitor p27(KIP1) in the nuclear compartment as a result of increased p27(KIP1) protein stability. We demonstrate that the mechanism of p27(KIP1) stabilization relied on inhibition of p27(KIP1) phosphorylation on tyrosine residues by c-Abl. We provide evidence that in neuroblastoma cell lines a significant fraction of cellular c-Abl is phosphorylated on Tyr-245, consistent with an open and active conformation. Notably, exposure to imatinib did not affect Tyr-245 phosphorylation. Given the low affinity of active c-Abl for imatinib, these data provide a molecular explanation for the relatively high imatinib concentrations required to inhibit neuroblastoma cell proliferation., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Prion diseases are neurodegenerative disorders characterized by the accumulation of a disease-associated prion protein and apoptotic neuronal death. Previous studies indicated that the ubiquitous expression of c-Abl tyrosine kinase transduces a variety of extrinsic and intrinsic cellular signals. In this study, we demonstrated that a synthetic neurotoxic prion fragment (PrP106-126) activated c-Abl tyrosine kinase, which in turn triggered the upregulation of MST1 and BIM, suggesting the activation of the c-Abl-BIM signaling pathway. The peptide fragment was found to result in cell death via mitochondrial dysfunction in neuron cultures. Knockdown of c-Abl using small interfering RNA protected neuronal cells from PrP106-126-induced mitochondrial dysfunction, production of reactive oxygen species, and apoptotic events inducing translocation of Bax to the mitochondria, cytochrome c release into the cytosol, and activation of caspase-9 and caspase-3. Blocking the c-Abl tyrosine kinase also prevented neuronal cells from PrP106-126-induced apoptotic morphological changes. This is the first study reporting that c-Abl tyrosine kinase as a novel upstream activator of MST1 and BIM plays an important role in prion-induced neuron apoptosis via mitochondrial dysfunction. Our findings suggest that c-Abl tyrosine kinase is a potential therapeutic target for prion disease.

Andrographolide is a diterpenoid compound isolated from Andrographis paniculata that exhibits anticancer activity. We previously reported that andrographolide suppressed v-Src-mediated cellular transformation by promoting the degradation of Src. In the present study, we demonstrated the involvement of Hsp90 in the andrographolide-mediated inhibition of Src oncogenic activity. Using a proteomics approach, a cleavage fragment of Hsp90α was identified in andrographolide-treated cells. The concentration- and time-dependent induction of Hsp90 cleavage that accompanied the reduction in Src was validated in RK3E cells transformed with either v-Src or a human truncated c-Src variant and treated with andrographolide. In cancer cells, the induction of Hsp90 cleavage by andrographolide and its structural derivatives correlated well with decreased Src levels, the suppression of transformation, and the induction of apoptosis. Moreover, the andrographolide-induced Hsp90 cleavage, Src degradation, inhibition of transformation, and induction of apoptosis were abolished by a ROS inhibitor, N-acetyl-cysteine. Notably, Hsp90 cleavage, decreased levels of Bcr-Abl (another known Hsp90 client protein), and the induction of apoptosis were also observed in human K562 leukemia cells treated with andrographolide or its active derivatives. Together, we demonstrated a novel mechanism by which andrographolide suppressed cancer malignancy that involved inhibiting Hsp90 function and reducing the levels of Hsp90 client proteins. Our results broaden the molecular basis of andrographolide-mediated anticancer activity., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Objective: To investigate the effect of dihydroartemisinin (DHA) on the BCR/ABL fusion gene in leukemia K562 cell., Methods: K562 cells were cultured in vitro. The rate of proliferation inhibition of cells treated with various concentrations of DHA were determined by using [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] (MTT) method. Expression of BCR/ABL fusion gene was analyzed by reverse transcription(RT-PCR) before and after DHA treatment. Apoptosis of K562 cells was detected by flow cytometry., Results: The growth of K562 cells was inhibited when the concentrations of DHA were 10-160 umol/L. With the added dose of DHA, the growth inhibition was remarkable, with the rate of inhibition risen from 52.76% to 94.65%. The expression of BCR/ABL fusion gene, as detected by RT-PCR after incubating the K562 cells with 20 umol/L DHA, measured as ΔCt = 4.45 ± 0.25 after 12 h and ΔCt = 5.23 ± 0.21 after 24 h, which was significantly lower compared with that of the control ( ΔCt = 4.23 ± 0.21, P < 0.05)., Conclusion: DHA can inhibit the proliferation of leukemia K562 cells and facilitate the induction of apoptosis by downregulating the expression of BCR/ABL fusion gene.

Inactivation of glycogen synthase kinase (GSK)-3 has been implicated in cancer progression. Previously, we showed an abundance of inactive GSK-3 in the human chronic myeloid leukemia (CML) cell line. CML is a hematopoietic malignancy caused by an oncogenic Bcr-Abl tyrosine kinase. In Bcr-Abl signaling, the role of GSK-3 is not well defined. Here, we report that enforced expression of constitutively active GSK-3 reduced proliferation and increased Bcr-Abl inhibition-induced apoptosis by nearly 1-fold. Bcr-Abl inhibition activated GSK-3 and GSK-3-dependent apoptosis. Inactivation of GSK-3 by Bcr-Abl activity is, therefore, confirmed. To reactivate GSK-3, we used glucosylceramide synthase (GCS) inhibitor PDMP to accumulate endogenous ceramide, a tumor-suppressor sphingolipid and a potent GSK-3 activator. We found that either PDMP or silence of GCS increased Bcr-Abl inhibition-induced GSK-3 activation and apoptosis. Furthermore, PDMP sensitized the most clinical problematic drug-resistant CML T315I mutant to Bcr-Abl inhibitor GNF-2-, imatinib-, or nilotinib-induced apoptosis by >5-fold. Combining PDMP and GNF-2 eliminated transplanted-CML-T315I-mutants in vivo and dose dependently sensitized primary cells from CML T315I patients to GNF-2-induced proliferation inhibition and apoptosis. The synergistic efficacy was Bcr-Abl restricted and correlated to increased intracellular ceramide levels and acted through GSK-3-mediated apoptosis. This study suggests a feasible novel anti-CML strategy by accumulating endogenous ceramide to reactivate GSK-3 and abrogate drug resistance.

Background & Aims: Nilotinib is a novel tyrosine kinase inhibitor of Bcr-Abl and other kinases. In this study, we have examined its activity as an anti-fibrotic agent., Methods: The in vitro effect of Nilotinib on rat and human HSCs was assessed using proliferation assays and Western blotting. The in vivo antifibrotic efficacy of Nilotinib was assessed in mice with liver fibrosis induced by CCl(4) and bile duct ligation (BDL)., Results: Nilotinib inhibited proliferation, migration, and actin filament formation, as well as the expression of α-SMA and collagen in activated HSCs. Nilotinib induced apoptosis of HSCs, which was correlated with reduced bcl-2 expression, increased p53 expression, cleavage of PARP, as well as increased expression of PPARγ and TRAIL-R. Nilotinib also induced cell cycle arrest, accompanied by increased expression of p27 and downregulation of cyclin D1. Interestingly, Nilotinib not only inhibited activation of PDGFR, but also TGFRII through Src. Nilotinib significantly inhibited PDGF and TGFβ-simulated phosphorylation of ERK and Akt. Furthermore, PDGF- and TGFβ-activated phosphorylated form(s) of Abl in human HSCs were inhibited by Nilotinib. In vivo, Nilotinib reduced collagen deposition and α-SMA expression in CCl(4) and BDL-induced fibrosis. These beneficial effects were associated with suppressed expression of procollagen-(I), TIMP-1, CD31, CD34, VEGF, and VEGFR. Nilotinib could induce HSC undergoing apoptosis in vivo, which was correlated with downregulation of bcl-2. We also observed reduced expression of phosphorylated ERK, Akt, and Abl in the Nilotinib-treated CCl(4) and BDL livers. In addition to its antifibrotic activity, the drug was hepatoprotective and reduced the elevations of ALT and AST after CCl(4) and BDL., Conclusions: These studies uncover a novel role of Bcr-Abl activity in treatment of liver fibrosis through multiple mechanisms and indicate that Nilotinib represents a potentially effective antifibrotic agent., (Copyright © 2011 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.)

BCR/ABL positive cells are known to be resistant to DNA damage induced by chemotherapy while they are sensitive to imatinib (IM), a tyrosine kinase inhibitor (TKI). To evaluate whether this drug can increase the activity of cytotoxic drugs on BCR/ABL positive cells, we measured the toxicity of cytosine arabinoside (ARA-C), hydroxyurea (HU) and melphalan (MEL), after a pretreatment of 24 h with IM on K562 cell line. The highest cytotoxic effect was seen when the TKI was followed by MEL; our results indicate that inhibition of BCR/ABL activity by IM increased the cytotoxicity of MEL by favoring the DNA damage induced by this drug and by shortening the time for DNA repair at the G2/M checkpoint. A stronger activation of some genes involved in both intrinsic and extrinsic apoptotic pathways was also observed with IM/MEL combination compared to IM or MEL alone. The drugs association was further tested in a type of BaF3 cells (TonB.210) where the BCR-ABL expression is inducible by doxycycline; in this model it was confirmed that a reduction of BCR/ABL activity resulted in an increased susceptibility to the cytotoxic effect of MEL. Furthermore, we studied the effect of IM/MEL treatment on the proliferative potential of myeloid progenitors of six CML patients at diagnosis. The analysis of CFU-GM and BFU-E colonies demonstrated that the IM/MEL combination was more effective than IM alone in reducing the overall number of colonies and the number of copies of BCR/ABL. In conclusion, our work shows that inhibition of BCR/ABL activity increases the toxicity of MEL and allows an efficient killing of leukemic cells, suggesting that a clinical development of this approach could have therapeutic advantages for CML patients., (© 2011 John Wiley & Sons A/S.)

With an understanding of the molecular changes that accompany cell transformation, cancer drug discovery has undergone a dramatic change in the past few years. Whereas most of the emphasis in the past has been placed on developing drugs that induce cell death based on mechanisms that do not discriminate between normal and tumor cells, recent strategies have emphasized targeting specific mechanisms that have gone awry in tumor cells. However, the identification of cancer-associated mutations in oncogenes and their amplification in tumors has suggested that inhibitors against such proteins might represent attractive substrates for targeted therapy. In the clinic, the success of imatinib (Gleevec®, STI571) and trastuzumab (Herceptin®), both firsts of their kind, spurred further development of new, second-generation drugs that target kinases in cancer. This review highlights a few important examples each of these types of therapies, along with some newer agents that are in various stages of development. Second-generation kinase inhibitors aimed at overriding emerging resistance to these therapies are also discussed., (© 2010 Mount Sinai School of Medicine.)

Germ cells are sensitive to genotoxins, and ovarian failure and infertility are major side effects of chemotherapy in young patients with cancer. Here we describe the c-Abl-TAp63 pathway activated by chemotherapeutic DNA-damaging drugs in model human cell lines and in mouse oocytes and its role in cell death. In cell lines, upon cisplatin treatment, c-Abl phosphorylates TAp63 on specific tyrosine residues. Such modifications affect p63 stability and induce a p63-dependent activation of proapoptotic promoters. Similarly, in oocytes, cisplatin rapidly promotes TAp63 accumulation and eventually cell death. Treatment with the c-Abl kinase inhibitor imatinib counteracts these cisplatin-induced effects. Taken together, these data support a model in which signals initiated by DNA double-strand breaks are detected by c-Abl, which, through its kinase activity, modulates the p63 transcriptional output. Moreover, they suggest a new use for imatinib, aimed at preserving oocytes of the follicle reserve during chemotherapeutic treatments.

Imatinib mesylate is effective against Ph chromosome-positive leukemia; however, resistance has been reported. High expression of bcr-abl in mRNA and protein levels, and other alterations were found in patients who experienced imatinib treatment failures and thus it is important to design alternative treatment strategies. The aim of this study was to evaluate the in vitro effect of berbamine, on imatinib-resistant chronic myelogenous leukemia (CML) K562 (K562-r) cells, and explore the mechanisms. The growth of K562-r cells was examined using the 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. Morphological analysis and DNA agarose electrophoresis were used to detect apoptosis in K562-r cells, and the extent of the cells in the sub-G1 cell cycle phase was measured using flow cytometry. The expression levels of BCR-ABL, phospho-BCR-ABL, and nuclear factor kappaB (NF-kappaB), IkappaBalpha, phospho-IkappaBalpha, IkappaB kinases alpha(IKKalpha), and Survivin were determined by Western blot. bcr-abl mRNA expression was determined by RT-PCR. MTT assays indicated that berbamine significantly inhibited the proliferation of K562-r cells. Cells with characteristics of apoptosis were confirmed by morphology examination and DNA agarose electrophoresis and percentage of apoptosis were increased after treatment with berbamine. The results also showed that berbamine was able to down-regulate BCR-ABL and phospho-BCR-ABL proteins by affecting bcr-abl mRNA expression and decrease expression of nuclear NF-kappaB, phospho-IkappaBalpha, IKKalpha, and Survivin. Collectively, berbamine could inhibit the proliferation of K562-r cells and induce apoptosis. The mechanisms may be related at least in part, to inhibit BCR-ABL and its downstream NF-kappaB signaling. Berbamine may provide an alternative candidate for the treatment of patients with CML resistant to imatinib therapy.

During therapy for chronic myeloid leukemia (CML), decline of the number of BCR-ABL transcripts has been shown to follow a biphasic pattern, with a fast phase followed by a slower phase. Hence, sustained remission requires a long phase of therapy. Data indicate that a combination of different available targeted drugs might prevent treatment failure due to drug resistance, especially at advanced stages of the disease. However, for long-term multiple-drug treatments, complications can arise from side effects. We investigate whether and how the number of drugs could be reduced during long-term therapy. Using computational models, we show that one or more drugs can be removed once the number of tumor cells is reduced significantly, without compromising the chances of sustained tumor suppression. Which drug to remove first depends on the number of mutations in the BCR-ABL gene that confer resistance to the drugs, as well as on how effectively the drugs inhibit Bcr-Abl protein tyrosine kinase activity and inhibit tumor growth. We further show that the number of CML cells at which the number of drugs can be reduced does not correlate with the two phases of decline of the BCR-ABL transcript numbers. Neither does it depend much on kinetic parameters of CML growth, except for the mutation rates at which resistance is generated. This is a significant finding because even without any information on most parameters, and using only the data on the number of cancer cells and the rate at which resistant mutants are generated, it is possible to predict at which stage of treatment the number of drugs can be reduced.

Background: Dasatinib is a multitargeted inhibitor of ABL, the SRC family, and other tyrosine kinases. We sought to evaluate the effects of this drug on cell proliferation, centrosomes, mitotic spindles, and cell cycle progression in vitro and in vivo., Design and Methods: Human dermal fibroblasts, Chinese hamster cells, human osteosarcoma cells, and blood and bone marrow mononuclear cells from 32 patients with chronic myeloid leukemia, gastrointestinal stromal tumor, and systemic mastocytosis as well as from six healthy individuals were investigated. The effects of dasatinib were compared with those of the ABL inhibitors imatinib and nilotinib, the SRC inhibitor PP2, and the ABL/LYN inhibitor INNO-406., Results: Dasatinib induced G(1) phase arrest in all cell lines and this was associated with a decline in cyclin D1 levels. In vitro, centrosomal aberrations, a decrease of mitotic spindles, and G(1) phase arrest were observed. In patients, centrosome alterations were found in a median of 17% (range, 10-22%) of cells with a decrease of spindles in 8/18 patients. In comparison, imatinib, nilotinib and PP2 led to centrosome aberrations without G(1) phase arrest. INNO-406 was associated with centrosome aberrations and cell cycle arrest in G(1) phase., Conclusions: Dasatinib blocks the G(1)/S transition and inhibits cell growth. It induces centrosomal aberrations and a decrease of mitotic spindles. The effects suggest an involvement of SRC and ABL inhibition.

Purpose of the Review: Monitoring for residual leukemic burden is an integral part of ongoing evaluation in chronic myeloid leukemia. With high rates of complete cytogenetic response induced by imatinib and other tyrosine kinase inhibitors, exploring refinements in current methods of minimal residual disease monitoring is relevant., Recent Findings: Estimation of leukemic burden can influence treatment decisions. Real-time RT-PCR has become the method of choice for quantifying bcr-abl transcripts in patients undergoing therapy. Efforts are in progress to standardize this technique. The numbers of bcr-abl transcripts can vary in serial specimens, and it is important that changes in treatment (e.g. referral for transplantation, imatinib dose increase, or switch to a new tyrosine kinase inhibitor) be based on consistent rises in transcript number over time rather than on a single change., Summary: Prognosis in newly diagnosed patients with chronic myeloid leukemia in chronic phase has dramatically improved with the availability of imatinib and other tyrosine kinase inhibitors. In parallel, methods of monitoring minimal residual disease are evolving. Although real-time RT-PCR is now a standard monitoring method, the clinical significance of frequent and more precise follow up in patients achieving a complete cytogenetic response requires better definition.

The treatment of chronic myelogenous leukemia (CML) has been revolutionized by the development of the small-molecule tyrosine kinase inhibitor imatinib. The primary target for this drug is the oncogenic BCR-ABL kinase. Five-year survival rates for patients in chronic phase CML is now greater than 80%. Patients who have advanced beyond the chronic phase to the accelerated phase or blast crisis, however, have not faired as well. Progression occurs for a variety of reasons, including late diagnosis, slow response to imatinib, and the development of imatinib-resistant clones. Imatinib resistance has, in part, been addressed with the introduction of the new BCR-ABL inhibitors, namely dasatinib and nilotinib. These drugs have shown efficacy in CML patients with wild-type BCR-ABL and some BCR-ABL mutants that are imatinib-resistant. Unfortunately, some BCR-ABL mutations remain resistant to these therapies and will require the development of alternative treatments, and other mechanisms of imatinib resistance besides BCR-ABL mutation exist. In the future, genetic and pharmacologic tests may allow the clinician to predict response to imatinib. More aggressive therapies are being considered for high-risk patients, including increased dosage of the current tyrosine kinase inhibitors, along with combination therapies. Aggressive therapy holds promise, as the data suggest that responses are improved. Unfortunately, toxicities are also increased, and thus a balance must be found to ensure safety and compliance. This is especially important for young CML patients, who hopefully will remain in remission for decades. Polymerase chain reaction analysis has become of primary importance as a means of assessing disease burden, and given the idiosyncrasies of this technique, standards must be established to allow results to be compared across different institutions. Additionally, the nature of advanced disease is being explored. Intriguingly, genetic analysis of transformed blasts from patients in blast crisis has identified numerous members of the Wnt/B-catenin pathway and JunB as being activated. Increased activity of these pathways correlates with poor response and eventual disease progression. In addition to these data, evidence is emerging associating survival of the quiescent blast cell with Wnt activity, leading to the hope that Wnt inhibitors will increase the likelihood of eradicating these cells. Other areas such as microRNA profiling and DNA methylation patterns are likely to provide important information.

Philadelphia (Ph) chromosome at (9; 22) reciprocal chromosomal translocation producing BCR-ABL fusion gene, emerges in almost all patients with chronic myeloid leukemia (CML). The protein product of BCR-ABL is a constitutively active tyrosine kinase that drives the abnormal proliferation of CML cells. Blast crisis (BC) is the terminal phase of CML, which is often associated with additional chromosomal and molecular secondary changes. Although the mechanisms responsible for transition of CML chronic phase (CP) into BC remain poorly understood, ample evidence suggests that it depends on synergy of BCR/ABL with other genes dysregulated during disease progression, and signaling pathways are abnormally activated by BCR/ABL. With the application of imatinib, a ABL-specific tyrosine kinase inhibitor, its remarkable therapeutic effects suggest that blast crisis transition will be postponed in most patients with CML. Rate of cumulative best response in CML-CP patients from the IRIS trial after 5 years are 98% for complete hematologic response, 92% for major cytogenetic response and 87% for complete cytogenetic response. However, a minority of CML-CP patients and most patients in progression either fail or respond suboptimally to imatinib. There are many distinct patterns of resistance, and ABL kinase mutations is a common finding associated with clinical resistance. Dasatinib and nilotinib can restore hematologic and cytogenetic remission in the majority of patients with primary failure or acquired resistance in chronic phase. This review illustrates the molecular mechanisms underlying transition to CML-BC, also addresses oneself to how and why imatinib resistance occurs.

Development of drug resistance has become a major obstacle for tyrosine kinase inhibitors (TKIs) in the treatment of Philadelphia chromosome-positive (Ph(+)) chronic myelogenous leukemia (CML) and other cancers. The BCR-ABL-T315I mutant does not respond to clinically available TKIs, although some newly developed anti-BCR-ABL-T315I TKIs are now being tested in patients. TKIs transiently inhibit kinase activity of BCR-ABL, but do not reduce the level of the BCR-ABL protein. Elimination of mutant BCR-ABL protein would provide a new therapeutic strategy for treating Ph(+) leukemia. We recently showed that inhibition of heat shock protein 90 (Hsp90) by a novel Hsp90 inhibitor, IPI- 504, causes BCR-ABL protein degradation, decreased numbers of leukemia stem cells, and prolonged survival of mice with CML induced by BCR-ABL-T315I. Here we discuss further the mechanisms and effectiveness of Hsp90 inhibition in suppression of survival and proliferation of leukemic progenitor and stem cells in CML mice, and the potential of this anti-Hsp90 strategy in treating CML patients, including those who have developed resistance to TKIs.

Purpose: Trichosanthin (TCS), an active component extracted from the root tubers of traditional Chinese medical herb Tian-Hua-Fen of the Cucurbitaceae family, has long been used for medical purpose in China; there is increasing interest in developing TCS as cancer therapeutic agents. The present study was to investigate the growth arrest of K562 cells and its molecular mechanisms, which the drugs induced by TCS and the possible functional interaction of TCS with imatinib (STI571) to K562 cells., Methods: Trypan blue exclusive staining was used to access the cell growth inhibition; western blot was used to evaluate the p210(Bcr-Abl), phosphorylated tyrosine kinase (PTK), and some signaling molecules involving in cell proliferation and apoptosis in K562 cells., Results: TCS and imatinib inhibited K562 cells at a time- and dose-dependent manners, respectively; TCS down-regulated p210(Bcr-Abl) at a time- and dose-dependent manners; TCS synergistically enhanced imatinib-induced K562 cell growth arrest and down-regulation of p210(Bcr-Abl), PTK activities, procaspase-3, Hsp90,NF-kappaB and PKC., Conclusion: The results suggest that TCS not only by itself involves but also synergizes activities of imatinib to induce K562 cell growth arrest, down-regulation of p210(Bcr-Abl) and its downstream signals and to stimulate the effect of the tyrosine kinase inhibition.

The development of imatinib is a milestone in the treatment of chronic myeloid leukemia (CML), and its therapeutic effect has been extensively investigated in CML patients carrying M-bcr and m-bcr BCR/ABL fusion transcripts. However, our knowledge about its therapeutic effect on CML patients with rare BCR/ABL fusion transcripts e19a2(u-bcr) remains sparse. Here, we report on two CML patients with e19a2 transcripts who rapidly progressed into the accelerated phase, further confirming the possibility that 19a2 might be associated with an unfavorable prognosis in CML. Moreover, these patients showed early response to imatinib treatment. Our study highlights the clinical potential of imatinib for this patient subgroup.

Background: Imatinib mesylate treatment causes remissions in a majority of patients with chronic myeloid leukemia (CML), but relapses are an increasing problem. We hypothesized that imatinib-resistant leukemic cells emerge from CML stem cells that acquire BCR-ABL gene mutations even before exposure to BCR-ABL-targeted agents such as imatinib., Methods: Lineage-negative (i.e., immature) CD34+ CD38- CML stem cell-enriched populations were isolated from five patients with chronic phase CML samples by fluorescence-activated cell sorting. To identify BCR-ABL gene mutations, complementary DNAs (cDNAs) prepared from purified CML stem cells were subjected to allele-specific amplification using primers corresponding to 16 kinase domain mutations, with normal bone marrow cells serving as negative controls. We also cloned and directly sequenced BCR-ABL cDNAs prepared from freshly isolated CML stem cells and from their progeny generated after 3-5 weeks of culture., Results: In 20%-33% of cDNA preparations from freshly isolated CML stem cell-enriched populations, both allele-specific amplification and direct sequencing methods revealed mutations in sequences corresponding to the BCR-ABL kinase domain. Mutations were not observed in cDNA sequences encoding the c-ABL kinase domain that were obtained from similar types of primitive normal cells. More than 70 different BCR-ABL mutations (including frameshift mutations and premature stop codons) were identified in the progeny of cultured CML stem cells. Analysis of individual clones derived from the cultured cells demonstrated that new BCR-ABL mutations were produced., Conclusions: Primary CML stem cells display instability of the BCR-ABL fusion gene both in vivo and in vitro. Thus, patients may possess leukemic stem cells with BCR-ABL kinase mutations before initiation of BCR-ABL-targeted therapies and would likely be predisposed to develop resistance to these agents.

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.

The deregulated kinase activity of p210-BCR/ABL oncoproteins, hallmark of chronic myelogenous leukaemia (CML), induces and sustains the leukaemic phenotype, and contributes to disease progression. Imatinib mesylate, a BCR/ABL kinase inhibitor, is effective in most of chronic phase CML patients. However, a significant percentage of CML patients develop resistance to imatinib and/or still progresses to blast crisis, a disease stage that is often refractory to imatinib therapy. Furthermore, there is compelling evidence indicating that the CML leukaemia stem cell is also resistant to imatinib. Thus, there is still a need for new drugs that, if combined with imatinib, will decrease the rate of relapse, fully overcome imatinib resistance and prevent blastic transformation of CML. We recently reported that the activity of the tumour suppressor protein phosphatase 2A (PP2A) is markedly inhibited in blast crisis CML patient cells and that molecular or pharmacologic re-activation of PP2A phosphatase led to growth suppression, enhanced apoptosis, impaired clonogenic potential and decreased in vivo leukaemogenesis of imatinib-sensitive and -resistant (T315I included) CML-BC patient cells and/or BCR/ABL+ myeloid progenitor cell lines. Thus, the combination of PP2A phosphatase-activating and BCR/ABL kinase-inhibiting drugs may represent a powerful therapeutic strategy for blast crisis CML patients.

Objectives: The objectives of this study were to determine the effects of Gleevec on p63 expression in head and neck squamous cell carcinoma (HNSCC) cell lines and to investigate the role of Gleevec in regulating p63 stabilization under DNA-damaging conditions., Methods: Immunohistochemical staining was performed to determine p63 expression in HNSCC tissue. Annexin V staining was used to assess the effects of p63 on early apoptosis. Immunoblotting was used to examine the effects of Gleevec on p63 protein levels in HNSCC cell lines in response to DNA damage. Immunofluorescence staining was performed to study the expression pattern of p63 and c-Abl., Results: In HNSCC, p63 protein levels are induced by DNA-damaging agents, including ionizing radiation, doxorubicin, and ultraviolet light. We demonstrate that Gleevec reduces p63/DeltaNp63 expression in a dose-dependent manner in HNSCC and overrides the protein induction by DNA-damaging agents. Overexpression of c-Abl in the absence of Gleevec results in higher levels of p63 than those treated with Gleevec, implicating c-Abl kinase activity as a regulator of p63 protein stability., Conclusions: Gleevec downregulates p63/DeltaNp63 levels in HNSCC in a dose-dependent manner under both normal and DNA-damaging conditions. This downregulation can be explained by Gleevec's inhibition of c-Abl, which destabilizes p63. Based on our data, treating cancers with high expression of TAp63 with Gleevec may result in the unfavorable inhibition of a tumor suppressor, whereas downregulation of DeltaNp63 would be advantageous. Further development of antibodies that can discriminate between TAp63 and DeltaNp63 will be needed to determine the specific effects of Gleevec on p63 in HNSCC.

Chronic myelogenous leukaemia (CML) and Philadelphia chromosome positive (Ph+) acute lymphoblastic leukaemia (ALL) are caused by the BCR-ABL oncogene. Imatinib inhibits the tyrosine kinase activity of the BCR-ABL protein and is an effective, frontline therapy for chronic-phase CML. However, accelerated or blast-crisis phase CML patients and Ph+ ALL patients often relapse due to drug resistance resulting from the emergence of imatinib-resistant point mutations within the BCR-ABL tyrosine kinase domain. This has stimulated the development of new kinase inhibitors that are able to over-ride resistance to imatinib. The novel, selective BCR-ABL inhibitor, AMN107, was designed to fit into the ATP-binding site of the BCR-ABL protein with higher affinity than imatinib. In addition to being more potent than imatinib (IC50< 30 nM) against wild-type BCR-ABL, AMN107 is also significantly active against 32/33 imatinib-resistant BCR-ABL mutants. In preclinical studies, AMN107 demonstrated activity in vitro and in vivo against wild-type and imatinib-resistant BCR-ABL-expressing cells. In phase I/II clinical trials, AMN107 has produced haematological and cytogenetic responses in CML patients, who either did not initially respond to imatinib or developed imatinib resistance. Dasatinib (BMS-354825), which inhibits Abl and Src family kinases, is another promising new clinical candidate for CML that has shown good efficacy in CML patients. In this review, the early characterisation and development of AMN107 is discussed, as is the current status of AMN107 in clinical trials for imatinib-resistant CML and Ph+ ALL. Future trends investigating prediction of mechanisms of resistance to AMN107, and how and where AMN107 is expected to fit into the overall picture for treatment of early-phase CML and imatinib-refractory and late-stage disease are discussed.

Although the Abelson (Abl) tyrosine kinase inhibitor imatinib mesylate has improved the treatment of breakpoint cluster region-Abl (Bcr-Abl)-positive leukemia, resistance is often reported in patients with advanced-stage disease. Although several Src inhibitors are more effective than imatinib and simultaneously inhibit Lyn, whose overexpression is associated with imatinib resistance, these inhibitors are less specific than imatinib. We have identified a specific dual Abl-Lyn inhibitor, NS-187 (elsewhere described as CNS-9), which is 25 to 55 times more potent than imatinib in vitro. NS-187 is also at least 10 times as effective as imatinib in suppressing the growth of Bcr-Abl-bearing tumors and markedly extends the survival of mice bearing such tumors. The inhibitory effect of NS-187 extends to 12 of 13 Bcr-Abl proteins with mutations in their kinase domain but not to T315I. NS-187 also inhibits Lyn without affecting the phosphorylation of Src, Blk, or Yes. These results suggest that NS-187 may be a potentially valuable novel agent to combat imatinib-resistant Philadelphia-positive (Ph+) leukemia.

In Philadelphia-positive (Ph(+)) leukemia, point mutations within the Bcr-Abl kinase domain emerged as a major mechanism of resistance to imatinib mesylate. We established a cell-based screening strategy for detection of clinically relevant point mutations using Bcr-Abl-transformed Ba/F3 cells. We identified 32 different single-point mutations within the kinase domain of Bcr-Abl. The pattern and frequency of mutations in this cell culture-based screen resembled the pattern and frequency observed in resistant patients. We then applied this screen to an alternative Abl kinase inhibitor. Using PD166326, the frequency of resistant colonies emerging at 5 to 10 times the median growth inhibition (IC50) of PD166326 was significantly lower than with imatinib. In addition, PD166326 produced a distinct pattern of Bcr-Abl mutations. The majority of mutations that came up with both imatinib and PD166326 could effectively be suppressed by increasing the dose of PD166326 to 50 to 500 nM. In contrast, only a few mutations could be suppressed by increasing the imatinib dose to 5 to 10 microM. However, 3 mutations affecting F317 displayed complete resistance to PD166326, but could be effectively inhibited by standard concentrations of imatinib. Thus, this robust and simple screening system provides a rational basis for combinatorial and sequential treatment strategies in targeted cancer therapy.

The constitutively active Abl kinase activity of the Bcr-Abl oncoprotein is causative for chronic myelogenous leukemia. Urea derivatives, structurally related to the therapeutic agent STI571, have been identified, which potently inhibit the tyrosine kinase activity of recombinant Abl. In particular a dimethylamino-aniline derivative (18) inhibited c-Abl transphosphorylation with an IC(50) value of 56 nM. Although this activity was not translated into cellular activity against the constitutively activated oncogenic Bcr-Abl, a number of compounds from this series potently inhibited cellular PDGFR autophosphorylation. It was also possible to differentiate between c-Abl and PDGFR kinase inhibition, with compound 22 being selective towards Abl and 23 selective for PDGFR.

The di-sulfonated di-phthalimidomethyl phthalocyanine zinc (ZnPcS2P2) was an amphiphilic photosensitizer for photodynamic therapy of cancer. According to fluorescence analytical method, extracting efficiencies of 2.0% SDS (Wt%), 1.5% Triton X-100 (phi) and DMF for ZnPcS2P2 in K562 cells were compared. The results showed that 1.5% Triton X-100 was the most efficient reagent for the first time extracting. Using 1.5% Triton X-100 as extracting reagent, the kinetic curves of cellular uptake of ZnPcS2P2 as well as LDL-ZnPcS2P2 complex by K562 cells were determined. Compared with ZnPcS2P2, the maximum cellular uptake of LDL-ZnPcS2P2 by K562 was doubled, which indicated that LDL was an effective targeting delivery vehicle for ZnPcS2P2.

Background and Objective: The tyrosinkinase inhibitor Imatinib is active in Philadelphia-positive (Ph+) leukemia. Mutations within the Bcr-Abl kinase domain represent the major cause for clinical resistance toward imatinib. We aimed to examine, whether the alternative Abl Kinaseinhibitor SKI-DV 2 - 43 may be capable of suppressing the growth of cells expressing mutant forms of Bcr-Abl., Methods: The proliferation of cells expressing wild-type and mutant forms of Bcr-Abl was measured in the presence of imatinib or the pyrido-pyrimidine SKI-DV 2 - 43., Results: The growth of a cell line expressing wild-type Bcr-Abl was suppressed with higher potency in the presence of SKI-DV 2 - 43 when compared to imatinib. Moreover, SKI-DV 2 - 43 effectively suppressed mutant forms of Bcr-Abl that cause imatinib resistance in patients., Conclusion: Therefore, alternative Abl kinase inhibitors might play an important role in the future therapy of Philadelphia-positive leukemias.

Activating mutations in the epidermal growth factor receptor (EGF-R) predict response to gefitinib. How does this recent discovery affect our outlook on selective (targeted) cancer therapy? It allows us to compare mutant EGF-R with Bcr-Abl as anticancer drug targets and to discuss the nature of oncogene addiction. It emphasizes molecular diagnostics to identify oncogene-addictive cancers. It also re-enforces the notion that most cancers with multiple oncogenic alterations (common cancers) will unlikely respond to selective drugs alone. In such cancers, one strategy is targeting cancer-non-specific, universal and vital structures, essential for life of all cells: microtubules, topoisomerases, histone deacetylases, the proteasome. But in order to be cancer-selective, these chemotherapeutic agents need to be combined with selective agents. Such combinations can be effective and selective in common cancers.

Present studies demonstrate that treatment with arsenic trioxide (AT) lowered ectopically expressed or endogenous levels of Bcr-Abl protein, as well as induced apoptosis of Bcr-Abl-expressing cultured and primary chronic myeloid leukemia cells, including those refractory to imatinib mesylate. Treatment with AT neither affected bcr-abl mRNA transcript levels nor promoted the proteasomal degradation of Bcr-Abl. Importantly, in [(35)S]methionine-labeled leukemia cells, exposure to AT rapidly lowered the levels of the newly synthesized Bcr-Abl, indicating inhibition of bcr-abl mRNA translation. Treatment with AT rapidly inhibited the activity of 3-phosphoinositide-dependent protein kinase-1, as well as of p70 S6 kinase-1. p70 S6 kinase-1 is known to be a positive regulator of the translation of a group of mRNAs that possesses a long and highly structured 5'-untranslated region (UTR) containing a tract of oligopyrimidines (TOP). Because bcr-abl mRNA was discovered to possess a long and highly structured 5'-UTR containing a 12-pyrimidine TOP sequence in its 5'-UTR, we determined the effect of AT in Jurkat cells with ectopic expression of a 5'-UTR-deleted mutant of the bcr-abl gene, i.e., Jurkat/Bcr-Abl (5'UTR-) cells. Treatment with AT neither lowered the levels of the 5'-UTR-deleted mutant of Bcr-Abl nor induced apoptosis of Jurkat/Bcr-Abl (5'UTR-) cells. Taken together, these findings demonstrate a novel mechanism by which AT down-regulates Bcr-Abl levels and induces apoptosis of Bcr-Abl-positive chronic myelogenous leukemia cells.

The BCR-ABL oncogene is the result of a reciprocal translocation between the long arms of chromosome 9 and 22 t(9; 22). There is good experimental evidence demonstrating that BCR-ABL is the single causative abnormality in chronic myeloid leukaemia (CML), making it a unique model for the development of molecular targets. In addition to CML, BCR-ABL transcripts can be found in a minority of acute lymphoblastic leukaemias and very rarely in acute myeloid leukaemia (AML). Elucidating the molecular mechanisms and downstream pathways of BCR-ABL has led to the design of several novel therapeutic approaches. In this review, molecular targeting of BCR-ABL will be discussed based on the inhibition of protein tyrosine kinase activity, antisense strategies and immunomodulation.

The ubiquitously expressed c-Abl tyrosine kinase is activated in the response of cells to genotoxic and oxidative stress. The present study demonstrates that reactive oxygen species (ROS) induce targeting of c-Abl to mitochondria. We show that ROS-induced localization of c-Abl to mitochondria is dependent on activation of protein kinase C (PKC)delta and the c-Abl kinase function. Targeting of c-Abl to mitochondria is associated with ROS-induced loss of mitochondrial transmembrane potential. The results also demonstrate that c-Abl is necessary for ROS-induced depletion of ATP and the activation of a necrosis-like cell death. These findings indicate that the c-Abl kinase targets to mitochondria in response to oxidative stress and thereby mediates mitochondrial dysfunction and cell death.

The Philadelphia (Ph) chromosome is found in approximately 3% of pediatric patients with acute lymphoblastic leukemia (ALL) and the percentage markedly increases in adult patients. The prognosis for this class of patients is poor, and no standard chemotherapy combination so far has demonstrated long-term efficacy. The Ph-translocation joins the BCR and ABL genes and leads to expression of a chimeric Bcr/Abl protein with enhanced tyrosine kinase activity. This increase in activity leads to malignant transformation by interference with basic cellular functions such as the control of proliferation, adherence to stroma and extracellular matrix, and apoptosis. One important pathway activated by Bcr/Abl is the Ras pathway. Ras proteins have to undergo a series of posttranslational modifications to become biologically active. The first modification is the farnesylation of the C-terminus catalyzed by farnesyl transferase. We studied the effect of the farnesyl transferase inhibitor SCH66336 in an in vivo murine model of Bcr/Abl-positive acute lymphoblastic leukemia. In the early leukemic phase, mice were randomly assigned to a treatment, a vehicle, and a nontreatment group. The treatment was well tolerated without any detectable side effects. All animals of the control groups died of leukemia/lymphoma within 103 days (range, 18-103 days). In contrast, 80% of the drug-receiving group survived without any signs of leukemia or lymphoma until termination of treatment, after a median treatment period of 200 days (range, 179-232 days). We conclude that farnesyl transferase inhibitor SCH66336 is able to revert early signs of leukemia and significantly prolongs survival in a murine ALL model.

Therapeutic agent STI571 (signal transduction inhibitor number 571) is a rationally developed, potent, and selective inhibitor for abl tyrosine kinases, including bcr-abl, as well c-kit and the platelet-derived growth factor receptor tyrosine kinases. Results of clinical trials to date have demonstrated the crucial role of the bcr-abl tyrosine kinase in chronic myelogenous leukemia (CML) pathogenesis and the potential of anticancer agents designed to target specific molecular abnormalities in human cancer. An initial phase I study of STI571 included 83 Ph(+) CML patients who had failed interferon-based therapy. Patients were required to be in chronic phase, defined liberally as less than 15% blasts in blood or bone marrow. Patients were treated with once-daily oral doses of STI571 in 14 successive dose cohorts ranging from 25-1,000 mg. In this phase I study, no dose-limiting toxicity was encountered and toxicity at all dose levels was minimal. The threshold for a maximally effective dose was found at 300 mg; for patients treated at or above this level, complete hematologic response was seen in 98% of patients, with complete cytogenetic responses in 13% and major cytogenetic responses in 31%. With a median duration of follow-up of 310 days, ongoing responses are evident in 96% of patients. In the phase II study of the accelerated phase of CML, 233 patients were treated with either 400 or 600 mg of STI571. With similar follow-up to the chronic phase trial, 91% of patients showed a hematological response; 63% of patients achieved a complete hematological response but not all patients had recovery of peripheral blood counts. In addition to the phase II clinical trials with STI571, a phase III trial randomizing newly diagnosed patients to either interferon with low-dose s.c. cytosine arabinoside versus STI571 is ongoing; this trial accrued rapidly and data collection is ongoing. Integration of STI571 into CML treatment algorithms will require long-term follow-up data from the ongoing phase II and III clinical studies.

Treatment-related acute myeloid leukemia (t-AML) following successful therapy of a primary malignancy has been recognized with increasing frequency among cancer survivors over the past several years. Many of these t-AML cases are associated with the use of intensive chemotherapy regimens that employ one or more agents which target eukaryotic topoisomerase II (topo II), and demonstrate non-random chromosomal translocations involving either the MLL (ALL-1, HRX) gene at 11q23 or the AML1 gene at 21q22. Although many investigators have speculated that these translocations are induced by the therapeutic use of topo II inhibitors, the molecular sequence of events by which topo II inhibitors might induce a chromosomal translocation are not well understood. We describe here the reproducible induction of highly specific, double-strand DNA cleavage at a specific site within the AML1 locus by topo II inhibitors. This DNA cleavage, which maps to a region of the AML1 locus frequently disrupted by chromosomal translocations, can be induced in several cell lines, with multiple different topo II inhibitors, indicating that this phenomenon is not restricted to a specific cell type or specific topo II inhibitor. It is conceivable that site-specific double-strand DNA cleavage within the AML1 locus induced by topo II inhibitors represents the initial molecular event leading to a chromosomal translocation and t-AML.