Biology of Chronic Myeloid Leukemia and Possible Therapeutic Approaches to Imatinib-Resistant Disease

BH3 mimetics and TKI combined therapy for Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) was considered for a long time one of the most hostile leukemia that was incurable for most of the patients, predominantly due to the extreme resistance to chemotherapy. Part of the resistance to cell death (apoptosis) is the result of increased levels of anti-apoptotic and decreased levels of pro-apoptotic member of the BCL-2 family induced by the BCR-ABL1 oncoprotein. BCR-ABL1 is a constitutively active tyrosine kinase responsible for initiating multiple and oncogenic signaling pathways. With the development of specific BCR-ABL1 tyrosine kinase inhibitors (TKIs) CML became a much more tractable disease. Nevertheless, TKIs do not cure CML patients and a substantial number of them develop intolerance or become resistant to the treatment. Therefore, novel anti-cancer strategies must be developed to treat CML patients independently or in combination with TKIs. Here, we will discuss the mechanisms of BCR-ABL1-dependent and -independent resistance to TKIs and the use of BH3-mimetics as a potential tool to fight CML.

Bioactive Lipids as Chronic Myeloid Leukemia’s Potential Biomarkers for Disease Progression and Response to Tyrosine Kinase Inhibitors

Chronic myelogenous leukemia (CML) is a myeloproliferative neoplasm that expresses the Philadelphia chromosome and constitutively activated Bcr-Abl tyrosine kinase in hematopoietic progenitor cells. Bcr-Abl tyrosine-kinase inhibitors (TKI) do not definitively cure all CML patients. The efficacy of TKI is reduced in CML patients in the blastic phase—the most severe phase of the disease—and resistance to this drug has emerged. There is limited knowledge on the underlying mechanisms of disease progression and resistance to TKI beyond BCR-ABL1, as well as on the impact of TKI treatment and disease progression on the metabolome of CML patients. The present study reports the metabolomic profiles of CML patients at different phases of the disease treated with TKI. The plasma metabolites from CML patients were analyzed using liquid chromatography, mass spectrometry, and bioinformatics. Distinct metabolic patterns were identified for CML patients at different phases of the disease and for those who were resistant to TKI. The lipid metabolism in CML patients at advanced phases and TKI-resistant patients is reprogrammed, as detected by analysis of metabolomic data. CML patients who were responsive and resistant to TKI therapy exhibited distinct enriched pathways. In addition, ceramide levels were higher and sphingomyelin levels were lower in resistant patients compared with control and CML groups. Taken together, the results here reported established metabolic profiles of CML patients who progressed to advanced phases of the disease and failed to respond to TKI therapy as well as patients in remission. In the future, an expanded study on CML metabolomics may provide new potential prognostic markers for disease progression and response to therapy.

Heme oxygenase-1 reduces the sensitivity to imatinib through nonselective activation of histone deacetylases in chronic myeloid leukemia

Resistance towards imatinib (IM) remains troublesome in treating many chronic myeloid leukemia (CML) patients. Heme oxygenase-1 (HO-1) is a key enzyme of antioxidative metabolism in association with cell resistance to apoptosis. Our previous studies have shown that overexpression of HO-1 resulted in resistance development to IM in CML cells, while the mechanism remains unclear. In the current study, the IM-resistant CML cells K562R indicated upregulation of some of the histone deacetylases (HDACs) compared with K562 cells. Therefore, we herein postulated HO-1 was associated with HDACs. Silencing HO-1 expression in K562R cells inhibited the expression of some HDACs, and the sensitivity to IM was increased. K562 cells transfected with HO-1 resisted IM and underwent obvious some HDACs. These findings related to the inhibitory effects of high HO-1 expression on the reactive oxygen species (ROS) signaling pathway that negatively regulated HDACs. Increased expression of HO-1 activated HDACs by inhibiting ROS production. In summary, HO-1, which is involved in the development of drug resistance in CML cells by regulating the expression of HDACs, is probably a novel target for improving CML therapy.

Validation of a rapid one-step high sensitivity real-time quantitative PCR system for detecting major BCR-ABL1 mRNA on an International Scale

BACKGROUND

Detection and quantitation of BCR-ABL1 transcripts are crucial for managing patients with chronic myeloid leukemia (CML). Although real-time quantitative polymerase chain reaction (RT-qPCR) can be measured on an International Scale (IS), this has not become fully universal. By using a WHO international standard panel established for calibrating secondary standards based on the IS, we have previously developed an RT-qPCR kit, ODK-1201, for quantification of major BCR-ABL1.

RESULTS

In this study, the reliability of kit-specific conversion factor 1.12 was validated by exchanging patients' samples between three local clinical laboratories and a reference laboratory. The mean bias of the local method after IS conversion was 1.6 fold lower than the reference method. The clinically-useful sensitivity of the kit was further evaluated for monitoring patients with deep molecular response. Based on the correlation of the IS values between ODK-1201 and the reference laboratory method, the detection level of the kit was estimated as 0.0032 % BCR-ABL1 (IS).

CONCLUSIONS

ODK-1201 is a highly sensitive one-step RT-qPCR system for detecting BCR-ABL1 on the IS in 2 h after RNA extraction, thus contributing to standardization of molecular monitoring in CML.

Optimizing combination therapies with existing and future CML drugs

Small-molecule inhibitors imatinib, dasatinib and nilotinib have been developed to treat Chromic Myeloid Leukemia (CML). The existence of a triple-cross-resistant mutation, T315I, has been a challenging problem, which can be overcome by finding new inhibitors. Many new compounds active against T315I mutants are now at different stages of development. In this paper we develop an algorithm which can weigh different combination treatment protocols according to their cross-resistance properties, and find the protocols with the highest probability of treatment success. This algorithm also takes into account drug toxicity by minimizing the number of drugs used, and their concentration. Although our methodology is based on a stochastic model of CML microevolution, the algorithm itself does not require measurements of any parameters (such as mutation rates, or division/death rates of cells), and can be used by medical professionals without a mathematical background. For illustration, we apply this algorithm to the mutation data obtained in [1], [2].

Bcr-Abl-independent mechanism of resistance to imatinib in K562 cells: Induction of cyclooxygenase-2 (COX-2) by histone deacetylases (HDACs)

Our previous studies have shown that overexpression of MDR1 and cyclooygenase-2 (COX-2) resulted in resistance development to imatinib in chronic myelogenous leukemia (CML) K562 (IR-K562) cells. In the present study, the regulatory mechanism of MDR1 induction by COX-2 was investigated. A gradual overexpression of MDR1 and COX-2 during the process of development was observed. Furthermore, down regulation of MDR1 upon COX-2 knockdown by siRNA showed a decrease in the PKC levels and activation of PKC by addition of PGE(2) to K562 cells, suggesting a role for PKC in the COX-2 mediated induction of MDR1. The present study demonstrates COX-2 induction by HDACs and MDR1 induction by COX-2 via PGE(2)-cAMP-PKC-mediated pathway.

Heterogeneity in chronic myeloid leukaemia dynamics during imatinib treatment: role of immune responses

Previous studies have shown that during imatinib therapy, the decline of chronic myeloid leukaemia BCR-ABL transcript numbers involves a fast phase followed by a slow phase in averaged datasets. Drug resistance leads to regrowth. In this paper, variation of treatment responses between patients is examined. A significant positive correlation is found between slopes of the fast and the slow phase of decline. A significant negative correlation is found between slopes of the slow phase of decline and the regrowth phase. No correlation is found between slopes of the fast phase of decline and the regrowth phase. A mathematical model that is successfully fitted to diverse clinical profiles explains these correlations by invoking the immune response as a key determinant of tumour decline during treatment. Boosting immunity during drug therapy could enhance the response to treatment in patients.

Combination therapies against chronic myeloid leukemia: short-term versus long-term strategies

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.

Combination of two but not three current targeted drugs can improve therapy of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a cancer of the hematopoietic system and has been treated with the drug Imatinib relatively successfully. Drug resistance, acquired by mutations, is an obstacle to success. Two additional drugs are now considered and could be combined with Imatinib to prevent resistance, Dasatinib and Nilotinib. While most mutations conferring resistance to one drug do not confer resistance to the other drugs, there is one mutation (T315I) that induces resistance against all three drugs. Using computational methods, the combination of two drugs is found to increase the probability of treatment success despite this cross-resistance. Combining more than two drugs, however, does not provide further advantages. We also explore possible combination therapies using drugs currently under development. We conclude that among the targeted drugs currently available for the treatment of CML, only the two most effective ones should be used in combination for the prevention of drug resistance.

Molecular basis explanation for imatinib resistance of BCR-ABL due to T315I and P-loop mutations from molecular dynamics simulations

BACKGROUND

Computational simulations have become powerful tools for understanding detailed interactions in biologic systems. To the authors' knowledge to date, the mechanism of imatinib resistance in BCR-ABL has not been clarified at the atomic level, and computational studies are required.

METHODS

Molecular dynamics (MD) simulations on the complex of imatinib with the wild-type, T315I mutant, and 10 other P-loop mutants of the tyrosine kinase BCR-ABL were performed to study the mechanism of imatinib resistance.

RESULTS

Simulations suggested that imatinib resistance of T315I results mainly comes from the breakdown of interactions between imatinib and both E286 and M290, contradictory to what was believed previously, in that the missing hydrogen bonding is the main contribution. The current results also demonstrated that the unfavorable electrostatic interaction between P-loop and imatinib is the main reason for resistance for the P-loop mutations. Furthermore, in Y253H, protonation of the histidine at the epsilon position is essential for rendering this mutation resistant to imatinib.

CONCLUSIONS

The current results indicated that large-scale simulations may offer insight and information that other simple modeling methods cannot provide regarding the problem of BCR-ABL imatinib resistance, especially in the case of conformational changes because of remote mutations. Imatinib resistance mechanisms that were not anticipated previously were revealed by analyzing the interactions between imatinib and individual residues based on simulation results. This results demonstrated that MD is a powerful way to verify and predict the clinical response or resistance to imatinib and to other potential drugs.

Treatment Options in Imatinib-Resistant Chronic Myelogenous Leukemia

Objective:

To discuss new therapeutic options available in the treatment of chronic myelogenous leukemia (CML) in patients who (ailed or were intolerant to imatinib therapy.

Data Sources:

Literature was accessed via MEDLINE (1966–May 2007). EMBASE (199l–3rd quarter 2007). the Proceedings of the American Society of Hematology (2000–2006), and the Proceedings of the American Society of Clinical Oncology (2000–2007). Search terms included imatinib, dasatinib, nilotinib. and chronic myelogenous leukemia.

Study Selection and Data Extraction:

Meeting abstracts and studies that reported preclinical and Phase 1, 2, and 3 trials published in English are included.

Data Synthesis:

Imatinib is the standard of care for CML; however, some patients develop resistance or are intolerant to the drug. Phase 1 and 2 clinical data for the more potent tyrosine kinase inhibitors, dasatinib and nilotinib, are promising. Hematologic and cytogenetic responses are reported with both. There does not appear to be cross-resistance between the drugs, although neither is effective against all mutations ol the hallmark molecular marker, the Philadelphia chromosome. Novel agents are also being examined for the treatment of patients with CML, including aurora kinase and farnesyl transferase inhibitors, as well as combination therapies.

Conclusions:

Dasatinib and nilotinib are second-tine options for patients who have CML and are resistant or intolerant to imatinib. Toxicity profiles between agents may differ. Clinical trials with these drugs and others are ongoing.

Stem cell regulation and the development of blast crisis in chronic myeloid leukemia: Implications for the outcome of Imatinib treatment and discontinuation

Chronic myeloid leukemia (CML) is a cancer of the hematopoietic system that is initiated by a single genetic alteration (the BCR-ABL fusion gene or Philadelphia chromosome) and progresses in several phases: during the chronic phase the number of cells grows slowly and the fraction of immature cells is low. During the accelerated phase and blast crisis, the population of CML cells and the fraction of immature cells rises sharply. The mechanisms that drive the transition from the chronic phase to blast crisis are not understood, and the requirement of genetic instability and further mutations has been suggested. Using mathematical models, I describe a theory that can account for the transition from the chronic phase to blast crisis without the need to invoke further mutations. The transition to blast crisis can be explained solely by feedback mechanisms that regulate the patterns of stem cell division, in particular the occurrence of symmetric versus asymmetric cell division. The model also has implications for the outcome of Imatinib treatment. According to the model, treatment can lead to the low level persistence of CML stem cells without assuming that these cells are less susceptible to drug-mediated activity, and this might explain why disease tends to relapse after treatment discontinuation even in the absence of acquired drug resistance. Further, the model defines conditions when Imatinib treatment might lead to the eradication of CML, which is relevant in the context of recent data that show absence of relapse as long as two years after treatment cessation.

Genomic profile of chronic myelogenous leukemia: Imbalances associated with disease progression

The expression of the chimeric BCR/ABL1 fusion gene resulting from t(9;22)(q34;q11) in chronic myelogenous leukemia (CML) is necessary for malignant transformation, but not sufficient to maintain disease progression. The appearance of various chromosomal and molecular alterations in the accelerated and terminal phase of CML is well documented, but evidence for causal relationship is largely lacking. We carried out a genome wide screening at a resolution of 1 Mb of 54 samples at different stages of CML together with 12 CML cell lines and found that disease progression is accompanied by a spectrum of recurrent genome imbalances. Among the most frequent are losses at 1p36, 5q21, 9p21, and 9q34 and gains at 1q, 8q24, 9q34, 16p, and 22q11, all of which were located with higher precision within the genome than previously possible. These genome imbalances are unique to CML cases with clinically manifested or suspected accelerated/blast stage alike, but not seen in chronic phase samples. Previously unrecognized cryptic imbalances occurring within the Ph-chromosome were also detected, although further scrutiny is required to pin-point gene involvement and seek association with disease features. Importantly, some of these imbalances were seen in the CD34(+) cells but not in the whole BM samples of patients in accelerated phase. Taken together, these findings highlight the potential of screening CD34(+) cells for genome wide imbalances associated with disease progression. Finally, the numerous single copy number variations recorded, many unique to this cohort of patients, raise the possible association of genome polymorphism and CML.

Effect of cellular quiescence on the success of targeted CML therapy

BACKGROUND

Similar to tissue stem cells, primitive tumor cells in chronic myelogenous leukemia have been observed to undergo quiescence; that is, the cells can temporarily stop dividing. Using mathematical models, we investigate the effect of cellular quiescence on the outcome of therapy with targeted small molecule inhibitors.

METHODS AND RESULTS

According to the models, the initiation of treatment can result in different patterns of tumor cell decline: a biphasic decline, a one-phase decline, and a reverse biphasic decline. A biphasic decline involves a fast initial phase (which roughly corresponds to the eradication of cycling cells by the drug), followed by a second and slower phase of exponential decline (corresponding to awakening and death of quiescent cells), which helps explain clinical data. We define the time when the switch to the second phase occurs, and identify parameters that determine whether therapy can drive the tumor extinct in a reasonable period of time or not. We further ask how cellular quiescence affects the evolution of drug resistance. We find that it has no effect on the probability that resistant mutants exist before therapy if treatment occurs with a single drug, but that quiescence increases the probability of having resistant mutants if patients are treated with a combination of two or more drugs with different targets. Interestingly, while quiescence prolongs the time until therapy reduces the number of cells to low levels or extinction, the therapy phase is irrelevant for the evolution of drug resistant mutants. If treatment fails as a result of resistance, the mutants will have evolved during the tumor growth phase, before the start of therapy. Thus, prevention of resistance is not promoted by reducing the quiescent cell population during therapy (e.g., by a combination of cell activation and drug-mediated killing).

CONCLUSIONS

The mathematical models provide insights into the effect of quiescence on the basic kinetics of the response to targeted treatment of CML. They identify determinants of success in the absence of drug resistant mutants, and elucidate how quiescence influences the emergence of drug resistant mutants.

Stochastic modeling of cellular colonies with quiescence: an application to drug resistance in cancer

Several cancers are thought to be driven by cells with stem cell like properties. An important characteristic of stem cells, which also applies to primitive tumor cells, is the ability to undergo quiescence, where cells can temporarily stop the cell cycle. Cellular quiescence can affect the kinetics of tumor growth, and the susceptibility of the cells to therapy. To study how quiescence affects treatment, we formulate a stochastic birth-death process with quiescence, on a combinatorial cellular mutation network, and consider the pre-treatment (growth) and treatment (decay) regimes. We find that, in the absence of mutations, treatment (if sufficiently strong) will proceed as a biphasic decline with the first (faster) phase driven by the elimination of the cycling cells and the second (slower) phase limited by the process of cell awakening. Other regimes are possible for weaker treatments. We also describe how the process of mutant generation is influenced by quiescence. Interestingly, for single-drug treatments, the probability to have resistance at start of treatment is independent of quiescence. For two or more drugs, the probability to have generated resistant mutants before treatment grows with quiescence. Finally, we study the influence of quiescence on the treatment phase. Starting from a given composition of mutants, the chances of treatment success are not influenced by the presence of quiescence.

Histone deacetylase inhibitors: biology and mechanism of action

Histone deacetylases (HDACs) and histone acetyltransferases are enzymes that regulate chromatin structure and function through the removal and addition, respectively, of the acetyl group from the lysine residues of core nucleosomal histones. This posttranslational modification of histones is an important process in the regulation of gene expression. Aberrant expression and recruitment and disrupted activities of HDACs and histone acetyltransferases have been found in malignant tissues, implicating their involvement in cancer. HDAC inhibitors (HDACIs) function through diverse mechanisms, including the promotion of cell cycle arrest and apoptosis and the inhibition of angiogenesis. Malignant cells appear more sensitive to the proapoptotic effects of HDACIs, underscoring the therapeutic potential of these agents. Multiple HDACIs are currently under investigation in clinical trials, including vorinostat (suberoylanilide hydroxamic acid), which was recently approved by the U.S. Food and Drug Administration for the treatment of cutaneous manifestations of cutaneous T-cell lymphoma in patients with progressive, persistent, or recurrent disease on or after 2 systemic therapies.

Bcr-Abl-independent imatinib-resistant K562 cells show aberrant protein acetylation and increased sensitivity to histone deacetylase inhibitors

Bcr-Abl-independent signaling pathways are known to be involved in imatinib resistance in some patients with chronic myelogenous leukemia (CML). In this study, to find new targets for imatinib-resistant CML displaying loss of Bcr-Abl kinase target dependence, we isolated imatinib-resistant variants, K562/R1, K562/R2, and K562/R3, which showed profound declines of Bcr-Abl levels and its tyrosine kinase activity, from K562 cells. Importantly, the imatinib resistance mechanism in these variants also included aberrant acetylation of nonhistone proteins such as p53, Ku70, and Hsp90 that was due to upregulation of histone deacetylases (HDACs) and down-regulation of histone acetyltransferase (HAT). In comparison with K562 cells, the imatinib-resistant variants showed up-regulation of HDAC1, -2, and -3 (class I HDACs) and class III SIRT1 and down-regulation of CBP/p300 and PCAF with HAT activity, and thereby p53 and cytoplasmic Ku70 were aberrantly acetylated. In addition, these were associated with down-regulation of Bax and up-regulation of Bcl-2. In contrast, the class II HDAC6 level was significantly decreased, and this was accompanied by an increase of Hsp90 acetylation in the imatinib-resistant variants, which was closely associated with loss of Bcr-Abl. These results indicate that alteration of the normal balance of HATs and HDACs leads to deregulated acetylation of Hsp90, p53, and Ku70 and thereby leads to imatinib resistance, suggesting the importance of the acetylation status of apoptosis-related nonhistone proteins in Bcr-Abl-independent imatinib resistance. We also revealed that imatinib-resistant K562 cells were more sensitive to suberoylanilide hydroxamic acid, an HDAC inhibitor, than K562 cells. These findings may have implications for HDAC as a molecular target in imatinib-resistant leukemia cells.

Dimethyl sulfoxide to vorinostat: development of this histone deacetylase inhibitor as an anticancer drug

In our quest to understand why dimethyl sulfoxide (DMSO) can cause growth arrest and terminal differentiation of transformed cells, we followed a path that led us to discover suberoylanilide hydroxamic acid (SAHA; vorinostat (Zolinza)), which is a histone deacetylase inhibitor. SAHA reacts with and blocks the catalytic site of these enzymes. Extensive structure-activity studies were done along the path from DMSO to SAHA. SAHA can cause growth arrest and death of a broad variety of transformed cells both in vitro and in tumor-bearing animals at concentrations not toxic to normal cells. SAHA has many protein targets whose structure and function are altered by acetylation, including chromatin-associated histones, nonhistone gene transcription factors and proteins involved in regulation of cell proliferation, migration and death. In clinical trials, SAHA has shown significant anticancer activity against both hematologic and solid tumors at doses well tolerated by patients. A new drug application was approved by the US Food and Drug Administration for vorinostat for treatment of cutaneous T-cell lymphoma. More potent analogs of SAHA have shown unacceptable toxicity.

Camptothecin acts synergistically with imatinib and overcomes imatinib resistance through Bcr-Abl independence in human K562 cells

In this study, we have tried to find new targets and effective drugs for imatinib-resistant chronic myelogenous leukemia (CML) cells displaying loss of Bcr-Abl kinase target dependence. The imatinib-resistant K562/R1, -R2 and -R3 cells showed profound declines of Bcr-Abl level and concurrently exhibited up-regulation of Bcl-2 and Ku70/80, and down-regulation of Bax, DNA-PKcs and BRCA1, suggesting that loss of Bcr-Abl after exposure to imatinib might be accompanied by other cell survival mechanism. K562/R3 cells were more sensitive to camptothecin (CPT)- and radiation-induced apoptosis than K562 cells, indicating hypersensitivity of imatinib-resistant cells to DNA damaging agents. Moreover, when K562 cells were treated with the combination of imatinib with CPT, the level of Bax and the cleavage of PARP-1 and DNA-PK were significantly increased in comparison with the effects of each drug. Therefore, our study suggests that CPT can be used to treat CML with loss of Bcr-Abl expression.

Dose modification of imatinib by monitoring the level of BCR-ABL transcript in chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a hematological malignancy that is characterized by the chromosome anomaly, t(9;22)(q34;q11). By this chromosomal translocation, a novel activated tyrosine kinase, BCR-ABL chimeric protein, is generated, and the protein is causative of the disease. Recently, Imatinib mesylate targeting to a BCR-ABL chimeric protein has been developed, and shown to achieve complete remission at a high rate. Patients are currently required to receive a fixed dose, 400 mg daily; however, it is possible that some of patients can maintain their remission with reduced doses of imatinib. In this study, we determined levels of BCR-ABL transcript in CML patients by real-time quantitative polymerase chain reaction analysis, and explored the possibility of individualization of therapeutic doses of imatinib. Thirty-five CML patients, including 17 newly diagnosed patients, 16 patients pre-treated with interferon-alpha, and 2 relapsed patients after allogeneic transplantation, were treated with imatinib. Complete cytogenetic response was achieved in 31 (89%) patients. Major molecular response (MMR) was achieved in 21 (60%). Complete molecular response (CMR) was achieved in 7 (20%). Imatinib was discontinued in 2 patients, one patient with MMR due to noncompliance and other patient sustaining CMR, but both patients relapsed 7 and 13 months later, respectively. The doses of imatinib were reduced in 7 patients due to its side effects, but 4 out of the 7 patients have sustained MMR, and 2 of them have sustained CMR for more than 23 months. These results indicate that some patients are able to maintain MMR with low-dose imatinib.

Bcr-Abl signaling through the PI-3/S6 kinase pathway inhibits nuclear translocation of the transcription factor Bach2, which represses the antiapoptotic factor heme oxygenase-1

The malignant phenotype of chronic myeloid leukemia (CML) is due to the abnormal tyrosine kinase activity of the Bcr-Abl oncoprotein. We have previously reported that expression of the Bach2 transcription factor, which induces apoptosis in response to oxidative stress, is greatly reduced in CML cells. Because these cells are resistant to apoptosis, we tested whether Bach2 could also be regulated through posttranslational mechanisms that promote inhibition of the apoptotic response to mutagenic stimuli in CML. We found that Bach2 is phosphorylated on S521 via the phosphatidylinositol-3/S6 kinase pathway, and substitution of this site to alanine leads to nuclear accumulation of the protein, indicating that this phosphorylation is important for its subcellular localization. Ectopic expression of the S521 mutant imparts greater impairment to CML cell growth than the wild-type factor. Furthermore, we showed that Bach2 transcriptionally represses heme oxygenase-1, an antiapoptotic factor up-regulated in CML. Because CML cells are known to produce high levels of intracellular reactive oxygen species, overexpression of heme oxygenase-1 resulting from inhibition of Bach2 activity may contribute to their genomic instability and leukemic phenotype.

Histone deacetylase inhibitors: discovery and development as anticancer agents

Histone deacetylase (HDAC) inhibitors are a new class of targeted anticancer agents. Several HDAC inhibitors are in clinical trials and have shown significant activity against a spectrum of both haematological and solid tumours at doses that are well tolerated by patients. HDACs and histone acetyltransferases can, by reversible acetylation, modify the structure and function of histones and proteins in transcription factor complexes, which are involved in the regulation of gene expression, as well as many non-histone proteins that are involved in regulating cell proliferation and cell death. HDAC inhibitors are a structurally diverse group of molecules; these agents selectively alter the expression of genes. HDAC inhibitors can induce cancer cell death, whereas normal cells are relatively resistant to HDAC inhibitor-induced cell death.

Blastic phase of chronic myelogenous leukemia

Chronic myelogenous leukemia (CML), also known as chronic myelocytic or chronic myeloid leukemia, is a clonal disorder of hematopoiesis that arises in a hematopoietic stem cell or early progenitor cell. This is characterized by the dysregulated production of mature nonlymphoid cells with normal differentiation. Eventually, in spite of the term chronic, there is progression to acute leukemia, usually of the myeloid variety, which is highly resistant to current therapies. Despite recent improvements in the treatment of early-stage disease, CML blast crisis (CMLBC) remains a therapeutic challenge. CMLBC is highly refractory to standard induction chemotherapy, with a response rate in myeloid blast crisis of less than 30%. Conventional chemotherapy has been much less successful in this disease compared with de novo acute leukemia, with a mean survival after diagnosis of blast crisis of only 2 to 4 months for nonresponders. Many regimens of chemotherapies have been tried in CMLBC, with minor success. Although imatinib was evaluated in patients with CMLBC, most CMLBC cases today arise in patients already on imatinib-based therapy and developing blastic phase on that therapy; thus there is no standard therapy for patients with CMLBC. Further studies of the mechanisms of transformation of chronic-phase CMLBC at a molecular level, and methods to target these molecular abnormalities, will determine the future direction of new treatment modalities. The prognosis of CML in blast crisis remains disappointing, despite great efforts. Currently, the most successful strategy for improving survival in CML is by prolonging the chronic phase and delaying the onset of blast crisis.

Targeted CML therapy: controlling drug resistance, seeking cure

Targeted cancer therapy with imatinib (Gleevec) has the capability to drive chronic myeloid leukemia (CML) into clinical remission. Some patients, particularly those with advanced disease, develop resistance to imatinib. To counteract this problem, two new BCR-ABL kinase inhibitors for imatinib-refractory disease are currently in clinical trials: the imatinib derivative AMN107 and the dual-specificity SRC/ABL inhibitor dasatinib. Using imatinib to reduce leukemic burden also facilitates the detailed investigation into how the persistence of CML disease depends on BCR-ABL signaling, particularly within the leukemic stem cell compartment. Mathematical models of drug resistance and disease relapse, in addition to experimental systems that recapitulate crucial aspects of advanced disease have deepened our understanding of CML biology. Together, these advances are contributing to a high level of disease control, and might ultimately lead to disease eradication.

Drug resistance in cancer: principles of emergence and prevention

Although targeted therapy is yielding promising results in the treatment of specific cancers, drug resistance poses a problem. We develop a mathematical framework that can be used to study the principles underlying the emergence and prevention of resistance in cancers treated with targeted small-molecule drugs. We consider a stochastic dynamical system based on measurable parameters, such as the turnover rate of tumor cells and the rate at which resistant mutants are generated. We find that resistance arises mainly before the start of treatment and, for cancers with high turnover rates, combination therapy is less likely to yield an advantage over single-drug therapy. We apply the mathematical framework to chronic myeloid leukemia. Early-stage chronic myeloid leukemia was the first case to be treated successfully with a targeted drug, imatinib (Novartis, Basel). This drug specifically inhibits the BCR-ABL oncogene, which is required for progression. Although drug resistance prevents successful treatment at later stages of the disease, our calculations suggest that, within the model assumptions, a combination of three targeted drugs with different specificities might overcome the problem of resistance.

Recent developments in cancer chemotherapy oriented towards new targets

Malignant diseases are one of the major causes of death in the western world. Patients are treated by surgery, radiation and chemotherapy. Chemotherapeutic treatment is used to decrease the tumour burden and to eliminate malignant cells. However, in most cases, resistance against chemotherapy develops. Therefore, there is a permanent need for new additional treatment strategies and chemotherapeutic combination regimens. In the present review article, the authors try to highlight the most promising approaches and summarise a selection of potential targets and compounds which might become alternative treatment options against malignant diseases. Due to the high number of scientific articles and the rapid developments in the area of cancer research, the authors can only mention a few selected targets and treatment options; however, the review focuses on new and notably important targets and compounds.

Mechanisms of BCR-ABL in the pathogenesis of chronic myelogenous leukaemia

Imatinib, a potent inhibitor of the oncogenic tyrosine kinase BCR-ABL, has shown remarkable clinical activity in patients with chronic myelogenous leukaemia (CML). However, this drug does not completely eradicate BCR-ABL-expressing cells from the body, and resistance to imatinib emerges. Although BCR-ABL remains an attractive therapeutic target, it is important to identify other components involved in CML pathogenesis to overcome this resistance. What have clinical trials of imatinib and studies using mouse models for BCR-ABL leukaemogenesis taught us about the functions of BCR-ABL beyond its kinase activity, and how these functions contribute to CML pathogenesis?

Oncogenes as Novel Targets for Cancer Therapy (Part I)

In the past 10 years, progress made in cancer biology, genetics, and biotechnology has led to a major transition in cancer drug design and development. There has been a change from an emphasis on non-specific, cytotoxic agents to specific, molecular-based therapeutics. Mechanism-based therapy is designed to act on cellular and molecular targets that are causally involved in the formation, growth, and progression of human cancers. These agents, which may have greater selectivity for cancer versus normal cells, and which may produce better anti-tumor efficacy and lower host toxicity, can be small molecules, natural or engineered peptides, proteins, antibodies, or synthetic nucleic acids (e.g. antisense oligonucleotides, ribozymes, and siRNAs). Novel targets are identified and validated by state-of-the-art approaches, including high-throughput screening, combinatorial chemistry, and gene expression arrays, which increase the speed and efficiency of drug discovery and development. Examples of oncogene-based, molecular therapeutics that show promising clinical activity include trastuzumab (Herceptin), imatinib (Gleevec), and gefitinib (Iressa). However, the full potential of oncogenes as novel targets for cancer therapy has not been realized and many challenges remain, from the validation of novel targets, to the design of specific agents, to the evaluation of these agents in both preclinical and clinical settings. In maximizing the benefits of molecular therapeutics in monotherapy or combination therapy of cancer, it is necessary to have an understanding of the underlying molecular abnormalities and mechanisms involved. This is the first part of a four-part review in which we discuss progress made in the last decade as it relates to the discovery of novel oncogenes and signal transduction pathways, in the context of their potential as targets for cancer therapy. This part delineates the latest discoveries about the potential use of growth factors and protein tyrosine kinases as targets for therapy. Later parts focus on intermediate signaling pathways, transcription factors, and proteins involved in cell cycle, DNA damage, and apoptotic pathways.

Comparison of tumor marker CA 242 with CA 19-9 and carcinoembryonic antigen (CEA) in pancreatic cancer

BACKGROUND/AIMS

Although there are a variety of tumor markers used for diagnosis of pancreatic carcinoma, the sensitivity and specificity of those markers have not yet reached an ideal level. The aim of this study was to compare the diagnostic value of CA 242 with CA 19-9 and CEA in the patients with pancreatic cancer.

METHODOLOGY

Serum CA 242, CA 19-9 and CEA levels were determined in 135 subjects in the following groups: Pancreatic cancer (n = 40), cholangiocellular carcinoma (n = 15), hepatocellular carcinoma (n = 10), cirrhosis (n = 7), chronic active hepatitis (n = 7), choledochal stone (n = 12), chronic pancreatitis (n = 9), acute pancreatitis (n = 6), and healthy controls (n = 29).

RESULTS

An elevated serum CA 242 concentration (> 20 U/mL) was found in 30 out of 40 (70%) (mean; 2163 +/- 838 U/mL) patients with pancreas cancer, in 11 out of 15 patients with cholangiocellular carcinoma (93.3%) (mean 916 +/- 529 U/mL), in none of patients with hepatocellular carcinoma and healthy controls. Slightly elevated CA 242 concentration was found in 6 out of 41 patients with benign hepatobiliary and pancreatic disease (range 0.4-97.8 U/mL) (1 acute pancreatitis, 2 chronic pancreatitis, 1 cirrhosis, 2 choledochal stone). Mean serum CA 242, CA 19-9 and CEA levels of the pancreas cancer group were significantly higher than those of the other groups except the cholangiocellular carcinoma group. There was no significant difference between the stage of pancreas cancer regarding mean serum CA 242, CA 19-9 and CEA level. There was positive correlation between serum CA 242 and CA 19-9 level. In the pancreas cancer, the sensitivity of CA 242, CA 19-9 and CEA was 75%, 80%, 40%, respectively and the specificity of those markers was 85.5%, 67.5% and 73%, respectively.

CONCLUSIONS

In conclusion, the advantage of CA 242 compared to CA 19-9 is that its specificity is higher than that of CA 19-9 in the diagnosis of pancreas cancer.

Chronic Myelogenous Leukemia and Myeloproliferative Disease

In Section I, Dr. Stephen O’Brien reviews the latest data on the clinical use of imatinib (STI571, Gleevec, Glivec) in CML. His review focuses on the use of imatinib in newly diagnosed chronic phase patients and summarizes cytogenetic and molecular response data, as well as use of the agent at high doses and in combination with other drugs. A brief summary of the prospective international Phase III studies that are currently ongoing is also provided, and the issues of resistance and definition of suboptimal therapeutic response are also covered. Finally, therapeutic decision-making and treatment strategy are considered.

In Section II, Dr. Ayalew Tefferi considers the latest developments in the biology and therapy of myeloid metaplasia/myelofibrosis. Dr. Tefferi covers what is currently understood of the biology of the disease and reviews established therapies for the condition as well as novel agents that are being used in clinical trials. The development of optimal management strategies for the disease is considered.

In Section III, Dr. Peter Valent reviews the classification of mast cell proliferative disorders and covers the clinical and pathological presentation of this group of neoplasms. He reviews the state-of-the-art regarding the molecular biology of mastocytosis along with diagnostic criteria and novel treatment concepts.