Imatinib resistance: obstacles and opportunities

The TKI Era in Chronic Leukemias

Tyrosine kinases are proteins involved in physiological cell functions including proliferation, differentiation, and survival. However, the dysregulation of tyrosine kinase pathways occurs in malignancy, including hematological leukemias such as chronic myeloid leukemia (CML) and chronic lymphocytic leukemia (CLL). Particularly, the fusion oncoprotein BCR-ABL1 in CML and the B-cell receptor (BCR) signaling pathway in CLL are critical for leukemogenesis. Therapeutic management of these two hematological conditions was fundamentally changed in recent years, making the role of conventional chemotherapy nearly obsolete. The first, second, and third generation inhibitors (imatinib, dasatinib, nilotinib, bosutinib, and ponatinib) of BCR-ABL1 and the allosteric inhibitor asciminib showed deep genetic and molecular remission rates in CML, leading to the evaluation of treatment discontinuation in prospective trials. The irreversible BTK inhibitors (ibrutinib, acalabrutinib, zanubrutinib, tirabrutinib, and spebrutinib) covalently bind to the C481 amino acid of BTK. The reversible BTK inhibitor pirtobrutinib has a different binding site, overcoming resistance associated with mutations at C481. The PI3K inhibitors (idelalisib and duvelisib) are also effective in CLL but are currently less used because of their toxicity profiles. These tyrosine kinase inhibitors are well-tolerated, do have some associated in-class side effects that are manageable, and have remarkably improved outcomes for patients with hematologic malignancies.

Ultra-deep sequencing mutation analysis of the BCR/ABL1 kinase domain in newly diagnosed chronic myeloid leukemia patients

Ultra-deep sequencing detects low-frequency genetic mutations with high sensitivity. We used this approach to prospectively examine mutations in the BCR/ABL1 tyrosine kinase from patients with newly diagnosed, chronic-phase chronic myeloid leukemia (CML) treated with the tyrosine kinase inhibitor nilotinib. Between May 2013 and November 2014, 50 patients from 18 institutions were enrolled in the study. We screened 103 somatic mutations and found that mutations in the P-loop domain were the most frequent (173/454 mutations in the P-loop) and noted the presence of the V299 L mutation (dasatinib-resistant/nilotinib-sensitive) in 98 % of patients (49/50). No patients had Y253H, E255 V, or F359 V/C/I mutations, which would recommend dasatinib rather than nilotinib treatment. The S417Y mutation was associated with lower achievement of a major molecular response (MMR) at 6 months, and the V371A mutation was associated with reduced MMR and MR^4.5 durations (MMR for 2 years: 100 % for no mutation vs. 75 % for mutation, P=0.039; MR^4.5 for 15 months: 94.1 % vs. 25 %, P=0.002). Patients with known nilotinib-resistant mutations had lower rates of MR^4.5 achievement. In conclusion, ultra-deep sequencing is a sensitive method for genetic-based treatment decisions. Based on the results of these mutational analyses, nilotinib treatment is a promising option for Korean patients with CML.

Wasabi Compound 6-(Methylsulfinyl) Hexyl Isothiocyanate Induces Cell Death with Coexisting Mitotic Arrest and Autophagy in Human Chronic Myelogenous Leukemia K562 Cells

A natural compound from Wasabia japonica, 6-(methylsulfinyl) hexyl isothiocyanate (6-MITC) was investigated for its anti-leukemia activity and mechanism of action. It was found that 6-MITC inhibited the viability of human chronic myelogenous leukemia K562 cells along with extensive mitotic arrest, spindle multipolarity, and cytoplasmic vacuole accumulation. The evidence of autophagy included the validation of autophagosomes with double-layered membranes under transmission electron microscopy, LC3I/II conversion, and the induction of G2/M phase arrest observed with acridine orange staining of treated cells, as well as the elevation of phosphorylated-histone H3 expression at the M phase. With regard to the expression of proteins related to mitosis, the down regulation of p-CHK1, p-CHK2, p-cdc25c, and p-cdc2, as well as the upregulation of cyclin B1, p-cdc20, cdc23, BubR1, Mad2, and p-plk-1 was observed. The knockdown of cdc20 was unable to block the effect of 6-MITC. The differentiation of k562 cells into monocytes, granulocytes, and megakaryocytes was not affected by 6-MITC. The 6-MITC-induced unique mode of cell death through the concurrent induction of mitosis and autophagy may have therapeutic potential. Further studies are required to elucidate the pathways associated with the counteracting occurrence of mitosis and autophagy.

Molecular screening and the clinical impacts of BCR-ABL KD mutations in patients with imatinib-resistant chronic myeloid leukemia

The present study aimed to detect the frequency of kinase domain (KD) mutations in order to evaluate their clinical significance and functional importance in 45 patients with chronic myeloid leukemia (CML) who were resistant to imatinib therapy. Sanger sequencing was used (45 patients), along with allele-specific oligonucleotide polymerase chain reaction (ASO-PCR; 3 patients), for the screening of mutations. BCR/ABL KD was amplified by nested PCR and sequencing was performed. Secondly, ASO-PCR was performed to confirm the results of the sequence analysis for E255K mutations. Mutations were detected in 11/45 patients (24.44%) via Sanger sequencing. D241G (4.4%), C369C (4.4%), K285N (2.2%), A380T (2.2%) and A366V (2.2%) mutations were detected. E255K (8.8%) was detected by ASO-PCR and Sanger sequencing. Mutations are a primary reason for suboptimal responses, loss of response and resistance to imatinib. In particular, the E255K mutation, which is characterized by resistance to imatinib and nilotinib, was detected in four patients. Analyzing the mutations and monitoring patients with CML may improve their prognosis and survival rate. ASO-PCR assays will be beneficial for the routine monitoring of mutations.

Identification of galectin-1 as a novel mediator for chemoresistance in chronic myeloid leukemia cells

Multidrug resistance protein-1 (MDR1) has been proven to be associated with the development of chemoresistance to imatinib (Glivec, STI571) which displays high efficacy in treatment of BCR-ABL-positive chronic myelogenous leukemia (CML). However, the possible mechanisms of MDR1 modulation in the process of the resistance development remain to be defined. Herein, galectin-1 was identified as a candidate modulator of MDR1 by proteomic analysis of a model system of leukemia cell lines with a gradual increase of MDR1 expression and drug resistance. Coincidently, alteration of galectin-1 expression triggers the change of MDR1 expression as well as the resistance to the cytotoxic drugs, suggesting that augment of MDR1 expression engages in galectin-1-mediated chemoresistance. Moreover, we provided the first data showing that NF-κB translocation induced by P38 MAPK activation was responsible for the modulation effect of galectin-1 on MDR1 in the chronic myelogenous leukemia cells. Galectin-1 might be considered as a novel target for combined modality therapy for enhancing the efficacy of CML treatment with imatinib.

MicroRNA-212/ABCG2-axis contributes to development of imatinib-resistance in leukemic cells

BCR-ABL-independent resistance against tyrosine kinase inhibitor is an emerging problem in therapy of chronic myeloid leukemia. Such drug resistance can be linked to dysregulation of ATP-binding cassette (ABC)-transporters leading to increased tyrosine kinase inhibitor efflux, potentially caused by changes in microRNA expression or DNA-methylation. In an in vitro-imatinib-resistance model using K-562 cells, microRNA-212 was found to be dysregulated and inversely correlated to ABC-transporter ABCG2 expression, targeting its 3'-UTR. However, the functional impact on drug sensitivity remained unknown. Therefore, we performed transfection experiments using microRNA-mimics and -inhibitors and investigated their effect on imatinib-susceptibility in sensitive and resistant leukemic cell lines. Under imatinib-treatment, miR-212 inhibition led to enhanced cell viability (p = 0.01), reduced apoptosis (p = 0.01) and cytotoxicity (p = 0.03). These effects were limited to treatment-naïve cells and were not observed in cells, which were resistant to various imatinib-concentrations (0.1 μM to 2 μM). Further analysis in treatment-naïve cells revealed that miR-212 inhibition resulted in ABCG2 upregulation and increased ABCG2-dependent efflux. Furthermore, we observed miR-212 promoter hypermethylation in 0.5 and 2 μM IM-resistant sublines, whereas ABCG2 methylation status was not altered. Taken together, the miR-212/ABCG2-axis influences imatinib-susceptibility contributing to development of imatinib-resistance. Our data reveal new insights into mechanisms initiating imatinib-resistance in leukemic cells.

Imatinib and Nilotinib increase glioblastoma cell invasion via Abl-independent stimulation of p130Cas and FAK signalling

Imatinib was the first targeted tyrosine kinase inhibitor to be approved for clinical use, and remains first-line therapy for Philadelphia chromosome (Ph+)-positive chronic myelogenous leukaemia. We show that treatment of human glioblastoma multiforme (GBM) tumour cells with imatinib and the closely-related drug, nilotinib, strikingly increases tyrosine phosphorylation of p130Cas, focal adhesion kinase (FAK) and the downstream adaptor protein paxillin (PXN), resulting in enhanced cell migration and invasion. Imatinib and nilotinib-induced tyrosine phosphorylation was dependent on expression of p130Cas and FAK activity and was independent of known imatinib targets including Abl, platelet derived growth factor receptor beta (PDGFRβ) and the collagen receptor DDR1. Imatinib and nilotinib treatment increased two dimensional cell migration and three dimensional radial spheroid invasion in collagen. In addition, silencing of p130Cas and inhibition of FAK activity both strongly reduced imatinib and nilotinib stimulated invasion. Importantly, imatinib and nilotinib increased tyrosine phosphorylation of p130Cas, FAK, PXN and radial spheroid invasion in stem cell lines isolated from human glioma biopsies. These findings identify a novel mechanism of action in GBM cells for two well established front line therapies for cancer resulting in enhanced tumour cell motility.

Treatment patterns and prognostic indicators of response to therapy among patients with chronic myeloid leukemia in Australia, Canada, and South Korea

OBJECTIVE

Given the multiple options for treatment of chronic-phase chronic myeloid leukemia (CML) with tyrosine kinase inhibitors, our objective was to understand treatment patterns in routine practice and prognostic indicators of response.

RESEARCH DESIGN AND METHODS

We conducted a retrospective medical record review of 681 patients with CML in Australia, Canada, and South Korea. Eligible patients had a diagnosis of chronic-phase CML, were Philadelphia chromosome and/or BCR-ABL positive, were aged 18 years or older, and had been treated with first-line imatinib therapy between January 2005 and September 2010. Data on patient demographics, medical history (e.g., comorbidities, Sokal score), and treatment characteristics (e.g., time to initiation, therapy duration) were abstracted. Descriptive analyses were stratified by country and therapy line. Prognostic indicators of response to imatinib were evaluated using multivariable logistic regression, adjusting for country, patient demographics, medical history, treatment characteristics, and side effects.

MAIN OUTCOME MEASURES

Hematologic, cytogenetic, and molecular responses at 3, 6, 12, and 18 months following initiation of each therapy line.

RESULTS

Patients' average age was 57 years, and 59% were male. Overall, imatinib was initiated approximately 4 months following CML diagnosis. Complete or major molecular response (C/MMR) at 6 months following imatinib initiation was 54% in Australia, 22% in Canada, and 38% in South Korea. At 18 months, over 60% of patients achieved C/MMR. Approximately 30% of patients discontinued imatinib primarily due to intolerance and lack of response. Among patients who received second-line treatment, dasatinib was used more frequently than nilotinib. Multivariable regression results indicated Sokal score was identified as a prognostic indicator of response to imatinib therapy at several time points.

LIMITATIONS

There are several limitations to this study. First, we selected a convenience sample of patients and physicians and therefore results may not be representative of the true population of patients with chronic-phase CML. Second, data were entered by the selected physician and could be subject to data entry errors or inaccuracies. Third, limited information was collected from the patient records, and it is possible that we did not capture additional prognostic or confounding factors related to the measured outcomes. Next, because this was an analysis of previously documented data (i.e., retrospective), we were unable to provide a priori definitions of response. Finally, multivariable analyses were limited to imatinib-related outcomes.

CONCLUSIONS

Treatment patterns and prognostic indicators differed by country. Health care providers, payers, and patients can utilize these results to inform treatment and policies aimed at improving the effectiveness of care for patients with chronic-phase CML.

Methylation analysis of the DAPK1 gene in imatinib-resistant chronic myeloid leukemia patients

Death-associated protein kinase-1 (DAPK1) is a pro-apoptotic gene that induces cellular apoptosis in response to internal and external apoptotic stimulants. The silencing of DAPK1 can result in uncontrolled cell proliferation, indicating that it may have a role in tumor suppression. DAPK1 activity can be inhibited by the cytosine methylation that occurs in its promoter region. These methylation changes in the promoter region of DAPK1 have been reported in a range of solid and hematological malignancies. In the present study, DAPK1 methylation was investigated in chronic myeloid leukemia patients (n=43) using bisulfite conversion followed by methylation-specific polymerase chain reaction. The present study included a number of patients who were identified to be resistant to the common chemotherapeutic agent imatinib (STI571, Gleevec^®, Glivec^®), exhibiting at least one mutation in the breakpoint cluster region-Abelson murine leukemia (BCR-ABL) gene. Thus, the patients in the present study were divided into two groups according to their response to imatinib therapy: Non-resistant (n=26) and resistant (n=17) to imatinib. Resistant patients were characterized by the presence of single or multiple mutations of the BCR-ABL gene: i) T315I, ii) M351T, iii) E255K, iv) T315I and M351T or v) T315I, M351T and E255K. The present study identified that: i) The incidence of DAPK1 methylation was significantly higher in the resistant patients compared with the non-resistant patients; ii) the extent of resistance varied between mutation types; and iii) there was no DAPK1 methylation in any of the healthy controls. These findings indicate that DAPK1 methylation may be associated with a signaling pathway for imatinib resistance in chronic myeloid leukemia.

Evaluation of novel aryloxyalkyl derivatives of imidazole and 1,2,4-triazole as heme oxygenase-1 (HO-1) inhibitors and their antitumor properties

A novel series of aryloxyalkyl derivatives of imidazole and 1,2,4-triazole, 17-31, was designed and synthesized as inhibitors of heme oxygenase-1 (HO-1) and heme oxygenase-2 (HO-2). Some of these compounds were found to be good inhibitors of HO-1, in particular those carrying an imidazole moiety as azolyl group and a 3-bromo or 4-iodophenyl as aryl moiety. The most potent compounds 6 and 30 were selected and studied for their antitumor properties in a model of LAMA-84 R cell line overexpressing HO-1 and resistant to imatinib mesylate (IM), a tyrosine-kinase inhibitor used in the treatment of multiple types of cancer, most notably Philadelphia Chromosome positive (Ph(+)) Chronic Myelogenous Leukemia (CML). Results show that both 6 and 30 sensitized LAMA-84 R cell line to antitumor properties of IM.

Differential effects of dosing regimen on the safety and efficacy of dasatinib: retrospective exposure-response analysis of a Phase III study

PURPOSE

Dasatinib is a prototypic short half-life BCR-ABL1 tyrosine kinase inhibitor. The recommended dose of dasatinib for chronic myeloid leukemia in chronic phase was changed from 70 mg twice daily to 100 mg once daily following a Phase III dose-optimization study. To better understand the superior benefit-risk profile of dasatinib 100 mg once daily, exposure-response was characterized for efficacy (major cytogenetic response) and safety (pleural effusion).

PATIENTS AND METHODS

Dasatinib exposure in patients with chronic myeloid leukemia in chronic phase was determined by population pharmacokinetic analysis of data from seven dasatinib clinical studies (N = 981), including the Phase III dose-optimization study (n = 567). Data from the Phase III study were then used to characterize exposure-response relationships for the four dasatinib treatment regimens investigated (100 mg once daily, 50 mg twice daily, 140 mg once daily, and 70 mg twice daily).

RESULTS

Major cytogenetic response was significantly (P < 0.01) associated with weighted average steady-state dasatinib plasma concentrations, and pleural effusion was significantly associated with trough concentration. Major cytogenetic response was also significantly associated with maintenance of uninterrupted dosing. The 100 mg once daily arm had the lowest steady-state trough concentration of the four dose arms investigated in the Phase III study, and although this arm also had the lowest weighted average steady-state dasatinib plasma concentration, it had the highest dose maintenance.

CONCLUSION

Dasatinib dose optimization to 100 mg once daily from 70 mg twice daily significantly minimizes adverse events while maintaining efficacy by exploiting differences in the measures of exposure associated with efficacy and safety.

Contribution of BCR-ABL kinase domain mutations to imatinib mesylate resistance in Philadelphia chromosome positive Malaysian chronic myeloid leukemia patients

Development of resistance to imatinib mesylate (IM) in chronic myeloid leukemia (CML) patients is mediated by different mechanisms that can be classified as BCR-ABL dependent or BCR-ABL independent pathways. BCR-ABL dependent mechanisms are most frequently associated with point mutations in tyrosine kinase domain (TKD) of BCR-ABL1 and also with BCR-ABL gene amplification. Many different types and frequencies of mutations have been reported in different studies, probably due to the different composition of study cohorts. Since no reports are available from Malaysia, this study was undertaken to investigate the frequency and pattern of BCR-ABL kinase domain mutations using dHPLC followed by sequencing, and also status of BCR-ABL gene amplification using fluorescence in situ hybridization (FISH) on 40 IM resistant Malaysian CML patients. Mutations were detected in 13 patients (32.5%). Five different types of mutations (T315I, E255K, Y253H, M351T, V289F) were identified in these patients. In the remaining 27 IM resistant CML patients, we investigated the contribution made by BCR-ABL gene amplification, but none of these patients showed amplification. It is presumed that the mechanisms of resistance in these 27 patients might be due to BCR-ABL independent pathways. Different mutations confer different levels of resistance and, therefore, detection and characterization of TKD mutations is highly important in order to guide therapy in CML patients.

The in-vitro antiproliferative effect of PRI-2191 and imatinib applied in combined treatment with cisplatin, idarubicin, or docetaxel on human leukemia cells

Imatinib mesylate (Gleevec, STI571) is a specific inhibitor of the Bcr/Abl fusion tyrosine kinase that exhibits potent antileukemic effects in chronic myelogenous leukemia. Bcr/Abl-positive K562 and Bcr/Abl-negative HL-60 human leukemia cells were used to investigate the effect of PRI-2191, a calcitriol analog, on the biological effects of imatinib combined with other anticancer drugs. The results show that PRI-2191 enhances the antiproliferative effect of imatinib on HL-60 cells. When these two agents together are applied with either docetaxel or cisplatin, but not with idarubicin, the antiproliferative effect could still be enhanced. Moreover, when the interaction between the chemotherapy agents was antagonistic or additive, PRI-2191 could even shift it to synergism. This effect correlated with an accumulation of HL-60 cells in the G0/G1 phase of the cell cycle and a decrease in the percentage of cells in the G2/M and S stage in the ternary combinations used. PRI-2191 did not influence apoptosis induced by imatinib alone or in ternary combinations with all the chemotherapy agents used. These results may suggest that the stronger antiproliferative effect of the combined treatment with PRI-2191 on HL-60 cells is related to cell cycle arrest rather than to the induction of apoptosis.

Four-channel asymmetric Real-Time PCR hybridization probe assay: a rapid pre-screening method for critical BCR-ABL kinase domain mutations

OBJECTIVES

Within the laboratory protocols, used for the study of BCR-ABL resistance mutations in chronic myeloid leukemia patients treated with Imatinib, direct sequencing remains the reference method. Since the incidence of patients with a mutation-related loss of response is not very high, it is very useful in the routine laboratory to perform a fast pre-screening method.

DESIGN AND METHODS

With this in mind, we have designed a new technique, based on a single Real-Time FRET-based PCR, followed by a study of melting peaks. This new tool, developed in a LightCycler 2.0, combines four different fluorescence channels for the simultaneous detection, in a single close tube, of critical mutations within the ABL kinase domain.

RESULTS

Assay evaluation performed on 33 samples, previously genotyped by sequentiation, resulted in full concordance of results.

CONCLUSIONS

This new methodology detects in a few steps the presence of critical mutations associated to Imatinib resistance.

BCR-ABL tyrosine kinase inhibitors in the treatment of Philadelphia chromosome positive chronic myeloid leukemia: a review

Chronic Myeloid Leukemia (CML) is a clonal disease characterized by the presence of the Philadelphia (Ph+) chromosome and its oncogenic product, BCR-ABL, a constitutively active tyrosine kinase, that is present in >90% of the patients. Epidemiologic data indicates that almost 5000 new cases are reported every year and 10% of these patients eventually succumb to the disease. The treatment of CML was revolutionized by the introduction of imatinib mesylate (IM, Gleevec), a BCR-ABL tyrosine kinase inhibitor (TKI). The clinical use of specific BCR-ABL inhibitors has resulted in a significantly improved prognosis, response rate, overall survival, and patient outcome in CML patients compared to previous therapeutic regimens. However, the complete eradication of CML in patients receiving imatinib was limited by the emergence of resistance mostly due to mutations in the ABL kinase domain and to a lesser extent by molecular residual disease after treatment. The second-generation BCR-ABL TKIs nilotinib (Tasigna) and dasatinib (Sprycel), showed significant activity in clinical trials in patients intolerant or resistant to imatinib therapy, except in those patients with the T315I BCR-ABL mutation. Identifying key components involved in the CML pathogenesis may lead to the exploration of new approaches that might eventually overcome resistance mediated to the BCR-ABL TKIs. Here, we present an overview about the current treatment of Ph+ CML patients with the TKIs and the obstacles to successful treatment with these drugs.

Activity of the potent dual Abl/Src tyrosine kinase inhibitor FB2 against Bcr-Abl positive cell lines in vitro and in vivo

We have previously shown the inhibition of the small-molecule inhibitor FB2 on imatinib-sensitive and resistance CML cell lines with the wild-type Bcr-Abl fusion gene. Here we report the potent and selective antiproliferation on FB2 on transfected Ba/F3 p210 cell lines expressing various isoforms of Bcr-Abl (wild-type, Y253F, T315I). FB2 which orients Bcr-Abl and Src kinase activities, is shown to override imatinib-resistance CML involving Y253F mutation in the Abl kinase domain of the fusion protein except T315I in vivo and in vitro. Thus, we present FB2 that displays potency toward Bcr-Abl and Src as the molecular target, and which could potentially be used to override drug resistance in CML.

Molecular mechanisms of acquired resistance to tyrosine kinase targeted therapy

In recent years, tyrosine kinases (TKs) have been recognized as central players and regulators of cancer cell proliferation, apoptosis, and angiogenesis, and are therefore considered suitable potential targets for anti-cancer therapies. Several strategies for targeting TKs have been developed, the most successful being monoclonal antibodies and small molecule tyrosine kinase inhibitors. However, increasing evidence of acquired resistance to these drugs has been documented, and extensive preclinical studies are ongoing to try to understand the molecular mechanisms by which cancer cells are able to bypass their inhibitory activity.This review intends to present the most recently identified molecular mechanisms that mediate acquired resistance to tyrosine kinase inhibitors, identified through the use of in vitro models or the analysis of patient samples. The knowledge obtained from these studies will help to design better therapies that prevent and overcome resistance to treatment in cancer patients.

Standard management of patients with chronic myeloid leukemia

The successful introduction of the tyrosine kinase inhibitors (TKIs) has revolutionized the treatment of patients with chronic myeloid leukemia (CML). Imatinib therapy induces high rates of complete cytogenetic and major molecular responses, and improves survival in CML. Following imatinib treatment, more than 90% of patients obtain complete hematologic response, and over 80% achieve a complete cytogenetic response. With 7 years of follow-up, the results are still very favorable, resulting in a major change in the natural history of the disease. Resistance to imatinib represents a clinical challenge. Although some clinical and biologic features have been found to be associated with a lower probability of response to imatinib, at present no precise markers allowing for the prediction of outcome for individual patients exist. The most common mechanisms of resistance to imatinib include BCR-ABL kinase domain mutations, amplification, and overexpression of the BCR-ABL oncogene, and clonal evolution with activation of additional transformation pathways. These mechanisms are eventually caused by the genomic instability, which characterizes the Philadelphia chromosome-positive clone. Several approaches to overcome resistance have been proposed. The understanding of at least some of the mechanisms of resistance to imatinib has led to a rapid development of new therapeutic agents that might overcome this resistance. Novel targeted agents designed to overcome imatinib resistance include second-generation TKIs such as dasatinib, nilotinib, bosutinib, bafetinib, and others. Other approaches are exploring combination therapy, with agents affecting different oncogenic pathways, and immune modulation. Herein, we review some of these targeted therapies, particularly those for which clinical data are already available.

Chronic myeloid leukemia therapy: focus on second-generation tyrosine kinase inhibitors

BACKGROUND

Chronic myeloid leukemia, the most common adult leukemia, is characterized by the Ph+ chromosome produced by the fusion of the BCR gene from chromosome 22 and the ABL gene from chromosome 9. Inhibition of the deleterious effects of this potent oncogene by the tyrosine kinase inhibitor (TKI) imatinib has revolutionized care of this disease, but intolerance and resistance does occur.

METHODS

The authors have reviewed both the preclinical and the clinical data concerning second-generation TKIs intended to circumvent or ameliorate issues with imatinib intolerance or resistance.

RESULTS

Two second-generation TKIs, dasatinib and nilotinib, are currently approved by the US Food and Drug Administration. Both have shown significant clinical activity in patients with chronic myeloid leukemia (CML) and Ph+ acute lymphoblastic leukemia (ALL) who are resistant or intolerant to imatinib or other therapies.

CONCLUSIONS

The TKIs are a superb example of an effective targeted approach for a malignant disease. As more clinical data become available and additional novel agents are developed, specific therapy and dosing strategies for individuals with CML will depend on the status of their disease, the anticipated side effects, and concurrent drug therapy.

First-line therapy for chronic myeloid leukemia: Past, present, and future

The development of Bcr-Abl tyrosine kinase inhibitors has dramatically changed the prognosis of patients with newly diagnosed chronic myeloid leukemia (CML). Standard-dose imatinib (400 mg/day in chronic phase, 600 mg/day in advanced CML) now dominates the management of this disease, producing considerably higher hematologic, cytogenetic, and molecular response rates than seen with previous drug therapies. However, although many patients respond well to standard-dose imatinib initially, some patients do not achieve adequate levels of response or discontinue therapy because of resistance. One approach to improving treatment response with first-line imatinib may be to increase the imatinib dose (800 mg/day), although recent trial data indicate that overall increases in response rates may be modest. Newer Bcr-Abl tyrosine kinase inhibitors can induce responses in patients with all phases of imatinib-resistant CML, even those with imatinib-resistant mutations in the BCR-ABL gene. Furthermore, in initial studies, first-line dasatinib or nilotinib treatment has produced response rates that compare favorably with historical controls treated with imatinib, although confirmation is required from head-to-head clinical trials. Future clinical approaches may include drug combinations, which may allow quiescent leukemia stem cells to be eradicated. Further improvements in drug treatment for first-line CML are expected during the next few years.

Development and targeted use of nilotinib in chronic myeloid leukemia

The development of imatinib has resulted in sustained hematologic and cytogenetic remissions in all phases of chronic myeloid leukemia (CML). Despite the high efficacy, relapses have been observed and are much more prevalent in patients with advanced disease. The most common mechanism of acquired resistance has been traced to Bcr-Abl kinase domain mutations. Several strategies have been developed to overcome the problem of imatinib resistance, including imatinib dose escalation, novel targeted agents and combination treatments. A second generation of tyrosine kinase inhibitors was developed, which displays increased potency towards Bcr-Abl and is able to target the majority of CML mutant clones. Nilotinib (Tasigna^®, AMN107, Novartis) is a close analog of imatinib with approximately 20-fold higher potency for BCR-ABL kinase inhibition. Preclinical and clinical investigations demonstrate that nilotinib effectively overcomes imatinib resistance, and has induced high rates of hematologic and cytogenetic responses in CML post imatinib failure, with a good tolerance. Nilotinib has been approved for CML patients in chronic and accelerated phases, post imatinib failure.

Acquired resistance to small molecule ErbB2 tyrosine kinase inhibitors

Breast cancers overexpressing the ErbB2 (HER2) receptor tyrosine kinase oncogene are treated with targeted therapies such as trastuzumab (Herceptin), an anti-ErbB2 antibody, and lapatinib (GW572016/Tykerb), a selective small molecule inhibitor of ErbB2 and epidermal growth factor receptor tyrosine kinases that was recently approved for ErbB2+ breast cancers that progressed on trastuzumab-based therapy. The efficacy of lapatinib as a monotherapy or in combination with chemotherapy, however, is limited by the development of therapeutic resistance that typically occurs within 12 months of starting therapy. In contrast to small molecule inhibitors targeting other receptor tyrosine kinases where resistance has been attributed to mutations within the targeted receptor, ErbB2 mutations have not been commonly found in breast tumors. Instead, acquired resistance to lapatinib seems to be mediated by redundant survival pathways that are activated as a consequence of marked inhibition of ErbB2 kinase activity. For example, inhibition of phosphatidylinositol3 kinase-Akt in lapatinib-treated cells leads to derepression of FOXO3A, a transcription factor that up-regulates estrogen receptor (ER) signaling, resulting in a switch in the regulation of survival factors (e.g., survivin) and cell survival from ErbB2 alone to ER and ErbB2 in resistant cells. In this review, we discuss the effects of lapatinib on signaling networks in ErbB2+ breast cancer cells to elucidate potential mechanisms of therapeutic resistance and strategies to overcome or prevent its development.

Human AQP5 plays a role in the progression of chronic myelogenous leukemia (CML)

Aquaporins (AQPs) have previously been associated with increased expression in solid tumors. However, its expression in hematologic malignancies including CML has not been described yet. Here, we report the expression of AQP5 in CML cells by RT-PCR and immunohistochemistry. While normal bone marrow biopsy samples (n = 5) showed no expression of AQP5, 32% of CML patient samples (n = 41) demonstrated AQP5 expression. In addition, AQP5 expression level increased with the emergence of imatinib mesylate resistance in paired samples (p = 0.047). We have found that the overexpression of AQP5 in K562 cells resulted in increased cell proliferation. In addition, small interfering RNA (siRNA) targeting AQP5 reduced the cell proliferation rate in both K562 and LAMA84 CML cells. Moreover, by immunoblotting and flow cytometry, we show that phosphorylation of BCR-ABL1 is increased in AQP5-overexpressing CML cells and decreased in AQP5 siRNA-treated CML cells. Interestingly, caspase9 activity increased in AQP5 siRNA-treated cells. Finally, FISH showed no evidence of AQP5 gene amplification in CML from bone marrow. In summary, we report for the first time that AQP5 is overexpressed in CML cells and plays a role in promoting cell proliferation and inhibiting apoptosis. Furthermore, our findings may provide the basis for a novel CML therapy targeting AQP5.

Overcoming kinase resistance in chronic myeloid leukemia

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

Strategies for overcoming imatinib resistance in chronic myeloid leukemia

Imatinib was the first treatment for chronic myeloid leukemia (CML) that specifically targeted the causative BCR-ABL oncoprotein, and represented a major therapeutic advance in this disease; however, some patients develop resistance or intolerance. Resistance can be classified as BCR-ABL-dependent (e.g., mutation in the BCR-ABL gene) or BCR-ABL-independent (alternative pathways of disease progression, e.g., SRC-family tyrosine kinases). The investigation of therapeutic options post-imatinib failure resulted in the development and regulatory approval of dasatinib, a BCR-ABL and SRC-family kinase inhibitor. Dasatinib is active across all phases of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia, and demonstrates activity in almost all imatinib-resistant mutations. Other therapeutic options are also under investigation, with nilotinib being the most clinically advanced. Nilotinib is an analog of imatinib with similar multiple kinase targets, but without inhibition of SRC, and reduced in vitro activity against BCR-ABL P-loop mutations compared with dasatinib. Similar to dasatinib, nilotinib has no activity against T315I mutations. The availability of dasatinib and development of other tyrosine kinase inhibitors provide positive prospects for patients with imatinib-resistant or -intolerant CML. Here, we discuss several of these new strategies for treating patients after imatinib failure.

Targeted therapy in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is characterized by the formation of the Philadelphia chromosome and oncogenic signaling by the resulting Bcr-Abl fusion protein. Understanding the molecular basis of CML has led to the development of highly effective targeted therapies that block Bcr-Abl tyrosine kinase activity. Imatinib, the current first-line therapy for CML, induces durable treatment responses in most patients. However, patients may develop imatinib resistance, which is often due to BCR-ABL mutations. With the availability of second generation tyrosine kinase inhibitors, an effective therapeutic option other than stem cell transplantation is available following imatinib failure. Randomized trial data suggest that dasatinib treatment is superior to imatinib dose escalation in patients with imatinib resistance. Nilotinib, a recently approved analogue of imatinib, has also demonstrated encouraging treatment responses in patients with imatinib-resistant CML. Other agents (including bosutinib and INNO-406) are in clinical development. With the potential availability of multiple treatment options for patients with CML, it may be possible to tailor treatment according to individual patient or disease characteristics, for example, BCR-ABL mutations. Future CML treatment may involve combination strategies. Overall, targeted agents have significantly improved the prognosis of patients diagnosed with CML.

Plasma pharmacokinetics and CYP3A12-dependent metabolism of c-kit inhibitor imatinib in dogs

Imatinib is a highly selective tyrosine kinase inhibitor, and is used for the treatment of chronic myeloid leukaemia (CML) and gastrointestinal stromal tumours (GISTs) in humans. The aim of this study is to determine the in vitro and in vivo pharmacokinetics of imatinib in dogs and which cytochrome P450 (CYPs) contribute to its metabolism. Imatinib was administered orally or intravenously to dogs and the time of the peak concentration (T(max)) of imatinib was 4-9 h. The mean half-life was 622 +/- 368 min, and the AUC was 1256 +/- 809 microM * min after oral administration. The range of C0 of intravenously injected dogs was 12-24 microM. The half-life and AUC after intravenous injection were 206 +/- 112 min and 1026 +/- 371 microM * min, respectively. Recombinant system of dog CYP3A12 and CYP2C21 showed that CYP3A12 contributed to the metabolism of imatinib. The inhibition of CYP3A-dependent activity using a rat anti-CYP3A antibody or ketoconazole revealed that CYP3A12 plays a major role in the metabolism of imatinib in dog liver microsomes.

Current and emerging treatment options in chronic myeloid leukemia

Treatments for chronic myeloid leukemia (CML) represent a success story in molecular medicine. The development of imatinib, a tyrosine kinase inhibitor (TKI) targeted against the causative Bcr-Abl oncoprotein in CML, has resulted in hematologic and cytogenetic remissions in all phases of CML. A significant proportion of patients are resistant to imatinib or develop resistance during treatment. This is often a result of mutated forms of the Bcr-Abl oncoprotein to which imatinib is unable to bind. Several strategies have been developed to overcome the problem of imatinib resistance, including high-dose imatinib, novel targeted agents, and combination treatments. Novel agents include dasatinib, a potent TKI that inhibits several critical oncogenic proteins and which has recently been approved for patients with CML who are resistant or intolerant to imatinib; and nilotinib, a potent selective Bcr-Abl kinase inhibitor currently in clinical development. Other agents in development include SKI-606 and INNO-406. Stem cell transplantation remains a useful option, although it is not generally used as first-line treatment. Overall, there are an increasing number of treatment options available for patients with CML.

BCR-ABL Activity Is Critical for the Immunogenicity of Chronic Myelogenous Leukemia Cells

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder caused by excessive granulopoiesis due to the formation of the constitutively active tyrosine kinase BCR-ABL. An effective drug against CML is imatinib mesylate, a tyrosine kinase inhibitor acting on Abl kinases, c-KIT, and platelet-derived growth factor receptor. Recently, a study revealed that patients treated with imatinib showed impaired CTL responses compared with patients treated with IFN-alpha, which might be due to a treatment-induced reduction in immunogenicity of CML cells or immunosuppressive effects. In our study, we found that inhibition of BCR-ABL leads to a down-regulation of immunogenic antigens on the CML cells in response to imatinib treatment, which results in the inhibition of CML-directed immune responses. By treating CML cells with imatinib, we could show that the resulting inhibition of BCR-ABL leads to a decreased expression of tumor antigens, including survivin, adipophilin, hTERT, WT-1, Bcl-x(L), and Bcl-2 in correlation to a decreased development of CML-specific CTLs. In contrast, this reduction in immunogenicity was not observed when a CML cell line resistant to the inhibitory effects of imatinib was used, but could be confirmed by transfection with specific small interfering RNA against BCR-ABL or imatinib treatment of primary CML cells.

Dasatinib for the treatment of Philadelphia chromosome-positive leukaemias

BCR-ABL, a constitutively active tyrosine kinase, causes chronic myeloid leukaemia (CML). Rational development of drugs targeting BCR-ABL has significantly improved the treatment of CML. Imatinib (a BCR-ABL tyrosine kinase inhibitor) produces haematological and cytogenetic remissions across all phases of CML and is the present standard of care. Imatinib resistance occurs in a significant proportion of patients and mechanisms of resistance include BCR-ABL mutations and activation of alternate oncogenic pathways. Dasatinib is a novel, potent, multi-targeted oral kinase inhibitor. Preclinical and clinical investigations demonstrate that dasatinib effectively overcomes imatinib resistance and has further improved the treatment of CML. Dasatinib was recently approved by the FDA for use in Philadelphia-positive leukaemias in patients who are resistant or intolerant to imatinib.

Trichosanthin down-regulated p210Bcr-Abl and enhanced imatinib-induced growth arrest in chronic myelogenous leukemia cell line K562

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.

Isodicentric Philadelphia chromosomes in imatinib mesylate (Gleevec)-resistant patients

Amplification/duplication of the BCR-ABL gene has been found to be one of the key factors leading to drug resistance to imatinib mesylate (IM). In the present study, we used G-banding to identify the presence of de novo identical isodicentric chromosomes in IM-resistant patients. Fluorescence in situ hybridization (FISH) analysis on interphase nuclei confirmed the heterogeneity and amplification of the fused BCR-ABL gene. FISH analysis superimposed on G-banding confirmed the presence of isodicentric Philadelphia chromosomes. The impact of the isodicentric Philadelphia chromosomes on genomic instability, heterogeneity, and amplification of the Philadelphia chromosomes in IM-resistant patients is discussed.

Dasatinib or high-dose imatinib for chronic-phase chronic myeloid leukemia after failure of first-line imatinib: a randomized phase 2 trial

Therapeutic options for chronic myelogenous leukemia (CML) resistant to 400 to 600 mg imatinib are limited. Escalating imatinib doses may overcome resistance. Dasatinib, a significantly more potent inhibitor of BCR-ABL, is safe and effective in this population. Patients with imatinib-resistant chronic-phase (CP) CML were randomized 2:1 to 140 mg dasatinib (n=101) or 800 mg imatinib (n=49). With a median follow up of 15 months, complete hematologic responses were observed in 93% and 82% of patients receiving dasatinib and high-dose imatinib (P=.034), respectively. Dasatinib resulted in higher major cytogenetic response rates (52%) than high-dose imatinib (33%) (P=.023); this included complete cytogenetic response in 40% and 16% (P=.004). Major molecular responses were also more frequent with dasatinib (16% versus 4%; P=0.038). Treatment failure (hazard ratio [HR], 0.16; P<.001) and progression-free survival (HR, 0.14; P<.001) both favored dasatinib. Superficial edema (42% versus 15%) and fluid retention (45% versus 30%) were more prevalent with imatinib; pleural effusion was more common with dasatinib (17% versus 0%). Grade 3 to 4 nonhematologic toxicity was minimal. Cytopenias were more frequent and severe with dasatinib. Dasatinib represents a safe and effective therapy for CP-CML resistant to conventional imatinib doses with improved cytogenetic and molecular response rates and progression-free survival relative to high-dose imatinib.