Knight Scholars Program: A Tiered Three-Year Mentored Training Program for Urban and Rural High School High School Students Increases Interest and Self-Efficacy in Interprofessional Cancer Research

Cancer research training programs build our future biomedical workforce. Training is often centered for students residing close to research institutions, making access more challenging for rural students. A cancer research training program was developed for high school students residing in five geographical regions across Oregon. Training was tiered in duration and intensity across the three years, including a one-week Introduction program and subsequent 10-week summer research training programs (Immersion and Intensive). A total of 60 students participated in in-person and/or virtual training, with Immersion students receiving mentored shadowing experiences in clinical care, public health, and outreach in their home communities. Laboratory rotations at a research-intensive institution enabled students to sample research environments before selecting an area of interest for Intensive training the following summer. Aligning with Self-Determination Theory, the Knight Scholars Program aims to build competence, relatedness, and autonomy of its trainees in biomedical sciences. The program exposed students to a wide range of interprofessional careers and collaborative teams, enabling scholars to envision themselves in various paths. Results show strong gains in interest and research self-efficacy for both Introduction and Immersion scholars, with findings highlighting the importance of representation within mentoring and training efforts.

Age-related mutations and chronic myelomonocytic leukemia

Chronic myelomonocytic leukemia (CMML) is a hematologic malignancy nearly confined to the elderly. Previous studies to determine incidence and prognostic significance of somatic mutations in CMML have relied on candidate gene sequencing, although an unbiased mutational search has not been conducted. As many of the genes commonly mutated in CMML were recently associated with age-related clonal hematopoiesis (ARCH) and aged hematopoiesis is characterized by a myelomonocytic differentiation bias, we hypothesized that CMML and aged hematopoiesis may be closely related. We initially established the somatic mutation landscape of CMML by whole exome sequencing followed by gene-targeted validation. Genes mutated in ⩾10% of patients were SRSF2, TET2, ASXL1, RUNX1, SETBP1, KRAS, EZH2, CBL and NRAS, as well as the novel CMML genes FAT4, ARIH1, DNAH2 and CSMD1. Most CMML patients (71%) had mutations in ⩾2 ARCH genes and 52% had ⩾7 mutations overall. Higher mutation burden was associated with shorter survival. Age-adjusted population incidence and reported ARCH mutation rates are consistent with a model in which clinical CMML ensues when a sufficient number of stochastically acquired age-related mutations has accumulated, suggesting that CMML represents the leukemic conversion of the myelomonocytic-lineage-biased aged hematopoietic system.

Functional RNAi screen targeting cytokine and growth factor receptors reveals oncorequisite role for interleukin-2 gamma receptor in JAK3-mutation-positive leukemia

To understand the role of cytokine and growth factor receptor-mediated signaling in leukemia pathogenesis, we designed a functional RNA interference (RNAi) screen targeting 188 cytokine and growth factor receptors that we found highly expressed in primary leukemia specimens. Using this screen, we identified interleukin-2 gamma receptor (IL2Rγ) as a critical growth determinant for a JAK3(A572V) mutation-positive acute myeloid leukemia cell line. We observed that knockdown of IL2Rγ abrogates phosphorylation of JAK3 and downstream signaling molecules, JAK1, STAT5, MAPK and pS6 ribosomal protein. Overexpression of IL2Rγ in murine cells increased the transforming potential of activating JAK3 mutations, whereas absence of IL2Rγ completely abrogated the clonogenic potential of JAK3(A572V), as well as the transforming potential of additional JAK3-activating mutations such as JAK3(M511I). In addition, mutation at the IL2Rγ interaction site in the FERM domain of JAK3 (Y100C) completely abrogated JAK3-mediated leukemic transformation. Mechanistically, we found IL2Rγ contributes to constitutive JAK3 mutant signaling by increasing JAK3 expression and phosphorylation. Conversely, we found that mutant, but not wild-type JAK3, increased the expression of IL2Rγ, indicating IL2Rγ and JAK3 contribute to constitutive JAK/STAT signaling through their reciprocal regulation. Overall, we demonstrate a novel role for IL2Rγ in potentiating oncogenesis in the setting of JAK3-mutation-positive leukemia. In addition, our study highlights an RNAi-based functional assay that can be used to facilitate the identification of non-kinase cytokine and growth factor receptor targets for inhibiting leukemic cell growth.

Bruton's tyrosine kinase is not essential for Bcr-Abl-mediated transformation of lymphoid or myeloid cells

Bcr-Abl, a constitutively active tyrosine kinase, is the cause of chronic myeloid leukemia (CML) and a subset of acute lymphoblastic leukemias (ALL). Bruton's tyrosine kinase (BTK), a member of the Tec family of tyrosine kinases with a crucial role in B-cell development, is consistently tyrosine phosphorylated in Bcr-Abl expressing murine pre B cells. BTK has been implicated in Bcr-Abl-mediated B-cell transformation and resistance to imatinib, implying that inhibiting BTK may be therapeutically beneficial. We decided to test whether BTK is a critical node in Bcr-Abl transformation and potential drug target in imatinib-resistant Bcr-Abl-positive cells. We depleted BTK in Ba/F3 and 32D cells expressing native and kinase domain (KD) mutant (E255K and T315I) Bcr-Abl, using shRNA. BTK levels were reduced to <10% of controls. However, no differences in viability and cell proliferation were observed and the response to imatinib was not altered. Consistent with this, proliferation and viability were unaffected by inhibition of BTK with reversible (PC-005) and irreversible (PCI-31523) small molecule inhibitors. Lastly, BTK inhibition did not affect the ability of Bcr-Abl to transform primary murine hematopoietic cells in colony forming and B-cell transformation assays. Collectively this data argues against a critical role for BTK in Bcr-Abl-mediated leukemogenesis.

Mutations of the BCR-ABL-kinase domain occur in a minority of patients with stable complete cytogenetic response to imatinib

Residual leukemia is demonstrable by reverse transcriptase-polymerase chain reaction in most patients with chronic myeloid leukemia who obtain a complete cytogenetic response (CCR) to imatinib. In patients who relapse during imatinib therapy, a high rate of mutations in the kinase domain of BCR-ABL have been identified, but the mechanisms underlying disease persistence in patients with a CCR are poorly characterized. To test whether kinase domain mutations are a common mechanism of disease persistence, we studied patients in stable CCR. Mutations were demonstrated in eight of 42 (19%) patients with successful amplification and sequencing of BCR-ABL. Mutation types were those commonly associated with acquired drug resistance. Four patients with mutations had a concomitant rise of BCR-ABL transcript levels, two of whom subsequently relapsed; the remaining four did not have an increase in transcript levels and follow-up samples, when amplifiable, were wild type. BCR-ABL-kinase domain mutations in patients with a stable CCR are infrequent, and their detection does not consistently predict relapse. Alternative mechanisms must be responsible for disease persistence in the majority of patients.

Dasatinib in patients with chronic phase chronic myeloid leukemia (CP-CML) who are resistant or intolerant to imatinib: Results of the CA180013 ’START-C’ Study

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Background: Dasatinib (BMS-354825) is a novel, oral, multi-targeted kinase inhibitor of BCR-ABL and SRC kinases with proven preclinical and clinical activity against imatinib resistant BCR-ABL mutations. Methods: CA180013 is an open-label Phase II study of dasatinib in imatinib-resistant (IM-R) or -intolerant (IM-I) patients (pts) with CP-CML. Between February-August 2005, 424 pts were recruited from 75 centers worldwide. Dasatinib was given at 70 mg twice daily (BID) with dose escalation to 90 mg BID in pts lacking response, and dose reductions to 50 and 40 mg BID for toxicity. Evaluations were weekly blood counts for the first 12 weeks; bone marrow cytology and cytogenetics every 3 months. The primary endpoint was rate of major cytogenetic response (MCyR; ≤35% Philadelphia pos. metaphases) in IM-R pts. Results: Data are currently available from the first 186 pts (127 IM-R, 59 IM-I) accrued prior to May 12, 2005. Median age was 59 yrs (range 24–79); 46% were male. Median time from diagnosis of CML was 64 months. Of the IM-R pts, 72% received IM >3 yrs, and 72% had >600 mg/day of IM. Overall, 70% had received prior interferon alpha. 62 (33%) pts achieved a prior MCyR to IM. With ≥6 months of follow up, 168 (90%) pts had a complete hematologic response (CHR). MCyR were achieved in 83 (45%) pts including 40 (31%) of IM-R pts, and 43 (73%) of IM-I pts. Mutations in the BCR-ABL domain were found in 65/176 (37%) pts; 57 (88%) achieved CHR, and 24 (37%) MCyR. Molecular response analysis is ongoing. 160 (86%) pts remain on study without progression. Grade 3/4 neutropenia or thrombocytopenia was reported in 83 (45%) pts and 85 (46%) pts with onset after 4–8 weeks of therapy in most pts. Temporary dose interruptions occurred in 146 (78%), and dose reductions in 96 (52%) pts with a median daily of 108 (range 19–169) mg. Non-hematologic toxicity consisted mainly of Grade 1/2 diarrhea, headache, superficial edema, and pleural effusion, with ≤2% Grade 3/4. There was no cross-intolerance between dasatinib and IM. Conclusions: Dasatinib demonstrated substantial hematologic and cytogenetic activity in IM-R and IM-I pts with CP-CML. An updated analysis of 424 pts with ≥6 months of follow up will be presented.

[Table: see text]

Long-term benefits of imatinib (IM) for patients newly diagnosed with chronic myelogenous leukemia in chronic phase (CML-CP): The 5-year update from the IRIS study

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Background: IM was proven to be superior to IFN+Ara-C for newly diagnosed patients (pts) with CML-CP (O’ Brien et al, NEJM 2003). 1,106 pts were randomized between June 2000 and Jan 2001 to either IM 400 mg or IFN+Ara-C with 553 pts to each treatment. This abstract is based on data collected up to 54 months after last patient had been recruited on IM. 60-months (5-year) data will be available for presentation. Methods: Evaluations included complete hematologic response (CHR), complete/partial cytogenetic response (CCyR/PCyR - defined as 0% / 1–35% Ph+ metaphases respectively), major cytogenetic response (MCyR=CCyR+ PCyR), major molecular response (MMR) - defined as ≥ 3 log reduction of BCR-ABL transcript levels from the standardized baseline, time to progression - defined as loss of CHR/MCyR, evolution to accelerated phase/blast crisis (AP/BC), or death due to any cause during treatment, and overall survival. Results: With a median follow-up of 54-months, 72% of the 553 randomized pts remain on initial IM treatment (5% of pts discontinued due to adverse events, 9.5% due to unsatisfactory therapeutic effect and 11% due to other reasons another 2.5% crossed over to IFN+Ara-C). Overall, the cumulative best response rates of CHR, MCyR and CCyR are 97%, 88% and 82%, respectively. The overall estimated survival was 90% (93% when censored at bone marrow transplant). An estimated 84% of pts have not progressed on treatment and 93% of pts were free from progression to AP/BC. The annual rate of progression to AP/BC of < 1% in the fourth year was lower than each of the first three years (1.5, 2.8, and 1.6%, respectively). Of the pts with MCyR at 12 months (n=436), an estimated 96% were free of progression to AP/BC at 54 months whereas it was only 81% for the 73 pts who did not achieve a MCyR at 12 months (p< 0.001). No patient with a MMR within 12 months progressed to AP/BC within 54 months. Conclusions: This analysis confirms the high rates and durability of responses to IM. Encouragingly, the rate of progression in the fourth year was lower than in each of the preceding three years. Results further demonstrate the beneficial effect of cytogenetic and molecular responses on long-term outcomes.

[Table: see text]

Zoledronate inhibits proliferation and induces apoptosis of imatinib-resistant chronic myeloid leukaemia cells

Although imatinib mesylate has revolutionized the treatment of chronic myeloid leukaemia (CML), resistance to the drug, manifesting as relapse after an initial response or persistence of disease, remains a therapeutic challenge. In order to overcome this, alternative or additional targeting of signaling pathways downstream of Bcr-Abl may provide the best option for improving clinical response. Bisphosphonates, such as zoledronate, have been shown to inhibit the oncogenicity of Ras, an important downstream effector of Bcr-Abl. In this study, we show that zoledronate is equally effective in inhibiting the proliferation and clonogenicity of both imatinib-sensitive and -resistant CML cells, regardless of their mechanism of resistance. This is achieved by the induction of S-phase cell cycle arrest and apoptosis, through the inhibition of prenylation of Ras and Ras-related proteins by zoledronate. The combination of imatinib and zoledronate also augmented the activity of either drug alone and this occurred in imatinib-resistant CML cells as well. Since zoledronate is already available for clinical use, these results suggest that it may be an effective addition to the armamentarium of drugs for the treatment of CML.

A single nucleotide polymorphism in the coding region of ABL and its effects on sensitivity to imatinib

We have identified a gene polymorphism (K247R) within or close to the P-loop of BCR-ABL, which leads to the substitution of arginine for lysine. We investigated the allelic frequency of K247R by screening 157 CML patients and 213 healthy blood donors with conventional sequencing, restriction enzyme digest and single strand conformational polymorphism analysis, and found the arginine allele to be rare. Three out of five CML patients with the arginine allele of K247R failed to achieve a major cytogenetic response to imatinib, suggesting that the arginine allele may have reduced sensitivity. However, despite K247R's position in or near to the P-loop, biochemical and cellular assays of imatinib and dasatinib sensitivity showed no alteration compared to wild type. Clinicians should be aware that possession of the arginine allele of K247R does not reflect a mutation that necessitates a change in the therapeutic strategy, unless there are other signs of inadequate response to drug.

Catalytic domains of tyrosine kinases determine the phosphorylation sites within c-Cbl

Catalytic (SH1) domains of protein tyrosine kinases (PTKs) demonstrate specificity for peptide substrates. Whether SH1 domains differentiate between tyrosines in a physiological substrate has not been confirmed. Using purified proteins, we studied the ability of Syk, Fyn, and Abl to differentiate between tyrosines in a common PTK substrate, c-Cbl. We found that each kinase produced a distinct pattern of c-Cbl phosphorylation, which altered the phosphotyrosine-dependent interactions between c-Cbl and CrkL or phosphatidylinositol 3'-kinase (PI3-K). Our data support the concept that SH1 domains determine the final sites of phosphorylation once PTKs reach their target proteins.

Detection of ABL kinase domain mutations with denaturing high-performance liquid chromatography

Mutations of the ABL kinase domain (KD) are common in patients with chronic myelogenous leukemia (CML) who develop resistance to imatinib. We developed an RT-PCR-based denaturing high-performance liquid chromatography (D-HPLC) assay to detect mutations of the ABL KD. Validation experiments using mixtures of wild type and mutant amplicons showed that the D-HPLC assay could detect mutant transcripts when they represented at least 15% of the total, and was thus twice as sensitive as automated sequencing. When D-HPLC was applied to 30 cDNAs from patients with imatinib resistance that had previously been characterized for KD mutations by direct sequencing of BCR-ABL RT-PCR products, there was concordance in 97% of samples. Resequencing confirmed the original mutations in all cases. In addition, sequencing of individual clones detected a mutation in one sample that had been mutation-positive by D-HPLC but wild type by conventional sequencing. In serial samples from the same individuals, D-HPLC detected mutations as early as 260 days before hematological relapse. D-HPLC is suitable for routine clinical monitoring of CML patients for emergence of KD mutations and may be useful for optimizing therapy. Early detection of emerging mutant clones may aid in guiding decisions regarding alternative treatment options.

Demonstration of Philadelphia chromosome negative abnormal clones in patients with chronic myelogenous leukemia during major cytogenetic responses induced by imatinib mesylate

Imatinib mesylate, an Abl-specific kinase inhibitor, produces sustained complete hematologic responses (CHR) and major cytogenetic responses (MCR) in chronic myeloid leukemia (CML) patients, but long-term outcomes in these patients are not yet known. This article reports the identification of clonal abnormalities in cells lacking detectable Philadelphia (Ph) chromosome/BCR-ABL rearrangements from seven patients with chronic- or accelerated-phase CML, who were treated with imatinib. All seven patients were refractory or intolerant to interferon therapy. Six of seven patients demonstrated MCR and one patient, who had a cryptic translocation, achieved low-level positivity (2.5%) for BCR-ABL by fluorescence in situ hybridization. The median duration of imatinib treatment before the identification of cytogenetic abnormalities in BCR-ABL-negative cells was 13 months. The most common cytogenetic abnormality was trisomy 8, documented in three patients. All patients had varying degrees of dysplastic morphologic abnormalities. One patient exhibited increased numbers of marrow blasts, yet consistently demonstrated no Ph-positive metaphases and the absence of morphologic features of CML. The presence of clonal abnormalities in Ph-negative cells of imatinib-treated CML patients with MCR and CHR highlights the importance of routine metaphase cytogenetic testing and long-term follow-up of all imatinib-treated patients.

Molecular and chromosomal mechanisms of resistance to imatinib (STI571) therapy

Selective inhibition of the BCR-ABL tyrosine kinase by imatinib (STI571, Glivec/Gleevec) is a promising new therapeutic strategy in patients with chronic myelogenous leukemia (CML). Despite significant hematologic and cytogenetic responses, resistance occurs, particularly in patients with advanced disease. We sought to determine the underlying mechanisms. Sixty-six patients with CML in myeloid blast crisis (n = 33), lymphoid blast crisis (n = 2), accelerated phase (n = 16), chronic phase (n = 13), and BCR-ABL-positive acute lymphoblastic leukemia (n = 2) resistant to imatinib were investigated. Median duration of imatinib therapy was 148 days (range 6-882). Patients were evaluated for genomic amplification of BCR-ABL, overexpression of BCR-ABL transcripts, clonal karyotypic evolution, and mutations of the imatinib binding site in the BCR-ABL tyrosine kinase domain. Results were as follows: (1) Median levels of BCR-ABL transcripts, were not significantly changed at the time of resistance but 7/55 patients showed a >10-fold increase in BCR-ABL levels; (2) genomic amplification of BCR-ABL was found in 2/32 patients evaluated by fluorescence in situ hybridization; (3) additional chromosomal aberrations were observed in 19/36 patients; (4) point mutations of the ABL tyrosine kinase domain resulting in reactivation of the BCR-ABL tyrosine kinase were detected in 23/66 patients. In conclusion, although the heterogeneous development of imatinib resistance is challenging, the fact that BCR-ABL is active in many resistant patients suggests that the chimeric oncoprotein remains a good therapeutic target. However, patients with clonal evolution are more likely to have BCR-ABL-independent mechanisms of resistance. The observations warrant trials combining imatinib with other agents.

Chronic myelogenous leukemia

The treatment recommendations for chronic myelogenous leukemia (CML) are evolving rapidly. In the past year, pegylated interferon and STI571 (Gleevec, imatinib mesylate), a Bcr-Abl tyrosine kinase inhibitor, have become commercially available and non-myeloablative stem cell transplants continue to be refined. Clinicians and patients face a bewildering array of treatment options for CML. In this article Dr. Sawyer reviews the clinical results with STI571 and ongoing investigations into mechanisms of resistance to STI571. Given the newness of STI571, a practical overview on the administration of STI571 is presented by Drs. Druker and Ford, focusing on aspects such as optimal dose, management of common side effects, and potential drug interactions. The most recent data on interferon-based regimens are reviewed by Dr. Baccarani in the third section. In the last section Dr. Goldman presents recent results of allogeneic stem cell transplants, including the reduced intensity conditioning regimens. Lastly, the proposed place of each of these treatments in the management of CML patients is addressed to assist in deciding amongst treatment options for CML patients.

Insights from pre-clinical studies for new combination treatment regimens with the Bcr-Abl kinase inhibitor imatinib mesylate (Gleevec/Glivec) in chronic myelogenous leukemia: a translational perspective

Clinical phase I/II studies with the Abl kinase inhibitor imatinib mesylate (Gleevec/Glivec, formerly STI571) for the treatment for chronic myelogenous leukemia (CML) demonstrated the safety and the remarkable efficacy of this molecularly targeted agent. However, a significant proportion of patients treated in the chronic phase of the disease after having failed interferon alpha (IFN) remain predominantly Philadelphia chromosome positive (Ph(+)), suggesting a risk of later relapses. Furthermore, results in blast crisis patients revealed a high frequency of relapses or resistance to imatinib. To circumvent resistance, improve response rates, or prolong survival, pre-clinical evaluations of combinations of imatinib with other agents have been pursued. Some of these have already been translated into clinical studies. Here, we first summarize evidence from pre-clinical studies on new combination regimens with imatinib in the treatment of CML. Second, we analyze preliminary clinical data of ongoing combination studies. Finally, we provide a summary of approaches that use novel antileukemic agents with molecularly characterized modes of action.

Chronic myeloid leukemia: current treatment options

The choice of primary treatment for patients with chronic myeloid leukemia (CML) diagnosed in chronic phase has become exceedingly difficult. There is little doubt that allogeneic stem cell transplantation can eradicate the leukemia and that a graft-versus-leukemia effect makes a major contribution to this result; conversely, only a minority of patients are eligible for transplantation, which still carries an appreciable risk for death or protracted illness. For most patients, interferon-alpha (IFN-alpha) prolongs life to some degree in comparison with hydroxyurea, but it is associated with considerable toxicity. The newly introduced tyrosine kinase inhibitor STI571 induces complete hematologic remission in almost all patients and is associated with a very high rate of cytogenetic response; its capacity to prolong life in comparison with IFN-alpha is not yet established. Here are reviewed some factors that predict survival after nontransplantation therapy and after allografting for CML in chronic phase. Two contrasting options are considered for managing the patient with newly diagnosed disease, and it can be concluded that, for now, allogeneic stem cell transplantation soon after diagnosis should continue to be offered as an option for selected patients. Further experience with the use of STI571 as a single agent or in combination with other antileukemic agents may alter the picture in the near future.

STI571: targeting BCR-ABL as therapy for CML

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

ST1571, a tyrosine kinase inhibitor for the treatment of chronic myelogenous leukemia: validating the promise of molecularly targeted therapy

The deregulated tyrosine kinase activity of the Bcr-Abl fusion protein has been established as the causative molecular event in chronic myelogenous leukemia (CML). Thus the Bcr-Abl tyrosine kinase is an ideal target for pharmacologic inhibition. ST1571 (formerly CGP57148B), is an Abl-specific tyrosine kinase inhibitor that in preclinical studies selectively kills Bcr-Abl-containing cells in vitro and in vivo. The results of clinical studies have demonstrated the potential of molecularly targeted therapies, and ST1571 is emerging as a new therapeutic agent for CML.

Chronic myelogenous leukaemia--new therapeutic principles

The deregulated tyrosine kinase activity of the BCR-ABL fusion protein is the cause of malignant transformation in almost all cases of chronic myelogenous leukaemia (CML), making BCR-ABL an ideal target for pharmacological inhibition. Signal transduction inhibitor (STI571) (formerly CGP57 148B), is an ABL specific, tyrosine kinase inhibitor. In preclinical studies, it has been shown to selectively kill BCR-ABL expressing cells, both in-vitro and in vivo. The results of clinical studies to date are highly encouraging and STI571 promises to be an important addition to the therapy of CML.

Current treatment approaches for chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a clonal hematopoietic stem cell disorder that progresses through distinct phases characterized by progressive loss of the differentiation of the malignant clone. Over the past 4 decades, it has been established that the Bcr-Abl protein, created as a consequence of a (9:22) chromosomal translocation, is the cause of the disease. Bcr-Abl functions as a constitutively activated tyrosine kinase, and this kinase activity is absolutely required for the transforming function of the Bcr-Abl protein. Thus, a specific inhibitor of the Bcr-Abl tyrosine kinase would be predicted to be an effective and selective therapeutic agent for CML. STI571, an Abl-specific tyrosine kinase inhibitor, has shown remarkable activity in all phases of CML. The clinical features, molecular pathogenesis, and current treatment options of CML are reviewed along with the development of STI571, the phase I clinical results, and the application of this paradigm to other malignancies.

The role of the tyrosine kinase inhibitor STI571 in the treatment of cancer

The tyrosine kinase inhibitor STI571 exemplifies the successful development of a rationally designed, molecularly targeted therapy for the treatment of cancer. This review details the steps in the development of this agent and highlights why this drug has been so successful in the treatment of chronic myelogenous leukaemia. Future directions including the mechanisms and management of resistance and new therapeutic strategies are discussed. Finally, the literature supporting the use of STI571 in other malignancies, including solid tumors is briefly reviewed.

STI571: a gene product-targeted therapy for leukemia

Chronic myelogenous leukemia (CML) is a clonal hematopoietic stem-cell disorder characterized by the (9:22) translocation and resultant production of the constitutively activated bcr-abl tyrosine kinase. Characterized clinically by marked myeloid proliferation, it invariably terminates in an acute leukemia. Conventional therapeutic options include interferon-based regimens and stem-cell transplantation, with stem-cell transplantation being the only curative therapy. Through rational drug development, STI571, a bcr-abl tyrosine kinase inhibitor, has emerged as a paradigm for gene product-targeted therapy, offering new hope for expanded treatment options for patients with CML.

Efficacy and safety of a specific inhibitor of the BCR-ABL tyrosine kinase in chronic myeloid leukemia

BACKGROUND

BCR-ABL is a constitutively activated tyrosine kinase that causes chronic myeloid leukemia (CML). Since tyrosine kinase activity is essential to the transforming function of BCR-ABL, an inhibitor of the kinase could be an effective treatment for CML.

METHODS

We conducted a phase 1, dose-escalating trial of STI571 (formerly known as CGP 57148B), a specific inhibitor of the BCR-ABL tyrosine kinase. STI571 was administered orally to 83 patients with CML in the chronic phase in whom treatment with interferon alfa had failed. Patients were successively assigned to 1 of 14 doses ranging from 25 to 1000 mg per day.

RESULTS

Adverse effects of STI571 were minimal; the most common were nausea, myalgias, edema, and diarrhea. A maximal tolerated dose was not identified. Complete hematologic responses were observed in 53 of 54 patients treated with daily doses of 300 mg or more and typically occurred in the first four weeks of therapy. Of the 54 patients treated with doses of 300 mg or more, cytogenetic responses occurred in 29, including 17 (31 percent of the 54 patients who received this dose) with major responses (0 to 35 percent of cells in metaphase positive for the Philadelphia chromosome); 7 of these patients had complete cytogenetic remissions.

CONCLUSIONS

STI571 is well tolerated and has significant antileukemic activity in patients with CML in whom treatment with interferon alfa had failed. Our results provide evidence of the essential role of BCR-ABL tyrosine kinase activity in CML and demonstrate the potential for the development of anticancer drugs based on the specific molecular abnormality present in a human cancer.

Activity of a specific inhibitor of the BCR-ABL tyrosine kinase in the blast crisis of chronic myeloid leukemia and acute lymphoblastic leukemia with the Philadelphia chromosome

BACKGROUND

BCR-ABL, a constitutively activated tyrosine kinase, is the product of the Philadelphia chromosome. This enzyme is present in virtually all cases of chronic myeloid leukemia (CML) throughout the course of the disease, and in 20 percent of cases of acute lymphoblastic leukemia (ALL). On the basis of the substantial activity of the inhibitor in patients in the chronic phase, we evaluated STI571 (formerly known as CGP 57148B), a specific inhibitor of the BCR-ABL tyrosine kinase, in patients who had CML in blast crisis and in patients with ALL who had the Ph chromosome.

METHODS

In this dose-escalating pilot study, 58 patients were treated with STI571; 38 patients had a myeloid blast crisis and 20 had ALL or a lymphoid blast crisis. Treatment was given orally at daily doses ranging from 300 to 1000 mg.

RESULTS

Responses occurred in 21 of 38 patients (55 percent) with a myeloid-blast-crisis phenotype; 4 of these 21 patients had a complete hematologic response. Of 20 patients with a lymphoid blast crisis or ALL, 14 (70 percent) had a response, including 4 who had complete responses. Seven patients with a myeloid blast crisis continue to receive treatment and remain in remission from 101 to 349 days after starting the treatment. All but one patient with a lymphoid blast crisis or ALL has relapsed. The most frequent adverse effects were nausea, vomiting, edema, thrombocytopenia, and neutropenia.

CONCLUSIONS

The BCR-ABL tyrosine kinase inhibitor STI571 is well tolerated and has substantial activity in the blast crises of CML and in Ph-positive ALL.

Mice lacking the homologue of the human 22q11.2 gene CRKL phenocopy neurocristopathies of DiGeorge syndrome

Heterozygous deletions within human chromosome 22q11 are the genetic basis of DiGeorge/velocardiofacial syndrome (DGS/VCFS), the most common deletion syndrome (1 in 4,000 live births) in humans. CRKL maps within the common deletion region for DGS/VCFS (ref. 2) and encodes an SH2-SH3-SH3 adapter protein closely related to the Crk gene products. Here we report that mice homozygous for a targeted null mutation at the CrkL locus (gene symbol Crkol for mice) exhibit defects in multiple cranial and cardiac neural crest derivatives including the cranial ganglia, aortic arch arteries, cardiac outflow tract, thymus, parathyroid glands and craniofacial structures. We show that the migration and early expansion of neural crest cells is unaffected in Crkol-/- embryos. These results therefore indicate an essential stage- and tissue-specific role for Crkol in the function, differentiation, and/or survival of neural crest cells during development. The similarity between the Crkol-/- phenotype and the clinical manifestations of DGS/VCFS implicate defects in CRKL-mediated signaling pathways as part of the molecular mechanism underlying this syndrome.

Chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a clonal hematopoietic stem cell disorder characterized by the (9:22) translocation and resultant production of the constitutively activated bcr-abl tyrosine kinase. Characterized clinically by marked myeloid proliferation, it invariably terminates in an acute leukemia. Interferon-based regimens and stem cell transplantations are the standard therapeutic options, with stem cell transplantation being the only curative therapy. As therapy for CML improves, molecular methods of monitoring response will become integrated in patient treatment. Through rational drug development, STI571, a bcr-abl tyrosine kinase inhibitor, has emerged as targeted therapy that offers new hope for expanded treatment options for patients with CML.

STI571: an inhibitor of the BCR-ABL tyrosine kinase for the treatment of chronic myelogenous leukaemia

The deregulated tyrosine kinase activity of the BCR-ABL fusion protein has been established as the causative molecular event in chronic myelogenous leukaemia. Thus, the BCR-ABL tyrosine kinase is an ideal target for pharmacological inhibition. STI571 (formerly CGP57148B), is an ABL-specific inhibitor of tyrosine kinase that, in preclinical studies, selectively killed BCR-ABL-containing cells in vitro and in vivo. Clinical studies have shown the potential of this specifically targeted therapy, and STI571 is emerging as an important new therapeutic agent for chronic myelogenous leukaemia.

Status of bcr-abl tyrosine kinase inhibitors in chronic myelogenous leukemia

The bcr-abl fusion protein is present in the vast majority of cases of chronic myelogenous leukemia, and the deregulated tyrosine kinase activity of this protein is essential for leukemic transformation. Thus, bcr-abl is an ideal target for pharmacologic inhibition. In preclinical studies, ST1571 (formerly CGP57148B), an abl-specific, tyrosine kinase inhibitor, selectively killed bcr-abl-expressing cells both in vitro and in vivo. In early clinical trials of ST1571, encouraging results have been obtained, and there is already a suggestion that ST1571 may soon need to be incorporated into treatment algorithms for chronic myelogenous leukemia.

Efficacy of STI571, an abl tyrosine kinase inhibitor, in conjunction with other antileukemic agents against bcr-abl-positive cells

Chronic myelogenous leukemia (CML), a malignancy of a hematopoietic stem cell, is caused by the Bcr-Abl tyrosine kinase. STI571(formerly CGP 57148B), an Abl tyrosine kinase inhibitor, has specific in vitro antileukemic activity against Bcr-Abl-positive cells and is currently in Phase II clinical trials. As it is likely that resistance to a single agent would be observed, combinations of STI571 with other antileukemic agents have been evaluated for activity against Bcr-Abl-positive cell lines and in colony-forming assays in vitro. The specific antileukemic agents tested included several agents currently used for the treatment of CML: interferon-alpha (IFN), hydroxyurea (HU), daunorubicin (DNR), and cytosine arabinoside (Ara-C). In proliferation assays that use Bcr-Abl-expressing cells lines, the combination of STI571 with IFN, DNR, and Ara-C showed additive or synergistic effects, whereas the combination of STI571 and HU demonstrated antagonistic effects. However, in colony-forming assays that use CML patient samples, all combinations showed increased antiproliferative effects as compared with STI571 alone. These data indicate that combinations of STI571 with IFN, DNR, or Ara-C may be more useful than STI571 alone in the treatment of CML and suggest consideration of clinical trials of these combinations.

Abl protein-tyrosine kinase inhibitor STI571 inhibits in vitro signal transduction mediated by c-kit and platelet-derived growth factor receptors

STI571 (formerly known as CGP 57148B) is a protein-tyrosine kinase inhibitor that is currently in clinical trials for the treatment of chronic myelogenous leukemia. STI571 selectively inhibits the Abl and platelet-derived growth factor (PDGF) receptor tyrosine kinases in vitro and blocks cellular proliferation and tumor growth of Bcr-abl- or v-abl-expressing cells. We have further investigated the profile of STI571 against related receptor tyrosine kinases. STI571 was found to potently inhibit the kinase activity of the alpha- and beta-PDGF receptors and the receptor for stem cell factor, but not the closely related c-Fms, Flt-3, Kdr, Flt-1, and Tek tyrosine kinases. Additionally, no inhibition of c-Met or nonreceptor tyrosine kinases such as Src and Jak-2 has been observed. In cell-based assays, STI571 selectively inhibited PDGF and stem cell factor-mediated cellular signaling, including ligand-stimulated receptor autophosphorylation, inositol phosphate formation, and mitogen-activated protein kinase activation and proliferation. These results expand the profile of STI571 and suggest that in addition to chronic myelogenous leukemia, STI571 may have clinical potential in the treatment of diseases that involve abnormal activation of c-Kit or PDGF receptor tyrosine kinases.

Inhibition of c-kit receptor tyrosine kinase activity by STI 571, a selective tyrosine kinase inhibitor

STI 571 (formerly known as CGP 57148B) is a known inhibitor of the c-abl, bcr-abl, and platelet-derived growth-factor receptor (PDGFR) tyrosine kinases. This compound is being evaluated in clinical trials for the treatment of chronic myelogenous leukemia. We sought to extend the activity profile of STI 571 by testing its ability to inhibit the tyrosine kinase activity of c-kit, a receptor structurally similar to PDGFR. We treated a c-kit expressing a human myeloid leukemia cell line, M-07e, with STI 571 before stimulation with Steel factor (SLF). STI 571 inhibited c-kit autophosphorylation, activation of mitogen-activated protein (MAP) kinase, and activation of Akt without altering total protein levels of c-kit, MAP kinase, or Akt. The concentration that produced 50% inhibition for these effects was approximately 100 nmol/L. STI 571 also significantly decreased SLF-dependent growth of M-07e cells in a dose-dependent manner and blocked the antiapoptotic activity of SLF. In contrast, the compound had no effect on MAP kinase activation or cellular proliferation in response to granulocyte-macrophage colony-stimulating factor. We also tested the activity of STI 571 in a human mast cell leukemia cell line (HMC-1), which has an activated mutant form of c-kit. STI 571 had a more potent inhibitory effect on the kinase activity of this mutant receptor than it did on ligand-dependent activation of the wild-type receptor. These findings show that STI 571 selectively inhibits c-kit tyrosine kinase activity and downstream activation of target proteins involved in cellular proliferation and survival. This compound may be useful in treating cancers associated with increased c-kit kinase activity.

All-trans-retinoic acid induces tyrosine phosphorylation of the CrkL adapter in acute promyelocytic leukemia cells

OBJECTIVE

All-trans-retinoic acid (RA) is a potent inducer of differentiation of acute promyelocytic leukemia (APL) cells in vitro and in vivo. It also exhibits synergistic effects with interferons on the induction of differentiation and growth inhibition in vitro. Recent studies showed that interferons engage a signaling pathway involving the CBL proto-oncogene and the CrkL adapter, which mediates interferon-induced growth inhibitory signals. The objective of this study was to determine whether the CBL-CrkL pathway is activated by treatment of the NB-4 and HL-60 acute leukemia cell lines with RA.

MATERIALS AND METHODS

The effects of RA treatment on CBL and CrkL phosphorylation, as well as on protein-protein interactions, were determined in studies involving immunoprecipitations of cell extracts with specific antibodies and Western blots. In addition, glutathione-S-transferase fusion proteins were used in binding studies to determine whether the SH2 domain of CrkL interacts with CBL in a RA-dependent manner and whether Rapl is activated by RA.

RESULTS

Treatment of NB-4 or HL-60 cells with RA resulted in strong tyrosine phosphorylation of CBL, which was time and dose dependent. Similarly, RA induced tyrosine phosphorylation of the CrkL adapter and the association of CrkL with CBL. The RA-dependent interaction of CrkL with CBL was mediated by binding of the SH2 domain of CrkL to tyrosine phosphorylated CBL, suggesting that CBL provides a docking site for engagement of CrkL in a RA-activated cellular pathway. The guanine exchange factor C3G was found to be associated with CrkL at similar levels before and after RA treatment, but Rapl activation downstream of C3G was not inducible by RA.

CONCLUSIONS

These findings demonstrate that the CBL-CrkL pathway is one of the mediators of the effects of RA on APL cells and suggest that one of the mechanisms of synergy between RA and interferons may involve regulation of components of this signaling cascade.

TEL/PDGFbetaR fusion protein activates STAT1 and STAT5: a common mechanism for transformation by tyrosine kinase fusion proteins

OBJECTIVE

TEL/PDGFbetaR is a tyrosine kinase fusion protein associated with the pathogenesis of chronic myelomonocytic leukemia. The following experiments were undertaken to understand the mechanisms whereby TEL/PDGFbetaR transforms cells.

MATERIALS AND METHODS

Activation of JAK and STAT proteins was studied in an interleukin 3 (IL-3)-dependent cell line, Ba/F3, transformed to IL-3 independence by TEL/PDGFbetaR.

RESULTS

TEL/PDGFbetaR activates STAT1 and STAT5 in transformed Ba/F3 cells through a JAK-independent pathway. Activation of STAT proteins requires the kinase activity of TEL/PDGFbetaR. JAK1, JAK2, JAK3, and TYK2 are not phosphorylated by TEL/PDGFbetaR. However, TEL/PDGFbetaR can phosphorylate STAT5 in transiently transfected COS cells, suggesting that TEL/PDGFbetaR may itself be the kinase involved in tyrosine phosphorylation of STAT proteins. In contrast, native PDGFbetaR stimulated by PDGF ligand does not activate STAT proteins to a significant degree in this hematopoietic context. STAT1 and STAT5 also are activated by TEL/ABL and TEL/JAK2 fusion proteins associated with human leukemia.

CONCLUSIONS

STAT activation may be a common mechanism of transformation by leukemogenic tyrosine kinase fusion proteins.

The linker phosphorylation site Tyr292 mediates the negative regulatory effect of Cbl on ZAP-70 in T cells

The protooncogene product Cbl has emerged as a negative regulator of tyrosine kinases. We have shown previously that Cbl binds to ZAP-70 through its N-terminal tyrosine kinase binding (TKB) domain. In this study, we demonstrate that overexpression of Cbl in Jurkat T cells decreases the TCR-induced phosphorylation of ZAP-70 and other cellular phosphoproteins. Coexpression of Cbl with ZAP-70 in COS cells reproduced the Cbl-induced reduction in the level of phosphorylated ZAP-70. The effect of Cbl was eliminated by the TKB-inactivating G306E mutation in Cbl as well as by a phenylalanine mutation of Tyr292 within the TKB domain binding site on ZAP-70. Notably, the oncogenic Cbl-70Z/3 mutant associated with ZAP-70, but did not reduce the levels of phosphorylated ZAP-70. Overexpression of Cbl, but not Cbl-G306E, in Jurkat T cells led to a decrease in the TCR-induced NF-AT luciferase reporter activity. Overexpression of the TKB domain itself, but not its G306E mutant, functioned in a dominant-negative manner and led to an increase in NF-AT reporter activity. Cbl-70Z/3-overexpressing cells exhibited an increase in both basal and TCR-induced NF-AT luciferase reporter activity, and this trend was reversed by the G306E mutation. Finally, by reconstituting a ZAP-70-deficient Jurkat T cell line, p116, we demonstrate that wild-type ZAP-70 is susceptible to the negative regulatory effect of Cbl, whereas the ZAP-70-Y292F mutant is resistant. Together, our results establish that the linker phosphorylation site Tyr292 mediates the negative regulatory effect of Cbl on ZAP-70 in T cells.

Rapid tyrosine phosphorylation and activation of Bruton's tyrosine/Tec kinases in platelets induced by collagen binding or CD32 cross-linking

Stimulation of the platelet nonintegrin collagen receptor, glycoprotein VI, evokes a signaling response similar to that induced by antigen receptor activation in B and T lymphocytes. A key transducer of the lymphocyte signaling pathways is the Bruton's tyrosine kinase (Btk)/Tec kinase family, which connects receptors to the elevation of intracellular-free calcium levels. An important signaling function for Btk in collagen-induced platelet activation in vitro was recently demonstrated by other researchers using Btk-deficient platelets from patients with X-linked agammaglobulinemia (XLA). Since Btk-deficiency does not induce an overt platelet-based bleeding disorder in vivo, collagen receptor responses may include other Btk/Tec kinase family members in normal platelets. Both Btk and Tec had increased tyrosine following stimulation of collagen receptors or CD32 cross-linking. Data from kinetic analyses and inhibitor studies and the use of phosphopeptide-specific antibodies recognizing 2 Btk regulatory phosphorylated tyrosine residues suggest a mechanism for coordinate recruitment of Btk and Tec through the immunoreceptor tyrosine-based activation motif, Src family kinases, and phosphatidylinositol 3-kinase. In XLA platelets, collagen treatment increased tyrosine phosphorylation of Tec and several other signaling proteins, including Lyn, Fyb, Slp-76, and the Wiskott-Aldrich syndrome protein. This indicates that important elements of the collagen signaling pathway proximal and distal to Btk and Tec are preserved despite the lack of functional Btk. The results are consistent with the conclusion that activation of Tec may sustain XLA platelet function in vivo, while some in vitro assays of nonintegrin collagen receptor signaling through the Btk/Tec kinase family reflect the additive dosage of the transducers. (Blood. 2000;95:1663-1670)

CrkL functions as a nuclear adaptor and transcriptional activator in Bcr-Abl-expressing cells

OBJECTIVE

To identify tyrosine phosphorylated proteins that interact with CrkL in Bcr-Abl-expressing cells and analyze the function of that association.

MATERIALS AND METHODS

Immunoprecipitation of CrkL was performed on lysates from parental cells (Rat-1, MO7e, or 32D) or Bcr-Abl-expressing cells (Rat-1p185, MO7p210, 32Dp210, K562) followed by immunoblotting for pTyr, Stat5, or CrkL. Interactions were confirmed in vitro using GST-CrkL fusion proteins. Electrophoretic mobility shift assays were performed on K562 nuclear extracts using a beta-casein promoter-derived probe. Supershift analysis was performed with CrkL, Stat5, Stat1, Grb2, and peptide-blocked CrkL and Stat5 antibodies. CrkL localization in Rat-1 and Rat-1p185 cells was detected with indirect immunofluorescence. Transcriptional activation was analyzed in COS7 cells transfected with a Stat-responsive luciferase reporter construct and Bcr-Abl, kinase-defective Bcr-Abl, CrkL, or Grb2.

RESULTS

We show that, in Bcr-Abl-expressing cells, CrkL+ interacts with tyrosine phosphorylated Stat5. Additionally, in the presence of Bcr-Abl, CrkL is found in the nucleus, can be detected in a Stat5/DNA complex, and increases transcriptional activation from a Stat-responsive reporter construct.

CONCLUSION

This suggests a novel role for CrkL, functioning as a nuclear adaptor protein that can associate with and activate Stat proteins in Bcr-Abl-expressing cells.

IFN-gamma activates the C3G/Rap1 signaling pathway

IFN-gamma transduces signals by activating the IFN-gamma receptor-associated Jak-1 and Jak-2 kinases and by inducing tyrosine phosphorylation and activation of the Stat-1 transcriptional activator. We report that IFN-gamma activates a distinct signaling cascade involving the c-cbl protooncogene product, CrkL adapter, and small G protein Rap1. During treatment of NB-4 human cells with IFN-gamma, c-cbl protooncogene product is rapidly phosphorylated on tyrosine and provides a docking site for the src homology 2 domain of CrkL, which also undergoes IFN-gamma-dependent tyrosine phosphorylation. CrkL then regulates activation of the guanine exchange factor C3G, with which it interacts constitutively via its N terminus src homology 3 domain. This results in the IFN-gamma-dependent activation of Rap1, a protein known to exhibit tumor suppressor activity and mediate growth inhibitory responses. In a similar manner, Rap1 is also activated in response to treatment of cells with type I IFNs (IFN-alpha, IFN-beta), which also engage CrkL in their signaling pathways. On the other hand, IFN-gamma does not induce formation of nuclear CrkL-Stat5 DNA-binding complexes, which are induced by IFN-alpha and IFN-beta, indicating that pathways downstream of CrkL are differentially regulated by different IFN subtypes. Taken altogether, our data demonstrate that, in addition to activating the Stat pathway, IFN-gamma activates a distinct signaling cascade that may play an important role in the generation of its growth inhibitory effects on target cells.

Abl kinase but not PI3-kinase links to the cytoskeletal defects in Bcr-Abl transformed cells

OBJECTIVE

The purpose of this study was to investigate the contribution of Abl kinase and phosphatidylinositol 3-kinase (PI3-kinase) to the altered adhesive properties and cytoskeletal defects in a Bcr-Abl transformed fibroblast cell model. MATERIALS ANID METHODS: Two fibroblast cell lines stably transfected with Bcr-Abl were compared to their parental counterparts for alterations in their adhesive properties in an attachment assay and for abnormalities in their cytoskeletal architecture by immunofluorescence microscopy. Cells then were treated with specific inhibitors of either the Abl kinase CGP57148 or the PI3-kinase LY294002 to determine whether these treatments would restore normal cytoarchitecture and adhesion.

RESULTS

[corrected] Significant defects in cytoskeletal architecture were observed using this fibroblast model of Bcr-Abl expression. Specific changes include loss of stress fibers and focal adhesions, which correlated with an adhesive defect. [corrected] Treatment of Bcr-Abl expressing cells with CGP57148, but not LY294002, resulted in reversion of cells to a near-normal phenotype, as assessed by immunofluorescence and attachment of Bcr-Abl transformed fibroblasts.

CONCLUSIONS

Our studies demonstrate that Bcr-Abl tyrosine kinase but not PI3- kinase activity is required for maintenance of cytoskeletal rearrangements resulting from Bcr-AbI expression. Further, inhibition of Abl kinase restored normal adhesive properties to the Bcr-Abl-expressing cells, demonstrating the contribution of Bcr-Abl kinase activity to abnormal cytoskeletal function.