Frequency of major molecular responses to imatinib or interferon alfa plus cytarabine in newly diagnosed chronic myeloid leukemia

Imatinib therapy in chronic myelogenous leukemia: strategies to avoid and overcome resistance

Imatinib is a molecularly targeted therapy that inhibits the oncogenic fusion protein BCR-ABL, the tyrosine kinase involved in the pathogenesis of chronic myelogenous leukemia (CML). Selective inhibition of BCR-ABL activity by imatinib has demonstrated efficacy in the treatment of CML, particularly in chronic phase. Some patients, however, primarily those with advanced disease, are either refractory to imatinib or eventually relapse. Relapse with imatinib frequently depends not only on re-emergence of BCR-ABL kinase activity but may also indicate BCR-ABL-independent disease progression not amenable to imatinib inhibition. Results from phase 2/3 trials suggest that rates of resistance and relapse correlate with the stage of disease and with the monitoring parameters--hematologic, cytogenetic and molecular response. These observations and more recent trials with imatinib, combined with insights provided by an increased understanding of the molecular mechanisms of resistance, have established the rationale for strategies to avoid and overcome imatinib resistance in the management of CML patients. To prevent resistance, early diagnosis and prompt treatment with appropriate initial dosing is essential. Management of resistance may include therapeutic strategies such as dose escalation to achieve individual optimal levels, combination therapy, as well as treatment interruption.

Clinical resistance to imatinib: mechanisms and implications

Although responses to imatinib in chronic phase chronic myelogenous leukemia have been durable in most cases, most patients in advanced disease develop resistance and relapse after a short duration of therapy. The mechanisms of drug resistance are diverse, but in most cases, mutations are found at the time of resistance that change amino acids within the kinase domain of BCR-ABL. Cytogenetic and molecular analyses at the time of resistance are suggested to guide therapy.

Optimizing Treatment of Chronic Myeloid Leukemia: A Rational Approach

Imatinib mesylate, a novel, molecularly targeted agent for the treatment of chronic myeloid leukemia (CML), has expanded the management options for this disease and provided a paradigm for the treatment of other cancers. Imatinib is a potent, specific inhibitor of BCR-ABL, the constitutively active protein tyrosine kinase critical to the pathogenesis of CML. A randomized, phase III comparison of imatinib with interferon-alfa plus cytarabine as initial treatment for newly diagnosed chronic-phase CML, which demonstrated significantly higher rates of disease response with less toxicity, better quality of life, and a significantly longer progression-free survival time, provided the most persuasive data supporting a major role for imatinib. Currently, allogeneic stem cell transplantation is the only treatment modality with long-term data demonstrating curative potential in CML. An option for less than half of CML patients and associated with substantial morbidity and mortality, transplantation may still be appropriate initial therapy for certain patients. Busulfan and hydroxyurea have no demonstrable effect on disease natural history. The interferon-plus-cytarabine combination can induce durable cytogenetic remissions and was previously the CML pharmacotherapy standard of care, but it is often poorly tolerated. Imatinib is now indicated as first-line therapy for CML in all phases.

Imatinib therapy in chronic myeloid leukemia

This article briefly recaps the key clinical trials that involve imatinib and focuses on the ongoing results and implications of these studies for future research in imatinib-based therapy in chronic myeloid leukemia (CML). Imatinib by no means has replaced allografting in the treatment of CML, although clearly, there has been a radical shift in thinking in considering which patients should undergo transplant. The route to cure with minimal toxicity may involve creative use of drug and transplant technologies.

Imatinib Mesylate in the Treatment of Chronic Myelogenous Leukemia

Imatinib mesylate binds to the inactive conformation of BCR-ABL tyrosine kinase, suppressing the Philadelphia chromosome-positive clone in chronic myelogenous leukemia (CML). Clinical studies of imatinib have yielded impressive results in the treatment of all phases of CML. With the higher rates of complete cytogenetic response with imatinib, molecular monitoring of disease has become mandatory in assessing response and determining prognosis. The practical aspects of the treatment of CML with imatinib are discussed. The emergence of imatinib resistance, albeit in a small percentage of patients, has prompted an evaluation of innovative treatment strategies.

Natural history and staging of chronic myelogenous leukemia

The natural history of chronic myelogenous leukemia (CML) has changed in recent years, partly due to earlier diagnosis but mostly as a consequence of the availability of effective therapies that have the potential to eradicate the Philadelphia chromosome-positive clone. Highly effective therapy with imatinib has changed the prognostic significance of clinical features traditionally associated with poor outcome. Achieving a complete cytogenetic response and a major molecular response early during the course of therapy with imatinib may be the most important factor in determining longterm outcome. Therefore, treatment modalities that increase the probability of achieving this goal should be pursued. This article describes the natural history of CML and its prognostic factors,with emphasis on changes due to the emergence of imatinib.

Growth autonomy and lineage switching in BCR-ABL-transduced human cord blood cells depend on different functional domains of BCR-ABL

The tyrosine kinase activity of p210BCR-ABL is essential to its leukemogenic potential, but the role of other functional domains in primary human hematopoietic cells has not been previously investigated. Here we show that infection of normal human CD34+ cord blood (CB) cells with a retroviral vector encoding p210BCR-ABL rapidly activates a factor-independent phenotype and autocrine interleukin-3/granulocyte colony-stimulating factor/erythropoietin production in the transduced cells. These changes are characteristic of primitive chronic myeloid leukemic (CML) cells and are important to the leukemogenicity of BCR-ABL-transduced murine hematopoietic stem cells. When BCR-ABL-transduced human CB cells were incubated with imatinib mesylate, an inhibitor of the p210BCR-ABL kinase, or when human CB cells were transduced with a BCR-ABL cDNA lacking the SH2 domain (p210DeltaSH2), factor independence was significantly reduced. In contrast, deletion of the SH2 domain had little impact on the p210BCR-ABL kinase-dependent promotion of erythropoietic differentiation also seen immediately following the BCR-ABL transduction of primitive human CB cells, but not in naturally occurring CML. Thus, p210BCR-ABL has distinct biological effects in primary human hematopoietic cells, which variably mimic features of human CML, and activation of these changes can show different dependencies on the integrity of the SH1 and SH2 domains of p210BCR-ABL.

The role of translation in neoplastic transformation from a pathologist's point of view

Increased cell proliferation, which is a hallmark of aggressive malignant neoplasms, requires a general increase in protein synthesis and a specific increase in the synthesis of replication-promoting proteins. Transient increase in the general protein synthesis rate, as well as preferential translation of specific mRNAs coding for growth promoting proteins (e.g. cyclin D1), takes place during normal mitogenic response. A number of extensively studied growth signal transduction pathways (Ras, PI3K, MAPK, mTOR-dependent pathways) activate the function and expression of various components of the translational machinery. In abnormal situations, constitutive activation of signal transduction pathways (e.g. oncogenic activation of Ras or Myc) leads to continuous upregulation of key elements of translational machinery. On the other hand, tumor suppressor genes (p53, pRb) downregulate ribosomal and tRNA synthesis, and their inactivation results in uncontrolled production of these translational components. During recent years, a significant effort has been dedicated to determining whether expression of translation factors is increased in human tumors using clinical biopsy specimens. The results of these studies indicate that expression of particular translation initiation factors is not always increased in human neoplasms. The pattern of expression is characteristic for a particular tumor type. For example, eIF-4E is usually increased in bronchioloalveolar carcinomas but not in squamous cell carcinomas of the lung. Interestingly, in certain highly proliferative and aggressive neoplasms (e.g. squamous cell carcinoma of the lung, melanoma), the expression of eIF-4E is barely detectable. These findings suggest that mechanisms for increasing general protein synthesis in various neoplasms differ significantly. Finally, the possibility of qualitative alterations in the translational machinery, rather than a simple increase in the activity of its components, is discussed along with the possibility of targeting those qualitative differences for tumor therapy.

Sustained cytogenetic response after discontinuation of imatinib mesylate in a patient with chronic myeloid leukaemia

A 58-yr-old woman was diagnosed with Ph(+) chronic myeloid leukaemia in May 2001. She was initially treated with hydroxyurea and subsequently with interferon-alpha (IFN-alpha). Imatinib mesylate was started in April 2002 after failure of IFN-alpha to induce a cytogenetic response. The patient remained on treatment with imatinib mesylate for 3 months during which she suffered daily fever resulting in discontinuation of the treatment. Response evaluation performed shortly after discontinuing imatinib mesylate revealed a complete cytogenetic remission and a substantial molecular response. Fifteen months later, she was still enjoying a major cytogenetic response. This case illustrates that a short course of imatinib mesylate may result in a sustained haematological and cytogenetic response.

High-dose imatinib mesylate therapy in newly diagnosed Philadelphia chromosome–positive chronic phase chronic myeloid leukemia

Imatinib mesylate (STI571) is effective in chronic phase chronic myelogenous leukemia (CML). However, most patients treated with 400 mg imatinib daily have variable levels of residual molecular disease. We treated 114 patients with newly diagnosed chronic phase CML with 400 mg imatinib twice daily. Overall, 109 patients (96%) had a major cytogenetic response (Philadelphia chromosome [Ph] < 35%), and 103 (90%) had a complete response (Ph 0%). With a median follow-up of 15 months, no patient has progressed to accelerated or blastic phase. The estimated 2-year survival rate was 94%. By quantitative polymerase chain reaction (QPCR) studies, 71 (63%) of 112 patients showed BCR-ABL/ABL percentage ratios decrease to less than 0.05%, and 31 (28%) to undetectable levels. Compared with standard-dose imatinib, high-dose imatinib was associated with significantly better complete cytogenetic response (P = .0005), major molecular response (QPRC < 0.05%; P = .00001), and complete molecular response (undetectable BCR-ABL; P = .001). High-dose imatinib was well tolerated but resulted in more frequent myelosuppression; 82% of patients continue to receive 600 mg or more of imatinib daily. In conclusion, high-dose imatinib induced higher rates of complete cytogenetic response and of molecular response in patients with newly diagnosed chronic phase CML. (Blood. 2004; 103:2873-2878)

The biology of CML blast crisis

Chronic myelogenous leukemia (CML) evolves from a chronic phase characterized by the Philadelphia chromosome as the sole genetic abnormality into blast crisis, which is often associated with additional chromosomal and molecular secondary changes. Although the pathogenic effects of most CML blast crisis secondary changes are still poorly understood, ample evidence suggests that the phenotype of CML blast crisis cells (enhanced proliferation and survival, differentiation arrest) depends on cooperation of BCR/ABL with genes dysregulated during disease progression. Most genetic abnormalities of CML blast crisis have a direct or indirect effect on p53 or Rb (or both) gene activity, which are primarily required for cell proliferation and survival, but not differentiation. Thus, the differentiation arrest of CML blast crisis cells is a secondary consequence of these abnormalities or is caused by dysregulation of differentiation-regulatory genes (ie, C/EBPalpha). Validation of the critical role of certain secondary changes (ie, loss of p53 or C/EBPalpha function) in murine models of CML blast crisis and in in vitro assays of BCR/ABL transformation of human hematopoietic progenitors might lead to the development of novel therapies based on targeting BCR/ABL and inhibiting or restoring the gene activity gained or lost during disease progression (ie, p53 or C/EBPalpha).

Imatinib produces significantly superior molecular responses compared to interferon alfa plus cytarabine in patients with newly diagnosed chronic myeloid leukemia in chronic phase

We analyzed molecular responses in 55 newly diagnosed chronic-phase chronic myeloid leukemia (CML) patients enrolled in a phase 3 study (the IRIS trial) comparing imatinib to interferon-alfa plus cytarabine (IFN+AraC). BCR-ABL/BCR% levels were measured by real-time quantitative RT-PCR and were significantly lower for the imatinib-treated patients at all time points up to 18 months, P<0.0001. The median levels for imatinib-treated patients continued to decrease and had not reached a plateau by 24 months. A total of 24 IFN+AraC-treated patients crossed over to imatinib. Once imatinib commenced, the median BCR-ABL/BCR% levels in these patients were not significantly different to those on first-line imatinib for the equivalent number of months. The incidence of progression in imatinib-treated patients, defined by hematologic, cytogenetic or quantitative PCR criteria, was significantly higher in the patients who failed to achieve a 1 log reduction by 3 months or a 2 log reduction by 6 months, P=0.002. A total of 49 patients were screened for BCR-ABL kinase domain mutations. Mutations were detected in two imatinib-treated patients who crossed over from IFN+AraC and both lost their imatinib response. In conclusion, first-line imatinib-treated patients had profound reductions in BCR-ABL/BCR%, which significantly exceeded those of IFN+AraC-treated patients and early measurements were predictive of subsequent response.

Reversal of bone marrow angiogenesis in chronic myeloid leukemia following imatinib mesylate (STI571) therapy

The effect of imatinib mesylate (imatinib) therapy on angiogenesis and myelofibrosis was investigated and compared with interferon (IFN) and hydroxyurea (HU) in 98 patients with newly diagnosed Philadelphia chromosome-positive/BCR-ABL(+) (Ph(+)/BCR-ABL(+)) chronic myeloid leukemia in first chronic phase and no other pretreatment. By means of immunostaining (CD34) and morphometry, a relationship between microvessel frequency and fiber density was detectable in initial bone marrow (BM) biopsies and sequential examinations after at least 8 months of therapy. First-line monotherapy with imatinib induced a significant reduction (normalization in comparison with controls) of microvessels and reticulin fibers. In most patients, decrease in BM vascularity was associated with a complete cytogenetic response. A significant anti-angiogenic effect was also observed after HU treatment, contrasting with IFN administration or combination regimens (IFN plus HU). In conclusion, our data support the anti-angiogenic capacity of imatinib by normalization of vascularity. In contrast, hematologic response following IFN treatment is independent from BM angiogenesis.

Chronic Myelogenous Leukemia and Myeloproliferative Disease

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

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

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

Mechanisms and implications of imatinib resistance mutations in BCR-ABL

PURPOSE OF REVIEW

Aside from bone marrow transplantation, a definitive cure for Philadelphia (Ph) chromosome-positive chronic myeloid leukemia (CML) has yet to be developed. Although Imatinib, the first molecularly targeted drug developed for CML has achieved a remarkable success, the emergence of resistance to this agent mitigates the prospect of a cure for this leukemia. Though a variety of resistance mechanisms can arise, in the majority of patients resistance coincides with reactivation of the tyrosine kinase activity of the BCR-ABL fusion oncoprotein. This can result from gene amplification and, more importantly, point mutations that disrupt the bind of imatinib to BCR-ABL itself. In this review, we aim to define and illuminate mechanisms of resistance and describe how drug resistance is shedding new light on kinase domain regulation.

RECENT FINDINGS

In light of recent studies and publications, it is now clear that Imatinib exerts its inhibitory action by stabilizing the inactive non ATP-binding conformation of BCR-ABL and that mutations even outside the kinase domain can lead to enhanced autophosphorylation of the kinase, thereby stabilizing the active conformation that resists imatinib binding. So far, 25 different substitutions of 21 amino acid residues of BCR-ABL have been detected in CML patients. In addition, it has been recently illustrated that mutations preexist the onset of treatment and that some confer a more aggressive disease phenotype. Finally it has been shown that molecular remission is almost never reached through Imatinib therapy.

SUMMARY

The most common mechanism of relapse for CML patients treated with Imatinib is the appearance of point mutations in the BCR-ABL oncogene that confer resistance to this drug. Insights into the emerging problem of resistance should promote the rational development of alternative, synergistic, and potentially curative treatment strategies.

Beyond chronic myelogenous leukemia

The myeloproliferative disorders (MPDs) are chronic malignant conditions originating from the clonal expansion of a multipotential hematopoietic stem cell. These diseases include polycythemia vera (PV), essential thrombocythenia, atypical chronic myeloid leukemia, idiopathic hypereosinophilic syndrome (HES), agnogenic myeloid metaplasia with myelofibrosis, and others. Receptor tyrosine kinases-the platelet-derived growth factor receptors (PDGFRs) and c-Kit-and their respective ligands have been implicated in the pathogenesis of MPDs. For example, a constitutively activated PDGFR fusion tyrosine kinase (FIP1L1-PDGFRA) was identified in some patients with HES, a disease characterized by sustained overproduction of eosinophils that has been classified by the World Health Organization as a chronic subtype of the MPDs. Imatinib is a selective inhibitor of PDGFRs, c-Kit, Abl and Arg protein-tyrosine kinases, as well as Bcr-Abl, the oncogenic tyrosine kinase that causes chronic myeloid leukemia. The efficacy of imatinib in treating HES, systemic mast cell disease, chronic myelomonocytic leukemia associated with PDGFRbeta fusion genes, and (to a lesser extent) PV and idiopathic myelofibrosis was reviewed from institutional experience and a review of the literature. In 3 studies that involved 11 patients with PV, 10 patients had reductions in phlebotomy with imatinib. Eight studies of 42 patients with HES indicated that 70% achieved complete hematologic remissions with imatinib. Four studies of 6 patients with MPD indicated responses with imatinib in 5 patients. Insight into the molecular pathogenesis of MPDs will improve the definitions of different disease categories and suggests that signal transduction inhibition is likely to be an increasingly important treatment option in the future.

Chronic myeloid leukemia—still a few questions

The study of chronic myeloid leukemia has yielded many insights, especially after the discovery of the Ph chromosome, into the pathogenesis of leukemia and other forms of malignant disease. Most recently, knowledge of the central function of the BCR-ABL fusion gene led to the development of a small molecule, imatinib, that has proved remarkably effective at reducing the number of leukemia cells in individual CML patients and promises to prolong life substantially in comparison with earlier treatments. However, many questions relating to this exciting new agent remain unanswered, for example, how exactly it works, how patients develop resistance and what can be done to prevent or delay its onset, and whether any patient can really be "cured" by its use.

Imatinib as a Paradigm of Targeted Therapies

Imatinib (Gleevec) exemplifies the successful development of a rationally designed, molecularly targeted therapy for the treatment of a specific cancer. This article reviews the identification of the BCR-ABL tyrosine kinase as a therapeutic target in chronic myeloid leukemia and the steps in the development of an agent to specifically inactivate this abnormality. The clinical trials results are reviewed along with a description of resistance mechanisms. As imatinib also inhibits the tyrosine kinase activity of KIT and the platelet-derived growth factor receptors, the extension of imatinib to malignancies driven by these kinases will be described. Issues related to clinical trials of molecularly targeted agents are discussed, including patient and dose selection. Last, the translation of this paradigm to other malignancies is explored.