The impact of clonal evolution on response to imatinib mesylate (STI571) in accelerated phase CML

Management of the advanced phases of chronic myelogenous leukemia in the era of tyrosine kinase inhibitors

Imatinib has revolutionised the management of chronic phase chronic myelogenous leukemia (CML). Unfortunately it has had less of an impact on the management of the advanced phases of CML. These historically difficult-to-treat phases of disease remain largely resistant to therapy. Even when responses are obtained with the tyrosine kinase inhibitors, they are brief, particularly in blast phase (BP) disease. Allogeneic stem cell transplantation is the only curative option for these patients and should be considered early as an integral part of the treatment plan. But transplant outcomes are dependent on cytogenetic and gross disease burden at the time of transplant. This review will compare and contrast the various tyrosine kinase- and non-tyrosine kinase inhibitor-based treatments for accelerated and BP CML before allogeneic transplantation.

Chronic myeloid leukaemia: the evolution of gene-targeted therapy

Chronic myeloid leukaemia (CML) was the first human cancer linked to an acquired chromosomal abnormality, subsequently shown to be a reciprocal translocation between chromosomes 9 and 22. The resulting fusion gene product, BCR-ABL, was shown to be the causative agent of the disease. CML has an incidence of around 1-2 cases per 100,000; in Australia, there are probably more than 200 new cases per year and more than 1300 prevalent cases. Treatment of CML with imatinib has been a powerful vindication of the concept of rational, gene-targeted drug design. Five-year published experience with imatinib at 400 mg orally daily demonstrates 89% overall survival and an estimated 93% freedom from disease progression. Adverse effects are mostly mild and transient. Higher doses of imatinib may be more efficacious and will be studied in upcoming clinical trials in Australia; however, imatinib is almost certainly not curative. Up to 28% of patients may have to stop imatinib because of intolerance or disease resistance, mostly due to point mutations of BCR-ABL. In this situation, many patients will respond to second- and third-generation tyrosine kinase inhibitors. Management of CML patients should involve close monitoring, especially in the first 2 years, with regular cytogenetics and quantitative polymerase chain reaction to optimise response and identify suboptimal responders as early as possible. Bone marrow transplantation remains the only known cure, but is reserved for patients whose kinase inhibitor therapy has failed, or who have advanced disease (accelerated phase or blastic transformation).

Part I: mechanisms of resistance to imatinib in chronic myeloid leukaemia

The introduction of selective tyrosine-kinase inhibitors (TKIs) for the treatment of chronic myeloid leukaemia has changed patient outcome and, consequently, management of this disease. Imatinib is now the treatment of choice for most newly diagnosed patients. Excellent responses, in terms of symptom control and haematological parameters, are usually obtained. However, failure to completely eradicate leukaemic cells and the escape of these cells from previous control has led to an intensive search for the mechanisms of resistance and subsequent treatments by which to overcome this resistance. Up to now, there has been considerable focus on the role of ABL-kinase-domain mutations as mediators of resistance to imatinib, thereby encouraging the development of a second generation of TKIs capable of inhibiting these mutant proteins. However, studies have increasingly shown that these mutations do not account for all cases of resistance and have a negligible role in the inability of TKIs to eradicate residual disease in patients who are good responders. More recently, attention has turned to the relative roles of drug bioavailability and drug efflux and drug influx proteins in the development of resistance to imatinib. This review is the first of two papers and discusses imatinib resistance and its potential causes. The second paper will focus on the assessment and subsequent management of patients with less than optimum responses to imatinib.

Constitutive overexpression of P-glycoprotein, rather than breast cancer resistance protein or organic cation transporter 1, contributes to acquisition of imatinib-resistance in K562 cells

PURPOSE

The purpose of this study was to investigate the contribution of drug transporters in acquired imatinib-resistance. Specifically, we focused on the efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), and an influx transporter, organic cation transporter 1 (OCT1).

MATERIALS AND METHODS

We established imatinib-resistant K562 cells (K562/IM). Real-time PCR or Western blot analyses were performed to examine the mRNA or protein levels. Alamar blue method was used in the cytotoxicity assay. The transport activities and intracellular imatinib levels were measured by flow cytometry and HPLC, respectively.

RESULTS

K562/IM displayed a 47-fold increase in resistance to imatinib over the parent K562 cells. Both P-gp and BCRP were overexpressed in K562/IM relative to K562. Furthermore, the intracellular imatinib level was markedly reduced in K562/IM. Interestingly, cyclosporin A, a P-gp inhibitor, but not fumitremorgin C, a BCRP inhibitor, restored both imatinib-sensitivity and the intracellular imatinib level. By contrast, no significant difference in the expression and function of OCT1 was observed between K562/IM and K562.

CONCLUSIONS

P-gp, rather than BCRP or OCT1, is partially responsible for the development of imatinib-resistance due to constitutive and functional overexpression, leading to reduced intracellular accumulation of imatinib in K562/IM.

Targeting immunosupportive cancer therapies: accentuate the positive, eliminate the negative

In this Commentary we aim to provide an overview of some specific examples of cancer therapeutics, including targeted approaches using monoclonal antibodies and kinase inhibitors, as well as highlight novel approaches for enhancing immunological responses against tumors. We point out that a fundamental property of the cancer cell, genomic instability, confounds the targeted therapies that aim to induce cell death directly while simultaneously enhancing the potential for immunological attack by creating a large number of neoantigens. We argue for combinatorial strategies with agents that target tumor cells to release these antigens together with innovative therapies that enhance immunological responses by interfering with inhibitory checkpoints.

Chromosomal abnormalities in Philadelphia chromosome negative metaphases appearing during imatinib mesylate therapy in patients with newly diagnosed chronic myeloid leukemia in chronic phase

The development of chromosomal abnormalities (CAs) in the Philadelphia chromosome (Ph)-negative metaphases during imatinib (IM) therapy in patients with newly diagnosed chronic myecloid leukemia (CML) has been reported only anecdotally. We assessed the frequency and significance of this phenomenon among 258 patients with newly diagnosed CML in chronic phase receiving IM. After a median follow-up of 37 months, 21 (9%) patients developed 23 CAs in Ph-negative cells; excluding -Y, this incidence was 5%. Sixteen (70%) of all CAs were observed in 2 or more metaphases. The median time from start of IM to the appearance of CAs was 18 months. The most common CAs were -Y and + 8 in 9 and 3 patients, respectively. CAs were less frequent in young patients (P = .02) and those treated with high-dose IM (P = .03). In all but 3 patients, CAs were transient and disappeared after a median of 5 months. One patient developed acute myeloid leukemia (associated with - 7). At last follow-up, 3 patients died from transplantation-related complications, myocardial infarction, and progressive disease and 2 lost cytogenetic response. CAs occur in Ph-negative cells in a small percentage of patients with newly diagnosed CML treated with IM. In rare instances, these could reflect the emergence of a new malignant clone.

Identification of BCR-ABL point mutations conferring resistance to the Abl kinase inhibitor AMN107 (nilotinib) by a random mutagenesis study

Patients with advanced stages of chronic myeloid leukemia (CML) often manifest imatinib mesylate resistance associated with point mutations in BCR-ABL. AMN107 is a new higher-potency inhibitor of BCR-ABL. To identify mutations in BCR-ABL that could result in resistance to AMN107, a cDNA library of BCR-ABL mutants was introduced into Ba/F3 cells followed by selection in AMN107 (0.125-0.5 μM). A total of 86 individual, drug-resistant colonies were recovered, and the SH3, SH2, and kinase domains of BCR-ABL were sequenced. A total of 46 colonies had single point mutations in BCR-ABL, with a total of 17 different mutations, all within the kinase domain. The other 40 colonies had multiple point mutations and were not analyzed further. Each of the 17 single point mutants were reconstructed by site-directed mutagenesis of native BCR-ABL and found to be approximately 2.5- to 800-fold more resistant to AMN107 than native BCR-ABL. The mutations included 6 known imatinib mesylate–resistant mutations, including T315I, which showed complete resistance to AMN107. Interestingly, most AMN107-resistant mutants were also resistant to imatinib mesylate. These results may predict some of the resistance mutations that will be detected in clinical trials with this kinase inhibitor.

Resistance to Targeted Therapy in Chronic Myelogenous Leukemia

The advent of the Bcr-Abl selective tyrosine kinase inhibitor imatinib mesylate (Glivec, Gleevec, Novartis, East Hanover, NJ) has substantially changed the treatment landscape for chronic myelogenous leukemia (CML). However, some patients, primarily those with advanced disease, are either initially refractory to imatinib or eventually develop imatinib resistance. Imatinib resistance or intolerance frequently depends on the re-emergence of Bcr-Abl kinase activity, but can also indicate Bcr-Abl-independent disease progression. Results from phase II/III trials suggest rates of resistance and relapse correlate with stage of disease and with the monitoring parameters: hematologic, cytogenetic, and molecular responses. To date, more than 40 different point mutations that code for distinct single amino acid substitutions in the Bcr-Abl kinase domain have been isolated from imatinib-resistant patients. These mutations affect amino acids involved in imatinib binding or in regulatory regions of the Bcr-Abl kinase domain, resulting in decreased sensitivity to imatinib while retaining aberrant kinase activity. Early mutation detection may aid in risk stratification and molecular-based treatment decisions. To overcome imatinib-resistant disease, novel tyrosine kinase inhibitors with activity against imatinib-resistant mutations and/or with inhibition of alternative pathways, such as Src activation, have recently been developed. Additional strategies include imatinib dose escalation, combination therapy, and treatment interruption to stop clonal selection of resistant cells.

Beneficial effects of combining nilotinib and imatinib in preclinical models of BCR-ABL+ leukemias

Drug resistance resulting from emergence of imatinib-resistant BCR-ABL point mutations is a significant problem in advanced-stage chronic myelogenous leukemia (CML). The BCR-ABL inhibitor, nilotinib (AMN107), is significantly more potent against BCR-ABL than imatinib, and is active against many imatinib-resistant BCR-ABL mutants. Phase 1/2 clinical trials show that nilotinib can induce remissions in patients who have previously failed imatinib, indicating that sequential therapy with these 2 agents has clinical value. However, simultaneous, rather than sequential, administration of 2 BCR-ABL kinase inhibitors is attractive for many reasons, including the theoretical possibility that this could reduce emergence of drug-resistant clones. Here, we show that exposure of a variety of BCR-ABL+ cell lines to imatinib and nilotinib results in additive or synergistic cytotoxicity, including testing of a large panel of cells expressing BCR-ABL point mutations causing resistance to imatinib in patients. Further, using a highly quantifiable bioluminescent in vivo model, drug combinations were at least additive in antileukemic activity, compared with each drug alone. These results suggest that despite binding to the same site in the same target kinase, the combination of imatinib and nilotinib is highly efficacious in these models, indicating that clinical testing of combinations of BCR-ABL kinase inhibitors is warranted.

Bcr-Abl resistance screening predicts a limited spectrum of point mutations to be associated with clinical resistance to the Abl kinase inhibitor nilotinib (AMN107)

In advanced-phase chronic myeloid leukemia (CML), resistance to imatinib mesylate is associated with point mutations in the BCR-ABL kinase domain. A new generation of potent ABL kinase inhibitors is undergoing clinical evaluation. It is important to generate specific resistance profiles for each of these compounds, which could translate into combinatorial and sequential treatment strategies. Having characterized nilotinib (AMN107) against a large panel of imatinib mesylate–resistant Bcr-Abl mutants, we investigated which mutants might arise under nilotinib therapy using a cell-based resistance screen. In contrast to imatinib mesylate, resistance to nilotinib was associated with a limited spectrum of Bcr-Abl kinase mutations. Among these were mutations affecting the P-loop and T315I. Rarely emerging resistant colonies at a concentration of 400 nM nilotinib exclusively expressed the T315I mutation. With the exception of T315I, all of the mutations that were identified were effectively suppressed when the nilotinib concentration was increased to 2000 nM, which falls within the peak-trough range in plasma levels (3.6-1.7 μM) measured in patients treated with 400 mg twice daily. Our findings suggest that nilotinib might be superior to imatinib mesylate in terms of the development of resistance. However, our study indicates that clinical resistance to nilotinib may be associated with the predominant emergence of T315I.

Role of Allogeneic Stem Cell Transplantation for Adult Chronic Myeloid Leukemia in the Imatinib Era

Due to superior survival in the short to medium term, the first-generation ABL kinase inhibitor imatinib mesylate has generally supplanted all other therapies as the initial treatment of choice in chronic phase chronic myeloid leukemia. The role of allogeneic stem cell transplantation (alloSCT) has shifted from a preferred first-line therapy to a possible second- or third-line therapy. However, despite generally excellent responses to imatinib, some patients respond poorly or lose response, and the risk-benefit equation in these cases may rapidly shift in favor of the alloSCT option. These patients need to be identified as soon as possible so that the alloSCT option can be applied while they are still in controlled chronic phase. Monitoring of imatinib response in patients who have suitable donors and are potentially eligible for alloSCT needs to be frequent, sensitive, and accurate. Clear criteria for switching from imatinib therapy to the alloSCT option should be established for each patient according to the specific risk profile of the transplant. The potential efficacy and safety of clinical trials combining reduced intensity alloSCT with ABL kinase inhibitor therapy warrants further consideration.

Limited clinical value of regular bone marrow cytogenetic analysis in imatinib-treated chronic phase CML patients monitored by RQ-PCR for BCR-ABL

Real-time quantitative polymerase chain reaction (PCR) for BCR-ABL mRNA in the peripheral blood (RQ-PCR) provides an accurate and reliable measure of response to therapy in chronic myeloid leukaemia (CML). We wanted to determine in what circumstances additional clinically relevant information was provided by simultaneous cytogenetic analysis in RQ-PCR monitored patients receiving imatinib treatment. We analysed 828 simultaneous RQ-PCR and bone marrow cytogenetic analyses from 183 patients with chronic phase CML with a median follow-up of 20 months. Cytogenetic progression was defined as Philadelphia (Ph)-positive clonal evolution, loss of complete cytogenetic response or an increase of > or = 20% Ph-positive cells. Cytogenetic progression occurred in 24/183 (13%) patients. At the time of cytogenetic progression, none of the 24 patients had a major molecular response (MMR; > or = 3-log reduction in BCR-ABL from standardised baseline). There were 320 RQ-PCR results from 95 patients indicating MMR. No abnormality was detected in any of the corresponding cytogenetic analyses. A policy of regular RQ-PCR monitoring with cytogenetic analysis targetted only to patients who have not achieved, or have lost MMR would represent a rational approach to monitoring and spare most patients the discomfort of multiple marrow aspirates. This approach depends upon availability of an accurate, reproducible RQ-PCR assay with ongoing quality assurance.

ABL Kinase Inhibitor Therapy for CML: Baseline Assessments and Response Monitoring

For chronic phase chronic myeloid leukemia (CML) patients treated with imatinib, the essential pre-therapy assessments include bone marrow morphology and cytogenetics as well as a baseline real-time quantitative PCR (RQ-PCR) for BCR-ABL. Baseline assessments of clinical relevance include Sokal and Hasford prognostic scores. Several other baseline assays are likely to be predictive of response, including IC50imatinib, organic cation transporter-1 (OCT-1) mRNA level, and gene expression profiles, but further confirmation is required. RQ-PCR assays of blood at least every 3 months once patients have commenced imatinib is recommended. This will facilitate early identification of suboptimal responders who may benefit from higher doses of imatinib or alternative therapy, and identify at an early stage patients with acquired resistance. Management of the latter group can be further guided by the findings from cytogenetics and BCR-ABL kinase domain mutation screening. Bone marrow cytogenetics is indicated at least every 6 months until the patient achieves major molecular response. RQ-PCR is only clinically useful if it is conducted under a rigorous quality control regimen so that fluctuations in the BCR-ABL level can be confidently attributed to a biological cause rather than assay variation. To further improve the clinical value of RQ-PCR monitoring, expression of results on an international scale is needed. This will facilitate a more uniform and rational approach to management of suboptimal response and loss of response.

Carcinogenesis, cancer therapy and chemoprevention

Carcinogenesis and cancer therapy are two sides of the same coin, such that the same cytotoxic agent can cause cancer and be used to treat cancer. This review links carcinogenesis, chemoprevention and cancer therapy in one process driven by cytotoxic agents (carcinoagents) that select either for or against cells with oncogenic alterations. By unifying therapy and cancer promotion and by distinguishing nononcogenic and oncogenic mechanisms of resistance, I discuss anticancer- and chemopreventive agent-induced carcinogenesis and tumor progression and, vice versa, carcinogens as anticancer drugs, anticancer drugs as chemopreventive agents and exploiting oncogene-addiction and drug resistance for chemoprevention and cancer therapy.

Inv(1)(p22q25) in chronic myeloproliferative disease: constitutional or clonal defect?

Pericentric inversion of chromosome 1 is uncommon in chronic myeloproliferative disease. We report the occurrence of an isolated inv(1)(p22q25) in two patients with BCR/ABL-negative chronic myeloproliferative disease. The inv(1) is an acquired clonal abnormality in one patient and a constitutional defect in the other. To our knowledge, this is the first report on the occurrence of inv(1)(p22q25) in hematolymphoid malignancies.

Characterization of AMN107, a selective inhibitor of native and mutant Bcr-Abl

The Bcr-Abl tyrosine kinase oncogene causes chronic myelogenous leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL). We describe a novel selective inhibitor of Bcr-Abl, AMN107 (IC50 <30 nM), which is significantly more potent than imatinib, and active against a number of imatinib-resistant Bcr-Abl mutants. Crystallographic analysis of Abl-AMN107 complexes provides a structural explanation for the differential activity of AMN107 and imatinib against imatinib-resistant Bcr-Abl. Consistent with its in vitro and pharmacokinetic profile, AMN107 prolonged survival of mice injected with Bcr-Abl-transformed hematopoietic cell lines or primary marrow cells, and prolonged survival in imatinib-resistant CML mouse models. AMN107 is a promising new inhibitor for the therapy of CML and Ph+ ALL.

PD166326, a novel tyrosine kinase inhibitor, has greater antileukemic activity than imatinib mesylate in a murine model of chronic myeloid leukemia

Imatinib mesylate is highly effective in newly diagnosed chronic myeloid leukemia (CML), but BCR/ABL (breakpoint cluster region/abelson murine leukemia)-positive progenitors persist in most patients with CML treated with imatinib mesylate, indicating the need for novel therapeutic approaches. In this study, we have used the murine CML-like myeloproliferative disorder as a platform to characterize the pharmacokinetic, signal transduction, and antileukemic properties of PD166326, one of the most potent members of the pyridopyrimidine class of protein tyrosine kinase inhibitors. In mice with the CML-like disease, PD166326 rapidly inhibited Bcr/Abl kinase activity after a single oral dose and demonstrated marked antileukemic activity in vivo. Seventy percent of PD166326-treated mice achieved a white blood cell (WBC) count less than 20.0 x 10(9)/L (20,000/microL) at necropsy, compared with only 8% of imatinib mesylate-treated animals. Further, two thirds of PD166326-treated animals had complete resolution of splenomegaly, compared with none of the imatinib mesylate-treated animals. Consistent with its more potent antileukemic effect in vivo, PD166326 was also superior to imatinib mesylate in inhibiting the constitutive tyrosine phosphorylation of numerous leukemia-cell proteins, including the src family member Lyn. PD166326 also prolonged the survival of mice with imatinib mesylate-resistant CML induced by the Bcr/Abl mutants P210/H396P and P210/M351T. Altogether, these findings demonstrate the potential of more potent Bcr/Abl inhibitors to provide more effective antileukemic activity. Clinical development of PD166326 or a related analog may lead to more effective drugs for the treatment of de novo and imatinib mesylate-resistant CML.

Management of early stage disease

More than 80% of newly diagnosed patients with chronic myeloid leukemia in chronic phase will achieve a complete cytogenetic response (CCR) with the standard dose of 400 mg imatinib daily. The probability of progression free survival is tightly correlated with the level of response, approaching 100% in those patients who achieve a reduction of BCR-ABL mRNA by at least 3-log at 12 months. High Sokal risk predicts poorer outcome, but on-treatment response parameters generally override pretherapeutic prognostic variables. Standard disease monitoring includes full blood counts, cytogenetics and quantitative RT-PCR for BCR-ABL mRNA but must be tailored to the level of response attained by a given patient. Conservative therapeutic milestones include a complete hematologic response at 3 months, a minor cytogenetic response at 6, a major cytogenetic response at 12 and CCR at 18, but a more aggressive approach may be justified in specific circumstances. Failure to achieve any of these milestones should trigger a re-assessment of the therapeutic strategy. Most patients with CCR remain positive by RT-PCR, and discontinuation of drug is usually followed by relapse, suggesting that imatinib fails to eradicate leukemic stem cells. The mechanisms underlying disease persistence are not well understood. Evidence is accumulating that early therapy intensification using high doses of imatinib (800 mg daily) or imatinib in combination with cytarabine or interferon-alpha may induce higher rates of RT-PCR negativity. Large studies will be required to determine whether this translates into improved progression free and overall survival.

Cytogenetic clonal evolution alone in CML relapse post-transplantation does not adversely affect response to imatinib mesylate treatment

Good prognosis after imatinib mesylate treatment has been reported if cytogenetic clonal evolution (CE) is the only criterion of accelerated phase (AP) chronic myelogenous leukemia (CML). To evaluate the impact of CE upon imatinib treatment in post-transplant settings, responses and toxicities in the relapsed AP-CE were analyzed in comparison with those in the relapsed chronic phase (CP). Both CP (n=7) and AP-CE patients (n=6) received imatinib mesylate in an oral dose of 400 mg/day. Complete cytogenetic responses were obtained in six patients of each group, CP (86%) and AC-CE (100%), while molecular remission was seen in 43 and 50%, respectively. Granulocytopenia or thrombocytopenia of grade III or more occurred in four (57%) and two (33%) patients with CP and AP-CE, respectively. Nonhematological adverse events were mild and tolerable in both groups and only one (7%) of the 13 patients experienced recurrent graft-versus-host disease after imatinib treatment. Although this is a relatively small group of patients, we suggest that imatinib mesylate should be considered as a front-line treatment for relapsed CML as it showed the high response rate and low toxicity. We also suggest that CE alone is not an important factor in the induction of cytogenetic and molecular remissions in post-transplant relapse.

Clonal evolution in chronic myelogenous leukemia

Clonal evolution (CE) may be a marker of disease progression in chronic myelogenous leukemia (CML) and is thought to reflect the genetic instability of the highly proliferative CML progenitors. The frequency of CE increases with advancing stage, rising from 30%in accelerated phase and up to 80% in blast crisis. Given its association with disease progression, CE is considered a feature that defines accelerated-phase CML; however, not all studies have demonstrated a uniformly poor outcome for patients with CE. Chromosomal abnormalities in Ph chromosome negative metaphases increasingly have been recognized in patients treated with imatinib. The true incidence of this phenomenon is not clear but appears to occur in 2% to 17% of imatinib-treated patients. Regardless of the precise mechanism and the long-term clinical implications, the findings described in this article underscore the importance of routine cytogenetic analysis for patients treated with imatinib. The continued study of these phenomena will help us to understand better the pathogenesis of CML and improve the long-term treatment of patients with CML.

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.

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.

Resistance to imatinib (Glivec): update on clinical mechanisms

Imatinib mesylate, an orally administered 2-phenylaminopyrimidine derivative that inhibits BCR/ABL tyrosine kinase activity, has shown great promise in the treatment of chronic myelogenous leukemia (CML). This small molecule, tyrosine kinase inhibitor, has also been shown to be effective against metastatic gastrointestinal stromal tumors (GISTs) expressing the stem cell factor (SCF) receptor kit. However, the threat of resistance in patients has prompted investigators to uncover the mechanisms whereby malignant cells develop resistance to imatinib, and has also led to the establishment of strategies designed to over-ride imatinib resistance. Here, we provide a comprehensive overview of the effectiveness of imatinib in the treatment of chronic, accelerated and blast crisis-phase CML, Philadelphia chromosome-positive (Ph+) acute lymphoid leukemia (ALL) and metastatic GIST. Established mechanisms of resistance to imatinib are discussed, as are novel therapeutic approaches to improving drug responsiveness by reversing development of imatinib resistance in patients.

Additional cytogenetic abnormalities in adults with Philadelphia chromosome-positive acute lymphoblastic leukaemia: a study of the Cancer and Leukaemia Group B

We analysed the nature and prognostic significance of secondary cytogenetic changes in 111 newly diagnosed adults with acute lymphoblastic leukaemia (ALL) and t(9;22)(q34;q11.2) or its variants. Secondary aberrations were seen in 75 (68%) patients. They included, in order of descending frequency: +der(22)t(9;22), +21, abnormalities of 9p, high hyperdiploidy (>50 chromosomes), +8, -7, +X and abnormalities resulting in loss of material from 8p, gain of 8q, gain of 1q and loss of 7p. Eighty patients (72%) had > or =1 normal metaphase in their karyotype. There were four balanced and 12 unbalanced translocations previously unreported in ALL with t(9;22). The t(2;7)(p11;p13) and der(18)t(8;18)(q11.2;p11.2) were seen in two cases each, and have never before been reported in haematological malignancy. All but four patients were treated on front-line Cancer and Leukaemia Group B clinical protocols. The presence of -7 as a sole secondary abnormality was associated with a lower complete remission (CR) rate (P = 0.004), while the presence of > or =3 aberrations was associated with a higher CR rate (P = 0.009) and +der(22)t(9;22) with a higher cumulative incidence of relapse (P = 0.02). It will be of interest to see if newly diagnosed t(9;22)-positive adult ALL patients with these and other secondary aberrations respond differently to treatment regimens that include imatinib mesylate.

Results of imatinib mesylate therapy in chronic myelogenous leukaemia with variant Philadelphia chromosome

Five to 10 per cent of patients with Philadelphia chromosome (Ph)-positive chronic myelogenous leukaemia (CML) have variant translocations involving chromosomes other than 9 and 22. We investigated the characteristics and outcome of patients with variant translocations treated with imatinib. Among 721 patients, 44 (6%) had variant translocations, involving one (n = 39) or two (n = 4) additional chromosomes. Nineteen patients (44%) were in chronic (12 previously untreated), 24 (55%) in accelerated and one (2%) in blastic phase. A major cytogenetic response was achieved in 14 (74%) patients treated in chronic phase and in 14 (58%) treated in accelerated phase. Six of 13 (46%) evaluable patients had deletion of derivative chromosome 9, and there was a trend for a lower response rate in these patients. We compared the 43 patients in chronic or accelerated phase to 678 patients with classic Ph treated with imatinib. The only significant difference in clinical characteristics was a higher frequency of accelerated phase among those with variant translocations (56%) compared with those with classic translocations (38%). No differences in outcome were evident. In a multivariate analysis, variant Ph translocations had no impact in response rate, overall survival or duration of response. We conclude that patients with variant Ph translocations have a similar prognosis to those with classic Ph translocations when treated with imatinib.

Imatinib Mesylate Resistance Through BCR-ABL Independence in Chronic Myelogenous Leukemia

Imatinib mesylate (IM) binds to the BCR-ABL protein, inhibiting its kinase activity and effectively controlling diseases driven by this kinase. IM resistance has been associated with kinase mutations or increased BCR-ABL expression. However, disease progression may be mediated by other mechanisms that render tumor cells independent of BCR-ABL. To demonstrate this potential, IM-resistant cells were found in chronic myelogenous leukemia patients with continuous BCR-ABL gene expression but undetectable BCR-ABL protein expression. These cells were unresponsive to IM and acquired BCR-ABL-independent signaling characteristics. IM resistance in some patients may be mediated through loss of kinase target dependence.

Apoptosis induced by molecular targeting therapy in hematological malignancies

Molecular targeting therapies for hematological malignant diseases such as monoclonal antibodies and small molecules have been reviewed. Imatinib mesylate (STI571) targets the tyrosine kinase activity of the BCR-ABL fusion protein in CML, and was superior to IFN-alpha plus low-dose cytarabine in newly diagnosed chronic-phase CML in a phase III randomized study. Imatinib induced apoptosis in BCR-ABL-positive cells in vitro, and activates several signaling pathways such as PI3K/Akt, STAT5 and Ras/MAPK. Combination therapies with imatinib and new strategies for downregulation of intracellular BCR-ABL protein levels have also been investigated from the phenomenon of resistance to imatinib. Anti-CD20 (rituximab) became the first monoclonal antibody approved for the treatment of a relapsed/refractory follicular/low-grade NHL and promising results were obtained from a phase III randomized study. Although antibody-dependent cell-mediated cytotoxicity and complement-mediated cytotoxicity are likely to be the major effectors of B-cell depletion in vivo, direct cytotoxicity by CD20 monoclonal antibody on B-cell lines in vitro has been reported. Anti-CD33 (Mylotarg) and FLT3 inhibitors for AML have also been used in clinical trials and signaling pathways induced by these agents are under intensive investigation. Arsenic trioxide, like all-TRANS-retinoic acid (ATRA), downregulates promyelocytic leukemia protein/retinoic acid receptor-alpha (PML/RARalpha) fusion protein and induced apoptosis in APL cells, and promising results were obtained from ATRA-resistant APL patients. Finally we show our promising in vitro and in vivo data of R-etodolac (a non-steroidal anti-inflammatory drug lacking cyclooxygenase inhibitor activity) against chronic lymphocytic leukemia (CLL) cells.

Clonal evolution and lack of cytogenetic response are adverse prognostic factors for hematologic relapse of chronic phase CML patients treated with imatinib mesylate

We followed 141 patients treated with imatinib mesylate (&gt; 300 mg) for chronicphase chronic myelogenous leukemia (CML) following failure of treatment with interferon. During 12 months from the start of imatinib mesylate treatment, 96.5% achieved a complete hematologic response, 47.0% achieved a major cytogenetic response, and 32.4% achieved a complete cytogenetic response. The proportion of patients with hematologic relapse was 10.9% at 12 months and 14.6% at 18 months. In a univariate Cox regression analysis, the only pretreatment characteristics that correlated with an increased risk of hematologic relapse were hemoglobin level less than 120 g/L (12 g/dL) (P = .02), increased bands in the peripheral blood (P = .01), and clonal evolution (P &lt; .0001). In a multivariate analysis, an elevated platelet count (P = .03) and clonal evolution (P &lt; .0001) were the only significant factors for hematologic relapse. During treatment, the absence of a major cytogenetic response within the first 6 months also significantly correlated with relapse (P = .03). Notably, patients failing to achieve a major cytogenetic response by 6 months had a significantly higher rate of hematologic relapse (27%) compared with those who achieved a major cytogenetic response by 6 months (3%), and patients with clonal evolution had a significantly higher risk of hematologic relapse (50%) than those without clonal evolution (9%).

A novel mechanism for imatinib mesylate-induced cell death of BCR-ABL-positive human leukemic cells: caspase-independent, necrosis-like programmed cell death mediated by serine protease activity

Caspase-independent programmed cell death can exhibit either an apoptosis-like or a necrosis-like morphology. The ABL kinase inhibitor, imatinib mesylate, has been reported to induce apoptosis of BCR-ABL-positive cells in a caspase-dependent fashion. We investigated whether caspases alone were the mediators of imatinib mesylate-induced cell death. In contrast to previous reports, we found that a broad caspase inhibitor, zVAD-fmk, failed to prevent the death of imatinib mesylate-treated BCR-ABL-positive human leukemic cells. Moreover, zVAD-fmk-preincubated, imatinib mesylate-treated cells exhibited a necrosis-like morphology characterized by cellular pyknosis, cytoplasmic vacuolization, and the absence of nuclear signs of apoptosis. These cells manifested a loss of the mitochondrial transmembrane potential, indicating the mitochondrial involvement in this caspase-independent necrosis. We excluded the participation of several mitochondrial factors possibly involved in caspase-independent cell death such as apoptosis-inducing factor, endonuclease G, and reactive oxygen species. However, we observed the mitochondrial release of the serine protease Omi/HtrA2 into the cytosol of the cells treated with imatinib mesylate or zVAD-fmk plus imatinib mesylate. Furthermore, serine protease inhibitors prevented the caspase-independent necrosis. Taken together, our results suggest that imatinib mesylate induces a caspase-independent, necrosis-like programmed cell death mediated by the serine protease activity of Omi/HtrA2.

Chromosomal abnormalities in Philadelphia chromosome-negative metaphases appearing during imatinib mesylate therapy in patients with Philadelphia chromosome-positive chronic myelogenous leukemia in chronic phase

BACKGROUND

Anecdotal cases of chromosomal abnormalities in Philadelphia chromosome (Ph)-negative metaphases have been reported in patients with chronic myelogenous leukemia (CML) in the chronic phase during treatment with interferon and, more recently, with imatinib. This phenomenon is different from true clonal evolution in that the additional cytogenetic abnormality occurs in Ph-negative cells.

METHODS

The authors analyzed their experience with 342 patients with CML in chronic phase treated with imatinib to investigate the frequency and significance of this event.

RESULTS

After a median follow-up of 30 months (range, 16-35 months), 21 patients (6%; 95% confidence interval, 0.04, 0.09) developed 25 chromosomal abnormalities in Ph-negative cells. Thirteen (54%) of these abnormalities were seen in 2 or more metaphases. The median time from the start of treatment with imatinib to the appearance of the abnormalities was 6 months (range, 3-22 months). The most common cytogenetic abnormality detected was trisomy 8 (33%). Twenty of 21 patients (95%) achieved a major (Ph < 35%) cytogenetic response (complete cytogenetic response in 13-62%). After a median follow-up of 22 months (range, 4-33 months), all 21 patients were alive, 20 of them in chronic phase and in complete hematologic response. None of the patients showed features of myelodysplasia.

CONCLUSIONS

Cytogenetic abnormalities occur in Ph-negative cells in a fraction of patients with CML in chronic phase treated with imatinib. With a short follow-up, no clear clinical consequences can be identified.

Transplantation for chronic myelogenous leukemia: yes, no, maybe so. . . An Oregon perspective

Chronic myelogenous leukemia (CML) is a hematopoietic stem cell disorder in which allogeneic stem cell transplantation remains the only curative option, but its use is limited by donor availability and treatment-related morbidity and mortality. Imatinib mesylate is a targeted agent for CML with efficacy to date, which is superior to all other nontransplant therapy and has limited toxicity. The curative potential of imatinib remains to be proven and may be limited to a small number of patients. Optimal decision making regarding the use of these divergent therapies has not been defined. This paper reviews critical data relevant to these treatment options and provides an approach to current management of the CML patient.

Chronic myelogenous leukemia

The treatment options for chronic myelogenous leukemia (CML) continue to evolve rapidly. Imatinib mesylate (Gleevec, Glivec, formerly STI571) has continued to show remarkable clinical benefits and the updated results with this agent are reviewed. As relapses using single agent imatinib have occurred, particularly in advanced phase patients, the issue of whether combinations of other antileukemic agents with imatinib may yield improved results is addressed. In addition, data on new agents that have potential in the treatment of CML are reviewed. These agents are presented in the context of their molecular mechanism of action. The most recent data for stem cell transplantation, along with advances in nonmyeloablative transplants, are also reviewed. In Section I, Drs. Stephen O'Brien and Brian Druker update the current status of clinical trials with imatinib and review ongoing investigations into mechanisms of resistance and combinations of imatinib with other agents. They also present their views on integration of imatinib with other therapies. In Section II, Dr. Jorge Cortes describes the most recent data on novel therapies for CML, including farnesyl transferase inhibitors, arsenic trioxide, decitabine, and troxatyl, among others. These agents are discussed in the context of their molecular mechanism of action and rationale for use. In Section III, Dr. Jerald Radich updates the results of stem cell transplants for CML, including emerging data on nonmyeloablative transplants. He also presents data on using microarrays to stratify patients into molecularly defined risk groups.

Detection of BCR-ABL mutations in patients with CML treated with imatinib is virtually always accompanied by clinical resistance, and mutations in the ATP phosphate-binding loop (P-loop) are associated with a poor prognosis

Imatinib-treated chronic myeloid leukemia (CML) patients with acquired resistance commonly have detectable BCR-ABL kinase domain mutations. It is unclear whether patients who remain sensitive to imatinib also have a significant incidence of mutations. We evaluated 144 patients treated with imatinib for BCR-ABL kinase domain mutations by direct sequencing of 40 accelerated phase (AP), 64 late chronic phase (> or = 12 months from diagnosis, late-CP), and 40 early-CP patients. Mutations were detected in 27 patients at 17 different residues, 13 (33%) of 40 in AP, 14 (22%) of 64 in late-CP, and 0 of 40 in early-CP. Acquired resistance was evident in 24 (89%) of 27 patients with mutations. Twelve (92%) of 13 patients with mutations in the adenosine triphosphate (ATP) binding loop (P-loop) died (median survival of 4.5 months after the mutation was detected). In contrast, only 3 (21%) of 14 patients with mutations outside the P-loop died (median follow-up of 11 months). As the detection of mutations was strongly associated with imatinib resistance, we analyzed features that predicted for their detection. Patients who commenced imatinib more than 4 years from diagnosis had a significantly higher incidence of mutations (18 [41%] of 44) compared with those treated within 4 years (9 [9%] of 100), P <.0001. Lack of a major cytogenetic response (MCR) was also associated with a higher likelihood of detecting a mutation; 19 (38%) of 50 patients without a MCR had mutations compared with 8 (8.5%) of 94 with an MCR, P <.0001. In conclusion, the detection of kinase domain mutations using a direct sequencing technique was almost always associated with imatinib resistance, and patients with mutations in the P-loop had a particularly poor prognosis.

Prognostic significance of cytogenetic clonal evolution in patients with chronic myelogenous leukemia on imatinib mesylate therapy

Cytogenetic clonal evolution (CE) is a known poor prognostic factor in Philadelphia chromosome-positive chronic myelogenous leukemia (Ph-positive CML). However, its prognostic relevance in the era of imatinib therapy is unknown. We investigated the independent prognostic relevance of CE in 498 patients with Ph-positive CML treated with imatinib for chronic or accelerated phases. One hundred twenty-one patients had CE alone (n = 70) or with other accelerated phase criteria (n = 51). Patients were compared in 4 categories: chronic phase (n = 295), CE only (n = 70), accelerated phase without CE (n = 82), and accelerated phase with CE (n = 51). Statistical methods used established methodologies for univariate and multivariate analyses. In chronic and accelerated phases of CML, CE was not associated with significant differences in major or complete cytogenetic response rates, but it was an independent poor prognostic factor for survival by multivariate analyses in both chronic (P =.005) and accelerated phase (P =.03). Multivariate analyses conducted at the 3-month landmark (including the 3-month cytogenetic response) identified the lack of cytogenetic response at 3 months to be a stronger independent poor prognostic factor for survival than CE for both chronic (major cytogenetic response versus other) and accelerated phase (any cytogenetic response versus other). We conclude that cytogenetic CE is not an important factor for achieving major or complete cytogenetic response with imatinib mesylate therapy, but it is an independent poor prognostic factor for survival in both chronic and accelerated phases of CML. The 3-month cytogenetic response to imatinib mesylate refined the prognostic relevance of such studies in patients on imatinib mesylate therapy.

Practical management of patients with chronic myeloid leukemia receiving imatinib

The introduction of imatinib, a specific inhibitor of the Bcr-Abl tyrosine kinase, has dramatically changed the management of chronic myeloid leukemia (CML). More than 10,000 patients worldwide have been treated with imatinib in clinical trials, and a large body of information has accumulated about the use of this drug. The purpose of this article is to review practical guidelines in regard to optimal dosing, monitoring, managing common side effects such as myelosuppression, and potential drug interactions. The treatment recommendations are intended to optimize therapy with imatinib while taking into account a patient's specific circumstances.

Cytogenetic studies in patients on imatinib

With the introduction of imatinib (Gleevec) (formerly STI571) for the treatment of chronic myeloid leukemia (CML), the various diagnostic methods used to monitor patients must be re-evaluated. Conventional cytogenetics has been the established method for follow-up of patients treated with interferon-alpha (IFN-alpha), and the prognostic value of major and complete cytogenetic remission was demonstrated in a number of studies. In patients on imatinib, these endpoints will likely remain valid, although longer observation is required. Cytogenetic remission as a time-dependent variable may aid risk stratification early on. Clonal evolution, ie, the presence of cytogenetic abnormalities in addition to the Philadelphia (Ph) chromosome, may provide important prognostic information as to the likely response to imatinib in all phases of CML but must be interpreted within the context of other disease characteristics. In some patients, clonal evolution is related to imatinib resistance. Information regarding the impact of specific types of additional cytogenetic abnormalities is still limited. Surprisingly, nonrandom karyotypic abnormalities have also been noted in the Ph-negative cells of some patients in cytogenetic remission. This is a novel phenomenon whose causality and prognostic implications require thorough and systematic evaluation.

Chronic myelogenous leukemia

Within the past few years, the introduction of imatinib mesylate (imatinib) has profoundly changed the management of patients with chronic myelogenous leukemia. This review article addresses the recent advances in the treatment of chronic myelogenous leukemia--in particular, maturing data on the use of imatinib in different phases of the disease; the optimal therapy of newly diagnosed patients; the emergence of resistance to imatinib and potential strategies to overcome this problem; and finally, the place of stem cell transplantation in current treatment algorithms.