Correlation between the proportion of Philadelphia chromosome-positive metaphase cells and levels of BCR-ABL mRNA in chronic myeloid leukaemia

Molecular monitoring of chronic myeloid leukemia: principles and interlaboratory standardization

Serial quantification of BCR-ABL1 messenger RNA (mRNA) is an important therapeutic indicator for patients with chronic myeloid leukemia, but historically, there has been substantial variation in results reported by different laboratories. To help improve the comparability of results, an international scale (IS) for BCR-ABL1 was proposed which is being implemented by testing laboratories worldwide. This is being achieved most commonly by the derivation of laboratory-specific conversion factors, but increasingly by the use of kits or reagents that are calibrated to the first World Health Organization International Genetic Reference Panel for quantitation of BCR-ABL1 mRNA. Recent attention has focused on the need to define and validate levels of deeper molecular response (MR) within the context of the IS. While there has been substantial progress in the alignment of results, BCR-ABL1 measurement is technically challenging and standardization is an ongoing process.

Management of the new patient with CML in chronic phase

Imatinib has been the preferred initial therapy for newly diagnosed chronic myeloid leukemia (CML) patients for the last 10 years. Today, other tyrosine kinase inhibitors (TKIs) are licensed for first-line use. In this paper we analyze the pros and cons of the various alternatives to imatinib and try to give some advice on the management of the newly diagnosed patient.

Initial choice of therapy among plenty for newly diagnosed chronic myeloid leukemia

Imatinib has been the preferred initial therapy for newly diagnosed chronic myeloid leukemia patients for the past 10 years. Recently, other, possibly better, tyrosine kinase inhibitors have been licensed for first-line use based on the early results of 2 large, randomized clinical trials. The pros and cons of the various alternatives to imatinib are analyzed herein, and I try to answer the question of are we ready to abandon imatinib and, if yes, then what treatment should a patient diagnosed today receive.

Current status of imatinib as frontline therapy for chronic myeloid leukemia

Imatinib is remarkably effective in treating newly diagnosed patients with chronic myeloid leukemia (CML) in chronic phase. However, at 5 years more than one third of the patients have abandoned the medication on account of side effects, lack of efficacy, or progression. Here we review the current results with imatinib, the prognostic factors for response, and issues associated with long-term treatment with imatinib such as pregnancy, adherence to therapy, and complete molecular responses.

Adherence is the critical factor for achieving molecular responses in patients with chronic myeloid leukemia who achieve complete cytogenetic responses on imatinib

PURPOSE

There is a considerable variability in the level of molecular responses achieved with imatinib therapy in patients with chronic myeloid leukemia (CML). These differences could result from variable therapy adherence.

METHODS

Eighty-seven patients with chronic-phase CML treated with imatinib 400 mg/d for a median of 59.7 months (range, 25 to 104 months) who had achieved complete cytogenetic response had adherence monitored during a 3-month period by using a microelectronic monitoring device. Adherence was correlated with levels of molecular response. Other factors that could influence outcome were also analyzed.

RESULTS

Median adherence rate was 98% (range, 24% to 104%). Twenty-three patients (26.4%) had adherence <or= 90%; in 12 of these patients (14%), adherence was <or= 80%. There was a strong correlation between adherence rate (<or= 90% or > 90%) and the 6-year probability of a 3-log reduction (also known as major molecular response [MMR]) in BCR-ABL1 transcripts (28.4% v 94.5%; P < .001) and also complete molecular response (CMR; 0% v 43.8%; P = .002). Multivariate analysis identified adherence (relative risk [RR], 11.7; P = .001) and expression of the molecular human organic cation transporter-1 (RR, 1.79; P = .038) as the only independent predictors for MMR. Adherence was the only independent predictor for CMR. No molecular responses were observed when adherence was <or= 80% (P < .001). Patients whose imatinib doses were increased had poor adherence (86.4%). In this latter population, adherence was the only independent predictor for inability to achieve an MMR (RR, 17.66; P = .006).

CONCLUSION

In patients with CML treated with imatinib for some years, poor adherence may be the predominant reason for inability to obtain adequate molecular responses.

Chronic myelogenous leukemia as a paradigm of early cancer and possible curative strategies

The chronological history of the important discoveries leading to our present understanding of the essential clinical, biological, biochemical, and molecular features of chronic myelogenous leukemia (CML) are first reviewed, focusing in particular on abnormalities that are responsible for the massive myeloid expansion. CML is an excellent target for the development of selective treatment because of its highly consistent genetic abnormality and qualitatively different fusion gene product, p210(bcr-abl). It is likely that the multiple signaling pathways dysregulated by p210(bcr-abl) are sufficient to explain all the initial manifestations of the chronic phase of the disease, although understanding of the circuitry is still very incomplete. Evidence is presented that the signaling pathways that are constitutively activated in CML stem cells and primitive progenitors cooperate with cytokines to increase the proportion of stem cells that are activated and thereby increase recruitment into the committed progenitor cell pool, and that this increased activation is probably the primary cause of the massive myeloid expansion in CML. The cooperative interactions between Bcr-Abl and cytokine-activated pathways interfere with the synergistic interactions between multiple cytokines that are normally required for the activation of stem cells, while at the same time causing numerous subtle biochemical and functional abnormalities in the later progenitors and precursor cells. The committed CML progenitors have discordant maturation and reduced proliferative capacity compared to normal committed progenitors, and like them, are destined to die after a limited number of divisions. Thus, the primary goal of any curative strategy must be to eliminate all Philadelphia positive (Ph+) primitive cells that are capable of symmetric division and thereby able to expand the Ph+ stem cell pool and recreate the disease. Several highly potent and moderately selective inhibitors of Bcr-Abl kinase have recently been discovered that are capable of killing the majority of actively proliferating early CML progenitors with minimal effects on normal progenitors. However, like their normal counterparts, most of the CML primitive stem cells are quiescent at any given time and are relatively invulnerable to the Bcr-Abl kinase inhibitors as well as other drugs. We propose that survival of dormant Ph+ stem cells may be the most important reason for the inability to cure the disease during initial treatment, while resistance to the inhibitors and other drugs becomes increasingly important later. An outline of a possible curative strategy is presented that attempts to take advantage of the subtle differences in the proliferative behavior of normal and Ph+ stem cells and the newly discovered selective inhibitors of Bcr-Abl. Leukemia (2003) 17, 1211-1262. doi:10.1038/sj.leu.2402912

Selection and characterization of BCR-ABL positive cell lines with differential sensitivity to the tyrosine kinase inhibitor STI571: diverse mechanisms of resistance

Targeting the tyrosine kinase activity of Bcr-Abl with STI571 is an attractive therapeutic strategy in chronic myelogenous leukemia (CML). A few CML cell lines and primary progenitors are, however, resistant to this compound. We investigated the mechanism of this resistance in clones of the murine BaF/3 cells transfected with BCR-ABL and in 4 human cell lines from which sensitive (s) and resistant (r) clones were generated by various methods. Although the resistant cells were able to survive in the presence of STI571, their proliferation was approximately 30% lower than that of their sensitive counterparts in the absence of the compound. The concentration of STI571 needed for a 50% reduction in viable cells after a 3-day exposure was on average 10 times higher in the resistant (2-3 μmol/L) than in the sensitive (0.2-0.25 μmol/L) clones. The mechanism of resistance to STI571 varied among the cell lines. Thus, in Baf/BCR-ABL-r, LAMA84-r, and AR230-r, there was up-regulation of the Bcr-Abl protein associated with amplification of the BCR-ABL gene. In K562-r, there was no Bcr-Abl overexpression, but the IC50 for the inhibition of Bcr-Abl autophosphorylation was increased in the resistant clones. Sequencing of the Abl kinase domain revealed no mutations. The multidrug resistance P-glycoprotein (Pgp) was overexpressed in LAMA84-r, indicating that at least 2 mechanisms of resistance operate in this cell line. KCL22-r showed neither Bcr-Abl up-regulation nor a higher threshold for tyrosine kinase inhibition by STI571. We conclude that BCR-ABL–positive cells can evade the inhibitory effect of STI571 by different mechanisms, such as Bcr-Abl overexpression, reduced intake mediated by Pgp, and, possibly, acquisition of compensatory mutations in genes other than BCR-ABL.

Selection and characterization of BCR-ABL positive cell lines with differential sensitivity to the tyrosine kinase inhibitor STI571: diverse mechanisms of resistance

Targeting the tyrosine kinase activity of Bcr-Abl with STI571 is an attractive therapeutic strategy in chronic myelogenous leukemia (CML). A few CML cell lines and primary progenitors are, however, resistant to this compound. We investigated the mechanism of this resistance in clones of the murine BaF/3 cells transfected with BCR-ABL and in 4 human cell lines from which sensitive (s) and resistant (r) clones were generated by various methods. Although the resistant cells were able to survive in the presence of STI571, their proliferation was approximately 30% lower than that of their sensitive counterparts in the absence of the compound. The concentration of STI571 needed for a 50% reduction in viable cells after a 3-day exposure was on average 10 times higher in the resistant (2-3 μmol/L) than in the sensitive (0.2-0.25 μmol/L) clones. The mechanism of resistance to STI571 varied among the cell lines. Thus, in Baf/BCR-ABL-r, LAMA84-r, and AR230-r, there was up-regulation of the Bcr-Abl protein associated with amplification of the BCR-ABL gene. In K562-r, there was no Bcr-Abl overexpression, but the IC50 for the inhibition of Bcr-Abl autophosphorylation was increased in the resistant clones. Sequencing of the Abl kinase domain revealed no mutations. The multidrug resistance P-glycoprotein (Pgp) was overexpressed in LAMA84-r, indicating that at least 2 mechanisms of resistance operate in this cell line. KCL22-r showed neither Bcr-Abl up-regulation nor a higher threshold for tyrosine kinase inhibition by STI571. We conclude that BCR-ABL–positive cells can evade the inhibitory effect of STI571 by different mechanisms, such as Bcr-Abl overexpression, reduced intake mediated by Pgp, and, possibly, acquisition of compensatory mutations in genes other than BCR-ABL.

Quantification of BCR-ABL transcripts in CML patients in cytogenetic remission after interferon-alpha-based therapy

We measured using a competitive quantitative polymerase chain reaction-capillary electrophoresis (PCR-CE)-based assay, the levels of bcr-abl transcripts in 44 patients with chronic myeloid leukemia (CML) after interferon-alpha (IFN-alpha) therapy, who achieved a major (10 patients, MCR group) or complete (34 patients, CCR group) cytogenetic response. All 34 CCR patients had molecular evidence of residual disease detected in bone marrow samples at the time of best karyotypic response. The median number of bcr-abl transcripts of 34 evaluable patients in the CCR group at the time of complete cytogenetic remission was 4/microg RNA (range 3-4600), while the median number of bcr-abl transcripts of 10 patients in the MCR group at the time of best cytogenetic response was 4490/microg RNA (range 600-23 900) (P = 0.000024). In nine CCR and five MCR patients we were able to quantify the amount of bcr-abl transcript both at diagnosis and after interferon therapy: no statistical difference (P = 0.18) was found between the two groups at diagnosis (median bcr-abl transcripts/microg RNA was 30 000 vs. 39 650, respectively). During IFN-alpha therapy, the two groups were evaluable at the time of major karyotypic conversion: at this point, there was a statistical difference of expression of bcr-abl transcript between the CCR group (17 patients) (median 2700; range 76-40 000) and the MCR group (10 patients) (median 4490; range 600-23 900), respectively (P = 0.046). No differences of bcr-abl amount of transcript were found in patients with CCR obtained either by IFN-alpha therapy alone (20 patients) vs. IFN-alpha plus ABMT (13 patients) (P = 0.47). We firstly demonstrated that although the CCR and MCR groups were clinically, cytogenetically and molecularly indistinguishable at diagnosis, the two groups could be recognized successfully during interferon therapy based on the level of bcr-abl transcript.

Leukemia patient-derived lymphoblastoid cell lines exhibit increased induction of leukemia-associated transcripts following high-dose irradiation

Improvement in diagnostic cytogenetic techniques has led to the recognition of an increasing number of leukemia-associated chromosomal translocations and inversions. These genetic lesions frequently are associated with the disruption of putative transcription factors and the production of hybrid transcripts that are implicated in leukemogenesis. Epidemiologic evidence suggests that some, but not all, individuals with a history of gamma-irradiation exposure are at increased risk of developing chronic myeloid leukemia (CML). CML is characterized by the Philadelphia chromosome and transcription of the resulting hybrid BCR-ABL gene. Utilizing the leukemia-associated BCR-ABL p210 transcript as a marker, we sought differences in the induction of illegitimate genetic recombination following high-dose gamma-irradiation of karyotypically normal lymphoblastoid cell lines (LCL) derived from individuals with and without a history of myeloid leukemias. Six LCL [4 leukemia patient derived [2 acute myeloid leukemia and 2 CML] and 2 from normal individuals were analyzed with reverse transcriptase polymerase chain reaction for BCR-ABL under stringent conditions following exposure to 0, 50, or 100 Gy of LET gamma-irradiation delivered via a Varian linear accelerator at 4 MV. Transcripts identical to disease-associated b2a2 and b3a2 transcripts were detected both spontaneously (background illegitimate genetic recombination) and following gamma-irradiation. Background BCR-ABL positivity was demonstrable in 4 of the 6 LCL, with no significant difference in detection between leukemic- and nonleukemic-derived LCL. Overall, increasing gamma-irradiation dose resulted in an increased frequency of BCR-ABL transcript detection (0 Gy vs 50 Gy vs 100 Gy,p = 0.0023, Chi-square test). Within the leukemic- but not the nonleukemic-derived LCL there was significantly greater BCR-ABL positivity after gamma-irradiation compared to unirradiated equivalents. Furthermore, the BCR-ABL positivity of both the AML- and CML-derived LCL after gamma-irradiation was significantly greater than that of the nonleukemic-derived LCL after gamma-irradiation. We speculate that this difference in the detection of illegitimate after gamma-irradiation recombination may be due to aberrant DNA double strand break repair mechanisms in individuals predisposed to the development of myeloid leukemias.

Interphase cytogenetics and competitive RT-PCR for residual disease monitoring in patients with chronic myeloid leukaemia during interferon-alpha therapy

There is a need for fast and sensitive methods to evaluate the response of patients with chronic myeloid leukaemia (CML) to interferon-alpha (IFN-alpha) therapy to complement cytogenetic analysis of Philadelphia (Ph) chromosome-positive metaphases. We have used interphase FISH (fluorescence in situ hybridization) and competitive RT-PCR (reverse transcriptase-polymerase chain reaction) techniques for detection of BCR-ABL-positive cells to measure suppression of leukaemic clone in a series of 51 follow-up samples from 24 CML patients undergoing IFN-alpha treatment. Interphase FISH analysis of the malignant clone in bone marrow using BCR and ABL probes was found to be highly correlated to conventional G-banding metaphase examination (r = 0.98). RT-PCR quantification of BCR-ABL mRNA transcripts in blood also showed a high degree of concordance with the proportion of Ph-positive metaphases (r = 0.93). In addition, the degree of cytogenetic response did not influence the equivalence between karyotype analysis and molecular methods. We concluded that interphase FISH and competitive RT-PCR provide reliable information on residual tumour burden and response to IFN-alpha in CML patients. These molecular methods may significantly improve the efficiency of residual disease monitoring during IFN-alpha therapy of CML.

Assessing residual leukaemia

For the great majority of patients with chronic myeloid leukaemia (CML), the Philadelphia (Ph) chromosome is a specific marker of the malignant clone. The standard method to assess the quality of remission in these patients is cytogenetic analysis of bone marrow derived metaphases. However, the molecular definition of the t(9;22) and its consequences has enabled other tests to be developed that can specifically detect CML cells. Fluorescence in situ hybridization (FISH) analyses chromosomes to detect either the juxtaposition of BCR and ABL sequences or the disruption of these genes; Southern blotting analyses genomic DNA to determine whether the BCR gene is rearranged; reverse-transcriptase polymerase chain reaction (RT-PCR) analyses RNA to determine the presence or absence of BCR-ABL transcripts; Western blotting analyses cell lysates to determine the presence or absence of BCR-ABL protein. Each of these techniques has particular advantages and pitfalls but in general they may be used to replace or at least to reduce the frequency of conventional cytogenetic analysis. Partly because of economic factors and the lack of standardization or effective quality control, these assays are still largely restricted to research laboratories. The sensitivity with which residual leukaemia can be detected suggests that FISH, Southern blotting and Western blotting are likely to be most useful in assessing patient response to interferon-alpha or other forms of treatment that typically induce partial remission. RT-PCR is by far the most sensitive assay and is probably most appropriate for monitoring patients who are in complete remission.

Variable numbers of BCR-ABL transcripts persist in CML patients who achieve complete cytogenetic remission with interferon-alpha

A substantial minority of patients with chronic myeloid leukaemia (CML) achieve a complete response to treatment with interferon-alpha (IFN-alpha), defined as the disappearance of Philadelphia chromosome positive metaphases or, for patients who are Philadelphia chromosome negative but BCR-ABL positive, the disappearance of the leukaemic clone as assayed by Southern blot. We have measured the levels of BCR-ABL transcripts in 20 such patients by quantitative PCR. Results were standardized for both quality and quantity of cDNA by quantification of ABL as an internal control. All 20 patients had evidence of residual disease; the median number of transcripts was 750/micrograms RNA (range 10-22,000) and the median BCR-ABL/ABL ratio was 0.17% (range 0.0008-3.6%). Our findings show that CML has not been eradicated in any patient and that the quantity of residual disease in complete responders may vary by as much as four orders of magnitude.