BACKGROUND Tyrosine kinase inhibitors (TKIs) have increased survival dramatically for patients with chronic myeloid leukemia (CML), but continuous administration of these drugs may elicit long-term toxicity. OBJECTIVE To investigate the incidence of vascular events in patients with CML treated with first- and second-generation TKIs. DESIGN Retrospective cohort study using nationwide population-based registries. SETTING Sweden. PATIENTS All patients diagnosed with chronic-phase CML in Sweden from 2002 to 2012 and treated with a TKI, and 5 age- and sex-matched control individuals per patient. MEASUREMENTS Relative risks, expressed as incidence rate ratios comparing patients with control individuals, were calculated. Events per 1000 person-years were assessed in interdrug comparisons. RESULTS 896 patients, 94.4% with documented TKI treatment, were followed for a median of 4.2 years. There were 54 arterial and 20 venous events in the CML cohort, corresponding to relative risks of 1.5 (95% CI, 1.1 to 2.1) and 2.0 (CI, 1.2 to 3.3), respectively. The event rate for myocardial infarction was higher in patients treated with nilotinib or dasatinib (29 and 19 per 1000 person-years, respectively) than in those receiving imatinib (8 per 1000 person-years), although data are limited and the CIs were wide and overlapped. Among 31 patients treated with a TKI who had myocardial infarction, 26 (84%) had at least 1 major cardiac risk factor diagnosed before the event occurred. LIMITATIONS Patients may have been exposed to multiple TKIs. Data on second- and third-generation TKIs were limited. CONCLUSION An increased risk for arterial and venous vascular events was seen in patients with CML treated with a TKI. Further study is needed to determine whether the risk for myocardial infarction increases with second-generation drugs. PRIMARY FUNDING SOURCE No external funding.

Recent studies suggest that a proportion of chronic myeloid leukemia (CML) patients in deep molecular remission can discontinue the tyrosine kinase inhibitor (TKI) treatment without disease relapse. In this multi-center, prospective clinical trial (EURO-SKI, NCT01596114) we analyzed the function and phenotype of T and NK cells and their relation to successful TKI cessation. Lymphocyte subclasses were measured from 100 imatinib-treated patients at baseline and 1 month after the discontinuation, and functional characterization of NK and T cells was done from 45 patients. The proportion of NK cells was associated with the molecular relapse-free survival as patients with higher than median NK-cell percentage at the time of drug discontinuation had better probability to stay in remission. Similar association was not found with T or B cells or their subsets. In non-relapsing patients the NK-cell phenotype was mature, whereas patients with more naïve CD56bright NK cells had decreased relapse-free survival. In addition, the TNF-α/IFN-γ cytokine secretion by NK cells correlated with the successful drug discontinuation. Our results highlight the role of NK cells in sustaining remission and strengthen the status of CML as an immunogenic tumor warranting novel clinical trials with immunomodulating agents.

Imatinib has revolutionized the treatment of chronic myeloid leukemia (CML). We evaluated clinical outcome and cost-effectiveness, using Swedish registry data based on patients with CML diagnosed 1973-2008. Outcome from three time periods (I: 1973-1979; II: 1991-1997; III: 2002-2008) associated with symptomatic treatment, interferon-α/stem cell transplant and implementation of imatinib, respectively, were compared and a lifetime cost-effectiveness model developed. Survival data from population registries, estimated resource use from clinical practice and quality of life estimates were employed. Substantial health gains were noted over time, paralleled by increased treatment costs. Median survival was 1.9, 4.0 and 13 years during the respective time periods. The incremental cost-effectiveness ratio (ICER) between periods III and II was €52,700 per quality-adjusted life year (QALY) gained. An estimated 80% price reduction of imatinib, related to patent expiry, would reduce this ICER to €22,700. Our data from four decades reveal dramatically improved survival in CML, paralleled by ICER levels generally accepted by health authorities.

Abstract Background: Tyrosine kinase inhibitors (TKIs) have significantly improved the treatment of CML. Even though TKI treatment is generally not considered curative, recent studies have shown that nearly half of CML patients who have achieve good and durable responses are able to stop the TKI treatment. However, patients who have successfully discontinued TKI treatment still have residual disease. We hypothesized that the immune system plays a role in treatment free remission (TFR), and our preliminary results in the EURO-SKI trial showed that patients who relapse early after imatinib discontinuation have decreased numbers and frequencies of NK cells. In EURO-SKI trial relapse was defined as the loss of major molecular response (MMR). We now aimed to analyze in more detail the phenotype and function of the NK cells in order to understand their role in TFR. Methods: Lymphocyte subclass analysis (the number of NK-, T- and B-cells) was performed at the time of therapy discontinuation and 1 month after the imatinib discontinuation in patients participating in the EURO-SKI stopping trial in the Nordic countries (n=105, results are presented from patients who have reached 6 months follow-up). More detailed immune phenotype and functional assays (NK-cell degranulation and secretion of Th1 type of cytokines IFN-γ/TNF-α) were analyzed from a proportion of patients (n=31). Results: Imatinib treated patients remaining in remission for 6 months (non-relapsing, n=48, median age 60,5 years) displayed an increased amount of NK cells at the time of drug discontinuation (18.6% vs. 11.0%, p=0.02, NK-cell count 0.25 x109 cells/L vs. 0.184 x109 cells/L m, p=0.059) compared to patients who relapsed early (before 5 months, n=29, median age 60,5 years). Furthermore, the NK cell frequency in non-relapsing patients was even higher than in healthy controls (11.5%, n=48, p=0.001). T and B cell counts and frequencies showed no differences between the groups. Detailed analysis of the NK cell compartment displayed a more mature phenotype for the NK cells in non-relapsing patients. Larger frequencies of NK cells from early relapsing patients was CD56bright compared to non-relapsing patients (4.8% vs. 2.7% of CD56 NK cells, p=0.04). Furthermore, patients who had higher frequencies of CD56bright NK cells than median had decreased TFR at 6 months (42%) compared to patients with lower frequency (70%, p=0.01). In addition, there was a trend towards more CD57pos (78% (n=21) vs. 66% (n=10), p=0.09) CD56dim NK cells in non-relapsing patients. To further study the mature NK cells in non-relapsing patients, recently identified markers (FceRgneg, PLZFneg, SYKneg, EAT-2neg) for adaptive NK cells were analyzed. Interestingly, there was a trend that non-relapsing patients had higher frequencies of adaptive-like NK cells. For example, non-relapsing patients had more CD56dim NK cells that had down regulated EAT-2 (2.8% (n=6) vs. 1.3% (n=5) of lymphocytes, p=0.03) and more CD56dim NK cells expressing NKG2D (11.2% vs. 2.6% of lymphocytes, p=0.02) and NKp46 (13.6% vs. 3.9% of lymphocytes, p=0.05). Moreover, after imatinib discontinuation the expression of transcription factor Eomes increased in the CD56dim NK cells of the early relapsing group (baseline MFI 2045 vs. 1 month 3480, p=0.06), while in non-relapsing group it seemed to even decrease (baseline MFI 2273 vs. 1 month 1980, p=0.13) pointing towards an adaptive phenotype. No significant differences between the groups were observed when degranulation against K562 cell line was studied. However, CD16neg NK cells from non-relapsing patients responded to K562 stimulation by secreting more TNFα/IFNγ compared to the early relapsing patients (21% vs. 13% of CD56pos CD16neg NK cells, p=0.01). Furthermore, patients whose CD16neg NK cells had higher than median TNFα/IFNγ secretion when stimulated with K562 cells showed an increased TFR at 6 months (78%) compared to patients who had lower TNFα/IFNγ secretion than median (37%, p=0.005). Conclusions: CML patients who successfully discontinued imatinib therapy displayed a higher number and frequency of peripheral blood mature, adaptive-like NK cells capable of secreting cytokines TNFα/IFNγ relative to relapsing patients. How such NK cells may contribute to maintenance of treatment free remission is still unknown. Nonetheless, our results warrant further clinical studies with NK-cell modulating agents. Disclosures Muller: Novartis: Honoraria, Other: Consulting or Advisory Role, Research Funding; ARIAD Pharmaceuticals Inc.: Honoraria, Other: Consulting & Advisory Role, Research Funding; BMS: Honoraria, Other: Consulting or Advisory Role, Research Funding. Hjorth-Hansen:Novartis: Honoraria; Ariad: Honoraria; Bristol-Myers Squibb: Research Funding; Pfizer: Honoraria, Research Funding. Saussele:Pfizer: Honoraria, Other: Travel grant; BMS: Honoraria, Other: Travel grant, Research Funding; Novartis Pharma: Honoraria, Other: Travel grant, Research Funding; ARIAD: Honoraria. Mahon:ARIAD: Consultancy; Novartis: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Honoraria; Pfizer: Consultancy. Porkka:Bristol-Myers Squibb: Honoraria; Celgene: Honoraria; Novartis: Honoraria; Pfizer: Honoraria. Richter:Ariad: Honoraria; Bristol-Myers Squibb: Honoraria; Novartis: Honoraria. Mustjoki:the Finnish Cancer Societies: Research Funding; Pfizer: Honoraria, Research Funding; Academy of Finland: Research Funding; Sigrid Juselius Foundation: Research Funding; Finnish Cancer Institute: Research Funding; Signe and Ane Gyllenberg Foundation: Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria, Research Funding.

We randomised 46 newly diagnosed patients with chronic myeloid leukaemia (median age 56) to receive dasatinib 100 mg QD or imatinib 400 mg QD and report outcome as an intention‐to‐treat analysis with 36 months follow‐up. Early cytogenetic and molecular responses were superior in the dasatinib group, with a tendency that imatinib patients caught up with time. For instance, MR 3.0 was reached at 3 months in 36% vs. 8% ( P = 0.02), at 12 months in 81% vs. 46% ( P = 0.02) and at 18 months in 73% vs. 65% (n.s.) of the patients in the two groups. In contrast, MR 4.5 was consistently superior in the dasatinib group at all time points from 6 months onwards, reaching 61% vs. 21% ( P < 0.05) at 36 months. Sixty‐four vs. 71% of the patients in the dasatinib and imatinib arms, respectively, remained on assigned drug. Dasatinib dose was frequently reduced, but with maintained excellent effect. One imatinib patient progressed to blastic phase, but no CML ‐related deaths occurred. In conclusion, our data compare favourably with those of the dasatinib registration study, DASISION . The fast and deep molecular responses induced by dasatinib compared with imatinib may be exploited to increase the proportion of patients who can achieve a treatment‐free remission after treatment discontinuation.

Abstract Background: Recent reports suggest that approximately 40% of CML patients who have achieved sustained complete molecular remission are able to stop TKI treatment without disease relapse. However, there are no predictive markers for successful therapy discontinuation. Therefore, we set up an immunological sub-study in the ongoing pan-European EURO-SKI stopping study. Our aim was to identify predictive biomarkers for relapse/non-relapse and to understand more on the mechanisms of immune surveillance in CML. Methods: The EURO-SKI study started in 2012, and patients included were at least three years on TKI and at least one year in MR4 or deeper before the study entry. Basic lymphocyte immunophenotyping (the number of NK-, T- and B-cells) was performed at the time of therapy discontinuation and 1, 6, and 12 months after the TKI stop and in case of relapse (defined as loss of MMR, BCR-ABL1>0.1% IS). In addition, from a proportion of patients more detailed immunophenotypic and functional analyses (cytotoxicity of NK-cells and secretion of Th1 type of cytokines IFN-γ/TNF-α) were done at the same times. Results: Thus far 119 Nordic patients (imatinib n=105, dasatinib n=12, nilotinib n=2) who have discontinued TKI treatment within the EURO-SKI study have been included in the lymphocyte subclass analysis (results are presented from patients who have reached 6 months follow-up). Immunophenotyping analysis demonstrates that imatinib treated patients who were able to maintain remission for 6 months (n=36) had increased NK-cell counts (0.26 vs. 0.15x109cells/L, p=0.01, NK-cell proportion 18.9% vs. 11%, p=0.005) at the time of drug discontinuation compared to patients who relapsed early (before 5 months n=22). Furthermore, the phenotype of NK-cells was more cytotoxic (more CD57+ and CD16+cells and less CD62L+cells), and also their IFN-γ/TNF-α secretion was enhanced (19.2% vs. 13%, p=0.02). Surprisingly, patients who relapsed more slowly (after 5 months, n=16) had similar baseline NK-cell counts (0.37x109cells/L), NK-cell proportion (21.2%), and phenotype and function as patients, who were able to stay in remission. No differences in the NK-cell counts were observed between patients who had detectable or undetectable BCR-ABL1 transcripts at the baseline (0.22 x109cells/L vs. 0.31 x109cells/L, p=0.61). Interestingly, NK-cell count was higher in patients with low Sokal risk score than in patients with intermediate risk (0.33 x109cells/L vs. 0.20 x109cells/L, p=0.04). Furthermore, there was a trend that male patients had a higher proportion of NK-cells than females (21.6% vs. 15.7%, p=0.06). Pretreatment with IFN-α or the duration of imatinib treatment did not have an effect on NK-cell count or proportion. In comparison to the imatinib group, dasatinib treated patients had higher NK-cell counts at the baseline (median 0.52x109cells/L vs. 0.26x109cells/L, p=0.02), and also the proportion of CD27 (median 50% vs. 16%, p=0.01) and CD57 expressing (median 79% vs. 74%, p=0.05) NK-cells was higher. The follow-up time of dasatinib treated patients is not yet long enough to correlate the NK-cell counts with the success of the treatment discontinuation. The absolute number of T-cells or their function did not differ significantly between relapsing and non-relapsing patients at the time of treatment discontinuation. However, both CD4+ and CD8+ T-cells tended to be more mature in patients who stayed in remission compared to patients who relapsed early (CD4+CD57+CD62L- median 5.7% vs. 2.4%, p=0.06, CD8+CD62L+CD45RA+ 13% vs. 26.7%, p=0.05). The analysis of follow-up samples showed that in patients who stayed in remission the Th1 type cytokine (IFN-γ/TNF-α) secretion of CD8+T-cells increased at 6 months compared to baseline (23.6 vs. 18.5%, p=0.07). Same phenomenon was observed in the late relapsing group at relapse compared to baseline (37.9 vs. 13.5%, p=0.03). No similar increase was observed in the early relapsing group. Conclusions: Low NK-cell numbers and poor cytokine secretion may predict early disease relapse after TKI discontinuation. However, patients who relapse later have high numbers of normally functioning NK-cells. Further research (detailed phenotypic analysis of NK- and T-cells including activating and inhibitory receptors and immune checkpoint molecules) and correlation of biomarker data with clinical parameters are ongoing to understand the ultimate determining factors of relapse. Disclosures Själander: Novartis: Honoraria. Hjorth-Hansen:Novartis: Honoraria; Bristol-myers Squibb: Honoraria; Ariad: Honoraria; Pfizer: Honoraria. Porkka:BMS: Honoraria; BMS: Research Funding; Novartis: Honoraria; Novartis: Research Funding; Pfizer: Research Funding. Mustjoki:Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria, Research Funding.

Introduction: The introduction of continuous tyrosine kinase inhibitor (TKI) treatment has dramatically improved progression-free survival for chronic phase chronic myeloid leukaemia (CML) patients. This success, however, has put the issue of long-term drug toxicity and safety into focus. Recent data from clinical studies have indicated an increased risk of cardiovascular events (CVE), including peripheral arterial occlusive disease, in CML patients receiving treatment with the TKIs nilotinib or ponatinib, as compared to imatinib (Giles et al, Leukemia 2013; Kim et al, Leukemia 2013; Cortes et al, New England Journal of Medicine 2013; FDA communication 2013).

Abstract Background The inhibition of oncogenic BCR-ABL1 kinase with tyrosine kinase inhibitors (TKIs) has significantly improved the prognosis of CML. Recent reports suggest that approximately 40 % of CML patients who have achieved optimal therapy response (complete molecular remission, CMR) can stop imatinib treatment without recurrence of detectable BCR-ABL1 transcripts. However, no predictive prognostic factors for successful therapy discontinuation have yet been identified. We therefore set up an immunological substudy in the ongoing pan-European EURO-SKI stopping study. We aimed to identify predictive biomarkers for relapse and non-relapse after TKI discontinuation. In addition, we aimed to understand more on the mechanisms of immune surveillance in CML and to study the effects of TKI treatment on the immune system. Materials and methods Patients in deep molecular remission (MR4, BCR-ABL < 0,01% IS) for at least one year and with TKI treatment for at least 3 years were eligible for the clinical study. Basic lymphocyte immunophenotyping (the proportions and absolute numbers of NK-, T- and B-cells) was performed at the university hospital laboratories at the time of therapy discontinuation, and 1, 6, and 12 months after the TKI discontinuation. In a proportion of patients a more detailed immunophenotypic (analysis of CD45RA, CD57, CD27 and CD62L expressions) and functional analyses were done from fresh blood samples in a central immunology laboratory (Helsinki) at the same time points. The cytotoxicity of NK-cells was studied by measuring the direct killing of target cells (K562) and by the degranulation assay (CD107a/b expression). The secretion of Th1 type of cytokines IFN-γ/TNF-α was studied from both T- and NK-cells. Results Thus far the basic lymphocyte subclass measurement has been analyzed from 62 patients who have discontinued TKI treatment within the EURO-SKI study. Functional analyses have been performed from 30 patients. 60 patients have used imatinib before treatment discontinuation and 2 patients dasatinib. At baseline, before the treatment discontinuation both CD4+ and CD8+ T-cell counts were within the normal range (median CD4+ 0.73x 109/L, range 0.11-2.4x 109/L; CD8+ 0.35x 109/L, 0.07-1.92 x 109/L). The TKI stop had no significant numerical or functional effect on T-cells, and at 1 month time-point the median T-cell counts were unchanged (CD4+ 0.73x 109/L; CD8+ 0.35x 109/L). Similarly, at the baseline, the median NK-cell count was within a normal range (0.26 x 109/L, range 0.04-1.04 x 109/L), and no significant change was observed 1 month after stopping the treatment (median 0.29 x 109/L). Furthermore, at the baseline and at the 1-month time-point the cytotoxicity of NK-cells and the cytokine secretion of T- and NK-cells did not significantly differ from the healthy controls when all patients were considered as a one group. However, when patients were divided in two groups based on the relapse status, the patients who eventually relapsed had significantly fewer NK-cells already at the baseline (Figure A; absolute count 0.18x 109/L vs. 0.32 109/L, p=0.008; proportions 11% vs. 21%, p=0.001). The phenotype of NK-cells also differed between the two groups, and the patients who relapsed had less NK-cells expressing CD57 (median 58% vs. 69%, p=0.046) and CD16 (median 67% vs. 83%, p=0.018) on the cell surface. Furthermore, the cytotoxicity of NK-cells was impaired in patients who failed to discontinue the TKI treatment successfully and no killing activity was observed in their samples (Figure B; alive K652 cells after co-incubation with effector cells 100% vs. 88%, p=0.07). No clear differences were observed in the function or the numbers of T-cells between relapsing and non-relapsing patients. Conclusions The NK-cell numbers and their function may predict disease relapse after TKI discontinuation. This may have impact on the future stopping trials. In addition, it further illustrates the importance of the immune system in the successful long-term treatment of CML. Disclosures: Ekblom: Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Hjorth-Hansen:Pfizer, BMS: Honoraria, Travel expenses Other. Porkka:BMS: Honoraria, Research Funding; Novartis: Honoraria, Research Funding. Richter:Bristol-Myers Squibb: Consultancy, Speakers Bureau; Novartis: Consultancy, Research Funding, Speakers Bureau. Mustjoki:Novartis: Honoraria; BMS: Honoraria, Research Funding.

Abstract Background Dasatinib is a potent BCR-ABL1 and SRC tyrosine kinase inhibitor, which in vitro is more effective against progenitor and putative leukemia stem cells than imatinib. This may translate into deeper molecular responses in vivo. Methods We randomized (1:1) 46 newly diagnosed CML patients to receive dasatinib 100 mg or imatinib 400 mg once daily. The primary endpoint of our study was treatment response in stem and progenitor cell fractions (Mustjoki et al, Leukemia 2013). We here summarize the clinical results of the study after a 24-month follow-up focusing on toxicity and standard response evaluation by quantitative BCR-ABL1 PCR and cytogenetics (NCT00852566 www.ClinicalTrials.gov). Results Both imatinib and dasatinib treated patients fared well with deeper and faster treatment responses than what has been reported in the registration studies. By karyotyping, dasatinib induced a faster response by 3 months (median of 5% of Ph+ cells in imatinib group vs. 0% in dasatinib group, p=0.01, n=21 in each group), but already by 12 months the difference disappeared, as all evaluable patients were in complete cytogenetic remission. The rate of molecular response MR3.0 was already at the 3 months time-point better in the dasatinib group (36% vs 8%, p=0.02; see Table), but within 18 months imatinib patients caught up the difference. In contrast, the achievement of deeper therapy responses, MR4.0 and MR4.5, was clearly different between the groups and increased over 24 months. After 18 months 64% and 71% of imatinib- and dasatinib-treated patients had achieved MR3.0 (p=0.59), while the MR4.0 rates were 23% and 62% (p=0.009) and MR4.5 rates 4% and 41% (0.003) (see Table below). The difference in median transcript levels was approximately 1 log (>10-fold difference) in all time-points after 3 months of therapy (see Table below). A total of 7 patients (30%) in both groups discontinued assigned treatment. Main drug-related toxicities were as expected. Dasatinib-induced serosal inflammation (pleural/pericardial effusions) was more frequent than in registration studies (6 patients, 27%). In 4 patients (18%) this led to therapy discontinuation, despite of drug interruption and dose reductions. In the imatinib group 3 patients discontinued due to drug-related toxicity (liver toxicity, rash and severe hypogammaglobulinemia with recurrent infections). Disease progression occurred in one dasatinib-treated patient (cytogenetic progression with the appearance of V299L mutation at month 9) and two imatinib-treated patients (blastic transformation at month 2 and molecular progression at month 18). The patient in blast phase has been transplanted and is currently in molecular remission. No CML-related deaths occurred, but one patient died from lung cancer. Interpretation Dasatinib induced faster and deeper molecular responses than imatinib and overall responses were better in both groups than in the registration studies. Relatively high rate of serosal toxicity was observed among the dasatinib-treated patients, but this had no adverse effect on response. Upcoming studies will show if the deeper treatment responses induced by dasatinib therapy translate into increased probability of successful therapy discontinuation. Disclosures: Hjorth-Hansen: Pfizer: Honoraria, Travel, Travel Other; Bristol-Myers Squibb: Honoraria, Research Funding, Travel, Travel Other; Novartis: Honoraria, Travel Other; Merck: Research Funding. Richter:Novartis: Consultancy, Honoraria, Research Funding, Travel Other; Bristol-Myers Squibb: Consultancy, Honoraria, Travel, Travel Other. Porkka:BMS: Consultancy, Research Funding, Speakers Bureau; Novartis: Consultancy, Research Funding, Speakers Bureau. Mustjoki:Novartis: Honoraria; BMS: Honoraria, Research Funding.

Clinical management guidelines on malignant disorders are generally based on data from clinical trials with selected patient cohorts. In Sweden, more than 95% of all patients diagnosed with chronic myeloid leukemia (CML) are reported to the national CML registry, providing unique possibilities to compile population-based information. This report is based on registry data from 2002 to 2010, when a total of 779 patients (425 men, 354 women; median age, 60 years) were diagnosed with CML (93% chronic, 5% accelerated, and 2% blastic phase) corresponding to an annual incidence of 0.9/100,000. In 2002, approximately half of the patients received a tyrosine kinase inhibitor as initial therapy, a proportion that increased to 94% for younger (<70 years) and 79% for older (>80 years) patients during 2007-2009. With a median follow-up of 61 months, the relative survival at 5 years was close to 1.0 for patients younger than 60 years and 0.9 for those aged 60 to 80 years, but only 0.6 for those older than 80 years. At 12 months, 3% had progressed to accelerated or blastic phase. Sokal, but not European Treatment and Outcome Study, high-risk scores were significantly linked to inferior overall and relative survival. Patients living in university vs nonuniversity catchment areas more often received tyrosine kinase inhibitors up front but showed comparable survival.

Imatinib has dramatically improved the clinical outcome in chronic myeloid leukemia, chronic phase (CMLcp), but a risk of resistance and serious disease progression still prevails. We have studied 45 newly diagnosed CMLcp patients initiated on imatinib, assessing treatment responses by interphase extral signal (ES)‐fluorescence in situ hybridization (FISH), quantitative real‐time (q‐RT) polymerase chain reaction (PCR), and chromosome banding analysis. In a landmark analysis, an early favorable response, defined as less than 10% BCR‐ABL‐positive cells by FISH after 3 months of treatment, was identified as a predictive marker of an improved long‐term clinical outcome. Of evaluable patients, 51% achieved this response. A large majority, 95% of such responders reached complete cytogenetic responses (CCyR) within 12 months and 100% event‐free survival (EFS) at 48 months, when compared with 67 and 65%, respectively, of patients with higher breakpoint cluster region ‐ Abelson (BCR‐ABL) positivity at 3 months ( P = 0.04; P = 0.006). No similar, significant correlations were noted between early disease assessments with PCR of BCR‐ABL mRNA transcripts or of cytogenetics versus a 12‐month CCyR or long‐term EFS. Our data, based on a limited patient cohort, indicate that (i) FISH can effectively be used in the early assessment of remaining Ph‐positive cells to identify patients at risk for a long‐term nonoptimal response to imatinib and that (ii) FISH may be more useful than PCR for this purpose. Am. J. Hematol., 2012. © 2012 Wiley Periodicals, Inc.

PURPOSE:Chronic myeloid leukemia (CML) management changed dramatically with the development of imatinib mesylate (IM), the first tyrosine kinase inhibitor targeting the BCR-ABL1 oncoprotein. In Sweden, the drug was approved in November 2001. We report relative survival (RS) of patients with CML diagnosed during a 36-year period.PATIENTS AND METHODS:Using data from the population-based Swedish Cancer Registry and population life tables, we estimated RS for all patients diagnosed with CML from 1973 to 2008 (n = 3173; 1796 males and 1377 females; median age, 62 years). Patients were categorized into five age groups and five calendar periods, the last being 2001 to 2008. Information on use of upfront IM was collected from the Swedish CML registry.RESULTS:Relative survival improved with each calendar period, with the greatest improvement between 1994-2000 and 2001-2008. Five-year cumulative relative survival ratios (95% CIs) were 0.21 (0.17 to 0.24) for patients diagnosed 1973-1979, 0.54 (0.50 to 0.58) for 1994-2000, and 0.80 (0.75 to 0.83) for 2001-2008. This improvement was confined to patients younger than 79 years of age. Five-year RSRs for patients diagnosed from 2001 to 2008 were 0.91 (95% CI, 0.85 to 0.94) and 0.25 (95% CI, 0.10 to 0.47) for patients younger than 50 and older than 79 years, respectively. Men had inferior outcome. Upfront overall use of IM increased from 40% (2002) to 84% (2006). Only 18% of patients older than 80 years of age received IM as first-line therapy.CONCLUSION:This large population-based study shows a major improvement in outcome of patients with CML up to 79 years of age diagnosed from 2001 to 2008, mainly caused by an increasing use of IM. The elderly still have poorer outcome, partly because of a limited use of IM.

Abstract Abstract 784 Background: In vitro studies have suggested that CML stem cells are resistant to tyrosine kinase inhibitors (TKIs), but in vivo effects in patients have not been prospectively assessed. Furthermore, the inter-individual variation of the leukemic stem cell pool at diagnosis and its possible prognostic value is unknown. Patients: 46 newly diagnosed CML-CP patients were randomized 1:1 to receive either dasatinib 100 mg or imatinib 400 mg QD. The primary endpoint was a comparison of the proportion of Ph+ cells in CD34+CD38− and CD34+CD38+ compartment at 6 months between the study arms. Key secondary endpoints were the fraction of Ph+ cells in the stem cell compartments at 1 and 3 months, and molecular and cytogenetic responses at 3, 6, 12 and 18 months. Experimental endpoints included the percentage of Ph+ cells in the stem cell compartment at diagnosis and its correlation with therapeutic response. Results: One patient in the imatinib arm and none in the dasatinib arm progressed to blast crisis within first 12 months. 4/22 of dasatinib patients have discontinued the treatment due to side-effects (mainly pleural effusion) and 1 patient due to insufficient response. 3/24 imatinib patients have discontinued the therapy (1 blast crisis, 1 side-effects, 1 other malignancy). Early cytogenetic responses were superior in the dasatinib arm: the median percentage of Ph+ cells in the bone marrow was 81% (imatinib) vs. 70% (dasatinib) at 1 month (p=0.15) and 5% vs. 0% at 3 months (p=0.0085). At 12 months all dasatinib (n=20) and 19/20 imatinib patients were in CCyR (results based on patients on treatment at 12 months). MMR rate was significantly higher in the dasatinib arm already at 6 months (70% vs. 20%, p=0.002) and similarly at 9 (75 vs. 26%, p=0.004) and 12 months (88% vs. 40%, p=0.009). Undetectable BCR-ABL1 transcripts (at least CMR4) were observed in 20% of the dasatinib patients at 6 months compared to none in the imatinib arm (p=0.11) and 44% in the dasatinib arm at 12 months compared to 7% in the imatinib arm (p=0.037). The median percentage of Ph+ cells, as measured by FISH (1000 cells analyzed), in the CD34+CD38− fraction at diagnosis was 79% (range 1–100%) compared to 96% (range 50–100%) in CD34+CD38+ fraction. The proportion of Ph+ cells in CD34+CD38− fraction correlated with WBC count (r=0.50, p<0.001), splenomegaly (r=0.43, p=0.0055), anemia (r=-0.44, p=0.004) and blood blast percentage (r=0.57, p=0.0001) at diagnosis. There was also a significant correlation between Ph+ cells in CD34+CD38− fraction at diagnosis and cytogenetic response at 1 (r=0.63, p<0.0001), 3 (r=0.48, p=0.0025) and 6 months (r=0.36, p=0.0271). Furthermore, leukemic stem cell burden at diagnosis correlated significantly with BCR-ABL1 transcript levels at 3 (r=0.54, p=0.0005), 6 (r=0.42, p=0.0088) and 9 months (r=0.40, p=0.0123). All patients who were not in MMR at 18 months, had >79% of Ph+ cells in CD34+CD38− fraction at diagnosis. During TKI therapy, the proportion of Ph+ cells decreased rapidly in the stem cell fractions. At 1 month, the median proportion of Ph+ cells was 14% and 56% in CD34+CD38− and CD34+CD38+ fractions compared to 69% in whole BM (p<0.0001, n=38). At 3 months, the respective numbers were 0.40%, 0.20% and 0.80% (p=0.087, n=33) and at 6 months 0%, 0% and 0.1% (p=0.23, n=41). Dasatinib-treated patients had significantly lower proportion of Ph+ cells in CD34+CD38+ fraction at 3 months than imatinib patients (0.05% vs. 0.68%, p=0.0318). A similar trend was also observed at 1 month (24% vs. 69%, p=0.05), but no difference existed at 6 months. In CD34+CD38− fraction the proportions of Ph+ cells did not differ significantly at 1 (11 vs. 17%, p=0.91), 3 (0.2 vs. 0.3%, p=0.44) or 6 months (0 vs. 0%, p=0.75). Conclusions: In comparison to imatinib, dasatinib induced superior therapeutic responses with remarkably high MMR and CMR rates. This was associated with a faster reduction of Ph+ cells at the progenitor CD34+CD38+ cell level. Surprisingly, both drugs rapidly depleted Ph+ cells from the more primitive CD34+CD38− compartment. The proportion of Ph+ stem cells at diagnosis varied significantly among individual patients and bore prognostic value. Patients with a low proportion of Ph+ leukemic stem cells at diagnosis achieved faster and better cytogenetic and molecular responses. The leukemic stem cell burden at diagnosis may be a biomarker predicting treatment outcome and reflecting key biological factors in CML. Disclosures: Mustjoki: Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Richter:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Dybedal:Novartis: Travel support. Fioretos:Cantargia AB: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Qlucore AB: Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Weiss Bjerrum:Novartis: Consultancy; Bristol-Myers Squibb: Consultancy. Simonsson:Novartis, BMS, Merck, Pfizer: Consultancy, Honoraria. Porkka:Novartis: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding. Hjorth-Hansen:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria.

Biologic and clinical observations suggest that combining imatinib with IFN-α may improve treatment outcome in chronic myeloid leukemia (CML). We randomized newly diagnosed chronic-phase CML patients with a low or intermediate Sokal risk score and in imatinib-induced complete hematologic remission either to receive a combination of pegylated IFN-α2b (Peg-IFN-α2b) 50 μg weekly and imatinib 400 mg daily (n = 56) or to receive imatinib 400 mg daily monotherapy (n = 56). The primary endpoint was the major molecular response (MMR) rate at 12 months after randomization. In both arms, 4 patients (7%) discontinued imatinib treatment (1 because of blastic transformation in imatinib arm). In addition, in the combination arm, 34 patients (61%) discontinued Peg-IFN-α2b, most because of toxicity. The MMR rate at 12 months was significantly higher in the imatinib plus Peg-IFN-α2b arm (82%) compared with the imatinib monotherapy arm (54%; intention-to-treat, P = .002). The MMR rate increased with the duration of Peg-IFN-α2b treatment (< 12-week MMR rate 67%, > 12-week MMR rate 91%). Thus, the addition of even relatively short periods of Peg-IFN-α2b to imatinib markedly increased the MMR rate at 12 months of therapy. Lower doses of Peg-IFN-α2b may enhance tolerability while retaining efficacy and could be considered in future protocols with curative intent.

Abstract Abstract 667 Background: Targeted tyrosine kinase inhibitor (TKI) therapy efficiently induces rapid hematologic and cytogenetic responses in most CML patients. In vitro studies have suggested that CML stem cells are resistant to TKIs and therefore the treatment may need to be life-long. However, the in vivo effects of TKIs on the leukemic stem cell pool in a patient population have not been prospectively assessed. In addition, the biological impact and prognostic value of leukemic stem cell burden at diagnosis is unknown. Aim: To analyze the proportions of Ph+ cells in the stem cell compartment in newly diagnosed CP CML patients at diagnosis, and correlate the initial leukemic stem cell burden to biological variables and hematological toxicity during first 3 months of TKI therapy. Patients and Methods: 42 newly diagnosed CP CML patients within the Nordic countries were randomized to receive either dasatinib 100 mg (n=21) or imatinib 400 mg (n=21) once daily. Stem cell assays were performed at diagnosis and at 1, 3, and 6 months from start of TKI therapy. After pre-selection of CD34+ cells from large volume bone marrow (BM) aspirates with paramagnetic beads, the CD34+ cells were fractionated into CD38 positive and negative pools using a sorting flow cytometer. The proportion of Ph+ cells in the stem cell fractions was assayed by counting 1000 cells with interphase FISH for BCR-ABL1. Results: Measurement of Ph+ stem cells was feasible in most patients at diagnosis and results from 36 evaluable patients will be presented. The median volume of BM aspirate was 28, 37, 40 and 36 ml at diagnosis, 1, 3, and 6 months after therapy start, respectively. The median yield of BM mononuclear cells was 1000, 110, 93 and 79 ×106, respectively. The median proportion of Ph+ cells was significantly lower in the more primitive CD34+CD38neg fraction when compared to the CD34+CD38+ fraction or to unfractionated BM (79%, range 0.6–100%; 96%, 50–100%; and 96%, 57–100%, respectively, p=0.0001). The proportion of Ph+ cells in the CD34+CD38neg fraction at diagnosis correlated with high leukocyte count (r=0.59, p<0.001), enlarged spleen size (r=0.46, p=0.005), low hemoglobin concentration (r=-0.46, p=0.006) and high blast percentage in peripheral blood (r=0.62, p<0.001). No correlation was found to age, gender, platelets, eosinophils or basophils. The proportion of Ph+ cells in the CD34+CD38+ fraction correlated only to the leukocyte count (r=0.33, p=0.049). Patients with high Sokal risk had a significantly higher proportion of Ph+ CD34+CD38neg stem cells at diagnosis as compared to low and intermediate risk patients (94% vs. 75%, respectively, p=0.036). Patients who had a higher leukemic stem cell burden (more than the median value of 79% of Ph+ CD34+CD38neg cells, Table 1) at diagnosis experienced more grade ≥2 hematological toxicity (neutropenia in particular) during first 3 months of TKI therapy as compared to patients with a lower (<79%) stem cell burden (62% vs. 19%, respectively, p=0.027). Conclusions: The proportion of Ph+ stem cells at the time of diagnosis varied from 1 to 100% between individual CML patients. It was correlated with hemoglobin concentration, leukocyte count, blast percentage and spleen size at diagnosis and with hematological toxicity during early course of treatment, mirroring paucity of healthy hematopoietic stem cell reservoir. The size of the leukemic stem cell pool at diagnosis may be a powerful prognostic marker and a major biological determinant for the high Sokal risk group. The effect of TKI therapy on the malignant stem cell pool size and correlation to therapy responses will be evaluated when all patients have reached the primary study endpoint of 6 months. Disclosures: Mustjoki: BMS, Novartis: Honoraria. Richter:BMS, Novartis: Consultancy, Honoraria. Simonsson:BMS, Novartis: Consultancy, Honoraria, Research Funding. Porkka:BMS, Novartis: Consultancy, Honoraria, Research Funding. Hjorth-Hansen:BMS, Novartis: Consultancy, Honoraria, Research Funding.

Abstract Abstract 205 Background: Little progress in terms of improving survival in patients with chronic myeloid leukemia (CML) was made until the introduction of interferon alpha and allogeneic stem cell transplantation for selected patients in the 1980s. The management changed dramatically with the development of imatinib mesylate, the first tyrosine kinase inhibitor (TKI) that targets the BCR-ABL1 oncoprotein. In Sweden clinical trials started in December 2000 and the drug was approved for clinical use in November 2001. This study evaluates the impact of treatment developments in CML by studying temporal trends in short-term and long-term excess mortality in a population-based setting. Materials and Methods: Using data from the nationwide, population-based Swedish Cancer Registry and Swedish population life-tables stratified by age, sex, and calendar time we characterized trends in relative survival for all patients diagnosed with CML in Sweden 1973–2008 (n=3,173; 1,796 men and 1,377 women; median age 62 years). Patients were categorized into five age groups (<50, 50–59, 60–69, 70–79 and >79 years) and five calendar periods (1973-1979, 1980–1986, 1987–1993, 1994–2000 and 2001–2008). Six hundred and nine stem cell transplants (539 allogeneic and 70 autologous) were reported to the EBMT registry during the study period. Results: Incidence remained stable over time with a consistent male predominance. Relative survival improved with calendar period with the greatest improvement in the last two calendar periods (figure). Five-year cumulative relative survival ratios (RSRs; 95% confidence intervals) were 0.21 (0.17-0.24), 0.23 (0.20-0.27), 0.37 (0.33-0.41), 0.54 (0.50-0.58) and 0.80 (0.75-0.83) in the five calendar periods, respectively. Ten-year RSRs were 0.06 (0.04-0.08) and 0.78 (0.73-0.83) in the first and last calendar periods, respectively. This improvement was confined to age groups up to 79 years of age but most pronounced in patients below 60 years. The 5-year RSRs for patients diagnosed 2001–2008 were 0.91 (0.85-0.94), 0.87 (0.78-0.92), 0.82 (0.72-0.90), 0.75 (0.61-0.86), and 0.25 (0.10-0.47) for the five age groups, respectively. Older age at diagnosis and male sex were associated with significantly higher excess mortality rates in models adjusted for potential confounding factors. Conclusion: In this large population-based study including > 3,000 CML patients survival increased significantly after 2001 (when imatinib mesylate was approved for clinical use in Sweden) for patients up to 79 years of age. Future studies are needed to assess if very old (>79 years) CML patients may benefit from an increased use of TKIs. Also newly introduced, targeted treatment options for CML need to be evaluated in future population-based studies. Disclosures: No relevant conflicts of interest to declare.

Abstract Abstract 4269 Background Imatinib (IMA) is recommended as first-line therapy for patients diagnosed with CML in chronic phase (CMLcp). Earlier studies, based particularly on the IRIS trial, have indicated that a favorable long-term clinical outcome to therapy is clearly associated with a reduction of the Ph-pos cell population and of BCR-ABL transcripts, as measured with cytogenetics and qPCR, respectively. We have investigated the response to IMA in a CMLcp cohort treated at our institution and assessed the prognostic value of early and repeated determinations BCR-ABL expression, using both ES-fluorescence in situ hybridization (FISH) and qPCR. Methods 45 pts with newly diagnosed CMLcp (24M/21F; median age 54 yrs, range 19-87; 5 with 9q del, 5 with variant translocations; Sokal scores 16 LR, 20 IR, 9 HR) were started on IMA (400mg qd) and hematologic, cytogenetic and molecular responses were followed at regular 3-mo intervals. Median IMA treatment time at follow-up was 29 mo (range 9-94). Regular cytogenetic karyotyping (CG) and interphase ES-FISH were performed on bone marrow cells (the latter method by scoring BCR-ABL in at least 500 cells on smear preparations, detection limit 20.2%) while qRT-PCR was performed on peripheral blood leukocyte samples with ABL as control gene. Major molecular response (MMR) was defined as the ratio BCR-ABL/control gene 20.1%. Results At diagnosis the pts displayed a median of 100% (range 67-100) Ph-pos metaphases by CG, while ES-FISH revealed BCR-ABL gene expression in a median of 86,2% (range 45,9-97,3) of cells. The response to IMA-treatment during the first 12 mo, displayed in Table I, included CCyR in 80%, MMR in 48% and reduction of FISH-detected BCR-ABL to <1% in 62% of analyzed pts. When first obtaining CCyR (negative CG), patients showed a median ES-FISH value of 0.27% (range 0 to 7.4%). Comparing pts who did vs. did not achieve CCyR at 12 mo (n= 36 vs. 9) FISH and PCR analyses at 3, 6 and 9 mo showed clear differences between the groups (Table II). At 24 months from start of IMA, 39 pts were evaluable for response and ‘events‘ (defined according to ELN as any of: death during study treatment, loss of CHR, loss of MCyR, progression to AP/BC or WBC>20). A total of 6 pts showed ‘event‘ (4 AP, 1 BC, 1 loss of CHR; 2 died). Table III provides landmark data related to FISH and PCR levels at 3 time points. Patients with a high remaining BCR-ABL expression were more likely to develop ‘events‘, as compared to those with low expression. Conclusion Our single centre data from an unselected CMLcp pat cohort on first-line IMA shows good clinical and molecular responses, well in line with results from the IRIS study. We propose that repeated, longitudinal interphase ES-FISH analyses may provide additional and important prognostic information regarding later targeted clinical endpoints. The method gives reliable information also early during the treatment, at higher disease penetration when PCR may be less reproducible, and can also be performed on peripheral blood samples. Disclosures: No relevant conflicts of interest to declare.

Previous studies have revealed markedly and consistently increased leukotriene C4 synthase (LTC4S) expression in myeloid cells from chronic myeloid leukaemia (CML) patients. This is of interest, because LTC4 has been shown not only to be a potent mediator of inflammation and allergy but also to stimulate human myelopoiesis, and to modulate proliferation and apoptosis in haematological as well as non-haematological cells. Imatinib mesylate is the standard choice of treatment in CML; however resistance is becoming an increasing problem. An alternative strategy or a complement to direct inhibition of p210 bcr-abl, may be to interfere with signalling molecules down-stream of p210 bcr-abl that contribute to the malignant transformation. We have previously described over-expression of LTC4S accompanied by elevated LTC4S activity in isolated CML cell fractions representing all stages of neutrophilic myelopoiesis as compared to normal cells of corresponding maturity (Tornhamre et al, 2003). LTC4S is not expressed in normal mature neutrophils, in contrast to mature neutrophils from CML patients where LTC4S is aberrantly expressed (Sjölinder et al, 2000). Here we report the first evidence of elevated LTC4 formation in vivo in CML patients by showing significantly increased urinary excretion of leukotriene E4 (LTE4) (Fig 1). Urine LTE4 level is a reflection of the whole body biosynthesis of LTC4 (Sala et al, 1990). A significant increase in urinary excretion of LTE4 was found in CML patients on a group level; 35·8 ± 2·1 (mean ± SEM) ng/mmol creatinine (range 24·4–52·5; n = 18) in healthy controls and 55·4 ± 6·7 ng LTE4/mmol creatinine (range 24·0–132·0; n = 23; P = 0·02) in CML patients. The range of values between patients is wide and a urine test could probably only be used as an indication to distinguish a pathological value from a normal one. This wide range in urinary LTE4 levels has also been seen in other studies of various inflammatory diseases. As the enzyme immunoassay method is not a absolute quantitative method to determine LTE4 concentrations, the results may be regarded as relative measures (Kumlin et al, 1995). However, for comparison between groups within a study this is sufficient. Urinary excretion of LTE4 in healthy individuals compared to CML patients. Urine samples were collected from normal controls (triangles; n = 18) and CML patients (n = 23). The group of patients consisted of newly diagnosed and hydroxycarbamide-treated (crosses; n = 9), IFN-treated (circles; n = 8) and IFN-failures included in this study (squares; n = 6). Urine samples were examined for LTE4 using enzyme immunoassay analysis and creatinine was measured with an alkaline colorimetric assay. Results are presented as ng LTE4/mmol creatinine. Statistical analysis was performed using two-sided Student's t-test for unpaired samples. *P < 0·05. Furthermore, LTC4S expression (reverse transcription polymerase chain reaction and immunoblot) and activity were assessed repeatedly in CD16(+) neutrophils from seven chronic phase interferon (IFN)-failing Philadelphia-positive (Ph+) CML patients during 9 months of imatinib mesylate treatment (starting dose 400 mg once daily). All patients showed complete haematological responses. Treatment with the p210 bcr-abl inhibitor imatinib mesylate led to a normalisation of the aberrant activity of LTC4S. Before treatment, neutrophilic LTC4S expression and activity were clearly elevated (12·6 ± 2·4 pmol LTC4/106 cells), but decreased significantly (2·9 ± 0·9; P = 0·005) already after 1 month of treatment and remained low throughout the study (Fig 2A and B). In contrast, presence of the Ph-chromosome (cytogenetics) and BCR-ABL1 [fluorescence in situ hybridization (FISH)] in bone marrow cells remained essentially unaltered or less markedly decreased in several patients. The difference in results between measuring LTC4S activity and cytogenetics and FISH may be explained by the fact that the latter two methods reflect not the activity of the enzyme, but the presence of the BCR-ABL1 gene. LTC4 synthase expression and activity in CD16(+) neutrophils and Ph-chromosome/BCR-ABL1 expression in bone marrow cells from seven α-inteferon-failing chronic phase CML patients (a–g) with BCR-ABL1(+) (A) or BCR-ABL1(−) (B) granulocytes, before and after 2 weeks, 1, 3, 6, and 9 months of treatment with imatinib mesylate. Patient b was excluded after 3 months of treatment due to blast crisis. The LTC4 synthase activity (filled rhombus) as well as protein (immunoblot) and RNA (reverse transcription polymerase chain reaction) expression was analysed in the CD16(+) neutrophil fractions obtained after magnetic-activated cell sorting. LTC4 synthase activity was determined by incubation with 10 μmol LTA4/l for 5 min and the formation of LTC4 was analysed by reversed-phase high-performance liquid chromatography. In parallel, the Ph-chromosome and BCR-ABL1 in bone marrow cells were detected with conventional cytogenetics (open squares) and FISH (open triangles), respectively, before and after 3, 6, and 9 months of imatinib mesylate treatment. (C) LTC4 synthase activity in CD16(+) neutrophils (left panel) and the parallel occurrence of BCR-ABL1, investigated by FISH, (right panel) in peripheral blood cells from CML patients before and after 7 and 14 d of treatment with imatinib mesylate. The lines represent individual patients and the bars show mean ± SEM (n = 4) of experiments performed in duplicate. Statistical analysis was performed using two-sided Student's t-test for paired samples. *P < 0·05, n.s. not significant. This significant effect of imatinib mesylate on LTC4S activity led us to further explore the time-response. This was analysed in isolated neutrophils from four additional Ph+ patients, three of whom were newly diagnosed. No effect could be seen after 7 d of treatment. However, after 14 d LTC4S activity was significantly decreased. Prior to treatment the isolated neutrophils showed LTC4 production of 15·3 ± 2·5 (mean ± SEM range 10–22) pmol/106 cells and after 14 d the activity had decreased to 3·2 ± 1·2 pmol/106 cells (P = 0·02). In parallel, the presence of BCR-ABL1 in peripheral blood cells was investigated by FISH. No change in the percentages of cells bearing BCR-ABL1 was seen after 14 d (Fig 2C): 76 ± 10% and 77 ± 8% (P = 0·79) before and after 14 d of imatinib mesylate treatment, respectively. This study examined the response of imatinib mesylate treatment on LTC4S in neutrophils. The observed normalised LTC4S preceded clinical response to imatinib mesylate. Our data suggest that aberrant LTC4S expression may be a down-stream step in p210 bcr-abl-induced signalling. It is possible that a decrease in LTC4S activity occurs even earlier in more immature myeloid cells. The reduction of LTC4S activity in response to imatinib mesylate was demonstrated in both newly diagnosed patients as well as IFN-failures. Neutrophils from two of the IFN-failed patients already possessed very low LTC4S activity prior to treatment. This finding was unexpected because all of, at least, 80 Ph+ CML patients in our previous studies demonstrated increased LTC4S activity. This discrepancy was investigated by May-Grünwald-Giemsa-FISH, which revealed that these two patients lacked BCR-ABL1+ granulocytes, in contrast to the other five. Instead the BCR-ABL1+ cells consisted mainly of erythroblasts. This further supports a coupling between p210 bcr-abl and LTC4S expression. The BCR-ABL1-negative granulocytes in these two patients may be explained by the fact that all patients in the study had received IFN prior to treatment with imatinib mesylate. It has been shown that IFN, occasionally in combination with cytosine arabinoside, can eliminate cytogenetically detectable Ph+ metaphases (Hochhaus et al, 2000). In line with this, we have earlier observed that in a patient achieving complete cytogenetic response after IFN treatment, the aberrant expression of LTC4S was also normalised. Imatinib mesylate can, in addition to p210, also inhibit other tyrosine kinase activities, such as c-kit and platelet-derived growth factor-receptor α/β. However, LTC4S was specifically up-regulated in neutrophils from CML patients with BCR-ABL1+ granulocytes. This suggests that it is the inhibition of this tyrosine kinase that is crucial for the normalisation of the aberrant LTC4S expression, although contribution from other signalling pathways cannot be excluded. In conclusion, CML patients showed an increased production of cysteinyl-LTs in vivo. In vivo treatment with imatinib mesylate normalised the increased LTC4S activity seen in CML neutrophils. This occurs faster than the cytogenetic response. The findings indicate that expression and activity of LTC4S may be a down-stream step in signal transduction events initiated by p210 bcr-abl. The role of LTC4S in the pathophysiology of CML and the validity of LTC4S as a clinical marker needs to be further elucidated. LTA4 methyl ester was a generous gift from Dr Robert Zipkin, Biomol Research Laboratories. Antisera against LTC4S were kindly provided by Merck Frosst. We would like to thank Ingrid Arvidsson for excellent technical assistance on the FISH analysis. This project was supported by grants from the King Gustav V Jubilee Foundation and Värmland County Council. C.R. was supported by personal grants from the Foundation of Haematological Research and Quality Control and the Foundation for Medical Research of Sophiahemmet Stockholm.