We conducted a systemic evaluation to describe the effect of minimal residual disease (MRD) kinetics on long-term allogeneic transplantation outcome by analyzing 95 adult transplants with Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph-positive ALL) who received first-line two courses of imatinib-based chemotherapy (median follow-up 5 years). MRD monitoring was centrally evaluated by real-time quantitative PCR (4.5 log sensitivity). After the first course of imatinib-based chemotherapy, 33 patients (34.7%) achieved at least major molecular response. On the basis of MRD kinetics by the end of two courses of imatinib-based chemotherapy, we stratified entire patients into four subgroups: early-stable molecular responders (EMRs, n=33), late molecular responders (LMRs, n=35), intermediate molecular responders (IMRs, n=9) and poor molecular responders (PMRs, n=18). Multivariate analysis showed that the most powerful factor affecting long-term transplantation outcome was MRD kinetics. Compared with EMRs, IMRs or PMRs had significantly higher risk of treatment failure in terms of relapse and disease-free survival (DFS). LMRs had a tendency toward a lower DFS. Quantitative monitoring of MRD kinetics during the first-line imatinib-based chemotherapy course is useful in identifying subgroups of Ph-positive ALL transplants at a high risk of relapse.
Combination of imatinib with conventional chemotherapy as the first-line treatment has demonstrated an improved complete hematological response (CHR) rate and an increased applicability in allogeneic stem cell transplantation (SCT), thus allowing a better short-term outcome in adults with Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph-positive ALL).1, 2, 3, 4, 5, 6 However, with an extended follow-up, a substantial proportion of transplant patients and practically almost all non-transplant patients continue to die as a result of relapse. Consequently, patients at the highest risk of relapse are likely to benefit from the identification of new criteria that enable predictable patient outcome.
After the introduction of imatinib, two groups have reported on the prognostic relevance of clinical, immunophenotypic and cytogenetic heterogeneities that affected the treatment outcome in adult Ph-positive ALL. Yanada et al.7 have found additional chromosome aberrations to be adverse prognostic factors in total patients. However, in their separate analysis, including only patients allografting in first CHR, this additional aberration was not correlated with outcome. Another study from Jaso et al.8 has also showed no adverse prognostic factors to allow risk stratification. Main limitations of these studies were the short follow-up duration (<3 years) and lack of analysis regarding the role of minimal residual disease (MRD) monitoring.
Some reports from the pre-imatinib era suggested a good correlation between MRD positivity or its quantitative level during the early phase of chemotherapy and treatment outcome in adults with Ph-positive ALL.9, 10, 11 Most published studies in the imatinib era include quantitative MRD findings,1, 2, 3, 4, 5, 6 but the long-term outcome in relation to molecular response to any given therapeutic approach remains to be determined. Previously, we conducted a prospective, phase 2 trial of allogeneic SCT following first-line imatinib-based chemotherapy in adults with Ph-positive ALL, and in the trial, BCR-ABL1 transcript levels in the bone marrow (BM) were routinely monitored using real-time quantitative PCR (RQ-PCR).12, 13, 14 Our interim analyzing data showed the positive impact of imatinib-based chemotherapy on the short-term SCT outcome13 and the potential of a 3-log MRD reduction (individual log reduction) at the end of the first four weeks of imatinib administration as a risk factor of outcome.14 However, in our previous reports, MRD kinetics, that is, the quality as well as the persistence of molecular response, by the end of whole pre-transplant imatinib-based chemotherapy courses were not included in the risk-factor analysis. Additional limitations included the small size of patient population and relatively short observation period. Moreover, MRD assessment was not performed according to the concept of international scale or log reduction from the standardized pooled baseline.15, 16
From this point of view, we conducted a systemic reevaluation to describe the significance of MRD reduction and its kinetics during a uniform first-line imatinib-based chemotherapy course as a risk factor of SCT outcome by analyzing 95 Ph-positive ALL transplants with a sufficient follow-up duration of 5 years.
Materials and methods
During the period between September 2000 and December 2009, 95 adults (median age 34 years (range 15–59 years)) with newly diagnosed Ph-positive ALL who received a uniform treatment protocol of allogeneic SCT following two courses of imatinib-based chemotherapy, as described previously in detail,13, 14 were included in this analysis. The treatment schedule is summarized in Table 1. A total of 18 patients were excluded because of poor-quality RNA (n=5), death during pre-transplant chemotherapy courses (n=9) and transplants receiving only one course of imatinib-based chemotherapy (n=4). All of the patients in the study provided written informed consent, and the study protocol was approved by the institutional review board of The Catholic University of Korea. This study was conducted in accordance with the Declaration of Helsinki. Data were analyzed as of December 2011.
As previously described,13, 14, 17, 18, 19 all of the patients were transplanted from a fully matched sibling or a suitably matched (⩽2 allele-mismatched) unrelated donor after the completion of two courses of imatinib-based chemotherapy. In brief, donor selection was based on high-resolution human leukocyte antigen genotyping using PCR-sequence-specific primer for both class I and II antigens. Donor and recipient pairs were considered matched when the pair was identical at A, B, C and DRB1 loci. The preparative regimen for patients in first CHR consisted of total body irradiation (13.2 Gy) and cyclophosphamide (120 mg/kg) and for patients beyond first CHR consisted of total body irradiation (12 Gy), cytarabine (12 g/m2) and melphalan (140 mg/m2). Patients aged >50 years or those with a comorbid condition (for example, active fungal infection or major organ dysfunction) were given a reduced-intensity regimen consisting of fludarabine (150 mg/m2) and melphalan (140 mg/m2). Graft-versus-host disease (GVHD) prophylaxis was attempted by administering calcineurin inhibitor (cyclosporine for all sibling transplants and tacrolimus for all unrelated transplants) plus methotrexate (10 mg/m2 on days 1, 3, 6 and 11). Antithymocyte globulin (2.5 mg/kg; Genzyme Transplant, Cambridge, MA, USA) was administered to the patients who received allele-mismatched unrelated donor grafts. If residual leukemia was detected in the absence of GVHD at 3 months after SCT, calcineurin inhibitors were rapidly discontinued. No prophylactic imatinib therapy was planned after SCT.
MRD monitoring for BCR-ABL1 transcript was centrally evaluated by RQ-PCR (4.5 log sensitivity) through handling of BM samples from all of the patients (Research Institute of Molecular Genetics, The Catholic University of Korea, Seoul, Korea). Samples were collected at diagnosis, at the end of each course of imatinib-based chemotherapy and then at 3, 6, 9, 12, 18, 24 and 36 months post SCT. The total RNA was extracted from BM mononuclear cells using Trizol (Invitrogen, Carlsbad, CA, USA), and the quality of RNA was checked by agarose-formaldehyde gel electrophoresis. A total of 2 μg RNA was used for reverse transcription using Transcriptor First Strand cDNA Synthesis Kit (Roche, Mannheim, Germany), and the cDNA synthesis was carried out by incubating at 25 °C for 10 min and at 42 °C for 60 min, and then the reaction was inactivated by heating at 99 °C for 5 min. After the cDNA synthesis, RQ-PCR for BCR-ABL1 quantitation was performed in a total volume of 20 μl containing 2 μl cDNA from the reverse transcription, 1 × PCR reaction mixture, 0.6 μM of each primer and 0.25 μM probe. As previously described,20 we designed one set of primers for each type of BCR-ABL1 transcript, ABL1 and TaqMan probes. RQ-PCR assay was carried out using LightCycler 480 Analyzer (Roche) under the following conditions: denaturation at 95 °C for 10 min for 1 cycle, and then denaturation at 95 °C for 10 s, annealing and elongation at 60 °C for 1 min for 50 cycles, followed by the cooling step at 45 °C for 5 min. The ABL1 control gene was also amplified in the same sample to enable correction for sample quality and quantity variations, and patient samples with an ABL1 control <10 000 copies were considered suboptimal and were excluded from this analysis. The median values of ABL1 transcripts were 226 750 (range 16 400–3 555 000). Both BCR-ABL1 and ABL1 were analyzed in duplicate, and results showing a discrepancy greater than two-folds were excluded and repeated from the cDNA synthesis step. The quantity of BCR-ABL1 transcripts was normalized for ABL1 expression, and the ratio of BCR-ABL1 to ABL1 was expressed as a percentage according to the concept of international scale for p210BCR-ABL1 and log reduction from the standardized pooled baseline for p190BCR-ABL1. We calculated the standardized pooled baseline by measuring the median percentage of pretreatment BCR-ABL1/ABL1 value of the total population expressing p190BCR-ABL1 (n=63; median 73.84%; range 13.08–401.54%). Thus, a BCR-ABL1/ABL1 of 0.07384% represented a reduction of 3-log from the standardized pooled baseline value. Negative results were confirmed by nested PCR.20
Definitions and evaluation of response
CHR was defined as the reconstitution of normal BM cellularity with <5% leukemic blasts together with an absolute neutrophil count of >1.5 × 109/l and a platelet count of >100 × 109/l. Major molecular response (MMR) was defined as a ratio of BCR-ABL1 to ABL1 ⩽0.1% on the international scale (for p210BCR-ABL1) or a reduction in BCR-ABL1 transcript level by at least 3-log from the standardized pooled baseline value (for p190BCR-ABL1).15, 16 Complete molecular response (CMR4.5, 4.5 log sensitivity) was defined as undetectable levels of BCR-ABL1 transcript. On the basis of MRD quality and its kinetics, we stratified entire patients into four subgroups as follows: (1) early-stable molecular responders (EMRs; patients showing early and persistent MMR or CMR4.5 by the end of two courses of imatinib-based chemotherapy), (2) late molecular responders (LMRs; patients showing a conversion of MRD levels from no MMR to MMR or CMR4.5 by the end of two courses of imatinib-based chemotherapy), (3) intermediate molecular responders (IMRs; patients showing late or persistent MRD levels of >0.1 to 1% (for p210BCR-ABL1) or<3-log to 2-log reduction (for p190BCR-ABL1) by the end of two courses of imatinib-based chemotherapy) and (4) poor molecular responders (PMRs; patients showing persistent MRD levels of>1% (for p210BCR-ABL1) or <2-log reduction (for p190BCR-ABL1) by the end of two courses of imatinib-based chemotherapy). Relapse was defined by the reappearance of ⩾5% leukemic cells in BM aspirates or extramedullary leukemia in patients with previously documented CHR. Patients were considered refractory if peripheral blood blasts or extramedullary disease had not been eliminated, or if BM blasts had not been reduced below 5%, or both. GVHD was diagnosed and graded using the previously published criteria.21, 22
We calculated the overall survival from the date of SCT until the date of death or the last follow-up. When calculating disease-free survival (DFS), both relapses and deaths in remission were counted as adverse events. Survival curves for overall survival and DFS were plotted using the Kaplan–Meier method and compared by the log-rank test. Relapse and nonrelapse mortality were calculated using cumulative incidence estimates to accommodate the following competing events23: death for relapse and relapse for nonrelapse mortality; the groups were compared with the Gray test.24 The prognostic significances of presenting and transplant covariates affecting overall survival and DFS were determined using the Cox proportional hazards model, including variables with a P-value <0.10 in earlier univariate testing. Factors were considered significant if they had an associated P-value of <0.05 as determined by the likelihood ratio test, using two-tailed significance testing. On the other hand, the prognostic significances of covariates affecting relapse and nonrelapse mortality were determined using the proportional hazards model for subdistribution of a competing risk.25 In these models, acute and chronic GVHD were considered as time-dependent covariates. The following variables were considered: patient age (<35 years versus ⩾35 years), sex (male versus female), presenting leukocyte count (<30 × 109/l versus ⩾30 × 109/l), extramedullary involvement (positive versus negative), additional chromosomal change (positive versus negative), transcript subtype (p190BCR-ABL1 versus others), hematological response after induction (CHR versus refractory), molecular response to the first course of imatinib-based chemotherapy (MMR versus CMR4.5 versus >0.1 to 1% (or <3-log to 2-log reduction) versus >1% (or <2-log reduction)), MRD kinetics by the end of two courses of imatinib-based chemotherapy (EMR versus LMR versus IMR versus PMR), disease status at SCT (first CHR versus >first CHR), time-to-transplantation (<142 days versus ⩾142 days), donor type (matched sibling donor versus matched unrelated donor versus mismatched unrelated donor), graft type (BM versus peripheral blood), donor–recipient sex match (female–male versus others), conditioning intensity (full intensity versus reduced intensity) and the presence of acute or chronic GVHD (positive versus negative).
Main presenting clinical and biological features at the time of diagnosis and SCT for all of the patients are given in Table 2. The median leukocyte count at diagnosis was 22.8 × 109/l (range 1.1–392.5 × 109/l). Karyotype analysis revealed additional chromosomal changes in 61 (64.2%) out of the 95 patients. As to breakpoints within the BCR gene, 63 patients (66.3%) had the p190BCR-ABL1 transcript (e1a2), 31 (32.6%) had the p210BCR-ABL1 (e13a2 (b2a2) or e14a2 (b3a2)) and 1 (1.1%) had the p230BCR-ABL1 (e19a2). Before the administration of imatinib (that is, after induction chemotherapy), 77 (81.1%) of the 95 patients had achieved CHR and the other 18 patients (18.9%) were refractory. At the end of the first course of imatinib-based chemotherapy, 90 (94.7%) out of the 95 patients had achieved CHR. After the completion of two courses of imatinib-based chemotherapy, all of the patients were transplanted from a fully matched sibling (n=55) or an unrelated (n=40); 18 fully matched and 22 allele-mismatched) donor at a median time of 142 days (range 114–291 days) from the start of induction chemotherapy. Of these, 88 patients (92.6%) received SCT in first CHR.
Overall SCT outcome
A total of 54 patients developed acute GVHD (43 grade II, 8 grade III and 3 grade IV). The cumulative incidence of acute GVHD at 5 years was 56.8±3.4%. Out of the 90 patients who survived at least 100 days with sustained engraftment after SCT, 52 developed chronic GVHD (20 limited and 32 extensive), which resulted in a 5-year cumulative incidence of 54.7±3.9%. After a median follow-up of 61 months (range 24–123 months) for surviving transplants, 61 patients (64.2%) remained alive. In all, 34 (35.8%) of the 95 patients succumbed; 16 died of causes other than leukemic relapse and the remaining 18 died of progressive leukemia. Twenty-two patients (18.9%) relapsed at a median of 11 months (range 2–87 months) after SCT, and four of them remained alive with durable DFS after salvage treatment. The 5-year cumulative incidence of relapse and nonrelapse mortality were 24.9±4.8% and 18.8±4.3%, respectively, and the 5-year DFS and overall survival rate were 61.5±5.1% and 63.7±5.2%, respectively (Figure 1).
Predictive potential of molecular response to imatinib-based chemotherapy: univariate analysis
At the end of the first course of imatinib-based chemotherapy, 33 out of the 95 patients (34.7%) achieved at least MMR (MMR (n=21; 22.1%) and CMR4.5 (n=12; 12.6%), respectively), compared with baseline value. Frequencies of showing MRD levels of >0.1 to 1% (or <3-log to 2-log reduction) and >1% (or<2-log reduction) at this time point were 27 (28.4%) and 35 (36.9%), respectively (Figure 2). Patients with MRD levels of >1% (or <2-log reduction) had a higher cumulative incidence of relapse (54.1% versus 5.3% at 5 years, P=0.007) and a lower DFS (29.9% versus 95.0% at 5 years, P=0.001) than those with MMR. No significant difference was found between patients with MMR and those with CMR4.5 in terms of relapse (5.3% versus 10.0%) and DFS (95.0% versus 75.0%). There was a marginal difference in DFS between patients with MMR and those with MRD levels of >0.1 to 1% (or <3-log to 2-log reduction) (95.0% versus 69.3%, P=0.048) (Figure 3).
Thirty-three patients who had MMR or CMR4.5 post first course of imatinib-based chemotherapy remained in stable molecular response at the end of the second course of imatinib-based chemotherapy, including nine patients who showed a further MRD reduction from MMR to CMR4.5. In all, 24 out of the 27 patients who had MRD levels of >0.1 to 1% (or <3-log to 2-log reduction) post first course of imatinib-based chemotherapy achieved new MMR (n=19) or CMR4.5 (n=5). In the remaining 35 patients with MRD levels of >1% (or <2-log reduction) post first course of imatinib-based chemotherapy, 11 achieved new MMR (n=10) or CMR4.5 (n=1). Overall, on the basis of MRD quality and its kinetics by the end of two courses of imatinib-based chemotherapy, frequencies of EMR, LMR, IMR and PMR were 33 (34.7%), 35 (36.9%), 9 (9.5%) and 18 (18.9%), respectively (Figure 2). The cumulative incidence of relapse at 5 years was significantly higher for IMR (36.5% versus 7.4%, P=0.012) or PMR (86.1% versus 7.4%, P<0.001) than EMR, whereas no significant difference was found between EMR and LMR (13.0%). Consequently, EMR had a higher DFS at 5 years than IMR (87.8% versus 55.6%, P=0.009) or PMR (87.8% versus 8.3%, P<0.001). A marginal difference in DFS was found between EMR and LMR (87.8% versus 64.2%, P=0.043) (Figure 4).
Multivariate analysis of prognostic factors affecting SCT outcome
As shown in Table 3, the most powerful predictive factor affecting relapse and DFS was MRD kinetics by the end of two courses of imatinib-based chemotherapy. The relative risk (RR) of relapse at 5 years was significantly higher for IMR (RR 9.01; 95% CI 1.63–49.69; P=0.012) or PMR (RR 32.95; 95% CI 6.78–160.21; P<0.001) than EMR, whereas no difference was found between EMR and LMR. Consequently, compared with EMR, IMR (RR 4.67; 95% CI 1.16–18.79; P=0.030) or PMR (RR 26.07; 95% CI 7.93–85.69; P<0.001) had a higher risk of treatment failure in terms of DFS. LMR had a tendency toward a lower DFS (RR 3.15; 95% CI 1.00–9.92; P=0.050). It is important to note that neither post-induction hematological response nor post first course of imatinib-based chemotherapy MRD levels showed statistical power on multivariate analysis. The absence of chronic GVHD was also found to be associated with a higher relapse risk (46.5% versus 9.4%; RR 3.60; 95% CI 1.27–10.21; P=0.016).
In the imatinib era, unlike in chronic myeloid leukemia, there is no clear definition of an appropriate MRD response during first-line imatinib-based chemotherapy on long-term outcome in Ph-positive ALL. To the best of our knowledge, our current report is the first to describe the significance of MRD kinetics during a uniform first-line imatinib-based chemotherapy as a risk factor of SCT outcome by analyzing the largest number of transplants with the longest follow-up duration in adults with Ph-positive ALL.
In the present study, we found a significant correlation between MRD kinetics and long-term SCT outcome. Only 2 out of 33 EMR during imatinib-based chemotherapy relapsed after SCT, and this translated into a better SCT outcome in terms of cumulative risk of relapse (7.4% for EMR versus 13.0% for LMR versus 36.5% for IMR versus 86.1% for PMR) and DFS (87.8% for EMR versus 64.2% for LMR versus 55.6% for IMR versus 8.3% for PMR). Conversely, MRD level at an early point in time (that is, at the end of the first course of imatinib-based chemotherapy) did not retain its statistical power on multivariate analysis. Our results suggest that an insufficient or a poor molecular response at an earlier time point of imatinib-based chemotherapy may be fairly compensated by the subsequent course of imatinib-based chemotherapy, and thus MRD kinetics is more closely related to SCT outcome. Indeed, 35 patients who had insufficient or poor molecular responses post first course of imatinib-based chemotherapy achieved new MMR or CMR4.5 at the end of the second course of imatinib-based chemotherapy. Interestingly, we observed no significant difference between patients with MMR and those with CMR4.5 in terms of relapse risk and DFS. Because PCR negativity should not be regarded as a true indication of complete leukemic cell clearance within the sensitivity limit of current RQ-PCR technology, we suggest that PCR negativity itself may be an inappropriate indicator to predict long-term outcome in adult Ph-positive ALL.
Our findings are partly supported by some reports from the pre-imatinib era. In a prospective multicenter LALA-94 trial,10 PCR assessment of MRD in 63 out of 103 Ph-positive ALL patients eligible for SCT showed that PCR negativity after two courses of chemotherapy was associated with longer CHR duration and survival (P=0.01). However, earlier assessment of MRD after one course of induction chemotherapy was found to be an inaccurate indicator of clinical outcome. The GIMEMA group11 also reported upon the significance of MRD level based on RQ-PCR findings in a prospective study of 45 Ph-positive ALL patients. In their study, good molecular responders with >2-log MRD reduction post induction and >3-log MRD reduction post consolidation showed a better treatment outcome (P<0.01). However, because these data were obtained from patients treated with chemotherapy alone, it would be interesting to verify whether quantitative MRD monitoring is useful for Ph-positive ALL patients in the imatinib era.
In the imatinib era, only limited information is available regarding the prognostic impact of MRD level on treatment outcome in adult Ph-positive ALL. Yanada et al.26 conducted a prospective MRD monitoring in adults with Ph-positive ALL treated with imatinib-combined chemotherapy (including both transplants and non-transplants) and reported upon the prognostic significance of MRD level at the end of induction (n=86 (day 28) and n=85 (day 63)) and after first consolidation (n=75). However, in their study, neither PCR negativity at the end of induction nor after consolidation was associated with a lower relapse rate or survival advantage (median follow-up 3.2 years), although patients whose MRD levels exceeded 1000 copies per μg had trends toward a higher relapse rate (P=0.070). Clinical factors that could account for this discrepancy between their results and ours are differences in imatinib administration schedule (concurrent versus alternative administration of imatinib during induction phase), subject population (inclusion of non-transplants versus transplants alone), conditioning and GVHD prophylaxis (heterogenous versus homogenous), follow-up duration (3.2 versus 5 years) and center effects (multicenter versus single center). Variations in methods used to quantify BCR-ABL1 (that is, RQ-PCR condition, control gene, confirmation of PCR negativity and reporting of RQ-PCR results) should also be considered. When quantitative MRD data are analyzed in relation to treatment outcome in Ph-positive leukemia, there is stcill substantial variation in the way in which RQ-PCR for BCR-ABL1 is performed and how results are reported in different laboratories worldwide despite the efforts to establish the standardized protocols for BCR-ABL1 mRNA quantitation.27 An International program is now underway to harmonize the reporting of results according to the international scale in chronic myeloid leukemia.15, 16, 28, 29 In this regard, our study is reliable in that the measurement of BCR-ABL1 transcripts by RQ-PCR was performed at central laboratory and expressed results according to the concept of both international scale (for p210BCR-ABL1) and log reduction from the standardized pooled baseline (for p190BCR-ABL1) for the first time in Ph-positive ALL.
A major limitation of our study is the lack of mutational analysis. To date, only limited data on prevalence of BCR-ABL1 tyrosine kinase domain mutations in imatinib-naïve patients with Ph-positive ALL are available.30, 31 Using a more sensitive technique than direct sequencing, the presence of low-level BCR-ABL1 tyrosine kinase domain mutations conferring imatinib resistance has been identified in a substantial proportion of imatinib-naïve patients with Ph-positive ALL with a high degree of concordance between the type of mutation detected at baseline and at relapse.30 Remarkably, these preexisting mutant clones did not predict a response rate or remission duration and were suppressed for certain time periods by imatinib-based chemotherapy, but they almost gave rise to eventual relapse. Even among patients without detectable mutations at diagnosis, relapse was commonly associated with the outgrowth of a mutant clone. These results provide a rationale for monitoring of both MRD and mutational status over the entire treatment courses and for selection of more effective treatment strategy to eliminate clones harboring BCR-ABL1 tyrosine kinase domain mutations in Ph-positive ALL.
In summary, our data indicate that quantitative monitoring of MRD kinetics during imatinib-based chemotherapy course is likely to be useful in identifying subgroups of Ph-positive ALL transplants at a high risk of relapse. Further systemic studies on large numbers of Ph-positive ALL patients with the same MRD language are mandatory to validate our results. If this is verified, high-risk subgroup of Ph-positive ALL patients may warrant closer MRD monitoring and kinetics-based therapeutic approaches (for example, early inclusion of newer tyrosine kinase inhibitors before SCT and maintenance of tyrosine kinase inhibitors after SCT) to modify the long-term SCT outcome.
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This research was supported by Seoul St Mary’s Clinical Medicine Research Program in the year of 2009 and 2010, respectively, through the Catholic University of Korea. Statistical analyses performed in this article were advised by the Catholic Medical Center Clinical Research Coordinating Center.
The authors declare no conflict of interest.
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Lee, S., Kim, D., Cho, B. et al. Impact of minimal residual disease kinetics during imatinib-based treatment on transplantation outcome in Philadelphia chromosome-positive acute lymphoblastic leukemia. Leukemia 26, 2367–2374 (2012). https://doi.org/10.1038/leu.2012.164
- Philadelphia chromosome-positive acute lymphoblastic leukemia
- allogeneic transplantation
- minimal residual disease kinetics
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