Hematologic and cytogenetic remission by STI571 (Glivec) in a patient relapsing with accelerated phase CML after second allogeneic stem cell transplantation

BCR-ABL1 transcripts (MR4.5) at post-transplant 3 months as an early predictor for long-term outcomes in chronic myeloid leukemia

BACKGROUND/AIMS

The aim of this study was to identify the role of BCR-ABL1 transcript level as a predictor for post-transplant relapse and outcome in patients who underwent allogeneic stem cell transplantation (SCT) for chronic phase (CP) chronic myeloid leukemia (CML).

METHODS

Of 101 patients receiving allograft in CML CP, 85 had available quantitative reverse transcriptase polymerase chain reaction data at post-transplant 3 months. These patients were divided into two groups according to molecular response (MR^4.5), defined as a BCR-ABL1 transcript level ≤ 0.0032% on the international scale, at 3 months based on receiver operating characteristic curve analysis of relapse.

RESULTS

The 4-year overall survival and event-free survival (EFS) were 80.6% and 57.3%, respectively, and the cumulative incidence of relapse at 4 years was 29.6% after a median follow-up of 126.4 months. We performed multivariate analyses including potential variables to evaluate the early predictive role of MR^4.5 at 3 months and found that MR^4.5 at 3 months was associated with a higher EFS (p = 0.028) and showed a trend for a lower relapse rate (p = 0.089).

CONCLUSIONS

our results imply that frequent molecular monitoring and immune suppressive therapy modulation are required for patients without reduction of BCR-ABL1 transcripts to this level after SCT.

Molecular monitoring of BCR-ABL transcripts after allogeneic stem cell transplantation for chronic myeloid leukemia

The monitoring of minimal residual disease (MRD) through low sensitivity real-time (RT) polymerase chain reaction (PCR) analysis of BCR-ABL transcripts allows early detection of chronic myeloid leukemia (CML) relapse after allogeneic hematopoietic stem cell transplantation (HSCT). The introduction of more sensitive techniques, such as RT quantitative (Q)-PCR, may lead to an overestimation of the risk of CML relapse. In this study, we reviewed the results of peripheral blood RT Q-PCR in CML patients who underwent allogeneic HSCT from 1983 to 2007. In our laboratory, RT Q-PCR analysis was routinely performed since 2002. Eighty-seven of 189 patients had available RT Q-PCR data; 63 patients had at least 3 RT Q-PCR analyses assessable. Fifty-two of 63 patients (83%) had, at least once, detectable transcript levels, but with an BCR-ABL/ABL ratio <.1% defined as <MR3 (molecular remission <0,1%), whereas 11 (17%) had persistent undetectable BCR-ABL transcripts. Six of 52 patients with <MR3 relapsed, defined as BCR-ABL transcript numbers >.1% confirmed by the finding of Ph+ cells in bone marrow. No patients with persistent undetectable transcripts relapsed (P = .19). Relapse did not correlate with the number of occurrences of <MR3 or with the time to the first <MR3 result. Finally, of 46 patients with detectable transcripts who did not relapse, 35 had undetectable transcripts at last contact. RT Q-PCR analysis had low specificity (19%) and low positive predictive value (12%) in predicting relapse of CML patients after allogeneic HSCT. Our data suggest that detection of low BCR-ABL transcript levels by RT Q-PCR analysis has a poor accuracy in defining the risk of CML relapse and should not be considered as the unique indication to treatment. Fluctuation of BCR-ABL transcripts levels is common as late as ≥10 years posttransplantation, possibly suggesting the long-term persistence of CML stem cells.

Cumulative clinical experience from a decade of use: imatinib as first-line treatment of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a malignant disease that originates in the bone marrow and is designated by the presence of the Philadelphia (Ph+) chromosome, a translocation between chromosomes 9 and 22. Targeted therapy against CML commenced with the development of small-molecule tyrosine kinase inhibitors (TKIs) exerting their effect against the oncogenic breakpoint cluster region (BCR)-ABL fusion protein. Imatinib emerged as the first successful example of a TKI used for the treatment of chronic-phase CML patients and resulted in significant improvements in response rate and overall survival compared with previous treatments. However, a significant portion of patients failed to respond to the therapy and developed resistance against imatinib. Second-generation TKIs nilotinib and dasatinib were to have higher efficiency in clinical trials in imatinib- resistant or intolerant CML patients compared with imatinib. Identification of novel strategies such as dose escalation, drug combination therapy, and use of novel BCR-ABL inhibitors may eventually overcome resistance against BCR-ABL TKIs. This article reviews the history of CML, including the treatment strategies used prediscovery of TKIs and the preclinical and clinical data obtained after the use of imatinib, and the second-generation TKIs developed for the treatment of CML.

Detection of Trisomy 8 in Donor-Derived Ph−Cells in a Patient with Ph+Chronic Myeloid Leukemia Successfully Treated with Imatinib (STI571) in Relapse after Allogeneic Transplantation

Recent data suggest that STI571 (Imatinib) induces complete cytogenetic responses in patients with chronic myeloid leukemia (CML) who relapse after allogeneic stem cell transplantation (SCT). However, little is known about molecular responses to STI571 and the duration of leukemia-free survival in these patients. We report on a 43 year old female patient who presented with a relapse from Ph+ CML in December 2000. Five years earlier she had received an SCT from an unrelated male donor in accelerated phase. At the time of relapse, she presented with marked leukocytosis (89,000 microl) and 10% blasts. In December 2000, therapy with Imatinib was started. After 3 months, the karyotype showed an XY, with trisomy 8 in about 50% of all metaphases, but without evidence of residual Ph+ cells. Moreover, in response to Imatinib, BCR/ABL transcripts decreased and were no longer detectable after 6 months. After a total observation period of 36 months, the patient is still in complete cytogenetic and molecular remission without signs of occurrence of a donor-type hematopoietic neoplasm or CML relapse. These data suggest that Imatinib is a useful agent for long term treatment of relapsed CML after allogeneic SCT.

Dynamics and potential impact of the immune response to chronic myelogenous leukemia

Recent mathematical models have been developed to study the dynamics of chronic myelogenous leukemia (CML) under imatinib treatment. None of these models incorporates the anti-leukemia immune response. Recent experimental data show that imatinib treatment may promote the development of anti-leukemia immune responses as patients enter remission. Using these experimental data we develop a mathematical model to gain insights into the dynamics and potential impact of the resulting anti-leukemia immune response on CML. We model the immune response using a system of delay differential equations, where the delay term accounts for the duration of cell division. The mathematical model suggests that anti-leukemia T cell responses may play a critical role in maintaining CML patients in remission under imatinib therapy. Furthermore, it proposes a novel concept of an "optimal load zone" for leukemic cells in which the anti-leukemia immune response is most effective. Imatinib therapy may drive leukemic cell populations to enter and fall below this optimal load zone too rapidly to sustain the anti-leukemia T cell response. As a potential therapeutic strategy, the model shows that vaccination approaches in combination with imatinib therapy may optimally sustain the anti-leukemia T cell response to potentially eradicate residual leukemic cells for a durable cure of CML. The approach presented in this paper accounts for the role of the anti-leukemia specific immune response in the dynamics of CML. By combining experimental data and mathematical models, we demonstrate that persistence of anti-leukemia T cells even at low levels seems to prevent the leukemia from relapsing (for at least 50 months). As a consequence, we hypothesize that anti-leukemia T cell responses may help maintain remission under imatinib therapy. The mathematical model together with the new experimental data imply that there may be a feasible, low-risk, clinical approach to enhancing the effects of imatinib treatment.

Neutropenic enterocolitis in adults: systematic analysis of evidence quality

OBJECTIVE

Neutropenic enterocolitis is a life-threatening complication occurring most frequently after intensive chemotherapy in acute leukaemias. The literature is heterogeneous and a systematic review is lacking.

METHODS

Following a systematic search we categorised all relevant reports according to their quality and extracted evidence to answer the questions: Which diagnostic criteria are appropriate? What is the incidence of neutropenic enterocolitis? Are there good quality studies supporting specific interventions: Which empiric antimicrobial therapy is recommendable? Is neutropenic enterocolitis without surgical emergency complications an indication for bowel resection?

RESULTS

We found and analysed 145 articles of these reports: 64 were reports of single cases, 30 papers reported of two or three cases, 13 were narrative reviews, 34 were retrospective case series of more than three cases and four were prospective diagnostic studies. There were no prospective trials or case control studies on the therapy of neutropenic enterocolitis. There was no consensus on diagnostic criteria. We discuss the difficulty to define diagnostic criteria without having a disease definition. Histology is mostly not available in the living patients. We suggest applying a combination of clinical and radiological criteria: fever, abdominal pain and any bowel wall thickening >4 mm detected by ultrasonography (US) or computed tomography. We calculated a pooled incidence rate from 21 studies of 5.3% (266/5058; 95% CI: 4.7%-5.9%) in patients hospitalised for haematological malignancies, for high-dose chemotherapy in solid tumours or for aplastic anaemia.

CONCLUSIONS

This systematic review provides diagnostic criteria for neutropenic enterocolitis, presents a quantitative synthesis on its incidence and discusses its treatment recommendations. Prospective studies are clearly warranted.

Cost-efficacy of imatinib versus allogeneic bone marrow transplantation with a matched unrelated donor in the treatment of chronic myelogenous leukemia: a decision-analytic approach

STUDY OBJECTIVE

To develop and populate a decision-analytic model for comparing the 2-year cost and efficacy of imatinib versus allogeneic bone marrow transplantation (BMT) with a matched unrelated donor in the treatment of a 35-year-old man with newly diagnosed, Philadelphia chromosome-positive (Ph[+]) chronic myelogenous leukemia (CML) in the chronic phase.

DESIGN

Markov cohort analysis and first-order Monte Carlo microsimulation.

MEASUREMENTS AND MAIN RESULTS

Direct medical costs were measured from the perspective of a third-party payer. Efficacy data and probabilities were obtained from survivability findings, most of which were derived from randomized controlled trials. We employed a 2-year time horizon with 3-month treatment cycles. The comparator was BMT with a matched unrelated donor, and the base case was defined as a 35-year-old, Ph(+) man with newly diagnosed CML. The Monte Carlo microsimulation indicated that the incremental cost:efficacy ratio was -$5000 for imatinib (95% confidence interval -$70,000-84,000). Analysis of the cost-efficacy plane revealed that imatinib was dominant over BMT in 84.69% of cases, whereas BMT dominated imatinib in 0.76% of cases. Trade-offs were warranted in the remaining cases. Sensitivity analyses of costs and discount rates found these results to be generally robust.

CONCLUSION

In most cases, imatinib was both less costly and more efficacious than BMT in the 2-year treatment of CML. Results of this investigation should be viewed in the context of emerging long-term clinical data. These data are necessary to assess cost-efficacy beyond the short-term time horizon of this study.

Early prediction of molecular remission by monitoring BCR-ABL transcript levels in patients achieving a complete cytogenetic response after imatinib therapy for posttransplantation chronic myelogenous leukemia relapse

Imatinib induces a high complete cytogenetic response (CCR) rate in relapsed chronic myelogenous leukemia. By analyzing minimal residual disease (MRD) under the levels of CCR, we tried to assess the molecular response after imatinib therapy. By using real-time quantitative reverse transcriptase-polymerase chain reaction (Q-RT-PCR), MRD was evaluated in 23 patients (3 in cytogenetic relapse, 6 in chronic phase, 9 in accelerated phase, and 5 in blast crisis) who were treated with standard-dose imatinib for relapsed chronic myelogenous leukemia after allogeneic stem cell transplantation. With a median therapy time of 399 days (range, 35-817 days), 19 (83%) patients achieved a CCR. Meanwhile, 11 (58%) of them achieved a molecular remission (MR), which was associated with improved survival. The Q-RT-PCR data were compared according to the best response (MR, n = 11; CCR, n = 8) in the patients achieving a CCR. The BCR-ABL/ABL ratios were similar in 2 groups at 3 months but were significantly different at 6 months (median, 0.0000012 for MR and 0.00022 for CCR; P =.003). The probability of a subsequent MR was significantly higher in patients with a lower BCR-ABL/ABL ratio at 6 months (100% for <0.0001 versus 33% for >/=0.0001; P =.006) or a greater reduction in the level between 3 and 6 months (log-reduction >/=1.0;, 100%; <1.0, 17%; P =.003). Q-RT-PCR is a reliable method for monitoring MRD: the early trends in the BCR-ABL/ABL ratio may be clinically useful in discriminating patients who will achieve an MR from those who will remain in CCR.

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

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

Allogeneic hematopoietic stem cell transplantation for chronic myeloid leukemia

There never has been a more difficult time to advise patients newly diagnosed with chronic myeloid leukemia (CML). Until recently,the options comprised noncurative but relatively nontoxic chemotherapy or potentially curative allogeneic stem cell transplantation,with its attendant morbidity and mortality. There now is the additional option of the tyrosine kinase inhibitor imatinib mesylate that has been in clinical practice for almost 5 years. Although data are emerging regarding the efficacy of imatinib, solid evidence of any prolongation in survival will be delayed for several years. Future management of CML will continue to depend on a combination of approaches that use allografting and targeted drug therapy. The next decade undoubtedly will witness the introduction of a number of agents capable of inhibiting signal transduction pathways,perhaps controlling CML in cases of primary or acquired imatinib resistance. In the absence of long-term outcome data for imatinib,it remains reasonable to propose that young patients with newly diagnosed CML who have HLA-identical or well-matched unrelated donors should undergo allogeneic transplantation using a standard or nonmyeloablative conditioning regimen. Similarly,patients who fail to respond to imatinib should be rescued with a transplant strategy. The role of molecular monitoring in these two strategies cannot be underestimated.

Preemptive treatment of minimal residual disease post transplant in CML using real-time quantitative RT-PCR: a prospective, randomized trial

Immunotherapy in the form of donor lymphocyte infusions in early-phase relapse might be advantageous as it induces a higher response, but this may be offset by increased toxicity, especially during the early period after transplantation. Among 45 consecutive patients receiving an allograft for CML, 13 patients were diagnosed to have molecular relapse (MRel), as defined by real-time quantitative reverse transcriptase-polymerase chain reaction, and another four patients were diagnosed to have cytogenetic relapse (CRel) within 6 months. Patients with MRel were randomly assigned to either a 'no therapy' group (group A, n=6), in which immunotherapy was reserved until CRel, or an 'immunotherapy' group (group B, n=7). In group A, all MRel progressed to CRel, and molecular remission (MR) was achieved in four (67%) after immunotherapy. The remaining two patients died of extensive GVHD and fungal pneumonia. In group B, only two MRel progressed to CRel and the remaining five (71%) achieved MR. Two patients died in the absence or loss of response. In patients relapsing directly into CRel (n=4), immunotherapy induced MR in two patients (50%). Earlier intervention played a role in preventing disease progression but this effect was not translated into better survival, which could have been overcome by imatinib mesylate, which induced MR and cytogenetic remission in nonresponders without toxicity.

Chronic myeloid leukemia: pathophysiology, diagnostic parameters, and current treatment concepts

Chronic myeloid leukemia (CML) is a stem cell disease characterized by excessive accumulation of clonal myeloid (precursor) cells in hematopoietic tissues. CML cells display the translocation t(9; 22) that creates the bcr/abl oncogene. The respective oncoprotein (= BCR/ABL) exhibits constitutive tyrosine kinase activity and promotes growth and survival in CML cells. Clinically, CML can be divided into three phases: the chronic phase (CP), the accelerated phase (AP), and the blast phase (BP) that resembles acute leukemia. Progression to AP and BP is associated with occurrence of additional genetic defects that cooperate with bcr/abl in leukemogenesis and lead to resistance against antileukemic drugs. The prognosis in CML is variable depending on the phase of disease, age, and response to therapy. The only curative approach available to date is stem cell transplantation. For those who cannot be transplanted, the BCR/ABL tyrosine kinase inhibitor STI571 (Glivec, Imatinib), interferon-alpha (with or without ARAC), or other cytoreductive drugs are prescribed. Currently available data show that STI571 is a superior compound compared to other drugs in producing complete cytogenetic and molecular responses. However, despite superior initial data and high expectations for an effect on survival, long term results are not available so far, and resistance against STI571 has been reported. Forthcoming strategies are therefore attempting to prevent or counteract STI571 resistance by co-administration of other antileukemic drugs. Whether these strategies will lead to curative drug therapy in CML in the future remains at present unknown.

Response to imatinib in patients who relapse after allogeneic stem cell transplantation for chronic myeloid leukemia

We studied 128 patients with chronic myeloid leukemia (CML) relapsing after allogeneic stem cell transplantation (SCT). Disease at the time of treatment with Imatinib was in chronic phase (CP) in 51 patients, accelerated phase (AP) in 31 and blastic crisis (BC) in 46. Of the 51 patients in CP, 14 were in cytogenetic and two in molecular relapses. The median interval between relapse and Imatinib therapy was 5 months (0-65). A total of 50 patients had failed treatment with donor lymphocyte infusions prior to Imatinib. The overall hemato-logical response rate was 84% (98% for patients relapsing in CP). The complete cytogenetic response (CCR) was 58% for patients in CP, 48% for AP and 22% for patients in BC. Complete molecular responses were obtained in 25 patients (26%), of whom 21 were in CP or AP. With a median follow-up of 9 months, the estimated 2-year survival for CP, AP and BC patients was 100, 86 and 12%, respectively. Out of 79 evaluable patients, 45 (57%) achieved full donor and 11 (14%) mixed chimerism after Imatinib. We conclude that Imatinib has significant activity against CML in relapse after allogeneic SCT. Durable cytogenetic and molecular remissions are obtainable in patients in CP.

Specific targeted therapy of chronic myelogenous leukemia with imatinib

Chronic myeloid leukemia (CML) is characterized by the Philadelphia translocation that fuses BCR sequences from chromosome 22 upstream of the ABL gene on chromosome 9. The chimerical Bcr-Abl protein expressed by CML cells has constitutive tyrosine kinase activity, which is essential for the pathogenesis of the disease. Imatinib, an ATP-competitive selective inhibitor of Bcr-Abl, has unprecedented efficacy for the treatment of CML. Most patients with early stage disease achieve durable complete hematological and complete cytogenetic remissions, with minimal toxicity. In contrast, responses are less stable in patients with advanced CML. This review highlights the pathogenesis of CML, its clinical features, and the development of imatinib as a specific molecularly targeted therapy. Aspects of disease monitoring and side effects are covered as well as resistance to imatinib and strategies to overcome resistance, such as alternative signal transduction inhibitors and drug combinations. Perspectives for further development are also discussed.

Stable molecular remission induced by imatinib mesylate (STI571) in a patient with CML lymphoid blast crisis relapsing after allogeneic stem cell transplantation

We report the response to the ABL kinase inhibitor imatinib mesylate (STI571) in a patient with chronic myeloid leukemia (CML) who relapsed twice after dose-reduced allogeneic stem cell transplantation (alloSCT) for B lymphoid blast crisis (BC) and failed to develop an antileukemic response despite grade 3 graft-versus-host disease (GvHD). Complete hematologic, cytogenetic and molecular responses were achieved within 9 weeks of therapy and are maintained after 27 months. Extensive chronic skin GvHD necessitating immunosuppressive therapy developed after 14 months. This case illustrates the ability of imatinib to induce sustained hematologic and molecular remissions in some patients relapsing with advanced stage CML after alloSCT.

Perspectives on the treatment of chronic phase and advanced phase CML and Philadelphia chromosome positive ALL(1)

Chronic myeloid leukaemia (CML) is a malignant disease of the bone marrow characterised by the presence of the Philadelphia (Ph) chromosome. About 20% of acute lymphoblastic leukaemia (ALL) patients also show this genetic abnormality. A new drug, imatinib (Glivec, Novartis Pharma AG, Basel, Switzerland, and formerly STI571) is having a profound effect on the treatment and management of all stages of CML and Philadelphia chromosome positive (Ph+) ALL. New treatment algorithms are being developed. Should imatinib replace or be combined with existing therapies? To address this question, we review the pros and cons of therapy with interferon-alpha (IFN-alpha), allogeneic transplantation, autologous transplantation, imatinib, and in the case of Ph+ ALL, chemotherapy and experimental approaches. Conservative and aggressive treatments will be discussed and new molecular methods of monitoring cytogenetic response and their significance will also be reviewed.

Imatinib mesylate (STI571) in preparation for allogeneic hematopoietic stem cell transplantation and donor lymphocyte infusions in patients with Philadelphia-positive acute leukemias

Chronic myeloid leukemia in blast crisis (BC) and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph(+) ALL) are associated with extremely poor outcome. Allogeneic transplantation during BC or active leukemia is most often unsuccessful due to high-rates of both treatment-related complications and relapse. Long-term results are significantly better if a second chronic phase or remission can be achieved prior to transplantation. Similarly, DLI given for the treatment of post-transplant relapse is more successful when given during a second remission. In this study we report our results with a previously unreported approach consisting of short-term treatment with imatinib mesylate (formerly, STI571) to induce or maintain remission, followed by allogeneic transplantation or DLI and the impact on transplantation/DLI outcome. Sixteen patients were treated either in preparation for transplantation (n = 12), for DLI (n = 1), or for both (n = 3). Ten had CML in BC; seven myeloid and three lymphoid BC. Six patients had Ph(+) ALL. The donors were matched unrelated (n = 9), matched siblings (n = 5) or haplo-identical (n = 2). Eleven of 15 patients given imatinib pre-transplant were transplanted in complete hematologic response. Engraftment and GVHD rates were not different from expected. Seven patients had grade II-III hepatic toxicity after transplantation. After a median follow-up of 10 months (range, 3-16 months) six remain alive, two after further therapy. The 1-year survival rate was 25%. Four patients were given imatinib prior to DLI, all had complete response. Two remain in remission >6 months from relapse. In conclusion, treatment with imatinib allows transplantation in a more favorable status or maintaining remission with low toxicity until transplantation is feasible. Pre-transplant imatinib seems safe and not associated with excess post-transplant complications. Imatinib may have substantial activity in combination with DLI. Further study of a larger group of patients is required to assess the impact on long-term outcome and the role of post-transplant imatinib in controlling residual disease.

Tyrosine kinase inhibitor STI571 in the treatment of Philadelphia chromosome-positive leukaemia failing myeloablative stem cell transplantation

Eight patients with Philadelphia chromosome-positive (Ph(+)) leukaemia relapsing from stem cell transplantation (SCT) (one syngeneic and seven allogeneic) were treated with the tyrosine kinase inhibitor STI571. Five patients relapsing as chronic myeloid leukaemia (CML) in chronic phase achieved a complete haematological response, with complete and major cytogenetic responses occurring in four and one cases, respectively. One patient became negative for BCR/ABL in the bone marrow. Three patients relapsed as acute leukaemia (two CML in myeloblastic crisis and one Ph(+) acute lymphoblastic leukaemia), all of whom achieved haematological and cytogenetic responses. One patient also became BCR/ABL negative. However, pancytopenia and graft-versus-host disease led to cessation of treatment in the remaining two cases, which was followed by disease recurrence refractory to further STI treatment. Our results showed that Ph(+) leukaemic relapses after SCT might respond well to STI571 therapy.

Effects of imatinib on bone marrow engraftment in syngeneic mice

Chronic myeloid leukemia (CML) and a subset of acute lymphoblastic leukemias arise from the genetic reciprocal translocation t(9;22), forming the BCR-ABL fusion gene. These lead to the expression of the constitutively active tyrosine kinase BCR-ABL, which is the causative oncogene for these leukemias. Allogeneic bone marrow transplantation (BMT) or stem cell transplantation (SCT) is currently considered the only curative treatment for chronic myeloid leukemia (CML). Recently, the selective tyrosine kinase inhibitor imatinib mesylate (Glivec, formerly STI-571) has been shown to induce durable hematologic and major cytogenetic responses in a high percentage of patients with chronic phase CML. In patients with advanced disease remissions are transient and most patients relapse despite continued imatinib treatment. Some of these patients go on to receive allogeneic BMT or SCT, during which administration of imatinib is usually discontinued as it is believed to interfere with bone marrow engraftment. In this study, we examined the effect of imatinib on hematopoietic engraftment in a syngeneic mouse model. We found that imatinib has no significant influence on hematopoietic recovery in lethally irradiated mice in vivo. Thus, our results suggest that continued administration of imatinib in the course of BMT or SCT may be a feasible therapeutic regimen.