Secondary basophilic leukemia in Ph-negative myeloid neoplasms: A distinct subset with poor prognosis.

During progression of myeloid neoplasms, the basophil compartment may expand substantially and in some of these patients, a basophilic leukemia is diagnosed. In patients with Ph-chromosome+ chronic myeloid leukemia, acceleration of disease is typically accompanied by marked basophilia. In other myeloid neoplasms, secondary leukemic expansion of basophils is rarely seen. We report on 5 patients who suffered from a myelodysplastic syndrome, myeloproliferative neoplasm, or acute leukemia and developed a massive expansion of basophils during disease progression. In 4 of 5 patients, peripheral blood basophil counts reached 40%, and the diagnosis "secondary basophilic leukemia" was established. As assessed by flow cytometry, neoplastic basophils expressed CD9, CD18, CD25, CD33, CD63, PD-L1, CD123, and CLL-1. In addition, basophils were found to display BB1 (basogranulin), 2D7, tryptase and KIT. In 4 of 5 patients the disease progressed quickly and treatment with azacitidine was started. However, azacitidine did not induce major clinical responses, and all patients died from progressive disease within 3 Y. In in vitro experiments, the patients´ cells and the basophilic leukemia cell line KU812 showed variable responses to targeted drugs, including azacitidine, venetoclax, hydroxyurea, and cytarabine. A combination of venetoclax and azacitidine induced cooperative antineoplastic effects in these cells. Together, secondary basophilic leukemia has a poor prognosis and monotherapy with azacitidine is not sufficient to keep the disease under control for longer time-periods. Whether drug combination, such as venetoclax+azacitidine, can induce better outcomes in these patients remains to be determined in future clinical studies.

Attenuation of canonical NF-κB signaling maintains function and stability of human Treg.

Nuclear factor 'κ-light-chain-enhancer' of activated B cells (NF-κB) signaling is a signaling pathway used by most immune cells to promote immunostimulatory functions. Recent studies have indicated that regulatory T cells (Treg) differentially integrate TCR-derived signals, thereby maintaining their suppressive features. However, the role of NF-κB signaling in the activation of human peripheral blood (PB) Treg has not been fully elucidated so far. We show that the activity of the master transcription factor forkhead box protein 3 (FOXP3) attenuates p65 phosphorylation and nuclear translocation of the NF-κB proteins p50, p65, and c-Rel following activation in human Treg. Using pharmacological and genetic inhibition of canonical NF-κB signaling in FOXP3-transgenic T cells and PB Treg from healthy donors as well as Treg from a patient with a primary NFKB1 haploinsufficiency, we validate that Treg activation and suppressive capacity is independent of NF-κB signaling. Additionally, repression of residual NF-κB signaling in Treg further enhances interleukin-10 (IL-10) production. Blockade of NF-κB signaling can be exploited for the generation of in vitro induced Treg (iTreg) with enhanced suppressive capacity and functional stability. In this respect, dual blockade of mammalian target of rapamycin (mTOR) and NF-κB signaling was accompanied by enhanced expression of the transcription factors FOXP1 and FOXP3 and demethylation of the Treg-specific demethylated region compared to iTreg generated under mTOR blockade alone. Thus, we provide first insights into the role of NF-κB signaling in human Treg. These findings could lead to strategies for the selective manipulation of Treg and the generation of improved iTreg for cellular therapy.

Inhibition of complement C1s in patients with cold agglutinin disease: lessons learned from a named patient program.

Cold agglutinin disease (CAD) causes predominantly extravascular hemolysis and anemia via complement activation. Sutimlimab is a novel humanized monoclonal antibody directed against classical pathway complement factor C1s. We aimed to evaluate the safety and efficacy of long-term maintenance treatment with sutimlimab in patients with CAD. Seven CAD patients treated with sutimlimab as part of a phase 1B study were transitioned to a named patient program. After a loading dose, patients received biweekly (once every 2 weeks) infusions of sutimlimab at various doses. When a patient's laboratory data showed signs of breakthrough hemolysis, the dose of sutimlimab was increased. Three patients started with a dose of 45 mg/kg, another 3 with 60 mg/kg, and 1 with a fixed dose of 5.5 g every other week. All CAD patients responded to re-treatment, and sutimlimab increased hemoglobin from a median initial level of 7.7 g/dL to a median peak of 12.5 g/dL (P = .016). Patients maintained near normal hemoglobin levels except for a few breakthrough events that were related to underdosing and which resolved after the appropriate dose increase. Four of the patients included were eventually treated with a biweekly 5.5 g fixed-dose regimen of sutimlimab. None of them had any breakthrough hemolysis. All patients remained transfusion free while receiving sutimlimab. There were no treatment-related serious adverse events. Overlapping treatment with erythropoietin, rituximab, or ibrutinib in individual patients was safe and did not cause untoward drug interactions. Long-term maintenance treatment with sutimlimab was safe, effectively inhibited hemolysis, and significantly increased hemoglobin levels in re-exposed, previously transfusion-dependent CAD patients.

Treatment with brentuximab vedotin plus bendamustine in unselected patients with CD30-positive aggressive lymphomas.

OBJECTIVES: A treatment regimen consisting of bendamustine and brentuximab vedotin (BV) has been described as a highly potent salvage therapy and as an effective induction therapy leading to high response rates before autologous stem cell transplantation (ASCT) in patients with classical Hodgkin lymphoma (cHL). In this retrospective analysis, we aimed to assess this therapy's efficacy in unselected patients with cHL and CD30+ peripheral T-cell lymphoma (PTCL). PATIENTS AND METHODS: Data of 28 patients with cHL and five patients with PTCL treated with a combination of bendamustine and BV at three Austrian tertiary cancer centers were analyzed. RESULTS: In patients with cHL, the ORR was 100% (78.6% CR, 21.4% PR). After 17 months median follow-up, median survival times were not reached; 1-year PFS was 81.9%, and 1-year OS was 95.7%. Thirteen eligible patients (46.4%) successfully underwent planned ASCT after salvage therapy with bendamustine and BV and subsequent high-dose chemotherapy. Three of the five PTCL patients achieved CR, while two did not respond and died during or shortly after therapy. CONCLUSION: A combination of bendamustine and BV is an effective salvage and induction therapy before ASCT in patients with relapsed/refractory cHL. Further research is warranted to evaluate the use in patients with PTCL.

Nivolumab for Newly Diagnosed Advanced-Stage Classic Hodgkin Lymphoma: Safety and Efficacy in the Phase II CheckMate 205 Study.

PURPOSE: Nivolumab, an anti-programmed death-1 monoclonal antibody, has demonstrated frequent and durable responses in relapsed/refractory classic Hodgkin lymphoma (cHL). We report results from Cohort D of the CheckMate 205 trial, which assessed nivolumab monotherapy followed by nivolumab plus doxorubicin, vinblastine, and dacarbazine (N-AVD) for newly diagnosed cHL. METHODS: Patients 18 years of age or older with untreated, advanced-stage (defined as III to IV and IIB with unfavorable risk factors) cHL were eligible for Cohort D of this multicenter, noncomparative, phase II trial. Patients received nivolumab monotherapy for four doses, followed by 12 doses of N-AVD; all doses were every 2 weeks, and nivolumab was administered at 240 mg intravenously. The primary end point was safety. Efficacy end points included objective response rate and modified progression-free survival, defined as time to disease progression/relapse, death, or next therapy. Chromosome 9p24.1 alterations and programmed death-ligand 1 expression were assessed in Hodgkin Reed-Sternberg cells in evaluable patients. RESULTS: A total of 51 patients were enrolled and treated. At diagnosis, 49% of patients had an International Prognostic Score of 3 or greater. Overall, 59% experienced a grade 3 to 4 treatment-related adverse event. Treatment-related febrile neutropenia was reported in 10% of patients. Endocrine immune-mediated adverse events were all grade 1 to 2 and did not require high-dose corticosteroids; all nonendocrine immune-mediated adverse events resolved (most commonly, rash; 5.9%). At the end of therapy, the objective response rate (95% CI) per independent radiology review committee was 84% (71% to 93%), with 67% (52% to 79%), achieving complete remission (five patients [10%] were nonevaluable and counted as nonresponders). With a minimum follow-up of 9.4 months, 9-month modified progression-free survival was 92%. Patients with higher-level Hodgkin Reed-Sternberg programmed death-ligand 1 expression had more favorable responses to N-AVD (P = .041). CONCLUSION: Nivolumab followed by N-AVD was associated with promising efficacy and safety profiles for newly diagnosed, advanced-stage cHL.

Inhibition of complement C1s improves severe hemolytic anemia in cold agglutinin disease: a first-in-human trial.

Cold agglutinin disease is a difficult-to-treat autoimmune hemolytic anemia in which immunoglobulin M antibodies bind to erythrocytes and fix complement, resulting in predominantly extravascular hemolysis. This trial tested the hypothesis that the anti-C1s antibody sutimlimab would ameliorate hemolytic anemia. Ten patients with cold agglutinin disease participated in the phase 1b component of a first-in-human trial. Patients received a test dose of 10-mg/kg sutimlimab followed by a full dose of 60 mg/kg 1 to 4 days later and 3 additional weekly doses of 60 mg/kg. All infusions were well tolerated without premedication. No drug-related serious adverse events were observed. Seven of 10 patients with cold agglutinin disease responded with a hemoglobin increase >2 g/dL. Sutimlimab rapidly increased hemoglobin levels by a median of 1.6 g/dL within the first week, and by a median of 3.9 g/dL (interquartile range, 1.3-4.5 g/dL; 95% confidence interval, 2.1-4.5) within 6 weeks (P = .005). Sutimlimab rapidly abrogated extravascular hemolysis, normalizing bilirubin levels within 24 hours in most patients and normalizing haptoglobin levels in 4 patients within 1 week. Hemolytic anemia recurred when drug levels were cleared from the circulation 3 to 4 weeks after the last dose of sutimlimab. Reexposure to sutimlimab in a named patient program recapitulated the control of hemolytic anemia. All 6 previously transfused patients became transfusion-free during treatment. Sutimlimab was safe, well tolerated, and rapidly stopped C1s complement-mediated hemolysis in patients with cold agglutinin disease, significantly increasing hemoglobin levels and precluding the need for transfusions. This trial was registered at www.clinicaltrials.gov as #NCT02502903.

Ultra-early response assessment in lymphoma treatment: [18F]FDG PET/MR captures changes in glucose metabolism and cell density within the first 72 hours of treatment.

PURPOSE: To determine whether, in patients with Hodgkin lymphoma (HL) or non-Hodgkin lymphoma (NHL), [18F]FDG PET/MR can capture treatment effects within the first week after treatment initiation, and whether changes in glucose metabolism and cell density occur simultaneously. METHODS: Patients with histologically proven HL or NHL were included in this prospective IRB-approved study. Patients underwent [18F]FDG PET/MR before, and then 48-72 h after (follow-up 1, FU-1) and 1 week after (FU-2) initiation of the first cycle of their respective standard chemotherapy (for HL) or immunochemotherapy (for NHL). Standardized [18F]FDG uptake values (SUVmax, SUVmean) and apparent diffusion coefficients (ADCmin, ADCmean) based on diffusion-weighted MRI, and metabolic and morphological tumour volumes (MTV, VOL) were assessed at each time-point. Multilevel analyses with an unstructured covariance matrix, and pair-wise post-hoc tests were used to test for significant changes in SUVs, ADCs, MTVs and VOLs between the three time-points. RESULTS: A total of 58 patients (11 with HL and 47 with NHL) with 166 lesions were analysed. Lesion-based mean rates of change in SUVmax, SUVmean, ADCmin, ADCmean, MTV and VOL between baseline and FU-1 were -46.8%, -33.3%, +20.3%, +14%, -46% and -12.8%, respectively, and between baseline and FU-2 were -65.1%, -49%, +50.7%, +32.4%, -61.1% and -24.2%, respectively. These changes were statistically significant (P < 0.01) except for the change in VOL between baseline and FU-1 (P = 0.079). CONCLUSION: In lymphoma patients, [18F]FDG PET/MR can capture treatment-induced changes in glucose metabolism and cell density as early as 48-72 h after treatment initiation.

Probability of remaining in unsustained complete remission after steroid therapy withdrawal in patients with primary warm-antibody reactive autoimmune hemolytic anemia.

BACKGROUND: Primary warm autoimmune hemolytic anemia (WAIHA) is a rare autoimmune disorder frequently responding to corticosteroid first-line treatment and effective second-line treatment options such as splenectomy or anti-CD20 antibody therapy. Disease management is frequently hampered by a lack of evidence. METHODS: We have investigated the probability of sustained treatment-free remission after steroid induction to facilitate clinical decision making regarding timing and necessity of second-line treatments. Response data from 31 patients with primary WAIHA initially treated with steroids were retrospectively analyzed. All patients responded by achieving a hemoglobin of at least 10 mg/dl. RESULTS: After steroid tapering and final withdrawal, 9 of 30 patients remained in unsustained complete remission (CR). The probability of remaining in CR after steroid treatment only was 38.2 % (2 SD 20.6 %) at 15 months. The median remission duration was 100 + months with a range of 12 + to 163 + months. Of note, none of the remaining patients still on steroids achieved CR beyond 15 + months. CONCLUSION: These data indicate that a considerable proportion of patients do not need further treatment and that relapses will not occur after 15 months in CR.

Evaluation of Diffusion-Weighted Magnetic Resonance Imaging for Follow-up and Treatment Response Assessment of Lymphoma: Results of an 18F-FDG-PET/CT-Controlled Prospective Study in 64 Patients.

PURPOSE: To determine the value of diffusion-weighted MRI (DWI-MRI) for treatment response assessment in 2-[18F]fluoro-2-deoxy-D-glucose (FDG)-avid lymphoma. EXPERIMENTAL DESIGN: Patients with FDG-avid Hodgkin (HL) or non-Hodgkin lymphoma (NHL) at pretherapeutic 18F-FDG-PET/CT, who had also undergone pretherapeutic whole-body DWI-MRI, were included in this prospective study. Depending on the histologic lymphoma subtype, patients received different systemic treatment regimens, and follow-up DWI-MRI and 18F-FDG-PET/CT were performed at one or more time points, depending on the clinical course. For each follow-up DWI-MRI, region-based rates of agreement, and rates of agreement in terms of treatment response (complete remission, partial remission, stable disease, or progressive disease), relative to the corresponding 18F-FDG-PET/CT, were calculated. RESULTS: Sixty-four patients were included: 10 with HL, 22 with aggressive NHL, and 32 with indolent NHL. The overall region-based agreement of DWI-MRI with 18F-FDG-PET/CT was 99.4%. For the 51 interim examinations (performed after 1-3 therapy cycles), region-based agreement of DWI-MRI with 18F-FDG-PET/CT was 99.2%, and for the 48 end-of-treatment examinations, agreement was 99.8%. No significant differences, in terms of region-based agreement between DWI-MRI and 18F-FDG-PET/CT, were observed between the three lymphoma groups (HL, aggressive NHL, indolent NHL; P = 0.25), or between interim and end-of-treatment examinations (P = 0.21). With regard to treatment response assessment, DWI-MRI agreed with 18F-FDG-PET/CT in 99 of 102 follow-up examinations (97.1%), with a κ value of 0.94 (P < 0.0001). CONCLUSIONS: In patients with FDG-avid lymphoma, DWI-MRI may be a feasible alternative to 18F-FDG-PET/CT for follow-up and treatment response assessment.

Serum-tryptase at diagnosis: a novel biomarker improving prognostication in Ph(+) CML.

Basophilia is an established prognostic variable in Ph-chromosome+ chronic myeloid leukemia (CML). However, in CML, basophils are often immature and thus escape microscopic quantification. We have previously shown that tryptase is produced and secreted by immature CML basophils. In the current study, serum samples of 79 CML patients (chronic phase=CP, n=69; accelerated/blast phase=AP/BP, n=10) treated with BCR/ABL inhibitors, were analyzed for their tryptase content. Serum-tryptase levels at diagnosis were found to correlate with basophil counts and were higher in AP/BP patients (median tryptase: 29.9 ng/mL) compared to patients with CP (11.7 ng/mL; p<0.05). In 20/69 patients with CP, progression occurred. The progression-rate was higher in patients with tryptase >15 ng/mL (31%) compared to those with normal tryptase levels (9%, p<0.05). To validate tryptase as new prognostic variable, we replaced basophils by tryptase levels in the EUTOS score. This modified EUTOS-T score was found to predict progression-free and event-free survival significantly better, with p values of 0.000064 and 0.00369, respectively, compared to the original EUTOS score (progression-free survival: p=0.019; event-free survival: p=0.156). In conclusion, our data show that the serum-tryptase level at diagnosis is a powerful prognostic biomarker in CML. Inclusion of tryptase in prognostic CML scores may improve their predictive value.

Cold antibody autoimmune hemolytic anemia and lymphoproliferative disorders: a retrospective study of 20 patients including clinical, hematological, and molecular findings.

A total of 20 patients with cold antibody hemolytic anemia were evaluated in a retrospective study of them, 15 had a monoclonal gammopathy of unknown significance (MGUS): 14 with MGUS of immunoglobulin M (IgM) subtype and 1 with immunoglobulin G subtype. One patient had smoldering Waldenström's macroglobulinemia, but four patients had no monoclonal protein and no evidence of lymphoma. However, in three of these patients, we were able to demonstrate a (mono-)clonal rearrangement of their immunoglobulin heavy and/or light chains. Of the 20 patients, 5 had IgHV34 nucleotide sequence indicating that the antibody was directed against the "I" antigen. Two patients exhibited a progressive increase of IgM over time, however without increasing hemolytic activity. Moreover, in two patients with long-term follow-up, we were able to correlate recurrent hemolytic activity with low environmental temperatures. Among four patients treated with rituximab, all four responded to treatment. However, treatment effect was only transient in all of them.

Evaluation of diffusion-weighted MRI for pretherapeutic assessment and staging of lymphoma: results of a prospective study in 140 patients.

PURPOSE: To determine the value of diffusion-weighted MRI (DWI-MRI) for pretherapeutic imaging of fluorodeoxyglucose (FDG)-avid lymphoma and lymphoma with variable FDG avidity. EXPERIMENTAL DESIGN: Treatment-naïve patients with lymphoma who were referred for whole-body staging were included in this prospective study. Group A included patients with FDG-avid lymphoma (e.g., Hodgkin, diffuse large B-cell, and follicular lymphoma), whereas Group B included patients with lymphoma of variable FDG avidity [e.g., extranodal marginal zone B-cell lymphoma of the mucosa-associated lymphoid tissue (MALT)]. All patients underwent DWI-MRI and 18F-FDG- positron emission tomography/computed tomography (PET/CT). Region-based sensitivity and agreement with Ann Arbor staging, relative to the reference standard, were calculated for DWI-MRI, and, in Group B, also 18F-FDG-PET/CT and contrast-enhanced (CE-) CT. RESULTS: In Group A (100 patients), DWI-MRI had a region-based sensitivity of 97%, and with regard to staging, agreed with the reference standard in 94 of 100 patients (κ, 0.92). In Group B (40 patients; 38 MALT lymphomas and 2 small lymphocytic lymphomas/chronic lymphocytic leukemias), DWI-MRI, 18F-FDG-PET/CT, and CE-CT had region-based sensitivities of 94.4%, 60.9%, and 70.7%, respectively. With regard to staging in Group B, DWI-MRI, 18F-FDG-PET/CT, and CE-CT agreed with the reference standard in 37 of 40, 26 of 40, and 24 of 40 patients, with κ values of 0.89, 0.52, and 0.43, respectively. CONCLUSIONS: In patients with FDG-avid lymphoma, DWI-MRI seems to be only slightly inferior to 18F-FDG-PET/CT with regard to pretherapeutic regional assessment and staging. In patients with lymphoma subtypes that show a variable FDG avidity (e.g., MALT lymphoma), DWI-MRI seems to be superior to both 18F-FDG-PET/CT and CE-CT.

Endogenous erythroid colony formation in chronic myeloid leukemia: a recurrent finding associated with persistent minimal residual disease under imatinib.

In vitro endogenous erythroid colony (EEC) formation is a common finding in BCR-ABL-negative myeloproliferative neoplasms. The aim of the present study was to determine the prevalence and the clinical significance of EEC growth in chronic myeloid leukemia (CML). Results of clonogeneic progenitor cell assays from 52 patients with newly diagnosed CML were correlated with disease characteristics at presentation and molecular response to imatinib. EECs (median 7 per dish, range 1-39) were detectable in 16 patients (31%). The proportion of patients with a high-risk Sokal score was lower in the EEC group (7% vs. 30%, respectively). The cumulative incidence of achieving a major molecular response after 2 years of imatinib was similar for both groups. However, patients with EECs were less likely to achieve a more profound decline of BCR-ABL transcripts. After 6 years of imatinib, the cumulative probability [95% CI] of reaching a ≥4 log reduction of BCR-ABL was 48% [16%; 92%] for patients of the EEC group and 84% [63%; 97%] for patients of the No EEC group. The probability [95% CI] of achieving a >4.5 log reduction of BCR-ABL after 7 years was 13% [2%; 61%] for patients with EECs and 52% [30%; 78%] for patients without EECs. In vitro EECs disappeared after achievement of a major molecular response in all evaluable patients. The data indicate that EEC formation is a recurrent finding in patients with CML which deserves further attention as a possible biomarker predicting the degree of molecular response to imatinib.

Identification of basophils as a major source of hepatocyte growth factor in chronic myeloid leukemia: a novel mechanism of BCR-ABL1-independent disease progression.

Chronic myeloid leukemia (CML) is a hematopoietic neoplasm characterized by the Philadelphia chromosome and the related BCR-ABL1 oncoprotein. Acceleration of CML is usually accompanied by basophilia. Several proangiogenic molecules have been implicated in disease acceleration, including the hepatocyte growth factor (HGF). However, little is known so far about the cellular distribution and function of HGF in CML. We here report that HGF is expressed abundantly in purified CML basophils and in the basophil-committed CML line KU812, whereas all other cell types examined expressed only trace amounts of HGF or no HGF. Interleukin 3, a major regulator of human basophils, was found to promote HGF expression in CML basophils. By contrast, BCR-ABL1 failed to induce HGF synthesis in CML cells, and imatinib failed to inhibit expression of HGF in these cells. Recombinant HGF as well as basophil-derived HGF induced endothelial cell migration in a scratch wound assay, and these effects of HGF were reverted by an anti-HGF antibody as well as by pharmacologic c-Met inhibitors. In addition, anti-HGF and c-Met inhibitors were found to suppress the spontaneous growth of KU812 cells, suggesting autocrine growth regulation. Together, HGF is a BCR-ABL1-independent angiogenic and autocrine growth regulator in CML. Basophils are a unique source of HGF in these patients and may play a more active role in disease-associated angiogenesis and disease progression than has so far been assumed. Our data also suggest that HGF and c-Met are potential therapeutic targets in CML.

Progressive peripheral arterial occlusive disease and other vascular events during nilotinib therapy in CML.

The second generation BCR/ABL kinase inhibitor nilotinib is increasingly used for the treatment of imatinib-resistant chronic myeloid leukemia (CML). So far, nilotinib is considered a well-tolerated drug with little if any side effects, although an increase in the fasting glucose level has been reported. We examined a series of 24 consecutive CML patients treated with nilotinib in our center for the development of non-hematologic adverse events. Three of these 24 CML patients developed a rapidly progressive peripheral arterial occlusive disease (PAOD) during treatment with nilotinib. In all three cases, PAOD required repeated angioplasty and/or multiple surgeries within a few months. No PAOD was known before nilotinib-therapy in these patients, although all three had received imatinib. In two patients, pre-existing risk factors predisposing for PAOD were known, and one of them had developed diabetes mellitus during nilotinib. In the other 21 patients treated with nilotinib in our center, one less severe PAOD, one myocardial infarction, one spinal infarction, one subdural hematoma, and one sudden death of unknown etiology were recorded. In summary, treatment with nilotinib may be associated with an increased risk of vascular adverse events, including PAOD development. In a subgroup of patients, these events are severe or even life-threatening. Although the exact mechanisms remain unknown, we recommend screening for pre-existing PAOD and for vascular risk factors such as diabetes mellitus in all patients before starting nilotinib and in the follow up during nilotinib-therapy.

High STAT5 levels mediate imatinib resistance and indicate disease progression in chronic myeloid leukemia.

In BCR-ABL1(+) leukemia, drug resistance is often associated with up-regulation of BCR-ABL1 or multidrug transporters as well as BCR-ABL1 mutations. Here we show that the expression level of the transcription factor STAT5 is another parameter that determines the sensitivity of BCR-ABL1(+) cells against tyrosine kinase inhibitors (TKIs), such as imatinib, nilotinib, or dasatinib. Abelson-transformed cells, expressing high levels of STAT5, were found to be significantly less sensitive to TKI-induced apoptosis in vitro and in vivo but not to other cytotoxic drugs, such as hydroxyurea, interferon-ß, or Aca-dC. The STAT5-mediated protection requires tyrosine phosphorylation of STAT5 independent of JAK2 and transcriptional activity. In support of this concept, under imatinib treatment and with disease progression, STAT5 mRNA and protein levels increased in patients with Ph(+) chronic myeloid leukemia. Based on our data, we propose a model in which disease progression in BCR-ABL1(+) leukemia leads to up-regulated STAT5 expression. This may be in part the result of clonal selection of cells with high STAT5 levels. STAT5 then accounts for the resistance against TKIs, thereby explaining the dose escalation frequently required in patients reaching accelerated phase. It also suggests that STAT5 may serve as an attractive target to overcome imatinib resistance in BCR-ABL1(+) leukemia.