RNA-based FLT3-ITD allelic ratio is associated with outcome and ex vivo response to FLT3 inhibitors in pediatric AML.

Controversy exists whether internal tandem duplication of FMS-like tyrosine kinase 3 (FLT3-internal tandem duplication [ITD]) allelic ratio (AR) and/or length of the ITD should be taken into account for risk stratification of pediatric acute myeloid leukemia (AML) and whether it should be measured on RNA or DNA. Moreover, the ITD status may be of relevance for selecting patients eligible for FLT3 inhibitors. Here, we included 172 pediatric AML patients, of whom 36 (21%) harbored FLT3-ITD as determined on both RNA and DNA. Although there was a good correlation between both parameters ARspearman = 0.62 (95% confidence interval, 0.22-0.87) and ITDlengthspearman = 0.98 (95% confidence interval, 0.90-1.00), only AR ≥ 0.5 and length ≥48 base pairs (bps) based on RNA measurements were significantly associated with overall survival (AR: Plogrank = .008; ITDlength: Plogrank = .011). In large ITDs (>156 bp on DNA) a remarkable 90-bp difference exists between DNA and RNA, including intron 14, which is spliced out in RNA. Ex vivo exposure (n = 30) to FLT3 inhibitors, in particular to the FLT3-specific inhibitor gilteritinib, showed that colony-forming capacity was significantly more reduced in FLT3-ITD-AR ≥ 0.5 compared with ITD-AR-low and ITD- patient samples (P < .001). RNA-based FLT3-ITD measurements are recommended for risk stratification, and the relevance of AR regarding eligibility for FLT3-targeted therapy warrants further study.

Clinical relevance of molecular aberrations in paediatric acute myeloid leukaemia at first relapse.

Outcome for relapsed paediatric acute myeloid leukaemia (AML) remains poor. Strong prognostic factors at first relapse are lacking, which hampers optimization of therapy. We assessed the frequency of molecular aberrations (FLT3, NRAS, KRAS, KIT, WT1 and NPM1 genes) at first relapse in a large set (n = 198) of relapsed non-French-American-British M3, non-Down syndrome AML patients that received similar relapse treatment. We correlated molecular aberrations with clinical and biological factors and studied their prognostic relevance. Hotspot mutations in the analysed genes were detected in 92 out of 198 patients (46·5%). In 72 of these 92 patients (78%), molecular aberrations were mutually exclusive for the currently analysed genes. FLT3-internal tandem repeat (ITD) (18% of total group) mutations were most frequent, followed by NRAS (10·2%), KRAS (8%), WT1 (8%), KIT (8%), NPM1 (5%) and FLT3-tyrosine kinase domain (3%) mutations. Presence of a WT1 aberration was an independent risk factor for second relapse (Hazard Ratio [HR] = 2·74, P = 0·013). In patients who achieved second complete remission (70·2%), WT1 and FLT3-ITD aberrations were independent risk factors for poor overall survival (HR = 2·32, P = 0·038 and HR = 1·89, P = 0·045 respectively). These data show that molecular aberrations at first relapse are of prognostic relevance and potentially useful for risk group stratification of paediatric relapsed AML and for identification of patients eligible for personalized treatment.

High-frequency type I/II mutational shifts between diagnosis and relapse are associated with outcome in pediatric AML: implications for personalized medicine.

Although virtually all pediatric patients with acute myeloid leukemia (AML) achieve a complete remission after initial induction therapy, 30%-40% of patients will encounter a relapse and have a dismal prognosis. To prevent relapses, personalized treatment strategies are currently being developed, which target specific molecular aberrations. To determine relevance of established AML type I/II mutations that may serve as therapeutic targets, we assessed frequencies of these mutations and their persistence during disease progression in a large group (n = 69) of paired diagnosis and relapse pediatric AML specimens. In 26 of 42 patients (61%) harboring mutations at either stage of the disease, mutation status changed between diagnosis and relapse, particularly in FLT3, WT1, and RAS genes. Presence or gain of type I/II mutations at relapse was associated with a shorter time to relapse (TTR), whereas absence or loss correlated with longer TTR. Moreover, an adverse outcome was found for patients with activating mutations at relapse, which was statistically significant for FLT3/ITD and WT1 mutations. These findings suggest that mutational shifts affect disease progression. We hence propose that risk stratification, malignant cell detection, and selection of personalized treatment should be based on status of type I/II mutations both at initial diagnosis and during follow-up.

Review of the relevance of aberrant antigen expression by flow cytometry in myeloid neoplasms.

This article reviews the use of aberrant antigen expression detected by flow cytometry in the diagnosis and clinical handling of acute myeloid leukaemia (AML) and the myelodysplastic syndromes (MDS). Such aberrancies offer a valuable tool for the proper classification of these myeloid malignancies according the World Health Organization 2008 classification. Aberrant antigen expression by flow cytometry is also important for prognostification. This review supports the view, that minimal residual disease detection methods that make use of such aberrancies should be part of the routine management of AML patients to guide therapy, but also suggests the introduction of flow cytometry in MDS for diagnosis and treatment decisions in the near future.

AML/Normal Progenitor Balance Instead of Total Tumor Load (MRD) Accounts for Prognostic Impact of Flowcytometric Residual Disease in AML.

Measurable residual disease (MRD) in AML, assessed by multicolor flow cytometry, is an important prognostic factor. Progenitors are key populations in defining MRD, and cases of MRD involving these progenitors are calculated as percentage of WBC and referred to as white blood cell MRD (WBC-MRD). Two main compartments of WBC-MRD can be defined: (1) the AML part of the total primitive/progenitor (CD34+, CD117+, CD133+) compartment (referred to as primitive marker MRD; PM-MRD) and (2) the total progenitor compartment (% of WBC, referred to as PM%), which is the main quantitative determinant of WBC-MRD. Both are related as follows: WBC-MRD = PM-MRD × PM%. We explored the relative contribution of each parameter to the prognostic impact. In the HOVON/SAKK study H102 (300 patients), based on two objectively assessed cut-off points (2.34% and 10%), PM-MRD was found to offer an independent prognostic parameter that was able to identify three patient groups with different prognoses with larger discriminative power than WBC-MRD. In line with this, the PM% parameter itself showed no prognostic impact, implying that the prognostic impact of WBC-MRD results from the PM-MRD parameter it contains. Moreover, the presence of the PM% parameter in WBC-MRD may cause WBC-MRD false positivity and WBC-MRD false negativity. For the latter, at present, it is clinically relevant that PM-MRD ≥ 10% identifies a patient sub-group with a poor prognosis that is currently classified as good prognosis MRDnegative using the European LeukemiaNet 0.1% consensus MRD cut-off value. These observations suggest that residual disease analysis using PM-MRD should be conducted.

Increased numbers of small circulating endothelial cells in renal cell cancer patients treated with sunitinib.

Mature circulating endothelial cell (CEC) as well as endothelial progenitor populations may reflect the activity of anti-angiogenic agents on tumor neovasculature or even constitute a target for anti-angiogenic therapy. We investigated the behavior of CECs in parallel with hematopoietic progenitor cells (HPCs) in the blood of renal cell cancer patients during sunitinib treatment. We analyzed the kinetics of a specific population of small VEGFR2-expressing CECs (CD45(neg)/CD34(bright)), HPCs (CD45(dim)/CD34(bright)), and monocytes in the blood of 24 renal cell cancer (RCC) patients receiving 50 mg/day of the multitargeted VEGF inhibitor sunitinib, on a 4-week-on/2-week-off schedule. Blood was taken before treatment (C1D1), on C1D14, C1D28, and on C2D1 before the start of cycle 2. Also plasma VEGF and erythropoietin (EPO) were determined. Remarkably, while CD34(bright) HPCs and monocytes decreased during treatment, CD34(bright) CECs increased from 69 cells/ml (C1D1) to 180 cells/ml (C1D14; P = 0.001) and remained high on C1D28. All cell populations recovered to near pre-treatment levels on C2D1. Plasma VEGF and EPO levels were increased on C1D14 and partly normalized to pre-treatment levels on C2D1. In conclusion, opposite kinetics of two circulating CD34(bright) cell populations, HPCs and small CECs, were observed in sunitinib-treated RCC patients. The increase in CECs is likely caused by sunitinib targeting of immature tumor vessels.

Identification of a small subpopulation of candidate leukemia-initiating cells in the side population of patients with acute myeloid leukemia.

In acute myeloid leukemia (AML), apart from the CD34(+)CD38(-) compartment, the side population (SP) compartment contains leukemic stem cells (LSCs). We have previously shown that CD34(+)CD38(-) LSCs can be identified using stem cell-associated cell surface markers, including C-type lectin-like molecule-1 (CLL-1), and lineage markers, such as CD7, CD19, and CD56. A similar study was performed for AML SP to further characterize the SP cells with the aim of narrowing down the putatively very low stem cell fraction. Fluorescence-activated cell sorting (FACS) analysis of 48 bone marrow and peripheral blood samples at diagnosis showed SP cells in 41 of 48 cases that were partly or completely positive for the markers, including CD123. SP cells in normal bone marrow (NBM) were completely negative for markers, except CD123. Further analysis revealed that the SP fraction contains different subpopulations: (a) three small lymphoid subpopulations (with T-, B-, or natural killer-cell markers); (b) a differentiated myeloid population with high forward scatter (FSC(high)) and high sideward scatter (SSC(high)), high CD38 expression, and usually with aberrant marker expression; (c) a more primitive FSC(low)/SSC(low), CD38(low), marker-negative myeloid fraction; and (d) a more primitive FSC(low)/SSC(low), CD38(low), marker-positive myeloid fraction. NBM contained the first three populations, although the aberrant markers were absent in the second population. Suspension culture assay showed that FSC(low)/SSC(low) SP cells were highly enriched for primitive cells. Fluorescence in situ hybridization (FISH) analyses showed that cytogenetically abnormal colonies originated from sorted marker positive cells, whereas the cytogenetically normal colonies originated from sorted marker-negative cells. In conclusion, AML SP cells could be discriminated from normal SP cells at diagnosis on the basis of expression of CLL-1 and lineage markers. This reveals the presence of a low-frequency (median, 0.0016%) SP subfraction as a likely candidate to be enriched for leukemia stem cells.

Standardization of flow cytometry in myelodysplastic syndromes: report from the first European LeukemiaNet working conference on flow cytometry in myelodysplastic syndromes.

The myelodysplastic syndromes are a group of clonal hematopoietic stem cell diseases characterized by cytopenia(s), dysplasia in one or more cell lineages and increased risk of evolution to acute myeloid leukemia (AML). Recent advances in immunophenotyping of hematopoietic progenitor and maturing cells in dysplastic bone marrow point to a useful role for multiparameter flow cytometry (FCM) in the diagnosis and prognostication of myelodysplastic syndromes. In March 2008, representatives from 18 European institutes participated in a European LeukemiaNet (ELN) workshop held in Amsterdam as a first step towards standardization of FCM in myelodysplastic syndromes. Consensus was reached regarding standard methods for cell sampling, handling and processing. The group also defined minimal combinations of antibodies to analyze aberrant immunophenotypes and thus dysplasia. Examples are altered numbers of CD34(+) precursors, aberrant expression of markers on myeloblasts, maturing myeloid cells, monocytes or erythroid precursors and the expression of lineage infidelity markers. When applied in practice, aberrant FCM patterns correlate well with morphology, the subclassification of myelodysplastic syndromes, and prognostic scoring systems. However, the group also concluded that despite strong evidence for an impact of FCM in myelodysplastic syndromes, further (prospective) validation of markers and immunophenotypic patterns are required against control patient groups as well as further standardization in multi-center studies. Standardization of FCM in myelodysplastic syndromes may thus contribute to improved diagnosis and prognostication of myelodysplastic syndromes in the future.

Immunophenotypic Detection of Measurable Residual (Stem Cell) Disease Using LAIP Approach in Acute Myeloid Leukemia.

Half of the patients with acute myeloid leukemia (AML), who achieve complete remission after chemotherapy treatment, will ultimately experience a relapse. Measurable residual disease (MRD) is an important post-treatment risk factor in AML, because it gives additional information about the depth of the remission. Within MRD, the small population of leukemic stem cells (LSCs) is thought to be at the base of the actual relapse. In this protocol, the flow cytometric detection of MRD and LSCs herein is outlined. We give a detailed overview of the sampling procedures for optimal multiparameter flow cytometry assessment of both MRD and LSC, using leukemia associated immunophenotypes (LAIPs) and LSC markers. Moreover, an overview of the gating strategies to detect LAIPs and LSC markers is provided. This protocol serves as guidance for flow cytometric detection of measurable residual (stem cell) disease necessary for proper therapeutic decision making in AML patients. © 2019 The Authors. Basic Protocol 1: Immunophenotypic LAIP detection for measurable residual disease monitoring Basic Protocol 2: Immunophenotypic detection of CD34+CD38- leukemic stem cells.

Can we incorporate MRD assessment into clinical practice in AML?

Measurable residual disease (MRD) can be assessed either by flow cytometry or molecular techniques. It has been proven to be highly prognostic in quite a number of prospective clinical studies. The recently published ELN MRD recommendations aim harmonize the approaches to MRD assessment in order to improve its overall quality. The predictive value leading to the usage as a surrogate endpoint for survival which would be instrumental for faster drug approvals has still to be proven. Nevertheless, many AML centers use MRD status to inform treatment.

Immunophenotypic measurable residual disease (MRD) in acute myeloid leukemia: Is multicentric MRD assessment feasible?

Flow-cytometric detection of now termed measurable residual disease (MRD) in acute myeloid leukemia (AML) has proven to have an independent prognostic impact. In a previous multicenter study we developed protocols to accurately define leukemia-associated immunophenotypes (LAIPs) at diagnosis. It has, however, not been demonstrated whether the use of the defined LAIPs in the same multicenter setting results in a high concordance between centers in MRD assessment. In the present paper we evaluated whether interpretation of list-mode data (LMD) files, obtained from MRD assessment of previously determined LAIPs during and after treatment, could reliably be performed in a multicenter setting. The percentage of MRD positive cells was simultaneously determined in totally 173 LMD files from 77 AML patients by six participating centers. The quantitative concordance between the six participating centers was meanly 84%, with slight variation of 75%-89%. In addition our data showed that the type and number of LAIPs were of influence on the performance outcome. The highest concordance was observed for LAIPs with cross-lineage expression, followed by LAIPs with an asynchronous antigen expression. Our results imply that immunophenotypic MRD assessment in AML will only be feasible when fully standardized methods are used for reliable multicenter assessment.

IMGN779, a Novel CD33-Targeting Antibody-Drug Conjugate with DNA-Alkylating Activity, Exhibits Potent Antitumor Activity in Models of AML.

The myeloid differentiation antigen CD33 has long been exploited as a target for antibody-based therapeutic approaches in acute myeloid leukemia (AML). Validation of this strategy was provided with the approval of the CD33-targeting antibody-drug conjugate (ADC) gemtuzumab ozogamicin in 2000; the clinical utility of this agent, however, has been hampered by safety concerns. Thus, the full potential of CD33-directed therapy in AML remains to be realized, and considerable interest exists in the design and development of more effective ADCs that confer high therapeutic indices and favorable tolerability profiles. Here, we describe the preclinical characterization of a novel CD33-targeting ADC, IMGN779, which utilizes a unique DNA-alkylating payload to achieve potent antitumor effects with good tolerability. The payload, DGN462, is prototypical of a novel class of purpose-created indolinobenzodiazeprine pseudodimers, termed IGNs. With low picomolar potency, IMGN779 reduced viability in a panel of AML cell lines in vitro Mechanistically, the cytotoxic activity of IMGN779 involved DNA damage, cell-cycle arrest, and apoptosis consistent with the mode of action of DGN462. Moreover, IMGN779 was highly active against patient-derived AML cells, including those with adverse molecular abnormalities, and sensitivity correlated to CD33 expression levels. In vivo, IMGN779 displayed robust antitumor efficacy in multiple AML xenograft and disseminated disease models, as evidenced by durable tumor regressions and prolonged survival. Taken together, these findings identify IMGN779 as a promising new candidate for evaluation as a novel therapeutic in AML. Mol Cancer Ther; 17(6); 1271-9. ©2018 AACR.

IGFBP7 Induces Differentiation and Loss of Survival of Human Acute Myeloid Leukemia Stem Cells without Affecting Normal Hematopoiesis.

Leukemic stem cells (LSCs) are thought to be the major cause of the recurrence of acute myeloid leukemia (AML) due to their potential for self-renewal. To identify therapeutic strategies targeting LSCs, while sparing healthy hematopoietic stem cells (HSCs), we performed gene expression profiling of LSCs, HSCs, and leukemic progenitors all residing within the same AML bone marrow and identified insulin-like growth factor-binding protein 7 (IGFBP7) as differentially expressed. Low IGFBP7 is a feature of LSCs and is associated with reduced chemotherapy sensitivity. Enhancing IGFBP7 by overexpression or addition of recombinant human IGFBP7 (rhIGFBP7) resulted in differentiation, inhibition of cell survival, and increased chemotherapy sensitivity of primary AML cells. Adding rhIGFBP7 reduced leukemic stem and/or progenitor survival and reversed a stem-like gene signature, but it had no influence on normal hematopoietic stem cell survival. Our data suggest a potential clinical utility of the addition of rhIGFBP7 to current chemotherapy regimens to decrease AML relapse rates.

VEGFR2 expressing circulating (progenitor) cell populations in volunteers and cancer patients.

Circulating cells of several lineages are thought to participate in angiogenesis and tumor growth. Experimental studies in tumor-bearing mice have pointed to the potential importance of VEGF-responding circulating (endothelial) progenitor cells in tumor growth. We have studied circulating CD31- and/or CD34-positive cell populations with a low to moderate VEGFR2 expression in human volunteers and cancer patients. We recognized four cell populations, which were further characterized by their content of major hematopoietic progenitor, monocytic, endothelial and platelet markers. After establishing the test-retest stability of the measurements in nine patients, we determined the frequencies of the various cell populations in a group of 20 volunteers and 14 cancer patients. Two populations were markedly increased in cancer patients. Small CD45(neg)/CD34(bright)/VEGFR2+ cells amounted to 12 and 64 cells/ml (P < 0.0001), respectively, and 246/ml and 578/ml VEGFR2+/CD45(bright) (/CD14+) monocytic cells were present in controls and cancer patients, respectively (P = 0.017). A third population of CD45(dim)/CD34(bright)/VEGFR2(low) cells amounted to 25 and 30 cells/ml (P = 0.38). Unexpectedly, a population of mainly anucleated CD45(low)/CD31(bright)/CD41(bright) cells was present in numbers of 9,076 and 16,697/ml (P = 0.04) in volunteers and cancer patients, which contained a VEGFR2(low) (compared to IgG isotype control) expressing population amounting to 1,142 and 1,642 cells/ml (P = 0.12). This fourth population probably reflects large platelets. The role of the herein identified VEGFR2+ circulating cell populations deserve further investigation in cancer patients treated with VEGF(R)-targeted therapies. Quantification of such cell populations in the blood of tumor patients may be valuable to monitor the efficacy of anti-angiogenic treatment.

Leukemic stem cells: identification and clinical application.

Leukemic stem cells (LSCs) in acute myeloid leukemia (AML) represent a low-frequency subpopulation of leukemia cells that possess stem cell properties distinct from the bulk leukemia cells, including self-renewal capacity and drug resistance. Due to these properties, LSCs are supposed to facilitate the development of relapse. The existence of LSCs is demonstrated by the ability to engraft and initiate human AML in immune-compromised mouse models. Although several lines of evidence suggest the complex heterogeneity of phenotypes displayed by LSC, many studies consider the CD34+/CD38- compartment as the most relevant. To increase the understanding of the true LSC, techniques such as multicolor flow cytometry, side-population assay and ALDH assay are utilized in many laboratories and could aid in this. A better understanding of different LSC phenotypes is necessary to enhance risk group classification, guide clinical decision-making and to identify new therapeutic targets. These efforts to eliminate LSC should ultimately improve the dismal AML outcome by preventing relapse development.

Graft-Versus-Leukemia Effect of Allogeneic Stem-Cell Transplantation and Minimal Residual Disease in Patients With Acute Myeloid Leukemia in First Complete Remission.

PURPOSE: The detection of minimal residual disease (MRD) in patients with acute myeloid leukemia (AML) may improve future risk-adapted treatment strategies. We assessed whether MRD-positive and MRD-negative patients with AML benefit differently from the graft-versus-leukemia effect of allogeneic hematopoietic stem-cell transplantation (alloHSCT). METHODS: A total of 1,511 patients were treated in subsequent Dutch-Belgian Hemato-Oncology Cooperative Group and the Swiss Group for Clinical Cancer Research AML trials, of whom 547 obtained a first complete remission, received postremission treatment (PRT), and had available flow cytometric MRD before PRT. MRD positivity was defined as more than 0.1% cells with a leukemia-associated immunophenotype within the WBC compartment. PRT consisted of alloHSCT (n = 282), conventional PRT by a third cycle of chemotherapy (n = 160), or autologous hematopoietic stem-cell transplantation (n = 105). RESULTS: MRD was positive in 129 patients (24%) after induction chemotherapy before proceeding to PRT. Overall survival and relapse-free survival were significantly better in patients without MRD before PRT compared with MRD-positive patients (65% ± 2% v 50% ± 5% at 4 years; P = .002; and 58% ± 3% v 38% ± 4%; P < .001, respectively), which was mainly because of a lower cumulative incidence of relapse (32% ± 2% compared with 54% ± 4%; P < .001, respectively). Multivariable analysis with adjustment for covariables showed that the incidence of relapse was significantly reduced after alloHSCT compared with chemotherapy or autologous hematopoietic stem cell transplantation (hazard ratio [HR], 0.36; P < .001), which was similarly exerted in both MRD-negative and MRD-positive patients (HR, 0.38; P < .001; and HR, 0.35; P < .001, respectively). CONCLUSION: The graft-versus-leukemia effect of alloHSCT is equally present in MRD-positive and MRD-negative patients, which advocates a personalized application of alloHSCT, taking into account the risk of relapse determined by AML risk group and MRD status, as well as the counterbalancing risk of nonrelapse mortality.

High stem cell frequency in acute myeloid leukemia at diagnosis predicts high minimal residual disease and poor survival.

PURPOSE: In CD34-positive acute myeloid leukemia (AML), the leukemia-initiating event originates from the CD34(+)CD38(-) stem cell compartment. Survival of these cells after chemotherapy may lead to minimal residual disease (MRD) and subsequently to relapse. Therefore, the prognostic impact of stem cell frequency in CD34-positive AML was investigated. EXPERIMENTAL DESIGN: First, the leukemogenic potential of unpurified CD34(+)CD38(-) cells, present among other cells, was investigated in vivo using nonobese diabetic/severe combined immunodeficient mice transplantation experiments. Second, we analyzed whether the CD34(+)CD38(-) compartment at diagnosis correlates with MRD frequency after chemotherapy and clinical outcome in 92 AML patients. RESULTS: In vivo data showed that engraftment of AML blasts in nonobese diabetic/severe combined immunodeficient mice directly correlated with stem cell frequency of the graft. In patients, a high percentage of CD34(+)CD38(-) stem cells at diagnosis significantly correlated with a high MRD frequency, especially after the third course of chemotherapy. Also, it directly correlated with poor survival. In contrast, total CD34(+) percentage showed no such correlations. CONCLUSIONS: Both in vivo data, as well as the correlation studies, show that AML stem cell frequency at diagnosis offers a new prognostic factor. From our data, it is tempting to hypothesize that a large CD34(+)CD38(-) population at diagnosis reflects a higher percentage of chemotherapy-resistant cells that will lead to the outgrowth of MRD, thereby affecting clinical outcome. Ultimately, future therapies should be directed toward malignant stem cells.

C-type lectin-like molecule-1: a novel myeloid cell surface marker associated with acute myeloid leukemia.

Acute myeloid leukemia (AML) has a poor prognosis due to treatment-resistant relapses. A humanized anti-CD33 antibody (Mylotarg) showed a limited response rate in relapsed AML. To discover novel AML antibody targets, we selected a panel of single chain Fv fragments using phage display technology combined with flow cytometry on AML tumor samples. One selected single chain Fv fragment broadly reacted with AML samples and with myeloid cell lineages within peripheral blood. Expression cloning identified the antigen recognized as C-type lectin-like molecule-1 (CLL-1), a previously undescribed transmembrane glycoprotein. CLL-1 expression was analyzed with a human anti-CLL-1 antibody that was generated from the single chain Fv fragment. CLL-1 is restricted to the hematopoietic lineage, in particular to myeloid cells present in peripheral blood and bone marrow. CLL-1 is absent on uncommitted CD34(+)/CD38(-) or CD34(+)/CD33(-) stem cells and present on subsets of CD34(+)/CD38(+) or CD34(+)/CD33(+) progenitor cells. CLL-1 is not expressed in any other tissue. In contrast, analysis of primary AMLs demonstrated CLL-1 expression in 92% (68 of 74) of the samples. As an AML marker, CLL-1 was able to complement CD33, because 67% (8 of 12) of the CD33(-) AMLs expressed CLL-1. CLL-1 showed variable expression (10-60%) in CD34(+) cells in chronic myelogenous leukemia and myelodysplastic syndrome but was absent in 12 of 13 cases of acute lymphoblastic leukemia. The AML reactivity combined with the restricted expression on normal cells identifies CLL-1 as a novel potential target for AML treatment.