Impact of hemodilution on flow cytometry based measurable residual disease assessment in acute myeloid leukemia

Measurable residual disease (MRD) measured in the bone marrow (BM) of acute myeloid leukemia (AML) patients after induction chemotherapy is an established prognostic factor. Hemodilution, stemming from peripheral blood (PB) mixing within BM during aspiration, can yield false-negative MRD results. We prospectively examined hemodilution by measuring MRD in BM aspirates obtained from three consecutive 2 mL pulls, along with PB samples. Our results demonstrated a significant decrease in MRD percentages between the first and second pulls (P = 0.025) and between the second and third pulls (P = 0.025), highlighting the impact of hemodilution. Initially, 39% of MRD levels (18/46 leukemia-associated immunophenotypes) exceeded the 0.1% cut-off, decreasing to 30% (14/46) in the third pull. Additionally, we assessed the performance of six published methods and parameters for distinguishing BM from PB samples, addressing or compensating for hemodilution. The most promising results relied on the percentages of CD16dim granulocytic population (scarce in BM) and CD117high mast cells (exclusive to BM). Our findings highlight the importance of estimating hemodilution in MRD assessment to qualify MRD results, particularly near the common 0.1% cut-off. To avoid false-negative results by hemodilution, it is essential to collect high-quality BM aspirations and preferably utilizing the initial pull for MRD testing.

Bimodal expression of potential drug target CLL-1 (CLEC12A) on CD34+ blasts of AML patients

Objectives

This study aims to retrospectively assess C‐lectin‐like molecule 1 (CLL‐1) bimodal expression on CD34+ blasts in acute myeloid leukemia (AML) patients (total N = 306) and explore potential CLL‐1 bimodal associations with leukemia and patient‐specific characteristics.

Methods

Flow cytometry assays were performed to assess the deeper immunophenotyping of CLL‐1 bimodality. Cytogenetic analysis was performed to characterize the gene mutation on CLL‐1‐negative subpopulation of CLL‐1 bimodal AML samples.

Results

The frequency of a bimodal pattern of CLL‐1 expression of CD34⁺ blasts ranged from 8% to 65% in the different cohorts. Bimodal CLL‐1 expression was most prevalent in patients with MDS‐related AML (P = .011), ELN adverse risk (P = .002), NPM1 wild type (WT, P = .049), FLT3 WT (P = .035), and relatively low percentages of leukemia‐associated immunophenotypes (P = .006). Additional immunophenotyping analysis revealed the CLL‐1⁻ subpopulation may consist of pre‐B cells, immature myeloblasts, and hematopoietic stem cells. Furthermore, (pre)‐leukemic mutations were detected in both CLL‐1⁺ and CLL‐1⁻ subfractions of bimodal samples (N = 3).

Conclusions

C‐lectin‐like molecule 1 bimodality occurs in about 25% of AML patients and the CLL‐1⁻ cell population still contains malignant cells, hence it may potentially limit the effectiveness of CLL‐1‐targeted therapies and warrant further investigation.

Applicability and reproducibility of acute myeloid leukaemia stem cell assessment in a multi-centre setting

Leukaemic stem cells (LSC) have been experimentally defined as the leukaemia‐propagating population and are thought to be the cellular reservoir of relapse in acute myeloid leukaemia (AML). Therefore, LSC measurements are warranted to facilitate accurate risk stratification. Previously, we published the composition of a one‐tube flow cytometric assay, characterised by the presence of 13 important membrane markers for LSC detection. Here we present the validation experiments of the assay in several large AML research centres, both in Europe and the United States. Variability within instruments and sample processing showed high correlations between different instruments (R_pearson > 0·91, P < 0·001). Multi‐centre testing introduced variation in reported LSC percentages but was found to be below the clinical relevant threshold. Clear gating protocols resulted in all laboratories being able to perform LSC assessment of the validation set. Participating centres were nearly unanimously able to distinguish LSC^high (>0·03% LSC) from LSC^low (<0·03% LSC) despite inter‐laboratory variation in reported LSC percentages. This study proves that the LSC assay is highly reproducible. These results together with the high prognostic impact of LSC load at diagnosis in AML patients render the one‐tube LSC assessment a good marker for future risk classification.

CD34+CD38- leukemic stem cell frequency to predict outcome in acute myeloid leukemia

Current risk algorithms are primarily based on pre-treatment factors and imperfectly predict outcome in acute myeloid leukemia (AML). We introduce and validate a post-treatment approach of leukemic stem cell (LSC) assessment for prediction of outcome. LSC containing CD34+CD38- fractions were measured using flow cytometry in an add-on study of the HOVON102/SAKK trial. Predefined cut-off levels were prospectively evaluated to assess CD34+CD38-LSC levels at diagnosis (n = 594), and, to identify LSC^low/LSC^high (n = 302) and MRD^low/MRD^high patients (n = 305) in bone marrow in morphological complete remission (CR). In 242 CR patients combined MRD and LSC results were available. At diagnosis the CD34⁺CD38^- LSC frequency independently predicts overall survival (OS). After achieving CR, combining LSC and MRD showed reduced survival in MRD^high/LSC^high patients (hazard ratio [HR] 3.62 for OS and 5.89 for cumulative incidence of relapse [CIR]) compared to MRD^low/LSC^high, MRD^high/LSC^low, and especially MRD^low/LSC^low patients. Moreover, in the NPM1mutant positive sub-group, prognostic value of golden standard NPM1-MRD by qPCR can be improved by addition of flow cytometric approaches. This is the first prospective study demonstrating that LSC strongly improves prognostic impact of MRD detection, identifying a patient subgroup with an almost 100% treatment failure probability, warranting consideration of LSC measurement incorporation in future AML risk schemes.

Comprehensive Protocol to Sample and Process Bone Marrow for Measuring Measurable Residual Disease and Leukemic Stem Cells in Acute Myeloid Leukemia

Response criteria in acute myeloid leukemia (AML) has recently been re-established, with morphologic examination utilized to determine whether patients have achieved complete remission (CR). Approximately half of the adult patients who entered CR will relapse within 12 months due to the outgrowth of residual AML cells in the bone marrow. The quantitation of these remaining leukemia cells, known as minimal or measurable residual disease (MRD), can be a robust biomarker for the prediction of these relapses. Moreover, retrospective analysis of several studies has shown that the presence of MRD in the bone marrow of AML patients correlates with poor survival. Not only is the total leukemic population, reflected by cells harboring a leukemia associated immune-phenotype (LAIP), associated with clinical outcome, but so is the immature low frequency subpopulation of leukemia stem cells (LSC), both of which can be monitored through flow cytometry MRD or MRD-like approaches. The availability of sensitive assays that enable detection of residual leukemia (stem) cells on the basis of disease-specific or disease-associated features (abnormal molecular markers or aberrant immunophenotypes) have drastically improved MRD assessment in AML. However, given the inherent heterogeneity and complexity of AML as a disease, methods for sampling bone marrow and performing MRD and LSC analysis should be harmonized when possible. In this manuscript we describe a detailed methodology for adequate bone marrow aspirate sampling, transport, sample processing for optimal multi-color flow cytometry assessment, and gating strategies to assess MRD and LSC to aid in therapeutic decision making for AML patients.

Leukemic stem cell frequency: a strong biomarker for clinical outcome in acute myeloid leukemia

INTRODUCTION

Treatment failure in acute myeloid leukemia is probably caused by the presence of leukemia initiating cells, also referred to as leukemic stem cells, at diagnosis and their persistence after therapy. Specific identification of leukemia stem cells and their discrimination from normal hematopoietic stem cells would greatly contribute to risk stratification and could predict possible relapses.

RESULTS

For identification of leukemic stem cells, we developed flow cytometric methods using leukemic stem cell associated markers and newly-defined (light scatter) aberrancies. The nature of the putative leukemic stem cells and normal hematopoietic stem cells, present in the same patient's bone marrow, was demonstrated in eight patients by the presence or absence of molecular aberrancies and/or leukemic engraftment in NOD-SCID IL-2Rγ-/- mice. At diagnosis (n=88), the frequency of the thus defined neoplastic part of CD34+CD38- putative stem cell compartment had a strong prognostic impact, while the neoplastic parts of the CD34+CD38+ and CD34- putative stem cell compartments had no prognostic impact at all. After different courses of therapy, higher percentages of neoplastic CD34+CD38- cells in complete remission strongly correlated with shorter patient survival (n=91). Moreover, combining neoplastic CD34+CD38- frequencies with frequencies of minimal residual disease cells (n=91), which reflect the total neoplastic burden, revealed four patient groups with different survival.

CONCLUSION AND PERSPECTIVE

Discrimination between putative leukemia stem cells and normal hematopoietic stem cells in this large-scale study allowed to demonstrate the clinical importance of putative CD34+CD38- leukemia stem cells in AML. Moreover, it offers new opportunities for the development of therapies directed against leukemia stem cells, that would spare normal hematopoietic stem cells, and, moreover, enables in vivo and ex vivo screening for potential efficacy and toxicity of new therapies.

Normal hematopoietic stem cells within the AML bone marrow have a distinct and higher ALDH activity level than co-existing leukemic stem cells

Persistence of leukemic stem cells (LSC) after chemotherapy is thought to be responsible for relapse and prevents the curative treatment of acute myeloid leukemia (AML) patients. LSC and normal hematopoietic stem cells (HSC) share many characteristics and co-exist in the bone marrow of AML patients. For the development of successful LSC-targeted therapy, enabling eradication of LSC while sparing HSC, the identification of differences between LSC and HSC residing within the AML bone marrow is crucial. For identification of these LSC targets, as well as for AML LSC characterization, discrimination between LSC and HSC within the AML bone marrow is imperative. Here we show that normal CD34+CD38– HSC present in AML bone marrow, identified by their lack of aberrant immunophenotypic and molecular marker expression and low scatter properties, are a distinct sub-population of cells with high ALDH activity (ALDH^bright). The ALDH^bright compartment contains, besides normal HSC, more differentiated, normal CD34+CD38+ progenitors. Furthermore, we show that in CD34-negative AML, containing solely normal CD34+ cells, LSC are CD34– and ALDH^low. In CD34-positive AML, LSC are also ALDH^low but can be either CD34+ or CD34–. In conclusion, although malignant AML blasts have varying ALDH activity, a common feature of all AML cases is that LSC have lower ALDH activity than the CD34+CD38– HSC that co-exist with these LSC in the AML bone marrow. Our findings form the basis for combined functionally and immunophenotypically based identification and purification of LSC and HSC within the AML bone marrow, aiming at development of highly specific anti-LSC therapy.

High prognostic impact of flow cytometric minimal residual disease detection in acute myeloid leukemia: data from the HOVON/SAKK AML 42A study

PURPOSE

Half the patients with acute myeloid leukemia (AML) who achieve complete remission (CR), ultimately relapse. Residual treatment-surviving leukemia is considered responsible for the outgrowth of AML. In many retrospective studies, detection of minimal residual disease (MRD) has been shown to enable identification of these poor-outcome patients by showing its independent prognostic impact. Most studies focus on molecular markers or analyze data in retrospect. This study establishes the value of immunophenotypically assessed MRD in the context of a multicenter clinical trial in adult AML with sample collection and analysis performed in a few specialized centers.

PATIENTS AND METHODS

In adults (younger than age 60 years) with AML enrolled onto the Dutch-Belgian Hemato-Oncology Cooperative Group/Swiss Group for Clinical Cancer Research Acute Myeloid Leukemia 42A study, MRD was evaluated in bone marrow samples in CR (164 after induction cycle 1, 183 after cycle 2, 124 after consolidation therapy).

RESULTS

After all courses of therapy, low MRD values distinguished patients with relatively favorable outcome from those with high relapse rate and adverse relapse-free and overall survival. In the whole patient group and in the subgroup with intermediate-risk cytogenetics, MRD was an independent prognostic factor. Multivariate analysis after cycle 2, when decisions about consolidation treatment have to be made, confirmed that high MRD values (> 0.1% of WBC) were associated with a higher risk of relapse after adjustment for consolidation treatment time-dependent covariate risk score and early or later CR.

CONCLUSION

In future treatment studies, risk stratification should be based not only on risk estimation assessed at diagnosis but also on MRD as a therapy-dependent prognostic factor.

Minimal residual disease detection defined as the malignant fraction of the total primitive stem cell compartment offers additional prognostic information in acute myeloid leukaemia

INTRODUCTION

Immunophenotypic detection of minimal residual disease (MRD) in bone marrow (BM) of acute myeloid leukaemia (AML) patients is of high prognostic relevance. Standard MRD percentage is assessed as a percentage of total white blood cells (WBCs) and is therefore highly dependent on WBC count. Peripheral blood (PB) contains more than five times lower MRD percentages. Therefore, PB in BM aspirates cause dilution of the MRD cells, possibly leading to false-negative results for BM MRD. The latter is avoided when relating the fraction of malignant primitive cells, identified by aberrant marker expression [aberrant primitive cells (aPC)], to the total population of primitive cells. Such a fraction may in addition reflect an important biological parameter.

METHODS

As this approach is thus independent of WBC count and the total size of the primitive compartment, we investigated the role of aPC fractions on overall and relapse-free survival (RFS) in 98 patients with AML under the age of 60.

RESULTS

We show that this approach identifies MRD-negative (as defined by % of WBC) but aPC-positive (as defined by % of primitive cells) patients with poor outcome after both first and second induction cycle of chemotherapy.

CONCLUSION

As a result, in cases with a primitive marker present, RFS is best predicted when combining standard MRD percentage with aPC fractions.

Abstract LB-45: High aldehyde dehydrogenase activity is a marker for normal hematopoietic stem cells but not leukemic stem cells in acute myeloid leukemia: novel therapeutic implications

Only a minority of cells, the leukemic stem cells (LSC), within AML are responsible for tumor growth and maintenance. Many patients experience relapse after therapy which originates from outgrowth of therapy resistant LSC. Therefore, eradication of LSC is necessary to cure AML. Both the normal hematopoietic stem cells (HSC) and LSC co-exist in the bone marrow (BM) of AML patients and success of anti-LSC strategies relies on specific elimination of LSC while sparing HSC. LSC are contained within the CD34+CD38-, the side population (SP) and the high aldehyde dehydrogenase (ALDH) activity compartments.

ALDH is a detoxifying enzyme responsible for oxidation of intracellular aldehydes and high ALDH activity results in resistance to alkylating agents such as cyclophosphamide. It has been shown that ALDH is highly expressed in both normal progenitor and stem cells and in AML blasts. In view of applicability of LSC specific therapies the detoxification by ALDH is clinically very important. A difference in ALDH activity between HSC and LSC might be used to preferentially kill LSC while sparing HSC.

To establish ALDH activity differences between HSC and LSC it should be possible to discriminate between them. We have shown that LSC can be identified and discriminated from HSC using stem cell-associated cell surface markers, such as CLL-1, lineage markers (CD7, CD19, CD56) and recently CD34/CD45 expression and cell size characteristics (Terwijn, Blood 111: 487, 2008). This offers the opportunity to identify co-existing LSC and HSC in the AML BM.

We now show that, although malignant AML blasts have varying ALDH activity, a common feature of all AML cases is that HSC that co-exist with LSC in BM of AML patients have a higher ALDH activity as compared to their malignant counterparts. We have analyzed ALDH activity in HSC and LSC, both present in the BM from 18 AML patients. In nine BM AML samples, defined as CD34negative (&lt;1%CD34+ blasts), the CD34+ compartment contained only normal CD34+CD38− HSC. The ALDH activity in these CD34+ HSC, is a factor 4,4 (range 1,7–18,9) higher than in LSC. In nine BM AML samples, defined as CD34positive AML, the CD34+CD38- HSC have a 7,7 fold (range 1,73–29,2 fold) higher ALDH activity as compared to putative LSC. In both CD34-positive and CD34-negative AML, we confirmed the identity of HSC and LSC by screening for molecular aberrancies present in AML blasts. The level of the ALDH activity of HSC within the AML BM is similar to that of HSC in NBM of healthy donors.

In conclusion, high ALDH activity is an unique marker of normal HSC within the AML BM (irrespective of AML phenotype) at diagnosis. Consequently, AML patients with high ALDH activity in HSC might benefit from treatment with agents that will be converted by ALDH enzymes, such as cyclophosphamide, whereby the difference between the activity in LSC and HSC will define the therapeutic window.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-45. doi:10.1158/1538-7445.AM2011-LB-45