Incidence, risk factors, and outcomes of second neoplasms in patients with acute promyelocytic leukemia: the PETHEMA-PALG experience.

The most important challenges in acute promyelocytic leukemia (APL) is preventing early death and reducing long-term events, such as second neoplasms (s-NPLs). We performed a retrospective analysis of 2670 unselected APL patients, treated with PETHEMA "chemotherapy based" and "chemotherapy free" protocols. Only de novo APL patients who achieved complete remission (CR) and completed the three consolidation cycles were enrolled into the analysis. Out of 2670 APL patients, there were 118 (4.4%) who developed s-NPLs with the median latency period (between first CR and diagnosis of s-NPL) of 48.0 months (range 2.8-231.1): 43.3 (range: 2.8-113.9) for s-MDS/AML and 61.7 (range: 7.1-231.1) for solid tumour. The 5-year CI of all s-NPLs was of 4.43% and 10 years of 7.92%. Among s-NPLs, there were 58 cases of s-MDS/AML, 3 cases of other hematological neoplasms, 57 solid tumours and 1 non-identified neoplasm. The most frequent solid tumour was colorectal, lung and breast cancer. Overall, the 2-year OS from diagnosis of s-NPLs was 40.6%, with a median OS of 11.1 months. Multivariate analysis identified age of 35 years (hazard ratio = 0.2584; p < 0.0001) as an independent prognostic factor for s-NPLs. There were no significant differences in CI of s-NPLs at 5 years between chemotherapy-based vs chemotherapy-free regimens (hazard ratio = 1.09; p = 0.932). Larger series with longer follow-up are required to confirm the potential impact of ATO+ATRA regimens to reduce the incidence of s-NPLs after front-line therapy for APL.

A phase 3 trial of azacitidine versus a semi-intensive fludarabine and cytarabine schedule in older patients with untreated acute myeloid leukemia.

BACKGROUND: Options to treat elderly patients (≥65 years old) newly diagnosed with acute myeloid leukemia (AML) include intensive and attenuated chemotherapy, hypomethylating agents with or without venetoclax, and supportive care. This multicenter, randomized, open-label, phase 3 trial was designed to assess the efficacy and safety of a fludarabine, cytarabine, and filgrastim (FLUGA) regimen in comparison with azacitidine (AZA). METHODS: Patients (n = 283) were randomized 1:1 to FLUGA (n = 141) or AZA (n = 142). Response was evaluated after cycles 1, 3, 6, and 9. Measurable residual disease (MRD) was assessed after cycle 9. When MRD was ≥0.01%, patients continued with the treatment until relapse or progressive disease. Patients with MRD < 0.01% suspended treatment to enter the follow-up phase. RESULTS: The complete remission (CR) rate after 3 cycles was significantly better in the FLUGA arm (18% vs 9%; P = .04), but the CR/CR with incomplete recovery rate at 9 months was similar (33% vs 29%; P = .41). There were no significant differences between arms in early mortality at 30 or 60 days. Hematologic toxicities were more frequent with FLUGA, especially during induction. The 1-year overall survival (OS) rate and the median OS were superior with AZA versus FLUGA: 47% versus 27% and 9.8 months (95% confidence interval [CI], 5.6-14 months) versus 4.1 months (95% CI, 2.7-5.5 months; P = .005), respectively. The median event-free survival was 4.9 months (95% CI, 2.8-7 months) with AZA and 3 months (95% CI, 2.5-3.5 months) with FLUGA (P = .001). CONCLUSIONS: FLUGA achieved more remissions after 3 cycles, but the 1-year OS rate was superior with AZA. However, long-term outcomes were disappointing in both arms (3-year OS rate, 10% vs 5%). This study supports the use of an AZA backbone for future combinations in elderly patients with AML.

A phase I-II study of plerixafor in combination with fludarabine, idarubicin, cytarabine, and G-CSF (PLERIFLAG regimen) for the treatment of patients with the first early-relapsed or refractory acute myeloid leukemia.

Clinical outcomes of patients with acute myeloid leukemia (AML) showing the first primary refractory or early-relapsed disease remain very poor. The Programa Español de Tratamientos en Hematología (PETHEMA) group designed a phase I-II trial using FLAG-Ida (fludarabine, idarubicin, cytarabine, and G-CSF) plus high-dose intravenous plerixafor, a molecule inducing mobilization of blasts through the SDF-1α-CXCR4 axis blockade and potentially leading to chemosensitization of the leukemic cells. We aimed to establish a recommended phase 2 dose (RP2D) of plerixafor plus FLAG-Ida, as well as the efficacy and safety of this combination for early-relapsed (first complete remission (CR/CRi) < 12 months) or primary refractory AML. Between 2012 and 2015, 57 patients were enrolled, and 41 received the RP2D (median age 52 years [range, 18-64]). Among these patients, 20 (49%) achieved CR/CRi, and 3 (7%) died during induction. CR/CRi rate was 50% (13/26) among primary refractory and 47% (7/15) among early relapse. Overall, 25 patients (61%) were allografted. Median overall and disease-free survivals were 9.9 and 13 months, respectively. In summary, the combination of plerixafor plus FLAG-Ida resulted in a relatively high CR/CRi rate in adult patients with primary refractory or early relapsed AML, with an acceptable toxicity profile and induction mortality rate, bridging the majority of patients to allogeneic stem cell transplantation. ClinicalTrials.gov Identifier: NCT01435343.

Correction to: A phase I-II study of plerixafor in combination with fludarabine, idarubicin, cytarabine, and G-CSF (PLERIFLAG regimen) for the treatment of patients with the first early-relapsed or refractory acute myeloid leukemia.

The name of Pau Montesinos was inadvertently presented as Pau Montesinos Fernández in the original article.The original version of this article was revised: The name of Pau Montesinos was inadvertently presented as Pau Montesinos Fernández.

Phase II trial to assess the safety and efficacy of clofarabine in combination with low-dose cytarabine in elderly patients with acute myeloid leukemia.

Previous studies have shown that clofarabine plus low-dose cytarabine (LDAC) could induce roughly 60 % of complete remissions (CR) with acceptable toxicity and induction mortality in elderly acute myeloid leukemia (AML) patients not suitable for intensive chemotherapy. The Programa Español de Tratamientos en Hematología group conducted a trial for patients diagnosed with untreated AML aged 60 years and older, using the combination of clofarabine (20 mg/m(2) × 5 days) plus low-dose cytarabine (20 mg/m(2) × 14 days). The protocol was flexible regarding the use of antifungal and antibacterial prophylaxis, and outpatient induction therapy was allowed. Although the planned recruitment goal was 75 patients, only 11 patients were enrolled (median age, 74 years) after observing high toxicity and unacceptable mortality (46 and 73 % at 4 and 8 weeks, respectively). The response assessment showed three CR (27 %), three resistant diseases (27 %), and five induction deaths (46 %). Induction was administered in an outpatient modality in five patients, while antifungal and antibacterial prophylaxis was not given in seven and five patients, respectively. In our context, induction therapy with the combination of clofarabine (20 mg/m(2)) plus LDAC was associated with high toxicity and unacceptable mortality in elderly AML patients, leading to the early interruption of the trial. Tight patients' clinical monitoring, follow-up, and intensive supportive care seem crucial to achieve at least acceptable clinical outcomes in elderly AML patients receiving clofarabine plus LDAC. This trial is registered at www.clinicaltrials.gov as no. NCT01193400.

Long-Term Outcomes After Autologous Versus Allogeneic Stem Cell Transplantation in Molecularly-Stratified Patients With Intermediate Cytogenetic Risk Acute Myeloid Leukemia: A PETHEMA Study.

Acute myeloid leukemia (AML) with intermediate risk cytogenetics (IRcyto) comprises a variety of biological entities with distinct mutational landscapes that translate into differential risks of relapse and prognosis. Optimal postremission therapy choice in this heterogeneous patient population is currently unsettled. In the current study, we compared outcomes in IRcyto AML recipients of autologous (autoSCT) (n = 312) or allogeneic stem cell transplantation (alloSCT) (n = 279) in first complete remission (CR1). Molecular risk was defined based on CEBPA, NPM1, and FLT3-ITD mutational status, per European LeukemiaNet 2017 criteria. Five-year overall survival (OS) in patients with favorable molecular risk (FRmol) was 62% (95% confidence interval [CI], 50-72) after autoSCT and 66% (95% CI, 41-83) after matched sibling donor (MSD) alloSCT (P = .68). For patients of intermediate molecular risk (IRmol), MSD alloSCT was associated with lower cumulative incidence of relapse (P < .001), as well as with increased nonrelapse mortality (P = .01), as compared to autoSCT. The 5-year OS was 47% (95% CI, 34-58) after autoSCT and 70% (95% CI, 59-79) after MSD alloSCT (P = .02) in this patient subgroup. In a propensity-score matched IRmol subcohort (n = 106), MSD alloSCT was associated with superior leukemia-free survival (hazard ratio [HR] 0.33, P = .004) and increased OS in patients alive 1 year after transplantation (HR 0.20, P = .004). These results indicate that, within IRcyto AML in CR1, autoSCT may be a valid option for FRmol patients, whereas MSD alloSCT should be the preferred postremission strategy in IRmol patients.

Concurrent intensive chemotherapy and imatinib before and after stem cell transplantation in newly diagnosed Philadelphia chromosome-positive acute lymphoblastic leukemia. Final results of the CSTIBES02 trial.

BACKGROUND: Imatinib, given concurrently or alternating with chemotherapy, has improved the response and survival of patients with Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph(+) ALL) but relapses are still frequent. The aim of this study was to evaluate the feasibility and results of giving imatinib concurrently with intensive chemotherapy, stem cell transplantation and post-transplant imatinib maintenance therapy in patients with newly diagnosed Ph(+) ALL. DESIGN AND METHODS: This was a phase II study of patients with newly diagnosed Ph(+) ALL given standard chemotherapy, together with imatinib (400 mg/day) until stem cell transplantation, followed by imatinib maintenance therapy for all patients regardless of the molecular status of the disease. RESULTS: Of the 30 patients included, 27 (90%) achieved complete remission, one was resistant to treatment and two died during induction therapy. The percentages of major and complete molecular responses were 86% and 21% after induction, and 81% and 65% after consolidation, respectively. Similar results were observed assessing minimal residual disease by flow cytometry. Of the 27 patients who achieved complete remission, 21 underwent stem cell transplantation (16 allogeneic, 5 autologous). Imatinib (400 mg/day) could be administered after transplantation for a median of 3.9 months in 12 patients, although it was interrupted in 10 patients (in 2 cases because of side effects of the drug). Nine patients relapsed, four before and five after stem cell transplantation and eight patients died of transplant-related causes. With a median follow-up of 4.1 years, the probabilities (95% CI) of disease-free and overall survival were 30% (15% to 45%) and 30% (16% to 45%), respectively. CONCLUSIONS: These results confirm that imatinib is an effective first-line treatment for adult Ph(+) ALL when given concurrently with chemotherapy, making stem cell transplantation feasible in a high proportion of patients. However, post-transplantation imatinib administration was limited, mainly because of transplantation-derived complications rather than drug-specific toxicity.

Minimal residual disease monitoring after allogeneic transplantation may help to individualize post-transplant therapeutic strategies in acute myeloid malignancies.

This study evaluates the prognostic value of minimal residual disease (MRD) monitoring by multiparametric flow cytometry in 41 patients with acute myeloid leukemia or myelodysplastic syndrome undergoing allogeneic transplantation. MRD assessment after transplant (day +100) allowed to discriminate different risk populations, being the most significant cut-off value for outcome level of MRD < or > or = 10(-3). Outcome was significantly better among patients with low (<10(-3)) versus high (> or = 10(-3)) MRD at day +100 after transplant. Thus, overall survival was 73% versus 25% at 4 years among patients with low versus high MRD at day +100 after transplant (P = 0.002); 74% of patients with low MRD were event free at 4 years as compared to 17% among patients with high MRD (P = 0.01). In multivariate analysis, MRD value as well as chronic GVHD significantly influenced outcome. In conclusion, MRD monitoring early post-transplant is an important tool for outcome prediction and should be considered in decision making after allogeneic transplantation.

Differences in ex-vivo Chemosensitivity to Anthracyclines in First Line Acute Myeloid Leukemia.

BACKGROUND: Induction schedules in acute myeloid leukemia (AML) are based on combinations of cytarabine and anthracyclines. The choice of the anthracycline employed has been widely studied in multiple clinical trials showing similar complete remission rates. MATERIALS AND METHODS: Using an ex vivo test we have analyzed if a subset of AML patients may respond differently to cytarabine combined with idarubicin, daunorubicin or mitoxantrone. Bone marrow (BM) samples of 198 AML patients were incubated for 48 hours in 96 well plates, each well containing different drugs or drug combinations at different concentrations. Ex vivo drug sensitivity analysis was made using the PharmaFlow platform maintaining the BM microenvironment. Drug response was evaluated as depletion of AML blast cells in each well after incubation. Annexin V-FITC was used to quantify the ability of the drugs to induce apoptosis, and pharmacological responses were calculated using pharmacokinetic population models. RESULTS: Similar dose-respond graphs were generated for the three anthracyclines, with a slight decrease in EC50 with idarubicin (p=1.462E-06), whereas the interpatient variability of either drug was large. To identify those cases of selective sensitivity to anthracyclines, potency was compared, in terms of area under the curve. Differences in anthracycline monotherapy potency greater than 30% from 3 pairwise comparisons were identified in 28.3% of samples. Furthermore, different sensitivity was detected in 8.2% of patients comparing combinations of cytarabine and anthracyclines. DISCUSSION: A third of the patients could benefit from the use of this test in the first line induction therapy selection, although it should be confirmed in a clinical trial specifically designed.

Immunophenotypic and cytogenetic comparison of Waldenstrom's macroglobulinemia with splenic marginal zone lymphoma.

Some B-cell lymphoproliferative disorders displaying a serum monoclonal immunoglobulin (Ig) M protein could be difficult to differentiate from Waldenstrom's macroglobulinemia (WM). We report on the immunophenotypic and cytogenetic characteristics of 85 patients with WM and compare them with 29 patients with splenic marginal zone lymphoma (SMZL). For immunophenotyping, WM and SMZL constantly expressed panB-cell markers (CD19, CD20, CD22, and surface Ig). However, there were differences in the k/l ratio (1.2:1 for SMZL and 4.5:1 for WM) and in some markers such as CD22 and CD11c, which were overexpressed in patients with SMZL compared with patients with WM, whereas CD25 was more frequently positive in WM (88% vs. 44%). The CD103 antigen was always negative in WM, whereas it was positive in 40% of SMZL cases. The monoclonal antibody FMC7 was usually positive in both entities: heterogeneous in WM but homogeneous in SMZL. The combination of CD25 and CD22 could differentiate between WM and SMZL. The principal molecular abnormality in WM is 6q deletion (30% in our experience), whereas in SMZL the most common abnormalities are loss of 7q (19%) along with +3q (19%) and +5q (10%). Interestingly, the incidence of IgH rearrangement was low in WM (12%) and SMZL (10%). Immunophenotypic and molecular cytogenetic studies could help to distinguish WM from SMZL.

Cell cycle analysis of Waldenstrom's macroglobulinemia.

Little is known about the DNA cell content and cell cycle characteristics of immunoglobulin (Ig) M monoclonal gammopathies. The autonomous clone appears to be rather heterogeneous, from mature B lymphocytes to plasma cells (PCs). We have evaluated the DNA cell content of 27 patients with IgM monoclonal gammopathies: 18 of them had Waldenstrom's macroglobulinemia (WM), and 9 were diagnosed with IgM-monoclonal gammopathy of undetermined significance (MGUS). To specifically analyze the cell cycle of the B lymphocyte and PC populations, we used a flow-cytometric double-staining technique with CD19/CD20/CD22 propidium iodide for B lymphocytes and CD38/CD138 propidium iodide for PCs. In 26 of 27 patients, both subsets of tumor cells (B lymphocyte and PC) showed a diploid DNA cell content (DNA index, 1). The median percentage of proliferating B lymphocytes, S-phase + G2/M-phase, was 1.8% (range, 0.4%-4.1%). This proliferative activity was significantly lower than that observed in nonmalignant cells (5.7%; range, 0.1%-14.2%; P = 0.004) in the same sample. No differences were observed when comparing the proliferative activity of WM with that of IgM MGUS (median, 1.7% vs. 2.2%, respectively). Cell cycle characteristics of PCs were simultaneously evaluated in 9 patients, with 1.8% cells in S phase or G2/M phase. In summary, the cell cycle analysis showed that IgM monoclonal gammopathies are low-proliferative disorders, with a DNA ploidy pattern (diploid) clearly different from that of multiple myeloma.

Classification and clinical behavior of blastic plasmacytoid dendritic cell neoplasms according to their maturation-associated immunophenotypic profile.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare subtype of leukemia/lymphoma, whose diagnosis can be difficult to achieve due to its clinical and biological heterogeneity, as well as its overlapping features with other hematologic malignancies. In this study we investigated whether the association between the maturational stage of tumor cells and the clinico-biological and prognostic features of the disease, based on the analysis of 46 BPDCN cases classified into three maturation-associated subgroups on immunophenotypic grounds. Our results show that blasts from cases with an immature plasmacytoid dendritic cell (pDC) phenotype exhibit an uncommon CD56- phenotype, coexisting with CD34+ non-pDC tumor cells, typically in the absence of extramedullary (e.g. skin) disease at presentation. Conversely, patients with a more mature blast cell phenotype more frequently displayed skin/extramedullary involvement and spread into secondary lymphoid tissues. Despite the dismal outcome, acute lymphoblastic leukemia-type therapy (with central nervous system prophylaxis) and/or allogeneic stem cell transplantation appeared to be the only effective therapies. Overall, our findings indicate that the maturational profile of pDC blasts in BPDCN is highly heterogeneous and translates into a wide clinical spectrum -from acute leukemia to mature lymphoma-like behavior-, which may also lead to variable diagnosis and treatment.

Role of minimal residual disease and chimerism after reduced-intensity and myeloablative allo-transplantation in acute myeloid leukemia and high-risk myelodysplastic syndrome.

We evaluated the impact of detection of minimal residual disease by flow cytometry (FCMRD) and CD3 chimerism in relapse in a cohort of 87 patients with acute myeloid leukemia or myelodysplastic syndrome undergoing stem cell transplantation. Patients with a positive FCMRD at day +100 after transplantation showed higher relapse rates and worse overall survival. In multivariate analysis, a positive FCMRD after transplantation was a significant predictor of relapse. Mixed chimerism showed a trend to statistical signification. We conclude that FCMRD at day 100 after SCT is the best predictor of relapse after SCT in patients with aggressive myeloid malignancies.

Pharmacological profiles of acute myeloid leukemia treatments in patient samples by automated flow cytometry: a bridge to individualized medicine.

BACKGROUND: We have evaluated the ex vivo pharmacology of single drugs and drug combinations in malignant cells of bone marrow samples from 125 patients with acute myeloid leukemia using a novel automated flow cytometry-based platform (ExviTech). We have improved previous ex vivo drug testing with 4 innovations: identifying individual leukemic cells, using intact whole blood during the incubation, using an automated platform that escalates reliably data, and performing analyses pharmacodynamic population models. PATIENTS AND METHODS: Samples were sent from 24 hospitals to a central laboratory and incubated for 48 hours in whole blood, after which drug activity was measured in terms of depletion of leukemic cells. RESULTS: The sensitivity of single drugs is assessed for standard efficacy (EMAX) and potency (EC50) variables, ranked as percentiles within the population. The sensitivity of drug-combination treatments is assessed for the synergism achieved in each patient sample. We found a large variability among patient samples in the dose-response curves to a single drug or combination treatment. CONCLUSION: We hypothesize that the use of the individual patient ex vivo pharmacological profiles may help to guide a personalized treatment selection.

Differential diagnosis of IgM MGUS and WM according to B-lymphoid infiltration by morphology and flow cytometry.

The distinction between IgM monoclonal gammopathy of undetermined significance (MGUS), asymptomatic Waldenstrom's macroglobulinemia (WM; aWM), and symptomatic WM (sWM) relies on two features: the presence of infiltration by lymphoplasmacytic lymphoma in the bone marrow (BM) biopsy and the existence of signs or symptoms attributable to the disease. Nevertheless, some patients lack a BM biopsy or it is not conclusive for diagnosis. In this study we have investigated 94 patients with IgM monoclonal gammopathies, in which a BM trephine biopsy and morphological and flow cytometry (FCM) evaluation of BM aspirate were available at diagnosis. We found a clear correlation between BM infiltration of B-lymphocytes assessed by morphology and by FCM with a Pearson correlation of 0.62 (P<.001). Moreover, in the absence of a BM trephine biopsy, the cut-off points that would help in the differential diagnosis between MGUS, aWM, and sWM would be 20% for morphology and 5% for FCM, both assessed in the BM aspirate.

Detailed immunophenotypic characterization of different major and minor subsets of peripheral blood cells in patients with paroxysmal nocturnal hemoglobinuria.

BACKGROUND: Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by a deficient expression of glycosylphosphatidylinositol-anchored proteins (GPI-APs), due to somatic mutations of the phosphatidylinositolglycan complementation Class A (PIG-A) gene. STUDY DESIGN AND METHODS: In this study, the expression of a high number of GPI-APs on different subsets of peripheral blood (PB) cells from 14 PNH patients and their potential association with underlying genetic abnormalities has been analyzed. RESULTS: This study confirms the existence of variable patterns of expression of different GPI-APs on both major and minor PB-cell subsets from PNH patients. The size of the PNH clone within PB neutrophils and monocytes was systematically higher than that of other cell populations. Genetic changes were detected in the PIG-A gene in 5 of 13 cases analyzed. Interestingly, the reactivity for many GPI-APs was significantly higher on different subsets of normal PB cells from PNH patients than those observed on healthy volunteers. CONCLUSION: The best combination of markers for the diagnostic screening of PNH would include evaluation of CD14 on monocytes and of CD16 on neutrophils, although further analysis of CD55 and CD59 expression may contain additional clinically useful information. Clear association between the genetic changes detected in the PIG-A gene in 5 of 13 cases analyzed, and the phenotypic profile of PNH cells has not been found. Additionally, an abnormally higher expression of several GPI-APs among normal residual cells from PNH patients in comparison to healthy donors was observed, suggesting that factors other than the PIG-A mutation could determine the phenotypic profile of PB cells in PNH.

Bortezomib induces selective depletion of alloreactive T lymphocytes and decreases the production of Th1 cytokines.

We explored the ability of the proteasome inhibitor bortezomib, which prevents nuclear factor kappaB (NF-kappaB) activation, to block T-cell activation, proliferation, and survival within alloreactive compared with resting T cells. For this purpose, T cells were stimulated with PHA, alphaCD3/alphaCD28, or allogeneic dendritic cells or through mixed lymphocyte cultures. NF-kappaB expression increased in activated T lymphocytes compared with resting T cells. Of interest, the higher the NF-kappaB expression, the more intense the proliferative blockade induced by bortezomib. Moreover, after mixed lymphocyte reaction (MLR) cultures, alloreactive T cells were 2 logs more sensitive to bortezomib-induced apoptosis than the resting T-cell counterpart. This effect was due to a selective induction of apoptosis among activated T cells that was related to caspase activation and cleavage of the antiapoptotic bcl-2 protein and was partially abolished by the addition of the pancaspase inhibitor Z-VAD-FMK. In addition, after secondary MLR, the number of activated T cells was significantly reduced among T lymphocytes previously cultured with bortezomib when cells from the same donor were used as stimulating cells. By contrast, when third-party donor cells were used as stimulating cells, no significant differences were observed between T lymphocytes previously exposed or not to the drug, indicating a highly specific depletion of T lymphocytes alloreactive against primary donor antigens. The addition of bortezomib decreased not only the proliferation and viability of activated T lymphocytes but also the levels of IFNgamma and IL-2, which were significantly decreased among activated T cells cultured with bortezomib at doses ranging from 10 to 100 nM. In conclusion, at concentrations reached in the clinical setting, bortezomib induces selective apoptosis and decreases Th1 response among alloreactive T lymphocytes while it barely affects unstimulated T cells. These results establish the basis for the clinical use of bortezomib in the management of graft-versus-host disease (GVHD).

Immunophenotypic analysis of myelodysplastic syndromes.

BACKGROUND AND OBJECTIVES: In contrast with hematologic malignancies in which the value of immunophenotypic studies is well established, information on the immunophenotypic characteristics of myelodysplastic syndromes (MDS) is scanty. The main goal of the present study was to explore the immunophenotypic differences between patients with MDS and normal individuals, including changes in distribution of cell lineages as well as phenotypic aberrations and blockades in cell maturation pathways. DESIGN AND METHODS: In MDS the proportion of bone marrow CD34+ cells was higher than in normal patients but the most immature progenitors (CD34+CD38-) were less represented. By contrast the proportion of myelomonocytic CD34+ cells was greater than in normal individuals, translating into an increased myeloid/non-myeloid CD34+ hematopoietic progenitor cell ratio. RESULTS: This suggests that in MDS, the majority of CD34+ cells are already committed to the myeloid lineage. Upon analyzing the granulo-monocytic differentiation pathway, MDS patients showed an increased proportion of monocytic cells with a decreased percentage of cells of neutrophil lineage, leading to a lower neutrophil/monocytic cell ratio. Maturational arrests in the monocytic but not in the neutrophil differentiation pathway were observed. In refractory anemia with excess blasts in transformation (RAEB-t) such blockades mainly occurred during the earliest stages of differentiation but in the other MDS subtypes they occurred in later stages. INTERPRETATION AND CONCLUSIONS: Phenotypic aberrations occurred in 90% of patients and a high proportion of cases showed >=2 aberrations. In summary, our results show that, in addition to an abnormal distribution of the bone marrow cell compartment, MDS patients frequently show aberrant phenotypes and maturational arrests. Some of these features may help in cases in which the diagnosis of MDS is questionable.