High-dose BCNU/Melphalan conditioning regimen before autologous stem cell transplantation in newly diagnosed multiple myeloma.

Single-agent high-dose melphalan (HDM, 200 mg/m2) has been the most commonly used conditioning regimen prior to autologous stem cell transplant, since its introduction in 1992. We used a more aggressive alkylator-based conditioning regimen in an attempt to overcome early relapse and combat drug resistance. We present a retrospective comparison and long-term follow-up of newly diagnosed patients with multiple myeloma (MM) treated with induction followed by either high-dose carmustine (BCNU) and HDM, or HDM alone, both followed by autologous stem cell transplant (ASCT). Between 1997 and 2002, 104 patients were treated with BCNU/HDM; from 2001 to 2008, 103 patients were treated with HDM alone. Median follow-up of survivors was 78 and 68 months for the BCNU/HDM and HDM groups, respectively. The median PFS was significantly increased with the BCNU/HDM regimen (40.4 vs 20.5 months, P<0.001). Median overall survival was increased with the BCNU/HDM regimen when compared with HDM alone (88.4 vs 67.2 months, P=0.07), but the difference was not statistically significant. Transplant-related mortality was similar in both groups (2.9% with BCNU and HDM vs 3.9% with HDM alone). Our findings suggest that the BCNU/HDM preparative regimen should be investigated further and potentially compared in a prospective randomized manner with HDM alone.

Potential for a novel manganese porphyrin compound as adjuvant canine lymphoma therapy.

PURPOSE: Manganese porphyrins are redox-active drugs and superoxide dismutase mimics, which have been shown to chemosensitize lymphoma, a cancer which frequently occurs in dogs. This study aimed to identify critical information regarding the pharmacokinetics and toxicity of Mn(III) meso-tetrakis (N-n-butoxyetylpyridium-2-yl) porphyrin, (MnTnBuOE-2-PyP5+, MnBuOE) in dogs as a prelude to a clinical trial in canine lymphoma patients. METHODS: A single-dose pharmacokinetic (PK) study in normal dogs was performed to determine the plasma half-life (t 1/2) of MnBuOE. A dose reduction study was performed to establish the maximum tolerated dose (MTD) of MnBuOE. The safety and PK of a multi-dosing protocol was assessed. RESULTS: Peak plasma drug concentration occurred 30 min post-injection. The t 1/2 was defined as 7 h. MnBuOE induced an anaphylactic reaction and prolonged tachycardia. The MTD was defined as 0.25 mg/kg. The dogs were given MTD 3×/week for 2-3 weeks. The highest recorded tissue drug levels were in the lymph nodes (4-6 µM), followed by kidney and liver (2.5, 2.0 uM, respectively). CONCLUSIONS: We obtained critical information regarding the PK and toxicity of MnBuOE in dogs. The acute drug reaction and tachycardia post-injection have not been described in other species and may be specific to canines. The high tissue drug levels in lymph nodes have not been previously reported. MnBuOE accumulation in lymph nodes has important implications for the utility of adjuvant MnBuOE to treat lymphoma. With MnBuOE lymph node accumulation, reduction in the dose and/or administration frequency could be possible, leading to reduced toxicity.

Alternative donors extend transplantation for patients with lymphoma who lack an HLA matched donor.

Alternative donor transplantation is increasingly used for high-risk lymphoma patients. We analyzed 1593 transplant recipients (2000-2010) and compared transplant outcomes in recipients of 8/8 allele HLA-A, -B, -C and DRB1 matched unrelated donors (MUDs; n=1176), 7/8 allele HLA mismatched unrelated donors (MMUDs; n=275) and umbilical cord blood donors (1 or 2 units UCB; n=142). Adjusted 3-year non-relapse mortality of MMUD (44%) was higher as compared with MUD (35%; P=0.004), but similar to UCB recipients (37%; P=0.19), although UCB had lower rates of neutrophil and platelet recovery compared with unrelated donor groups. With a median follow-up of 55 months, 3-year adjusted cumulative incidence of relapse was lower after MMUD compared with MUD (25% vs 33%, P=0.003) but similar between UCB and MUD (30% vs 33%; P=0.48). In multivariate analysis, UCB recipients had lower risks of acute and chronic GVHD compared with adult donor groups (UCB vs MUD: hazard ratio (HR)=0.68, P=0.05; HR=0.35; P<0.001). Adjusted 3-year OS was comparable (43% MUD, 37% MMUD and 41% UCB). These data highlight the observation that patients with lymphoma have acceptable survival after alternative donor transplantation. MMUD and UCB can extend the curative potential of allotransplant to patients who lack suitable HLA matched sibling or MUD.

Leukemic stem cells: a review.

A small subset of cells in a patient with leukemia, termed leukemic stem cells (LSCs) have been shown to be responsible for the proliferation of disease. LSCs are thought to derive from normal hematopoietic stem cells, but are phenotypically distinguishable in that they are CD90(-), CD117(-), and CD123(+). Research in mouse models provides several potential therapeutics to target these cells in human patients. Eliminating LSCs should provide an efficient, potentially curative treatment option for leukemia patients.

Pre-engraftment syndrome after myeloablative dual umbilical cord blood transplantation: risk factors and response to treatment.

High fevers and/or rashes prior to neutrophil engraftment are frequently observed after umbilical cord blood (UCB) transplantation, and the condition is referred to as pre-engraftment syndrome (PES). Few studies have evaluated the risk factors for and treatment response to PES. Therefore, we retrospectively characterized PES in 57 consecutive engrafted patients (≥ 12 years old) who received myeloablative dual UCB transplantation. All patients received TBI (≥ 13.2 Gy)-based myeloablative conditioning. Tacrolimus (n=35) or CYA (n=22) combined with mycophenolate mofetil was used as GVHD prophylaxis. PES was defined as the presence of non-infectious fever (≥ 38.5 °C) and/or rash prior to or on the day of neutrophil engraftment. The incidence (95% confidence interval) of PES was 77% (66-88%). The incidence of PES was significantly higher in patients who received CYA as a GVHD prophylaxis than those who received tacrolimus (P<0.001), and this association was confirmed in the multivariate analysis. The occurrence of PES did not impact OS or tumor relapse, although it may have increased non-relapse mortality (P=0.071). The incidence of acute GHVD or treatment-related mortality was not influenced by the choice to use corticosteroids to treat PES. This study suggests that use of CYA for GVHD prophylaxis increases the risk of PES following dual UCB transplantation.

MK-0457, an Aurora kinase and BCR-ABL inhibitor, is active in patients with BCR-ABL T315I leukemia.

MK-0457, an Aurora kinase and BCR-ABL inhibitor, was studied on a Phase I/II study in 77 patients with refractory hematologic malignancies. The average number of cycles per patient was 3 (range 1-21). Maximum tolerated doses for a 5-day short infusion and continuous infusion regimens were 40 mg/m(2)/h and 144 mg/m(2)/h, respectively. Drug-related adverse events (AEs) included transient mucositis and alopecia. Eight of 18 patients with BCR-ABL T315I-mutated chronic myelogenous leukemia (44%) had hematologic responses and one of three patients (33%) with Philadelphia chromosome-positive acute lymphoblastic leukemia obtained complete remission. MK-0457 has important activity in patients with leukemias expressing the highly resistant T315I BCR-ABL mutation.

Preemptive dosing of plerixafor given to poor stem cell mobilizers on day 5 of G-CSF administration.

Plerixafor, given on day 4 of G-CSF treatment is more effective than G-CSF alone in mobilizing hematopoietic progenitor cells. We tested a strategy of preemptive plerixafor use following assessment of the peak mobilization response to 5 days of G-CSF. Patients were eligible for plerixafor if, on day 5 of G-CSF, there were <7 circulating CD34+ cells/µL or if <1.3 × 10(6) CD34+ cells/kg were collected on the first day of apheresis. Plerixafor (0.24 mg/kg s.c.) was given on day 5 of G-CSF followed by apheresis on day 6. This was repeated for up to two additional doses of plerixafor. The primary end point of the study was the percentage of patients who collected at least 2 × 10(6) CD34+ cells/kg. Twenty candidates for auto-SCT enrolled on the trial. The circulating CD34+ cell level increased a median of 3.1 fold (range 1-8 fold) after the first dose of plerixafor and a median of 1.2 fold (range 0.3-6.5 fold) after the second dose of plerixafor. In all, 15 out of 20 (75%) patients achieved the primary end point. In conclusion, the decision to administer plerixafor can be delayed until after the peak mobilization response to G-CSF has been fully assessed.

Outcomes of a 1-day nonmyeloablative salvage regimen for patients with primary graft failure after allogeneic hematopoietic cell transplantation.

Primary graft failure after allogeneic hematopoietic cell transplantation is a life-threatening complication. A shortened conditioning regimen may reduce the risk of infection and increase the chance of survival. Here, we report the outcome of 11 patients with hematologic diseases (median age, 44; range, 25-67 years, seven males) who received a 1-day reduced-intensity preparative regimen given as a re-transplantation for primary graft failure. The salvage regimen consisted of fludarabine, cyclophosphamide, alemtuzumab and TBI, all administered 1 day before re-transplantation. All patients received T-cell replete PBSCs from the same or a different haploidentical donor (n=10) or from the same matched sibling donor (n=1). Neutrophil counts promptly increased to >500/µL for 10 of the 11 patients at a median of 13 days. Of these, none developed grade III/IV acute GVHD. At present, 8 of the 11 patients are alive with a median follow-up of 11.2 months from re-transplantation and 5 of the 8 are in remission. In conclusion, this series suggests that our 1-day preparative regimen is feasible, leads to successful engraftment in a high proportion of patients, and is appropriate for patients requiring immediate re-transplantation after primary graft failure following reduced-intensity transplantation.

Differential impact of inhibitory and activating Killer Ig-Like Receptors (KIR) on high-risk patients with myeloid and lymphoid malignancies undergoing reduced intensity transplantation from haploidentical related donors.

The impact of activating KIR (aKIR) and inhibitory KIR (iKIR) on OS, relapse-related mortality (RRM) and acute GVHD (aGVHD) was prospectively studied in 84 adults with high-risk hematologic malignancies receiving reduced intensity conditioning (RIC) T-cell depleted hematopoietic SCT (HSCT) from haploidentical related donors. In this clinical model, freedom from RRM is dependent on GVL effect. Patients were divided into myeloid (n=49) and lymphoid (n=35) malignancy groups. KIR-ligand and ligand-ligand models were studied in both GVH and rejection directions and statistically correlated with outcome measures. In the myeloid group, OS was higher (P=0.009) and RRM was lower (P=0.036) in patients missing HLA-C group2 ligand to donor iKIR. OS was higher if patients had >1 missing ligand (P=0.018). In lymphoid malignancy, missing ligand to donor KIR had no impact on OS or RRM. However, OS was better with donor aKIR 2DS2 (P=0.028). There was a trend towards shorter OS in recipient with KIR 2DS1, 2DS5 and 3DS1, although sample sizes were too small to provide inferential statistics. Findings in lymphoid malignancy patients should be further studied. These results suggest that the absence of appropriate HLA ligands in the recipient to donor iKIR may induce GVL without aGVHD in myeloid malignancy patients undergoing TCD-RIC transplants.

Prognostic factors for outcomes in allogeneic transplantation for CML in the imatinib era: a CIBMTR analysis.

Allogeneic hematopoietic SCT is an effective treatment in accelerated (AP) or blast phase (BP) CML. Imatinib (IM) has transient but significant activity in advanced phases of CML, which may permit early allografting for responding patients. To identify prognostic factors in allograft recipients previously treated with IM, we analyzed 449 allogeneic hematopoietic SCTs performed from 1999 to 2004 in advanced-phase CML, using the data reported to the Center for International Blood and Marrow Transplant Research. CML patients in second chronic phase (CP2, n=184), AP (n=185) and BP (n=80) received HLA-identical sibling (27%), related (3%), or matched or mismatched unrelated donor (70%), peripheral blood (47%) or BM (53%) hematopoietic SCT after myeloablative (78%) or non-myeloablative (22%) conditioning. In all, 52% in CP2, 49% in AP and 46% in BP received IM before hematopoietic SCT. Disease-free survival was 35-40% for CP2, 26-27% for AP and 8-11% for BP. Cumulative incidence of acute and chronic GVHD and TRM were not affected by the stages of CML or pre-hematopoietic SCT IM exposure. Multivariate analyses showed that conventional prognostic indicators remain the strongest determinants of transplant outcomes. In conclusion, there are no new prognostic indicators of the outcomes of allogeneic hematopoietic SCT for advanced-phase CML in the IM era.

Response and toxicity of donor lymphocyte infusions following T-cell depleted non-myeloablative allogeneic hematopoietic SCT from 3-6/6 HLA matched donors.

We report the outcome of early donor lymphocyte infusions (DLIs) after T-cell depleted non-myeloablative transplantation using stem cells from HLA-matched or mismatched donors. Sixty-nine patients with high-risk hematologic malignancies received DLI following fludarabine, CY and alemtuzumab with infusion of stem cells from a matched sibling (52) or partially matched family member donor (17). Patients received the first infusion at a median of 50 days after transplant, and doses ranged from 1 x 10(4) CD3+ cells/kg to 3.27 x 10(8) CD3+ cells/kg, depending on clinical status and the physician's discretion. A median cell dose of 1 x 10(5) CD3+ cells/kg in the mismatched setting and 1 x 10(6) CD3+ cells/kg in the matched sibling setting appears safe with only 1 of 7 (14%) and 4 of 31 patients (13%), respectively, experiencing severe acute GVHD at these doses. Importantly, 38% of patients with persistent disease before DLI attained a remission after infusion. Nine of the 69 patients remain alive and disease-free 32-71 months after the first DLI. In conclusion, low doses of DLI can be safely provided soon after T-cell depleted non-myeloablative therapy and provide a chance of remission. However, long-term survival still remains poor, primarily because of relapse in these patients.

Apolipoprotein E genotype as a determinant of survival in chronic lymphocytic leukemia.

Survival of chronic lymphocytic leukemia (CLL) cells requires sustained activation of the antiapoptotic PI-3-K/Akt pathway, and many therapies for CLL cause leukemia cell death by triggering apoptosis. Blood lipoprotein particles are either pro- or antiapoptotic. High-density lipoprotein particles are antiapoptotic through sphingosine-1-phosphate receptor 3-mediated activation of the PI-3-K/Akt pathway. Apolipoprotein E4 (apoE4)-very low density lipoproteins (VLDL) increase apoptosis, but the apoE2-VLDL and apoE3-VLDL isoforms do not. As increased B-cell apoptosis favors longer survival of CLL patients, we hypothesized that APOE4 genotype would beneficially influence the clinical course of CLL. We report here that women (but not men) with an APOE4 genotype had markedly longer survival than non-APOE4 patients. VLDL is metabolized to low-density lipoprotein through lipoprotein lipase. Higher levels of lipoprotein lipase mRNA in these CLL patients correlated with shorter survival. The beneficial effect of APOE4 in CLL survival is likely mediated through APOE4 allele-specific regulation of leukemia cell apoptosis. The APOE allele and genotype distribution in these CLL patients is the same as in unaffected control populations, suggesting that although APOE genotype influences CLL outcome and response to therapy, it does not alter susceptibility to developing this disease.

Outcomes of a second non-myeloablative allogeneic stem cell transplantation following graft rejection.

Following initial graft rejection, a second attempt at allogeneic immunotherapy is often contemplated, but data on the success is limited. We therefore report on 11 patients with hematologic malignancies, renal cell cancer or marrow failure who underwent a second reduced-intensity regimen for primary or secondary graft failure. Nine of the 11 patients initially engrafted with the second attempt including two of four who used the same donor. One of the patients engrafted after the third attempt using a different donor and conditioning regimen. There were two treatment-related deaths. Four patients died from progressive disease 1-9 months after the second transplant. Two patients are still in recovery phase less than 1 year from the second transplant. Long-term remission is possible and three patients are alive in complete remission.

Successful allogeneic engraftment of mismatched unrelated cord blood following a nonmyeloablative preparative regimen.

Reduction in the toxicity of allogeneic transplantation with nonmyeloablative induction regimens has expanded the scope of practice to older and more debilitated patients. However, the limited availability of matched sibling donors requires that alternative donor sources be investigated. Reported here are 2 cases of patients with advanced hematologic malignancies without matched siblings, partially matched family members, or matched unrelated donors who successfully underwent nonmyeloablative conditioning therapy followed by infusion of partially matched, unrelated-donor cord blood cells. The patients are in remission and remain 100% donor as assessed by short tandem repeat analysis of the marrow 6 and 12 months following transplantation.

Nuclear factor-kappaB is constitutively activated in primitive human acute myelogenous leukemia cells.

Human acute myelogenous leukemia (AML) is thought to arise from a rare population of malignant stem cells. Cells of this nature, herein referred to as leukemic stem cells (LSCs), have been documented for nearly all AML subtypes and appear to fulfill the criteria for stem cells in that they are self-renewing and give rise to the cells found in many leukemic populations. Because these cells are likely to be critical for the genesis and perpetuation of leukemic disease, the present studies sought to characterize unique molecular properties of the LSC population, with particular emphasis on the transcription factor, nuclear factor-kappaB (NF-kappaB). Previous experiments have shown that unstimulated human CD34(+) progenitor cells do not express NF-kappaB. In contrast, primary AML CD34(+) cells display readily detectable NF-kappaB activity as assessed by electrophoretic mobility shift assay and gene expression studies. Furthermore, detailed analyses of enriched AML stem cells (CD34(+)/CD38(-)/CD123(+)) indicate that NF-kappaB is also active in the LSC population. Given the expression of NF-kappaB in leukemic, but not normal primitive cells, the hypothesis that inhibition of NF-kappaB might induce leukemia-specific apoptosis was tested by treating primary cells with the proteasome inhibitor MG-132, a well-known inhibitor of NF-kappaB. Leukemic CD34(+)/CD38(-) cells displayed a rapid induction of cell death in response to MG-132, whereas normal CD34(+)/CD38(-) cells showed little if any effect. Taken together, these data indicate that primitive AML cells aberrantly express NF-kappaB and that the presence of this factor may provide unique opportunities to preferentially ablate LSCs.

Hematopoietic engraftment and survival in adult recipients of umbilical-cord blood from unrelated donors.

BACKGROUND: Umbilical-cord blood from unrelated donors who are not HLA-identical with the recipients can restore hematopoiesis after myeloablative therapy in children. We studied the use of transplantation of umbilical-cord blood to restore hematopoiesis in adults. METHODS: Sixty-eight adults with life-threatening hematologic disorders received intensive chemotherapy or total-body irradiation and then transplants of HLA-mismatched umbilical-cord blood. We evaluated the outcomes in terms of hematologic reconstitution, the occurrence of acute and chronic graft-versus-host disease (GVHD), relapses, and event-free survival. RESULTS: Of the 68 patients, 48 (71 percent) received grafts of umbilical-cord blood that were mismatched for two or more HLA antigens. Of the 60 patients who survived 28 days or more after transplantation, 55 had neutrophil engraftment at a median of 27 days (range, 13 to 59). The estimated probability of neutrophil recovery in the 68 patients was 0.90 (95 percent confidence interval, 0.85 to 1.0). The presence of a relatively high number of nucleated cells in the umbilical-cord blood before it was frozen was associated with faster recovery of neutrophils. Severe acute GVHD (of grade III or IV) occurred in 11 of 55 patients who could be evaluated within the first 100 days after transplantation. Chronic GVHD developed in 12 of 33 patients who survived for more than 100 days after transplantation. The median follow-up for survivors was 22 months (range, 11 to 51). Of the 68 patients, 19 were alive and 18 of these (26 percent) were disease-free 40 months after transplantation. The presence of a high number of CD34+ cells in the graft was associated with improved event-free survival (P=0.05). CONCLUSIONS: Umbilical-cord blood from unrelated donors can restore hematopoiesis in adults who receive myeloablative therapy and is associated with acceptable rates of severe acute and chronic GVHD.

Expression of tumor-suppressor genes interferon regulatory factor 1 and death-associated protein kinase in primitive acute myelogenous leukemia cells.

Previous studies indicate that human acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells. Cells of this nature can initiate and maintain leukemic cell growth in both long-term cultures and nonobese diabetic/severe combined immune-deficient mice. To characterize the biology of primitive AML cells, gene expression screens were performed with 7 primary AML and 3 normal specimens. For each sample, stem cell populations (CD34(+)/CD38(-)) were isolated and used to synthesize radiolabeled complementary DNA (cDNA). AML vs normal probes were then hybridized to cDNA arrays containing genes related to cancer and apoptosis. Of approximately 1400 genes analyzed, 2 tumor-suppressor genes were identified that were overexpressed in all 7 of the AML CD34(+)/CD38(-) cell populations: death-associated protein kinase and interferon regulatory factor 1. Expression of each gene was confirmed by reverse-transcription polymerase chain reaction and immunoblot analysis. It is proposed that tumor-suppressor proteins play a role in the biology of primitive AML cells. (Blood. 2001;97:2177-2179)

Radiation dose selection in Hodgkin's disease patients with large mediastinal adenopathy treated with combined modality therapy.

PURPOSE: To determine the effective dose of consolidation radiation in Hodgkin's disease (HD) patients with large mediastinal adenopathy (LMA) treated with combined modality therapy (CMT). METHODS AND MATERIALS: Eighty-three HD patients with LMA receiving CMT between 1983 and 1997 at Duke University and Yale University were identified. Patients underwent complete clinical staging. The staging breakdown was: IA, 4 patients; IB, 1 patient; IIA, 25 patients; IIB, 33 patients; IIIA, 3 patients; IIIB-6 patients; IVA, 2 patients; and IVB, 9 patients. All patients received induction chemotherapy (CT) as follows: MOPP/ABV(D), 31 patients; BCVPP, 15 patients; ABVD, 24 patients; MOPP, 3 patients; and other regimens, 10 patients. Following 6 cycles of CT, patients were restaged and classified as having either complete response (CR) or induction failure (IF). Post-CT gallium scans were obtained in 52 patients. Patients with residual radiographic abnormalities were classified as having CR if they were gallium-negative and clinically well otherwise. Following induction CT, 78 patients had a CR. There were 5 IFs. Consolidation irradiation was administered to all sites of initial involvement in patients who had achieved CR. RT dose varied. Patients were grouped into the following dose ranges: < or = 20 Gy, 12 patients; 20-25 Gy, 24 patients; 25-30 Gy, 30 patients; > or = 30 Gy, 12 patients. RESULTS: Overall survival and failure-free survival were both 76% at 10 years. Of the 78 CR patients, 15 failed. Patterns of failure were in-field alone, 8 patients; out of field alone, 2 patients; and combined, 5 patients. Failure patterns by RT dose were: < or = 20 Gy, 0/12; 20-25 Gy, 7/24; 25-30 Gy, 5/30; > or = 30 Gy, 3/11. There was no apparent correlation between RT dose and subsequent failure. Post chemotherapy gallium scans were helpful in predicting for failure. Of 48 patients in whom the gallium was negative after chemotherapy, there were 6 failures, compared with 9 failures among 30 patients in whom gallium was not done after chemotherapy (p = 0.066). Additionally, patients receiving adriamycin-based chemotherapy regimens had improved outcomes compared to those not receiving adriamycin (p = 0.03.) CONCLUSIONS: These retrospective data suggest that low-dose radiotherapy following CR achieved with induction chemotherapy (particularly when documented with gallium scanning) may be as effective as higher doses for bulky HD at presentation. Phase III trials are necessary for confirmation of this hypothesis.

The interleukin-3 receptor alpha chain is a unique marker for human acute myelogenous leukemia stem cells.

Recent studies suggest that the population of malignant cells found in human acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells (LSCs). LSCs have been documented for nearly all AML subtypes and have been phenotypically described as CD34+/CD38- or CD34+/HLA-DR-. Given the potentially critical role of these primitive cells in perpetuating leukemic disease, we sought to further investigate their molecular and cellular characteristics. Flow cytometric studies using primary AML tissue showed that the interleukin-3 receptor alpha chain (IL-3Ralpha or CD123) was strongly expressed in CD34+/CD38- cells (98 +/- 2% positive) from 16 of 18 primary specimens. Conversely, normal bone marrow derived CD34+/CD38- cells showed virtually no detectable expression of the CD123 antigen. To assess the functional role of IL-3Ralpha positive cells, purified CD34+/CD123+ leukemia cells were transplanted into immune deficient NOD/SCID mice. These experiments showed that CD123+ cells were competent to establish and maintain leukemic populations in vivo. To begin to elucidate a biological role for CD123 in leukemia, primary AML samples were analyzed with respect to signal transduction activity in the MAPK, Akt, and Stat5 pathways. Phosphorylation was not detected in response to IL-3 stimulation, thereby suggesting CD123 is not active in conventional IL-3-mediated signaling. Collectively, these data indicate that CD123 represents a unique marker for primitive leukemic stem cells. Given the strong expression of this receptor on LSCs, we propose that targeting of CD123 may be a promising strategy for the preferential ablation of AML cells.