Nilotinib exerts equipotent antiproliferative effects to imatinib and does not induce apoptosis in CD34+ CML cells

Blood ◽  
2007 ◽  
Vol 109 (9) ◽  
pp. 4016-4019 ◽  
Author(s):  
Heather G. Jørgensen ◽  
Elaine K. Allan ◽  
Niove E. Jordanides ◽  
Joanne C. Mountford ◽  
Tessa L. Holyoake
Keyword(s):  
Chronic Myeloid Leukemia ◽  
Progenitor Cells ◽  
Imatinib Mesylate ◽  
Myeloid Leukemia ◽  
Caspase 3 ◽  
Inhibitory Concentration ◽  
Antiproliferative Effects ◽  
Cd34 Cells ◽  
Stem And Progenitor Cells ◽  
Predominant Effect

Abstract Chronic myeloid leukemia (CML) stem and progenitor cells overexpress BcrAbl and are insensitive to imatinib mesylate (IM). We therefore investigated whether these cells were efficiently targeted by nilotinib. In K562, the inhibitory concentration (IC50) of nilotinib was 30 nM versus 600 nM for IM, consistent with its reported 20-fold-higher potency. However, in primary CD34+ CML cells, nilotinib and IM were equipotent for inhibition of BcrAbl activity, producing equivalent but incomplete reduction in CrkL phosphorylation at 5 μM. CML CD34+ cells were still able to expand over 72 hours with 5 μM of either drug, although there was a concentration-dependent restriction of amplification. As for IM, the most primitive cells (CFSEmax) persisted and accumulated over 72 hours with nilotinib and remained caspase-3 negative. Furthermore, nilotinib with IM led to further accumulation of this population, suggesting at least additive antiproliferative effects. These results confirmed that, like IM, the predominant effect of nilotinib is antiproliferative rather than proapoptotic.

Persisting Chronic Myeloid Leukemia Stem and Progenitor Cells From Patients in Major Molecular Remission Under Imatinib Are Characterized by Low BCR/ABL Expression.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 2207-2207
Author(s):  
Ashu Kumari ◽  
Cornelia Brendel ◽  
Thorsten Volkmann ◽  
Sonja Tajstra ◽  
Andreas Neubauer ◽  
...  
Keyword(s):  
Stem Cells ◽  
Stem Cell ◽  
Chronic Myeloid Leukemia ◽  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Soft Agar ◽  
Molecular Remission ◽  
Stem And Progenitor Cells ◽  
First Diagnosis

Abstract Abstract 2207 Poster Board II-184 Introduction: Treatment with the Abl-kinase specific inhibitor imatinib (IM) is very effective in chronic myeloid leukemia (CML). However, IM presumably fails to eradicate CML stem cells (HSC) leading to disease persistence and relapse after IM-discontinuation. Although causes of CML persistence under imatinib remain ill defined, quiescence and BCR/ABL-overexpression of CML stem and progenitor cells have been suggested as underlying mechanisms. We here set out to identify means to directly study persistence mechanisms in residual BCR/ABL-positive progenitor and stem cell clones from chronic phase CML patients in major molecular remission (mmR) under imatinib. Methods: Bone marrow specimens of twenty-one CML patients in at least major molecular remission (mmR) according to the international scale, first diagnosis (FD) patients (n=5) and healthy donors (n=4) were sorted into HSC, common myeloid progenitors (CMP), granulocyte/macrophage progenitors (GMP) and megakaryocate-erythrocyte progenitors (MEP) and BCR-ABL mRNA expression was directly assessed by quantitative real time (qPCR) and/or nested PCR (mRNA of 4.000 sorted cells). Alternatively, HSC, CMP, GMP and MEP were seeded into soft agar and mRNA was extracted from individual colony forming units (CFU) to assess BCR/ABL-mRNA expression by qPCR. Moreover, CFU of sub-fractions of first diagnosis CML patients were treated in vitro with IM at 3mM and BCR/ABL-expression of surviving CFU was compared with the BCR/ABL expression levels of mock-treated CML-CFU. In total, 595 soft agar colonies were analyzed. Results: By nested PCR, BCR/ABL-mRNA was readily detectable in the HSC compartments of 7 of 10 (7/10) CML patients in mmR. BCR/ABL was also detected in the CMP-, GMP-, and MEP-compartments in 6, 10 and 8 of the 10 patients, respectively. Real time qRT-PCR suggested only a trend toward stronger BCR/ABL positivity of the HSC compartment when compared to the other progenitor compartments (table 1). A detailed analysis of the BCR/ABL-expression of individual CFU from HSC-, CMP-, GMP-, and MEP-compartments of mmR patients revealed that persisting CML-CFU expressed significantly less BCR/ABL than first diagnosis CML-CFU obtained before imatinib therapy (table 1). This finding could be recapitulated in vitro: primary CML-CD34+ cells of first diagnosis CML patients (n=4) were seeded into soft agar in the presence or absence of 3 uM imatinib. After 14 days BCR/ABL expression only of BCR/ABL-positive CFU was compared. BCR/ABL-positive CML-CFU (n=30) that had survived imatinib exposure expressed significantly less BCR/ABL than mock-treated CML-CFU (n=175) (p<0.001). Work is in progress providing in vitro evidence that selection/induction of low BCR/ABL expression in immature progenitor and stem cells is a new mechanism of imatinib persistence in mmR patients via reducing oncogenic addiction from BCR/ABL. Conclusions: We showed that BCR/ABL-persistence is not confined to the quiescent CML-stem cell compartment, but seems to affect also the highly proliferative progenitor compartments. More intriguingly, persisting CML-HSC and -precursor cells express remarkably low levels of BCR/ABL when compared to first diagnosis HSC and progenitors, implying that low BCR/ABL expression reduces imatinib sensitivity in vivo. The simple model of selection / induction of low BCR/ABL expression as mechanism of imatinib persistence in CML would explain the low propensity of disease progression after achieving mmR, and the low genetic instability of CML clones from mmR patients. Disclosures: No relevant conflicts of interest to declare.

scholarly journals Bortezomib induces apoptosis in primitive chronic myeloid leukemia cells including LTC-IC and NOD/SCID repopulating cells

Blood ◽  
2010 ◽  
Vol 115 (11) ◽  
pp. 2241-2250 ◽  
Author(s):  
Nicholas B. Heaney ◽  
Francesca Pellicano ◽  
Bin Zhang ◽  
Lisa Crawford ◽  
Su Chu ◽  
...  
Keyword(s):  
Chronic Myeloid Leukemia ◽  
Progenitor Cells ◽  
Cell Lines ◽  
Myeloid Leukemia ◽  
Kinase Inhibitors ◽  
Therapeutic Option ◽  
Proteasome Inhibitors ◽  
Chronic Phase ◽  
Proteasome Activity ◽  
Stem And Progenitor Cells

AbstractChronic myeloid leukemia (CML) is treated effectively with tyrosine kinase inhibitors (TKIs); however, 2 key problems remain—the insensitivity of CML stem and progenitor cells to TKIs and the emergence of TKI-resistant BCR-ABL mutations. BCR-ABL activity is associated with increased proteasome activity and proteasome inhibitors (PIs) are cytotoxic against CML cell lines. We demonstrate that bortezomib is antiproliferative and induces apoptosis in chronic phase (CP) CD34+ CML cells at clinically achievable concentrations. We also show that bortezomib targets primitive CML cells, with effects on CD34+38−, long-term culture-initiating (LTC-IC) and nonobese diabetic/severe combined immunodeficient (NOD/SCID) repopulating cells. Bortezomib is not selective for CML cells and induces apoptosis in normal CD34+38− cells. The effects against CML cells are seen when bortezomib is used alone and in combination with dasatinib. Bortezomib causes proteasome but not BCR-ABL inhibition and is also effective in inhibiting proteasome activity and inducing apoptosis in cell lines expressing BCR-ABL mutations, including T315I. By targeting both TKI-insensitive stem and progenitor cells and TKI-resistant BCR-ABL mutations, we believe that bortezomib offers a potential therapeutic option in CML. Because of known toxicities, including myelosuppression, the likely initial clinical application of bortezomib in CML would be in resistant and advanced disease.

Functional ABCG2 is overexpressed on primary CML CD34+ cells and is inhibited by imatinib mesylate

Blood ◽  
2006 ◽  
Vol 108 (4) ◽  
pp. 1370-1373 ◽  
Author(s):  
Niove E. Jordanides ◽  
Heather G. Jorgensen ◽  
Tessa L. Holyoake ◽  
Joanne C. Mountford
Keyword(s):  
Chronic Myeloid Leukemia ◽  
Cell Line ◽  
Imatinib Mesylate ◽  
Cell Population ◽  
Myeloid Leukemia ◽  
Philadelphia Chromosome ◽  
Drug Transporter ◽  
Cd34 Cells ◽  
Considerable Debate ◽  
Intracellular Concentrations

Abstract Imatinib mesylate (IM) therapy for chronic myeloid leukemia (CML) has transformed the treatment of this disease. However, the vast majority of patients, despite major responses, still harbor Philadelphia chromosome–positive (Ph+) cells. We have described a population of primitive Ph+ cells that are insensitive to IM and may be a source of IM resistance. Cell line studies have suggested that the drug transporter ABCG2 may be a mediator of IM resistance, however there is considerable debate about whether IM is an ABCG2 substrate or inhibitor. We demonstrate here that primitive CML CD34+ cells aberrantly overexpress functional ABCG2 but that cotreatment with IM and an ABCG2 inhibitor does not potentiate the effect of IM. We definitively show that IM is an inhibitor of, but not a substrate for, ABCG2 and that, therefore, ABCG2 does not modulate intracellular concentrations of IM in this clinically relevant cell population.

Evaluation of Ph+/CD34+ Residual Cells in Chronic Myeloid Leukemia Patients after Long Lasting Treatment with Imatinib Mesylate.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 1999-1999
Author(s):  
Monica Bocchia ◽  
Elisabetta Abruzzese ◽  
Micaela Ippoliti ◽  
Simona Calabrese ◽  
Alessandro Gozzetti ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Chronic Myeloid Leukemia ◽  
Imatinib Mesylate ◽  
Median Time ◽  
False Positive ◽  
Myeloid Leukemia ◽  
Prolonged Treatment ◽  
Published Data ◽  
Imatinib Treatment ◽  
Cd34 Cells

Abstract Although the success of imatinib mesylate therapy represents an exciting advance in targeted cancer therapy, it has still to be determined whether responses to this p210 inhibitor in chronic myeloid leukemia (CML) patients will be durable. In fact most of clinical studies agree on the evidence of a persistent molecular disease in the majority of treated patients and altough the absolute level of bcr-abl transcript may vary over the treatment, yet a molecular complete response is of rare observation. In addition, discontinuation of imatinib exerts always in rapid loss of response. In accordance to this the persistence of malignant progenitors in patients in complete cytogenetic response (CCR) after short term imatinib treatment, has been recently demonstrated. In particular, Bathia et al. showed in 12/15 patients studied after a median time of 10 months of imatinib treatment a median of 11% of residual CML CD34+ progenitors in the bone marrow (by FISH Dual Fusion bcr/abl analysis)while only 3/15 patients had no detectable residual CD34+ cells. Less is known about residual Ph+/CD34+ cells surviving after a prolonged therapy with this targeting drug. Thus, we evaluated the amount of bone marrow residual CD34+ cells in 17 CML patients in stable CCR after a long lasting treatment with imatinib. At the time of evaluation, the patients were on conventional dose (400mg) Imatinib for a median time of 48 months (range 36–58 months) having achieved a CCR status (conventionally defined as the complete absence of t(9;22) on caryotypic analysis) within 3 to 6 months of treatment. However all of them still showed molecular disease as detected by nested RT-PCR. Bone marrow CD34+ cell-enriched populations were selected from mononuclear cells using immunomagnetic column separation and were evaluated after cytospin by FISH using a bcr-abl Dual Color Extra Signal Probe(LSI bcr-abl ES, Vysis), that is able to detect bcr-abl fusion in interphase nuclei with a false positive signal rate close to 0. A minimum of 100 CD34+ nuclei per each sample were evaluated. Interestingly, in 8/17 patients no Ph+/CD34+ cells were detected, while in the remaining 9/17 patients a median of 2% (range 0.5–11%) of bcr-abl positive progenitors were still observed. In this small selected serie of patients prolonged treatment with imatinib appears to be correlated with a lower, yet detectable, amount of residual bone marrow Ph+/CD34+ cells when compared to previously published data. This result could be partly explained with the different specificity and sensitivity of the probe used (bcr/abl ES<1% false positive; bcr-abl Dual Fusion 8–10% false positive) The clinical significance of these data as well as the role of this cell target to monitor minimal residual disease in CML needs to be evaluated on a larger serie of patients.

Gene Expression Signatures Associated with Treatment and Resistance to Imatinib Mesylate in Chronic Myeloid Leukemia Patients.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 433-433 ◽  
Author(s):  
Vivian Oehler ◽  
S. Branford ◽  
E. Pogosova-Agadjanyan ◽  
N. Shah ◽  
C. Sawyers ◽  
...  
Keyword(s):  
Gene Expression ◽  
Chronic Myeloid Leukemia ◽  
Differential Expression ◽  
Imatinib Mesylate ◽  
Myeloid Leukemia ◽  
Secondary Resistance ◽  
Total Rna ◽  
Expression Of Genes ◽  
Cd34 Cells ◽  
Study Gene Expression

Abstract Imatinib mesylate (IM) has dramatically changed the treatment approach for patients with chronic myeloid leukemia (CML). However, ~20% of chronic phase (CP) patients are initially resistant to IM, and among patients who achieve a complete cytogenetic response (CCyR), a small minority relapse back into CP or progress to advanced disease. Abl tyrosine kinase domain mutations are the major cause of secondary resistance. Oligonucleotide microarray analysis was used to study gene expression patterns associated with primary IM resistance and relapse after initial successful response to IM. Samples included total RNA from diagnostic samples of 25 CML CP patients within 6 months of diagnosis and at the time of failure (7 patients); total RNA from 18 patients who relapsed after initial CCyR with documented Abl point mutations (12 CP and 6 BC); and 10 myeloid progenitor samples (sorted by CD34 and CD38 status) from an IM naïve and IM resistant (R) patient, both in blast crisis. Results: For primary IM resistance, analysis of paired samples before and after treatment revealed that primary failure was associated with the differential expression of genes associated with RNA post-transcriptional modification, protein synthesis, cellular growth and proliferation, and cell death. Two genes with the highest differential expression included the apotosis resistance genes API5 and TRAF5. TRAF5 was also increased in patients who relapsed after initial CCyR, while API5 showed significantly increased expression in sorted CD34+ cells from an IMR patient. In secondary resistance patients, a set of drug transporters including ABCA2, ABCA3, MDR1, and ABCC3 had increased expression and hOCT1 decreased expression relative to 42 IM naïve CP patients. In vitro experiments compared K562 resistant (R) and sensitive (S) cell lines over time exposed to IM. The K562 IMR cell line showed a 1.5 log higher expression of ABCG2 and a 1 log higher expression of TRAF5 compared to the K562 IMS cells. However, sequential clinical samples of 16 IM non-responders vs. 14 CCyR patients showed no change in ABCG2 expression, possibly because ABCG2 is expressed only in early progenitor cells, not differentiated cells. The importance of using CD34+ cells was demonstrated in 7 array studies comparing gene expression in CD34+ selected cells from an IMR patient compared to an IM naïve patient. IM resistance was associated with increased expression of genes associated with cell cycle, DNA recombination and repair, and proliferation. Genes associated with apoptosis resistance included increased expression of API5, survivin, and decreased expression of BAK1. Protein serine/threonine and tyrosine kinases associated with cell proliferation/survival and tumor progression with increased expression in the IMR patient included: AURKB, AKT3, BUB1B, CDC2, CHEK1, MAPK9, STK6, TTK, and WEE1. Conclusion: Gene expression studies suggest that primary resistance to IM therapy is associated with resistance to apoptosis; relapse on IM is associated with activation of drug transporter genes and genes associated with disease progression; in studying disease response and resistance, primitive hematopoetic cells are critical for analysis.

SFK Inhibition with Dasatinib Results In Selective Targeting of Primitive Human Acute Myeloid Leukemia Stem and Progenitor Cells.

Blood ◽  
2010 ◽  
Vol 116 (21) ◽  
pp. 1053-1053
Author(s):  
Cedric Emmanuel Dos Santos ◽  
Tinisha McDonald ◽  
Liang Li ◽  
Allen Lin ◽  
Ya-Huei Kuo ◽  
...  
Keyword(s):  
Stem Cell ◽  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Growth And Survival ◽  
Cd34 Cells ◽  
Dependent Inhibition ◽  
Stem And Progenitor Cells ◽  
Control Versus ◽  
Dose Dependent Inhibition ◽  
Dose Dependent

Abstract Abstract 1053 The Src family tyrosine kinases (SFKs) are abnormally activated in AML compared to normal CD34+ hematopoietic progenitors. Studies using pharmacological and siRNA approaches indicate an important role for the SFK Lyn in AML progenitor cell growth and survival (Dos Santos et al., 2008). However the role of SFKs in AML leukemic stem cell (LSC) growth and survival is not clear. SFK activity in Lin- CD34+ CD38- primitive stem/progenitor cells, Lin- CD34+ CD38+ committed progenitors, and Lin-CD34- cells from primary human AML (n=14) and cord blood (CB) (n=6) samples was measured by analyzing SFK phosphorylation using flow cytometry. We observed significant increase of SFK phosphorylation in AML compared to CB Lin- CD34+CD38- cells (3.8±1.9 versus 1.9±0.7, p=0.006), Lin- CD34+ CD38+ (3.9±1.7 versus 1.9±0.6, p=0.013) and Lin- CD34- cells (3.4±1.8 versus 1.1±0.5, p=0.0005). Dasatinib, a potent SFK and ABL kinase inhibitor, is approved for clinical use in chronic myeloid leukemia. We evaluated the effect of SFK inhibition using Dasatinib on the growth and viability of AML stem and progenitor cells. Dasatinib exposure resulted in dose-dependent inhibition of SFK phosphorylation in each subpopulation (in CD34+ CD38-, 3.0 for the control versus 1.9 with 100nM and 1.6 with 500nM, in CD34+ CD38+, 2.8 for the control versus 1.9 with 100nM and 1.6 with 500nM) after 30 minutes of drug treatment). The addition of Dasatinib (10-500nM) to methylcellulose progenitor assays resulted in dose-dependent inhibition of AML colony forming cell (CFC) growth (83.9±16.1% inhibition with 500nM, and 48.1±6.8% inhibition with 10nM Dasatinib, n=8), to a greater extent than CB CFC to (CFU-GM inhibition 62.6±1.5% with 500nM, and 1.1±16.3% with 10nM Dasatinib, n=4). Short-term exposure to Dasatinib (10-500nM) for 48 hours also resulted in significantly greater inhibition of AML CFC (73.6±13% with 500nM, and 41.7±10.8% with 10nM Dasatinib, n=8) compared to CB CFC (CFU-GM inhibition 23±9.1% with 500nM, and 1.3±11.3% with 10nM Dasatinib, n=4). Importantly Dasatinib treatment (200nM) also resulted in reduction of AML stem/primitive progenitor growth in long term culture-initiating cells (LTC-IC) assays (56±23, 8 % inhibition, p=0.003, n=4), suggesting that SFK inhibition may inhibit AML stem cell maintenance. The effect of Dasatinib on apoptosis was evaluated by labeling cells with Annexin V and DAPI. Treatment with Dasatinib resulted in significant increase in apoptosis of Lin- AML cells (41.5% ±10.7 of apoptosis with 200nM Dasatinib versus 25%±10.8 for the control, p=0,004, n=5) We studied the effects of Dasatinib on differentiation of Lin- CB (n=3) and AML cells (N=5) cultured with SCF, IL-3, GM-CSF, G-CSF and EPO. In normal CD34+, Dasatinib (100 nM) treatment resulted in increased CD33+ and CD14+ cells and reduced CD34+, CD11b+, CD15+, GPA+ and CD71+ cell numbers, indicating that SFK increased monocytic but reduced granulocytic and erythroid differentiation. Treatment of AML cells with Dasatinib resulted in markedly reduced numbers of CD34+, CD33+ and CD71+ cells, but increased numbers of CD11b cells, in 3 of 5 samples, indicating a trend towards increased granulocytic differentiation in contrast to normal progenitors. Our results indicate that SFK activity is increased in primary human AML stem and progenitor cells and suggest that SFK blockade with Dasatinib may reduce maintenance of AML LSC/ primitive progenitors, through inhibition of progenitor proliferation, induction of apoptosis and enhancement of differentiation. These results support further evaluation of SFK blockade with Dasatinib for targeting of AML stem and progenitor cells in preclinical and clinical studies. Disclosures: Bhatia: Novartis: Consultancy, Honoraria.

scholarly journals STAT5 is required for long-term maintenance of normal and leukemic human stem/progenitor cells

Blood ◽  
2007 ◽  
Vol 110 (8) ◽  
pp. 2880-2888 ◽  
Author(s):  
Hein Schepers ◽  
Djoke van Gosliga ◽  
Albertus T. J. Wierenga ◽  
Bart J. L. Eggen ◽  
Jan Jacob Schuringa ◽  
...  
Keyword(s):  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Cell Number ◽  
Hematopoietic Stem ◽  
Fold Reduction ◽  
Cd34 Cells ◽  
Stem And Progenitor Cells ◽  
Precise Role ◽  
Term Maintenance

Abstract The transcription factor STAT5 fulfills a distinct role in the hematopoietic system, but its precise role in primitive human hematopoietic cells remains to be elucidated. Therefore, we performed STAT5 RNAi in sorted cord blood (CB) and acute myeloid leukemia (AML) CD34+ cells by lentiviral transduction and investigated effects of STAT5 downmodulation on the normal stem/progenitor cell compartment and the leukemic counterpart. STAT5 RNAi cells displayed growth impairment, without affecting their differentiation in CB and AML cultures on MS5 stroma. In CB, limiting-dilution assays demonstrated a 3.9-fold reduction in progenitor numbers. Stem cells were enumerated in long-term culture-initiating cell (LTC-IC) assays, and the average LTC-IC frequency was 3.25-fold reduced from 0.13% to 0.04% by STAT5 down-regulation. Single-cell sorting experiments of CB CD34+/CD38− cells demonstrated a 2-fold reduced cytokine-driven expansion, with a subsequent 2.3-fold reduction of progenitors. In sorted CD34+ AML cells with constitutive STAT5 phosphorylation (5/8), STAT5 RNAi demonstrated a reduction in cell number (72% ± 17%) and a decreased expansion (17 ± 15 vs 80 ± 58 in control cultures) at week 6 on MS5 stroma. Together, our data indicate that STAT5 expression is required for the maintenance and expansion of primitive hematopoietic stem and progenitor cells, both in normal as well as leukemic hematopoiesis.

Selective Targeting Of Inv(16)+ AML Stem Progenitor Cells By Inhibiting HDAC8

Blood ◽  
2013 ◽  
Vol 122 (21) ◽  
pp. 224-224
Author(s):  
Jing Qi ◽  
Sandeep Singh ◽  
Qi Cai ◽  
Ling Li ◽  
Hongjun Liu ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Myeloid Leukemia ◽  
Leukemic Cells ◽  
Control Group ◽  
Cd34 Cells ◽  
Stem And Progenitor Cells ◽  
Primary Bone ◽  
Primary Bone Marrow ◽  
Acute Myeloid

Abstract Chromosomal inversion inv(16)(p13.1q22) which leads to the fusion of the transcription factor gene CBFb and the MYH11 gene, occurs in over 8% of acute myeloid leukemia (AML) cases. The fusion product CBFβ-SMMHC (CM) inhibits differentiation of hematopoietic stem and progenitor cells (HSPCs) and creates pre-leukemic populations predisposed to acute myeloid leukemia (AML) transformation. The mutations of tumor suppressor p53 occur in approximately half of all cases of human cancer, but TP53 mutations are relatively rare in inv(16) AML. We have previously shown that CM expression leads to reduced acetylation of p53 and impaired p53 target gene activation through formation of aberrant protein complex with p53 and HDAC8 (Blood, 2012,120: A772.). Here, we showed that CM interacts with p53 both in CM transformed mouse primary bone marrow cells as well as in AML stem and progenitor cells from inv(16) patients. When HDAC8 selective pharmacological inhibitor 22d directed against its catalytic sites (ChemMedChem 2012, 7:10, 1815-24;) were used to treat inv(16) mouse primary bone marrow progenitor cells and inv(16)+ CD34+ stem progenitor cells from patients, Ac-p53 levels were remarkably increased as shown by western blot. We further assessed the p53 target genes expression after HDAC8 inhibitor 22d treatment by qRT-PCR assay in inv(16)+ CD34+ stem progenitor cells (n=8), and observed variable levels of activation in p53 targets (Fold activation: p21:2.25-fold, hdm2:1.17-fold, 14-3-3σ: 3.12-fold, puma: 2.39-fold), indicating p53 was re-activated. Similar results were also shown in CM transformed mouse bone marrow progenitor cells. Importantly, we found that 22d treatment significantly inhibit the growth of inv(16)+ AML CD34+ cells (n=9) rather than normal CD34+ cells (n=7) , (AML IC50= 6.509 μM, vs Normal cells IC50=13.83 μM, p=0.0003). Meanwhile, 22d selectively induces apoptosis of inv(16)+ AML stem and progenitor cells while sparing normal HSPCs (AML LD50= 10.24 μM, vs NL LD50= 46.36 μM, p=0.001). To evaluate whether the effect of HDAC8i is mediated by p53, we knocked down p53 with a lentiviral vector expressing shRNA against p53 (or non-silencing shRNA) in AML CD34+ cells, and treated the cells with HDAC8 inhibitor 22d (5-20 µM). We showed that despite the inter-sample variability, knocking down p53 expression in all AML samples tested (n=3) led to reduced HDAC8i-induced apoptosis, suggesting that p53 contributes to the apoptosis effect induced by HDAC8i (22d) in inv(16)+ AML cells. Importantly, by taking advantage of our conditional knock-in mouse model (Cbfb56M/+/Mx1-Cre), which develops AML under induced expression of CBFß-SMMHC (Cancer Cell, 2006, 9:1, 57-68), we were able to perform the ex vivo treatment assay by treating primary leukemic cells (marked with dTomato) with either DMSO (as vehicle control) or with HDAC8 inhibitor 22d (10μM) for 48h, followed by transplantion into congenic mice (control group n=8, treatment group n=7). We observed reduced short-term engraftment of leukemic cells that are treated with 22d (10 μM) at 4 weeks post-transplantation in the peripheral blood (Donor cell%: control group=5.99%, treatment group=0.178%, P=0.0093). Interestingly, engraftment of cord blood CD34+ cells at 16 weeks post-bone marrow transplantation was not reduced after treatment with 22d (10 μM) (human CD45+ %: control=66.2% versus treatment=63.4%, p=0.9), indicating the effect by HDAC8 inhibition is selective for leukemic cells. In conclusion, we have identified a novel mechanism whereby CBFβ-SMMHC inhibits p53 fucntion, and may further implicate inhibition of HDAC8 as a promising approach to selectively target inv(16)+ AML stem and progenitor cells. Disclosures: No relevant conflicts of interest to declare.

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