Tyrosine Kinase Inhibitors Resistance In Chronic Myeloid Leukemia Research Articles

Mechanisms underlying therapeutic resistance of tyrosine kinase inhibitors in chronic myeloid leukemia.

Chronic myeloid leukemia (CML) is a malignant clonal disease involving hematopoietic stem cells that is characterized by myeloid cell proliferation in bone marrow and peripheral blood, and the presence of the Philadelphia (Ph) chromosome with BCR-ABL fusion gene. Treatment of CML has dramatically improved since the advent of tyrosine kinase inhibitors (TKI). However, there are a small subset of CML patients who develop resistance to TKI. Mutations in the ABL kinase domain (KD) are currently recognized as the leading cause of TKI resistance in CML. In this review, we discuss the concept of resistance and summarize recent advances exploring the mechanisms underlying CML resistance. Overcoming TKI resistance appears to be the most successful approach to reduce the burden of leukemia and enhance cures for CML. Advances in new strategies to combat drug resistance may rapidly change the management of TKI-resistant CML and expand the prospects for available therapies.

Mechanism of Late Recurrences of Chronic Myeloid Leukemia after Discontinuation of TKI Therapy: BCR::ABL1 Ins35bp Loss-of-Function Splicing Mutation and Regulatory T Cells

Introduction: Treatment-free remission (TFR) as one of the goals of treatment of BCR::ABL-positive chronic phase chronic myeloid leukemia (CML) has been proposed. Actually, approximately 40-60% of CML patients maintain TFR without recurrence after discontinuation of tyrosine kinase inhibitors (TKIs). In some of such patients, there are fluctuated cases in which BCR::ABL positive clones continue to be detected for long periods at levels of international scale (IS) BCR::ABL < 0.1%. Such fluctuated cases show no apparent molecular recurrence and maintain TFR for a long time, but they have also been shown to be more likely to eventually develop late recurrence (Haematologica 2022, Blood adv. 2020). We have reported that a higher percentage of PD-1 positive CD8 + T cells before and after TKI discontinuation is associated with molecular recurrence, and together with the possibility of assessing host immune response capacity after TKI discontinuation by using regulatory T (Treg) cells (Fujioka, et al. Cancers 2021). On the other hands, mutations in BCR::ABL caused by the alternative splicing, such as BCR::ABL Ins35bp, are known to be one of the TKI resistance in CML. Loss of BCR::ABL kinase activity is thought to result in loss of TKI sensitivity and thus failure to achieve deep molecular remission (Yuda, et al. Cancer Science 2017). In this study, we analyzed the mutations that cause splicing variants such as BCR::ABL Ins35bp and host immune response in CML patients to elucidate molecular mechanisms in the long-term maintenance of TFR and late recurrence. Methods: Twenty-four CML patients who discontinued TKI treatment for any reason at Akita University Hospital were included in the study. The trends of IS level after TKI discontinuation were monitored, and immune profile analysis by multi-color flow cytometry (FCM) was performed in all patients. Mutation analysis of the BCR::ABL region including splicing variants was performed in selected patients. Mutation analysis was performed on samples with sufficient cell count and IS > 0.0032% (MR 4.5), and the BCR::ABL region of these samples was amplified by long nested PCR and then analyzed by the next-generation sequencer (NGS). Results: First, BCR::ABL Ins35bp was evaluated by mutation analysis. Among CML patients who underwent mutation analysis, BCR::ABL Ins35bp was detected in all cases in which the samples could perform analysis at newly diagnosis (ND). Even after TKI discontinuation, a certain percentage of BCR::ABL Ins35bp was detected in all patients, and the percentage of BCR::ABL Ins35bp increased at the time of recurrence regardless of whether early or late. These results suggest that function-dead BCR::ABL Ins35bp clones may remain in CML patients with detectable BCR::ABL under and after TKI treatment, and eventually leading to recurrence with transcriptional promotion of native BCR::ABL. On the other hand, there were cases in which the BCR::ABL Ins35bp was lost at the time of recurrence. There is a possibility of immunological involvement in such cases. CML patients with detectable BCR::ABL in the TFR for a long time had a decreased immunosuppressive Treg cells, while analysis using late recurrence increased Treg cells prior to recurrence (figure). These suggest the presence of cases in which early recurrence is suppressed by host immune responses. Conclusion: Although the loss of function mutations such as BCR::ABL Ins35bp was previously thought to occur after TKI administration as a cause of TKI resistance, this analysis reveals that the clone is included from the ND before TKI treatment. Examining the BCR::ABL Ins35bp clone at ND may be possible to predict the possibility of recurrence after TFR. Host immune capacity, as assessed by the kinetics of Treg after TKI discontinuation, may contribute to prolonged TFR. Thus Treg monitoring may be important in predicting molecular recurrence, especially in BCR::ABL Ins35bp cases.

Genetic Landscape of Chronic Myeloid Leukemia and a Novel Targeted Drug for Overcoming Resistance.

Tyrosine kinase inhibitors (TKIs) exemplify the success of molecular targeted therapy for chronic myeloid leukemia (CML). However, some patients do not respond to TKI therapy. Mutations in the kinase domain of BCR::ABL1 are the most extensively studied mechanism of TKI resistance in CML, but BCR::ABL1-independent mechanisms are involved in some cases. There are two known types of mechanisms that contribute to resistance: mutations in known cancer-related genes; and Philadelphia-associated rearrangements, a novel mechanism of genomic heterogeneity that occurs at the time of the Philadelphia chromosome formation. Most chronic-phase and accelerated-phase CML patients who were treated with the third-generation TKI for drug resistance harbored one or more cancer gene mutations. Cancer gene mutations and additional chromosomal abnormalities were found to be independently associated with progression-free survival. The novel agent asciminib specifically inhibits the ABL myristoyl pocket (STAMP) and shows better efficacy and less toxicity than other TKIs due to its high target specificity. In the future, pooled analyses of various studies should address whether additional genetic analyses could guide risk-adapted therapy and lead to a final cure for CML.

BCR/ABL1ΔE7-8-9 isoform contributes to tyrosine kinase inhibitor resistance in chronic myeloid leukemia.

In chronic myeloid leukemia (CML) patients, the involvement of the BCR/ABL1 isoform in tyrosine kinase inhibitors (TKIs) resistance has attracted lots of attention. In this work, a novel isoform that encoded truncated protein due to the deletion of ABL1 exon7, 8, and 9 was reported and named BCR/ABL1ΔE7-8-9 here. This isoform was detected only in 10.2% of CML patients with inadequate responses to TKIs. BCR/ABL1Δexon7-8-9 isoform promoted S phase cell proliferation and reduced the expression of fusion gene and ABL1 phosphorylation level more slowly than that of control cells after TKIs treatment. The novel isoform has the qualities of a functional tyrosine kinase, localized in the cytoplasm, and could not be imported into the nucleus by TKIs. These results indicated that BCR/ABL1Δexon7-8-9 showed poorer sensitivity to imatinib and nilotinib than wild-type BCR/ABL1. According to molecular docking studies, nilotinib and imatinib present different binding sites and have a lower binding capacity with BCR/ABL1ΔE7-8-9 protein than the wild type. Our findings suggested that the novel isoform BCR/ABL1ΔE7-8-9 may contribute to TKIs resistance in CML due to its weakened TKIs binding ability. It enriched the mechanism of spliceosome involved in TKIs resistance. Monitoring the expression of BCR/ABL1ΔE7-8-9 helps guide the treatment of CML patients in the clinic.

Proteasome 26S subunit, non-ATPases 1 (PSMD1) and 3 (PSMD3), play an oncogenic role in chronic myeloid leukemia by stabilizing nuclear factor-kappa B

Tyrosine kinase inhibitors (TKIs) targeting BCR-ABL1 have revolutionized therapy for chronic myeloid leukemia (CML), paving the way for clinical development in other diseases. Despite success, targeting leukemic stem cells and overcoming drug resistance remain challenges for curative cancer therapy. To identify drivers of kinase-independent TKI resistance in CML, we performed genome-wide expression analyses on TKI-resistant versus sensitive CML cell lines, revealing a nuclear factor-kappa B (NF-κB) expression signature. Nucleocytoplasmic fractionation and luciferase reporter assays confirmed increased NF-κB activity in the nucleus of TKI-resistant versus sensitive CML cell lines and CD34+ patient samples. Two genes that were upregulated in TKI-resistant CML cells were proteasome 26S subunit, non-ATPases 1 (PSMD1) and 3 (PSMD3), both members of the 19S regulatory complex in the 26S proteasome. PSMD1 and PSMD3 were also identified as survival-critical genes in a published small hairpin RNA library screen of TKI resistance. We observed markedly higher levels of PSMD1 and PSMD3 mRNA in CML patients who had progressed to the blast phase compared with the chronic phase of the disease. Knockdown of PSMD1 or PSMD3 protein correlated with reduced survival and increased apoptosis in CML cells, but not in normal cord blood CD34+ progenitors. Luciferase reporter assays and immunoblot analyses demonstrated that PSMD1 and PSMD3 promote NF-κB protein expression in CML, and that signal transducer and activator of transcription 3 (STAT3) further activates NF-κB in scenarios of TKI resistance. Our data identify NF-κB as a transcriptional driver in TKI resistance, and implicate PSMD1 and PSMD3 as plausible therapeutic targets worthy of future investigation in CML and possibly other malignancies.

Combating TKI resistance in CML by inhibiting the PI3K/Akt/mTOR pathway in combination with TKIs: a review.

Chronic myeloid leukemia (CML), a myeloproliferative hematopoietic cancer, is caused by a genetic translocation between chromosomes 9 and 22. This translocation produces a small Philadelphia chromosome, which contains the Bcr-Abl oncogene. The Bcr-Abl oncogene encodes the BCR-ABL protein, upregulates various signaling pathways (JAK-STAT, MAPK/ERK, and PI3K/Akt/mTOR), and out of which the specifically highly active pathway is the PI3K/Akt/mTOR pathway. Among early treatments for CML, tyrosine kinase inhibitors (TKIs) were found to be the most effective, but drug resistance against kinase inhibitors led to the discovery of novel alternative therapies. At this point, the PI3K/Akt/mTOR pathway components became new targets due to stimulation of this pathway in TKIs-resistant CML patients. The current review article deals with reviewing the scientific literature on the PI3K/Akt/mTOR pathway inhibitors listed in the National Cancer Institute (NCI) drug dictionary and proved effective against multiple cancers. And out of those enlisted inhibitors, the US FDA has also approved some PI3K inhibitors (Idelalisib, Copanlisib, and Duvelisib) and mTOR inhibitors (Everolimus, Sirolimus, and Temsirolimus) for cancer therapy. So far, several inhibitors have been tested, and further investigations are still ongoing. Even in Imatinib, Nilotinib, and Ponatinib-resistant CML cells, a dual PI3K/mTOR inhibitor, BEZ235, showed antiproliferative activity. Therefore, by considering the literature data of these reviews and further examining some of the reported inhibitors, which proved effective against the PI3K/Akt/mTOR signaling pathway in multiple cancers, may improve the therapeutic approaches towards TKI-resistant CML cells where the respective signaling pathway gets upregulated.

Metabolic Plasticity Is an Essential Requirement of Acquired Tyrosine Kinase Inhibitor Resistance in Chronic Myeloid Leukemia.

Simple SummaryTyrosine kinase inhibitors (TKIs), such as imatinib, have become the standard initial treatment of choice for chronic myeloid leukemia (CML) patients. However, one obstacle to face is that a significant proportion of patients presents poor response to TKIs, or acquires resistance resulting in disease relapses. Mutations in BCR-ABL1 protein are a well described mechanism of resistance but other not well established mechanisms outside BCR-ABL1 mutations are emerging as important in the acquisition of resistance. Abnormal metabolism of CML cells that acquire resistance to imatinib has been pointed out as a putative downstream key event, but deep studies aimed to unveil metabolic adaptations associated with acquired resistance are still lacking. Here, we perform an exhaustive study on metabolic reprogramming associated with acquired imatinib resistance and we identify metabolic vulnerabilities of CML imatinib resistant cells that could pave the way for new therapies targeting TKI failure. Tyrosine kinase inhibitors (TKIs) are currently the standard chemotherapeutic agents for the treatment of chronic myeloid leukemia (CML). However, due to TKI resistance acquisition in CML patients, identification of new vulnerabilities is urgently required for a sustained response to therapy. In this study, we have investigated metabolic reprogramming induced by TKIs independent of BCR-ABL1 alterations. Proteomics and metabolomics profiling of imatinib-resistant CML cells (ImaR) was performed. KU812 ImaR cells enhanced pentose phosphate pathway, glycogen synthesis, serine-glycine-one-carbon metabolism, proline synthesis and mitochondrial respiration compared with their respective syngeneic parental counterparts. Moreover, the fact that only 36% of the main carbon sources were utilized for mitochondrial respiration pointed to glycerol-phosphate shuttle as mainly contributors to mitochondrial respiration. In conclusion, CML cells that acquire TKIs resistance present a severe metabolic reprogramming associated with an increase in metabolic plasticity needed to overcome TKI-induced cell death. Moreover, this study unveils that KU812 Parental and ImaR cells viability can be targeted with metabolic inhibitors paving the way to propose novel and promising therapeutic opportunities to overcome TKI resistance in CML.

PVR and ICAM-1 on Blast Crisis CML Stem and Progenitor Cells with TKI Resistance Confer Susceptibility to NK Cells.

The BCR-ABL1 fusion gene generating an oncogenic tyrosine kinase is a hallmark of chronic myeloid leukemia (CML), which can be successfully targeted by BCR-ABL1 tyrosine kinase inhibitors (TKIs). However, treatment-free remission has been achieved in a minority of patients due to evolving TKI resistance and intolerance. Primary or acquired resistance to the approved TKIs and progression to blast crisis (BC), thus, remain a major clinical challenge that requires alternative therapeutic strategies. Here, we first demonstrate that donor natural killer (NK) cells prepared using a protocol adopted in clinical trials can efficiently eliminate CML-BC blasts, with TKI resistance regardless of BCR-ABL1 mutations, and preferentially target CD34+CD38− leukemic stem cells (LSC), a potential source of disease relapse. Mechanistically, the predominant expression of PVR, a ligand for the NK cell-activating DNAM-1 receptor, in concert with ICAM-1, a ligand for NK cell adhesion, confer this susceptibility to NK cells, despite the lack of ligands for NKG2D, a principal NK cell activating receptor, as an immune evasion mechanism. With these mechanistic insights, our findings provide a proof-of-concept that donor NK cell-based therapy is a viable strategy for overcoming TKI resistance in CML, particularly the advanced, multi-TKI-resistant CML with dismal outcome.

An Updated Safety and Efficacy Results of Phase 1 Study of HQP1351, a Novel 3rd Generation of BCR-ABL Tyrosine Kinase Inhibitor (TKI), in Patients with TKI Resistant Chronic Myeloid Leukemia

Background: HQP1351, a novel, orally active potent 3rd generation TKI, was designed for treatment of patients with chronic myeloid leukemia (CML) resistant to current TKI-therapies including those with T315I mutation. This is an updated report of the safety and efficacy of HQP1351. Methods: It is an open-label, dose escalation and expansion phase 1 trial of HQP1351 which was designed to determine maximum tolerated dose (MTD) and to identify dose-limiting toxicities (DLTs) in patients with CML in the chronic phase (CP) or accelerated phase (AP) resistant or intolerant to ≥ 2 prior TKIs or those with T315I mutation after ≥1 prior TKI. HQP1351 was orally administered once every other day (QOD) in 28-day cycles at 11 dose cohorts ranging from 1mg to 60 mg. The eligible patients received continuous treatments until disease progression to AP or BP, developing intolerant toxicity, consent withdrawal, or death. Blood samples were collected at certain time points on Day 1-2 and Day 27-28 during cycle 1 for PK analyses. Results: From 26 October, 2016 to 27 May, 2019, 101 patients including 87 CP patients and 14 AP patient were enrolled in this study. Seventy-one (70.3%) patients were male. Median duration of follow-up was 12.8 (range, 1.2-31.5) months. Median age was 40 (range, 20-64) years. Median interval from CML diagnosis to starting HQP1351 treatment was 5.8 (range, 0.3-15.2) years. Eighty-three (83.8%) patients received ≥2 prior lines of TKI-therapy. Sixty-two (61.4%) patients harbored T315I mutation. Seventeen patients who initially assigned to the dose escalation cohorts ranging from 1mg to 20 mg moved to 30 mg or higher dose cohorts via intra-patient dose escalation. Fifty-six patients enrolled in the dose expansion part of the trial, including 30mg,40mg and 50mg dose cohorts. Two out of 3 patients at 60mg dose cohort experienced DLT and 50mg QOD was considered as the MTD. During the follow-up period, HQP1351 was well-tolerated in each dose cohort with an exception of 60 mg cohort. All patients experienced ≥1 treatment related adverse events (TRAEs), most of the non-hematologic TRAEs were reported as grade 1 or grade 2. The most common hematologic TRAE of grade 3/4 was thrombocytopenia (49.5%). The incidences of TRAEs tended to be dose-dependent. No death and grade 5 AEs occurred. The incidence of common TRAEs (≥ 10% any grade) are summarized in Table1. HQP1351 showed potent anti-leukemic activities in CML patients. In the 68 evaluable patients with non-CHR at baseline, 63 (92.6%) achieved CHR including 52 out of 55 (94.5%) CP patients and 11 out of 13 (84.6%) AP patients, respectively. In the 95 evaluable patients with non-CCyR at baseline, 56 out of 81 (69.1%) CP patients achieved MCyR including 49 (60.5%) CCyR; and 6 out of 14 (42.9%) AP patients, achieved MCyR including 5 (35.7%) CCyR, respectively. In the 100 evaluable patients, 32 out of 86 (37.2%) CP patients and 5 out of 14 (35.7%) AP patients achieved MMR, respectively. HQP1351 showed highly efficacious in the patients with T315I mutation (Figure A, B). The probability and the depth of response increased with prolonged treatment period (Figure C). As the cut-off date of May 27 2019, 9 patients (5 CP and 4 AP) withdrawn from the study, including progression to AP or BP (n=5), intolerant AEs (n=2), consent withdrawal (n=1), and newly diagnosis of breast cancer (n=1). The progression free survival (PFS) rate at 18-month was 94% in the CP patients and 61% in the AP patients (Figure D). PK analyses indicated an approximately dose proportional increase in exposure (AUC and Cmax) following oral administration of HQP1351 at doses from 1mg to 60 mg. The peak concentration of HQP1351 was reached at 4-8h. The elimination appeared to be linear with a mean terminal T1/2 of 17.5 to 42.5 h. Slight to moderate accumulation was observed on Day 27 following multiple dosing. Reduction of CRkL phosphorylation in PBMCs, a biomarker of BCR-ABL inhibition, has shown to be dose and time dependent in 53 evaluable patients treated with HQP1351. Conclusions: HQP1351 was well tolerated and exhibited significant and durable antitumor activity in the patients with TKI-resistant CML, including those with T315I mutation. Two pivotal studies of HQP1351 in CML-CP and CML-AP patients with T315I mutation are ongoing in China. Disclosures Chen: HealthQuest Pharma Inc.: Employment. Niu:HealthQuest Pharma Inc.: Employment. Men:HealthQuest Pharma Inc.: Employment. Wang:HealthQuest Pharma Inc.: Employment. JI:HealthQuest Pharma Inc.: Employment. Huang:HealthQuest Pharma Inc.: Employment. Yang:Ascentage Pharma Group: Employment. Zhai:Ascentage Pharma Group Inc.: Employment; HealthQuest Pharma Inc.: Employment.

The AHI-1-BCR-ABL-DNM2 Complex Mediates Mitochondrial Dynamics in Drug-Resistant BCR-ABL+ Cells

Chronic myeloid leukemia (CML) is driven by the BCR-ABL1 oncoprotein with constitutively active protein-tyrosine kinase activity, perturbing multiple signaling pathways. Although therapies with tyrosine kinase inhibitors (TKIs) can effectively treat early phase CML, relapses and emergence of TKI resistance are problematic, due to BCR-ABL kinase domain mutations and TKI unresponsive quiescent leukemic stem cells (LSCs). These observations point towards a need for alternate treatment strategies to prevent the development of resistant LSCs. We previously demonstrated that Abelson helper integration site-1 (AHI-1) is a highly deregulated protein in CML LSCs and that its WD40-repeat domain physically interacts with BCR-ABL, enhancing leukemia-initiating activity. AHI-1 also contains an SH3 domain, which mediates TKI resistance in LSCs. This domain interacts with dynamin-2 (DNM2) and forms a complex with BCR-ABL, to enhance the phosphorylation and activity of DNM2. The AHI-1-BCR-ABL-DNM2 complex is shown to regulate leukemic properties in patient LSCs, including increased ROS production, endocytosis and autophagy. Interestingly, deletion of the Ahi-1 SH3 domain (Ahi-1 SH3Δ) results in a defect in Ahi-1 localization, with most being present in the nucleus. To test whether Ahi-1 SH3 domain activity directly affects cytoplasmic anchoring and localization, we have generated two Ahi-1 mutants, using site-directed mutagenesis: a mutation in the key tryptophan residue (W939A) involved in SH3 domain binding and in a non-conserved surface residue (M906A), as a negative control, based on the crystal structure of the AHI-1 SH3 domain. Interestingly, the cytoplasm-to-nucleus signal ratio of Ahi-1 W939A was significantly reduced compared to the negative control or wildtype Ahi-1, as assessed by immunofluorescence and confocal microscopy (70% reduction, p<0.0001), indicating that changes in localization of Ahi-1 SH3Δ may result in disruption of the complex and allow for new interactions with nuclear proteins. Investigating changes in the proteome may help uncover downstream effects of the AHI-1-BCR-ABL-DNM2 complex and its biological role in mediating TKI resistance. Advanced antibody microarray analysis was then used to investigate differences in the proteome and phosphorylation landscape of BCR-ABL+ cells co-transduced with wildtype Ahi-1 or Ahi-1 SH3Δ. This system quantifies the differences in expression and phosphorylation states of key signaling proteins simultaneously, using 878 antibodies in duplicate. Twenty leads were identified by the following criteria: a large signal difference of at least 1.5-fold change, high signal strength for high expression, and low error between duplicates. These leads were validated by Western blot analysis and several of them were confirmed. Particularly, phosphorylation of cyclin-dependent kinase 1 (CDK1), a key player in cell cycle control and mitochondrial dynamics, was greatly reduced in cells expressing wildtype Ahi-1 compared to Ahi-1 SH3Δ, indicating that AHI-1-mediated phosphorylation changes in CDK1 may contribute to regulation of mitochondrial functions. Indeed, BCR-ABL-transduced cells co-expressing wildtype Ahi-1 showed increased mitochondria potential in response to TKI treatment or serum starvation, in MitoTracker analysis (p<0.05). However, this was not observed in BCR-ABL-transduced cells co-expressing the Ahi-1 SH3Δ mutant. A similar trend was also observed in immunofluorescence confocal microscopy analysis of the mitochondrial importer receptor, TOM20. To further study the role of DNM2 in mediating mitochondrial dynamics associated with AHI-1 and BCR-ABL, CRISPR-Cas9 mediated DNM2 knockdown was performed in TKI-resistant cells, using two different DNM2-targeting guide RNAs; these resulted in significant reduction in DNM2 (78% & 75%) in Western blot analysis. The knockdown cells showed a reduction in viability (60% reduction) and increased sensitivity to TKI treatment compared to the control (90% vs. 30% reduction) after 48 hours and changes in mitochondrial activity were also observed in these cells. These results support a role for mitochondrial dynamics in the AHI-1-BCR-ABL-DNM2 complex-mediated TKI response and that targeting key biological processes regulated by the AHI-1-BCR-ABL-DNM2 complex and its pathways may lead to new therapeutic strategies to overcome TKI resistance in CML. Disclosures Pelech: Kinexus Bioinformatics Corporation: Equity Ownership.

Efficacy of OR21, a Novel Oral Demethylating Agent in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a clonal hematopoietic stem cell disease induced by t(9;22)(q34;q11) translocation. The prognosis of patients with CML has dramatically improved since tyrosine kinase inhibitors (TKIs) were introduced, and recent studies show that approximately 40 to 50 % of CML patients achieved deep molecular response (DASISION and ENESTnd trial) within several years. However, therapeutic options for patients with CML who are resistant for TKIs are limited. Besides BCR-ABL kinase domain mutation, somatic mutations associated with epigenetic gene alteration (e.g. TET, DNMT3A) are reportedly involved in TKI resistance and disease progression. Thus, we investigated the efficacy of DNA demethylating agents in CML. Azacitidine (AZA) and decitabine (DAC), currently available as DNA demethylating agents, have low bioavailability for oral administration because they are easily degraded by cytidine deaminase. Then, we are developing a novel demethylating agent, OR21, with possible oral absorbability as a prodrug of DAC. In vivo analysis using cynomolgus monkeys, the area under the curve of DAC after intraduodenally administration of DAC (1.5mg/kg, 6.6µmol/kg) or OR21 (2.25mg/kg, 6.6µmol/kg) was 0.01 µM·h, 0.298 µM·h respectively, indicating OR21 have high oral absorbability. To assess the demethylating activity of OR21 for CML, we performed western blot analysis and bisulfite pyrosequencing assay to measure LINE1 methylation, using CML cell-lines (K562, BV173). OR21 decreased DNMT1 protein level as a result of demethylating and LINE1 methylation in K562 and BV173 comparable to DAC. Next, we performed cell growth inhibition and cell apoptosis assay after 72 hours exposure of OR21 to assess anti-tumor effect in vitro. OR21 inhibited cell growth comparable to DAC in CML cell-lines (K562, BV173, KCL22, MYL) in a dose-dependent manner. Notably, OR21 inhibited the cell growth and apoptosis against BV173 with an extremely low concentration (IC50; 5nM) than that of AZA (IC50; 122nM) via significant accumulation of reactive oxygen species. Whereas, OR21 weakly induced cell apoptosis against K562. OR21 induced G2/M phase cell-cycle arrest in CML cell-lines except for BV173 via pRb downregulation. These results indicated mechanisms of anti-tumor effect in OR21 were induction of cell apoptosis (BV173) or cell cycle arrest (K562, KCL22, MYL). Because of the different mechanism of action, we assessed whether OR21 and TKIs combination can enhance the anti-tumor effect of CML. We investigated the combination effects of OR21 with imatinib (IM) or dasatinib (DAS) in K562. Combination index values at IC80 calculated by Calucusyn software showed 0.642±0.129 (with IM), 1.182±0.2 (with DAS), respectively. Accordingly, OR21 combined with TKIs showed at least additive or synergistic effect for K562. TKI resistance, which can be associated with somatic mutations leading to epigenetic gene alteration or loss of function of p53, is an obstacle for molecular remission in patients with CML, thus we examined the effects of OR21 in TKI resistant cell lines or efficacy on p53 mutational status. OR21 inhibited the cell growth in IM-resistant cell-line MYL-R, a derivative of MYL, which had overexpression of Lyn, and Ba/F3 BCR-ABLT315I, which exogenously expressed Bcr-Abl (T315I), indicating OR21 could overcome TKI resistance in CML. OR21 or cytarabine did not enhance cell apoptosis against K562, MYL and KCL22 (p53 deficient or mutant cell lines) combination with nutlin-3a (MDM-2 inhibitor), while increased cell apoptosis was observed in BV173 (p53 wild type) treated with cytarabine and nutlin-3a, but not with OR21 and nutlin-3a. These results suggested the effects of OR21 did not depend on p53 mutational status. Finally, we used a mouse xenograft model to evaluate anti-tumor effect of OR21 in vivo. BALB/c Rag-2/JAK3 double-deficient (BRJ) mice were injected intravenously via tail vein with 5 ×106 BV173 cells. OR21 were administered at a dose of 2.7mg/kg (equivalent to DAC 1.0mg/kg in AUC) and PBS (vehicle) twice weekly. OR21 significantly prolonged survival in a xenograft mice model (median 35 days vs not reached, P<0.01). In conclusion, a novel orally available demethylating agent OR21 is effective for CML cells including TKI resistant clones. The efficacy and safety of OR21 for CML is expected to be verified by early-phase clinical trials. Disclosures Kamachi: Ohara Pharmaceutical Co.: Research Funding. Ureshino:OHARA Pharmaceutical Co.: Research Funding. Yoshida:OHARA Pharmaceutical Co., Ltd.: Research Funding. Kurahashi:Ohara Pharmaceutical Co.: Employment. Watanabe:Ohara Pharmaceutical Co.: Research Funding. Okada:Japan Agency for Medical Research and Development: Research Funding; Bristol-Myers Squibb: Research Funding. Kimura:Ohara Pharmaceutical Co.: Research Funding; Novartis: Honoraria, Research Funding.

Molecular Mechanisms of Resistance to Tyrosine Kinase Inhibitors.

Chronic myeloid leukemia (CML) patients with constitutive activity of BCR-ABL1 oncoprotein frequently derive significant clinical benefits from tyrosine kinase inhibitors (TKIs). Point mutations in the ABL1 kinase domain (KD) are an important mechanism of TKI resistance in CML. In this review, we present molecular mechanisms of TKI resistance paying particular attention to drug resistance which allows for a survival advantage in CML. Sensitive disease monitoring is a required standard of care for management of CML. Screening of these mutations fail to explain 20-40% of resistant cases where activation of different survival pathways must be the main reason for resistance. Eliminating TKI resistance appears to be the most successful therapeutic way to decrease leukemic disease burden and potentiate cure. Advances on novel strategies for identifying and confronting drug resistance are rapidly altering management of CML that are resistant to TKI and expanding the landscape of available therapies.

High Recombination Activating Gene (RAG) Expression and RAG Mediated Recombination Is Associated with Oncogenic Rearrangement Observed with Tyrosine Kinase Inhibitor Resistant CML

BackgroundPatients with chronic myeloid leukemia (CML) who develop blast crisis are resistant to TKI therapy. A key focus in CML research is the identification of genetic factors that promote blast crisis and TKI resistance. By using an integrative genomic approach we recently reported frequent structural variation in CML patients, particularly at lymphoid blast crisis (LBC) (Blood, 2018). Developing lymphocytes are uniquely equipped with a molecular toolkit capable of programmed DNA damage and structural variant formation; the V(D)J recombination pathway. Recombination activating genes (RAG1 and RAG2) are involved in cleavage and recombination of immunoglobulin genes to provide diversity in antibodies and T cell receptors. Off target RAG activity is reported to occur in lymphoid malignancies and cause oncogenic structural rearrangements. However, RAG expression and the extent of RAG mediated structural variation in CML are largely uncharacterized.AimTo elucidate the role of RAG mediated recombination as a source of oncogenic structural rearrangement in CML.MethodsIn a study of samples of 49 patients at chronic phase (CP) diagnosis (Dx), 20 at LBC and 19 at myeloid blast crisis (MBC), we performed whole exome sequencing and/or RNAseq. Bioinformatic analyses included fusion detection (Manta & STAR), gene expression analysis (EdgeR), and copy number variation analysis (in house developed tool). Unsupervised motif detection of sequences surrounding breakpoints was performed with MEME, and fusions were visualized with Jcircos. To identify off target RAG mediated mutation we interrogated the breakpoints of structural variants, excluding those associated with the Philadelphia translocation and those solely involving antigen receptor gene rearrangement.Results33 structural variants were identified in 22 patients with samples at Dx and/or blast crisis involving genes regularly mutated in hematologic malignancy such as MLL, MECOM, RUNX1 and IKZF1. Differential expression analysis between patients at Dx, MBC and LBC revealed >1000 genes that were differentially expressed, P<0.001. Amongst the most upregulated genes at LBC were RAG1, RAG2 and DNTT, reflected in the most enriched gene ontology: V(D)J recombination. V(D)J recombination genes were sufficient alone in stratifying LBC samples from CP Dx and MBC [Figure 1A]. Furthermore, RAG expression was detectably elevated at Dx in most patients who subsequently developed LBC [Figure 1B].To identify off target RAG mediated structural variation, we interrogated the sequence surrounding breakpoints of the 33 identified structural variants (66 breakpoints) that were separated into groups of high (n=40) and low (n=26) RAG expression. The canonical heptamer CACAGTG recombination signal sequence (RSS) was enriched only at breakpoints in the high RAG group with sequences from 31/40 breakpoints making up the motif [Figure 1C]. The comparable group of low RAG breakpoints were not enriched for the motif. Another indicator of RAG involvement is the addition of non-template nucleotides at the breakpoint, consistent with DNTT nucleotidyl-transferase activity. This was detected almost exclusively at breakpoints in patients with high RAG, 16/20 structural variants, compared to 1/13 with low RAG.There was an association between high RAG expression and structural variants bearing the RAG RSS motif at breakpoints with nucleotide additions. The most frequently observed of these were intragenic deletions of IKZF1 in 7 patients, including 3 at Dx with subsequent LBC, 3 at LBC and 1 at MBC. A number of novel structural variants also had the RAG recombination signature: IKZF1-IGK fusion, RUNX1 deletion and recurrent HBS1L-MYB intergenic locus deletion.RAG mediated structural variation was further investigated on a genomic level using putative low level fusions identified from RNAseq. By assessing the immunoglobulin/T cell receptor genes, as known sites of RAG activity, we found enrichment of interchromosomal fusions involving these genes in samples with high RAG, compared to Dx and MBC with low RAG [Figure 1D]. An agnostic motif enrichment search at sequences surrounding putative fusions showed enrichment of a motif resembling the recombination signal sequence.ConclusionOur results implicate RAG as a driver of structural variation and the formation of novel oncogenic rearrangements in CML. [Display omitted] DisclosuresHughes:Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees. Branford:Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Qiagen: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Cepheid: Honoraria.

Studying Methylation Status of Protein Tyrosine Phosphatase Receptor Gamma (PTPRG) in Chronic Myeloid Leukemia Patients in the State of Qatar

Background:Chronic myeloid leukemia (CML) is a clonal myeloid stem cell disorder associated with increased myeloid cells spectrum in peripheral blood. The disease is due to the genetic translocation t(9;22)(q34;q11.2) encoding for the BCR-ABL1 fusion oncogene which is the hallmark of CML. The resultant chimeric protein P210 kD of BCR-ABL1 causes a constitutive activation of tyrosine kinase domain that leads to continuous activation of several cell signaling pathways.The CML is currently treated with several generations of tyrosine kinase inhibitors (TKIs). However, treatment failure, resistance and disease progression are still reported and the molecular mechanisms still remain poorly understood.On the other hand, protein tyrosine phosphatase receptor gamma (PTPRG) is the natural regulatory mechanism to tyrosine kinase family, acts as tumor suppressor gene, well documented to be downregulated in CML patients while its re-expression led to reduced tyrosine phosphorylation and induced apoptosis in CML cells.PTPRG is also an important protein in the JAK/STAT pathway, has a vital role in the downstream of BCR-ABL protein kinase. Several studies suggested that the aberrant DNA methylation is involved in TKIs resistance in CML leading to leukemic clone escape and disease propagation.In different types of tumors, several mechanisms have been suggested for the loss of function of PTPRG activities such as mutations, deletions, or loss of heterozygosity LOH and gene silencing by methylation.Aim:In this study, we aimed to evaluate and characterize the methylation status of PTPRG gene function in 15 CML Patients 1- Group A: CML patients at diagnosis and 2- Group B) CML patients failed treatments according to European Leukemia Net (ELN) 2013 criteria and 10 healthy individuals. (Labeled as Normal in the figure 1)Materials and Methods:Thirty subsequent peripheral blood samples have been collected from the 15 CML patients at diagnosis and at time of failure. Genomic DNA was isolated from 400 ml of blood on EDTA tubes using Maxwell DNA Purification Kits, the quality and quantity of DNA were measured on Nanodrop Spectrophotometer and gel electrophoresis. The methylation-specific PCR and bisulfite-sequencing were adapted, EZ DNA Methylation Kit was utilized for bisulfite-conversion. Methylation Primers were designed to target the promotor and intronic CpG islands of the PTPRG gene. Gradient PCR was performed to detect the two bands of 321bp and 218bp for CpG islands of promoter and intron of PTPRG respectively. Bisulfite-specific PCR technique was carried out to amplify a fragment of bisulfite-converted DNA that has been pre-defined by the designed bisulfite-specific primers.The Methylation PCR amplified product was sequenced via Sanger Sequencing (Applied Biosystems 3130 Genetic Analyzer). The methylation analysis was performed to study the methylation status of the CpG islands using the ESME (Epigenetic Sequencing Methylation) Analysis Software.Results:Results of the methylation in 25 CpG sites of Promoter region of PTPRG:There are significant differences in the mean of methylation status amongst CML patients at diagnosis and healthy individuals of CpG promoter of PTPRG (p value is ˂0.04). However, there are no significant differences in the mean of methylation status between patients failed treatments and other groups (Figure1)Results of the methylation in 26 CpG sites of Intronic region of PTPRG:There are also significant differences in the mean of methylation status amongst all groups (p value is ˂0.003). (Figure1)Discussion and Conclusions:This is the first prospective study to evaluate epigenetic changes of PTPRG gene from CML patients in Qatar. We previously documented that the expression level of PTPRG is inversely correlated with the expression of BCR-ABL1 in CML patients.In this study, the mechanism behind the downregulation of PTPRG in our cohorts of CML patients was explored and the aberrant methylation of PTPRG could be a mechanism associated with CML disease progression and resistance to TKIs.However, further studies are still needed including larger patient's cohort and CML patients responding to the TKIs.Finally, the methylation of PTPRG in the CML patients at diagnosis and failed treatment may allow the identification of new therapeutic targets and using methylation inhibitors could be also suggested to prevent or reverse TKIs resistance and restore TKIs sensitivity. DisclosuresNo relevant conflicts of interest to declare.

Interleukin 1 Beta As a Promising Therapeutic Target in ABL Tyrosine Kinase Inhibitor Resistant Chronic Myeloid Leukemia Cells

Introduction: Although ABL tyrosine kinase inhibitor (TKI), imatinib, nilotinib and dasatinib have demonstrated the potency against chronic myeloid leukemia (CML) and Philadelphia chromosome (Ph) positive acute lymphoblastic leukemia (Ph+ALL) patients, resistance to ABL TKI can develop in the many patients. It has already reported that ABL kinase domain mutations have been implicated in the pathogenesis of ABL TKI resistance, however, it is fully not known the molecular mechanism of drug resistance ABL TKIs. Therefore, new approach against ABL TKI resistant cells may improve the outcome of Ph-positive leukemia patients. Interleukin-1 (IL-1) is a proinflammatory cytokine and central mediator of innate immunity. IL-1β also controls essential cell responses. Because enhanced IL-1β signaling is reported in patients of hematological malignancies, IL-1β may be the promising therapeutic value in ABL TKI resistant CML patients.Materials and methods: In this study, we established ABL TKI-resistant in vitro cell line models (K562 imatinib-R, K562 nilotinib-R, K562 dasatinib-R, K562 ponatinib-R). We also investigated whether IL-1β was involved in ABL TKI resistant Ph-positive leukemia cells and cytokines were induced by IL-1β in human umbilical vein endothelial cells (HUVEC).Results: We analyzed the relationship of IL-1β signaling pathways and ABL TKI sensitivity by microarray gene expression data from the online Gene Expression Omnibus (GEO). IL-1β is related to imatinib sensitivity and resistant in CML patients from the public microarray datasets of GSE14671. We next examined ABL TKI resistant cell lines (K562 imatinib-R, K562 nilotinib-R, K562 dasatinib-R, K562 ponatinib-R) in this study. BCR-ABL point mutation was not found in ABL TKI resistant cells. BCR-ABL expression levels were not increased in ABL TKI resistant K562 cells. These cells were highly resistant to ABL TKIs compare to K562 cells (K562 imatinib-R: imatinib 2μM, nilotinib-R: nilotinib 2μM, dasatinib-R: dasatinib 100nM, ponatinib-R: ponatinib 50nM). We investigated gene expression profiles in cultured ABL TKI resistant K562 cells by DNA microarray. We found gene expression of IL-1β and IL-1β mediated signaling pathway was increased ABL TKI resistant K562 cells. IL-1β gene amplification was confirmed by RT-PCR analysis. Nuclear Factor kappa-light-chain-enhancer of activated B cells (NF-kB) acts as a central mediator of inflammatory responses. Because bortezomib is a proteasome inhibitor whose anti-cancer action is partly mediated through inhibition of NF-κB, we examined the bortezomib in ABL TKI resistant cells. Combined treatment of ABL TKI resistant cells with ponatinib or imatinib and bortezomib caused more cytotoxicity than each drug alone. Caspase 3/7 activity and cellular cytotoxicity was also increased. ABL TKIs are also associated with vascular adverse events (VAEs) in CML, we next investigated the in vitro effects of ABL TKIs on cultured HUVEC. We found gene expression of IL-1β was increased after ABL TKI especially dasatinib and ponatinib treatment. IL-1β was increased in the cell culture supernatant after ABL TKIs treatment. In the immunoblot analysis, p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) phosphorylation was increased by IL-1β in the time dependent manner. We also found that gene expression of IL-6, IL-8, intercellular adhesion molecule 1 (ICAM1) and monocyte chemotactic protein-1 (MCP-1) was enhanced by IL-1β stimulation.Conclusion: The IL-1β signaling pathway is involved in ABL TKI sensitivity and drug resistant in CML cells and plays a key role in cytokine production of the HUVEC. We also provide the promising clinical relevance as a candidate drug for treatment of ABL TKI resistant leukemia patients. DisclosuresOhyashiki:Asahikase: Research Funding; Taiho Pharmaceutical KK: Honoraria, Research Funding; Dainippon Sumitomo KK,: Honoraria, Research Funding; MSD,: Honoraria, Research Funding; Bristol Meyer Squibb KK,: Honoraria, Research Funding; Ono Pharmaceutical KK,: Honoraria, Research Funding; Celegene KK,: Honoraria, Research Funding; Pfizer KK,: Honoraria, Research Funding; Kyowakko Kirin KK,: Research Funding; Nihon-Seiyaku,: Research Funding; Eizai,: Research Funding; Chugai KK,: Honoraria, Research Funding; Takeda Pharmaceutical KK,: Honoraria, Research Funding; Asteras KK,: Research Funding; Jansen Pharma KK,: Research Funding; Nippon-shinyaku,: Honoraria, Research Funding; Novartis KK,: Honoraria, Research Funding.

PFKFB3 Is a Crucial Target in the Treatment of Tyrosine Kinase Inhibitor Resistant Chronic Myelogenous Leukemia

Resistance to the BCR-ABL tyrosine kinase inhibitor (TKI) imatinib mesylate (IM) remains challenge for the treatment of chronic myeloid leukemia (CML). IM resistance often results from unknown mechanisms with wild type BCR-ABL that have no effects on TKIs binding to ABL kinase domain. The basis of such BCR-ABL-independent IM resistance remains to be elucidated. To gain insight into BCR-ABL-independent IM resistance mechanisms, we performed an initial bioinformatics screen on over represented CML genes, followed by a quantitative PCR screen of genes that were elevated in TKIs resistant CML samples. We identified a total of 33 candidate genes that were highly expressed in TKIs resistant patients. Among these genes, 6-Phosphofructo-2-Kinase/Fructose-2,6-Biphosphatase 3 (PFKFB3) controlling the limiting step of glycolysis, was found to strongly associated with TKIs resistance. RNA interference of the expression of PFKFB3 and pharmacological inhibition of its kinase activity markedly increased the sensitivity of TKIs resistant CML cells to TKIs. Furthermore, pharmacological inhibition of PFKFB3 prevented CML cells growth and significantly improved the survival of both allograft and xenograft CML mice. ChIP-seq data analysis combined with subsequent knockdown experiment demonstrated that the Ets transcription factor PU.1 regulates the elevated expression of PFKFB3 in TKIs resistant CML cells. Collectively, our results identify a therapeutically targetable mechanism of BCR-ABL-independent TKIs resistant CML. DisclosuresNo relevant conflicts of interest to declare.

Telomerase Inhibition with Imetelstat Eradicates β-Catenin Activated Blast Crisis Chronic Myeloid Leukemia Stem Cells

IntroductionLeukemia stem cells (LSCs) in chronic myeloid leukemia (CML) are generated from progenitors that have aberrantly activated self-renewal pathways thereby resulting in tyrosine kinase inhibitor (TKI) resistance. The telomerase complex, consisting of a reverse transcriptase subunit (TERT), an RNA template subunit (TERC), and a protective shelterin scaffold, transcriptionally modulates the Wnt/b-catenin self-renewal pathway. Many malignancies, including BCR-ABL TKI resistant blast crisis CML (BC CML), exhibit robust telomerase activity thereby prompting the development of imetelstat, a competitive inhibitor of telomerase enzymatic activity. Imetelstat is a covalently lipidated 13-mer oligonucleotide that binds with high affinity to the TERC subunit. Recent clinical trials showed early signs of efficacy in myeloproliferative neoplasms. However, the role of imetelstat in selective self-renewing LSC inhibition in CML had not been elucidated. Thus, we performed progenitor RNA sequencing (RNA-seq), stromal co-cultures and humanized LSC primagraft studies to investigate the capacity of imetelstat to selectively inhibit LSC self-renewal and to determine the mechanism of action.Methods and ResultsCytoscape analysis of RNA-seq data derived from FACS-purified progenitors from human blast crisis (BC; n=9) compared with chronic phase (CP; n=8) CML and primary normal (n=6) samples revealed transcriptional upregulation of b-catenin, LEF1, TCF7L1, ABL1 and other key genes within the TERT interactome suggesting a role for TERT activation in human BC LSC generation. Human progenitor LSC-supportive SL/M2 stromal co-culture experiments revealed that combined treatment with a potent BCR-ABL TKI, dasatinib at 1 nM, and imetelstat at 1 or 5 mM significantly inhibited (p<0.001, ANOVA) in vitro self-renewal of BC CML (n=5) compared with age-matched normal bone marrow progenitors. Treatment of primagraft mouse models of human BC CML with 30 mg/kg of imetelstat three times a week for four weeks resulted in a significant reduction in bone marrow progenitor LSC burden compared with mismatch treated controls (p=0.04). Furthermore, qRT-PCR showed decreased β-catenin transcript levels in imetelstat compared with vehicle-treated samples. Finally, FACS analysis revealed a significant reduction in activated b-catenin protein levels in engrafted human myeloid progenitors following imetelstat treatment in the TKI resistant bone marrow niche.ConclusionsNiche responsive interactions between the telomerase complex and the Wnt/b-catenin self-renewal pathway sensitize b-catenin activated LSC to imetelstat in both in vitro and in vivo humanized pre-clinical BC CML models thereby providing a strong rationale for LSC eradication trials involving imetelstat. DisclosuresHuang:Janssen Research & Development, LLC: Employment, Other: I am an employee of Janssen and a stock owner . Jamieson:UC San Diego: Other: I received funding from Janssen Research & Development, LLC.

Characterization and Targeting of Neoplastic Stem Cells in Ph + Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm characterized by the presence of an oncogenic fusion gene, BCR–ABL1. This fusion gene produces a cytoplasmic protein with tyrosine kinase activity that acts as a main driver of oncogenesis and abnormal proliferation of myeloid cells in CML. Targeted therapy with BCR–ABL1 tyrosine kinase inhibitors (TKIs) such as imatinib is followed by long-term responses in most patients. However, despite continuous treatment, relapses occur, suggesting the presence of TKI-resistant neoplastic stem cells in these patients. Here, we discuss potential mechanisms and signaling molecules involved in the prosurvival and self-renewal capacity of CML neoplastic stem cells as well as antigens expressed by these cells. Several of these signaling molecules and cell surface antigens may serve as potential targets of therapy and their use may overcome TKI resistance in CML in the future.

Abstract 4507: SL-501, a next-generation targeted therapy directed to IL-3R, inhibits the growth of tyrosine kinase inhibitor-resistant CML cells

Abstract Although imatinib and other tyrosine kinase inhibitors (TKIs) achieve high response rates in patients with chronic myeloid leukemia (CML), patients can fail these therapies or develop specific mutations that render TKIs ineffective. Moreover, TKIs have limited effectiveness against CML cancer stem cells (CSCs). It was previously shown that SL-401, a novel targeted therapy directed to the interleukin 3 receptor (IL-3R), inhibits the growth of CML tumor bulk and CSCs, both of which over express IL-3R. SL-501 is a next-generation targeted therapy also directed to IL-3R. SL-501 has approximately 4-fold increased binding affinity for IL-3R and a more than 30-fold increase in potency against acute myeloid leukemia (AML) cells compared to SL-401. In this study, we assessed the activity of SL-501 against a panel of TKI-sensitive and TKI-resistant CML cell lines. KBM5, KBM5-STI, K562, and BV173 cells were treated with SL-501 (range: 5 - 21,000 pg) for 24 or 48 hours and assessed for viability and apoptosis. We found that SL-501 reduced the absolute cell number of all cell lines tested in a dose-dependent manner with IC50s of 20 pM, 21 pM, 31.5 nM, and 3.9 nM, respectively; these concentrations are readily achieved at tolerable doses in the clinic. Moreover, SL-501 significantly reduced the number of imatinib-resistant KBM5-STI viable cells by more than 5-fold after 48 hours. Using caspase and lactate dehydrogenase (LDH) release assays, we also found that SL-501 alone induced apoptosis of all cell lines tested by more than 40% after 24 hours. The combination of SL-501 with TKIs (imatinib, dasatinib, or nilotinib) had an enhanced effect on apoptosis in KBM5, K562, and BV173 cells 24 hours post-treatment. Taken together, these findings indicate that SL-501, a novel next-generation IL-3-targeted therapeutic, may be effective at treating TKI-resistant CML. These promising results together with the IL-3R expression pattern on CML blasts and CSCs warrant further development of SL-501 in CML and, in particular, TKI-resistant CML. Citation Format: Christopher Brooks, Kenneth Hoberman, Ivan Bergstein, Eric Rowinsky. SL-501, a next-generation targeted therapy directed to IL-3R, inhibits the growth of tyrosine kinase inhibitor-resistant CML cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4507. doi:10.1158/1538-7445.AM2014-4507

RPS27a promotes proliferation, regulates cell cycle progression and inhibits apoptosis of leukemia cells

Ribosomal protein S27a (RPS27a) could perform extra-ribosomal functions besides imparting a role in ribosome biogenesis and post-translational modifications of proteins. The high expression level of RPS27a was reported in solid tumors, and we found that the expression level of RPS27a was up-regulated in advanced-phase chronic myeloid leukemia (CML) and acute leukemia (AL) patients. In this study, we explored the function of RPS27a in leukemia cells by using CML cell line K562 cells and its imatinib resistant cell line K562/G01 cells. It was observed that the expression level of RPS27a was high in K562 cells and even higher in K562/G01 cells. Further analysis revealed that RPS27a knockdown by shRNA in both K562 and K562G01 cells inhibited the cell viability, induced cell cycle arrest at S and G2/M phases and increased cell apoptosis induced by imatinib. Combination of shRNA with imatinib treatment could lead to more cleaved PARP and cleaved caspase-3 expression in RPS27a knockdown cells. Further, it was found that phospho-ERK(p-ERK) and BCL-2 were down-regulated and P21 up-regulated in RPS27a knockdown cells. In conclusion, RPS27a promotes proliferation, regulates cell cycle progression and inhibits apoptosis of leukemia cells. It appears that drugs targeting RPS27a combining with tyrosine kinase inhibitor (TKI) might represent a novel therapy strategy in TKI resistant CML patients.