A novel treatment strategy targeting polo-like kinase 1 in hematological malignancies

Exploitation and Verification of a Stroma- and Metastasis-Associated Risk Prognostic Signature in Pancreatic Adenocarcinoma

Pancreatic adenocarcinoma (PAAD), one of the most malignant tumors, not only has abundant mesenchymal components, but is also characterized by an extremely high metastatic risk. The purpose of this study was to construct a model of stroma- and metastasis-associated prognostic signature, aiming to benefit the existing clinical staging system and predict the prognosis of patients. First, stroma-associated genes were screened from the TCGA database with the ESTIMATE algorithm. Subsequently, transcriptomic data from clinical tissues in the RenJi cohort were screened for metastasis-associated genes. Integrating the two sets of genes, we constructed a risk prognostic signature by Cox and LASSO regression analysis. We then obtained a risk score by a quantitative formula and divided all samples into high- and low-risk groups based on the scores. The results demonstrated that patients with high-risk scores have a worse prognosis than those with low-risk scores, both in the TCGA database and in the RenJi cohort. In addition, tumor mutation burden, chemotherapeutic drug sensitivity and immune infiltration analysis also exhibited significant differences between the two groups. In exploring the potential mechanisms of how stromal components affect tumor metastasis, we simulated different matrix stiffness in vitro to explore its effect on EMT key genes in PAAD cells. We found that cancer cells stimulated by high matrix stiffness may trigger EMT and promote PAAD metastasis.

Identification of small molecules that mitigate vincristine-induced neurotoxicity while sensitizing leukemia cells to vincristine

Vincristine (VCR) is one of the most widely prescribed medications for treating solid tumors and acute lymphoblastic leukemia (ALL) in children and adults. However, its major dose-limiting toxicity is peripheral neuropathy that can disrupt curative therapy. Peripheral neuropathy can also persist into adulthood, compromising quality of life of childhood cancer survivors. Reducing VCR-induced neurotoxicity without compromising its anticancer effects would be ideal. Here, we show that low expression of NHP2L1 is associated with increased sensitivity of primary leukemia cells to VCR, and that concomitant administration of VCR with inhibitors of NHP2L1 increases VCR cytotoxicity in leukemia cells, prolongs survival of ALL xenograft mice, but decreases VCR effects on human-induced pluripotent stem cell-derived neurons and mitigates neurotoxicity in mice. These findings offer a strategy for increasing VCR's antileukemic effects while reducing peripheral neuropathy in patients treated with this widely prescribed medication.

The landscape of gene co-expression modules correlating with prognostic genetic abnormalities in AML

Background

The heterogenous cytogenetic and molecular variations were harbored by AML patients, some of which are related with AML pathogenesis and clinical outcomes. We aimed to uncover the intrinsic expression profiles correlating with prognostic genetic abnormalities by WGCNA.

Methods

We downloaded the clinical and expression dataset from BeatAML, TCGA and GEO database. Using R (version 4.0.2) and ‘WGCNA’ package, the co-expression modules correlating with the ELN2017 prognostic markers were identified (R² ≥ 0.4, p < 0.01). ORA detected the enriched pathways for the key co-expression modules. The patients in TCGA cohort were randomly assigned into the training set (50%) and testing set (50%). The LASSO penalized regression analysis was employed to build the prediction model, fitting OS to the expression level of hub genes by ‘glmnet’ package. Then the testing and 2 independent validation sets (GSE12417 and GSE37642) were used to validate the diagnostic utility and accuracy of the model.

Results

A total of 37 gene co-expression modules and 973 hub genes were identified for the BeatAML cohort. We found that 3 modules were significantly correlated with genetic markers (the ‘lightyellow’ module for NPM1 mutation, the ‘saddlebrown’ module for RUNX1 mutation, the ‘lightgreen’ module for TP53 mutation). ORA revealed that the ‘lightyellow’ module was mainly enriched in DNA-binding transcription factor activity and activation of HOX genes. The ‘saddlebrown’ module was enriched in immune response process. And the ‘lightgreen’ module was predominantly enriched in mitosis cell cycle process. The LASSO- regression analysis identified 6 genes (NFKB2, NEK9, HOXA7, APRC5L, FAM30A and LOC105371592) with non-zero coefficients. The risk score generated from the 6-gene model, was associated with ELN2017 risk stratification, relapsed disease, and prior MDS history. The 5-year AUC for the model was 0.822 and 0.824 in the training and testing sets, respectively. Moreover, the diagnostic utility of the model was robust when it was employed in 2 validation sets (5-year AUC 0.743–0.79).

Conclusions

We established the co-expression network signature correlated with the ELN2017 recommended prognostic genetic abnormalities in AML. The 6-gene prediction model for AML survival was developed and validated by multiple datasets.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-021-02914-2.

Chromosomal Instability in Acute Myeloid Leukemia

Chromosomal instability (CIN), the increasing rate in which cells acquire new chromosomal alterations, is one of the hallmarks of cancer. Many studies highlighted CIN as an important mechanism in the origin, progression, and relapse of acute myeloid leukemia (AML). The ambivalent feature of CIN as a cancer-promoting or cancer-suppressing mechanism might explain the prognostic variability. The latter, however, is described in very few studies. This review highlights the important CIN mechanisms in AML, showing that CIN signatures can occur largely in all the three major AML types (de novo AML, secondary-AML, and therapy-related-AML). CIN features in AML could also be age-related and reflect the heterogeneity of the disease. Although most of these abnormalities show an adverse prognostic value, they also offer a strong new perspective on personalized therapy approaches, which goes beyond assessing CIN in vitro in patient tumor samples to predict prognosis. Current and emerging AML therapies are exploring CIN to improve AML treatment, which includes blocking CIN or increasing CIN beyond the limit threshold to induce cell death. We argue that the characterization of CIN features, not included yet in the routine diagnostic of AML patients, might provide a better stratification of patients and be extended to a more personalized therapeutic approach.

RNAi prodrugs decrease elevated mRNA levels of Polo-like kinase 1 in ex vivo cultured primary cells from pediatric acute myeloid leukemia patients

Polo-like kinase 1 (Plk1) is an important regulator of the cell cycle and it is frequently overexpressed in cancer cells. Several small molecule inhibitors have been developed to target Plk1 and some of them have reached clinical trials in adults with acute myeloid leukemia (AML). Pediatric AML patients have a poor prognosis and survivors suffer from long-term side effects. As adult AML cells have an elevated expression of Plk1, AML is a disease candidate for Plk1 inhibition. However, the relative success of clinical trials have been hampered by adverse reactions. Herein, PLK1-targeting RNA interference (RNAi) prodrugs that enter cells without a transfection reagent are used to target PLK1 selectively in primary cells from pediatric AML patients. We show that PLK1 and PLK4 mRNA expression are significantly higher in pediatric AML patients when compared to healthy donors and that PLK1 is downregulated by on average 50% using RNAi prodrugs without a significant effect on other PLK family members. In addition, the RNAi prodrug-induced decrease in PLK1 can be used to potentiate the effect of cytarabine. In summary, PLK1-targeting RNAi prodrugs can decrease the elevated levels of PLK1 in primary cells from pediatric AML patients and sensitize pediatric AML cells to chemotherapeutics.

Novel Targeted Therapeutics in Acute Myeloid Leukemia: an Embarrassment of Riches

PURPOSE OF REVIEW

Acute myeloid leukemia (AML) is an aggressive malignancy of the bone marrow that has a poor prognosis with traditional cytotoxic chemotherapy, especially in elderly patients. In recent years, small molecule inhibitors targeting AML-associated IDH1, IDH2, and FLT3 mutations have been FDA approved. However, the majority of AML cases do not have a targetable mutation. A variety of novel agents targeting both previously untargetable mutations and general pathways in AML are currently being investigated. Herein, we review selected new targeted therapies currently in early-phase clinical investigation in AML.

RECENT FINDINGS

The DOT1L inhibitor pinometostat in KMT2A-rearranged AML, the menin inhibitors KO-539 and SYNDX-5613 in KMT2Ar and NPM1-mutated AML, and the mutant TP53 inhibitor APR-246 are examples of novel agents targeting specific mutations in AML. In addition, BET inhibitors, polo-like kinase inhibitors, and MDM2 inhibitors are promising new drug classes for AML which do not depend on the presence of a particular mutation. AML remains in incurable disease for many patients but advances in genomics, epigenetics, and drug discovery have led to the development of many potential novel therapeutic agents, many of which are being investigated in ongoing clinical trials. Additional studies will be necessary to determine how best to incorporate these novel agents into routine clinical treatment of AML.

Targeting Plk1 with siRNNs in primary cells from pediatric B-cell acute lymphoblastic leukemia patients

B-cell acute lymphoblastic leukemia (B-ALL) accounts for nearly one fifth of all childhood cancers and current challenges in B-ALL treatment include resistance, relapse and late-onset side effects due to the chemotherapy. To overcome these hurdles, novel therapies need to be investigated. One promising target is Polo-like kinase 1 (Plk1), a key regulator of the cell cycle. In this study, the Plk family expression is investigated in primary peripheral blood and bone marrow mononuclear cells from ten pediatric B-ALL patients. For the first time, short interfering RiboNucleic Neutrals (siRNNs) that enter cells without a transfection reagent are used to target Plk1 mRNA in primary cells from pediatric B-ALL patients. Our results show that the expression of Plk1 and Plk4 is significantly higher in pediatric B-ALL patients compared to healthy donors. Moreover, treatment of primary peripheral blood and bone marrow mononuclear cells from pediatric B-ALL patients, cultured ex vivo, with Plk1-targeting siRNNs results in cleavage of Plk1 mRNA. Importantly, the Plk1 knockdown is specific and does not affect other Plk members in contrast to many small molecule Plk1 inhibitors. Thus, Plk1 is a potential therapeutic target in pediatric B-ALL and selective targeting of Plk1 can be achieved by the use of siRNNs.

Inhibition of PLK1 by capped-dose volasertib exerts substantial efficacy in MDS and sAML while sparing healthy haematopoiesis

INTRODUCTION

Targeting the cell cycle machinery represents a rational therapeutic approach in myelodysplastic syndromes (MDS) and secondary acute myeloid leukemia (sAML). Despite substantial response rates, clinical use of the PLK inhibitor volasertib has been hampered by elevated side effects such as neutropenia and infections.

OBJECTIVES

The primary objective was to analyse whether a reduced dose of volasertib was able to limit toxic effects on the healthy haematopoiesis while retaining its therapeutic effect.

METHODS

Bone marrow mononuclear cells (BMMNCs) of patients with MDS/sAML (n = 73) and healthy controls (n = 28) were treated with volasertib (1 μM to 1 nM) or vehicle control. Short-term viability analysis was performed by flow cytometry after 72 hours. For long-term viability analysis, colony-forming capacity was assessed after 14 days. Protein expression of RIPK3 and MCL-1 was quantified via flow cytometry.

RESULTS

Reduced dose levels of volasertib retained high cell death-inducing efficacy in primary human stem and progenitor cells of MDS/sAML patients without affecting healthy haematopoiesis in vitro. Interestingly, volasertib reduced colony-forming capacity and cell survival independent of clinical stage or mutational status.

CONCLUSIONS

Volasertib offers a promising therapeutic approach in patients with adverse prognostic profile. RIPK3 and MCL-1 might be potential biomarkers for sensitivity to volasertib treatment.

The balance between mitotic death and mitotic slippage in acute leukemia: a new therapeutic window?

Mitosis is the process whereby an eukaryotic cell divides into two identical copies. Different multiprotein complexes are involved in the fine regulation of cell division, including the mitotic promoting factor and the anaphase promoting complex. Prolonged mitosis can result in cellular division, cell death, or mitotic slippage, the latter leading to a new interphase without cellular division. Mitotic slippage is one of the causes of genomic instability and has an important therapeutic and clinical impact. It has been widely studied in solid tumors but not in hematological malignancies, in particular, in acute leukemia. We review the literature data available on mitotic regulation, alterations in mitotic proteins occurring in acute leukemia, induction of prolonged mitosis and its consequences, focusing in particular on the balance between cell death and mitotic slippage and on its therapeutic potentials. We also present the most recent preclinical and clinical data on the efficacy of second-generation mitotic drugs (CDK1-Cyclin B1, APC/CCDC20, PLK, Aurora kinase inhibitors). Despite the poor clinical activity showed by these drugs as single agents, they offer a potential therapeutic window for synthetic lethal combinations aimed to selectively target leukemic cells at the right time, thus decreasing the risk of mitotic slippage events.

Antileukemia Effects of Notch-Mediated Inhibition of Oncogenic PLK1 in B-Cell Acute Lymphoblastic Leukemia

In B-cell acute lymphoblastic leukemia (B-ALL), activation of Notch signaling leads to cell-cycle arrest and apoptosis. We aimed to harness knowledge acquired by understanding a mechanism of Notch-induced cell death to elucidate a therapeutically viable target in B-ALL. To this end, we identified that Notch activation suppresses Polo-like kinase 1 (PLK1) in a B-ALL-specific manner. We identified that PLK1 is expressed in all subsets of B-ALL and is highest in Philadelphia-like (Ph-like) ALL, a high-risk subtype of disease. We biochemically delineated a mechanism of Notch-induced PLK1 downregulation that elucidated stark regulation of p53 in this setting. Our findings identified a novel posttranslational cascade initiated by Notch in which CHFR was activated via PARP1-mediated PARylation, resulting in ubiquitination and degradation of PLK1. This led to hypophosphorylation of MDM2Ser260, culminating in p53 stabilization and upregulation of BAX. shRNA knockdown or pharmacologic inhibition of PLK1 using BI2536 or BI6727 (volasertib) in B-ALL cell lines and patient samples led to p53 stabilization and cell death. These effects were seen in primary human B-ALL samples in vitro and in patient-derived xenograft models in vivo These results highlight PLK1 as a viable therapeutic target in B-ALL. Efficacy of clinically relevant PLK1 inhibitors in B-ALL patient-derived xenograft mouse models suggests that use of these agents may be tailored as an additional therapeutic strategy in future clinical studies.

Targeted Polo-like Kinase Inhibition Combined With Aurora Kinase Inhibition in Pediatric Acute Leukemia Cells

BACKGROUND

Recent studies have shown that cell cycle events are tightly controlled by complex and shared activities of a select group of kinases. Among these, polo-like kinases (Plks) are regulatory mitotic proteins that are overexpressed in several types of cancer and are associated with poor prognosis.

MATERIALS AND METHODS

We have evaluated, in preclinical in vitro studies, the activity of a panel of Plk inhibitors against cell lines derived from refractory pediatric leukemia, as well as primary leukemia cells, in culture. Through in vitro growth inhibition studies, Western blot analysis for the expression and activation of key regulators of cell growth and survival and gene silencing studies, we specifically examined the ability of these agents to induce cytotoxicity through the activation of apoptosis and their capacity to interact and modulate the expression and phosphorylation of Aurora kinases.

RESULTS

Our findings show that the various Plk-1 inhibitors in development show potential utility for the treatment of pediatric leukemia and exhibit a wide range of phosphorylation and target modulatory capabilities. Finally, we provide evidence for a complex interregulatory relationship between Plk-1 and Aurora kinases enabling the identification of synergy and biologic correlates of drug combinations targeting the 2 distinct enzyme systems.

DISCUSSION

This information provide the rationale for the evaluation of Plk-1 as an effective target for therapeutics in refractory pediatric leukemia and indicate compensatory activities between Plk-1 and Aurora kinases, providing insight into some of the complex mechanisms involved in the process of cell division.

Enhanced polo-like kinase 1 expression in myelodysplastic syndromes

Background

Cancer is characterized by uncontrolled cellular proliferation, and Polo-like kinase 1 (PLK1), a key regulator of the cell cycle, is overexpressed in many cancers, including acute leukemia and lymphoma. However, the dynamics of PLK1 transcription in myelodysplastic syndromes (MDS) are unknown. This study aimed to investigate the transcript dynamics of PLK1 and determine its role in the pathophysiology of MDS.

Methods

PLK1 mRNA obtained from the bone marrow samples of 67 patients with MDS, 16 patients with secondary acute myeloid leukemia (sAML), and 10 healthy controls were analyzed using quantitative real-time PCR and compared according to various clinical parameters.

Results

The median PLK1 expression levels differed slightly, but not significantly, between MDS and sAML patients [661.21 (range, 29.38-8,987.31) vs. 1,462.05 (32.22-5,734.09), respectively], but were significantly higher (P<0.001) than the levels in the healthy controls [19.0 (1.60-49.90)]. Further analyses of PLK1 levels according to the WHO classification of MDS, prognostic risk groups, karyotype risk groups, marrow blast percentage, and depth of cytopenia did not reveal any significant associations. In patients progressing to sAML, PLK1 expression levels differed significantly according to the presence or absence of resistance to hypomethylation treatment (2,470.58 vs. 415.98, P=0.03).

Conclusion

PLK1 is upregulated in MDS patients; however, its role in the pathophysiology of MDS is unclear. Gene upregulation in cases with pharmacotherapeutic resistance warrants further investigation.

Polo-like kinases and acute leukemia

Acute leukemia is a common malignancy among children and adults worldwide and many patients suffer from chronic health issues using current therapeutic approaches. Therefore, there is a great need for the development of novel and more specific therapies with fewer side effects. The family of Polo-like kinases (Plks) is a group of five serine/threonine kinases that play an important role in cell cycle regulation and are critical targets for therapeutic invention. Plk1 and Plk4 are novel targets for cancer therapy as leukemic cells often express higher levels than normal cells. In contrast, Plk2 and Plk3 are considered to be tumor suppressors. Several small molecule inhibitors have been developed for targeting Plk1 inhibition. Despite reaching phase III clinical trials, one of the ATP-competitive Plk1 inhibitor, volasertib, did not induce an objective clinical response and even caused lethal side effects in some patients. In order to improve the specificity of the Plk1 inhibitors and reduce off-target side effects, novel RNA interference (RNAi)-based therapies have been developed. In this review, we summarize the mechanisms of action of the Plk family members in acute leukemia, describe preclinical studies and clinical trials involving Plk-targeting drugs and discuss novel approaches in Plk targeting.

RO3280: A Novel PLK1 Inhibitor, Suppressed the Proliferation of MCF-7 Breast Cancer Cells Through the Induction of Cell Cycle Arrest at G2/M Point

BACKGROUND

As a member of serine/threonine-protein kinase, Polo.like kinase 1 (PLK1) plays crucial roles during mitosis and also contributes to DNA damage response and repair. PLK1 is aberrantly expressed in many types of tumor cells and increased levels of PLK1 are closely related to tumorigenesis and poor clinical outcomes. Therefore, PLK1 is accepted as one of the potential targets for the discovery of novel anticancer agents. The objective of this study was to assess the cytotoxic effects of a novel PLK1 inhibitor, RO3280, against MCF-7, human breast cancer cells; HepG2, human hepatocellular carcinoma cells; and PC3, human prostate cancer cells, as well as non-cancerous L929 fibroblast cells.

METHODS

Antiproliferative activity of RO3280 was examined using the XTT assay. Flow cytometry assay was performed to evaluate cell cycle distribution, apoptosis, multicaspase activity, mitochondrial membrane potential, and DNA damage response. Apoptosis with fluorescence imaging studies was also examined.

RESULTS

According to the results of XTT assay, although RO3280 displayed potent cytotoxicity in all treated cancer cells, the most sensitive cell line was identified as MCF-7 cells that were selected for further studies. The compound induced a cell cycle arrest in MCF-7 cells at G2/M phase and significantly induced apoptosis, multicaspase activity, DNA damage response, and decreased mitochondrial membrane potential of MCF-7 cells.

CONCLUSION

Overall, RO3280 induces anticancer effects promoted mainly by DNA damage, cell cycle arrest, and apoptosis in breast cancer cells. Further studies are needed to assess its usability as an anticancer agent with specific cancer types.

Polo-like kinase inhibitor volasertib marginally enhances the efficacy of the novel Fc-engineered anti-CD33 antibody BI 836858 in acute myeloid leukemia

Acute myeloid leukemia (AML) is the second most common type of leukemia in adults. Incidence of AML increases with age with a peak incidence at 67 years. Patients older than 60 years have an unfavorable prognosis due to resistance to conventional chemotherapy. Volasertib (BI 6727) is a cell-cycle regulator targeting polo-like kinase which has been evaluated in clinical trials in AML. We evaluated effects of volasertib in primary patient samples and NK cells. At equivalent doses, volasertib is cytotoxic to AML blasts but largely spares healthy NK cells. We then evaluated the effect of volasertib treatment in combination with BI 836858 on primary AML blast samples using antibody-dependent cellular cytotoxicity (ADCC) assays. Volasertib treatment of NK cells did not impair NK function as evidenced by comparable levels of BI 836858 mediated ADCC in both volasertib-treated and control-treated NK cells. In summary, volasertib is cytotoxic to AML blasts while sparing NK cell viability and function. Higher BI 836858 mediated ADCC was observed in patient samples pretreated with volasertib. These findings provide a strong rationale to test combination of BI 836858 and volasertib in AML.

RNAi prodrugs targeting Plk1 induce specific gene silencing in primary cells from pediatric T-acute lymphoblastic leukemia patients

Epidemiological studies of childhood leukemia survivors reveal an alarmingly high incidence of chronic health disabilities after treatment, therefore, more specific therapies need to be developed. Polo-like kinase 1 (Plk1) is a key player in mitosis and a target for drug development as it is upregulated in multiple cancer types. Small molecules targeting Plk1 are mainly ATP-competitors and, therefore, are known to elicit side effects due to lack of specificity. RNA interference (RNAi) is known for its high catalytic activity and target selectivity; however, the biggest barrier for its introduction into clinical use is its delivery. RNAi prodrugs are modified, self-delivering short interfering Ribonucleic Neutrals (siRNNs), cleaved by cytoplasmic enzymes into short interfering Ribonucleic Acids (siRNAs) once inside cells. In this study we aimed to investigate the potential of siRNNs as therapeutic tools in T-acute lymphoblastic leukemia (T-ALL) using T-ALL cell lines and patient-derived samples. We demonstrate for the first time that RNAi prodrugs (siRNNs) targeting Plk1, can enter pediatric T-ALL patient cells without a transfection reagent and induce Plk1 knockdown on both protein and mRNA levels resulting in G2/M-arrest and apoptosis. We also show that siRNNs targeting Plk1 generate less toxicity in normal cells compared to the small molecule Plk1 inhibitor, BI6727, suggesting a potentially good therapeutic index.

Differential gene expression profiling linked to tumor progression of splenic marginal zone lymphoma

The genetic events that lead to aggressive transformation of cases of splenic marginal zone lymphoma (SMZL) after the chronic clinical stage have not been well understood. We aimed to find candidate genes associated with aggressive features of SMZL. We have successfully established two SMZL cell lines, designated SL-15 and SL-22, derived from the same patient's tumor clone in chronic and aggressive phases, respectively. Microarray analysis identified cell cycle-associated genes-specifically PLK1-as the most significantly upregulated in primary aggressive SMZL cells compared with cells from chronic phase. EPHA4 and MS4A1 (CD20) were found to be downregulated dramatically. These gene expression patterns were reproduced in both cell lines. Genetic knockdown of PLK1 resulted in inhibition of cell proliferation and induction of apoptosis in SL-22 cells, which expressed higher levels of PLK1 than SL-15 cells. SL-22 cells needed higher concentrations of chemical PLK1 inhibitors to achieve greater effects. In addition, we found homozygous deletion of the MS4A1 gene as a newly identified molecular mechanism of CD20-negative conversion. Our findings are expected to stimulate further studies on whether PLK1 could be a potential therapeutic target for this tumor. Furthermore, cases with CD20-negatively converted lymphomas should be screened for the genomic loss of MS4A1.

Identification of volasertib-resistant mechanism and evaluation of combination effects with volasertib and other agents on acute myeloid leukemia

Volasertib, a selective PLK1 inhibitor, was effective for acute myeloid leukemia (AML) patients in clinical trials. However, its efficacy was limited in mono-therapy, and a higher incidence of fatal events was revealed in the combination with low-dose cytarabine. Thus, optimization of combination therapy with volasertib and other agents is necessary for its clinical development, and the predictive factors for response or resistance to volasertib remain largely unknown. In this study, we investigated the resistance mechanism in volasertib-resistant cell lines and the combination effects with other agents, such as azacitidine (AZA), on AML cells. We identified that mutations in the ATP-binding domain of PLK1 and expression of MDR1 conferred resistance to volasertib. In the combination therapy, the effects of AZA differed among cells, but were prominent in the cells with higher GI₅₀ values of volasertib in mono-therapy. Furthermore, we identified that the cells in G2/M phase were more sensitive to volasertib, and the PI3K/AKT pathway was up-regulated upon administration of volasertib. Combination therapies with the agents that caused cell cycle accumulation in G2/M phase or with PI3K inhibitor were highly potent against AML cells. Our findings provide strategies for further clinical development of volasertib and PLK inhibitors for AML.

RNAi-mediated knockdown of MCM7 gene on CML cells and its therapeutic potential for leukemia

MCM7 is one of the subunits of MCM2-7 complex, which is essential to DNA replication licensing and the control of cell cycle progression. It has been demonstrated that MCM7 participates in mRNA transcription and DNA damage regulation as well. MCM7 gene is found to be over-expressed in multiple cancers, but there are few reports about its effect in leukemia. Recent studies have proven that MCM7 expression has a relationship with diagnosis and prognosis, which has led to their potential clinical application as a marker for cancer screening. RNA interference (RNAi) is a biological process in which RNA molecules inhibit gene expression, typically by causing the destruction of specific mRNA molecules. It is a valuable research tool, which is widely used in cell culture and living organisms as well as in medicine recent years. It is indicated that RNAi application for targeting functional carcinogenic molecules, tumor resistance to chemotherapy and radiotherapy is required in cancer treatment. Gene products knockdown by RNAi technology exerts anti-proliferative and pro-apoptotic effects upon cell culture systems, animal models and in clinical trials in the most studies. In the present study, we found that MCM7 highly expressed in K562 cells rather than that in normal neutrophils. Thus, lentivirus-mediated shRNA targeting MCM7 was used to suppress its endogenous expression in K562 cells and develop a novel therapeutic strategy for leukemia.

Combined Cancer Immunotherapy Against Aurora Kinase A

Aurora kinase A (AURKA) is a centrosomal protein that is overexpressed in a number of human malignancies and can contribute to tumor progression. As we used this protein as a target of DNA immunization, we increased its immunogenicity by the addition of the PADRE helper epitope and decreased its potential oncogenicity by mutagenesis of the kinase domain. For in vitro analysis of induced immune responses in mice, we identified the Aurka(220-228) nonapeptide representing an H-2Kb epitope. As DNA vaccination against the Aurka self-antigen by a gene gun did not show any antitumor effect, we combined DNA immunization with anti-CD25 treatment that depletes mainly regulatory T cells. Whereas 1 anti-CD25 dose injected before DNA vaccination did not enhance the activation of Aurka-specific splenocytes, 3 doses administered on days of immunizations augmented about 10-fold immunity against Aurka. However, an opposite effect was found for antitumor immunity-only 1 anti-CD25 dose combined with DNA vaccination reduced tumor growth. Moreover, the administration of 3 doses of anti-CD25 antibody alone accelerated tumor growth. Analysis of tumor-infiltrating cells showed that 3 anti-CD25 doses not only efficiently depleted regulatory T cells but also activated helper T cells and CD3(-)CD25(+) cells. Next, we found that blockade of the PD-1 receptor initiated 1 week after the first immunization was necessary for significant inhibition of tumor growth with therapeutic DNA vaccination against Aurka combined with depletion of CD25 cells. Our results suggest that combined cancer immunotherapy should be carefully evaluated to achieve the optimal antitumor effect.

New drugs in acute myeloid leukemia

The standard therapy for acute myeloid leukemia (AML) has not changed meaningfully for the past four decades. Improvements in supportive care and modifications to the dose and schedule of existing agents have led to steady improvements in outcomes. However, developing new therapies for AML has been challenging. Although there have been advances in understanding the biology of AML, translating this knowledge to viable treatments has been slow. Active research is currently ongoing to address this important need and several promising drug candidates are currently in the pipeline. Here, we review some of the most advanced and promising compounds that are currently in clinical trials and may have the potential to be part of our future armamentarium. These drug candidates range from cytotoxic chemotherapies, targeted small-molecule inhibitors, and monoclonal antibodies.

Polo-like kinase inhibitors in hematologic malignancies

Polo-like kinases (Plk) are key regulators of the cell cycle and multiple aspects of mitosis. Two agents that inhibit the Plk signaling pathway have shown promising activity in patients with hematologic malignancies and are currently in phase III trials. Volasertib is a Plk inhibitor under evaluation combined with low-dose cytarabine in older patients with acute myeloid leukemia (AML) ineligible for intensive induction therapy. Rigosertib, a dual inhibitor of the Plk and phosphatidylinositol 3-kinase pathways, is under investigation in patients with myelodysplastic syndrome (MDS) who have failed azacitidine or decitabine treatment. The prognosis for patients with AML, who are ineligible for intensive induction therapy, and for those with MDS refractory/relapsed after a hypomethylating agent, remains poor. Novel approaches, such as Plk inhibitors, are urgently needed for these patients. Here, we provide a comprehensive overview of the current state of development of Plk inhibitors for the treatment of hematologic malignancies.

In vitro anticancer properties and biological evaluation of novel natural alkaloid jerantinine B

Natural products play a pivotal role in medicine especially in the cancer arena. Many drugs that are currently used in cancer chemotherapy originated from or were inspired by nature. Jerantinine B (JB) is one of seven novel Aspidosperma indole alkaloids isolated from the leaf extract of Tabernaemontana corymbosa. Preliminary antiproliferative assays revealed that JB and JB acetate significantly inhibited growth and colony formation, accompanied by time- and dose-dependent apoptosis induction in human cancer cell lines. JB significantly arrested cells at the G2/M cell cycle phase, potently inhibiting tubulin polymerisation. Polo-like kinase 1 (PLK1; an early trigger for the G2/M transition) was also dose-dependently inhibited by JB (IC50 1.5 µM). Furthermore, JB provoked significant increases in reactive oxygen species (ROS). Annexin V+ cell populations, dose-dependent accumulation of cleaved-PARP and caspase 3/7 activation, and reduced Bcl-2 and Mcl-1 expression confirm apoptosis induction. Preclinical in silico biopharmaceutical assessment of JB calculated rapid absorption and bioavailability >70%. Doses of 8-16 mg/kg JB were predicted to maintain unbound plasma concentrations >GI50 values in mice during efficacy studies. These findings advocate continued development of JB as a potential chemotherapeutic agent.

Acute myeloid leukemia--major progress over four decades and glimpses into the future

In this Review, the progress in research and therapy of acute myeloid leukemia is detailed.

Mitotic entry: Non-genetic heterogeneity exposes the requirement for Plk1

The quest to develop novel antimitotic chemotherapy agents has led to the generation of several small molecule inhibitors targeting Plk1, a protein kinase required for multiple aspects of cell division. Previous studies have shown that upon exposure to Plk1 inhibitors, cells enter mitosis, delay briefly in prophase and then arrest in mitosis due to an inability to undergo centrosome separation. Here, we show that four different classes of Plk1 inhibitor block mitotic entry in several cancer cell lines and non-transformed RPE-1 cells. The proportion of cells that arrest in G2 is cell line and concentration dependent, and is subject to non-genetic heterogeneity. Following inhibitor washout, the G2 block is alleviated and cells enter mitosis but then fail to complete cell division indicating that most Plk1 inhibitors are not fully reversible. An exception is CYC140844; in contrast to five other inhibitors examined here, this novel Plk1 inhibitor is fully reversible. We discuss the implications for developing Plk1 inhibitors as chemotherapy agents and research tools.

Targeting polo-like kinase 1 in acute myeloid leukemia

Polo-like kinase 1 (Plk1) plays a number of important roles in the passage of cells through mitosis. It is expressed at high levels in a variety of malignancies, including acute myeloid leukemia (AML). Inhibition of Plk1 results in cell cycle arrest and apoptosis, and has anti-tumor effects in pre-clinical models. A number of Plk1 inhibitors have been developed, some of which have entered clinical trials. Of these, volasertib (BI6727) has been most extensively studied clinically in AML. Volasertib has demonstrated antileukemic activity in AML, both as a single agent and when combined with low-dose cytarabine. It is well tolerated, with the major toxicity being reversible myelosuppression. A recently completed phase III clinical trial in older AML patients will address the question of whether adding this agent to low-dose cytarabine is associated with a survival advantage.

Therapeutic polo-like kinase 1 inhibition results in mitotic arrest and subsequent cell death of blasts in the bone marrow of AML patients and has similar effects in non-neoplastic cell lines

Polo-like kinase 1 (PLK1) is an important regulator of the cell cycle and is overexpressed in various solid and hematological malignancies. Small molecule inhibitors targeting PLK1, such as BI2536 or BI6727 (Volasertib) are a promising therapeutic approach in such malignancies. Here, we show a loss of specifically localized PLK1 in AML blasts in vivo, accompanied by mitotic arrest with transition into apoptosis, in bone marrow biopsies of AML patients after treatment with BI2536. We verify these results in live cell imaging experiments with the AML cell line HL-60, and demonstrate that non-neoplastic, immortalized lymphoblastoid cells are also sensitive to PLK1 inhibition. It is demonstrated that normal granulopoietic precursors have similar PLK1 expression levels as leukemic blasts. These results are in line with the adverse effects of PLK1 inhibition and underline the great potential of PLK1 inhibitors in the treatment of AML.

Efficacy and mechanism of action of volasertib, a potent and selective inhibitor of Polo-like kinases, in preclinical models of acute myeloid leukemia

Polo-like kinase 1 (Plk1), a member of the Polo-like kinase family of serine/threonine kinases, is a key regulator of multiple steps in mitosis. Here we report on the pharmacological profile of volasertib, a potent and selective Plk inhibitor, in multiple preclinical models of acute myeloid leukemia (AML) including established cell lines, bone marrow samples from AML patients in short-term culture, and subcutaneous as well as disseminated in vivo models in immune-deficient mice. Our results indicate that volasertib is highly efficacious as a single agent and in combination with established and emerging AML drugs, including the antimetabolite cytarabine, hypomethylating agents (decitabine, azacitidine), and quizartinib, a signal transduction inhibitor targeting FLT3. Collectively, these preclinical data support the use of volasertib as a new therapeutic approach for the treatment of AML patients, and provide a foundation for combination approaches that may further improve and prolong clinical responses.

Molecular targeting of the oncoprotein PLK1 in pediatric acute myeloid leukemia: RO3280, a novel PLK1 inhibitor, induces apoptosis in leukemia cells

Polo-like kinase 1 (PLK1) is highly expressed in many cancers and therefore a biomarker of transformation and potential target for the development of cancer-specific small molecule drugs. RO3280 was recently identified as a novel PLK1 inhibitor; however its therapeutic effects in leukemia treatment are still unknown. We found that the PLK1 protein was highly expressed in leukemia cell lines as well as 73.3% (11/15) of pediatric acute myeloid leukemia (AML) samples. PLK1 mRNA expression was significantly higher in AML samples compared with control samples (82.95 ± 110.28 vs. 6.36 ± 6.35; p < 0.001). Kaplan-Meier survival analysis revealed that shorter survival time correlated with high tumor PLK1 expression (p = 0.002). The 50% inhibitory concentration (IC50) of RO3280 for acute leukemia cells was between 74 and 797 nM. The IC50 of RO3280 in primary acute lymphocytic leukemia (ALL) and AML cells was between 35.49 and 110.76 nM and 52.80 and 147.50 nM, respectively. RO3280 induced apoptosis and cell cycle disorder in leukemia cells. RO3280 treatment regulated several apoptosis-associated genes. The regulation of DCC, CDKN1A, BTK, and SOCS2 was verified by western blot. These results provide insights into the potential use of RO3280 for AML therapy; however, the underlying mechanisms remain to be determined.

BI 6727 and GSK461364 suppress growth and radiosensitize osteosarcoma cells, but show limited cytotoxic effects when combined with conventional treatments

Polo-like kinase 1 (PLK1), a key regulator of mitosis, is often overexpressed in childhood cancers and is associated with poor prognosis. Previous reports have shown that inhibition of PLK1 might serve as a promising anticancer treatment for osteosarcoma. In this study, we tested the second-generation PLK1 inhibitors BI 6727 and GSK461364 in HOS and MG-63 cell lines, both as a single agent and in combination with methotrexate, cisplatin, vinblastine, doxorubicin, or ionizing radiation. Both PLK1 inhibitors worked equally in terms of cell growth arrest, apoptosis induction, and radiosensitization. Combining BI 6727 or GSK461364 with conventional treatments, however, showed trivial synergistic antitumor effects in vitro. Our results reinforce the potential use of PLK1 inhibitors for a pharmacologic intervention in osteosarcoma, although their applicability in polychemotherapeutic regimens deserves further investigation.

Discovery and development of the Polo-like kinase inhibitor volasertib in cancer therapy

Owing to their integral involvement in cell cycle regulation, the Polo-like kinase (Plk) family, particularly Plk1, has emerged as an attractive therapeutic target in oncology. In recent years, several Plk1 inhibitors have been developed, with some agents showing encouraging results in early-phase clinical trials. This review focuses on volasertib (BI 6727; an investigational agent), a potent and selective Plk inhibitor. Volasertib has shown promising activity in various cancer cell lines and xenograft models of human cancer. Trials performed to date suggest that volasertib has clinical efficacy in a range of malignancies, with the most promising results seen in patients with acute myeloid leukemia (AML). Encouragingly, recent phase II data have demonstrated that volasertib combined with low-dose cytarabine (LDAC) was associated with higher response rates and improved event-free survival than LDAC alone in patients with previously untreated AML. Based on these observations, and its presumably manageable safety profile, volasertib is currently in phase III development as a potential treatment for patients with AML who are ineligible for intensive remission induction therapy. Given that many patients with AML are of an older age and frail, this constitutes an area of major unmet need. In this review, we discuss the biologic rationale for Plk1 inhibitors in cancer, the clinical development of volasertib to date in solid tumors and AML, and the future identification of biomarkers that might predict response to volasertib and help determine the role of this agent in the clinic.

PLK1 expression and BI 2536 effects in childhood acute lymphoblastic leukemia

BACKGROUND

Polo-like kinase 1 (PLK1) is a conserved kinase that mediates various mitotic events. Compelling data have repeatedly demonstrated its upregulation in different neoplasia, being frequently associated with poor prognosis. However, in childhood acute lymphoblastic leukemia (ALL), no studies have yet been conducted.

PROCEDURE

PLK1 expression and association with biological features were evaluated in 65 consecutively diagnosed childhood ALL samples by quantitative real-time PCR. Moreover, the effects of a specific PLK1 inhibitor, BI 2536, was tested against a panel of nine ALL cell lines at nanomolar concentrations (10, 50, 100 nM).

RESULTS

The mRNA expression of PLK1 showed great variability in pediatric ALL, but no difference was evidenced compared to normal bone marrow. Additionally, no association was found between PLK1 mRNA expression with any clinical or biological features. Alternatively, high mRNA expression of PLK1 was present in ALL cell lines. In vitro treatment with BI 2536 strongly diminished growth, while presenting significant reduction in colony formation capacity and increased apoptosis rates. Moreover, strong G2/M arrest was detected suggesting important impaired proliferation after treatment.

CONCLUSIONS

PLK1 mRNA expression level is not associated with prognosis in childhood ALL; however, considering the great variability observed in the sample and the in vitro experiments presented herein, BI 2536 treatment might serve as a promising therapeutic to enhance the efficacy of conventional treatment modalities in some childhood ALL cases.

c-Src activation through a TrkA and c-Src interaction is essential for cell proliferation and hematological malignancies

Although the kinase receptor TrkA may play an important role in acute myeloid leukemia (AML), its involvement in other types of leukemia has not been reported. Furthermore, how it contributes to leukemogenesis is unknown. Here, we describe a molecular network that is important for TrkA function in leukemogenesis. We found that TrkA is frequently overexpressed in other types of leukemia such as acute lymphoblastic leukemia (ALL), chronic myelogenous leukemia (CML), and myelodysplastic syndrome (MDS) including AML. In addition, TrkA was overexpressed in patients with MDS or secondary AML evolving from MDS. TrkA induced significant hematological malignancies by inducing PLK-1 and Twist-1, and enhanced survival and proliferation of leukemia, which was correlated with activation of the phosphatidylinositol 3-kinase/Akt/mTOR pathway. Moreover, endogenous TrkA associated with c-Src complexes was detected in leukemia. Suppression of c-Src activation by TrkA resulted in markedly decreased expression of PLK-1 and Twist-1 via suppressed activation of Akt/mTOR cascades. These data suggest that TrkA plays a key role in leukemogenesis and reveal an unexpected physiological role for TrkA in the pathogenesis of leukemia. These data have important implications for understanding various hematological malignancies.

Inhibiting Polo-like kinase 1 causes growth reduction and apoptosis in pediatric acute lymphoblastic leukemia cells

This study investigated Polo-like kinase 1, a mitotic regulator often over-expressed in solid tumors and adult hematopoietic malignancies, as a potential new target in the treatment of pediatric acute lymphoblastic leukemia. Polo-like kinase 1 protein and Thr210 phosphorylation levels were higher in pediatric acute lymphoblastic leukemia (n=172) than in normal bone marrow mononuclear cells (n=10) (P<0.0001). High Polo-like kinase 1 protein phosphorylation, but not expression, was associated with a lower probability of event-free survival (P=0.042) and was a borderline significant prognostic factor (P=0.065) in a multivariate analysis including age and initial white blood cell count. Polo-like kinase 1 was necessary for leukemic cell survival, since short hairpin-mediated Polo-like kinase 1 knockdown in acute lymphoblastic leukemia cell lines inhibited cell proliferation by G2/M cell cycle arrest and induced apoptosis through caspase-3 and poly (ADP-ribose) polymerase cleavage. Primary patient cells with a high Polo-like kinase 1 protein expression were sensitive to the Polo-like kinase 1-specific inhibitor NMS-P937 in vitro, whereas cells with a low expression and normal bone marrow cells were resistant. This sensitivity was likely not caused by Polo-like kinase 1 mutations, since only one new mutation (Ser335Arg) was found by 454-sequencing of 38 pediatric acute lymphoblastic leukemia cases. This mutation did not affect Polo-like kinase 1 expression or NMS-P937 sensitivity. Together, these results indicate a pivotal role for Polo-like kinase 1 in pediatric acute lymphoblastic leukemia and show potential for Polo-like kinase 1-inhibiting drugs as an addition to current treatment strategies for cases expressing high Polo-like kinase 1 levels.

TrkC promotes survival and growth of leukemia cells through Akt-mTOR-dependent up-regulation of PLK-1 and Twist-1

It has been suggested that activation of receptor PTKs is important for leukemogenesis and leukemia cell response to targeted therapy in hematological malignancies including leukemia. PTKs induce activation of the PI3K/Akt/mTOR pathway, which can result in prevention of apoptosis. Here, we describe an important role of the TrkC-associated molecular network in the process of leukemogenesis. TrkC was found to be frequently overexpressed in human leukemia cells and leukemia subtypes. In U937 human leukemia cells, blockade of TrkC using small hairpin RNA (shRNA) specific to TrkC or K562a, a specific inhibitor of TrkC, resulted in a significant decrease in growth and survival of the cells, which was closely associated with reduced mTOR level and Akt activity. In addition, TrkC enhances the survival and proliferation of leukemia, which is correlated with activation of the PI3K/Akt pathway. Moreover, TrkC significantly inhibits apoptosis via induction of the expression of PLK-1 and Twist-1 through activation of AKT/mTor pathway; therefore, it plays a key role in leukemogenesis. These findings reveal an unexpected physiological role for TrkC in the pathogenesis of leukemia and have important implications for understanding various hematological malignancies.

In vitro targeting of Polo-like kinase 1 in bladder carcinoma: comparative effects of four potent inhibitors

Despite the improvements in neoadjuvant chemotherapy, the outcome of patients with advanced bladder cancer has changed very little over the past 30 years. In the present study we tested and compared the in vitro antitumor activities of four different inhibitors of Polo-like kinase 1 (PLK1) (BI 2536, BI 6727, GW843682X, and GSK461364), against 3 bladder carcinoma cell lines RT4, 5637 and T24. The impact on radiosensitivity and drug interactions in simultaneous treatments with cisplatin, methotrexate, and doxorubicin were also investigated. Our results showed that PLK1 inhibition prevented cell proliferation and clonogenicity, causing significant inhibition of invasion of tumor cells, though modest differences were observed between drugs. Moreover, all PLK1 inhibitors induced G 2/M arrest, with the subsequent induction of death in all 3 cell lines. Drug interactions studies showed auspicious results for all PLK1 inhibitors when combined with the commonly used cisplatin and methotrexate, though combinations with doxorubicin showed mostly antagonistic effects. Comparably, the four PLK1 inhibitors efficiently sensitized cells to ionizing radiation. Our findings demonstrate that irrespective of the inhibitor used, the pharmacological inhibition of PLK1 constrains bladder cancer growth and dissemination, providing new opportunities for future therapeutic intervention. However, further laboratorial and pre-clinical tests are still needed to corroborate the usefulness of using them in combination with other commonly used chemotherapeutic drugs.

The Polo-Like Kinase 1 (PLK1) inhibitor NMS-P937 is effective in a new model of disseminated primary CD56+ acute monoblastic leukaemia

CD56 is expressed in 15-20% of acute myeloid leukaemias (AML) and is associated with extramedullary diffusion, multidrug resistance and poor prognosis. We describe the establishment and characterisation of a novel disseminated model of AML (AML-NS8), generated by injection into mice of leukaemic blasts freshly isolated from a patient with an aggressive CD56(+) monoblastic AML (M5a). The model reproduced typical manifestations of this leukaemia, including presence of extramedullary masses and central nervous system involvement, and the original phenotype, karyotype and genotype of leukaemic cells were retained in vivo. Recently Polo-Like Kinase 1 (PLK1) has emerged as a new candidate drug target in AML. We therefore tested our PLK1 inhibitor NMS-P937 in this model either in the engraftment or in the established disease settings. Both schedules showed good efficacy compared to standard therapies, with a significant increase in median survival time (MST) expecially in the established disease setting (MST = 28, 36, 62 days for vehicle, cytarabine and NMS-P937, respectively). Importantly, we could also demonstrate that NMS-P937 induced specific biomarker modulation in extramedullary tissues. This new in vivo model of CD56(+) AML that recapitulates the human tumour lends support for the therapeutic use of PLK1 inhibitors in AML.

Polo-like kinases in AML

New therapies targeting critical elements of the cell cycle open novel and attractive avenues for the treatment of cancer patients. At present, the number of clinical trials that are registered with the European Organization for Research and Treatment of Cancer (EORTC) and with the US National Cancer Institute, which investigate the efficacy of Polo-like kinase 1 (Plk1) inhibitors against solid tumors and different types of leukemia is growing. Plks are important regulators of mitotic progression. Plk1, the best characterized mammalian Plk, has become an attractive target for cancer drug development, because most types of cancer appear to be addicted to the non-oncogene Plk1. Here, the authors discuss the role of Plk1 and the potential tumor suppressor gene Plk2 in acute myeloid leukemia (AML).

Thrombomodulin protects endothelial cells from a calcineurin inhibitor-induced cytotoxicity by upregulation of extracellular signal-regulated kinase/myeloid leukemia cell-1 signaling

OBJECTIVE

We have recently reported that recombinant human soluble thrombomodulin (rTM) counteracted capillary leakage associated with engraftment, as well as sinusoidal obstructive syndrome after hematopoietic stem cell transplantation. These observations prompted us to explore whether rTM possessed cytoprotective effects on endothelial cells.

METHODS AND RESULTS

Exposure of human umbilical vein endothelial cells to rTM induced expression of antiapoptotic protein myeloid leukemia cell-1 through the activation of extracellular signal-regulated kinase in these cells. Additional studies found that exposure of human umbilical vein endothelial cells to cyclosporine A and FK506, an immunosuppressant used for the individuals receiving hematopoietic stem cell transplantation, induced apoptosis, which was attenuated when human umbilical vein endothelial cells were exposed to these agents in the presence of rTM. Further studies using deletion mutants of thrombomodulin (TM) identified that the epidermal growth factor domain of TM possessed cytoprotective effects. A single nucleotide substitution at codon 376 or 424 of TM, which impairs the ability of TM to produce activated protein C or bind to thrombin, respectively, did not hamper the cytoprotective effects of TM, which suggested that cytoprotective effects of rTM were distinctive from those of activated protein C.

CONCLUSIONS

TM may be useful for prevention, as well as treatment of endothelial cell damage after hematopoietic stem cell transplantation.

NMS-P937, an orally available, specific small-molecule polo-like kinase 1 inhibitor with antitumor activity in solid and hematologic malignancies

Polo-like kinase 1 (PLK1) is a serine/threonine protein kinase considered to be the master player of cell-cycle regulation during mitosis. It is indeed involved in centrosome maturation, bipolar spindle formation, chromosome separation, and cytokinesis. PLK1 is overexpressed in a variety of human tumors and its overexpression often correlates with poor prognosis. Although five different PLKs are described in humans, depletion or inhibition of kinase activity of PLK1 is sufficient to induce cell-cycle arrest and apoptosis in cancer cell lines and in xenograft tumor models. NMS-P937 is a novel, orally available PLK1-specific inhibitor. The compound shows high potency in proliferation assays having low nanomolar activity on a large number of cell lines, both from solid and hematologic tumors. NMS-P937 potently causes a mitotic cell-cycle arrest followed by apoptosis in cancer cell lines and inhibits xenograft tumor growth with clear PLK1-related mechanism of action at well-tolerated doses in mice after oral administration. In addition, NMS-P937 shows potential for combination in clinical settings with approved cytotoxic drugs, causing tumor regression in HT29 human colon adenocarcinoma xenografts upon combination with irinotecan and prolonged survival of animals in a disseminated model of acute myelogenous leukemia in combination with cytarabine. NMS-P937, with its favorable pharmacologic parameters, good oral bioavailability in rodent and nonrodent species, and proven antitumor activity in different preclinical models using a variety of dosing regimens, potentially provides a high degree of flexibility in dosing schedules and warrants investigation in clinical settings.

Targeting of polo-like kinases and their cross talk with Aurora kinases--possible therapeutic strategies in human acute myeloid leukemia?

INTRODUCTION

Five human polo-like kinases (PLKs) have been identified, and PLK1 - 4 seem to interact with Aurora kinases and act as cell cycle regulators in both normal and malignant human cells.

AREAS COVERED

The present review describes i) experimental evidence for a role for PLKs and Aurora kinases in human leukemogenesis and ii) the results from clinical studies of PLK and Aurora kinase inhibitors in the treatment of human acute myeloid leukemia (AML). The review was based on searches in the PubMed and the ClinicalTrials.gov databases. These inhibitors have antiproliferative and proapoptotic effects in AML cells. Hematological and gastrointestinal toxicities are frequently dose limiting, and this may limit the use of these agents in combination with conventional AML therapy. Aurora kinase inhibitors seem to be most effective for patients with high expression of the target kinases, and the same may be true for PLK inhibitors.

EXPERT OPINION

PLK inhibition is a promising strategy for the treatment of AML. Future clinical studies have to clarify i) whether this strategy is most effective for certain subsets of patients; ii) whether multikinase inhibitors targeting several cell cycle regulators should be preferred; and iii) how this therapeutic strategy eventually should be combined with conventional antileukemic chemotherapy.

Management of cutaneous T cell lymphoma: new and emerging targets and treatment options

Cutaneous T cell lymphomas (CTCL) clinically and biologically represent a heterogeneous group of non-Hodgkin lymphomas, with mycosis fungoides and Sézary syndrome being the most common subtypes. Over the last decade, new immunological and molecular pathways have been identified that not only influence CTCL phenotype and growth, but also provide targets for therapies and prognostication. This review will focus on recent advances in the development of therapeutic agents, including bortezomib, the histone deacetylase inhibitors (vorinostat and romidepsin), and pralatrexate in CTCL.

Induction of antitumor immunity using dendritic cells electroporated with Polo-like kinase 1 (Plk1) mRNA in murine tumor models

Polo-like kinase 1 (Plk1), a serine-threonine kinase, plays a key role in the regulation of the cell cycle. Elevated Plk1 expression in various cancers is correlated with poor prognosis and poor patient survival rates. Several Plk1 inhibitors are currently being developed as potential treatments for cancer. In the present study, we investigated whether dendritic cells (DC) electroporated with mouse Plk1RNA (mPlk1RNA/DC) can induce Plk1-specific immune responses and exert antitumor effects in various murine tumor models. Overexpression of Plk1 protein was confirmed in several mouse and human tumor cell lines and various cancer tissues. Furthermore, Plk1-specific CD4(+) and CD8(+) T cells were induced by vaccination with mPlk1RNA/DC and the cytotoxic activity of the T cells was demonstrated against several Plk1-expressing tumor cell lines. Vaccination with mPlk1RNA/DC inhibited the growth of MC-38 and B16F10 tumors in C57BL/6 mice and the growth of CT26 tumors in BALB/c mice. Depletion of CD8(+) T cells reversed the inhibition of tumor growth by mPlk1RNA/DC vaccination. Homologous human Plk1RNA-electroporated DC also inhibited tumor growth in MC-38 tumor-bearing mice. In addition, Plk1-specific cytotoxic T lymphocytes from PBMC of healthy donors could be induced using autologous monocyte-derived DC electroporated with RNA encoding the whole gene of human Plk1. Taken together, the results of the present study suggest that Plk1 could be a universal tumor antigen recognized by cytotoxic T lymphocytes for cancer immunotherapy.

Polo-like kinase 1 inhibitors in mono- and combination therapies: a new strategy for treating malignancies

Polo-like kinase 1 (Plk1) inhibitors belong to a new class of drugs for the treatment of malignant diseases. They selectively act against a target (Plk1) which is involved in different stages of mitosis such as centrosome maturation, spindle formation, chromosome separation and cytokinesis. Because Plk1 is mainly expressed in proliferating tissues and overexpressed in cancers, its inhibition is potentially less prone to toxicities associated with current antimitotic agents, which also act on nondividing cells. Several Plk1 inhibitors are being evaluated as cancer treatment drugs. Based on the essential role of Plk1 during mitosis, Plk1 inhibitors target all rapidly dividing cells irrespective of their tumor suppressor or oncogene mutations. In this article, their mechanisms of action, efficacy and toxicity profile are discussed.

The polo-like kinase inhibitor BI 2536 exhibits potent activity against malignant plasma cells and represents a novel therapy in multiple myeloma

OBJECTIVE

Polo-like kinase 1 (Plk1) is a regulator of the cell cycle that has been implicated in the pathology of many cancers. We have investigated whether this kinase plays a role in multiple myeloma (MM) using the Plk1 inhibitor BI 2536.

MATERIALS AND METHODS

We have used six MM cell lines and six patient-derived samples to determine the effects of the Plk1 inhibitor, BI 2536, on cell viability, apoptosis, and cytokinesis. We have also examined the effect of the microenvironment on these parameters and the effects of BI 2536 in combination with other antimyeloma agents.

RESULTS

We show that MM cell lines and patient samples express PLK1 and that cell death by apoptosis occurs when Plk1 is inhibited. Cells treated with BI 2536 accumulate in the G(2)/M phase of the cell cycle causing endoduplication. The effects of BI 2536 are not abrogated when cells are cultured on extracellular matrix components, in the presence of interleukin-6, or with bone marrow stromal cells.

CONCLUSIONS

Plk1 inhibition leads to cell death in MM cell lines and patient myeloma samples. Our data suggest that inhibition of Plk1 may have potential use as a therapeutic strategy in multiple myeloma.

Small molecule kinase inhibitor screen identifies polo-like kinase 1 as a target for neuroblastoma tumor-initiating cells

Neuroblastoma (NB) is an often fatal pediatric tumor of neural crest origin. We previously isolated NB tumor-initiating cells (NB TIC) from bone marrow metastases that resemble cancer stem cells and form metastatic NB in immunodeficient animals with as few as ten cells. To identify signaling pathways important for the survival and self-renewal of NB TICs and potential therapeutic targets, we screened a small molecule library of 143 protein kinase inhibitors, including 33 in clinical trials. Cytostatic or cytotoxic drugs were identified that targeted PI3K (phosphoinositide 3-kinase)/Akt, PKC (protein kinase C), Aurora, ErbB2, Trk, and Polo-like kinase 1 (PLK1). Treatment with PLK1 siRNA or low nanomolar concentrations of BI 2536 or BI 6727, PLK1 inhibitors in clinical trials for adult malignancies, were cytotoxic to TICs whereas only micromolar concentrations of the inhibitors were cytotoxic for normal pediatric neural stem cells. Furthermore, BI 2536 significantly inhibited TIC tumor growth in a therapeutic xenograft model, both as a single agent and in combination with irinotecan, an active agent for relapsed NB. Our findings identify candidate kinases that regulate TIC growth and survival and suggest that PLK1 inhibitors are an attractive candidate therapy for metastatic NB.

RNAi phenotype profiling of kinases identifies potential therapeutic targets in Ewing's sarcoma

Background

Ewing's sarcomas are aggressive musculoskeletal tumors occurring most frequently in the long and flat bones as a solitary lesion mostly during the teen-age years of life. With current treatments, significant number of patients relapse and survival is poor for those with metastatic disease. As part of novel target discovery in Ewing's sarcoma, we applied RNAi mediated phenotypic profiling to identify kinase targets involved in growth and survival of Ewing's sarcoma cells.

Results

Four Ewing's sarcoma cell lines TC-32, TC-71, SK-ES-1 and RD-ES were tested in high throughput-RNAi screens using a siRNA library targeting 572 kinases. Knockdown of 25 siRNAs reduced the growth of all four Ewing's sarcoma cell lines in replicate screens. Of these, 16 siRNA were specific and reduced proliferation of Ewing's sarcoma cells as compared to normal fibroblasts. Secondary validation and preliminary mechanistic studies highlighted the kinases STK10 and TNK2 as having important roles in growth and survival of Ewing's sarcoma cells. Furthermore, knockdown of STK10 and TNK2 by siRNA showed increased apoptosis.

Conclusion

In summary, RNAi-based phenotypic profiling proved to be a powerful gene target discovery strategy, leading to successful identification and validation of STK10 and TNK2 as two novel potential therapeutic targets for Ewing's sarcoma.