Enhanced radiosensitivity of androgen-resistant prostate cancer: AZD1152-mediated Aurora kinase B inhibition

Role of AURKB Inhibition in Reducing Proliferation and Enhancing Effects of Radiotherapy in Triple-Negative Breast Cancer

Breast cancer is a leading cause of cancer-related deaths in females. Triple-negative breast cancer (TNBC) subtype is the most aggressive form of breast cancer that lacks biomarkers and effective targeted therapies. Its high degree of heterogeneity as well as innate and acquired resistance to treatment creates further barriers in achieving positive clinical outcomes in TNBC. Thus, development of novel treatment approaches in TNBC is of high clinical significance. Multimodality approaches with targeted agents and radiotherapy (RT) are promising for increasing efficacy of treatment and circumventing resistance. Here we examined anticancer effects of the Aurora Kinase B (AURKB) inhibitor AZD1152 as a single agent and in combination with RT using various TNBC cell lines, MDA-MB-468, MDA-MB-231 and SUM-159. We observed that AZD1152 alone effectively inhibited colony formation in TNBC cell lines. The combination of AZD1152 at IC50 concentrations together with ionizing radiation further reduced colony formation as compared to the single agent treatment. Our data support the notion that inhibition of the AURKB pathway is a promising strategy for treatment and radiosensitization of TNBC and warrants further translational studies.

Revisiting Aurora Kinase B: A promising therapeutic target for cancer therapy

Cancer continues to be a major health concern globally, although the advent of targeted therapy has revolutionized treatment options. Aurora Kinase B is a serine-threonine kinase that has been explored as an oncology therapeutic target for more than two decades. Aurora Kinase B inhibitors show promising biological results in in-vitro and in-vivo experiments. However, there are no inhibitors approved yet for clinical use, primarily because of the side effects associated with Aurora B inhibitors. Several studies demonstrate that Aurora B inhibitors show excellent synergy with various chemotherapeutic agents, radiation therapy, and targeted therapies. This makes it an excellent choice as an adjuvant therapy to first-line therapies, which greatly improves the therapeutic window and side effect profile. Recent studies indicate the role of Aurora B in some deadly cancers with limited therapeutic options, like triple-negative breast cancer and glioblastoma. Herein, we review the latest developments in Aurora Kinase B targeted research, with emphasis on its potential as an adjuvant therapy and its role in some of the most difficult-to-treat cancers.

Mitotic kinase inhibitors as Therapeutic Interventions for Prostate Cancer: Evidence from In vitro Studies

Prostate cancer is one of the devastating diseases characterized by genetic changes leading to uncontrolled growth and metastasis of the cells of the prostate gland and affects men worldwide. Conventional hormonal and chemotherapeutic agents are effective in mitigating the disease if diagnosed at an early stage. All dividing eukaryotic cells require mitotic progression for the maintenance of genomic integrity in progeny populations. The protein kinases, upon activation and de-activation in an ordered fashion, lead to spatial and temporal regulation of the cell division process. The entry into mitosis along with the progression into sub-phases of mitosis is ensured due to the activity of mitotic kinases. These kinases include Polo-Like-Kinase 1 (PLK1), Aurora kinases, and Cyclin-Dependent-Kinase 1 (CDK1), among others. The mitotic kinases, among others, are usually overexpressed in many cancers and can be targeted using small molecule inhibitors to reduce the effects of these regulators on mechanisms, such as regulation of genomic integrity and mitotic fidelity. In this review, we attempted to discuss the appropriate functions of mitotic kinases revealed through cell culture studies and the impact of their respective inhibitors derived in pre-clinical studies. The review is designed to elucidate the growing field of small molecule inhibitors and their functional screening or mode of action at the cellular and molecular level in the context of Prostate Cancer. Therefore, studies performed specifically on cells of Prostatic-origin are narrated in this review, culminating in a comprehensive view of the specific field of mitotic kinases that can be targeted for therapy of Prostate cancer.

HI-511 overcomes melanoma drug resistance via targeting AURKB and BRAF V600E

Rationale: Melanoma is an aggressive tumor of the skin and drug resistance is still a major problem in melanoma therapy. Novel targets and effective agents to overcome drug resistant melanoma are urgently needed in clinical therapy.

Methods: Gene Expression Omnibus (GEO) database analysis, pathway enrichment analysis, and survival rate analysis were utilized to identify a candidate target. An anchorage-independent cell growth assay, flow cytometry, Western blot, and a xenograft mouse model were used to study the function of Aurora kinase B (AURKB) in both drug-sensitive and drug-resistant melanoma. Next, HI-511, a novel dual-target inhibitor targeting both AURKB and BRAF V600E, was designed and examined by an in vitro kinase assay. Methods as indicated above in addition to a BRAF V600E/PTEN-loss melanoma mouse model were used to demonstrate the effect of HI-511 on melanoma development in vitro and in vivo.

Results: AURKB is highly expressed in melanoma and especially in vemurafenib-resistant melanoma and the expression was correlated with patient survival rate. Knocking down AURKB inhibited cell growth and induced apoptosis in melanoma, which was associated with the BRAF/MEK/ERKs and PI3-K/AKT signaling pathways. Importantly, we found that HI-511, a novel dual-target inhibitor against AURKB and BRAF V600E, suppresses both vemurafenib-sensitive and vemurafenib-resistant melanoma growth in vitro and in vivo by inducing apoptosis and mediating the inhibition of the BRAF/MEK/ERKs and PI3K/AKT signaling pathways.

Conclusion: AURKB is a potential target for melanoma treatment. HI-511, a novel dual-target inhibitor against both AURKB and BRAF V600E, could achieve durable suppression of melanoma growth, even drug-resistant melanoma growth.

Aurora kinases and DNA damage response

It is well established that Aurora kinases perform critical functions during mitosis. It has become increasingly clear that the Aurora kinases also perform a myriad of non-mitotic functions including DNA damage response. The available evidence indicates that inhibition Aurora kinase A (AURKA) may contribute to the G₂ DNA damage checkpoint through AURKA's functions in PLK1 and CDC25B activation. Both AURKA and Aurora kinase B (AURKB) are also essential in mitotic DNA damage response that guard against DNA damage-induced chromosome segregation errors, including the control of abscission checkpoint and prevention of micronuclei formation. Dysregulation of Aurora kinases can trigger DNA damage in mitosis that is sensed in the subsequent G₁ by a p53-dependent postmitotic checkpoint. Aurora kinases are themselves linked to the G₁ DNA damage checkpoint through p53 and p73 pathways. Finally, several lines of evidence provide a connection between Aurora kinases and DNA repair and apoptotic pathways. Although more studies are required to provide a comprehensive picture of how cells respond to DNA damage, these findings indicate that both AURKA and AURKB are inextricably linked to pathways guarding against DNA damage. They also provide a rationale to support more detailed studies on the synergism between small-molecule inhibitors against Aurora kinases and DNA-damaging agents in cancer therapies.

Combination Therapy of Prostate Cancer by Oncolytic Adenovirus Harboring Interleukin 24 and Ionizing Radiation

Prostate cancer is a common malignant tumor and the second leading cause of cancer-related death in men. Radiation therapy is a curative treatment for localized prostate cancer and has a limited effect for castration-resistant prostate cancer (CRPC). Interleukin 24 (IL-24) has a radiosensitizing effect in cancer cells. Our previous studies showed that ZD55-IL-24, an oncolytic adenovirus harboring IL-24, had better anti-tumor effect with no toxicity to normal cells. In this study, we evaluated the synergistic anti-tumor effect of oncolytic adenovirus ZD55-IL-24 combined with radiotherapy in prostate cancer. In Vitro and In Vivo experiments showed that the combined therapy significantly inhibited the growth of prostate cancer and provoked apoptosis of prostate cancer cells. In conclusion, the combination of ionizing radiation and oncolytic adenovirus expressing IL24 could achieve synergistic anti-tumor effect on prostate cancer, and is a promising strategy for prostate cancer therapy.

Mitosis inhibitors in anticancer therapy: When blocking the exit becomes a solution

Current microtubule-targeting agents (MTAs) remain amongst the most important antimitotic drugs used against a broad range of malignancies. By perturbing spindle assembly, MTAs activate the spindle assembly checkpoint (SAC), which induces mitotic arrest and subsequent apoptosis. However, besides toxic side effects and resistance, mitotic slippage and failure in triggering apoptosis in various cancer cells are limiting factors of MTAs efficacy. Alternative strategies to target mitosis without affecting microtubules have, thus, led to the identification of small molecules, such as those that target spindle Kinesins, Aurora and Polo-like kinases. Unfortunately, these so-called second-generation of antimitotics, encompassing mitotic blockers and mitotic drivers, have failed in clinical trials. Our recent understanding regarding the mechanisms of cell death during a mitotic arrest pointed out apoptosis as the main variable, providing an opportunity to control the cell fates and influence the effectiveness of antimitotics. Here, we provide an overview on the second-generation of antimitotics, and discuss possible strategies that exploit SAC activity, mitotic slippage/exit and apoptosis induction, in order to improve the efficacy of anticancer strategies that target mitosis.

Sensitization of prostate cancer to radiation therapy: Molecules and pathways to target

Radiation therapy is used to treat cancer by radiation-induced DNA damage. Despite the best efforts to eliminate cancer, some cancer cells survive irradiation, resulting in cancer progression or recurrence. Alteration in DNA damage repair pathways is common in cancers, resulting in modulation of their response to radiation. This article focuses on the recent findings about molecules and pathways that potentially can be targeted to sensitize prostate cancer cells to ionizing radiation, thereby achieving an improved therapeutic outcome.

Reactive oxygen species generation and increase in mitochondrial copy number: new insight into the potential mechanism of cytotoxicity induced by aurora kinase inhibitor, AZD1152-HQPA

Aurora-B kinase overexpression plays important roles in the malignant progression of prostate cancer (PCa). AZD1152-HQPA, as an inhibitor of Aurora-B, has recently emerged as a promising agent for cancer treatment. In this study, we aimed to investigate the effects of AZD1152-HQPA on reactive oxygen species (ROS) generation and mitochondrial function in PCa. We used AZD1152-HQPA (Barasertib), a highly potent and selective inhibitor of Aurora-B kinase. The effects of AZD1152-HQPA on cell viability, DNA content, cell morphology, and ROS production were studied in the androgen-independent PC-3 PCa cell line. Moreover, the mitochondrial copy number and the expression of genes involved in cell survival and cancer stem cell maintenance were investigated. We found that AZD1152-HQPA treatment induced defective cell survival, polyploidy, micronuclei formation, cell enlargement, and cell death by significant overexpression of p73, p21 and downregulation of cell cycle-regulatory genes in a drug concentration-dependent manner. Moreover, AZD1152 treatment led to an excessive ROS generation and an increase in the mitochondrial copy number not only in PC-3 but also in several other malignant cells. AZD1152 treatment also led to downregulation of genes involved in the maintenance of cancer stem cells. Our results showed a functional relationship between the aurora kinase inhibition, an increase in mitochondrial copy number, and ROS generation in therapeutic modalities of cancer. This study suggests that the excessive ROS generation may be a novel mechanism of cytotoxicity induced by the aurora kinase inhibitor, AZD1152-HQPA.

Aurora kinases: novel therapy targets in cancers

Aurora kinases, a family of serine/threonine kinases, consisting of Aurora A (AURKA), Aurora B (AURKB) and Aurora C (AURKC), are essential kinases for cell division via regulating mitosis especially the process of chromosomal segregation. Besides regulating mitosis, Aurora kinases have been implicated in regulating meiosis. The deletion of Aurora kinases could lead to failure of cell division and impair the embryonic development. Overexpression or gene amplification of Aurora kinases has been clarified in a number of cancers. And a growing number of studies have demonstrated that inhibition of Aurora kinases could potentiate the effect of chemotherapies. For the past decades, a series of Aurora kinases inhibitors (AKIs) developed effectively repress the progression and growth of many cancers both in vivo and in vitro, suggesting that Aurora kinases could be a novel therapeutic target. In this review, we'll first briefly present the structure, localization and physiological functions of Aurora kinases in mitosis, then describe the oncogenic role of Aurora kinases in tumorigenesis, we shall finally discuss the outcomes of AKIs combination with conventional therapy.

Cyclin K dependent regulation of Aurora B affects apoptosis and proliferation by induction of mitotic catastrophe in prostate cancer

Cyclin K plays a critical role in transcriptional regulation as well as cell development. However, the role of Cyclin K in prostate cancer is unknown. Here, we describe the impact of Cyclin K on prostate cancer cells and examine the clinical relevance of Cyclin K as a biomarker for patients with prostate cancer. We show that Cyclin K depletion in prostate cancer cells induces apoptosis and inhibits proliferation accompanied by an accumulation of cells in the G2/M phase. Moreover, knockdown of Cyclin K causes mitotic catastrophe displayed by multinucleation and spindle multipolarity. Furthermore, we demonstrate a Cyclin K dependent regulation of the mitotic kinase Aurora B and provide evidence for an Aurora B dependent induction of mitotic catastrophe. In addition, we show that Cyclin K expression is associated with poor biochemical recurrence-free survival in patients with prostate cancer treated with an adjuvant therapy. In conclusion, targeting Cyclin K represents a novel, promising anti-cancer strategy to induce cell cycle arrest and apoptotic cell death through induction of mitotic catastrophe in prostate cancer cells. Moreover, our results indicate that Cyclin K is a putative predictive biomarker for clinical outcome and therapy response for patients with prostate cancer.

The Aurora kinase inhibitors in cancer research and therapy

Compounds that affect enzymatic function of kinases are valuable for the understanding of the complex biochemical processes in cells. Aurora kinases (AURKs) play a key role in the control of the mitosis. These kinases are frequently deregulated in different human cancers: overexpression, amplifications, translocations and deletions were reported in many cancer cell lines as well as patient tissues. These findings steered a rigorous hunt for small-molecule AURK inhibitors not only for research purposes as well as for therapeutic uses. In this review, we describe a number of AURK inhibitors and their use in cancer research and/or therapy. We hope to assist researchers and clinicians in deciding which inhibitor is most appropriate for their specific purpose. The review will also provide a broad overview of the clinical studies performed with some of these inhibitors (if such studies have been performed).

CHMP4C Disruption Sensitizes the Human Lung Cancer Cells to Irradiation

Human lung cancer is highly invasive and the most malignant among human tumors. Adenocarcinoma as a specific type of non-small cell lung cancer occurs with high frequency and is also highly resistant to radiation therapy. Thus, how to avoid radiation resistance and improve radiotherapy effectiveness is a crucial question. In the present study, human lung cancer A549 and H1299 cells were irradiated using γ-rays from a Co60 irradiator. Protein expression was detected by Western blotting. Cell cycle and apoptosis were measured by flow cytometry. Surviving fraction was determined by colony formation assay. γH2AX and 53BP1 foci formation were examined by fluorescence microscopy. In the results, we show that CHMP4C, a subunit of Endosomal sorting complex-III (ESCRT-III), is involved in radiation-induced cellular response. Radiation-induced Aurora B expression enhances CHMP4C phosphorylation in non-small cell lung cancer (NSCLC) cells, maintaining cell cycle check-point and cellular viability as well as resisting apoptosis. CHMP4C depletion enhances cellular sensitivity to radiation, delays S-phase of cell cycle and reduces ionizing radiation (IR)-induced γH2AX foci formation. We found that Aurora B targets CHMP4C and inhibition of Aurora B exhibits similar effects with silencing of CHMP4C in radioresistance. We also confirm that CHMP4C phosphorylation is elevated after IR both in p53-positive and-negative cells, indicating that the close correlation between CHMP4C and Aurora B signaling pathway in mediating radiation resistance is not p53 dependent. Together, our work establishes a new function of CHMP4C in radiation resistance, which will offer a potential strategy for non-small cell lung cancer by disrupting CHMP4C.

Daurinol Enhances the Efficacy of Radiotherapy in Lung Cancer via Suppression of Aurora Kinase A/B Expression

The aurora kinases constitute one family of serine/threonine kinases whose activity is essential for mitotic progression. The aurora kinases are frequently upregulated in human cancers and are associated with sensitivity to chemotherapy in certain ones. In the present study, we investigated whether aurora kinases could be a target to overcome radioresistance or enhance the radiosensitivity of lung cancer. For that purpose, we determined the therapeutic potential of daurinol, an investigational topoisomerase inhibitor, alone and in combination with radiation, by observing its effect on aurora kinases. Daurinol decreased cell viability and proliferation in human colon and lung cancer cells. Gene expression in daurinol-treated human colon cancer cells was evaluated using RNA microarray. The mRNA expression of 18 genes involved in the mitotic spindle check point, including aurora kinase A (AURKA) and aurora kinase B (AURKB), was decreased in daurinol-treated human colon cancer cells as compared with vehicle-treated cells. As expected, radiation increased expression levels of AURKA and AURKB. This increase was effectively attenuated by siRNAs against AURKA and AURKB, which suppressed cell growth and increased apoptosis under radiation. Furthermore, the expression of AURKA and AURKB was suppressed by daurinol in the presence or absence of radiation in colon and lung cancer cells. Daurinol alone or in combination with radiation decreased lung cancer growth in xenograft mouse models. Our data clearly confirm the antitumor and radiosensitizing activity of daurinol in human lung cancer cells through the inhibition of AURKA and AURKB.

Close correlation between MEK/ERK and Aurora-B signaling pathways in sustaining tumorigenic potential and radioresistance of gynecological cancer cell lines

Both Aurora-A and -B kinases have been implicated in tumorigenesis; and as such, they represent an attractive therapeutic target. Recent studies found that Aurora-A is a downstream target of mitogen-activated protein kinase 1/ERK2, while Aurora-B has been found to be a prognostic/predictive therapeutic target for epithelial cancer. In a wide range of human cancers, the Ras/Raf/MEK/ERK/MAP kinase pathway is enhanced and the cellular response to growth signals is known to increase. The purpose of this study was to investigate whether the MEK/ERK cascade regulates tumorigenic signaling and radioresistance via the Aurora-B-mediated pathway in a panel of gynecological cancer cell lines. Exponentially growing human endometrial (Ishikawa), cervical (HeLa), cervical (CASKI) and vulva (SiHa) cancer cells were used in culture treated with either control or MEK/ERK inhibitor or AZD1152 before and after irradiation. Western blotting, ERK1/2 siRNA transfection, growth assay in modified monolayer, Annexin V and migration/invasion assays were performed. The specific MEK/ERK inhibitor U0126 decreased the tumorigenic potential and improved the radiation response in all cellular models. The modulation of radioresponse upon U0126 treatment positively correlated with the inhibition of phospho-ERKs and the reduction of Aurora-B kinase expression. In addition, upon U0126 treatment DNA-PKcs protein expression was found to be downregulated, indicating that the improved radiation response may be caused by decreased DNA double-strand damage repair mechanisms. The knockdown of ERK by siRNA confirmed the MEK/ERK-dependent Aurora-B kinase expression. The use of AZD1152, a selective Aurora-B inhibitor, counteracted tumorigenic potential and radioresistance phenotype by highly increasing apoptotic mechanisms in all gynecological cancer cell lines used. Evidence from our experiments show that tumorigenic potential and radiation response in gynecological cancer cells may ensue from a MEK/ERK or Aurora-B inhibition. Together with the close correlation of MEK/ERK and Aurora-B protein expression, this study underlines the potential role of a MEK/ERK/Aurora-B axis whose interruption recovers the antitumor effects of radiotherapy.

Differential response of DU145 and PC3 prostate cancer cells to ionizing radiation: role of reactive oxygen species, GSH and Nrf2 in radiosensitivity

BACKGROUND

Radioresistance is the major impediment in radiotherapy of many cancers including prostate cancer, necessitating the need to understand the factors contributing to radioresistance in tumor cells. In the present study, the role of cellular redox and redox sensitive transcription factor, Nrf2 in the radiosensitivity of prostate cancer cell lines PC3 and DU145, has been investigated.

MATERIALS AND METHODS

Differential radiosensitivity of PC3 and DU145 cells was assessed using clonogenic assay, flow cytometry, and comet assay. Their redox status was measured using DCFDA and DHR probes. Expression of Nrf2 and its dependent genes was measured by EMSA and real time PCR. Knockdown studies were done using shRNA transfection.

RESULTS

PC3 and DU145 cells differed significantly in their radiosensitivity as observed by clonogenic survival, apoptosis and neutral comet assays. Both basal and inducible levels of ROS were higher in PC3 cells than that of DU145 cells. DU145 cells showed higher level of basal GSH content and GSH/GSSG ratio than that of PC3 cells. Further, significant increase in both basal and induced levels of Nrf2 and its dependent genes was observed in DU145 cells. Knock-down experiments and pharmacological intervention studies revealed the involvement of Nrf2 in differential radio-resistance of these cells.

CONCLUSION

Cellular redox status and Nrf2 levels play a causal role in radio-resistance of prostate cancer cells.

GENERAL SIGNIFICANCE

The pivotal role Nrf2 has been shown in the radioresistance of tumor cells and this study will further help in exploiting this factor in radiosensitization of other tumor cell types.

Aurora B inhibitor barasertib and cytarabine exert a greater-than-additive cytotoxicity in acute myeloid leukemia cells

Barasertib, an aurora B inhibitor, terminates cell division, introduces polyploidy, and consequently causes apoptosis. In the present study, we evaluated the effect of the combination of barasertib and cytarabine (ara-C), a key agent for leukemia chemotherapy, on leukemic cells in vitro. Human leukemia HL-60 cells and HL-60/ara-C20 cells, a 20-fold ara-C-resistant variant, were used. The 50% growth inhibitory concentrations of an active metabolite of barasertib, barasertib-hydroxyquinazoline-pyrazol-aniline (Barasertib-HQPA), and ara-C were 51 nM and 300 nM for HL-60 cells and 70 nM and 5300 nM for HL-60/ara-C20 cells, respectively. Barasertib-HQPA induced polyploidy with a subsequent induction of sub-G1 phase apoptosis, indicating the M-phase specific cytotoxicity. Cells treated with the S-phase specific ara-C accumulated in S phase and subsequently died through apoptosis. When HL-60 cells were treated with barasertib-HQPA and ara-C in combination, a greater-than-additive apoptosis was induced. This enhancement was obtained when the cells were treated with barasertib-HQPA prior to ara-C (37.9% sub-G1) or with both concurrently (31.2% sub-G1), but not with ara-C prior to barasertib-HQPA (17.8% sub-G1). The combination effects were similarly obtained in HL-60/ara-C20 cells with 19.7% sub-G1 for barasertib-HQPA→ara-C, 18.4% sub-G1 for both concurrently, and 13.8% sub-G1 for ara-C→barasertib-HQPA, and another leukemic U937 cells with 25.4% sub-G1 for barasertib-HQPA→ara-C, 28.2% sub-G1 for both concurrently, and 16.0% sub-G1 for ara-C→barasertib-HQPA. Barasertib-HQPA inhibited aurora B autophosphorylation and histone H3 phosphorylation in all the cell lines. Barasertib-HQPA did not inhibit DNA synthesis, allowing ara-C incorporation into DNA for its cytotoxicity. Thus, barasertib-HQPA and ara-C provided a greater-than-additive cytotoxicity in leukemic cells in vitro.

Selective enhancing effect of early mitotic inhibitor 1 (Emi1) depletion on the sensitivity of doxorubicin or X-ray treatment in human cancer cells

Chemotherapy and radiation in addition to surgery has proven useful in a number of different cancer types, but the effectiveness in normal tissue cannot be avoided in these therapies. To improve the effectiveness of these therapies selectively in cancer tissue is important for avoiding side effects. Early mitotic inhibitor 1 (Emi1) is known to have the function to inhibit anaphase-promoting complex/cyclosome ubiquitin ligase complex, which ubiquitylates the cell cycle-related proteins. It recently has been shown that Emi1 knockdown prevents transition from S to G2 phase by down-regulating geminin via anaphase-promoting complex/cyclosome activation. At present, anticancer drugs for targeting DNA synthesis to interfere with rapidly dividing cells commonly are used. As Emi1 depletion interferes with completion of DNA synthesis in cancer cells, we thought that Emi1 knockdown might enhance the sensitivity for anticancer agents. Here, we confirmed that Emi1 siRNA induced polyploidy for preventing transition from S to G2 phase in several cancer cell lines. Then, we treated Emi1 depleted cells with doxorubicin. Interestingly, increased apoptotic cells were observed after doxorubicin treatment in Emi1 siRNA-treated cancer cells. In addition, Emi1 depletion enhanced the sensitivity of x-ray irradiation in cancer cells. Importantly, synergistic effect of Emi1 knockdown in these combination therapies was not observed in normal cells. These results suggest that Emi1 siRNA can be a useful tool for enhancing of sensitivity of cancer cells to anticancer reagents and radiation.

Antimitotic agents for the treatment of patients with metastatic castrate-resistant prostate cancer

INTRODUCTION

Metastatic castrate-resistant prostate cancer (mCRPC) is the second deadliest cancer in men. The group of taxanes, which target microtubules of mitotic cells, is currently the only chemotherapy which has proven to increase overall survival in mCRPC patients. Other mitotic inhibitors are being explored for their clinical potential in mCRPC treatment.

AREAS COVERED

In this review, we summarize recent developments in the application of mitotic inhibitors for mCRPC from a clinical perspective. The four main groups of mitotic inhibitors currently being tested in clinical trials are microtubule-inhibitors, polo-like kinase 1 inhibitors, aurora kinase inhibitors and kinesin-spindle protein inhibitors. Compounds of these groups of inhibitors that are in clinical development for mCRPC are discussed. For this extensive overview, relevant literature was searched in PubMed and retrieved from clinicaltrials.gov and presentations at ASCO/AACR meetings.

EXPERT OPINION

In general, mitotic inhibitors are clinically well tolerated but exert limited antitumor activity compared to preclinical study results. However, efficacy of mitotic inhibitors is improving, either by personalizing treatment, by introducing more active compounds, by decreasing resistance of cancer cells against mitotic inhibitors or by using mitotic inhibitors in combination therapies.

AURKA suppression induces DU145 apoptosis and sensitizes DU145 to docetaxel treatment

The palliative therapy effect by docetaxel for CRPC patients makes it urgent to improve the therapy. It was suggested that PI3K and androgen receptor-directed combination therapy may be effective for prostate cancer (PCa) patients PTEN negative. However, for those patients PTEN positive, the mechanism of anti-apoptosis survival of cancer cells is not yet well defined. Amplification of AURKA has been detected in 5% of PCa. In this work, Du145, a PTEN positive PCa cell model, was employed to investigate the role of aurora kinase a (AURKA) on cell growth. Inhibition of AURKA expression by shRNA markedly reduced prostate cancer cell viability. Furthermore, we demonstrate that AURKA inhibition induced a remarkable downregulation of AKT activity and Bax induction. Moreover, specific inhibition of the activity of AURKA, but not other aurora family members, by small molecular chemical inhibitors induced significant cell killing effects. Notably, AURKA inhibition sensitized prostate cancer cells to docetaxel treatment. Our work suggests that AURKA-directed monotherapy or combination therapy with docetaxel could be a potent treatment for PCa patients in future.

Mitosis-targeting therapies: a troubleshooting guide

Several mitotic kinases and kinesins are currently considered as cancer targets based on their critical role during the cell division cycle and their significant level of expression in human tumors. Yet, their use is limited by the lack of selectivity against tumor cells, the low percentage of mitotic cells in many human tumors, and dose-limiting side-effects. As a consequence, initial clinical trials have shown limited responses. Despite these drawbacks, inhibiting mitosis is a promising strategy that deserves further development. Future advances will benefit from more specific inhibitors with better pharmacodynamic properties, a clear physiological characterization and cell-type-specific requirements of old and new mitotic targets, and rational strategies based on synthetic lethal interactions to improve selectivity against tumor cells.

Identification of aurora kinase B and Wee1-like protein kinase as downstream targets of (V600E)B-RAF in melanoma

BRAF is the most mutated gene in melanoma, with approximately 50% of patients containing V600E mutant protein. (V600E)B-RAF can be targeted using pharmacological agents, but resistance develops in patients by activating other proteins in the signaling pathway. Identifying downstream members in this signaling cascade is important to design strategies to avoid the development of resistance. Unfortunately, downstream proteins remain to be identified and therapeutic potential requires validation. A kinase screen was undertaken to identify downstream targets in the (V600E)B-RAF signaling cascade. Involvement of aurora kinase B (AURKB) and Wee1-like protein kinase (WEE1) as downstream proteins in the (V600E)B-RAF pathway was validated in xenografted tumors, and mechanisms of action were characterized in size- and time-matched tumors. Levels of only AURKB and WEE1 decreased in melanoma cells, when (V600E)B-RAF, mitogen-activated protein kinase 1/2, or extracellular signal-regulated kinase 1/2 protein levels were reduced using siRNA compared with other identified kinases. AURKB and WEE1 were expressed in tumors of patients with melanoma at higher levels than observed in normal human melanocytes. Targeting these proteins reduced tumor development by approximately 70%, similar to that observed when inhibiting (V600E)B-RAF. Furthermore, protein or activity levels of AURKB and WEE1 decreased in melanoma cells when pharmacological agents targeting upstream (V600E)B-RAF or mitogen-activated protein kinase were used to inhibit the (V600E)B-RAF pathway. Thus, AURKB and WEE1 are targets and biomarkers of therapeutic efficacy, lying downstream of (V600E)B-RAF in melanomas.

Suberoylanilide hydroxamic acid affects γH2AX expression in osteosarcoma, atypical teratoid rhabdoid tumor and normal tissue cell lines after irradiation

PURPOSE

Osteosarcoma and atypical teratoid rhabdoid tumors are tumor entities with varying response to common standard therapy protocols. Histone acetylation affects chromatin structure and gene expression which are considered to influence radiation sensitivity. The aim of this study was to investigate the effect of the combination therapy with the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) and irradiation on atypical teratoid rhabdoid tumors and osteosarcoma compared to normal tissue cell lines.

METHODS

Clonogenic assay was used to determine cell survival. DNA double-strand breaks (DSB) were examined by pulsed-field electrophoresis (PFGE) as well as by γH2AX immunostaining involving flow cytometry, fluorescence microscopy, and immunoblot analysis.

RESULTS

SAHA lead to an increased radiosensitivity in tumor but not in normal tissue cell lines. γH2AX expression as an indicator for DSB was significantly increased when SAHA was applied 24 h before irradiation to the sarcoma cell cultures. In contrast, γH2AX expression in the normal tissue cell lines was significantly reduced when irradiation was combined with SAHA. Analysis of initial DNA fragmentation and fragment rejoining by PFGE, however, did not reveal differences in response to the SAHA pretreatment for either cell type.

CONCLUSION

SAHA increases radiosensitivity in tumor but not normal tissue cell lines. The increased H2AX phosphorylation status of the SAHA-treated tumor cells post irradiation likely reflects its delayed dephosphorylation within the DNA damage signal decay rather than chromatin acetylation-dependent differences in the overall efficacy of DSB induction and rejoining. The results support the hypothesis that combining SAHA with irradiation may provide a promising strategy in the treatment of solid tumors.