VX-680, a potent and selective small-molecule inhibitor of the Aurora kinases, suppresses tumor growth in vivo

Expression of Aurora A (but not Aurora B) is predictive of survival in breast cancer

PURPOSE

The cell cycle mediators Aurora A and B are targets of drugs currently in clinical development. As with other targeted therapies in breast cancer, response to therapy might be associated with target expression in tumors. We therefore assessed expression of Aurora A and B in breast tumors and studied associations with clinical/pathologic variables.

EXPERIMENTAL DESIGN

Tissue microarrays containing primary specimens from 638 patients with 15-year follow-up were employed to assess expression of Aurora A and B using our automated quantitative analysis method; we used cytokeratin to define pixels as breast cancer (tumor mask) within the array spot and measured Aurora A and B expression within the mask using Cy5-conjugated antibodies.

RESULTS

Aurora A and B expression was variable in primary breast tumors. High Aurora A expression was strongly associated with decreased survival (P = 0.0005). On multivariable analysis, it remained an independent prognostic marker. High Aurora A expression was associated with high nuclear grade and high HER-2/neu and progesterone receptor expression. Aurora B expression was not associated with survival.

CONCLUSIONS

Aurora A expression defines a population of patients with decreased survival, whereas Aurora B expression does not, suggesting that Aurora A might be the preferred drug target in breast cancer. Aurora A expression in early-stage breast cancer may identify a subset of patients requiring more aggressive or pathway-targeted treatment. Prospective studies are needed to confirm the prognostic role of Aurora A as well as the predictive role of Aurora A expression in patients treated with Aurora A inhibitors.

Perturbation of Incenp function impedes anaphase chromatid movements and chromosomal passenger protein flux at centromeres

Incenp is an essential mitotic protein that, together with Aurora B, Survivin, and Borealin, forms the core of the chromosomal passenger protein complex (CPC). The CPC regulates various mitotic processes and functions to maintain genomic stability. The proper subcellular localization of the CPC and its full catalytic activity require the presence of each core subunit in the complex. We have investigated the mitotic tasks of the CPC using a function blocking antibody against Incenp microinjected into cells at different mitotic phases. This method allowed temporal analysis of CPC functions without perturbation of complex assembly or activity prior to injection. We have also studied the dynamic properties of Incenp and Aurora B using fusion protein photobleaching. We found that in early mitotic cells, Incenp and Aurora B exhibit dynamic turnover at centromeres, which is prevented by the anti-Incenp antibody. In these cells, the loss of centromeric CPC turnover is accompanied by forced mitotic exit without the execution of cytokinesis. Introduction of anti-Incenp antibody into early anaphase cells causes abnormalities in sister chromatid separation through defects in anaphase spindle functions. In summary, our data uncovers new mitotic roles for the CPC in anaphase and proposes that CPC turnover at centromeres modulates spindle assembly checkpoint signaling.

Molecular basis of drug resistance in aurora kinases

Aurora kinases have emerged as potential targets in cancer therapy, and several drugs are currently undergoing preclinical and clinical validation. Whether clinical resistance to these drugs can arise is unclear. We exploited a hypermutagenic cancer cell line to select mutations conferring resistance to a well-studied Aurora inhibitor, ZM447439. All resistant clones contained dominant point mutations in Aurora B. Three mutations map to residues in the ATP-binding pocket that are distinct from the "gatekeeper" residue. The mutants retain wild-type catalytic activity and were resistant to all of the Aurora inhibitors tested. Our studies predict that drug-resistant Aurora B mutants are likely to arise during clinical treatment. Furthermore, because the plasticity of the ATP-binding pocket renders Aurora B insensitive to multiple inhibitors, our observations indicate that the drug-resistant Aurora B mutants should be exploited as novel drug targets.

Reversine, a novel Aurora kinases inhibitor, inhibits colony formation of human acute myeloid leukemia cells

The demonstration that the small synthetic molecule reversine [2-(4-morpholinoanilino)-N6-cyclohexyladenine] promotes the dedifferentiation of committed cells into multipotent progenitor-type cells has raised hopes on the exploitation of this small chemical tool for the generation of stem cells. Here, we show that reversine causes a failure in cytokinesis and induces polyploidization. These effects of reversine are due to the inhibition of Aurora A and B, two related kinases that are implicated in several aspects of mitosis and that are frequently amplified and overexpressed in human tumors. Reversine inhibits the phosphorylation of histone H3, a direct downstream target of Aurora kinases. Similarly to the Aurora kinase inhibitor VX-680, which has recently entered phase II clinical trials for cancer treatment, reversine inhibited colony formation of leukemic cells from patients with acute myeloid leukemia but was significantly less toxic than VX-680 on cells from healthy donors. The crystal structure of the reversine-Aurora B kinase complex shows that reversine is a novel class of ATP-competitive Aurora kinase inhibitors. Thus, although our studies raise serious doubts on the application of reversine in regenerative medicine, they support the paradigm that reversine might be a useful agent in cancer chemotherapy.

Target validation and biomarker identification in oncology : the example of aurora kinases

The strong link between gene expression of mitotic Aurora kinases and cancer has stimulated a very high interest in developing Aurora kinase inhibitors for cancer therapy. Validation of Aurora kinases as targets, and development of pharmacodynamic biomarkers for inhibitors of Aurora kinases, provides an example of how target validation can help the drug discovery process, and also of how to interpret results depending on the technology used. In this review, we outline the principal tools, concepts, and strategies of target and biomarker validation for Aurora kinases, with emphasis on validation results derived from RNA-interference experiments. These data were essential for the decision to enter the next steps in drug development and for the selection of the appropriate biomarkers for clinical trials.

Current status of therapy for chronic myeloid leukemia: a review of drug development

Chronic myeloid leukemia (CML) has led the way for developing rational drug development in cancer. Most cases of CML diagnosed and treated in chronic phase are extremely well controlled with imatinib monotherapy, and primary resistance is very uncommon. Even though the treatment failure rate is low, the emergence of drug resistance and the lack of eradication of the hematopoietic stem cell clone has prompted a wave of drugs to address one or both these problems. Several clinical trials (Phase I and II) of dasatinib or nilotinib in the treatment of imatinib-resistant or -intolerant Ph chromosome-positive leukemia have already reported a remarkable rate of hematologic response greater than 90% for chronic-phase patients. These drugs minimize the risk of acquired drug resistance that is particularly seen within the first 24-36 months of therapy, and can prevent early failure in these patients, Furthermore, rational, noncross-resistant combinations that include a T315I inhibitor and drugs that can eradicate the hematopoietic stem cell clone may extend the coverage to virtually all patients with bcr-abl. Here we review the 6-year impact of the 'magic pill', Gleevec, (Glivec), including the emerging problems with its treatment, the efficacy data of dasatinib and nilotinib and the very promising data of the newer generation of drugs for CML.

A RasGAP SH3 peptide aptamer inhibits RasGAP-Aurora interaction and induces caspase-independent tumor cell death

The Ras GTPase-activating protein RasGAP catalyzes the conversion of active GTP-bound Ras into inactive GDP-bound Ras. However, RasGAP also acts as a positive effector of Ras and exerts an anti-apoptotic activity that is independent of its GAP function and that involves its SH3 (Src homology) domain. We used a combinatorial peptide aptamer approach to select a collection of RasGAP SH3 specific ligands. We mapped the peptide aptamer binding sites by performing yeast two-hybrid mating assays against a panel of RasGAP SH3 mutants. We examined the biological activity of a peptide aptamer targeting a pocket delineated by residues D295/7, L313 and W317. This aptamer shows a caspase-independent cytotoxic activity on tumor cell lines. It disrupts the interaction between RasGAP and Aurora B kinase. This work identifies the above-mentioned pocket as an interesting therapeutic target to pursue and points its cognate peptide aptamer as a promising guide to discover RasGAP small-molecule drug candidates.

Modulation of kinase-inhibitor interactions by auxiliary protein binding: crystallography studies on Aurora A interactions with VX-680 and with TPX2

VX-680, also known as MK-0457, is an ATP-competitive small molecule inhibitor of the Aurora kinases that has entered phase II clinical trials for the treatment of cancer. We have solved the cocrystal structure of AurA/TPX2/VX-680 at 2.3 A resolution. In the crystal structure, VX-680 binds to the active conformation of AurA. The glycine-rich loop in AurA adopts a unique bent conformation, forming a pi-pi interaction with the phenyl group of VX-680. In contrast, in the published AurA/VX-680 structure, VX-680 binds to AurA in the inactive conformation, interacting with a hydrophobic pocket only present in the inactive conformation. These data suggest that TPX2, a protein cofactor, can alter the binding mode of VX-680 with AurA. More generally, the presence of physiologically relevant cofactor proteins can alter the kinetics, binding interactions, and inhibition of enzymes, and studies with these multiprotein complexes may be beneficial to the discovery and optimization of enzyme inhibitors as therapeutic agents.

Discovery of a potent and selective aurora kinase inhibitor

This communication describes the discovery of a novel series of Aurora kinase inhibitors. Key SAR and critical binding elements are discussed. Some of the more advanced analogues potently inhibit cellular proliferation and induce phenotypes consistent with Aurora kinase inhibition. In particular, compound 21 (SNS-314) is a potent and selective Aurora kinase inhibitor that exhibits significant activity in pre-clinical in vivo tumor models.

The Aurora kinase family in cell division and cancer

The Aurora protein kinase family (consisting of Aurora-A, -B and -C) is an important group of enzymes that controls several aspects of cell division in mammalian cells. Dysfunction of these kinases has been associated with a failure to maintain a stable chromosome content, a state that can contribute to tumourigenesis. Additionally, Aurora-A is frequently found amplified in a variety of tumour types and displays oncogenic activity. On the other hand, therapeutic inhibition of these kinases has shown great promise as potential anti-cancer treatment, most likely because of their essential roles during cell division. This review will focus on our present understanding of the different roles played by these kinases, their regulation throughout cell division, their deregulation in human cancers and on the progress that is made in targeting these important regulators in the treatment of cancer.

Phosphorylation of HsMis13 by Aurora B kinase is essential for assembly of functional kinetochore

Chromosome movements in mitosis are orchestrated by dynamic interactions between spindle microtubules and the kinetochore, a multiprotein complex assembled onto centromeric DNA of the chromosome. Here we show that phosphorylation of human HsMis13 by Aurora B kinase is required for functional kinetochore assembly in HeLa cells. Aurora B interacts with HsMis13 in vitro and in vivo. HsMis13 is a cognate substrate of Aurora B, and the phosphorylation sites were mapped to Ser-100 and Ser-109. Suppression of Aurora B kinase by either small interfering RNA or chemical inhibitors abrogates the localization of HsMis13 but not HsMis12 to the kinetochore. In addition, non-phosphorylatable but not wild type and phospho-mimicking HsMis13 failed to localize to the kinetochore, demonstrating the requirement of phosphorylation by Aurora B for the assembly of HsMis13 to kinetochore. In fact, localization of HsMis13 to the kinetochore is spatiotemporally regulated by Aurora B kinase, which is essential for recruiting outer kinetochore components such as Ndc80 components and CENP-E for functional kinetochore assembly. Importantly, phospho-mimicking mutant HsMis13 restores the assembly of CENP-E to the kinetochore, and tension developed across the sister kinetochores in Aurora B-inhibited cells. Thus, we reason that HsMis13 phosphorylation by Aurora B is required for organizing a stable bi-oriented microtubule kinetochore attachment that is essential for faithful chromosome segregation in mitosis.

A pentacyclic aurora kinase inhibitor (AKI-001) with high in vivo potency and oral bioavailability

Aurora kinase inhibitors have attracted a great deal of interest as a new class of antimitotic agents. We report a novel class of Aurora inhibitors based on a pentacyclic scaffold. A prototype pentacyclic inhibitor 32 (AKI-001) derived from two early lead structures improves upon the best properties of each parent and compares favorably to a previously reported Aurora inhibitor, 39 (VX-680). The inhibitor exhibits low nanomolar potency against both Aurora A and Aurora B enzymes, excellent cellular potency (IC50 < 100 nM), and good oral bioavailability. Phenotypic cellular assays show that both Aurora A and Aurora B are inhibited at inhibitor concentrations sufficient to block proliferation. Importantly, the cellular activity translates to potent inhibition of tumor growth in vivo. An oral dose of 5 mg/kg QD is well tolerated and results in near stasis (92% TGI) in an HCT116 mouse xenograft model.

Concomitant inhibition of Mdm2-p53 interaction and Aurora kinases activates the p53-dependent postmitotic checkpoints and synergistically induces p53-mediated mitochondrial apoptosis along with reduced endoreduplication in acute myelogenous leukemia

Aberrant expression of Aurora kinases and inactivation of wild-type p53 by Mdm2 overexpression are frequent molecular events in acute myelogenous leukemia (AML), and preclinical data for inhibition of Aurora kinases or Mdm2 are promising. However, it remains largely unknown whether the viability of cells exposed to Aurora kinase inhibitors depends on the p53 status. We investigated the interaction of Aurora kinases and p53 pathways after their simultaneous blockades using a small-molecule pan-Aurora kinase inhibitor, MK-0457, and a selective small-molecule antagonist of Mdm2, Nutlin-3. We found that MK-0457, which itself activates p53 signaling, acts synergistically with Nutlin-3 to induce apoptosis in wild-type p53 AML cell lines OCI-AML-3 and MOLM-13 but not in p53-null HL-60 cells. MK-0457 and Nutlin-3 showed synergism in inducing p53, conformational change of Bax and Deltapsi(m) loss, suggesting an involvement of p53-mediated mitochondrial apoptosis. Nutlin-3 constrained endoreduplication after Aurora inhibition via activation of a p53-dependent postmitotic checkpoint and p21 induction in pseudo-G1 cells. Our findings provide the molecular rationale for concomitant targeting of Aurora kinases and Mdm2 in AML where TP53 mutations are rare and downstream p53 signaling is mostly intact.

Progress in the treatment of adults with acute lymphoblastic leukemia

PURPOSE OF REVIEW

Despite improvements in the achievement of complete remission and progress in the supportive care of adults with acute lymphoblastic leukemia during the last decade, the majority of patients have eventually relapsed with overall survival in adult acute lymphoblastic leukemia of only 30-40%. However, the recent approach of adapting therapy according to biologic features appears to be resulting in significant progress for specific subsets of adults with acute lymphoblastic leukemia.

RECENT FINDINGS

The present review highlights some of these new risk-adapted approaches focusing on recent advances in treatment of Philadelphia chromosome positive acute lymphoblastic leukemia and mature B-cell acute lymphoblastic leukemia using biologically targeted therapies, and new approaches to treatment of acute lymphoblastic leukemia in older adolescents and young adults, adopting therapeutic strategies employed in the successful treatment of children with acute lymphoblastic leukemia. A recent study that examines the role of allogeneic stem cell transplant for adults with acute lymphoblastic leukemia in first remission will also be reviewed.

SUMMARY

The subset-specific approach to therapy of adult acute lymphoblastic leukemia is beginning to result in promising improvements in survival. Future improvements in survival of adults with acute lymphoblastic leukemia will depend on participation in large cooperative group trials using biologically defined protocols for this relatively rare and heterogeneous group of diseases.

Bora and the kinase Aurora a cooperatively activate the kinase Plk1 and control mitotic entry

A central question in the study of cell proliferation is, what controls cell-cycle transitions? Although the accumulation of mitotic cyclins drives the transition from the G2 phase to the M phase in embryonic cells, the trigger for mitotic entry in somatic cells remains unknown. We report that the synergistic action of Bora and the kinase Aurora A (Aur-A) controls the G2-M transition. Bora accumulates in the G2 phase and promotes Aur-A-mediated activation of Polo-like kinase 1 (Plk1), leading to the activation of cyclin-dependent kinase 1 and mitotic entry. Mechanistically, Bora interacts with Plk1 and controls the accessibility of its activation loop for phosphorylation and activation by Aur-A. Thus, Bora and Aur-A control mitotic entry, which provides a mechanism for one of the most important yet ill-defined events in the cell cycle.

Stem cell and kinase activity-independent pathway in resistance of leukaemia to BCR-ABL kinase inhibitors

BCR-ABL tyrosine kinase inhibitors, such as imatinib (Gleevec) are highly effective in treating human Philadelphia chromosome-positive (Ph⁺) chronic myeloid leukaemia (CML) in chronic phase but not in terminal acute phase; acquired drug resistance caused mainly by the development of BCR-ABL kinase domain mutations prevents cure of the leukaemia. In addition, imatinib is ineffective in treating Ph⁺ B-cell acute lymphoblastic leukaemia (B-ALL) and CML blast crisis, even in the absence of the kinase domain mutations. This type of drug resistance that is unrelated to BCR-ABL kinase domain mutations is caused by the insensitivity of leukaemic stem cells to kinase inhibitors such as imatinib and dasatinib, and by activation of a newly-identified signalling pathway involving SRC kinases that are independent of BCR-ABL kinase activity for activation. This SRC pathway is essential for leukaemic cells to survive imatinib treatment and for CML transition to lymphoid blast crisis. Apart from BCR-ABL and SRC kinases, stem cell pathways must also be targeted for curative therapy of Ph⁺ leukaemia.

PHA-680626 exhibits anti-proliferative and pro-apoptotic activity on Imatinib-resistant chronic myeloid leukemia cell lines and primary CD34+ cells by inhibition of both Bcr-Abl tyrosine kinase and Aurora kinases

Emergence of resistance to Imatinib complicates the treatment of chronic myeloid leukemia (CML). Second-generation Bcr-Abl inhibitors are capable to overcome resistance mediated by most mutations except T315I. As this mutation is causative for approximately 20% of clinically observed resistances, the need for novel treatment strategies becomes obvious. Here, we report on a novel kinase inhibitor PHA-680626 exhibiting strong inhibitory activity on both Bcr-Abl and Aurora kinases. Significant anti-proliferative and pro-apoptotic effects were observed in human BCR-ABL positive cell lines and murine BaF3 cells ectopically expressing wt BCR-ABL or the Imatinib-resistant BCR-ABL mutants M351T, E255K and, T315I. Treatment with PHA-680626 decreased phosphorylation of CrkL and histone H3. As CrkL represents a typical downstream target of Bcr-Abl while histone H3 phosphorylation is an indicator for Aurora kinase B activity, these findings indicate that effects of PHA-680626 are mediated via inhibition of both pathways. Moreover, high anti-proliferative activity of PHA-680626 was observed in primary CD34+ cells derived from CML patients at diagnosis or in blast crisis as well as from an individual harbouring the T315I mutation. Thus, both Bcr-Abl and Aurora kinase inhibition contribute to the efficacy of PHA-680626 against Imatinib-resistant BCR-ABL positive leukemias, particularly those harbouring the T315I mutation.

Activity of a novel Aurora kinase inhibitor against the T315I mutant form of BCR-ABL: in vitro and in vivo studies

Despite promising results from clinical studies of ABL kinase inhibitors, a challenging problem that remains is the T315I mutation against which neither nilotinib nor dasatinib show significant activity. In the present study, we investigated the activity of a novel Aurora kinase inhibitor, VE-465, against leukemia cells expressing wild-type BCR-ABL or the T315I mutant form of BCR-ABL. We observed a dose-dependent reduction in the level of BCR-ABL autophosphorylation in VE-465-treated cells. Exposure to the combination of VE-465 and imatinib exerted an enhanced apoptotic effect in K562 cells. Combined treatment with VE-465 and imatinib caused more attenuation of the levels of phospho-AKT and c-Myc in K562 cells. Further, the isobologram indicated the synergistic effect of simultaneous exposure to VE-465 and imatinib in K562 cells. To assess the in vivo efficacy of VE-465, athymic nude mice were injected intravenously with BaF3 cells expressing wild-type BCR-ABL or the T315I mutant form. The vehicle-treated mice died of a condition resembling acute leukemia by 28 days; however, nearly all mice treated with VE-465 (75 mg/kg, twice daily; intraperitoneally for 14 days) survived for more than 56 days. Histopathological analysis of vehicle-treated mice revealed infiltration of the spleen. In contrast, histopathological analysis of organs from VE-465-treated mice demonstrated normal tissue architecture. Taken together, the present study shows that VE-465 exhibits a desirable therapeutic index that can reduce the in vivo growth of T315I mutant form and wild-type BCR-ABL-expressing cells in an efficacious manner.

Aurora kinase A controls meiosis I progression in mouse oocytes

Aurora kinase A (AURKA), which is a centrosome-localized serine/threonine kinase crucial for cell cycle control, is critically involved in centrosome maturation and spindle assembly in somatic cells. Active T288 phosphorylated AURKA localizes to the centrosome in the late G(2) and also spreads to the minus ends of mitotic spindle microtubules. AURKA activates centrosomal CDC25B and recruits cyclin B1 to centrosomes. We report here functions for AURKA in meiotic maturation of mouse oocytes, which is a model system to study the G(2) to M transition. Whereas AURKA is present throughout the entire GV-stage oocyte with a clear accumulation on microtubule organizing centers (MTOC), active AURKA becomes entirely localized to MTOCs shortly before germinal vesicle breakdown. In contrast to somatic cells in which active AURKA is present at the centrosomes and minus ends of microtubules, active AURKA is mainly located on MTOCs at metaphase I (MI) in oocytes. Inhibitor studies using Roscovitine (CDK1 inhibitor), LY-294002 (PI3K inhibitor) and SH-6 (PKB inhibitor) reveal that activation of AURKA localized on MTOCs is independent on PI3K-PKB and CDK1 signaling pathways and MOTC amplification is observed in roscovitine- and SH-6-treated oocytes that fail to undergo nuclear envelope breakdown. Moreover, microinjection of Aurka mRNA into GV-stage oocytes cultured in 3-isobutyl-1-methyl xanthine (IBMX)-containing medium to prevent maturation also results in MOTC amplification in the absence of CDK1 activation. Overexpression of AURKA also leads to formation of an abnormal MI spindle, whereas RNAi-mediated reduction of AURKA interferes with resumption of meiosis and spindle assembly. Results of these experiments indicate that AURKA is a critical MTOC-associated component involved in resumption of meiosis, MTOC multiplication, proper spindle formation and the metaphase I-metaphase II transition.

Vorinostat synergistically potentiates MK-0457 lethality in chronic myelogenous leukemia cells sensitive and resistant to imatinib mesylate

Interactions between the dual Bcr/Abl and aurora kinase inhibitor MK-0457 and the histone deacetylase inhibitor vorinostat were examined in Bcr/Abl(+) leukemia cells, including those resistant to imatinib mesylate (IM), particularly those with the T315I mutation. Coadministration of vorinostat dramatically increased MK-0457 lethality in K562 and LAMA84 cells. Notably, the MK-0457/vorinostat regimen was highly active against primary CD34(+) chronic myelogenous leukemia (CML) cells and Ba/F3 cells bearing various Bcr/Abl mutations (ie, T315I, E255K, and M351T), as well as IM-resistant K562 cells exhibiting Bcr/Abl-independent, Lyn-dependent resistance. These events were associated with inactivation and down-regulation of wild-type (wt) and mutated Bcr/Abl (particularly T315I). Moreover, treatment with MK-0457 resulted in accumulation of cells with 4N or more DNA content, whereas coadministration of vorinostat markedly enhanced aurora kinase inhibition by MK-0457, and preferentially killed polyploid cells. Furthermore, vorinostat also interacted with a selective inhibitor of aurora kinase A and B to potentiate apoptosis without modifying Bcr/Abl activity. Finally, vorinostat markedly induced Bim expression, while blockade of Bim induction by siRNA dramatically diminished the capacity of this agent to potentiate MK-0457 lethality. Together, these findings indicate that vorinostat strikingly increases MK-0457 activity against IM-sensitive and -resistant CML cells through inactivation of Bcr/Abl and aurora kinases, as well as by induction of Bim.

Aurora kinases as anticancer drug targets

The human aurora family of serine-threonine kinases comprises three members, which act in concert with many other proteins to control chromosome assembly and segregation during mitosis. Aurora dysfunction can cause aneuploidy, mitotic arrest, and cell death. Aurora kinases are strongly expressed in a broad range of cancer types. Aurora A expression in tumors is often associated with gene amplification, genetic instability, poor histologic differentiation, and poor prognosis. Aurora B is frequently expressed at high levels in a variety of tumors, often coincidently with aurora A, and expression level has also been associated with increased genetic instability and clinical outcome. Further, aurora kinase gene polymorphisms are associated with increased risk or early onset of cancer. The expression of aurora C in cancer is less well studied. In recent years, several small-molecule aurora kinase inhibitors have been developed that exhibit preclinical activity against a wide range of solid tumors. Preliminary clinical data from phase I trials have largely been consistent with cytostatic effects, with disease stabilization as the best response achieved in solid tumors. Objective responses have been noted in leukemia patients, although this might conceivably be due to inhibition of the Abl kinase. Current challenges include the optimization of drug administration, the identification of potential biomarkers of tumor sensitivity, and combination studies with cytotoxic drugs. Here, we summarize the most recent preclinical and clinical data and discuss new directions in the development of aurora kinase inhibitors as antineoplastic agents.

Aurora kinase B is a predictive factor for the aggressive recurrence of hepatocellular carcinoma after curative hepatectomy

BACKGROUND

Patterns of cancer recurrence hold the key to prognosis after curative resection. This retrospective study aimed to identify a predictor and therapeutic candidate for aggressive recurrence of hepatocellular carcinoma (HCC).

METHODS

Primary HCC tissues from 107 patients who had curative resection were analysed. Genome-wide gene expression profiles were investigated using a microarray technique, and clustering analysis was carried out based on the first diagnosis of recurrence according to the Milan criteria. Immunohistochemical expression and array-based comparative genomic hybridization (array-CGH) were also assessed.

RESULTS

Microarray analysis revealed overexpression of Aurora kinase B, a chromosome passenger protein kinase, as the most significant predictor of the aggressive recurrence of HCC. Aurora kinase B protein expression was significantly associated with aggressive recurrence (P < 0.001) and prognosis (P < 0.001). Multivariable analysis identified Aurora kinase B as the only independent predictor of aggressive recurrence of HCC (P = 0.031). Array-CGH analysis showed that genomic instability was closely related to Aurora kinase B expression (P = 0.011).

CONCLUSION

Aurora kinase B is an effective predictor of aggressive HCC recurrence, in relation to the genomic instability. It might be worth considering as a molecular target for the adjuvant therapy of HCC.

Getting to the stem of chronic myeloid leukaemia

Tyrosine kinase inhibitor (TKI) therapy for chronic myeloid leukaemia (CML) is the consummate success story for targeted therapy, yet relapse is a nearly inevitable consequence of cessation or interruption of therapy. Primitive TKI-refractory CML stem cells are the likely source of these relapses, as they provide sanctuary for the Philadelphia chromosome. In advanced disease, their progressively anaplastic progeny ultimately maintain CML independently of the CML haematopoietic stem cell (HSC). Interestingly, there are at least two distinct cell types capable of self-renewal in different phases of CML: first, a primitive HSC with BCR-ABL mutation, which maintains the more indolent chronic-phase disease and, second, a coexisting mutated progenitor cell which acquires stem cell characteristics responsible for rapid cell expansion in advanced disease.

Simultaneous targeting of Aurora kinases and Bcr-Abl kinase by the small molecule inhibitor PHA-739358 is effective against imatinib-resistant BCR-ABL mutations including T315I

The emergence of resistance to imatinib (IM) mediated by mutations in the BCR-ABL domain has become a major challenge in the treatment of chronic myeloid leukemia (CML). Here, we report on studies performed with a novel small molecule inhibitor, PHA-739358, which selectively targets Bcr-Abl and Aurora kinases A to C. PHA-739358 exhibits strong antiproliferative and proapoptotic activity against a broad panel of human BCR-ABL–positive and –negative cell lines and against murine BaF3 cells ectopically expressing wild-type (wt) or IM-resistant BCR-ABL mutants, including T315I. Pharmacologic synergism of IM and PHA-739358 was observed in leukemia cell lines with subtotal resistance to IM. Treatment with PHA-739358 significantly decreased phosphorylation of histone H3, a marker of Aurora B activity and of CrkL, a downstream target of Bcr-Abl, suggesting that PHA-739358 acts via combined inhibition of Bcr-Abl and Aurora kinases. Moreover, strong antiproliferative effects of PHA-739358 were observed in CD34+ cells derived from untreated CML patients and from IM-resistant individuals in chronic phase or blast crisis, including those harboring the T315I mutation. Thus, PHA-739358 represents a promising new strategy for treatment of IM-resistant BCR-ABL-positive leukemias, including those harboring the T315I mutation. Clinical trials investigating this compound in IM-resistant CML have recently been initiated.

Cell polarization during monopolar cytokinesis

During cytokinesis, a specialized set of proteins is recruited to the equatorial region between spindle poles by microtubules and actin filaments, enabling furrow assembly and ingression before cell division. We investigate the mechanisms underlying regional specialization of the cytoskeleton in HeLa cells undergoing drug-synchronized monopolar cytokinesis. After forced mitotic exit, the cytoskeleton of monopolar mitotic cells is initially radially symmetric but undergoes a symmetry-breaking reaction that simultaneously polarizes microtubules and the cell cortex, with a concentration of cortical furrow markers into a cap at one side of the cell. Polarization requires microtubules, F-actin, RhoA, Myosin II activity, and Aurora B kinase activity. Aurora B localizes to actin cables in a gap between the monopolar midzone and the furrow-like cortex, suggesting a communication between them. We propose that feedback loops between cortical furrow components and microtubules promote symmetry breaking during monopolar cytokinesis and regional specialization of the cytoskeleton during normal bipolar cytokinesis.

Effects of DNAzymes targeting Aurora kinase A on the growth of human prostate cancer

Aurora kinase A has been demonstrated to be involved in the malignant progression of many types of cancer including prostate cancer, we therefore hypothesized that Aurora kinase A might work as a valuable target for prostate cancer treatment. To test this hypothesis, we used DNAzyme technology to inhibit Aurora kinase A expression and evaluated the effects of DNAzymes as therapeutic agents to treat prostate cancer. In an in vitro cleavage assay, we found that a DNAzyme (DZ2) targeting Aurora kinase A could effectively cleave Aurora kinase A mRNA. When transfected into the prostate cancer cell line PC3, DZ2 was found to strongly inhibit the expression of Aurora kinase A examined by western blot analysis, and thus suppressed cell growth, arrested the progression of cell cycle, induced cell apoptosis and attenuated cell migration, as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium assay, flow cytometry and Boyden chamber assay. Through in vivo study, we also found that DZ2 could significantly inhibit the growth of human prostate cancer xenografts in nude mice. In conclusion, DZ2 could effectively attenuate malignant progression of prostate cancer both in vitro and in vivo, suggesting that DNAzyme targeting Aurora kinase A may be used as a valuable therapy to treat prostate cancer.

Aurora kinase inhibitors: identification and preclinical validation of their biomarkers

Aurora kinases are key regulators of mitosis and inhibitors being developed by a wide range of pharmaceutical and biotechnology companies for the treatment of cancer. Tumor cells respond differentially on inhibition of different Aurora kinase family members and these differences have to be considered in the clinical development of small-molecule inhibitors with respect to the chosen indications, the schedules or the selection of appropriate end points and they should also guide the development of biomarkers. Preclinical validation of potential biomarkers for Aurora kinase inhibitors led to a first application in clinical trials, as exemplified for the phosphorylation of histone H3 to follow Aurora-B inhibition. This review discusses the criteria for translation into the clinic and the value of pharmacodynamic biomarkers and their potential, but also their limitations to be used as surrogate markers for clinical end points.

Tumor cell dependence on Ran-GTP-directed mitosis

Deregulated cell division is a hallmark of cancer, but whether tumor cells become dependent on specific mitotic mechanisms is not known. Here, we show that the small GTPase Ran, a regulator of mitotic spindle formation, is differentially overexpressed in human cancer as compared with normal tissues, in vivo. Acute silencing of Ran in various tumor cell types causes aberrant mitotic spindle formation, mitochondrial dysfunction, and apoptosis. This pathway does not require p53, Bax, or Smac, but is controlled by survivin as a novel Ran target in cancer. Conversely, loss of Ran in normal cells is well tolerated and does not result in mitotic defects or loss of cell viability. Therefore, tumor cells can become dependent on Ran signaling for cell division, and targeting this pathway may provide a novel and selective anticancer strategy.

Aurora A kinase RNAi and small molecule inhibition of Aurora kinases with VE-465 induce apoptotic death in multiple myeloma cells

The expression of RHAMM and other centrosome-associated genes are known to correlate with the extent of centrosome amplification in multiple myeloma, and with poor prognosis. RHAMM has a significant interaction with TPX2, a protein which regulates the localization and action of Aurora A kinase (AURKA) at the spindle poles. AURKA is known to be a central determinant of centrosome and spindle function and is a target for cancer therapy. Given these observations, we investigated the role of Aurora kinases as therapeutic targets in myeloma. Here we report that AURKA is expressed ubiquitously in myeloma, to varying degrees, in both cell lines and patients' bone marrow plasma cells. siRNA targeting AURKA induces apoptotic cell death in myeloma cell lines. The Aurora kinase inhibitor VE-465 also induces apoptosis and death in myeloma cell lines and primary myeloma plasma cells. The combination of VE-465 and dexamethasone improves cell killing compared with the use of either agent alone, even in cells resistant to the single agents. The phenotype of myeloma cells treated with VE-465 is consistent with published reports on the effects of Aurora kinase inhibition. Aurora kinase inhibitors should be pursued as potential treatments for myeloma.

Current Therapies for Chronic Myeloid Leukemia

Management strategies for patients with chronic-phase chronic myeloid leukemia (CML) have changed dramatically since the introduction of imatinib into clinical trials in 1998. Imatinib is generally accepted, at present, to be the most appropriate initial therapy for newly diagnosed chronic-phase CML; however, a proportion of patients will not respond adequately. Many of these patients may benefit from alternative treatment strategies, including second- and third-generation BCR-ABL kinase inhibitors and allogeneic hematopoietic stem cell transplantation (HSCT). Additionally, with continued improvements in molecular monitoring, it is much more clinically routine to measure ongoing treatment efficacy or characterize pending disease relapse via molecular analysis. The challenge is to now combine molecular monitoring information with timely treatment decisions to achieve the best possible outcomes. Additionally, unanswered questions about HSCT remain, and include (1) What is the role of allogeneic HSCT in CML? (2) What type of transplant, reduced-intensity or myeloablative, should be performed? The goal of this article is to provide an overview of where we stand in the treatment of CML in 2008.

Discovery of selective aminothiazole aurora kinase inhibitors

Aurora family kinases regulate important events during mitosis including centrosome maturation and separation, mitotic spindle assembly, and chromosome segregation. Misregulation of Aurora kinases due to genetic amplification and protein overexpression results in aneuploidy and may contribute to tumorigenesis. Here we report the discovery of new small molecule aminothiazole inhibitors of Aurora kinases with exceptional kinase selectivity and report a 1.7 A cocrystal structure with the Aurora B:INCENP complex from Xenopus laevis. The compounds recapitulate the hallmarks of Aurora kinase inhibition, including decreased histone H3 serine 10 phosphorylation, failure to complete cytokinesis, and endoreduplication.

PHA-739358, a potent inhibitor of Aurora kinases with a selective target inhibition profile relevant to cancer

PHA-739358 is a small-molecule 3-aminopyrazole derivative with strong activity against Aurora kinases and cross-reactivities with some receptor tyrosine kinases relevant for cancer. PHA-739358 inhibits all Aurora kinase family members and shows a dominant Aurora B kinase inhibition-related cellular phenotype and mechanism of action in cells in vitro and in vivo. p53 status-dependent endoreduplication is observed upon treatment of cells with PHA-739358, and phosphorylation of histone H3 in Ser(10) is inhibited. The compound has significant antitumor activity in different xenografts and spontaneous and transgenic animal tumor models and shows a favorable pharmacokinetic and safety profile. In vivo target modulation is observed as assessed by the inhibition of the phosphorylation of histone H3, which has been validated preclinically as a candidate biomarker for the clinical phase. Pharmacokinetics/pharmacodynamics modeling was used to define drug potency and to support the prediction of active clinical doses and schedules. We conclude that PHA-739358, which is currently tested in clinical trials, has great therapeutic potential in anticancer therapy in a wide range of cancers.

Oncogene addiction: setting the stage for molecularly targeted cancer therapy

In pugilistic parlance, the one-two punch is a devastating combination of blows, with the first punch setting the stage and the second delivering the knock-out. This analogy can be extended to molecularly targeted cancer therapies, with oncogene addiction serving to set the stage for tumor cell killing by a targeted therapeutic agent. While in vitro and in vivo examples abound documenting the existence of this phenomenon, the mechanistic underpinnings that govern oncogene addiction are just beginning to emerge. Our current inability to fully exploit this weakness of cancer cells stems from an incomplete understanding of oncogene addiction, which nonetheless represents one of the rare chinks in the formidable armor of cancer cells.

MK615 inhibits pancreatic cancer cell growth by dual inhibition of Aurora A and B kinases

AIM: To investigate the anti-neoplastic effect of MK615, an anti-neoplastic compound isolated from Japanese apricot, against human pancreatic cancer cells in vitro.

METHODS: Three human pancreatic cancer cell lines PANC-1, PK-1, and PK45H were cultured with MK615 at concentrations of 600, 300, 150, and 0 &mgr;g/mL. Growth inhibition was evaluated by cell proliferation assay, and killing activity was determined by lactate dehydrogenase (LDH) assay. Expression of Aurora A and B kinases was detected by real-time polymerase chain reaction (PCR) and Western blotting. Cell cycle stages were evaluated by flow cytometry.

RESULTS: The growth inhibitory rates of MK615 at 150, 300, and 600 &mgr;g/mL were 2.3% ± 0.9%, 8.9% ± 3.2% and 67.1% ± 8.1% on PANC1 cells, 1.3% ± 0.3%, 8.7% ± 4.1% and 45.7 ± 7.6% on PK1 cells, and 1.2 ± 0.8%, 9.1% ± 2.1% and 52.1% ± 5.5% on PK45H cells, respectively (P <0.05). The percentage cytotoxicities of MK615 at 0, 150, 300, and 600 &mgr;g/mL were 19.6% ± 1.3%, 26.7% ± 1.8%, 25.5% ± 0.9% and 26.4% ± 0.9% in PANC1 cells, 19.7% ± 1.3%, 24.7% ± 0.8%, 25.9% ± 0.9% and 29.9% ± 1.1% in PK1 cells, and 28.0% ± 0.9%, 31.2% ± 0.9%, 30.4% ± 1.1% and 35.3 ± 1.0% in PK45H cells, respectively (P < 0.05). Real-time PCR and Western blotting showed that MK615 dually inhibited the expression of Aurora A and B kinases. Cell cycle analysis revealed that MK615 increased the population of cells in G2/M phase.

CONCLUSION: MK615 exerts an anti-neoplastic effect on human pancreatic cancer cells in vitro by dual inhibition of Aurora A and B kinases.

Strategies for overcoming imatinib resistance in chronic myeloid leukemia

Imatinib was the first treatment for chronic myeloid leukemia (CML) that specifically targeted the causative BCR-ABL oncoprotein, and represented a major therapeutic advance in this disease; however, some patients develop resistance or intolerance. Resistance can be classified as BCR-ABL-dependent (e.g., mutation in the BCR-ABL gene) or BCR-ABL-independent (alternative pathways of disease progression, e.g., SRC-family tyrosine kinases). The investigation of therapeutic options post-imatinib failure resulted in the development and regulatory approval of dasatinib, a BCR-ABL and SRC-family kinase inhibitor. Dasatinib is active across all phases of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia, and demonstrates activity in almost all imatinib-resistant mutations. Other therapeutic options are also under investigation, with nilotinib being the most clinically advanced. Nilotinib is an analog of imatinib with similar multiple kinase targets, but without inhibition of SRC, and reduced in vitro activity against BCR-ABL P-loop mutations compared with dasatinib. Similar to dasatinib, nilotinib has no activity against T315I mutations. The availability of dasatinib and development of other tyrosine kinase inhibitors provide positive prospects for patients with imatinib-resistant or -intolerant CML. Here, we discuss several of these new strategies for treating patients after imatinib failure.

Tubulin-associated proteins: Aurora and Polo-like kinases as therapeutic targets in cancer

Tubulin is a very important target for cancer-fighting therapies; therefore, the cancer research community continues to adopt new ways of developing the therapeutic potential of tubulin and tubulin-associated proteins. Two families of tubulin-associated kinases, Aurora and Polo-like, have received significant attention regarding how they contribute to tumorigenesis and can be targeted with selective small molecule inhibitors. Aurora and Polo-like kinases play essential roles in centrosome separation, chromosome alignment and segregation, and cytokinesis. Inhibition of any of these kinases results in abnormal mitotic events (which vary depending on the particular family member) and eventually leads to apoptosis. Because of the biological consequences of inhibiting these kinases, several Aurora or Polo-like selective inhibitors have advanced to various stages of preclinical and clinical development; the most advanced are currently in phase 2 clinical trials.

Spatio-Temporal Expression Patterns of Aurora Kinases A, B, and C and Cytoplasmic Polyadenylation-Element-Binding Protein in Bovine Oocytes During Meiotic Maturation1

Maturation of immature bovine oocytes requires cytoplasmic polyadenylation and synthesis of a number of proteins involved in meiotic progression and metaphase-II arrest. Aurora serine-threonine kinases--localized in centrosomes, chromosomes, and midbody--regulate chromosome segregation and cytokinesis in somatic cells. In frog and mouse oocytes, Aurora A regulates polyadenylation-dependent translation of several mRNAs such as MOS and CCNB1, presumably by phosphorylating CPEB, and Aurora B phosphorylates histone H3 during meiosis. We analyzed the expression of three Aurora kinase genes--AURKA, AURKB, and AURKC--in bovine oocytes during meiosis by reverse transcription followed by quantitative real-time PCR and immunodetection. Aurora A was the most abundant form in oocytes, both at mRNA and protein levels. AURKA protein progressively accumulated in the oocyte cytoplasm during antral follicle growth and in vitro maturation. AURKB associated with metaphase chromosomes. AURKB, AURKC, and Thr-phosphorylated AURKA were detected at a contractile ring/midbody during the first polar body extrusion. CPEB, localized in oocyte cytoplasm, was hyperphosphorylated during prophase/metaphase-I transition. Most CPEB degraded in metaphase-II oocytes and remnants remained localized in a contractile ring. Roscovitine, U0126, and metformin inhibited meiotic divisions; they all induced a decrease of CCNB1 and phospho-MAPK3/1 levels and prevented CPEB degradation. However, only metformin depleted AURKA. The Aurora kinase inhibitor VX680 at 100 nmol/L did not inhibit meiosis but led to multinuclear oocytes due to the failure of the polar body extrusion. Thus, in bovine oocyte meiosis, massive destruction of CPEB accompanies metaphase-I/II transition, and Aurora kinases participate in regulating segregation of the chromosomes, maintenance of metaphase-II, and formation of the first polar body.

Polo and Aurora kinases: lessons derived from chemical biology

During the cell division cycle, mitotic entry, spindle assembly, chromosome segregation, and cytokinesis must all be carefully coordinated to ensure that the two daughter cells inherit all the genetic material required for further growth and development. Central to this coordination are several protein kinases including Aurora A, Aurora B, and the Polo-like kinase, Plk1. A number of small-molecule Aurora and Plk1 inhibitors have been developed because these kinases are seen as attractive anticancer drug targets. These inhibitors are now being widely used as chemical biology tools to understand how these kinases ensure faithful genome transmission.

Chromatin-modifying proteins in cancer

Chromatin-modifying proteins mold the genome into areas that are accessible for transcriptional activity and areas that are transcriptionally silent. This epigenetic gene regulation allows for different transcriptional programs to be conducted in different cell types at different timepoints-despite the fact that all cells in the organism contain the same genetic information. A large amount of data gathered over the last decades has demonstrated that deregulation of chromatin-modifying proteins is etiologically involved in the development and progression of cancer. Here we discuss how epigenetic alterations influence cancer development and review known cancer-associated alterations in chromatin-modifying proteins.

The selectivity of protein kinase inhibitors: a further update

The specificities of 65 compounds reported to be relatively specific inhibitors of protein kinases have been profiled against a panel of 70-80 protein kinases. On the basis of this information, the effects of compounds that we have studied in cells and other data in the literature, we recommend the use of the following small-molecule inhibitors: SB 203580/SB202190 and BIRB 0796 to be used in parallel to assess the physiological roles of p38 MAPK (mitogen-activated protein kinase) isoforms, PI-103 and wortmannin to be used in parallel to inhibit phosphatidylinositol (phosphoinositide) 3-kinases, PP1 or PP2 to be used in parallel with Src-I1 (Src inhibitor-1) to inhibit Src family members; PD 184352 or PD 0325901 to inhibit MKK1 (MAPK kinase-1) or MKK1 plus MKK5, Akt-I-1/2 to inhibit the activation of PKB (protein kinase B/Akt), rapamycin to inhibit TORC1 [mTOR (mammalian target of rapamycin)-raptor (regulatory associated protein of mTOR) complex], CT 99021 to inhibit GSK3 (glycogen synthase kinase 3), BI-D1870 and SL0101 or FMK (fluoromethylketone) to be used in parallel to inhibit RSK (ribosomal S6 kinase), D4476 to inhibit CK1 (casein kinase 1), VX680 to inhibit Aurora kinases, and roscovitine as a pan-CDK (cyclin-dependent kinase) inhibitor. We have also identified harmine as a potent and specific inhibitor of DYRK1A (dual-specificity tyrosine-phosphorylated and -regulated kinase 1A) in vitro. The results have further emphasized the need for considerable caution in using small-molecule inhibitors of protein kinases to assess the physiological roles of these enzymes. Despite being used widely, many of the compounds that we analysed were too non-specific for useful conclusions to be made, other than to exclude the involvement of particular protein kinases in cellular processes.