Recent advances in the development of Aurora kinases inhibitors in hematological malignancies

Abstract

Over the last two decades, since the discovery of Drosophila mutants in 1995, much effort has been made to understand Aurora kinase biology. Three mammalian subtypes have been identified thus far which include the Aurora A, B and C kinases. These regulatory proteins specifically work at the cytoskeleton and chromosomal structures between the kinetochores and have vital functions in the early phases of the mitotic cell cycle. Today, there are multiple phase I and phase II clinical trials as well as numerous preclinical studies taking place looking at Aurora kinase inhibitors in both hematologic and solid malignancies. This review focuses on the preclinical and clinical development of Aurora kinase inhibitors in hematological malignancy and discusses their therapeutic potential.

Introduction

Mitosis is the complex biological process by which chromosomes in a cell nucleus are separated into two daughter cells after a completion of a cell cycle. Since the survival of a cell depends on the accuracy of mitosis, multiple checkpoint systems have evolved to ensure the correct coordination of this process. Errors in these mechanisms can lead to genomic instability, which is an important aspect of tumorigenesis [Keen and Taylor, 2004]. It is not surprising that many cancer cells result after an imbalance in a vital regulatory protein at a given cell cycle checkpoint. Several of these regulatory proteins include mitotic kinases, including the Aurora kinase family [Kitzen et al. 2010].

The Aurora kinase family originally emerged in 1995 from a Drosophila phenotypic screen for defects in mitotic spindle function. Aurora mutants were so named because of the similarity of their disordered mitotic spindles to the Aurora Borealis, or Northern lights [Sausville, 2004], a natural phenomenon in the polar regions. Yeasts have a single Aurora kinase, whereas mammals have three: Aurora A, B and C. The regulatory proteins, Aurora kinases specifically work at the cytoskeleton and chromosomal structures between the kinetochores [Carmena and Earnshaw, 2003].

Aurora kinase A

Also known as the polar Aurora, it associates with the centrosomes that are separating during late S/early G2 phases (Figure 1). The Aurora A gene lays within a region of chromosome 20q13, which is amplified in many tumors. Regulation of Aurora A is complex and involves both phosphorylation/dephosphorylation and degradation. Furthermore, overexpression of an active mutant of Aurora A in rat1 cells induced neoplastic transformation, indicating that Aurora A is an oncogene [Kitzen et al. 2010]. The oncogenic potential of Aurora A probably results from two distinct functions of the kinase: (1) chromosome segregation as well as control of genomic stability and (2) regulation of entry into mitosis [Warner et al. 2003]. Aurora B inhibition phenotype is dominant because it prevents mitotic checkpoint arrest caused by Aurora A kinase inhibition (Figure 1); therefore, overall Aurora A expression may not be a significant factor governing response in cases in which pan-inhibition may potentially occur [Hilton and Shapiro, 2014].

Figure 1.

(A) Aurora A kinase concentrations increase during mid-prophase and the kinase associates with the mitotic poles and adjacent microtubules, which is critical for the establishment of the microtubule spindle and centrosome duplication. (B) Aurora B kinase, a part of the chromosomal passenger complex (CPC), works at the level of the centromere ensuring proper chromosomal alignment which is required for microtubule stabilization.

Aurora kinase B

Human Aurora B was initially identified in a polymerase chain reaction screen for kinases that were overexpressed in tumors. It is considered to be a chromosomal passenger protein, which is a necessity for a number of processes during mitosis. Aurora B expression and activity in proliferating tissues are cell cycle regulated: expression peaks at the G2–M transition, and kinase activity is maximal during mitosis [Bischoff et al. 1998]. Aurora B is also regulated in a similar fashion to Aurora A, but also involves two additional proteins which make up the chromosomal passenger complex (CPC). Inner centromere protein and survivin function to target the kinase, and the movement of the passenger complex from centromeres, to central spindle, to midbody, which presumably reflects movement of the kinase to act on different substrates [Carmena and Earnshaw, 2003].

Aurora kinase C

The Aurora C gene has been identified only in testis and has been located within a region of chromosome 19q13. It was initially thought to be involved in meiotic spindle formation and its localization was restricted to centrosomes from anaphase through to cytokinesis [Kimura et al. 1998]. Like the Aurora B, the Aurora C is also similarly activated. Very little is known about its functionality.

Aurora kinase inhibitors and hematologic malignancies

Over the last decade or so, there has been significant interest in development of Aurora kinases as a therapeutic target for cancer. All three of the Aurora kinases identified in humans, A, B and C, are affected in different cancer types. The overexpression of these kinases has been detected in many solid and hematologic malignancies and thus they have become targets of many small molecule therapies (Table 1).

Table 1.

Aurora kinase inhibitor’s in clinical investigation.

Drug	Compound	Manufacturer	Aurora A, B and/or C inhibitor	Diseases in which the drug is being investigated	Reported DLT
Alisertib		Millennium Pharmaceuticals	A	MM, NHL, CML, AML and solid tumors	Mucositis, neutropenia, alopecia, elevated LFTs
AT-9283		Astex Pharmaceuticals	A, B	CML, AML and solid tumors	TLS, infection, HTN
Barasertib		AstraZeneca	B	AML and solid tumors	Neutropenia, stomatitis
ENMD-2076		EntreMed Inc.	A	MM, AML and solid tumors	HTN, nausea/vomiting, fatigue
MK-5108		Vertex Pharmaceuticals	A	NHL and solid tumors	Neutropenia
MSC 1992371A		Rigel Pharmaceuticals	A, B, C	AML and CML	Neutropenia, infection, mucositis, diarrhea
Danusertib		Nerviano Pharmaceuticals	A, B, C	MM, CML and solid tumors	Neutropenia, HTN, diarrhea, anorexia, fatigue, alopecia
Tozasertib		Merck and Vertex Pharmaceuticals	A, B, C	MM, AML and solid tumors	Diarrhea, constipation, nausea, vomiting, stomatitis

DLT, dose-limiting toxicity; MM, multiple myeloma; AML, acute myeloid leukemia; CML, chronic myelogenous myeloma; LFT, liver function test; TLS, tumor lysis syndrome; HTN, hypertension; NHL, non-Hodgkin’s lymphoma.

MLN8237

Manufactured as Alisertib by Millennium Pharmaceuticals, MLN8237 is a second-generation, orally bioavailable, highly selective small molecule which inhibits the serine/threonine protein kinase Aurora kinase A. Alisertib treated cells show defects of alignment during metaphase, slowly moving chromosomes in anaphase, and chromatin bridges during telophase, resulting in aneuploidy [Hoar et al. 2007].

Preclinical data

Gorgun and colleagues demonstrated in vitro and in vivo activity in multiple myeloma (MM) cells as well as a xenograft murine model. Treatment of cultured MM cells resulted in mitotic spindle abnormalities, mitotic accumulation, as well as inhibition of cell proliferation through apoptosis and senescence. Combined with dexamethasone, doxorubicin or bortezomib, synergistic/additive anti-myeloma activity was observed. Tumor burden was significantly reduced (p = 0.007) and overall survival was significantly increased (p < 0.005) in animals treated with 30 mg/kg alisertib for 21 days [Gorgun et al. 2010].

Laboratory investigations also supported testing alisertib in non-Hodgkin lymphoma (NHL). Qi et al. demonstrated that the majority of mantle cell lymphoma (MCL) specimens had high levels of Aurora expression. Aurora A and B were also found elevated in 13 aggressive B-cell NHL lines. The association of increased expression of both Aurora A and B genes with reduced survival may be due to their role in more rapid tumor cell growth in MCL and correlates well with decreased survival in MCL. Combination studies suggested that the combination of alisertib and docetaxel enhanced apoptosis by ~3-4-fold in cell culture compared to single agents [Qi et al. 2011]. Mahadevan and colleagues observed synthetic lethality with the addition of rituximab to alisertib and vincristine in diffuse large B-cell lines [Mahadevan et al. 2012]. Further, the combination prevented tumor relapse in a murine model of Myc and Bcl-2 overexpressing lymphoma [Mahadevan et al. 2014]. In addition to the results in B-cell lymphoma, Qi and colleagues also found that treatment with alisertib inhibited murine T-cell lines by inducing endo-reduplication in a dose- and time-dependent manner. They postulated this effect was in part due to inhibition of Aurora B in addition to Aurora A [Qi et al. 2013].

Also in 2011, Kelly and colleagues evaluated alisertib in chronic myelogenous leukemia (CML) and concluded that MLN8237 possessed equipotent activity against Ba/F3 cells and primary CML cells expressing unmutated and mutated forms of breakpoint cluster region-Abelson kinase (BCR-ABL). In addition, inhibition of Aurora A with MLN8237 significantly increased the in vitro and in vivo efficacy of nilotinib. Targeted knockdown of Apollon, an apopotic protein, sensitized CML cells to nilotinib-induced apoptosis, indicating that this is an important factor underlying alisertib’s ability to increase the efficacy of nilotinib [Kelly et al. 2011].

Clinical data

Phase I evaluations, by Kelly and colleagues, demonstrated dose-limiting myelosuppression after sequential cohorts of patients received alisertib, oral powder form, 25–90 mg for 14 or 21 consecutive days plus 14 or 7 days’ rest, or at a starting dose of 40 mg once daily for 14 days plus 14 days’ rest, all in 28-day cycles. Subsequent cohorts received enteric coated tablets of alisertib, 30–50 mg twice daily for 7 days plus 14 days’ rest in 21 day cycles. A similar reported side effect of myelosuppression was reported in the study by Venkatakrishanan and colleagues in the East Asian population. Additional commonly observed toxicities included; grade 3 stomatitis, and somnolence which was related to gamma aminobutyric acid A alpha-1 benzodiazepine receptor binding, was initially observed and ultimately improved with twice daily dosing. These studies recommended phase II dosing of alisertib with 50 mg BID for 7 days in 21-day cycles [Kelly et al. 2011; Venkatakrishanan et al. 2015].

Given the upregulation of Aurora A in aggressive lymphomas and the supporting efficacy in vitro, Friedberg and colleagues treated 48 patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL), mantle-cell lymphoma (MCL), transformed follicular lymphoma, Burkitt’s lymphoma, and peripheral T-cell lymphoma with alisertib, administered at the recommended phase II dose. The overall response rate was 27%, including responses in 3 of 21 patients with DLBCL, 3 of 13 with MCL, 1 of 1 with Burkitt’s lymphoma, 2 of 5 with transformed follicular lymphoma, and 4 of 8 with T-cell lymphoma [Friedberg et al. 2014]. The most common grade 3–4 adverse events reported were neutropenia in over half of the patients, anemia and thrombocytopenia in a third of the patients and a few with stomatitis, febrile neutropenia and fatigue. The toxicity profile was similar to that reported in early-phase trials of patients with solid tumors, in which neutropenia and stomatitis were the most commonly observed toxicities with a similar dose and schedule [Cervantes et al. 2012; Dees et al. 2012].

Further development of alisertib proceeded in B- and T-cell lymphomas based on responses observed across the lymphoma histologies. A phase I combination of alisertib with rituximab and vincristine focused on aggressive B-cell lymphomas attempting to replicate the notable laboratory efficacy. Of 35 patients enrolled, 28 had DLBCL, 1 transformed follicular, 1 Burkitt and 1 mantle cell lymphoma. Alisertib administered at 50 mg twice daily for 7 days was well tolerated with rituximab. With the addition of vincristine, the dose was lowered to 40 mg twice daily without any subsequent new treatment emergent toxicities. The dose limiting toxicities continued to be largely related to myelosuppression and the primary reason for study discontinuation was disease progression. An interim analysis demonstrated that of 12 patients receiving alisertib with rituximab, 3 clinical responses occurred. Of 8 patients receiving the triplet, 3 additional responses were reported. As such, the combination appears well tolerated and may warrant further phase 2 testing given the early activity in relapsed DLBCL [Kelly et al. 2014]. Early data from a phase II trial investigating the addition of rituximab to alisertib in this heavily pre-treated population revealed significant grade 3 neutropenia, fatigue, mucositis as well as a few cases of grade 3 cystitis, pancreatitis, and pneumonitis. However the combination was overall reported to be well tolerated with one responding patient with refractory CD5+ DLBCL able to proceed to allogeneic transplant [Kami et al. 2014].

Single-agent development of alisertib has proceeded as well in T-cell lymphomas. A phase II study again suggested alisertib to be well-tolerated in this population with myelosuppression accounting for the majority of grade 3 and 4 adverse events. These toxicities included neutropenia, anemia and thrombocytopenia in one third of the patients. A total of 30% of peripheral T-cell lymphoma (PTCL) patients achieved an objective response, including patients with Peripheral T cell Lymphoma (PTCL) not other specified, angioimmunoblastic, anaplastic large cell and adult T-cell lymphomas [Barr et al. 2015]. Given these results, a global phase III trial randomizing patients to alisertib or investigator’s choice of romidepsin, gemcitibine or pralatrexate was developed had recently completed accrual [ClinicalTrials.gov identifier: NCT01482962]. A recent announcement from the sponsor stated that the primary endpoint is unlikely to be met, suggesting alisertib did not provide a superior progression-free survival over standard of care. Combination studies are developing alisertib and histone deacetylase inhibitor doublets given notable synergy specific to T-cell lines [Zullo et al. 2015]. Based on this, two phase I trials are combining alisertib with romidepsin [Fanale et al. 2014] and vorinostat [Siddiqi et al. 2015], respectively.

Clinical investigation in myeloid malignancies has occurred as well. An exploratory phase II study was done with alisertib in acute myeloid leukemia (AML). In this study, the researchers looked at 57 patients with AML or high-grade myelodysplastic syndrome that received alisertib 50 mg BID for 7 days in 21-day cycles. Only one complete response lasting over a year was observed. No responses were observed in MDS patients. Adverse events which were seen in more than a third of the study patients included diarrhea, fatigue, nausea, febrile neutropenia, and stomatitis [Goldberg et al. 2014].

Even more recently, interim data for an ongoing phase I study examining the outcome of using Alisertib in combination with 7 + 3 induction chemotherapy in patients with AML, showed that 13 of 14 treated patients had demonstrated marrow ablation at the mid-induction point of therapy, and none required re-induction with 5 + 2. Delayed thrombocytopenia has been the dose-limiting toxicity reported thus far [Fathi et al. 2014].

AT-9283

Agents targeting both Aurora A and B are being developed as well. A multi-kinase inhibitor manufactured by Astex Pharmaceuticals, AT9283 inhibits Aurora kinases A and B, JAK2 (Janus kinase 2) and the kinase BCR-ABL, which has shown subsequent inhibition of cellular division and proliferation and the induction of apoptosis in tumor cells that overexpress these kinases.

Preclinical data

Dawson and colleagues studied AT9283 in murine leukemia models and found that the drug potently inhibited proliferation and Jak2-related signaling in Jak2-dependent cell lines as well as inhibiting the formation of erythroid colonies from hematopoietic progenitors isolated from myeloproliferative disease patients with Jak2 mutations. They also observed that inhibition of both Jak2 and Aurora B was observed in the model systems used, indicating a dual mechanism of action [Dawson et al. 2010].

The in vitro anti-myeloma activity of AT9283 alone and in combination with lenalidomide and the in vivo efficacy by using a xenograft mouse model of human MM was studied by a group of researchers out of Harvard Medical Center in Boston. They found that AT9283 induced cell-growth inhibition and apoptosis in MM. Features consistent with both Aurora kinase A and Aurora kinase B inhibition we observed. In vivo studies showed decreased MM cell growth and prolonged survival in AT9283-treated mice compared with controls [Santo et al. 2011].

Similarly as described with alisertib, cells treated with AT9283 exhibited endo-reduplication confirming the mechanism of action of an Aurora B inhibitor. Also, treatment of NHL-B cell lines with AT9283 induced apoptosis in a dose- and time-dependent manner and inhibited cell proliferation [Qi et al. 2012].

Clinical data

A phase I study identified the maximum tolerated dose of AT9283 as 324 mg/m²/72 h after researchers administered a continuous 72-hour infusion every 21 days. Doses were escalated by a standard 3 + 3 design. A high incidence atrial fibrillation was observed, often during severe infections such as pneumonia, and during electrolyte disturbance but not during AT9283 infusion, so the relationship between these events and AT9283 treatment was unclear [Foran et al. 2014].

AT9283 is also being investigated in a phase II setting in chemotherapy refractory, MM patient population in a trial being sponsored by the NCIC Clinical Trials Group in Canada.

AZD1152

AZD1152, also known as Barasertib, is an orally bioavailable AstraZeneca compound which has been shown to be a highly specific inhibitor of Aurora B.

Preclinical data

Walsby and colleagues [Walsbyet al. 2008] assessed Aurora kinase gene expression profiles, including the investigation of Barasertib, in 101 tumor cell samples from patients with AML. Barasertib was investigated for its in vitro effects on cell viability, histone H3 phosphorylation, cell cycle and morphology in AML cell lines and primary AML samples. The drug was found to induce growth arrest and the accumulation of hyperploid cells in AML cell lines and primary AML cultures. Furthermore, it inhibited histone H3 phosphorylation and ultimately resulting in the induction of apoptosis [Tsuboi et al. 2011]. Oke and colleagues also confirmed Barasertib to inhibit Aurora B activity in AML cell lines and in some human primary leukemic cells, as evidenced by complete inhibition of His H3 phosphorylation at submicromolar concentrations via in vitro studies [Oke et al. 2009].

Clinical data

In a phase I/II study to determine the maximum tolerated dose of Barasertib in patients with newly diagnosed or relapsed AML the maximum tolerated dose was reported to be 1200 mg daily. Part A of the study administered as a continuous 7-day infusion every 21 days to evaluate the aforementioned maximum tolerated dose and part B was conducted to evaluate the efficacy of the drug. The most commonly reported grade ⩾3 events were febrile neutropenia and stomatitis/mucosal inflammation [Lowenberg et al. 2011].

A second phase I study conducted in Japan also studied the safety, efficacy and pharmacokinetic profile of Barasertib in patients with advanced AML. Barasertib (50–1200 mg) was administered as a continuous 7-day intravenous infusion every 21 days. No dose-limiting toxicities were reported and 1200 mg was also reported as the highest planned dose to be studied. Neutropenia and febrile neutropenia were the most commonly reported adverse events as well [Tsuboi et al. 2011].

ENMD-2076

Developed by EntreMed Inc., ENMD-2076 is an orally active Aurora A kinase inhibitor proven to have potent activity with Aurora A and multiple tyrosine kinases linked to cancer and inflammatory diseases. It has also shown activity against FLT3, c-KIT, c-FMS and VEGFR-2 and -3. The drug exerts its effects through multiple mechanisms of action, including anti-proliferative activity and the inhibition of angiogenesis. ENDMD-2076 exhibits favorable pharmacokinetic profiles and has activity in patients with AML and MM [How and Yee, 2012].

Preclinical data

Wang and colleagues investigated the activity of ENMD-2076 against MM cells in vitro and in vivo. The drug inhibited the phosphoinositide 3-kinase/AKT pathway and downregulated survivin and X-linked inhibitor of apoptosis as early as 6 hours post-therapy. With longer treatment, 24–48 hours, ENMD-2076 also inhibited Aurora A and B kinases, and induced G2/M cell cycle arrest. Immunohistochemical staining of excised tumors showed significant reduction in phospho-Histone 3 (pH3), Ki-67, and angiogenesis, and also a significant increase in cleaved caspase-3 at all dose levels compared to tumors from vehicle-treated mice [Wang et al. 2010].

ENMD-2076 has also been observed to be toxic to AML cells in vitro by inhibiting cell growth and inducing apoptosis, via activation of Caspase pathway and regulation of pro-apoptotic and anti-apoptotic proteins [Cao et al. 2012].

Clinical data

Interim results from an ongoing phase I study with ENMD-2076 in relapsed or refractory MM patients were presented in 2010. Data showed that daily oral administration of ENMD-2076 was seemingly well tolerated. No dose-limiting toxicities were observed. Of the nine evaluable patients, three patients had stable disease and one patient achieved a partial response.

In addition, results for the ENMD-2076 phase I study in patients with relapsed or refractory leukemia was presented by Yee and colleagues. Data for 20 evaluable patients demonstrated that daily oral administration of ENMD-2076 was associated with reductions in bone marrow blast counts of 11%, 14%, 23%, and 65%. One patient achieved a CRi (complete remission with incomplete hematological recovery) and three patients achieved a morphologic leukemia-free state (MLFS).

Though phase I studies for ENMD-2076 are ongoing in the setting of hematological malignancies, the drug has completed a phase I study in patients with solid tumors and is currently in a multicenter phase II study in ovarian cancer. Dose levels of 60, 80, 120, 200, and 160 mg/m² were evaluated. Two patients experienced grade 3 hypertension at 200 mg/m², and additional grade 3 neutropenia events limited tolerability at this dose. An intermediate dose of 160 mg/m² was determined to be the maximum tolerated dose. The most common drug-related adverse events included hypertension, nausea/vomiting, and fatigue [Diamond et al. 2011].

MK-5108

MK-5108, developed by Vertex Pharmaceuticals, is a novel small molecule of potent inhibitory activity against Aurora-A.

Preclinical data

In 2011, Kretzner and colleagues studied MK-5108 studied growth and survival of multiple lymphomas cell lines either the drug alone or in combination with vorinostat, a histone deacetylase inhibitor. Both drugs were found to strongly inhibit cell cycle effects on both Hodgkin’s lymphoma and non-Hodgkin’s lymphoma cells in in vitro studies [Kretzner et al. 2011].

Clinical data

Although there is no phase I trial completed for MK-5108 in hematological malignancies, the drug was tested in patients with advanced solid tumors either as a single agent or in combination with docetaxel. Febrile neutropenia and myelotoxicity were reported as DLTs in the combination treatment regimen. However, no significant toxicities were reported in the monotherapy arm [Minton et al. 2010].

MSC 1992371A

MSC1992371A, also known as R763 or AS703569, owned by Rigel Pharmaceuticals, is a potent adenine triphosphate-competitive inhibitor of Aurora kinase A, B and C. Its mechanism of action is via disrupting mitotic spindle activity, blocking cell separation, and leading to polyploidy and cell death [Mita et al. 2014].

Preclinical data

Preclinically, MSC1992371A has demonstrated potent antitumor activity as single agent and in combination treatment in leukemia cell lines, freshly isolated leukemia cells, and leukemia xenograft models. At low nanomolar concentrations, MSC1992371A also inhibits other kinases involved in cell survival and proliferation including FLT3, BCR-ABL1, and BCR-ABL1 with T315I mutation. It also inhibits JAK2 kinase, but at higher concentrations [McLaughlin et al. 2010].

Clinical data

In 2013, Graux and colleagues conducted a phase I dose-escalation trial in advanced hematological malignancies. Patients received escalating doses either on days 1–3 and 8–10 or on days 1–6 of a 21-day cycle. The maximum tolerated doses were 37 and 28 mg/m²/day, respectively. Dose-limiting toxicities included severe neutropenia with infection and sepsis, mucositis/stomatitis, and diarrhea. Complete responses occurred in three patients. The use of the agent was limited by a very narrow therapeutic window as the doses at which signs of clinical benefit were seen in the dose-escalation part of the trial resulted in unacceptable toxicity in the expansion cohorts. Less-toxic doses needed in the disease-specific cohorts lacked efficacy and in many cases could not prevent early progression. Despite the limited therapeutic window, the maximum tolerated dose was determined as 37 mg/m²/day MSC1992371A with intermittent 3-day dosing and 28 mg/m²/day during 6-day continuous dosing [Graux et al. 2013].

PHA-739358

PHA-739358 also known as Danusertib, the drug produced by Nerviano Medical Sciences, is a pan-Aurora kinase inhibitor that features a pyrrolopyrazole scaffold which had previously been identified as an ATP-mimetic pharmacophore suited for kinase binding. Danusertib also inhibits several receptor tyrosine kinases such as Abl, Ret, FGFR-1 and TrkA. IT has also been reported to have some synergistic qualities while being used in combination with Imatinib in CML patients [Fancelli et al. 2005].

Preclinical data

Danusertib has been shown to inhibit all Aurora kinase family members and show a dominant Aurora B kinase inhibition-related cellular phenotype and mechanism of action in cells in vitro and in vivo. Balabanov and colleagues treated K562 cells with Danusertib at concentrations ranging from 0.01 to 5 µM for 24 hours and analyzed changes in the downstream target of BCR-ABL and Aurora kinase. Starting at low concentrations, a dose-dependent inhibition of BCR-ABL activity was observed, whereas inhibition of Aurora kinase activity required higher concentrations, pointing to a therapeutic window between the two effects [Balabanov et al. 2011].

Preclinical efficacy and toxicity studies were have also been performed in nude mice transplanted with several human xenografts, using maximum tolerated doses of 60 mg/kg/day for 5 days, 30 mg/kg/day for 10 days, or 45 mg/kg/day for 10 days. Only mild weight loss and myelosuppression was noted in that particular study. Danusertib has also been tested in a rat model having 9,10-dimethylbenz-A-anthracene (DMBA)-induced mammary carcinomas. At 25 mg/kg, TGI was measured as 75% and a complete cure was achieved in one rat [Kollareddy et al. 2012].

Clinical data

In the setting of hematological malignancies, a phase I trial evaluating the anti-tumor activity of Danusertib in MM patients who had a history of at least two previous lines of treatment for the disease was recently terminated due to low accrual rates. However, in the solid oncologic setting the drug has been shown to have favorable outcomes in multiple cancers and most frequent drug-related non-laboratory adverse events were fatigue, asthenia, nausea, diarrhea, anorexia, vomiting, alopecia, constipation and pyrexia. And, laboratory adverse events included hematological toxicity, hypoalbuminemia and transaminitis [Schöffski et al. 2015].

VX-680

VX-680, also known as MK0457, VE465 and Tozasertib and owned by Merck and Vertex Pharmaceuticals, is a pan-Aurora kinase inhibitor that disrupts mitosis and induces apoptosis in cycling cells and has potent preclinical efficacy in a broad spectrum of leukemias, lymphomas and solid tumors.

Preclinical data

Preclinically, tumor regression has been achieved with multiple solid tumor types as well as AML. These data have suggested profound inhibition of tumor growth was achieved with VX-680 at well-tolerated doses, and no signs of mechanism-independent toxicity were observed [Harrington et al. 2004; Bebbington et al. 2009; Karthigeyan et al. 2011].

Tozasertib has also been reported to have apoptosis-inducing synergy in combination with Vorinostat in multiple lymphoma cell lines [Okabe et al. 2009; Kretzner et al. 2011; Nguyen et al. 2011] and remains an area of interest of future clinical exploration.

Clinical data

In a phase I/II dose-escalation study, 77 patients with refractory AML were treated. During the phase I portion, 24 and 40 mg/m²/h doses were given as 5-day continuous IV infusion versus a 24-hour infusion. In patients receiving the 5-day regimen, the maximum tolerated dose was determined to be 40 mg/m²/h while the maximum tolerated dose was determined to be at least 144 mg/m²/h. Infections, most commonly pneumonia and cellulitis, were the most commonly observed adverse event. Other adverse events included gastrointestinal related side effects such as diarrhea, nausea, vomiting, constipation, stomatitis and abdominal pain [Giles et al. 2013].

Other Aurora kinase inhibitors under investigation

BI 811283

BI 811283 is a small molecule drug that selectively binds to the ATP binding pocket of the Aurora B Kinase protein, inhibiting its function in cell division [Gürtler et al. 2010]. BI 811283 is developed by Boehringer Ingelheim for use as an antineoplastic therapy and is in the early stages of clinical development along with undergoing some of the first in human trials in patients with AML.

GSK 1070916

GlaxoSmithKline’s GSK1070916 is an ATP competitive drug which binds to and inhibits the activity of Aurora kinases B and C, ultimately resulting in inhibition of cellular division and a decrease in the proliferation of tumor cells that overexpress the Aurora kinases B and C. Antitumor effects have so far been observed in 10 human tumor xenograft models including breast, colon, lung and two leukemia models [Hardwicke et al. 2009; Adams et al. 2010; Medina et al. 2010].

PHA-680632

Developed by Nerviano Medical Sciences, PHA-680632 is a potent inhibitor of Aurora A, Aurora B and Aurora C kinases. The drug has shown to be active on a wide range of cancer cell lines and shows significant tumor growth inhibition in different animal tumor models at well-tolerated doses [Soncini et al. 2006]. Preclinical treatment of cells with anti-Aurora A small interfering RNA (siRNA), but not anti-Aurora B siRNA induced accumulation of active caspase 9 and 3, which is an indicative of predominant Aurora A inhibition-related apoptosis [Tao et al. 2007].

More recently, Mazzera and colleagues demonstrated that Aurora kinases physically and functionally interact with the key regulators of biological pathways in MM. The pharmacological blockade of Aurora kinase activity induces TRAIL, (also known as Apo2L) which is a member of the tumor necrosis factor family of death receptor ligands, sensitization in MM because it retracts the TRAIL-induced activation of nuclear factor κB thereby destroying the cancer cells survival pathway [Mazzera et al. 2013].

Mechanisms of Aurora kinase inhibitor resistance

Although targeted therapies have largely impacted the oncologic realm over the last decade, proving to have more tolerable side effects when compared with chemotherapy, primary drug resistance still remains a major obstacle.

Although very little is known about drug resistance of Aurora kinase inhibitors in the hematologic malignancy setting, several studies have proposed mechanisms in multiple solid tumors. One study reported that colon and pancreatic carcinoma cell lines became resistant to AZD1152 over the course of 3 months exposure. Liquid chromatography–mass spectrometry analysis showed decreased drug accumulation in cytoplasm of these resistant cells [Guo et al. 2009]. Another study in breast cancer cell lines found that five, point mutations, in an Aurora kinase B inhibitor lead to implications in drug binding [Girdler et al. 2008].

Hilton and Shapiro reported Aurora kinase expression did not correlate with any clinical benefit nor resistance. Instead, overall response to a specific therapeutic agent, with either Aurora A, B or a pan-Aurora kinase inhibitor, is likely codependent on the length of mitotic arrest and the activation of apoptotic pathways, both factors that are independent of Aurora A kinase expression [Hilton and Shapiro, 2014].

Conclusion

Over the last two decades, and since the discovery of Drosophila mutants, much effort has been made to understand Aurora kinase biology. These enzymes have vital functions in the early phases of the mitotic cell cycle. Current research has demonstrated that over expression of these kinases have proven to be valuable in targeted therapy development and disease stabilization. Today, there are multiple phase I and II clinical trials as well as numerous preclinical studies taking place looking at Aurora kinase inhibitors in both hematologic and solid malignancies.

In early clinical experience the principal toxicity observed has been myelosuppression; with neutropenia constituting the most frequent dose-limiting toxicity though these targeted agents still have a more favorable side-effect profile when compared to the conventional chemotherapy regimens. Given early efficacy demonstrated with alisertib in NHL, ongoing randomized clinical trials will further define its therapeutic role, most notably in PTCL.

Further topics of research interest are the general mechanisms of tumor cell resistance to Aurora kinase inhibitors, as none have yet been identified, as well as the effects of combining other cytotoxic agents with Aurora kinase inhibitors. Knowing the drug resistance patterns to AKIs and synergistic qualities of these drugs when used in combination with standard chemotherapy would help us achieve more powerful antitumor strategies in the future.