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.

SummaryAurora kinases are potential targets for cancer therapy. Previous studies have validated Aurora kinase A as a therapeutic target in multiple myeloma (MM), and have demonstrated in vitro anti‐myeloma effects of small molecule Aurora kinase inhibitors that inhibit both Aurora A and B. This study demonstrated that Aurora B kinase was strongly expressed in myeloma cell lines and primary plasma cells. The selective Aurora B inhibitor AZD1152‐induced apoptotic death in myeloma cell lines at nanomolar concentrations, with a cell cycle phenotype consistent with that reported previously for Aurora B inhibition. In some cases, AZD1152 in combination with dexamethasone showed increased anti‐myeloma activity compared with the use of either agent alone. AZD1152 was active against sorted CD138+ BM plasma cells from myeloma patients but also, as expected, was toxic to CD138− marrow cells from the same patients. In a murine myeloma xenograft model, AZD1152‐inhibited tumour growth at well‐tolerated doses and induced cell death in established tumours, with associated mild, transient leucopenia. AZD1152 shows promise in these preclinical studies as a novel treatment for MM.

BACKGROUND: We have previously found that in multiple myeloma there is amplification of the centrosome, the organelle that nucleates the mitotic spindle. We have reported that RHAMM is a component of the centrosome which interacts with TPX2, the protein that targets Aurora A kinase to the mitotic spindle. We have also shown that RHAMM expression and alternative splicing in myeloma correlate with centrosome amplification, aggressive disease and poor survival. These findings led us to speculate that Aurora A and possibly other Aurora kinases are potential therapeutic targets in myeloma. METHODS: We examined the expression of Aurora A, B and C kinases in 5 myeloma cell lines and autoMACS-purified CD138+ myeloma bone marrow plasma cells from 20 patients. We assessed the anti-proliferative and pro-apoptotic effects of Aurora A knockdown in myeloma cell lines with RNA interference. We investigated the anti-myeloma activity of two potent, selective Aurora kinase inhibitors, VE-465 (Merck/Vertex) and AZD1152 (AstraZeneca), in 5 myeloma cell lines, in CD138+ bone marrow plasma cells from 2 myeloma patients, and in a NOD/SCID murine xenograft model. RESULTS: Aurora A, B and C kinases are ubiquitously expressed in both myeloma cell lines and myeloma bone marrow plasma cells. Expression levels vary among patients. Aurora A and B are expressed in myeloma plasma cells at levels comparable to that seen in the CD138- cells from the same marrow sample, and comparable to the levels seen in normal marrow from control individuals. Aurora C, while expressed at low levels, is consistently ectopically overexpressed in myeloma plasma cells relative to coexisting CD138- cells and normal marrow. In myeloma cell lines, Aurora A knockdown with RNA interference induces apoptosis and cell killing. In all five myeloma cell lines tested, and in myeloma bone marrow plasma cells from two patients, both VE-465 and AZD1152 induce apoptosis and myeloma cell killing at nanomolar concentrations, to varying degrees (20–80% reduction in cell viability). VE-465 is known to inhibit all three Aurora kinases with comparable specificity, while AZD1152 is known to inhibit Aurora B and C more selectively than Aurora A. Despite these differences in activity, both compounds have comparable pre-clinical efficacy against myeloma. Myeloma cell lines treated with either agent demonstrate a phenotype consistent with target inhibition. Both drugs show additive effects on killing of cell lines and primary myeloma cells when combined with dexamethasone, even in dexamethasone-resistant cells. Anti-myeloma activity was seen with single agent Aurora kinase inhibition in the murine model, at well tolerated doses. CONCLUSIONS: Aurora kinases are potential therapeutic targets in myeloma. Aurora kinase inhibitors comprise an emerging class of anti-cancer drug therapy that deserves further evaluation for myeloma patients.

BACKGROUND: The centrosome is the cellular organelle that nucleates the mitotic spindle. Polo-like kinase 1 (Plk1), a centrosome-associated serine/threonine kinase, serves as a key regulator of multiple steps in mitosis. Plk1 is overexpressed in a broad spectrum of tumor types, and its expression often correlates with poor patient prognosis. We have previously observed centrosome amplification in myeloma, and have validated other components of the myeloma centrosome and mitotic apparatus as therapeutic targets. The present study explored the expression of Plk1 in myeloma and the effect of BI2536, a potent and selective inhibitor of Plk1, on myeloma cells in the pre-clinical setting. METHODS: Plk1 expression was assayed in a panel of myeloma cell lines (MMCL) and AutoMACS-purified CD138+ patient bone marrow plasma cells (BMPC). The anti-myeloma effects of BI2536, alone or in combination with conventional agents, were assayed on myeloma cells with proliferation (MTS) and apoptosis (Annexin V/propidium iodide) assays. The phenotype of treated cells was examined with DNA content analysis and immunofluorescence microscopy. The efficacy of BI2536 monotherapy was evaluated in NOD/SCID mice bearing RPMI8226 xenografts. RESULTS: Plk1 is ubiquitously expressed in myeloma to varying degrees in both MMCL and BMPC. BI2536 inhibited the proliferation of MMCL (RPMI 8226, U266, LP-1 and KMS-11) and patient BMPC at nanomolar concentrations. The addition of BI2536 was able to overcome resistance to dexamethasone. Bortezomib in combination with BI2536 had significantly increased anti-myeloma effects compared to the use of either agent alone. BI2536-treated MMCL accumulated 4N DNA content prior to undergoing apoptosis. The phenotype of BI2536-treated cells is consistent with inhibition of Plk1, showing prometaphase arrest and monopolar mitotic spindles in a dose-dependent fashion. BI2536 induces regression of human myeloma xenografts in NOD/SCID mice. Taken together, BI2536 is a promising new agent for the treatment of multiple myeloma. This work provides further evidence that Plk1 and the amplified myeloma centrosome are targets for therapy.