What can we learn about

What can we learn about the anticancer therapeutic efficacy of Aurora kinase inhibitors from our experience with the Bcr-Abl inhibitors and the long term clinical consequence of their use in treating leukemias? As discussed, all Aurora kinase inhibitors developed to date are ATP-competitive inhibitors although their chemical structures are different in each instance. It is, therefore, possible that clinical resistance due to mutations in Aurora kinase genes may arise in patients after prolonged treatment with the inhibitors. This possibility has gained credence from recent identification of mutations in Aurora-B gene in cells exposed to an Aurora kinase inhibitor [109]. In this study, HCT116 colorectal cancer cells developing resistance to ZM447439 after being treated for 3weeks revealed mutations in the form of amino Solithromycin substitution, Y156H, G160E, G160V and H250Y in ATP-binding pocket in the kinase domain of Aurora-B. Indeed, transient expression of Aurora-B allele harboring these mutations resulted in resistance to not only ZM447439 but also VX-680/MK-0457 and AZD1152. Mutation at glycine 216 (G216) in Aurora-A and an equivalent site, G160 in Aurora-B also confers resistance to VX-680/MK-0457 [42]. However, prolonged VX-680/MK-0457 treatment of cells expressing G216L mutant Aurora-A or G160L mutant of Aurora-B showed discriminate results in which Aurora-A mutant cells died in contrast to survival of Aurora-B mutant cells. These observations imply that protecting cells from failure to undergo cytokinesis, in other words, minimizing the extent of aneuploidy is crucial for cell survival. Interestingly, both Aurora-A and -B mutant cells remained sensitive to Aurora-A specific inhibitor MLN8054 although Aurora-B mutant is insensitive to this inhibitor in the in vitro enzymatic assay. These findings deserve consideration in the development of next generation of Aurora kinase inhibitors and signify the relevance of both Aurora-A and -B as anticancer therapeutic targets, which regulate the cellular phenotypes through complex regulatory interactions. The aforementioned results predict that MLN8054 can target VX-680/MK-0457 resistant cancer cells, and Aurora-A may develop mutations at sites different from those of Aurora-B to acquire resistance against the inhibitors. In fact, multiple cancer associated mutations in Aurora-A have been identified, of which, some alter the kinase activity and others create or abolish protein interaction [110], [111], [112]. As mentioned earlier, I31F polymorphism in Aurora-A can activate NF-κB pathway through preferential UBE2N interaction to down-regulate IκBα [28], [113]. Another V57I polymorphism in Aurora-A shows reduced kinase activity and correlates with aneuploidy. The I31F and V57I haplotype is associated with an increase in esophageal cancer risk [111]. V174M mutant Aurora-A is constitutively active due to stabilization of the activation loop whereas S155R mutant of Aurora-A looses interaction with TPX2 resulting in reduced kinase activity as well as mis-localization on mitotic spindle [112]. Moreover, over expression of the kinase inactivated allele of Aurora-A does not interfere with cell proliferation unless its kinase activity is completely lost [114]. To date, the association between Aurora-A mutants and the druggability of Aurora kinase inhibitors in enzymatic assays and in vivo studies remain unclear. In contrast to Aurora-A, no cancer associated mutation of Aurora-B has yet been identified. Taken together, the experimental findings demonstrate that contrary to Aurora-B, whose activity is essential for cell viability, both hyper- and reduced activity mutants of Aurora-A are able to induce chromosome instability, suggesting that Aurora-A may be exerting its oncogenic function through both its kinase activity dependent and independent functions. In this regard, it is significant that Aurora-A protein is essential in MYCN gene amplified neuroblastoma cells in which Aurora-A stabilizes N-Myc protein through a direct interaction in a kinase activity independent manner [33]. In this study, Hesperadin at a concentration sufficient for inhibition of Aurora-A kinase activity failed to destabilize N-Myc. On the other hand, a recent study in pediatric preclinical testing program found that the neuroblastoma cells without MYCN gene amplification remain sensitive to Aurora-A inhibitor MLN8237 thus documenting that N-Myc protein stabilization by Aurora-A is a restricted phenomenon occurring in a minor proportion of neuroblastoma cases [34].