Outcome of Aurora Kinase Inhibition of Acute Myeloid Leukemia by AT9283 Is Dependent upon the Presence or Absence of Mutations in Type 1 Oncogenic Kinase Signalling Pathways.

Recent reports suggest that Aurora kinases (AK) A and B are overexpressed in a proportion of patients with AML and that the level of overexpression correlates with their sensitivity to AK inhibition in vitro. Inhibition of AKB results in mitotic exit in the absence of cell division resulting in polyploidy whilst AKA is responsible for the fidelity of mitotic spindle assembly and phosphorylates p53 at Ser315 leading to its ubiquitination by Mdm2 and subsequent proteolysis. Inhibition of this process increases p53 stability inducing cell-cycle arrest with 4N DNA. Clinical studies of the dual AK inhibitor AT9283 indicate that it is active in the treatment of a proportion of patients with relapsed/refractory AML. In order to explore the molecular basis of this varied sensitivity we investigated the effect of AT9283 in a panel of 10 AML cell lines from a variety of genetic backgrounds including a proportion driven by ras, Flt3 or c-kit mutations. Two phenotypes were observed;

1. Accumulation of cells in the G2/M phase (4N) of the cell cycle followed by apoptosis, or

2. Accumulation of cells with >4N DNA (polyploid) followed by apoptosis.

Cell lines that exhibit the former phenotype were those driven by mutation in an oncogenic kinase such as ras, c-kit or Flt3. Further analysis of Cyclin B levels suggest that profile 1) results from a G2 block occurring as a consequence of a dominant AKA inhibitory effect in these cell lines. Profile 2) results from AKB inhibition; where cells continue to undergo rounds of DNA replication in the absence of cell division. Inhibition of both of these signaling pathways has been confirmed in AML blasts taken from patients treated with AT9283 in the ongoing clinical program. These results suggest that AT9283 triggers the mitotic checkpoint and induces apoptosis in patients harboring mutations in FLT3 or c-kit via AKA inhibition. These AML cases have been shown to be more likely to exhibit normal cytogenetics and this profile may be important in sustaining rapid peripheral blast proliferation typical of this subtype of AML. Cell lines that respond to treatment with AT9283 by becoming polyploid may be manifesting the effect of predominant AKB inhibition. The balance between these outcomes may reflect the corresponding levels of expression of AKA and AKB in individual subtypes of AML along with factors such as TP53 mutation being associated with genomic instability supporting the development of complex karyotypic abnormalities. Such preliminary findings indicate that patients with mutations in oncogenic signaling pathways may be particularly sensitive to treatment with AT9283 due to the presence of an intact mitotic checkpoint and a dominant Aurora A inhibitory phenotype. This hypothesis is currently being explored through the analysis of biological studies obtained from the ongoing clinical program.