Cell cycle checkpoints protect the fidelity of DNA replication and division and ensure the correct ordering of cell cycle events. Once the information encoded in DNA is lost, it cannot be replaced, therefore these pathways are vital for maintaining genomic integrity and preventing carcinogenesis 
. There are several checkpoints regulating cell cycle progression: those that are activated during the G1-, S- or G2-phase of the cell cycle in response to DNA damage. This DNA damage can arise either as a result of endogenous stimuli or through external mechanisms (including genotoxic stress or chemotherapeutic drugs). In addition, a second kind of checkpoint, here termed the mitotic spindle checkpoint, is activated during every cell cycle and only silenced once all chromosomes are properly attached to a bipolar spindle and ensures accurate chromosome segregation and protects against aneuploidy.
DNA damaging agents, such as cisplatin, carboplatin, irinotecan and doxorubicin, along with ionizing radiation are the mainstays of cancer therapy. While they have different mechanisms of action, they all directly or indirectly induce DNA damage thereby activating DNA damage checkpoints and induce cell cycle arrest in G1, S, or at the G2-M transition. In mammalian cells, the key effector proteins are p53 and the checkpoint kinases Chk1 and Chk2. A large proportion of human cancers exhibit dysregulation of p53 function (e.g. by p53 mutation or loss of p53 expression) and therefore are unable to activate transcription of the CDK inhibitor, p21, which is required for arrest in G1. These human tumors are thought to be highly reliant on the Chk kinases to protect them in response to DNA damaging insults 
. Chk1 is required for the signal evoked by damaged DNA to prevent entry into mitosis; it is widely assumed that Chk1 inhibitors kill cells by overriding this constraint allowing entry into a lethal mitosis.
Damage sensors that recognize double strand breaks or protein complexes that recognize replication stress activate the transducing kinases ATM and ATR. In turn, these kinases directly activate the effector kinases Chk1 and Chk2. Chk1 and Chk2 negatively regulate the Cdc25 family of phosphatases thereby preventing cell cycle progression as well as directly modulating repair proteins resulting in increased lesion repair. Chk1 appears to be the crucial effector kinase as both biochemical and genetic studies have demonstrated it to be indispensible for the S and G2/M checkpoints 
. Chk1 inhibition, therefore, represents a novel therapeutic strategy to increase the lethality of DNA-damaging chemotherapeutic drugs in p53 pathway defective cancers. Abrogation of the remaining intact checkpoint should result in increased tumor cell death. This “synthetic lethality” approach should increase the therapeutic index of chemotherapeutic drug as normal cells remain protected by their functional p53 pathway. This approach has started to be tested clinically with small molecule inhibitors of Chk1 (AZD7762, PF-477736, XL844 and SCH900776) currently undergoing Phase I clinical evaluation in combination with gemcitabine, irinotecan and cytarabine 
. Recent work has suggested that Chk1 may also be required for the normal operation of the spindle assembly checkpoint 
, which may account for the ability of the Chk1 inhibitor PF-477736 to potentiate the efficacy of docetaxel in xenografts 
Spindle checkpoint function and thus accurate mitosis relies on the Mad proteins Mad1, Mad2 and BubR1, the Bub proteins Bub1 and Bub3, the mitotic kinases Aurora A and Aurora B, as well as Chk1 
. Several antimitotic drugs including the taxanes and the vinca
alkaloids, via their effects on microtubules, prevent the formation of a normal mitotic spindle, resulting in spindle checkpoint activation. These agents impose mitotic arrest, usually leading to apoptosis either in mitosis or, more often, in the post-mitotic G1-phase following mitotic escape 
The Aurora family of Ser/Thr kinases consists of three members designated Aurora A, B and C, all of which play a role in mitotic progression 
. All three Aurora kinases are implicated in cancer development and progression, and their overexpression is common in a wide variety of human cancers 
. Aurora kinases have become popular targets for cancer drug discovery with at least thirteen small molecule inhibitors currently in Phase I and II clinical evaluation 
. Two of the molecules that have demonstrated the potential of this approach are VX680 (MK-0457, Vertex/Merck) and AZD1152 (AstraZeneca). Inhibiting Aurora B results in premature exit from mitosis, failed cytokinesis followed by induction of reduplication (that is the formation of cells with a >4N DNA content). Histone H3 phosphorylation is a widely used biomarker of Aurora B activity 
. Through their ability to induce mitotic checkpoint malfunction, Aurora kinase inhibitors synergize with agents that target the mitotic spindle, such as paclitaxel and nocodazole 
Using fragment screening, structure guided drug design and kinase cross screening we have identified VER-150548, a potent small molecule inhibitor of both Chk1 and Chk2, and Aurora A and Aurora B kinases. Here we demonstrate that in unperturbed cells, VER-150548 induced a cellular phenotype consistent with Aurora kinase inhibition but in the presence of DNA damage, a Chk1 inhibitor phenotype. We have therefore utilised VER-150548 as a useful chemical probe to further understand the interplay between these two signalling pathways and the temporal factor that determines the predominant cellular signalling pathway.