ATR (“ATM and Rad3 related”) kinase is a protein kinase involved in cellular responses to DNA damage.  ATR kinase acts with ATM (“ataxia telangiectasia mutated”) kinase and many other proteins to regulate a cell's response to DNA damage, commonly referred to as the DNA Damage Response (“DDR”).  The DDR stimulates DNA repair, promotes survival and stalls cell cycle progression by activating cell cycle checkpoints, which provide time for repair.  Without the DDR, cells are much more sensitive to DNA damage and readily die from DNA lesions induced by endogenous cellular processes such as DNA replication or exogenous DNA damaging agents commonly used in cancer therapy. 
Healthy cells can rely on a host of different proteins for DNA repair including the DDR kinase ATR.  In some cases these proteins can compensate for one another by activating functionally redundant DNA repair processes.  On the contrary, many cancer cells harbour defects in some of their DNA repair processes, such as ATM signaling, and therefore display a greater reliance on their remaining intact DNA repair proteins which include ATR. 
In addition, many cancer cells express activated oncogenes or lack key tumour suppressors, and this can make these cancer cells prone to dysregulated phases of DNA replication which in turn cause DNA damage.  ATR has been implicated as a critical component of the DDR in response to disrupted DNA replication.  As a result, these cancer cells are more dependent on ATR activity for survival than healthy cells.  Accordingly, ATR inhibitors may be useful for cancer treatment, either used alone or in combination with DNA damaging agents, because they shut down a DNA repair mechanism that is more important for cellular survival in many cancer cells than in healthy normal cells. 
In fact, disruption of ATR function (e.g. by gene deletion) has been shown to promote cancer cell death both in the absence and presence of DNA damaging agents.  This suggests that ATR inhibitors may be effective both as single agents and as potent sensitizers to radiotherapy or genotoxic chemotherapy. 
ATR peptide can be expressed and isolated using a variety of methods known in the literature (see e.g., Ünsal-Kaçmaz et al, PNAS 99: 10, pp 6673-6678, May 14, 2002; see also Kumagai et al.  Cell 124, pp 943-955, Mar. 10, 2006; Unsal-Kacmaz et al.  Molecular and Cellular Biology, February 2004, p 1292-1300; and Hall-Jackson et al.  Oncogene 1999, 18, 6707-6713). 
For all of these reasons, there is a need for the development of potent and selective ATR inhibitors for the treatment of cancer, either as single agents or as combination therapies with radiotherapy or genotoxic chemotherapy.