Chk2 is a checkpoint kinase involved in the ATM-Chk2 checkpoint pathway (http://discover.nci.nih.gov/mim). This pathway is activated by genomic instability and DNA damage and results in either cell death (apoptosis) or arrest of the cell cycle to allow DNA repair to occur. Chk2 is activated by phosphorylation and in turn phosphorylates its downstream targets, including p53 and Cdc25C. Currently there are no specific inhibitors of Chk2 in the clinic. The rationale for developing specific Chk2 inhibitors is primarily due to the dual role of Chk2 on cell cycle checkpoints and on apoptosis via phosphorylation of its downstream substrates including Cdc25A, Cdc25C, p53, BRCA1, E2F1, and PML. Chk2 provides an unexplored therapeutic target in cancer cells with inherent defects in G1 checkpoint function. By virtue of Chk2's role in both cell cycle checkpoint regulation and apoptosis, selective inhibition of Chk2 could improve the therapeutic index of DNA-damaging agents. This may be especially true in p53-deficient tumors where the p53-dependent apoptotic response is deficient. In normal tissues Chk2 may act as a pro-apoptotic effector, thus Chk2 inhibitors may protect normal tissues (Reviewed in Pommier et al. 2005 Curr Pharm Des 11, 2855). In collaboration with DTP, we have developed a drug screening program for specific Chk2 inhibitors using a fluorescence polarization assay, IMAP. This assay detects the degree of phosphorylation of a fluorescently-linked substrate by Chk2 and any subsequent inhibition. Over 100,000 compounds have been tested from the Open Synthetic Repository (OSR) library and a number of leads have been identified. Here we present further in vitro data showing the specific inhibition of Chk2 kinase activity by the lead compounds using an in vitro kinase assay. Overall these data provide a promising basis for studies using the lead compounds in combination with current DNA-damaging agents used in the clinic.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA