In vitro activity: KU-55933 inhibits DNA-PK and PI3K with IC50 of 2.5 μM and 16.6 μM, respectively. Besides, KU-55933 also prevents the activity of mTOR with IC50 of 9.3 μM. KU-55933 is active at the cellular level in ablating a well-characterized ATM-dependent phosphorylation event. KU-55933 has a dose-dependent effect in inhibiting this ATM-dependent phosphorylation event with IC50 of 300 nM. KU-58050 does not prevent the ATM-dependent phosphorylation of p53 serine 15 until a dose of 30 μM. Addition of KU-55933 has no appreciable effects on UV-induced phosphorylation of H2AX on serine 139, NBS1 on serine 343, CHK1 on serine 345, and SMC1 on serine 966. In stark contrast to the UV responses, KU-55933 ablates the ionizing radiation-induced phosphorylation of these ATM substrates. KU-55933 sensitizes HeLa cells to a range of ionizing radiation doses. KU-55933 inhibits the phosphorylation of Akt induced by growth factors in cancer cells. KU-55933 suppresses the proliferation of cancer cells. Furthermore, suppression of ATM by KU-55933 improves survival, probably via prevention of downstream activation of TAp63α.

Kinase Assay: ATM for use in the in vitro assay is obtained from HeLa nuclear extract by immunoprecipitation with rabbit polyclonal antiserum raised to the COOH-terminal 400 amino acids of ATM in buffer containing 25 mM HEPES (pH 7.4), 2 mM MgCl2, 250 mM KCl, 500 μM EDTA, 100 μM Na3VO4, 10% v/v glycerol, and 0.1% v/v Igepal. ATM-antibody complexes are isolated from nuclear extract by incubating with protein A-Sepharose beads for 1 hour and then through centrifugation to recover the beads. In the well of a 96-well plate, ATM-containing Sepharose beads are incubated with 1 μg of substrate glutathione S-transferase–p53N66 (NH2-terminal 66 amino acids of p53 fused to glutathione S-transferase) in ATM assay buffer [25 mM HEPES (pH 7.4), 75 mM NaCl, 3 mM MgCl2, 2 mM MnCl2, 50 μM Na3VO4, 500 μM DTT, and 5% v/v glycerol] at 37 °C in the presence or absence of inhibitor. After 10 minutes with gentle shaking, ATP is added to a final concentration of 50 μM and the reaction continued at 37 °C for an additional 1 hour. The plate is centrifuged at 250 × g for 10 minutes (4 °C) to remove the ATM-containing beads, and the supernatant is removed and transferred to a white opaque 96-well plate and incubated at room temperature for 1.5 hours to allow glutathione S-transferase-p53N66 binding. This plate is then washed with PBS, blotted dry, and analyzed by a standard ELISA technique with a phospho-serine 15 p53 antibody. The detection of phosphorylated glutathione S-transferase-p53N66 substrate is performed in combination with a goat antimouse horseradish peroxidase-conjugated secondary antibody. Enhanced chemiluminescence solution is used to produce a signal and chemiluminescent detection is carried out

Cell Assay: U2OS cells are exposed to ionizing radiation (3, 5, or 15 Gy) or UV (5 or 50 J/m2) and the ATM response determined by Western blot analysis of p53 serine 15 phosphorylation and stabilization of wild-type p53. Whole cell extracts are obtained from each time point, proteins separated by SDS-PAGE, and the ATM-specific increase in phosphorylated serine 15 measured with a p53 phospho-serine 15 specific antibody. Overall p53 stabilization with time is also observed with a p53-specific antibody (DO-1). Similarly, for studying ATM-dependent phosphorylations on H2AX, CHK1, NBS1, and SMC1, the following antibodies are used: CHK1 phospho-serine 345 and NBS1 phospho-serine 343 antibodies. Histone H2A (H-124) and CHK1 antibodies are also used, as well as SMC1 and SMC1 phospho-serine 966 antibodies. For determination of a cellular IC50 for KU-55933, the peak response time for p53 serine 15 phosphorylation of 2 hours is used to monitor inhibition of ATM. KU-55933 is titrated onto cells and preincubated for 1 hour before ionizing radiation. Using scanning densitometry, the percentage inhibition relative to vehicle control is calculated, and the IC50 value is calculated as for the in vitro determinations.

Cancer Res. 2009 Apr 15;69(8):3510-9; Mol Cancer Ther. 2010 Jan;9(1):113-25.