RT Journal Article SR Electronic T1 Chlorambucil Cytotoxicity in Malignant B Lymphocytes Is Synergistically Increased by 2-(Morpholin-4-yl)-benzo[h]chomen-4-one (NU7026)-Mediated Inhibition of DNA Double-Strand Break Repair via Inhibition of DNA-Dependent Protein Kinase JF Journal of Pharmacology and Experimental Therapeutics JO J Pharmacol Exp Ther FD American Society for Pharmacology and Experimental Therapeutics SP 848 OP 855 DO 10.1124/jpet.106.118356 VO 321 IS 3 A1 Lilian Amrein A1 Martin Loignon A1 Anne-Christine Goulet A1 Michael Dunn A1 Bertrand Jean-Claude A1 Raquel Aloyz A1 Lawrence Panasci YR 2007 UL http://jpet.aspetjournals.org/content/321/3/848.abstract AB Chlorambucil (CLB) treatment is used in chronic lymphocytic leukemia (CLL) but resistance to CLB develops in association with accelerated repair of CLB-induced DNA damage. Phosphorylated histone H2AX (γH2AX) is located at DNA double-strand break (DSB) sites; furthermore, it recruits and retains damage-responsive proteins. This damage can be repaired by nonhomologous DNA end-joining (NHEJ) and/or homologous recombinational repair (HR) pathways. A key component of NHEJ is the DNA-dependent protein kinase (DNA-PK) complex. Increased DNA-PK activity is associated with resistance to CLB in CLL. We used the specific DNA-PK inhibitor 2-(morpholin-4-yl)-benzo[h]chomen-4-one (NU7026) to sensitize CLL cells to chlorambucil. Our results indicate that in a CLL cell line (I83) and in primary CLL-lymphocytes, chlorambucil plus NU7026 has synergistic cytotoxic activity at nontoxic doses of NU7026. CLB treatment results in G2/M phase arrest, and NU7026 increases this CLB-induced G2/M arrest. Moreover, a kinetic time course demonstrates that CLB-induced DNA-PK activity was inhibited by NU7026, providing direct evidence of the ability of NU7026 to inhibit DNA-PK function. DSBs, visualized as γH2AX, were enhanced 24 to 48 h after CLB and further increased by CLB plus NU7026, suggesting that the synergy of the combination is mediated by NU7026 inhibition of DNA-PK with subsequent inhibition of DSB repair. The American Society for Pharmacology and Experimental Therapeutics