We report for the first time the use of a selective small-molecule inhibitor of DNA repair to potentiate
topoisomerase II (
topo II)
poisons, identifying
DNA-dependent protein kinase (
DNA-PK) as a potential target for
leukemia therapy.
Topo II
poisons form cleavable complexes that are processed to
DNA double-strand breaks (DSBs).
DNA-PK mediates nonhomologous end joining (NHEJ). Inhibition of this
DSB repair pathway may sensitize cells to
topo II
poisons. We investigated the effects of a novel
DNA-PK inhibitor,
NU7026 (2-(morpholin-4-yl)-benzo[h]chomen-4-one), on the response to
topo II
poisons using K562
leukemia cells.
NU7026 (10 microM) potentiated the growth inhibition of
idarubicin,
daunorubicin,
doxorubicin,
etoposide,
amsacrine (mAMSA), and mitroxantrone with potentiation factors at 50% growth inhibition ranging from approximately 19 for mAMSA to approximately 2 for
idarubicin (potentiation of
etoposide was confirmed by clonogenic assay). In contrast,
NU7026 did not potentiate
camptothecin or
cytosine arabinoside (araC).
NU7026 did not affect the levels of
etoposide-induced
topo IIalpha or beta cleavable complexes.
NU7026 alone had no effect on cell cycle distribution, but
etoposide-induced accumulation in G2/M was increased by
NU7026. A concentration-dependent increase in
etoposide-induced
DSB levels was increased by
NU7026. The mechanism of
NU7026 potentiation of
topo II
poisons involves inhibition of NHEJ and a G2/M checkpoint arrest.