Benzisoquinolinedione (
nafidimide;
NSC 308847) is an
investigational drug currently in phase I clinical testing. We have studied the antileukemic activity in vitro, the cellular
drug transport, and the molecular mechanism of action with
DNA of this new compound. By
agarose gel electrophoresis, we verified that
nafidimide is an
intercalating agent, through its alteration of the electrophoretic migration of
DNA products produced by the relaxing action of
DNA topoisomerase I. Concentrations of up to 100 microM of
nafidimide did not produce
topoisomerase I-mediated DNA cleavage.
Nafidimide produced
DNA single-strand breaks (SSB), double-strand breaks, and
DNA-
protein cross-links in human
myeloid leukemia cells (measured with filter elution). The ratio of SSB/
DNA-
protein cross-links was 1.32 +/- 0.36, a value similar to that produced by 4'-(9-acridinylamino)methanesulfon-m-anisidide (
m-AMSA), suggesting that
nafidimide, like
m-AMSA, produced
protein-associated
DNA-strand breaks through a
topoisomerase II-mediated reaction. The production of double-strand breaks by
nafidimide also suggests the involvement of
topoisomerase II in the
drug-induced DNA cleavage. The cytotoxic activity of
nafidimide was quantified in human
myeloid leukemia cell lines differing by
a factor of 70 in their cytotoxic sensitivity to
m-AMSA. The
m-AMSA-resistant line was less than 2-fold resistant to
nafidimide. Cellular
drug uptake was rapid and reached a steady state level in 30 min at 37 degrees C. At the end of exposure,
drug egress was rapid, as was the disappearance of the
DNA SSB. Rapid cellular uptake of
nafidimide, with low retention at the end of exposure and rapid rejoining of
DNA SSB suggest that prolonged cellular exposure may be necessary for optimal antitumor effect. In vitro cloning data suggest that
nafidimide may be a therapeutic option for patients with
leukemia resistant to
m-AMSA.