Camptothecin (
CPT) and its derivatives target mammalian
DNA topoisomerase I (top1) and are among the most effective novel anticancer drugs. However, the activity of CPTs is limited by several factors, including
drug inactivation by
lactone ring opening,
tumor drug resistance, and toxicity in patients. Novel top1 inhibitors are being searched to overcome such limitations and expand the anticancer spectrum of camptothecins.
MJ-III-65 (
NSC 706744) is among the most promising indenoisoquinolines to date. In this study, we show that
MJ-III-65 enhances top1 cleavage complexes by both inhibiting their reversal (religation) more efficiently than
CPT and by enhancing their formation. The top1 DNA cleavage complexes induced by
MJ-III-65 exhibit a different distribution pattern compared with
CPT and exhibit different base sequence preferences immediately around the top1 cleavage sites. Although CPTs have a preference for
thymine at the (-1) position and
guanine at the (+1) position of the top1-mediated DNA cleavage sites,
MJ-III-65 can accommodate different base pairs at the (-1), (+1), or (+2) position, with a preference for a
cytosine at the (-1) position on the scissile strand. Another difference with CPTs is the activity of
MJ-III-65 against
CPT-resistant top1
enzymes, implying that the
amino acid residue interactions with top1 are different for
MJ-III-65 and CPTs. As with
CPT,
MJ-III-65 is inactive against
vaccinia top1. This study shows the specific molecular interactions of
MJ-III-65 with top1 and demonstrates that
MJ-III-65 is a potentially useful top1 inhibitor that enhances and traps top1 cleavage sites not sensitive to CPTs.