BACKGROUND: The cumulative cardiotoxicity of anthracyclines is thought to result 
from the generation of free radicals. New DNA topoisomerase II inhibitors less 
prone to redox reactions, such as mitoxantrone and more recently the 
anthrapyrazoles, were developed to circumvent this toxicity.
PURPOSE: Two anthrapyrazoles currently in clinical evaluation, DuP 941 
(Losoxantrone) and DuP 937, were compared to other topoisomerase II inhibitors 
with respect to their cytotoxic potency and selectivity and with respect to 
topoisomerase II inhibition.
METHODS: Cytotoxicity was tested in the 60 cell lines of the National Cancer 
Institute preclinical antitumor drug discovery screen (NCI screen). The potency 
of anthrapyrazoles to inhibit purified topoisomerase II was determined. The 
specificity of drug-induced topoisomerase II pattern of cleavage, one of the 
cellular determinants of cytotoxicity, was investigated in human c-myc DNA.
RESULTS: Using the COMPARE analysis, we found that the most closely related 
cytotoxic profiles in the NCI screen were between the anthrapyrazoles and 
mitoxantrone. Among topoisomerase II inhibitors, the cytostatic potency was by 
decreasing order: mitoxantrone; doxorubicin, which was slightly greater than DuP 
941, azatoxin; DuP 937; and amsacrine, which was much greater than VP-16. The 
potency of mitoxantrone and anthrapyrazoles to generate DNA double-strand 
breaks, by induction of the topoisomerase II cleavable complexes in nuclear 
extracts, was in agreement with cytotoxicity. Sequencing of drug-induced 
topoisomerase II cleavages in c-myc DNA showed a common cleavage pattern for 
anthrapyrazoles and mitoxantrone. This pattern was different from the patterns 
obtained with other topoisomerase II inhibitors.
CONCLUSION: At the molecular and cellular levels, anthrapyrazoles are potent 
topoisomerase II inhibitors closely related to mitoxantrone.
IMPLICATIONS: These results validate the COMPARE analysis using the NCI screen 
to predict molecular mechanisms of drug action. Anthrapyrazoles, which are 
unlikely to produce free radicals, might be useful in the same indications as 
mitoxantrone, especially for patients with cardiac risks, for pediatric 
patients, and for patients treated with intensified protocols.