Topoisomerase II Inhibition and Cytotoxicity of the Anthrapyrazoles DuP 937 and DuP 941 (Losoxantrone) in the National Cancer Institute Preclinical Antitumor Drug Discovery Screen

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 anthrapy razoles 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 mitox antrone and anthrapyrazoles to generate DNA double strand breaks, by induction of the topoisomerase II cleavable complexes in nuclear extracts, was in agreement with cyto toxicity. 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. [J Natl Cancer Inst 86: 1239–1244, 1994]