DNA TOPOISOMERASE II-DEPENDENT CYTOTOXICITY OF ALKYLAMINOANTHRAQUINONES AND THEIR N-OXIDES

We studied the role of DNA topoisomerase II in the biological actions of a series of novel alkyl-aminoanthraquinones, including N-oxide deri vatives designed as prodrugs liable to bioreductive activation in hypo xic tumour cells. Drug structures were based upon the DNA-binding anti cancer topoisomerase II poison mitoxantrone with modifications to the alkylamino side chains. The agents included AQ4, 1,4-bis {[2-(dimethyl amino)ethyl]amino}5,8-dihydroxy-an- thracene-9,10-dione, and AQ6, 1{[2 -dimethylamino)- ethyl]amino}4-{[2[(hydroxyethyl)amino]ethyl]- amino}5 ,8-dihydroxy-anthracene-9,10-dione, together with the corresponding mo no-N-oxide (AQ6NO) and di-N-oxide (AQ4NO). The R(3)(+)-O- modification renders the terminal nitrogen group electrically neutral and was foun d to reduce AQ6NO or effectively abolish AQ4NO-DNA binding. Comparativ e studies were carried out using two SV40-transformed fibroblast cell lines, MRC5-V1 and AT5BIVA, the latter being a relative overproducer o f DNA topoisomerase II alpha. The inhibition of DNA topoisomerase II d ecatenation activity ranked according to DNA-binding capacity. A simil ar ranking was found for drug-induced DNA-protein cross-linking in int act cells, depending upon topoisomerase II availability. Inhibition of DNA synthesis in S-phase synchronized cultures ranked in the order of AQ6 > mitoxantrone >> AQ6NO and was independent of topoisomerase II a vailability. Cytotoxicity of acute 1-h exposures for all agents except the inactive AQ4NO was enhanced in the topoisomerase II-overproducing cell line. The results indicate an important role for enzyme targetin g in anthraquinone action. However, DNA synthesis inhibition and cytot oxicity were greater than expected for AQ6, given its topoisomerase- a nd DNA-interaction properties, and parallel studies have provided evid ence of an additional role for enhanced subcellular accumulation and n uclear targeting. The inactivity of AQ4NO and the retention of only pa rtial activity of AQ6NO, allied with the effective topoisomerase II-ta rgeting and high cytotoxic potential of their presumed metabolites, fa vour their use as prodrugs in tumour cells with enhanced bioreductive potential.