Structure-activity studies of novobiocin analogs as modulators of the cytotoxicity of etoposide (VP-16)

We have previously reported that the antibiotic novobiocin enhanced the tox icity of the anticancer agent etoposide (VP-16) to several drug-sensitive a nd -resistant tumor cell lines. The increase in VP-16 cytotoxicity produced by novobiocin was not due to the combined effects of these agents on topoi somerase II, but to inhibition by novobiocin of VP-16 efflux, which in turn led to increased accumulation of VP-16 and increased formation of potentia lly lethal VP-16-stabilized topoisomerase II-DNA covalent complexes. We hav e now identified novobiocin analogs that are essentially equivalent to novo biocin as inhibitors of the activity of topoisomerase II, but that are more potent than novobiocin (a) as modulators of the cytotoxicity of VP-16 to W EHI-3B leukemia and A549 lung carcinoma cells and (b) in increasing VP-16 a ccumulation in these cell lines. Thus, removal of the sugar moiety of novob iocin to form novobiocic acid enhanced the potency of the antibiotic as a m odulator of VP-16, whereas the substituted coumarin ring alone (U-7587) was devoid of VP-16 modulatory activity. Modifications of the side chain of no vobiocin significantly influenced modulatory activity, with cyclonovobiocic acid, which was formed from novobiocic acid by acid-catalyzed cycloadditio n, being the most active in enhancing the cytotoxicity of VP-16. The increa sed potency of novobiocic acid and cyclonovobiocic acid as modulators of VP -16 activity was achieved with no change from novobiocin in the capacity of these analogs to inhibit the catalytic activity of mammalian topoisomerase II, indicating a change in the specificity of these analogs.