We have previously reported that the
antibiotic novobiocin enhanced the toxicity of the
anticancer agent etoposide (VP-16) to several drug-sensitive and -resistant tumor cell lines. The increase in
VP-16 cytotoxicity produced by
novobiocin was not due to the combined effects of these agents on
topoisomerase II, but to inhibition by
novobiocin of
VP-16 efflux, which in turn led to increased accumulation of
VP-16 and increased formation of potentially lethal VP-16-stabilized
topoisomerase II-
DNA covalent complexes. We have now identified
novobiocin analogs that are essentially equivalent to
novobiocin 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 WEHI-3B
leukemia and A549 lung
carcinoma cells and (b) in increasing
VP-16 accumulation in these cell lines. Thus, removal of the
sugar moiety of
novobiocin to form novobiocic
acid enhanced the potency of the
antibiotic as a modulator of
VP-16, whereas the substituted
coumarin ring alone (U-7587) was devoid of
VP-16 modulatory activity. Modifications of the side chain of
novobiocin significantly influenced modulatory activity, with cyclonovobiocic
acid, which was formed from novobiocic
acid by
acid-catalyzed cycloaddition, being the most active in enhancing the cytotoxicity of
VP-16. The increased 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.