INVITRO AND INTRACELLULAR INHIBITION OF TOPOISOMERASE-II BY THE ANTITUMOR AGENT MERBARONE

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  • 15 May 1989
    • journal article
    • research article
    • Vol. 49  (10) , 2578-2583
Abstract
Merbarone has previously been shown to have antitumor activity of unknown mechanism in P388 and L1210 tumor models (A.D. Brewer et al., Biochem. Pharmacol., 34:2047-2050, 1985) and is currently undergoing Phase I clinical trials. Here we report that merbarone is an inhibitor of topoismerase II. Merbarone inhibited purified mammalian topoisomerase II with a 50% inhibitory concentration of 20 .mu.M, as assessed by ATP-dependent unknotting of P4 phage DNA or relaxation of supercoiled pBR322 plasmid. In contrast to the type II enzyme, inhibition of catalytic activity of topoisomerase I required about 10-fold higher concentrations of merbarone, with a 50% inhibitory concentration of approximately 200 .mu.M. Unlike epipodophyllotoxin analogues and certain DNA intercalative agents which stabilize the topoisomerase II-DNA "cleavable complex", merbarone did not cause detectable topoisomerase II-induced DNA cleavage. Furthermore, merbarone inhibited the production by amsacrine or identical conditions novobiocin did not decrease these breaks, setting merbarone apart from a novobiocin-like class of topoisomerase II inhibitor. In L1210 cells, merbarone produced only small numbers of protein-associated DNA strand breaks, and only at very high concentrations. Merbarone reduced in a concentration-dependent manner the number of amsacrine- or teniposide-stimulated protein-associated DNA strand breaks in L1210 cells or their isolated nuclei. The data suggest that merbarone represents a novel type of topoisomerase II inhibitor.