title: In vitro and intracellular inhibition of topoisomerase II by the antitumor agent merbarone.
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 topoisomerase II. Merbarone inhibited purified mammalian topoisomerase II with a 50% inhibitory concentration of 20 microM, 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 microM. 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 teniposide of topoisomerase II-associated DNA strand breaks; under 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.
authors.affiliation: Department of Molecular Pharmacology, Smith Kline and French Laboratories, Swedeland, Pennsylvania 19406.
authors.forename: F H
authors.initials: FH
authors.lastname: Drake
authors.forename: G A
authors.initials: GA
authors.lastname: Hofmann
authors.forename: S M
authors.initials: SM
authors.lastname: Mong
authors.forename: J O
authors.initials: JO
authors.lastname: Bartus
authors.forename: R P
authors.initials: RP
authors.lastname: Hertzberg
authors.forename: R K
authors.initials: RK
authors.lastname: Johnson
authors.forename: M R
authors.initials: MR
authors.lastname: Mattern
authors.forename: C K
authors.initials: CK
authors.lastname: Mirabelli
mesh_terms: D000677:Amsacrine / Q000494:pharmacology; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D002166:Camptothecin / Q000494:pharmacology; D004247:DNA / Q000378:metabolism; D004249:DNA Damage; D007942:Leukemia, Experimental / Q000201:enzymology; D051379:Mice; D013713:Teniposide / Q000494:pharmacology; D013858:Thiobarbiturates / Q000494:pharmacology*; D059004:Topoisomerase I Inhibitors; D059005:Topoisomerase II Inhibitors
chemical_list: D000970:Antineoplastic Agents; D013858:Thiobarbiturates; D059004:Topoisomerase I Inhibitors; D059005:Topoisomerase II Inhibitors; D000677:Amsacrine; D004247:DNA; D013713:Teniposide; D002166:Camptothecin; C045459:merbarone