Cellular resistance to the antitumor DNA topoisomerase II inhibitor S16020-2: Importance of the N-[2(dimethylamino)ethyl] carbamoyl side chain

The new olivacine derivative S16020-2 (NSC-659687) is a DNA topoisomerase I I inhibitor endowed with a remarkable antitumor activity against various ex perimental tumors. In vitro physicochemical properties of this compound, in particular its interaction with DNA and DNA topoisomerase II, were very si milar to those of ellipticine derivatives, except for a strictly ATP-depend ent mechanism of cleavable complex induction. From the Chinese hamster lung fibroblast cell line DC-3F, a subline resistant to S16020-2, named DC-3F/S 16, was selected by adding stepwise increasing concentrations of the drug t o the cell growth medium. Whereas DC-3F/9-OH-E cells, a DC-3F subline resis tant to 9-hydroxy-ellipticine, are cross-resistant to S16020-2, DC-3F/S16 c ells are only very weakly cross-resistant to ellipticine derivatives, indic ating that, despite their structural similarity, these compounds may differ in their mechanisms of action. Uptake and efflux rates of S16020-2 were id entical in the resistant and the sensitive cells. Topoisomerase II alpha wa s expressed at the same level in both sensitive and resistant cells, wherea s expression of the beta-enzyme was approximately 50% lower in the resistan t cells. Sequencing of both alpha- and beta-isoform cDNAs revealed a point mutation that converts Arg(486) to a Gly in the alpha cDNA, whereas the bet a cDNA was not modified. This amino acid substitution in a highly conserved sequence of the enzyme appears to be responsible for the resistance to S16 020-2. Comparative analysis of the properties of the ellipticine and S16020 -2- resistant cells suggests that S16020-2, which is a DNA intercalator, mi ght also interact with this enzyme amino acid sequence through its side cha in.