Mechanism of action of the dual topoisomerase-I and -II inhibitor TAS-103 and activity against (multi)drug resistant cells

TAS-103 is a recently developed dual inhibitor of topoisomerase-I (topo-I) and topoisomerase-II (topo-II). TAS-103 has documented cytotoxicity in vitr o and antitumor activity against a variety of mouse, rat,and human xenograf ts in vivo. Purpose: To determine TAS-103 activity against (multi)drug resi stant cells in vitro and to delineate its mechanism of action. Methods: TAS -103 was evaluated for activity against three human multidrug-resistant cel l lines representing resistance mediated by P-glycoprotein (Pgp)-, multidru g resistance protein (MRP)I and lung resistance protein (LRP) as well as on e camptothecin-resistant cell line associated with a mutated topo-I enzyme. Drug sensitivity fallowing short (2 h), intermediate (6-8 h) and long term (24 h) exposures were compared. The mechanism of action was studied by eva luating inhibition of topoisomerase-I and -II specific DNA relaxation assay s, drug-induced DNA/protein cross-link formation,:nd competitive DNA interc alation with ethidium bromide. Results: Increasing the exposure time only m odestly potentiated TAS-103 cytotoxicity (3-5 fold) demonstrating a lack of strong exposure duration dependency. TAS-103 cytotoxicity was not affected by the presence of Tiny of the drug resistance mechanisms studied. TAS-103 inhibits topo-I and -II activity in DNA relaxation assays, but in our assa y system TAS-103 was found to have only a weak ability to induce DNA-protei n crosslinks. DNA migration patterns in agarose gel electrophoresis indicat e that TAS-103 fan interact directly with DNA. Also its ability to displace ethidium bromide which has intercalated into the DNA provides an indicatio n on the nature of drug-DNA interaction. Conclusions: TAS-103 cytotoxicity is not affected by the presence of Pgp, MRP, LRP or mutations in the CAM bi nding region of the topo-I enzyme and its growth-inhibitory effect appears to be weakly dependent on exposure duration. The presented evidence suggest that the inhibitory effects of TAS-103 on topo-I and -II may in part be re lated to its DNA binding rather than primarily through stabilization of top o-I or -II intermediates with DNA through specific binding to the enzymes.