The investigation of in vitro inhibition effects of bortezomib in combined with etoposide and camptotheci̇n on eukaryotic dna topoisomerase enzymes

DNA topoisomerases are ubiquitous enzymes that regulate conformational changes in DNA topology during essential cellular processes such as replication, transcription and recombination and for this reason, have been characterized as the cellular targets of a number of anti-cancer drugs. Camptothecin is the best known topoisomerase I inhibitor anticancer drug. Etoposide is a potent topoisomerase II inhibitor that is used to treat a wide spectrum of human cancers. Bortezomib is a powerful proteasome inhibitor which used in the treatment of hematological malignancies. However, the inhibitory effect of bortezomib on topoisomerase enzymes has remained sufficiently undetermined. In this thesis study, we investigated the inhibition effects of bortezomib on human topoisomerase I and II enzymes both alone and in combination modes with topoisomerase I inhibitor camptothecin and topoisomerase II inhibitor etoposide, after all, their inhibition effects were compared. The interactions of bortezomib and topoisomerase inhibitors with human type I and type II topoisomerase enzymes and combined studies were evaluated by using relaxation assay in cell-free systems, and the results were analyzed on agarose gel electrophoresis. IC50 values (50% inhibition concentrations) of the drugs on topoisomerase I and II enzymes were calculated using the S probit analysis program. In the result, bortezomib showed very weak inhibition effect on human topoisomerase I enzyme (IC50= 87.11 mM). On the other hand, it possesses the strong inhibitory effect on human topoisomerase II enzyme (IC50= 1.41 mM). Our results indicated that bortezomib is effective not only on proteasome but also on topoisomerase II. Bortezomib showed an increased effect at lower doses with synergistic effect when used in combination with camptothecin and etoposide than when used alone. Our findings belongs to combined application of these drugs will shed light to combined drug regimens that are to be used in further clinic for cancer treatment.