Down-regulation of topoisomerase II by camptothecin does not prevent additive activity of the topoisomerase II inhibitor etoposide in vitro.
Topoisomerases (Topo) I and II are cellular enzymes that catalyze the relaxation of topologically strained DNA and that are involved in a number of DNA-related processes. Their complete inhibition by Topo I and II inhibitors gives promise for improvements in the treatment of malignant diseases. However, preclinical studies showed down-regulation of Topo II protein expression by Topo I inhibitors, which may preclude the useful application of combined topoisomerase inhibition in the clinic. We investigated the efficacy of the combination of etoposide (ETP) and camptothecin (CPT) in human gastric and lung cancer cell lines with different sensitivity towards ETP. The cytotoxicity of different drugs was assessed by the sulforhodamine B assay. Drug interactions were evaluated by isobologram analysis. The polymerase chain reaction and flow cytometry were employed for examination of the mdr1 (multidrug resistance type 1) phenotype. As reported by others, incubation of the P glycoprotein (P-gp)-negative tumor cell lines with the Topo I inhibitor CPT resulted in a significant down-regulation of Topo II protein expression. This was obviously due to changes in the cell cycle distribution of the cells induced by the treatment, with a marked increase of cells in G2/M phase and a consecutive decrease of S phase cells. Despite these biochemical changes, isobologram analysis showed additive cytotoxic activity of CPT and ETP in all the cell lines, independent of whether the drug incubation was performed simultaneously or sequentially. These data indicate that down-regulation of Topo II protein by CPT does not prevent additive activity of CPT and ETP in vitro, and therefore combined Topo I and II inhibition may be useful for investigation in clinical trials.
Stahl
M
M
Department of Internal Medicine (Cancer Research), West German Cancer Center, University of Essen, Germany.
Kasimir-Bauer
S
S
Harstrick
A
A
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000096:biosynthesis*; D002166:Camptothecin / Q000633:toxicity*; D002453:Cell Cycle / Q000187:drug effects*; D002470:Cell Survival / Q000187:drug effects*; D004250:DNA Topoisomerases, Type II / Q000096:biosynthesis*; D004317:Doxorubicin / Q000633:toxicity; D004347:Drug Interactions; D005047:Etoposide / Q000633:toxicity*; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D007700:Kinetics; D008175:Lung Neoplasms; D017239:Paclitaxel / Q000633:toxicity; D016133:Polymerase Chain Reaction; D012235:Rhodamines; D013274:Stomach Neoplasms; D059004:Topoisomerase I Inhibitors; D059005:Topoisomerase II Inhibitors; D014407:Tumor Cells, Cultured; D014750:Vincristine / Q000633:toxicity
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D012235:Rhodamines; D059004:Topoisomerase I Inhibitors; D059005:Topoisomerase II Inhibitors; C022027:lissamine rhodamine B; D014750:Vincristine; D005047:Etoposide; D004317:Doxorubicin; D004250:DNA Topoisomerases, Type II; D017239:Paclitaxel; D002166:Camptothecin