Synergistic effects of topoisomerase I inhibitor, 7-ethyl-10-hydroxycamptothecin, and irradiation in a cisplatin-resistant human small cell lung cancer cell line.
7-ethyl-10-[4-(1-piperidyl)-1-piperidyl] carbonyloxy-camptothecin, a topoisomerase I (topo I) inhibitor, is one of the most active agent against lung cancer, and its radiosensitizing effect has been reported recently. We evaluated a combination in vitro effect of irradiation and 7-ethyl-10-hydroxy-CPT (SN-38), an active metabolite of 7-ethyl-10-[4- (1-piperidyl)-1-piperidyl] carbonyloxy-camptothecin, on a human small cell lung cancer cell line (SBC-3) and its cisplatin-resistant subline (SBC-3/CDDP). Growth-inhibitory effects of irradiation with or without SN-38 were determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. A modified isobologram method was used to evaluate the treatment interaction. The combination of irradiation and SN-38 showed a synergistic inhibitory effect on the growth of SBC-3/CDDP despite its cross-resistance to irradiation and SN-38. In contrast, the same combination showed only an additive effect on the growth of parental SBC-3 cells. There was no significant difference in topo I protein expression between these two cell lines. In SBC-3 cells, topo I catalytic activity was suppressed by 4 Gy of irradiation, without a decrease of nuclear topo I protein, whereas the exposure of SBC-3 cells to 1 microM SN-38 subsequent to irradiation showed no remarkable additional effects on both topo I activity and protein content. On the other hand, in SBC-3/CDDP cells, topo I activity was unchanged by irradiation, but the subsequent exposure to SN-38 gave rise to a decrease in topo I activity, which was accompanied by a significant decrease in the topo I protein content (P = 0.02). These observations may indicate that SN-38 induces sequestration of topo I onto DNA in radiation-treated SBC-3/CDDP cells and suggest that the synergistic effect of irradiation and SN-38 in SBC-3/CDDP cells was considered attributable to DNA repair-related enhanced recruitment of topo I onto the damaged DNA.
Kohara
Hiroyuki
H
Department of Internal Medicine II, Okayama University Medical School, Okayama 700-8558, Japan. hkohara@kb4.so-net.ne.jp
Tabata
Masahiro
M
Kiura
Katsuyuki
K
Ueoka
Hiroshi
H
Kawata
Kazuhiko
K
Chikamori
Masakazu
M
Aoe
Keisuke
K
Chikamori
Kenichi
K
Matsushita
Akio
A
Harada
Mine
M
D000970:Antineoplastic Agents / Q000627:therapeutic use*; D002166:Camptothecin / Q000031:analogs & derivatives / Q000627:therapeutic use*; D018288:Carcinoma, Small Cell / Q000201:enzymology / Q000473:pathology / Q000628:therapy*; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects; D002945:Cisplatin / Q000627:therapeutic use*; D003131:Combined Modality Therapy; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D019008:Drug Resistance, Neoplasm; D004357:Drug Synergism; D004791:Enzyme Inhibitors / Q000627:therapeutic use*; D005562:Formazans; D006801:Humans; D015151:Immunoblotting; D000077146:Irinotecan; D008175:Lung Neoplasms / Q000201:enzymology / Q000473:pathology / Q000628:therapy*; D013778:Tetrazolium Salts; D059004:Topoisomerase I Inhibitors; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000528:radiation effects*
D000970:Antineoplastic Agents; D004791:Enzyme Inhibitors; D005562:Formazans; D013778:Tetrazolium Salts; D059004:Topoisomerase I Inhibitors; C079782:MTT formazan; D000077146:Irinotecan; D004264:DNA Topoisomerases, Type I; D002945:Cisplatin; D002166:Camptothecin