Radiation enhancement by the combined use of topoisomerase I inhibitors, RFS-2000 or CPT-11, and topoisomerase II inhibitor etoposide in human lung cancer cells.
OBJECTIVE
We have tested the camptothecin analogs, RFS-2000 or CPT-11, in combination with both etoposide and ionizing radiation in vitro to examine the radiation enhancing potential of topoisomerase I plus topoisomerase II (Topo I+Topo II) inhibition in human cancer cells.


METHODS
H460 human lung carcinoma cells were plated and treated with 10nM RFS-2000 or 4.5microM CPT-11 for 4h. Cells were then irradiated with various doses and treated with 1microM etoposide for 1.5h. Cell survival and sublethal damage recovery (SLDR) were determined by clonogenic assay. 7-aminoactinomycin D (7-AAD) staining and flow cytometry were used to analyze cell viability/apoptosis after combined treatment of drugs with radiation.


RESULTS
Survival experiments showed radiation dose enhancement ratios (DER) of 1.26, 1.34, and 1.63 for RFS-2000, etoposide, and RFS-2000 plus etoposide, respectively; the corresponding DER values were 1.30, 1.39, and 1.65 for CPT-11, etoposide, and CPT-11 plus etoposide. The analysis of cell viability/apoptosis using 7-AAD staining and flow cytometry showed an additive effect. Greater inhibition of SLDR was observed with RFS-2000 plus etoposide than with either agent separately, but CPT-11 plus etoposide showed a more modest effect upon SLDR.


CONCLUSIONS
These data show that the combination of Topo I inhibitors, RFS-2000 or CPT-11 plus Topo II inhibitor etoposide, is a more effective radiation enhancer than either agent alone in human lung cancer cells. The mechanism of radiation enhancement may involve inhibition of SLDR with RFS-2000 plus etoposide, but other mechanisms may be involved in the combined treatment including CPT-11.
Kim
Jae Sung
JS
Department of Radiation Oncology, The Vanderbilt Clinic B-902, Vanderbilt University Medical Center, 1301 22nd Avenue S., Nashville, TN 37232-5671, USA.
Amorino
George P
GP
Pyo
Hongryull
H
Cao
Qianwen
Q
Choy
Hak
H
D000970:Antineoplastic Agents / Q000494:pharmacology*; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D002470:Cell Survival / Q000187:drug effects; D003114:Colony-Forming Units Assay; D003131:Combined Modality Therapy; D004305:Dose-Response Relationship, Drug; D004338:Drug Combinations; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005047:Etoposide / Q000494:pharmacology*; D006801:Humans; D000077146:Irinotecan; D008175:Lung Neoplasms; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D059004:Topoisomerase I Inhibitors; D059005:Topoisomerase II Inhibitors; D014407:Tumor Cells, Cultured
D000970:Antineoplastic Agents; D004338:Drug Combinations; D004791:Enzyme Inhibitors; D011838:Radiation-Sensitizing Agents; D059004:Topoisomerase I Inhibitors; D059005:Topoisomerase II Inhibitors; D005047:Etoposide; D000077146:Irinotecan; C079905:rubitecan; D002166:Camptothecin