Radiation lethality enhancement with 9-aminocamptothecin: comparison to other topoisomerase I inhibitors.
OBJECTIVE
Preclinical studies have demonstrated differences in potency, solubility, and tumor specific activity among the camptothecin (CPT) analogues. 9-Aminocamptothecin (9-AC) has demonstrated greater potency in animal studies than other clinically available Topoisomerase I (Topo I) inhibitors. We sought to determine: (a) if 9-AC enhanced the lethal effects of ionizing radiation to a greater extent than other Topo I inhibitors; and (b) the biological and biochemical characteristics of the enhancement.


METHODS
Quiescent radioresistant human melanoma (U1-Mel) cells were x-irradiated (1-7 Gy) and exposed to various concentrations of 9-AC (0.1-100 microM), either before (for 4 h), during, or after (for 4 h) irradiation. Survival was determined via colony forming assays and normalized to correct for drug cytotoxicity. The effect of 9-AC on radiation-related potential lethal damage repair PLDR) (PLDR) was also measured. A modification of the SDS-KCl assay was used to quantify DNA-Topo I complexes.


RESULTS
Enhancement of radiation lethality was observed using confluent U1-Mel cells. The sensitizer enhancement ratio (SER) after a 4 h postirradiation exposure of 10 microM 9-AC was 2.5 at 10% survival. Toxicity from the drug alone was greater than topotecan (TPT), but less than CPT. The radiation synergy effect was: (a) dependent on drug concentration (> or = 2 microM); (b) dependent on timing, with enhancement present only when the drug was present at the time of, or shortly after, radiation; and (c) irreversible, with inhibition of PLDR. Exposure to 9-AC during or after irradiation substantially elevated the number of DNA-Topo I complexes (four- to tenfold) over control levels and correlated with enhanced loss of survival.


CONCLUSIONS
9-Aminocamptothecin enhanced radiation lethality in vitro at low drug concentrations with characteristics similar to other Topo I inhibitors. A greater SER, but greater lethality with the drug alone, was obtained in comparison to TPT. The clinical implications of these findings remain unexplored.
Department of Human Oncology, University of Wisconsin-Madison, 53792, USA.
J P
JP
Lamond
M
M
Wang
T J
TJ
Kinsella
D A
DA
Boothman
D000970:Antineoplastic Agents / Q000008:administration & dosage / Q000494:pharmacology*; D002166:Camptothecin / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D008545:Melanoma / Q000201:enzymology / Q000532:radiotherapy; D011836:Radiation Tolerance / Q000187:drug effects*; D013997:Time Factors; D059004:Topoisomerase I Inhibitors; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000201:enzymology / Q000528:radiation effects
D000970:Antineoplastic Agents; D059004:Topoisomerase I Inhibitors; C080125:9-aminocamptothecin; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin