title: The effect of DB-67, a lipophilic camptothecin derivative, on topoisomerase I levels in non-small-cell lung cancer cells.
abstract: OBJECTIVE
  To determine the in vitro drug sensitivity of two non-small-cell lung cancer cell lines after treatment with the novel lipophilic camptothecin derivative, 7- tert-butyldimethylsilyl-10-hydroxycamptothecin (DB-67), to determine if topoisomerase I protein levels decrease after treatment with DB-67, and to assess the duration and extent of topoisomerase I modulation after DB-67 exposure, in order to provide information about drug resistance that may be useful in determining an appropriate dosing schedule for DB-67.
  
  
  METHODS
  The growth inhibition of the non-small-cell lung cancer cell lines A549 and H460 after exposure to DB-67 was evaluated with the MTS assay. A549 and H460 cells were treated for various times with DB-67 and topoisomerase I levels were determined by western blot analysis. In addition, A549 and H460 cells were treated with DB-67 for 24 h and topoisomerase I levels were determined by western blot analysis daily for 1 week after drug removal.
  
  
  RESULTS
  DB-67 inhibited the growth of both A549 and H460 cells grown in culture; the A549 cells were more resistant to the cytotoxic effects of DB-67 than H460 cells. Notably, A549 cells had approximately one-half the baseline topoisomerase I than H460 cells. Topoisomerase I protein levels significantly decreased after 8-18 h of exposure to DB-67. Both A549 and H460 cells treated with DB-67 for 24 h had only negligible amounts of topoisomerase I at the end of treatment. However, within 24 h of drug removal topoisomerase I levels returned to near baseline levels in both cell lines.
  
  
  CONCLUSIONS
  The decrease in topoisomerase I levels caused by DB-67 may represent a mechanism of resistance to this novel camptothecin derivative. Dosing DB-67 once every 48-72 h may maximize the interaction of the drug with topoisomerase I and should be considered as a potential dosing schedule in the preclinical and clinical development of this compound.
authors.affiliation: Division Pharmacy Practice and Science, 202 College of Pharmacy, University of Kentucky, Lexington, KY 40536-0082, USA.
authors.forename: Aimee K
authors.initials: AK
authors.lastname: Bence
authors.forename: Cynthia A
authors.initials: CA
authors.lastname: Mattingly
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Burke
authors.forename: Val R
authors.initials: VR
authors.lastname: Adams
mesh_terms: D002166:Camptothecin / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology*; D002289:Carcinoma, Non-Small-Cell Lung / Q000201:enzymology*; D004264:DNA Topoisomerases, Type I / Q000378:metabolism*; D004354:Drug Screening Assays, Antitumor; D006801:Humans; D008175:Lung Neoplasms / Q000201:enzymology*; D017646:Organosilicon Compounds / Q000737:chemistry / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D017646:Organosilicon Compounds; C415575:7-tert-butyldimethylsilyl-10-hydroxycamptothecin; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin