title: Resistance to paclitaxel mediated by P-glycoprotein can be modulated by changes in the schedule of administration.
abstract: OBJECTIVE
  Increasing use of paclitaxel in clinical oncology has stimulated interest in its mechanisms of resistance and ways to overcome these. Studies were performed with paclitaxel to determine the role of P-glycoprotein in drug sensitivity, and the effect of schedule on relative resistance. We have previously reported that prolonged exposure to P-glycoprotein substrates decreases relative resistance in multidrug resistant cells.
  
  
  METHODS
  Using both unselected and drug-selected cell lines, cross-resistance and cytotoxicity reversal studies using cyclosporin A were performed. In multidrug-resistant cells, cross-resistance was evaluated after 3-, 24-, and 96-h exposures to paclitaxel.
  
  
  RESULTS
  Cross-resistance to paclitaxel in P-glycoprotein-expressing sublines was shown to be comparable to that of other drugs transported by P-glycoprotein. Sensitivity to paclitaxel could be modulated by cyclosporin A in unselected cell lines expressing P-glycoprotein and not in P-glycoprotein-negative cell lines. Resistance to paclitaxel was reduced tenfold by increasing the duration of exposure in P-glycoprotein-expressing cells. This effect was not observed in a paclitaxel-resistant cell line which does not express P-glycoprotein.
  
  
  CONCLUSIONS
  These studies extend observations on the schedule dependence of paclitaxel cytotoxicity and the role of P-glycoprotein in mediating paclitaxel sensitivity. The schedule dependence of relative resistance suggests that infusional paclitaxel may help in overcoming P-glycoprotein-mediated resistance.
authors.affiliation: Medicine Branch, National Cancer Institute, NIH, Bethesda, MD 20892, USA.
authors.forename: Z
authors.initials: Z
authors.lastname: Zhan
authors.forename: S
authors.initials: S
authors.lastname: Scala
authors.forename: A
authors.initials: A
authors.lastname: Monks
authors.forename: C
authors.initials: C
authors.lastname: Hose
authors.forename: S
authors.initials: S
authors.lastname: Bates
authors.forename: T
authors.initials: T
authors.lastname: Fojo
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000972:Antineoplastic Agents, Phytogenic / Q000008:administration & dosage / Q000494:pharmacology*; D001943:Breast Neoplasms; D002277:Carcinoma; D016923:Cell Death / Q000187:drug effects; D003110:Colonic Neoplasms; D004334:Drug Administration Schedule; D018432:Drug Resistance, Multiple / Q000502:physiology*; D019008:Drug Resistance, Neoplasm / Q000502:physiology*; D005260:Female; D006801:Humans; D010051:Ovarian Neoplasms; D017239:Paclitaxel / Q000008:administration & dosage / Q000494:pharmacology*; D014407:Tumor Cells, Cultured / Q000187:drug effects*
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000972:Antineoplastic Agents, Phytogenic; D017239:Paclitaxel